V Bolgova

S Kolesnikov

Andrey Leonov

S Lefebvre

Stéphane Veronika Bolgova

Sami Lefebvre

Nidhal Hlioui

François Boucenna

Andrey Costa

Leonov

S Hlioui

A Lefebvre

Méthodes D'

Bolgova Veronika

Hlioui Sami

Stéphane Lefebvre

THE CONTEXT OF THE THESIS International Conference Papers With Peer Review

Keywords: ASD Adjustable-Speed Drive. BJT Bipolar Junction Transistor IGBT Insulated Gate Bipolar Transistor KI Potassium Iodide. LEV PWM Pulse Width Modulation. RLC Resistance, Inductance, Capacitance. RMS Root Mean Square. Si Silicon. SiC Silicon Carbide. SRM Switched Reluctance Machine TEAM Thermal, Electrical, Ambient and Mechanical stresses. UV Ultraviolet. VDF Variable Frequency Drive. VPI Vacuum Pressure Impregnation. WBG Wide Bandgap

Development of testing methods for winding

I would like to thank my committee members, Professor Daniel Roger, Professor Oleg Nos, for examining thoroughly my manuscript, and the rest of the committee, Professor Gilbert Teyssedre and Associate Professor Talgat Gazizov, for fruitful discussions during the dissertation defence.

It is with the immense gratitude that I acknowledge my research supervisors, Andrey Leonov, Stéphane Lefebvre and Sami Hlioui. Thank you for your support and encouragement, for your guidance, patience, persistent help in both scientific and administrative issues that have been indispensable for the realization of my Ph.D. I wish to thank my Russian colleagues from the Tomsk Polytechnic University, Andrey Bukharkin, Stanislav Kolesnikov and Tatiana Soldatenko, for your presence, advices and helping hand when I was in Tomsk. I would like to give special gratitude to Boris Ubiyennykh to sharing your immense knowledge, enthusiasm and creativity.

I have had the great pleasure of working with my French colleagues from the SATIE. I am grateful to Yoann Pascal for your patience and help. Many thanks also go to my other colleagues, Mahamat Chabakata, Shuangfeng Zhang, Salim Asfirane, Matthieu Landel,... I am very grateful to Sophie ABRIET for your support and help with administrative problems quickly and efficiently. I would like to thank Adrien Dittrick, Tatiana Babicheva and Ekaterina Katia for being my friends and encouraging me from the beginning of my thesis.

Last but not the least, I would like to acknowledge with gratitude, the support and L'étude expérimentale et les simulations numériques réalisées dans le cadre de cette thèse se concentrent essentiellement sur les effets des ondes de tension à forts dV/dt et de la fréquence de commutation élevée sur la robustesse de l'isolation du bobinage des machines électriques alimentées par des onduleurs. Par conséquent, les travaux touchent un certain nombre de questions liées à l'entrainement à vitesse variable, à l'électronique de puissance, aux machines électriques, aux matériaux diélectriques et aux décharges partielles (DPs). La pertinence de ce travail est due à la mise en oeuvre de composants à semi-conducteurs grand gap, tels que le Carbure de silicium (SiC), dans les convertisseurs de puissance qui peuvent intensifier les contraintes électriques dans les moteurs entrainés, accélérant la défaillance d'isolation d'enroulement du moteur.

Le Chapitre I est consacré à un état de l'art sur les machines électriques basse tension et leurs systèmes d'isolation. Il synthétise les facteurs affectant l'isolation du bobinage des machines électriques lors de leur fabrication (la compatibilité des fils d'enroulement et des compositions d'imprégnation) ainsi que leur fonctionnement (des contraintes thermiques, électriques, et mécaniques, ainsi que des conditions ambiantes). Une attention particulière devrait être accordée aux contraintes électriques dues à la tension d'alimentation du moteur entraîné par un onduleur. Comme on peut le voir dans la littérature, l'isolation des bobinages des moteurs alimentés par des convertisseurs statiques basés sur la modulation de largeur d'impulsion (MLI) est soumise à des contraintes électriques provenant de la forme d'onde de la tension d'alimentation. Les phénomènes de nuisances v RÉSUMÉ DÉTAILLÉ EN FRANÇAIS électromagnétiques sur l'ensemble de la chaîne de conversion, les ondes réfléchies dans le câble d'alimentation, ainsi que la non-adaptation des impédances du câble et de l'enroulement de la machine, peuvent conduire à l'apparition de surtensions aux bornes de la machine. Les surtensions peuvent être suffisamment élevées pour dépasser la tension d'apparition des décharges partielles (TADP). L'apparition de ces décharges partielles entraine alors la dégradation des isolants qui, à son tour, amène à une augmentation de la fréquence des impulsions de DPs et de leurs amplitudes. Il en résulte une dégradation de l'isolation entre les spires qui, à terme, peut avoir pour conséquence la défaillance de la machine. Cela est particulièrement critique pour l'isolation de Type I utilisée dans les machines basse tension (jusqu'à 700 V de tension nominale efficace).

Le développement de l'électronique de puissance ne s'arrête pas comme on peut le constater avec l'implémentation des semi-conducteurs à large bande interdite, tels que le SiC, qui permet la réalisation de convertisseurs de forte densité de puissance notamment pour les systèmes d'entrainement à vitesse variable des machines électriques. En raison de vitesses de commutation plus élevées que les composants Silicium (Si), une fréquence de commutation plus importante peut être obtenue avec des dispositifs à large bande interdite, ce qui conduit à des contraintes électriques en dV/dt plus sévères sur l'isolation des enroulements des moteurs alimentés par de tels convertisseurs. Les enroulements de la machine peuvent alors subir des surtensions plus élevées ce qui peut augmenter le risque de DP et accélérer le vieillissement ainsi que la destruction de l'isolant. À cet égard, les contraintes liées à la forme d'ondes de tension à forts dV/dt générée par les onduleurs à base de SiC et leurs effets sur l'isolation du bobinage nécessitent d'une étude profonde.

Faisant suite à l'état de l'art, le Chapitre II présente une étude des surtensions transitoires dans un moteur alimenté par un onduleur qui permet d'estimer les contraintes électriques provoquant la dégradation accélérée de l'isolation du bobinage du moteur. À cet égard, un modèle hautes fréquences (HF) existant dans la littérature a été utilisé pour des simulations du système d'entrainement à vitesse variable. Les modèles HF proposés dans la littérature sont généralement destinés à examiner et prédire les phénomènes transitoires dans les machines asynchrones qui sont les machines électriques les plus utilisées pour les applications industrielles. Or, d'autres types de machines utilisés dans les systèmes d'entrainement à vitesse variable doivent être considérées. Par conséquent, notre étude se concentre sur la modélisation de la machine à réluctance variable (MRV) qui est particulièrement intéressante en raison de sa construction redondante et tolérante aux pannes, sa commande particulière utilisant un convertisseur de puissance en demipont asymétrique et son bobinage concentré contrairement aux moteurs asynchrones.

De plus, l'absence dans la littérature de modèles de MRV pour l'analyse de leurs comportements à haute fréquence en fait un objet d'études intéressant. vi RÉSUMÉ DÉTAILLÉ EN FRANÇAIS En se basant sur les modèles hautes fréquences développés pour les moteurs asynchrones triphasés, le moteur à réluctance variable à double saillances (triphasé, à 6 dents statorique et à 4 dents rotoriques, V DC = 300 V) a été modélisé par des circuits électriques équivalents aux paramètres RLC discrets. L'identification des paramètres du modèle HF est basée sur des mesures de l'impédance en mode commun et mode différentiel réalisées en utilisant l'analyseur d'impédance (Agillent Technologies E5061B, 5 Hz -3 GHz) dans la gamme de fréquences de 30 kHz à 30 MHz (Figure 1). En faisant une validation du modèle dans le domaine fréquentiel, nous avons observé une bonne corrélation entre les résultats mesurés et simulés présentés ci-dessous dans la Figure 2. Nous avons étudié les surtensions transitoires à l'entrée du moteur combinant le circuit équivalent du moteur suggéré et un modèle de câble d'alimentation composé des associations de cellules élémentaires proposé dans la littérature. Les phases de la machine à réluctance variable pouvant être alimentées indépendamment, nous avons effectué les observations des fronts de tension lors des commutations en mode monophasé. L'onduleur est modélisé par un générateur d'impulsions idéal avec des temps de montée/descente et donc dV/dt spécifiés. Les résultats des simulations confirment que les fronts raides de tension engendrent une surtension importante. Il est à noter qu'en cas de temps de montée extrêmement court, jusqu'à 10 ns, même un mètre de câble d'alimentation représente une longueur critique (Figure 3). De plus, l'augmentation de la longueur de câble intensifie la surtension transitoire, affecte la fréquence d'oscillation et allonge les oscillations transitoires (Figure 4). L'étude réalisée est en corrélation avec les travaux précédents et met en évidence l'importance de la caractérisation des effets des forts dV/dt. vii RÉSUMÉ DÉTAILLÉ EN FRANÇAIS Ces observations révèlent que la tension impulsionnelle ayant un temps de montée très court et une fréquence de commutation élevée donnent lieu à une concentration d'ozone plus élevée et à une augmentation significative de la température ce qui raccourcit extrêmement la durée de vie de l'isolation. Malheureusement, les effets des forts dV/dt et de la fréquence de commutation élevée n'ont pas pu être dissociés ce qui a compliqué l'analyse de l'effet du seul dV/dt.

Les résultats expérimentaux présentés ci-dessus m'ont motivé à examiner de manière plus approfondie les processus de détérioration de l'isolation entre spires exposée à une tension impulsionnelle à forts dV/dt, ainsi que les caractéristiques des décharges partielles comme je le décris dans le Chapitre V. Les éprouvettes sous forme de paire torsadée ont été utilisées pour mener cette étude phénoménologique.

En premier lieu, les surtensions causées par la longueur de câblage entre le générateur d'impulsions à base de SiC et la paire torsadée au cours des essais ont été modélisées et analysées en se basant sur l'approche électromagnétique et l'approximation de la ligne de transmission à faibles pertes (Figure 9). Les résultats obtenus mettent en xiii RÉSUMÉ DÉTAILLÉ EN FRANÇAIS évidence l'importance de minimiser la longueur du câble pour limiter les oscillations de tension et les surtensions (Figure 10) ce qui est en phase avec les observations présentées dans le Chapitre II. Les mesures de courant, de température et de spectre d'émission optique montrent une énergie élevée des décharges à barrière diélectrique augmentant avec l'accroissement de la fréquence de commutation, ainsi qu'avec l'intensification du champ électrique (également démontrée par des calculs électrostatiques effectués en utilisant la modélisation par éléments finis). Les images microscopiques des décharges (Figure 12a) et de l'érosion de surface de l'isolant (Figure 12b) mettent en évidence la structure filamentaire des décharges à barrière en les associant à l'érosion de l'isolant par les attaques chimiques dues aux filaments. Ainsi, le mécanisme de destruction de l'isolation entre spires Les principaux résultats et contributions de la présente thèse sont les suivants :

• Élaboration d'un modèle à haute fréquence destiné à étudier des surtensions transitoires aux bornes d'une machine à réluctance variable alimentée par un onduleur.

• Conception d'un banc d'essai qui comprend un générateur d'impulsions à base de SiC fournissant une tension impulsionnelle à forts dV/dt (jusqu'à 60 kV/µs appliqué à l'éprouvette) avec une amplitude de tension jusqu'à 1700 V et une fréquence de commutation maximale jusqu'à 125 kHz.

• Conception d'une nouvelle forme d'éprouvette pour l'essai des fils émaillés de section rectangulaire.

• Conception d'un banc d'essai synthétique associant deux sources de tension ce qui offre la possibilité d'appliquer la forme d'onde de tension désirée à l'éprouvette et en même temps de contrôler sa température, sa pression et son milieu.

• Estimation des défauts d'isolation à l'aide de la visualisation des défauts lorsque les éprouvettes sont placées sous la basse pression et soumises aux ondes de tension à forts dV/dt dépassant la TADP.

• Caractérisation des décharges partielles dans l'air entre les spires de paires torsadées comme des décharges à barrière diélectrique. Compte tenu du fait que la paire torsadée représente un modèle physique de l'enroulement de la machine, les décharges partielles dans l'enroulement peuvent également être considérées et étudiées comme des décharges à barrière diélectrique.

• Révélation de la structure filamentaire des décharges à barrière diélectrique dans les paires torsadées testées sous tension impulsionnelle à forts dV/dt avec haute fréquence de commutation, ce qui provoque l'apparition de traces d'érosion périodiques sur la surface de l'émail.

• Démonstration de fait qu'en cas de décharges à barrière dans les paires torsadées testées sous tension impulsionnelle à forts dV/dt à haute fréquence de commutation, le mécanisme de défaillance d'origine électrothermique peut prédominer sur celui d'origine électrochimique.

xvii

RÉSUMÉ DÉTAILLÉ EN FRANÇAIS

Le travail effectué ouvre des perspectives envisageables au terme de cette recherche parmi lesquelles nous pouvons citer :

• Poursuivre le travail sur la modélisation HF de la machine à réluctance variable intégrant les modèles de l'onduleur et du câble d'alimentation en étendant la gamme de fréquences. Approfondir l'étude de surtension en proposant un modèle permettant de déterminer les répartitions de tension dans le bobinage concentré et distribué.

• Étudier la distribution probable des défauts dans l'isolation du bobinage qui, en association avec la répartition de tension, permettra de prévoir avec plus de précision les zones présentant un risque d'apparition de décharges partielles.

• Approfondir l'étude expérimentale des mécanismes de défaillance de l'isolation entre spires soumise aux ondes de tension à forts dV/dt et hautes fréquences de commutation en contraintes environnementales telles que la basse pression. À l'issue de cette étude, développer un modèle mathématique de la durée de vie de l'isolation compte tenu des nouvelles contraintes imposées par l'utilisations des semiconducteurs à grand gap tels que le SiC.

• Approfondir l'étude des décharges à barrières diélectriques : déterminer un bilan de puissance (échauffement, production d'ozone, rayonnement UV, pertes diélectriques), examiner l'effet de la permittivité relative et de la capacité des fils sur l'apparition des décharges, améliorer le modèle des barrières diélectrique dans la paire torsadée en tenant compte des capacités dynamiques qui évoluent en présence de décharges, ainsi que la résistance variable caractérisant l'ionisation de l'air.

xviii

General introduction

Context

Electrical insulation quality plays a vital role in providing electrical machines reliability. This essential component of electrical machines operates under combined Thermal, Electrical, Ambient and Mechanical stresses (TEAM) [1], [2], [3], [4] and [5]. It results in ageing and gradual deterioration of insulation properties that can reduce machine's service lifetime. As reported by [2], [6], [7] and [8], one of the most common reasons of induction motor breakdowns is a stator winding turn insulation fault which accounts for 36-37% of all the failures. Thus, the turn insulation, consisting of enamelled wire and impregnating compound, is the weakest element of the machine's insulation system.

The impact of these factors on dielectrics ageing and deterioration depends on the electrical machine's environmental conditions (temperature, humidity and pressure), the power supply and the control system. In case of sinusoidal 50-Hz AC supply voltage, thermal stresses are the main cause of low voltage electrical machines insulation deterioration [2] and [3]. This is due to the fact, that voltage magnitude and surges are not high enough to cause an insulation breakdown or an ionization process.

Nowadays, the energy consumption due to electrical machines utilized in industry represents 40% of the total worldwide generated electrical energy [9]. The objective to reduce the environmental impact of transportation system inspire the hybrid, electric vehicles, and more electric aircrafts technologies continuing development. These advancements can also increase the electrical energy consumption that demands high-efficient motor drives design and electrical machines eco-conception [10], [11], [12], [13] and [14].

Aforementioned challenges highlight the importance of efficient variable-frequency control allowing to reduce energy losses and to adapt the electrical machine torque and speed to the load. At the same time, electrical machines being a part of Variable Frequency Drives (VDFs) are subjected to repetitive surges issued from impulse voltage waveform with steep rising and falling edges when supplied from power converters [3], [4] and [15]. The arising electrical stresses can drastically accelerate the electrical insulation ageing and lead to Partial Discharges (PDs) inception in low voltage electrical machines [16], [17], [18] and [19]. This phenomenon, previously considered only in high voltage electrical equipment, provokes complex physico-chemical processes and decreases notably the

GENERAL INTRODUCTION

low voltage insulation service life.

The negative effect of impulse voltage becomes more important with the implementation of Wide Bandgap (WBG) power semiconductors as Silicon Carbide (SiC). The modern advances in semiconductors technology allow to create powerful inverters with higher switching speed and frequency range in comparison with Silicon technology, providing losses reduction, compactness, and lower weight [12], [20], [21], [22], [23], [24] and [25]. Due to these features SiC power semiconductor devices are especially attractive for motor drives in automotive, aerospace, and industrial applications where power density and weight are the main factors for power electronics dimensioning [12], [23] and [26].

Nevertheless, resulting electrical stresses are severe for the controlled motors: high dV/dt pulses with very fast rise time (about 10-50 ns), raise of common mode currents and electromagnetic radiation [24], [27] and [28]. Due to this fact, PDs will occur with higher probability and the motor insulation ageing will be accelerated shortening the service life.

Since the presence of PDs in inverter-fed low voltage motors have been recognized, numerous studies were related to characterize PDs, inception conditions and the influence of their presence on the insulation [16], [17], and to compare enamel winding wires' endurance to partial discharges (also referred as corona resistance) [29], [30], [31], [32], or to detect insulation defects [33]. Insulation lifetime models depending on the applied stresses and their interaction are described in [34] and [35]. Thermal, mechanical and electrical stresses effects on turn insulation were studied separately in [36] using experimental setups, without consideration of factors interaction effect. In [37] analysis and predetermination of voltage distribution and spikes in motor windings were carried out.

In thesis [4] the voltage distribution modelling in simplified motor winding represented by equivalent circuit and the method to evaluate the risk of PDs occurrence are proposed.

The use of WBG power electronics in motor drives requires new methods and approaches to study PDs effect on motor winding insulation. In reference [27] the experimental setup is performed using SiC components reproducing environmental and operational conditions. Nevertheless, the fact that the tests are conducted on complex motor drive system makes them relatively expensive and difficult to use in order to study the physical processes of dielectric ageing, or to utilize in motors conception. Therefore, ageing and deterioration process in insulation subjected to impulse voltage, in particular, with high dV/dt and high commutation frequency are not thoroughly investigated. Moreover, considering the influence of thermal stresses affecting the insulation during motor operation, the process of electro-thermal ageing needs to be considered.

GENERAL INTRODUCTION

Objectives and methods

Having regard to the above, this study is dedicated to investigate the constraints applied to solid dielectrics used in inverter fed motors and their effects on motor windings considering the high dV/dt and high switching frequency specific to SiC-based inverters.

To achieve these goals, the work is focused on the following axes:

• Develop models for determine voltage transients and surges at machine's terminals characterized by high dV/dt.

• Define the methodology for estimating the endurance of insulating materials used in low voltage electrical machines fed by inverters. In particularly, the turn insulation endurance in presence of partial discharges will be studied.

• Design the experimental test bench for reproducing the constraints imposed by impulse voltage waveform with high dV/dt and high switching frequency. The temperature effect also should be considered.

• Investigate the processes of electro-thermal insulation ageing and destruction due to partial discharges activity in inverter fed machines.

Main contributions

One of the main contributions of this work consists in revealing the influence of combined electrical and thermal effects on the enamel insulation resistance to partial discharges and insulation lifespan. It should be noted, that the high frequency impulse voltage waveform with steep edges is a limiting factor that determines the applicability of insulating materials in inverter fed motors. This is confirmed by experimental results of endurance tests when an accelerated deterioration and a change in insulation destruction mechanism were observed on wires' specimens subjected to voltage pulses having high dV/dt ratio and high switching frequency.

The partial discharges arising in air gaps between turns of winding wire's specimens subjected to high dV/dt impulse voltage having high commutation frequency have been characterized as filamentary dielectric barrier discharges; having regard that the twisted pair represents a physical model of machine's winding part, the PDs within the winding may be also considered and studied as dielectric barrier discharges.

The proposed testing methods and experimental benches allow to conduct accelerated ageing and endurance tests of enamel insulation under electrical stresses engendered by conventional Silicon (Si) and modern SiC inverters. Furthermore, a novel specimen's shape for testing enamelled rectangular winding wires is proposed that expands the range of studied motors.

GENERAL INTRODUCTION

Another contribution is a method of detecting insulation defects such as air gaps and cracks testing under the impulse voltage at low pressure. The random and artificial defects have been detected during the experimental study of partial discharges phenomenon on winding wire's specimens.

Outline

This thesis is organized into five chapters. The final part provides the main conclusions drawn from this work and future research axes.

The first chapter is devoted to factors affecting the insulation reliability of lowvoltage electrical machines powered by inverters. The influence of defects and turn insulation components compatibility on mechanical and electric strength is considered. The negative effects of variable frequency control on the motor insulation system are identified, and the main solutions of the mentioned problems proposed in literature are discussed.

In the second chapter the high frequency model of the electric drive system composed by inverter, supply cable and motor is suggested. The main goal is to provide the analysis of overvoltage at motor terminals caused by high dV/dt and impedance mismatch between the inverter, cable and the powered motor, which plays an important role for motor insulation robustness. This study is focused on the modelling of a three-phase

Switched

Introduction

Variable Frequency Drives (VDFs) are widely used throughout industry, electrical energy production and transportation systems, providing energy savings and equipment performance improvement due to adjustable speed control of electrical machines.

Electrical machines operate converting electrical energy in mechanical energy in "motor" mode or inversely in "generator" mode (regenerative braking). Schematic representation of typical system usually used in industrial applications composed of an induction motor fed by a power converter through a long supply cable is given in Figure I.1 [38].

Figure I.1: Typical motor drive system with a long supply cable [38].

Depending on applications, the distance between machine and feeding converter can be shorter. For example, in automotive systems adopted electric propulsion, the traction electric motor and the controlling power electronic block can be disposed in close proximity. Figure I.2 depicts traction and charging power systems installed on-board of Renault's electric vehicle: ZOE [39].

Another example of propulsion system for electric vehicles is a wheel motor technology [40] and [41]. The Michelin Active Wheel concept (Figure I.3) was firstly presented at "Challenge Bibendum 2004's edition". In-wheel electric drive integration is a promising technology, which combines mechanical and electrical components giving opportunities for suspension's mass reducing and driving strategies optimization. With the purpose to obtain more compact embedded systems, deep converter-integration concepts, consisting of electrical machine and integrated VDF, are designed [14] and [42]. INVERTERS: LITERATURE SURVEY (b) vehicle powertrain scheme [41].

Regardless of integration technology, electrical machine being part of electric drive, constitutes the final element of a chain of energy conversion; it is at the interface between the power converter and the mechanical system. Therefore, it constitutes a preferred route for the transmission of electromagnetic interferences in the environment [12], [20], [22], [23], [24], [25] and [26]. Having regard to the above, this Chapter presents a literature survey on low voltage motors and their insulation systems, detailing the key factors affecting motor winding insulation. There, the influence of impulse supply voltage on the electrical machine controlled by PWM inverters will be studied and negative effects caused by long supply cable will be discussed.

Electrical machines

Electric motors consume large part of worldwide produced energy. They cover a vast number of applications everywhere in industry, transport, and household appliance, with different sizes and supply voltages. The electric motor is also the central component in Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs). Concerning automotive applications, there are many types of motors that could be used in electric traction drives HEVs and EVs [14] and [45] (see Figure I.6).

Regarding aeronautics and aerospace applications, a particular attention is given when selecting electric motor type and drive topology. It is especially crucial in More Electric Aircraft (MEA) where electric driven systems replace conventional mechanical, hydraulic, or pneumatic systems providing technical and economic improvements. An overview of motor drive technologies used for safety-critical MEA applications is given in [46]. According to this overview, the feasible candidate machines are limited to motors

Induction machine

The fields of application of induction machines comprise a significant part of the industry applications, owing to their reliability, ruggedness, robust construction, and lower cost of maintenance compared with synchronous electrical machines and DC motors, and also their ability to operate in hostile environments [1], [45], [47] and [48]. In automotive drives IMs are utilized in such vehicles as Renault Kangoo, Daimler Chrysler Durango, Chevrolet Silverado, BMW X5, and Tesla [47]. One of the most widely utilized type of IM is the well-known cage (or squirrel cage) induction motor. The cage induction motor predominance is attributed to the simplicity and ruggedness of the rotor [1]. Figures I.9 and I.10 show the construction and magnetic circuit of an induction motor respectively [8].

Induction motors main drawbacks are high losses (stator copper losses, core (iron) losses, stray and mechanical losses), low efficiency, and low power factor. To improve their efficiency and performance, IMs should be controlled through power converters as

shown in Figure I.1 and different minimum-loss control schemes and strategies can be utilized, as described in [49] and [50]. INVERTERS: LITERATURE SURVEY

Switched reluctance machine

Compared with mature motor technologies such as the induction motor, DC shunt motor, and the more recent brushless Permanent Magnet Synchronous Machine (PMSM), SRM offers a competitive alternative to its counterparts [14] and [51]. The numerous advantages such as high torque density, simple construction, robustness and low manufacturing cost make this type of machines attractive for researchers and manufacturers [52].

The most used conventional SRM have different numbers of salient poles on the stator and rotor. There are two windings of different phases in the each slot of the stator.

The rotor does not contain any windings or excitation system which allows reaching very high speeds [53]. Moreover, by contrast with PMSM, this machine do not contain costly rare earth permanent magnets. As an example, the construction of three-phase and I.11c respectievely [54].

The SRM operation principle is based on variable reluctance. The torque is produced by the tendency of the rotor poles to align with the excited stator poles (see Fig-

ure I.11b).
During rotation of a rotor, the reluctance and, consequently, the phase inductance vary between their minimal and maximal values. When operated as a motor, the phase windings are excited during the period in which the inductance is increasing as the rotor turns. When operated as a generator, the phases are commutated on and off during the period when the inductance is reduced as the rotor turns. The higher the ratio of aligned inductance to the unaligned inductance, the higher the torque [55] and [56]. Based on the previously mentioned, the torque T of three-phase SRM can be defined as follows:

T = 1 2 i 2 a d L a d θ + 1 2 i 2 b d L b d θ + 1 2 i 2 c d L c d θ (I.1)
where i i and L i are the corresponding phase currents and inductances; θ is an electrical angle that defines a rotor position. As shown in [52] the mutual inductance between phases for conventional SRM can be neglected due to its winding configuration.

According to the expression I.1, the torque is directly proportional to ∂L ∂θ

and I 2 ,
where the last one is always positive regardless of the sign of excited current, so the excitation can be unipolar. Unipolar operation requires nonstandard power electronic modules: SRM is mostly driven by asymmetric half-bridge power converter [57] (see Figure I.12).

Another converter's topology can also be employed depending on applications and requirements, e.g., bipolar excitation can be used to increase the torque density and reduce torque pulsations [56] There are different types of SRM, where the stator and rotor poles number combinations and phase's number determine the machine and drive configurations and performances. Increasing stator/rotor poles number allows to produce the higher power factor and torque density with lower torque ripple. On the other hand, it reduces the constant power capability and the overload capability. A larger number of phases improves the reliability and fault tolerance, but complicates the inverter and requires a higher operating frequency for a given speed [55] and [56].

Overviews [14], [45], [47], [56], [58] and research works [53], [START_REF] Andrada | Switched Reluctance Drives for Electric Vehicle Applications[END_REF], [60] and [61] on automotive propulsion systems demonstrate that switched reluctance motors are recognized to have a potential for EV/HEV applications. Examples of SRM drives automotive application are Land Rover Defender, presented at the 2013 Geneva Motor Show [62], the Australian hybrid ECOmmodore and hybrid aXcessaustralia Low Emission Vehicle (LEV) [60].

The main disadvantages of this type of electrical machines are the high torque ripple, the acoustic noise and vibrations (especially at low speeds), undesirable in modern vehicles [51]. These drawbacks can be minimized by optimization of control strategy (torque/current control) and motor design. The SRM with controlled torque can find application where standard variable frequency drives are common.

Electrical machine winding

As can be seen from the foregoing, two types of electrical machines (cage induction motor and switched reluctance motor) are considered in this work. Thus, only stator winding insulation systems will be discussed. wire or magnet wire. This wire is wound continuously (by using a winding machine or by hand) through slots in the stator magnet core to form a coil: turns of wire connected in series, arranged to link or produce magnetic flux. These turns are placed randomly in coils, thus the term "random". The first and the last turns can be adjacent, and in this case turn insulation is subjected to the entire voltage of the coil [1]. The winding insulation system's components will be detailed in Section 4. In case of conventional switched reluctance motor, winding is wound around each tooth of the stator that allows to minimize magnetic coupling between phases and make easier electrical feeding [51] and [52].

Concentrated winding

Both round and rectangular sections wires can be used to constitute winding coils.

The use of rectangular winding wires allows to increase a slot filling factor (or space factor), the ratio between the total conductor area in a slot and slot area, as shown in Fig-

ure I.18 [67]. This parameter determines the motor's rated power. By improving the slot filling factor the power density may be increased and the DC winding losses may be reduced [68].

The coil concentrated winding configuration simplifies mechanical design and manufacturing process as well as machine's maintainability by offering a stator modularity possibility [68]. It makes possible to arrange turns in winding coils in order to assure better electric field distribution compared with random wound winding. The multiphase concentrated winding can improve the electrical machine's fault tolerance [69]. Nevertheless, the turn insulation tends to be exposed to higher mechanical stresses during the coil-forming process, creating insulation stretches and cracks [1].

Furthermore, distributed windings demonstrate higher torque and power density performances compared with concentrated winding [68]. The stator winding topology should be selected regarding electrical machines speed and power range, and respecting torque, volume and efficiency requirements.

CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY

Electrical machine winding insulation system

Electrical machine's insulation system is a combination of dielectric materials. According to IEC 60034-18-1:2010 [70], its role is to provide insulation of conducting parts from each other and grounded ones, to prevent short-circuits between winding turns, and thus to ensure reliable electrical machine operation.

In the following sections components of stator winding insulation systems, as well as the main factors affecting them during machines operation will be discussed.

As previously noted, winding insulation system comprises slot insulation, phase to phase, phase to ground and turn insulation. The slot insulation, phase to phase and phase to ground insulation are usually made of paper or organic laminated materials based on polymers (Nomex®, Mylar®) providing good dielectric strength, flexibility, mechanical toughness and thermal conductivity. This insulation has a greater thickness compared with the turn insulation composed of winding wire and impregnating composition [1], [3],

[4] and [71]. Whereas the turn insulation is mostly subjected to failures, this component of insulation system will be studied in detail.

Winding wires

Winding wires having thin enamel insulation (enamelled wires) are mostly used in low voltage electrical machines windings. The thinner insulation allows to assure a higher slot filling factor. Moreover, overall dimensions of motor can be decreased by reducing the insulation thickness while keeping the energetic parameters [72].

The enamelled winding wire is a copper or aluminium conductor coating with a polymer film insulation composed of one, two, or three types of enamel varnish -insulating layers (Figure I.19). In modern winding wires the combination of different chemical compositions is often used to provide the required electrical, thermal and mechanical parameters [START_REF] Çåëåíåöêèé | Þðèé Àíäðååâè÷. Ñîâðåìåííûå òåíäåíöèè â ïðîèçâîäñòâå ýìàëèðîâàííûõ ïðîâîäîâ[END_REF]. Enamel varnish is a solution of high-molecular-weight film-forming compounds in organic volatile compounds, where the volume of a dry (solid) substrate in average corresponds to 28-32% [START_REF]Âàñèëüåâ, Å.Á. Ñîñòîÿíèå ïðîèçâîäñòâà è íàó÷íî-òåõíè÷åñêèå àñïåêòû ðàçâèòèÿ îáìîòî÷íûõ ïðîâîäîâ â Ðîññèè[END_REF] and [START_REF] Aymonino | Contribution à l'étude du comportement des systèmes d'isolation des machines tournantes à courant alternatif fonctionnant sous très hautes températures (200-400[END_REF]. Depending on required winding wire insulation INVERTERS: LITERATURE SURVEY properties and motor operation conditions, different chemical compositions can be applied. The most common are the enamel varnish based on polyester (PE), polyimide (PI), polyurethane (PU), polyamide (PA), polyamide-imide (PAI), and polyester-imide (PEI).

The enamel film should be applied to the wire continuously and as thinly as possible. Firstly, it allows to have a good filling factor of wound stator, as it was mentioned above. Secondly, it prevents the falling drops of liquid varnish which tend to distribute unevenly over the wire. Therefore, each insulating layer (of one chemical composition) consists of a number of very thin coats (Figure I.20), which are applied sequentially passing through an enamel applicator. This reduces amount of defects and micro-voids in the enamel insulation resulting from the volatilization of low-molecular additives and solvents. The number of these single layers can reach up to 20-30 µm and the total thickness of the enamel film comes to be from 30 to 60 µm [START_REF]Âàñèëüåâ, Å.Á. Ñîñòîÿíèå ïðîèçâîäñòâà è íàó÷íî-òåõíè÷åñêèå àñïåêòû ðàçâèòèÿ îáìîòî÷íûõ ïðîâîäîâ â Ðîññèè[END_REF] and [START_REF] Aymonino | Contribution à l'étude du comportement des systèmes d'isolation des machines tournantes à courant alternatif fonctionnant sous très hautes températures (200-400[END_REF]. Depending on the insulation range of thickness or grade, enamelled wires are categorized in accordance with the IEC 60317-0-1:2013 [START_REF]Specifications for particular types of winding wires -Part 0-1: General requirements[END_REF]. The insulation grade is defined by external wire's diameter D (D grade1 < D grade2 < D grade3 ). Therefore, properties which depend on the thickness of the insulation (e.g., electrical breakdown voltage, resistance to abrasion, springiness, etc.) vary from one grade to the next. A higher grade number means that wire has higher electric strength. The copper round enamelled wire is the most prevailing winding wire type by reason of production process features and product quality. [48] and [START_REF]Electrical Insulation -Thermal evaluation and designation[END_REF]. According to [START_REF] Çåëåíåöêèé | Þðèé Àíäðååâè÷. Ñîâðåìåííûå òåíäåíöèè â ïðîèçâîäñòâå ýìàëèðîâàííûõ ïðîâîäîâ[END_REF] and [START_REF]Âàñèëüåâ, Å.Á. Ñîñòîÿíèå ïðîèçâîäñòâà è íàó÷íî-òåõíè÷åñêèå àñïåêòû ðàçâèòèÿ îáìîòî÷íûõ ïðîâîäîâ â Ðîññèè[END_REF] in Russia enamelled wires having the thermal classes of 155 and 180 are mainly produced. Furthermore, higher winding wire thermal class allows to increase electrical machines power density or to reduce their size and weight that is especially crucial for traction drives.

Winding impregnation

With the purpose to consolidate winding's turns, the winding is impregnated with a liquid impregnating composition (compound, resin or varnish). Epoxy, polyester or silicon resins are often used. The impregnation reduces air voids inside stator slots and between winding wire's turns, prevents winding moisture and contamination, improves winding mechanical strength and heat transfer.

For wound stator impregnating, dipping, trickling or Vacuum Pressure Impregnation (VPI) technologies can be used. In dipping method, the winding is fully immersed in reservoir containing liquid impregnating varnish until air gaps filling with following curing in an oven and cooling. Trickling impregnation process is performed at a very slowly rotating wound stator and the impregnating material is dripped over. During this process a current circulating in winding increases its temperature due to Joule effect to cure the impregnant. In VPI impregnation processing, the wound stator is placed under vacuum to remove air from winding gaps and pores, and placed in impregnation resin bath. Then the excessive pressure is applied to force impregnating material penetration into the stator winding. After that, draining and baking processes follow to cure the impregnant. VPI impregnation allows to improve air gaps filling between conductor strands minimizing voids in turn insulation and providing a greater bond strength [1].

As an alternative to impregnation self-adherent enamelled wires can be used. Their thermo-adhesive external layer, e.g., made of aromatic polyamide, allows to solidify winding turns by bonding at elevated temperature. To this effect, winding heating is usually achieved by circulating current that cures wires' enamel top coat [3].

Impregnating compositions and wires enamel insulation compatibility

As far as the winding insulation system includes the winding wire enamel film and the impregnating composition, winding insulation reliability is determined by quality and compatibility of both insulating materials. Differences in chemical and physicomechanical properties of these components may cause negative effects on insulation system performance [START_REF] Aymonino | Contribution à l'étude du comportement des systèmes d'isolation des machines tournantes à courant alternatif fonctionnant sous très hautes températures (200-400[END_REF].

The impregnant, the enamel film and the conductor are held together by adhesive forces. These forces create intermolecular bonding between contacting surfaces and provide system mechanical integrity. System components are forced to be deformed together under thermal cycling or external loads, but differences in materials' coefficient of thermal linear expansion and modulus of elasticity lead to unequal deformations. As a result, subsurface stresses appear in insulation. Engendered mechanical integrity deterioration facilitates penetration of moisture, oxygen and corrosive components into windings, intensifying turn insulation ageing processes and decreasing breakdown voltage [START_REF] Çåëåíåöêèé | Þðèé Àíäðååâè÷. Ñîâðåìåííûå òåíäåíöèè â ïðîèçâîäñòâå ýìàëèðîâàííûõ ïðîâîäîâ[END_REF] and [START_REF]Âàñèëüåâ, Å.Á. Ñîñòîÿíèå ïðîèçâîäñòâà è íàó÷íî-òåõíè÷åñêèå àñïåêòû ðàçâèòèÿ îáìîòî÷íûõ ïðîâîäîâ â Ðîññèè[END_REF].

Solvents contained in impregnating compositions affect enamel insulation during impregnation process and subsequent curing. As a result, enamel insulation softening, swelling or cracking may occur. Swollen enamel film adhesion often decreases while heating that leads to contact imperfections with conductor. Solvent negative influence increases with heating and mechanical stresses in enamel insulation caused by wires stretching and bonding when stator is wound [START_REF]Âàñèëüåâ, Å.Á. Ñîñòîÿíèå ïðîèçâîäñòâà è íàó÷íî-òåõíè÷åñêèå àñïåêòû ðàçâèòèÿ îáìîòî÷íûõ ïðîâîäîâ â Ðîññèè[END_REF]. Depending on impregnating composition (varnish, resin, compound, with solvent or solvent-free), impregnation technique and conductor strands density, turn insulation may have different microstructures.

A complete filling of interturn space allows to obtain a monolithic turn insulation microstructure (Figure I.21a). When an impregnant is partially leaked or solvent is evaporated during curing process air pockets (cavities) appear in impregnated winding. The cured impregnating material is distributed around wires as a film with uneven thickness.

Such insulation microstructure is called cellular (Figure I.21b) [71].

The real winding usually has mixed microstructure. The monolithic and cellular microstructures ratio depends on used impregnating material, its technological features (viscosity, non-volatile matter content), impregnating method and quality, as well as on wire's diameter, slot filling factor and winding design. Turn insulation microstructure, The minimum level of impregnant and enamel film adhesion should ensure windings integrity. Adhesion increasing leads to rising internal mechanical stresses in the impregnating material and affects defect formation processes. Therefore, turn insulation destruction processes start with impregnating material cracking caused by internal mechanical stresses. When a crack reaches enamel insulation, its propagation mechanism depends on adhesive forces in "impregnating material -enamelled wire" interface and eventual surfaces imperfections. The mechanism is described in [71]. In front of the crack tip, which is a stress concentrator, the local internal mechanical stresses magnitude is significantly higher than in the rest of the polymer's volume. If interface adhesive force is higher than internal mechanical stresses in impregnating material, the crack can pass through the enamel film and reach the wire's conductor ( (b) crack in impregnating material with delamination from enamel film [71].

Having regard on above stated, turn insulation components should be selected considering their chemical and physico-mechanical properties and their compatibility characterized by interface adhesive forces that will allow to obtain the insulation system having minimum defects.

Operational factors affecting winding insulation

During production and operation processes, insulation is influenced by Thermal, Electrical, Ambient and Mechanical stresses (TEAM), including also pressure, moisture, and corrosive substances [1], [START_REF] Aymonino | Contribution à l'étude du comportement des systèmes d'isolation des machines tournantes à courant alternatif fonctionnant sous très hautes températures (200-400[END_REF], [78], [START_REF] Ãîëüäáåðã | Íàäåaeíîñòü ýëåêòðè÷åñêèõ ìàøèí. Ì[END_REF] and [START_REF]Íàöèîíàëüíûé èññëåäîâàòåëüñêèé Òîìñêèé ïîëèòåõíè÷åñêèé óíèâåðñèòåò[END_REF] (see Figure I.24 [START_REF]Íàöèîíàëüíûé èññëåäîâàòåëüñêèé Òîìñêèé ïîëèòåõíè÷åñêèé óíèâåðñèòåò[END_REF]). It results in ageing and time-depended deterioration of insulation properties that can reduce motor's service lifetime.

Environmental humidity influences on electrical machines time between failures, because moisture adsorption and sorption processes in insulating materials reduce its dielectric strength. Moisture effect is especially significant in poorly impregnated winding. In this case, moisture penetrates into unfilled space and diffuses into insulation's pores. Reducing insulation resistance and dielectric strength, moisture creates conditions for leakage currents appearance. Chemically active substances, such as oils and solvent vapours, influence on insulation leading to its deterioration. A dust contained in the air has an abrasive effect on insulation and reduces its insulating resistance [3] and [START_REF] Ãîëüäáåðã | Íàäåaeíîñòü ýëåêòðè÷åñêèõ ìàøèí. Ì[END_REF]. forces on current). Cycles of mechanical and thermomechanical loads damage the insulation system integrity [START_REF]Íàöèîíàëüíûé èññëåäîâàòåëüñêèé Òîìñêèé ïîëèòåõíè÷åñêèé óíèâåðñèòåò[END_REF].

In case of sinusoidal 50-Hz AC supply voltage, the temperature effect is considered as one of the main operational factor causing the low-voltage insulation properties degradation [1], [2], [3], [8] and [START_REF] Tshiloz | Real-time insulation lifetime monitoring for motor windings[END_REF]. The low-voltage winding insulation is primarily determined by the electrical stress [1].

Prolonged heating during the electrical machine operation is inevitably accompanied by the winding insulation system thermal ageing and deterioration provoked by chemical reactions of oxidation in air-cooled machines [1]. The chemical reaction rate can be estimated using empirical Arrhenius law. In [START_REF] Thomas W Dakin | Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon[END_REF] it is proposed to use the Arrhenius chemical reaction rates relationship formulated in the exponential form to predict insulation life depending on expected service temperature:

L = Ae -B T (I.2)
where L is insulation life time in hours; T is the absolute temperature in Kelvin, A and B are constants dependent on environmental conditions.

CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY

The known "10 °C half-life rule" approximates this equation: every 10 °C increase in operating temperature above the winding insulation system thermal class (temperature limit) reduces the expected insulation service life by half [1], [48] and [START_REF] Lipo | Introduction to AC Machine Design[END_REF] (Figure I.25 [START_REF] Lipo | Introduction to AC Machine Design[END_REF]).

Failures in motor windings

Electrical machines insulation should be selected considering constraints mentioned above and be enable to ensure specified service lifetime. As reported by [2], [6],

[7] and [8] a inter-turn short circuits between turns of the same phase (turn-to-turn fault); b short circuited winding (coil-to-coil fault); c short circuits between winding and stator core at the end of the stator slot (coil to ground fault); d short circuits between winding and stator core in the middle of the stator slot;

e short circuit at the leads;

f short circuit between phases (phase-to-phase fault).

Effect of impulse supply voltage on winding insulation

The impact of described above factors on dielectrics ageing and deterioration depends on the electrical machine's environmental conditions, the power supply and the control system. As previously noted, in case of sinusoidal 50-Hz AC supply voltage, thermal stresses are the main cause of low voltage electrical machines insulation deterioration. This is due to the fact, that voltage magnitude and surges are not high enough to cause an insulation breakdown or an ionization process.

Numerous studies [1], [3], [4], [15], [START_REF] Fabiani | Aging investigation of motor winding insulation subjected to PWM-supply through PD measurements[END_REF], [START_REF] Liu | Overvoltage mitigation of submersible motors with long cables of different lengths[END_REF], [START_REF] Nussbaumer | Transient distribution of voltages in induction machine stator windings resulting from switching of power electronics[END_REF], [START_REF] Martin Kaufhold | Electrical stress and failure mechanism of the winding insulation in PWMinverter-fed low-voltage induction motors[END_REF] and [START_REF] Kaufhold | Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters[END_REF] The impulse voltage waveform with steep rising and falling edges creates a nonlinear voltage distribution along feeding motor windings. Indeed, overvoltage at motor terminals can occur due to reflected waves phenomena in cable line caused by mismatching of inverter, supply cable and motor winding impedances [15], [START_REF] Fabiani | Aging investigation of motor winding insulation subjected to PWM-supply through PD measurements[END_REF], [START_REF] Liu | Overvoltage mitigation of submersible motors with long cables of different lengths[END_REF], [START_REF] Nussbaumer | Transient distribution of voltages in induction machine stator windings resulting from switching of power electronics[END_REF], [START_REF] Kaufhold | Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters[END_REF] and [START_REF] Melfi | Effect of surge voltage risetime on the insulation of low-voltage machines fed by PWM converters[END_REF].

Overvoltage at motor terminals depending on voltage rise time and drive to motor cable length is shown in Figure I.30 [START_REF] Melfi | Effect of surge voltage risetime on the insulation of low-voltage machines fed by PWM converters[END_REF]. Moreover, in a random wound motor winding, the first and the last turns of one or more coils may be adjacent and be exposed to the entire terminal voltage [3], [START_REF] Fabiani | Aging investigation of motor winding insulation subjected to PWM-supply through PD measurements[END_REF], [START_REF] Nussbaumer | Transient distribution of voltages in induction machine stator windings resulting from switching of power electronics[END_REF] and [START_REF] Kaufhold | Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters[END_REF]. The increased voltage amplitudes and the transient voltages uneven distribution lead to rise in voltage amplitude stressing the turn insulation up to ten times compared with machines operated at sine AC voltage of 50 or 60 Hz frequency [START_REF] Kaufhold | Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters[END_REF].

The negative effect of repetitive impulse voltage becomes more important with the implementation of wide bandgap power semiconductors as Silicon Carbide in inverters.

The modern advances in semiconductors technology allow to create inverters with high temperature capabilities, lower conduction and switching losses, higher switching speed and frequency range in comparison with Si technology, providing compactness and lower weight [12], [20], [21], [22], [23], [24] and [26]. Due to these features, SiC power semiconductor devices are very attractive for motor drives in automotive, aerospace, especially for MEA concepts, as well as in industrial applications where power density and weight are one of the main factors for power electronics sizing [12], [20], [22] and [26]. Nevertheless, resulting electrical stresses are severe for the controlled motors: dV/dt with very low rise/fall times (10-50 ns), raise of common mode currents and electromagnetic radiation [3], [24], [27], [28] and [START_REF] Barater | Multistress Characterization of Fault Mechanisms in Aerospace Electric Actuators[END_REF].

Overvoltage in inverter-fed motor winding can be higher than the value of Partial Discharge Inception Voltage (PDIV) that provokes partial discharges in winding insulation leading to early insulation degradations and/or failures.

CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY

According to [START_REF] Melfi | Effect of surge voltage risetime on the insulation of low-voltage machines fed by PWM converters[END_REF] and [START_REF] Êó÷èíñêèé | Ãåîðãèé Ñòàíèñëàâîâè÷. ×àñòè÷íûå ðàçðÿäû â âûñîêîâîëüòíûõ êîíñòðóêöèÿõ[END_REF]

L = cV -n (I.3)
where c is a material constant and n is the power law constant.

CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY

A lot of researches have been devoted to develop PDs detection methods, to eliminate electromagnetic interference effects and to analyse measured characteristics [96],

[105], [START_REF] Hekmati | Proposed method of partial discharge allocation with acoustic emission sensors within power transformers[END_REF], [START_REF] Danouj | Using a new generation of piezoelectric sensors for partial discharge detection[END_REF], [START_REF] Mountassir | Quantification of the performance of iterative and non-iterative computational methods of locating partial discharges using RF measurement techniques[END_REF], [START_REF] Zhang | Research on current sensor adapted for partial discharge detection in inter-turn insulation of wind turbine generator[END_REF], [START_REF] Wu | An overview of state-of-the-art partial discharge analysis techniques for condition monitoring[END_REF] and [START_REF] Siebler | Optical-only Detection of Partial Discharge with Fluorescent Polymer Optical Fiber Sensors[END_REF]. In electrical detection methods ultra-high frequency sensors or antennas, high-frequency current transformers and coupling capacitors are employed. Partial discharges in high voltage cables can be detected using acoustic technique (piezoelectric sensor). Optical detection (optical fiber based sensors, ultraviolet cameras) and chemical methods (e.g., analysis of gases dissolved in transformer oil and liquid chromatography) are also proposed.

Partial discharges measurements are often used for insulation condition and fault diagnostics in high voltage cable lines, electrical machines and transformers. This allows to identify insulation defects at the earliest stages of its occurrence, to monitor their propagation, to evaluate insulation current state and further operation. In [START_REF] Fabiani | Aging investigation of motor winding insulation subjected to PWM-supply through PD measurements[END_REF] it is proposed to use partial discharge measurements also for low-voltage electrical machines condition monitoring. However, in case of electric motors having Type I insulation system [63],

which are not expected to withstand PDs activity, the presence of partial discharges might indicate a critical insulation condition.

In the order to eliminate or limit electrical discharges in electrical machine's winding different solutions may be employed. Firstly, the risk of partial discharge inception can be decreased by limiting electrical stresses in motor winding insulation using filters to adapt cable impedance and inverter-fed motor impedance [3], [4] and [START_REF] Vermaelen | Contribution à la modélisation et à la réduction des perturbations conduites dans les systèmes d'entraînement à vitesse variable[END_REF]. It reduces

Conclusion

The present Chapter was devoted to an overview of factors affecting low voltage electrical machines fed by inverters. Firstly, different types of electrical machines which could be used in automotive and aeronautical applications are shortly discussed focusing on induction machines as commonly used and switched reluctance machines as promising competitive alternative to another electrical machines' types. For these machines construction and components of distributed and concentrated stator windings are given.

It is shown, that the turn insulation composed of winding wire enamel insulation and impregnating material is the weakest element of winding insulation system. In this regard, the described technological parameters and operational stresses is presented in relation with their impact on the turn insulation.

The conducted literature survey reveals the negative influence of impulse supply voltage on winding insulation in electrical machines fed by inverters compared with sinusoidal voltage supplied machines. Arising overvoltage and uneven voltage distribution creates conditions for partial discharge ignition in winding insulation.

The implementation of wide bandgap power semiconductors as SiC in recent years

CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY

allows to create very powerful inverters with high switching speed and frequency enhancing electric drives efficiency and reducing their size and weight. However, resulting electrical stresses are severe for controlled motors: dV/dt with ns-level of rising and falling time, high switching frequency, raise of common mode currents and electromagnetic radiation. Due to this fact, partial discharges will occur with higher probability, and the winding insulation ageing and failure will be accelerated.

Thus, the aim of this work is to investigate the influence of voltage waveform on turn insulation ageing and destruction processes, highlighting the effect of high dV/dt and switching frequency. To achieve this objective, test benches allowing to perform insulation electro-thermal accelerated ageing tests were designed. These test benches make it possible to reproduce the stresses affecting the winding insulation of motors fed by inverters based on Silicon technology as well as inverters using high speed SiC switches.

Preceding the experimental study, a high frequency model of electric drive system composed by an inverter, a supply cable and a switched reluctance machine is proposed.

Introduction

With the objective to estimate the electrical stresses, arising in inverter-fed electric motor windings and causing their insulation degradation, the motor terminal transient overvoltage should be studied. The overvoltage at motor terminals is caused by impedance mismatch between the inverter, cable and the powered motor provoking propagation and reflexion phenomena along the cable [1], [2] and [3]. Overvoltage magnitude depends on the impedance of the machine, cable length and impulse voltage rise and fall times [1]. Hence, the use of fast switching WBG electronic devices leads to higher dV/dt and consequently to higher overvoltages [4], [5], giving rise to partial discharges and accelerating the insulation degradation.

To study and predict transient phenomena in PWM drives a High Frequency (HF) model-based approach is proposed in literature [4], [6], [7], [8], [9], [10] and [11]. The analysis of electric drive's high frequency behaviour includes a study of ElectroMagnetic Interference (EMI) considering the Common Mode (CM) and the Differential Mode (DM) for overvoltage transients.

In this regard, the present chapter is oriented toward the high frequency modelling of the electric drive system composed by inverter, supply cable and electric motor considering CM and DM parameters. This study is focused on the modelling of a three-phase SRM. The suggested HF equivalent circuit model of SRM is based on RLC-parameters extracted from CM and DM impedance measurements. In order to complement this model the power cable model proposed in [12] is used. Considering that in the present electric drive model the study of a single commutation effect is performed, the inverter is modelled by an ideal pulse generator.

High frequency electrical machine modelling 2.1 High frequency motor models: literature review

Since the transient overvoltage problems in variable frequency drives has been known in 1980 [1], many research works were dedicated to this phenomena. The analysis and prediction of overvoltage requires high frequency modelling, the maximum frequency of which depends on the rise time t r . The frequency range is defined considering that the differential mode voltage spectrum contains a rise time frequency component determined as f r = 1 πt r [3], or that the highest significant frequency of the spectrum corresponds to the inverse of the voltage rise time f max = 1 t r [13]. Numerous HF motor models have been proposed for overvoltage and EMI analysis, as well as for EMI filters conception. The existing models have a similar topology but different calculation methods; some of them are presented below.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

In paper [14], a HF model of three-phase AC motor's stator winding is proposed [15] and [16]. In order to obtain a global solution taking into account the high-and low-frequency phenomena of the inverter-motor system, the HF model is connected to the dq dynamic model [15]. The HF

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

model was extended in [16] to other types of AC motors such as synchronous reluctance (without interior permanent magnets), and brushless motors with acceptable accuracy using "trial and error" procedure for model parameters identification. Moreover, the integral HF model of AC PWM drive system for EMI analysis is discussed in paper [17] where a simplified motor model is used considering the predominant effect of the input capacitance due to the winding insulation and the motor frame resistance. Basing on above described model, the improved HF motor model is proposed in [11]. A series R t , L t , C t branch is introduced as shown in A high frequency lumped parameters model of induction motor similar to [16] and [9] was recently used to study overvoltage at motor terminals in SiC electric drives in [4].

The same impedance measurement technique in frequency range from 1 kHz to 100 MHz is used, but the model lumped parameters values are obtained from the Laplace domain transfer function formulas. A power cable HF model with lumped parameters is also provided that allows to model the overvoltage in the electric drive with different cable lengths.

The evolution of overvoltage depending on the impulse voltage rise time and cable length is given in this paper and shown in Figure II.7. The fact that the overvoltage never reaches 2 times of DC bus voltage level is explained by losses presented in power cables. The similar results have been obtained by simulations in [5].

Other techniques and approaches for electric motor modelling can be found in literature. In paper [7] an analytical model of Adjustable-Speed Drive (ASD) motor was elaborated using a developed mathematical formula basing on Maxwell's equations; the simulation was performed using Matlab Simulink software.

In [18], the HF model of synchronous machine is proposed. Parameters of the equivalent distributed circuit are derived from the measured voltage waveforms at the beginning and the end of the machine's winding basing on the transmission line theory.

The winding voltage is obtained by calculating the developed mathematical model.

A combined low and high frequency model of induction motor is developed in [12]. Impedance measurements to establish the equivalent circuits were performed. In order to determine the low frequency parameters, the tests were carried out at 50 Hz. The inductive effects as well as skin and proximity effects are observed up to 100 kHz, and capacitive couplings are defined accurately up to 100 kHz. In theses [13], [19] and [20], the parameters of proposed electrical machine winding HF model are identified using Finite Element Method (FEM). The advantage of this method results in the possibility of predictive analysis of transient overvoltage and EMI propagation knowing the electrical machine geometry. On the other hand, the FEM analysis is often conducted on stator winding prototypes, and the model complexity for realsize electrical machine can be a limiting factor of this method.

The presented models allow to investigate machine behaviour in frequency and time domains. Typically, the models are developed for induction motors. The frequency depended parameters (caused by skin and proximity effect) can be represented by additional fixed values in the model circuit in order to provide the time-domain analysis.

Motor winding impedance has distributed parameters, but the utilization of distributed parameters is avoided in HF modelling because it increases significantly the calculation time. Instead, a number of cells or equivalent circuit per winding phase with lumped parameters is often used. The model frequency range and accuracy depends on the number of circuit parameters. Equivalent circuit of electric motor and associated parameters can be established from the motor impedance characteristic measurements and/or its geometry. In this work, the HF motor modelling is based on methods described in [9] and [11],

where the equivalent circuit parameters are extracted from CM and DM impedance measurements.

Switched reluctance motor modelling

As it was shown in Chapter I, the switched reluctance machine is a promising and attractive electrical machine which may be used in many industrial and transport applications. The main advantages of this machine are its simple construction, robust structure, and its reliability. In contrast with the induction machine having distributed (random wound) stator winding, the conventional switched reluctance machine has concentrated stator windings made with round or rectangular wires. The concentrated windings offer the possibility to make fault-tolerant multiphase electrical machine: the larger the number of phases, the greater the reliability and the lower torque pulsations. In turn, the large number of phases requires high operating frequency and accurate control of switches.

The fact that the rotor does not contain windings or magnets makes SRM suitable for very high speeds that also necessitates high switching frequency [21], [22]. On the other hand, the literature review shows the absence of high frequency model proposed for this machine's type that makes it a relevant task.

Proposed motor models

The high frequency models proposed for SRM modelling correspond to the equivalent circuit models described in [9] and [11]. Two types of per phase circuits having relatively simple structure as shown in Figures II.8a

Motor impedance measurements

The studied electrical machine is a three-phase switched reluctance motor having six slots, four poles and concentated winding made with rectangular wires.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

The tests were performed connecting the impedance analyzer to different phases, and it was observed that the three phases have almost the same frequency response.

Measurements performed at different rotor positions shown that it slightly affects the CM impedance. On the other hand, the changing in DM impedance magnitude and phase (keeping the similar curves shapes) was observed in frequency range from 30 kHz to 5 MHz which is related to the SRM construction and operation. The motor's inductance depends on rotor position: maximum inductance when rotor poles are aligned with the stator poles, and minimum inductance at the unaligned position [23].

In the present study this changing is not considered and the model parameters are extracted from the motor impedances measured when rotor was stationary and its poles were aligned with the stator poles. The obtained impedances are provided in Figures II.12

Model parameters identification

The parameters of switched reluctance motor model are identified using the characteristic points from CM and DM impedance measurements. As for induction machines studied in [9] and [11], the SRM impedance is mainly capacitive in common mode. The point CM 3 corresponding to the first series resonance indicates an influence of CM inductance of studied motor, and the last series resonance is mainly due to the effect of measuring cable connections and motor internal feed lines stray inductance.

Coming now to the measured differential mode impedance. At low frequencies and up to the parallel resonance (point DM 2 ), the DM impedance of the studied SRM is mainly inductive. Then, the series resonance at point DM 4 , which may be associated with winding inter-turn effects, is observed. After that, the DM impedance becomes mainly capacitive until the last series resonance (point DM 3 ) due to the effect of stray inductance.

According to the calculation methods described in [9] and [11], the model parameters discussed in Section 2.2.1 are derived as follows.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

Basing on the equation

C = 1 2πfZ
, the low-and high frequency CM capacitances C t ot al (point CM 2 ) and C HF (point CM 1 ) are derived from the measured CM impedance.

Then, the resonance frequency f res1 and corresponded impedance magnitude Z mi n1 (point CM 3 ) are identified. Using the obtained values, the following model parameters are calculated:

C g1 = 1 3 C HF (II.1) C g2 = 1 3 (C total -C HF ) (II.2)
The CM inductance L CM required for further calculations is determined from the resonance at point CM 3 :

L CM = 1 12π 2 C g2 f 2 res1 (II.
L zu = 3 16π 2 C g1 f 2 res2 (II.5) R g1 = 2 3 Z min2 (II.6)
Using the common mode and differential mode inductances L CM and L DM , the inductance L d can be calculated.

L d = L CM + 4 9 L DM (II.7)
The parasitic resistance R g2 can be approximately calculated by II.8 or more accurately -by II.9. In the present study, the second equation is used.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE R g2 ≈ 1 3 Z min1 (II.8) R g2 = 3Z min1 - 9R e (2πf res1 L CM ) 2 R 2 e + 36π 2 f 2 res1 L 2 CM (II.9)
Finally, with the purpose to catch the resonance at point DM 4 , according to [11] the parameters R t , L t and C t are calculated as follows.

R t = Z min3 cos θ Z3 (II.10) C t ≈ 1 6 (C g1 + C g2 ) (II.11) L t = 1 4π 2 C t f 2 res3
(II.12)

The Table II

Comparison of measured and simulated motor impedances

With the objective to test the effectiveness of the chosen models for the studied switched reluctance motor modelling as well as the model parameters identification method, the measured and simulated motor impedances are compared. To this purpose, the frequency-domain numerical analysis was performed using LTSpice powered by Matlab software.

Firstly, the switched reluctance motor is modelled by the simplified equivalent circuit proposed in [9]. lation. In spite of this, an adjustment of model parameters is still needed to fit the simulation results to the measured ones considering the fact, that the choice of certain characteristic points and parameters' values is arbitrary. In this regard, the "trial-and-error" method or optimization algorithms are often used in literature, e.g. in [14], [16] and [9], in order to increase the model efficiency.

Table II.2 demonstrates the model parameters influence on the simulated impedance curves basing on the performed simulations. These observations correlate with [4] where the equivalent circuit model similar with [9] is studied. It should be noted that the changing in the model parameters values affects both common and differential modes. However, it is observed that improving in simulation accuracy of one mode results in decorrelation of another mode.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

The parameters representing the skin and proximity effects should be frequencydependent for better HF model accuracy, but it complicates the time-domain analysis.

Moreover, specific features of SRM concerning its winding construction and consequent minimum magnetic coupling between phases, as well dependence of the machine reluctance on its rotor position [22] are not considered in the studied models. At the same time, it is noted that differences observed at last impedance resonance can be neglected considering the significant influence of measuring cable connections.

Although the mentioned above imperfections, the conducted study in frequencydomain shows that the extended HF equivalent circuit model may be used for modelling switched reluctance motors. Therefore, this motor model is retained for further timedomain modelling.

High frequency cable modelling

In the order to predict the overvoltage at inverter-fed motor terminals, an accurate high frequency model of the feeding cable is also required. As for the electric motor, the cable impedance behaviour over the frequency range should be reproduced. Different approaches to model cables for electric drives exist in literature. Cable models having distributed [5], [10] and [24] or lumped parameters [4], [8], [11], [12], [25] and [26] are proposed for time-domain analysis. In order to provide a good accuracy of lumpedparameter model considering the distributed parameters effect, an adequate number of cells or segments corresponding to elementary cable length should be used. The model parameters values can be extracted from differential mode impedance measurements (open circuit and short circuit impedances) [4], [12] or calculated using the finite-element method [5], [10], [24], transmission-line modelling technique [11] and modal transformation method [26], or evaluated by applying evolutionary algorithms [25]. By analogy with the HF motor model, the validation of proposed cable model is performed through the HF simulated and measured impedances comparison. The models taking into account skin and proximity effects as well as dielectric losses show the best accuracy.

With the purpose to perform the transient overvoltage analysis, the HF cable model proposed in [12], which provides a good compromise between model complexity and efficiency, is chosen. This model represents a cascade of cells (equivalent circuits) with lumped parameters and is compatible with the described above equivalent circuit model of switched reluctance motor. In this model, the lumped parameters R-L are cascaded in order to reproduce the skin and proximity effects. The dielectric losses are neglected. In order to approximate the distributed parameters effects along the cable, a multicell model should be used for a long cable modelling. In this case, one cell or equivalent circuit should correspond to electrically short cable length l at the maximum signal frequency f max . According to [27] l = 1 10 λ, where λ is the wavelength which in cable line is defined by (II.13).

λ = v 1 f max (II.13)
The signal propagation velocity v depends on the surrounding medium. Electromagnetic waves in cable line can be expressed in terms of the cable insulation permittivity

r : v = c r (II.14)
where c is the speed of electromagnetic propagation in free space. The permittivity of polyethylene is equal to 2.3, thus the propagation velocity v ∼ = 2 × 10 8 m/s.

Thus, for the frequency of 30 MHz, the minimal cable length adequately represented by its equivalent circuit should not exceed 0.67 m. In case of impulse voltage having very short rise time, e.g. t r = 10 ns, the induced waveform should have significant spectral content up to around 1/10 ns = 100 MHz [27]. Therefore, the corresponded cable length modelled by the equivalent circuit model should be no more than 0.2 m.

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

In this Chapter, we focus on the motor modelling, therefore the cable parameters used in our simulations are based on values obtained by C. Vermaelen in [12]. In his work RLC-parameters of the proposed equivalent circuit correspond to 1 m of cable length for the frequency range up to 10 MHz. As discussed above, in order to investigate the effect of very short rise time, the minimal cable length should not exceed 0.2 m. Thus, the HF model corresponding to cable of one meter length is composed by five equivalent circuits and the RLC-parameters values are divided by 5. Table II.3 provides the parameters values of the equivalent circuit.

Table II.3: HF cable model parameters basing on [12].

Model parameter Value

Phase resistance

R ph1 , Ω 0.012 R ph2 , Ω 1.8 R ph3 , Ω 200 Shield resistance R sh1 , Ω 0.012 R sh2 , Ω 0.6 R sh3 , Ω 60

Simulation of transient overvoltage in inverter-fed motor drive

In order to analyse the overvoltage phenomena at inverter-fed motor terminals the numerical study is performed. The system, consisting of a pulse voltage source, the equivalent circuit of feeding cable and the motor equivalent circuit, is simulated in timedomain using LTSpice software. The inverter voltage is approximated as trapezoidal pulses (the inverter internal impedances are neglected) with specified rise and fall times as in [8],

[17] and [5]. Moreover, considering the particularity of switched reluctance motor supply-

CHAPTER II. HIGH-FREQUENCY MODELLING OF INVERTER-FED MOTOR DRIVE

ing when its phases are energized independently, a single-phase study may be sufficient.

To study the effects of a single commutation the inverter can be modelled by an ideal pulse generator.

Firstly, the voltages applied to motor winding (the phase is energized) are simulated using simplified and extended motor models based on [9] and [11] respectively. The The insulation accelerated ageing is engendered. The mean time to breakdown is used as the end-point criteria for the insulation endurance evaluation.

Review of methods for testing the inverter-fed motors winding insulation

Various testing methods are proposed to investigate the effect of impulse voltage generated by PWM based inverters on electrical machines insulation. These tests focus on winding insulation owing to the fact that it is the weakest point of electrical machine. The experimental studies are carried out both on electric motors [1], [2], [3], [4], [5], [6], [7], [8] and on different winding wires' specimens. With the objective to test phase-to-phase and phase-to-ground insulation, motorettes fabricated using winding wires and intertape are used in [8] (see Figure III.1) and [9]. For qualification of hairpin motors insulation having bar windings, formettes and coupled bars (shown in Figure III.2) are proposed in [10] to test phase-to phase and turn-to-turn insulation respectively. The turn-to-turn insulation is identified as the weakest part of winding insulating system in [10] and [11].

Turn insulation tests are mainly conducted on twisted pairs which are standardized specimens for round section enamelled wires [9], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] and [28]. These specimens should be prepared in accordance with the standard IEC 60851-5 [29] (see Figure III.3). Otherwise, in works [20], [30], [31], [32], [33], [34], [35], [36] and [37] The mentioned studies are related to PDs and insulation failure mechanism investigation in low voltage electrical machines fed by inverters for various applications including automotive [10], [19] and aeronautical ones [3], [6], [7], [8], [9], [13] and [22].

In [5], [20], [23], [30], [38], [39] and [40] partial discharges characteristics, inception conditions, and the influence of PDs activity on the insulation lifetime are investigated.

The effects of testing voltage waveform parameters on PDs features and winding wires endurance are shown in [8], [21], [31], [32], [33], [34], [35] and [37]. These works show that the higher is the PDIV, the longer will be the insulation service life in PWM drive environment. The PDs patterns and inception voltage under repetitive impulse voltage changes significantly comparing to the sinusoidal voltage. It was observed, that the very short impulse voltage rise time, e.g., 20 ns in [39], may lead to higher PDIV. At the same time, the faster rise time and higher frequencies shorten the insulation lifetime due to the increased PDs repetition rate and PDs magnitude. It is also noted, that the endurance of turn insulation can be increased using the wires with thicker enamel layer. In [15], [16], [17] and [21] the endurance of conventional and corona resistant (nano-filled) enamelled winding wires are compared. The corona resistant wires demonstrate the better endurance under partial discharges and they are recommended to use in inverter-fed motors. In [26] the impact of ozone generated by PDs activity as a factor accelerating the insulation ageing is analysed. Insulation lifetime models depending on the applied stresses (voltage amplitude, frequency and temperature) and their interaction are described in [25], [41] and [42]. The design of experiments and response surface methods are used to establish the lifetime models. The effect of the initial conditions of winding wires is studied in [25], where the lifetime of mechanically damaged wires (light, medium and heavy manually wound wires) is evaluated. It is shown that the heavy wound dramatically affects the insulation lifetime, and a bi-modal lifetime distribution is observed in this case. Mechanical, thermal and electrical stresses effects on turn insulation were studied in [27]. In this paper different types of enamels and impregnating compositions are compared in terms of breakdown voltage and PDIV. A novel fabrication method of enamelled wire using the enamel extrusion and the following polymerization by an ultraviolet lamp is proposed. The works [9], [12], [13], [22], [45] and [46] are intended to the investigation of temperature, humidity and pressure effects on PDIV in motor winding insulation. These studies show that the humidity has lower impact on partial discharges inception than the pressure and the temperature. A technique to improve winding insulation design and, consequently, to reduce partial discharge activity in inverter-fed motors is proposed in [13] and [28]. The proposal consists in adding a thin silver conducting paint on enamelled wire outer surface. This solution provides the increasing in the the partial discharge inception voltage. In works [8], [19], [43], [44] methods for determination of voltage distribution and overvoltage in motor windings are given. This type of study is conducted in order to predict the partial discharges occurrence determining the potential overvoltage in motor winding, or to prevent their ignition improving the voltage distribution within the winding.

The use of wide bandgap power electronics in motor drives requires new approaches to study PDs effects on motor winding insulation. SiC-based inverters may intensify electrical stresses on driven motors' winding insulation because of their higher switching speeds and switching frequencies comparing with silicon electronic components. Due to this fact, PDs will occur with higher probability, and the motor insulation ageing and failure will be accelerated. In reference [6], the experimental study is conducted on the electric motor using SiC-based inverter and thermal vacuum chamber reproducing environmental and operational conditions. Nevertheless, the fact that the tests are carried out on the complex motor drive system makes them relatively expensive and difficult to use in order to study the physical processes of dielectric ageing. Therefore, ageing and deterioration process in insulation subjected to PWM voltage waveform, in particular impulse voltage with high dV/dt and high switching frequency, are not thoroughly investigated.

Moreover, taking into account influence of thermal stresses affecting the insulation during motor operation, the simultaneous effect of these factors and the process of electrothermal ageing need to be considered.

Thus, the aim of this experimental work is to investigate the influence of voltage waveform on enamel insulation endurance, highlighting the effect of high dV/dt and switching frequency, as well as the effect of rise in ambient temperature. To achieve these objectives, experimental benches allowing to perform insulation endurance tests have been designed. The test benches make it possible to reproduce the stresses affecting the winding insulation of motors fed by inverters, generating repetitive impulse voltage with µsand ns-level of rise/fall time, that corresponds to conventional Si and modern SiCbased inverters respectively.

3 Testing methodology

Test objects: winding wires

In standard IEC TS 60034-18-41 [47] two types of winding insulation are defined. In the present research the winding insulation of Type I is studied. As previously mentioned, the weakest element of winding insulation system is the turn insulation, consisting of enamelled winding wire and impregnating compound. Basing on consideration that partial discharges can occur in defects of winding impregnation such as voids and cracks, the enamel insulation resistance to partial discharges determines the turn insulation endurance. In this regard, the endurance tests are mostly conducted on unimpregnated enamelled winding wires presented below.

The commercially available conventional wires and one corona resistant wire are tested. The specific composition of the corona resistant nano-filled wire makes possible a better resistance to degradation caused by PDs, providing improvement in terms of chemical erosion by partial discharges, electrical breakdown and endurance behaviour, as well as space charge mitigation, comparing to conventional enamelled wires [15], [17],

[48] and [49].

For this study, enamelled winding wires manufactured by JSC "Sibkabel" (part of "Cable Alliance Holding", Russia) of brands PETD2-K-180 (corona resistant wire) and In this regard, this specimen type wasn't chosen for further tests. As differentiated from the single coil, two pieces of wire are superimposed while winding.

Specimens preparation

It creates a contact on wires' wide sides. In the both described configurations the wires' ends should be kept separated or insulated in order to avoid a flashover during the test. The proposed coil's shape specimen has no analogues and it allows to model the turn insulation in concentrated winding configuration made of rectangular section wire.

Breakdown voltage tests

The breakdown voltage value (given in kV or V/µm) is usually provided in enamelled winding wires' specifications in order to characterize the insulation electrical propri- According to [29] test voltage can be applied between the wire's conductor and the metal shot. Therefore, the tested specimen is placed in a bath made of dielectric material

Developed and installed in TPU, Russia

To place a wire specimen two cuts are made on the bath's opposite sides at a distance of at least 10 mm from the shot level to prevent it's spilling out. The cuts' width should be in 1.5-2.5 larger of the tested wire's diameter. The specimen in the bath is covered by the shot. The shot's diameter is no more than double the wire's diameter. It ensures a close contact between the shot and specimen's surface. Grounded metal electrodes can be placed parallel to the bath's bottom or on its long sides. The sinusoidal voltage is applied to the wire specimen at the one end while the other end is not connected (in the air). The bath's dimensions are sufficient to fully submerge the tested specimen into the shot. It is important to note, that the specimen ends should be sufficiently long in order to avoid a flashover. In this regard, the rectangular wire specimen's length is increased up to 300 mm and the wire's end in the air is additionally insulated by a fluoroplastic band.

Endurance tests

The proposed methodology is based on insulation endurance tests, in which the test object is exposed to one or more ageing factors related to service conditions. It gives an opportunity to study processes of dielectric materials degradation and failure mechanism, to find out the impact of applied stresses and to conduct the withstand capabil-ity comparative analysis of tested winding wires. This experimental study focuses on enamel insulation ageing investigation incorporating both voltage and temperature effects as they are significant ageing factors in inverter fed motors according to [11], [50],

[51] and [52]. The tests are conducted on above described wires' specimens for the purpose to assess the enamel insulation endurance to partial discharges.

To provoke the electrical ageing of enamel insulation, sinusoidal or impulse voltages are applied to wires' specimens using different electrical benches. The voltage amplitude is set higher than the PDIV level determined by preliminary tests when partial The tests are carried out with the following goals:

discharges
• to evaluate the influence of voltage waveform parameters on insulation ageing and life time by comparing the obtained mean times to breakdown;

• to estimate and compare the ability of different winding wires to withstand partial discharges activity depending on applied stresses;

• to study the effect of high temperature accelerating insulation ageing and breakdown when combined with electrical stresses.

The experimental results and their analysis will be discussed in Chapter IV.

Endurance test under sinusoidal voltage

The experimental bench similar to described in Section 3.3 can be used to conduct the wires insulation endurance tests. The "wire -shot" tests are performed in accordance with the method proposed in [55] and [56], where the resistance of enamelled wires to

Endurance test under low dV/dt PWM impulse voltage: µs -level rise time

The turn insulation endurance tests performed under sinusoidal voltage exceeded PDIV allow to evaluate insulation withstand capability while damaged by partial discharges.

However, the partial discharges' features and insulation destruction process depend on voltage waveform parameters and inverter realization. Thus, to qualify and evaluate insulation system of inverter-fed motors impulse voltage testing is recommended in [47].

The presented in this section electrical bench has been designed in the interest • V a -steady state impulse voltage amplitude (ultimate value), that corresponds to DC bus voltage V dc when the machine is fed by inverter, V a = V dc ;

• V os -overshoot voltage, defined as the amplitude of the peak voltage in excess of the steady state impulse voltage;

• V pk/pk -peak to peak voltage;

• V pk -peak voltage; for bipolar voltage impulses, it is half the peak to peak voltage;

• t r -rise time of impulse voltage defined as time required to rise from 10% to 90% of the steady state voltage; from 10% to 90% of 2V a for bipolar impulse.

The rate of voltage change dV/dt is defined here as the voltage rise from 10% and 90% 2V a over the time t r at the start of the impulse. MOTORS FED BY INVERTERS This setup already allowed us to apply very steep voltage rates, which can affect the partial discharges' properties and served as a first prototype of high dV/dt test bench.

Whereas the previous studies [33] and [60] show that bipolar pulses can have a greater effect on insulation lifetime than unipolar pulses, so another pulse generator assembly was designed and used for winding wires insulation endurance tests. This experimental bench is described in the following section.

B H-bridge inverter topology

The designed electrical test bench acts like a pulse generator assembled with a measurement unit and a thermal chamber where tested specimens are installed. The In the present setup tested specimens are installed in the thermal chamber that provides a temperature control and serves as a grounded metal screen protecting the inverter from electromagnetic interferences issued from partial discharges. To improve this protection, the inverter is put in the grounded aluminium cage (Faraday cage). The effect of cable length on the transient overvoltage when applying impulse voltage with short rise time as shown in Chapter II should be considered. In this regard, the electrical connections between the inverter's PCB and the tested specimen is made using a 1 m length cable. For this purpose, the cable insulated by polytetrafluoroethylene enabling operation at high temperatures was chosen. MOTORS FED BY INVERTERS Initially, measurements were carried out using a RC-divider connected at one of the specimen's connection point in the thermal chamber and the ground. There was no specimen connected in order to avoid the signal disturbances issued from partial discharges while the voltage is measured.

Partial discharges detection tests

The endurance tests of winding wires' enamel insulation were carried out on the described experimental benches. The main ageing factor was the sinusoidal or repetitive impulse voltage. Furthermore, the electrical stresses were combined with the thermal ones that accelerates the insulation ageing. The applied voltage activates the ionization processes in the insulation engendering partial discharges in air gaps between turns of enamelled wires. In this study, partial discharges signal measurements have been performed with the aim to demonstrate the presence of PDs activity during the endurance tests. The optical, electromagnetic and chemical PD detection methods are used.

Optical method

The optical method is based on detection of radiation emitted as a result of dielectric ionization, excitation and recombination occurring during the electrical discharges activity. The radiation associated with electrical discharges appears in the Ultraviolet (UV) spectrum, the visible light and the infrared region. Depending on dielectric materials, surface condition and discharge type, different optical detectors or sensors can be used: photomultipliers, optical fiber sensors, UV cameras [61], [62], [63] and [64].

In this work the optical detection of partial discharges is carried out using an electron-optical defectoscope Filin-7 [64] shown in

CHAPTER III. ENDURANCE TESTING METHODS FOR TURN INSULATION OF MOTORS FED BY INVERTERS

The partial discharges on specimens under sinusoidal voltage varying from 2 to 5 kV RMS have the weakest radiation level. The most luminous discharges are observed during the test on the high dV/dt experimental bench (see Figure III.33). This effect can be associated with voltage changing rate and high frequency of impulse voltage resulting in high energy discharges.

Electromagnetic method

The detection of PDs is also performed using an antenna considering that partial discharges activity has an impact on the electromagnetic field in very high frequency (30-300 MHz) and ultra-high frequency (300-3000 MHz) ranges [65], [19], [61] and [66].

The antenna is made with a 150 Ω coaxial cable PK 150-7-11 (Russia) for radio frequency signal transmission of 6 m length. It's electrical parameters are given in Table III

Chemical method

It is widely known that partial discharges generate ozone and other chemically active species. Ozone is highly corrosive gas, its effect leads to insulating material oxidation, breaking of organic polymers' C-C bonds, and facilitate dielectric breakdown [26] and [67].

Ozone concentration measurements are proposed in [68] and [69] as an insulation fault diagnostic method based on the detection of air derivatives such as ozone and Oxides of Nitrogen (NOx), inside hydro-generator stator cores or transformers.

In described experimental benches two wires of twisted pair or coil present two electrodes covered by enamel insulation and have small air gaps between turns. When the high potential difference is applied between these electrodes the test bench acts like a plasma reactor generating ozone from the ambient air. The development of such plasma reactor intended for ozone generation is reported by [70] and [71].

In this regard, the experimental setup is implemented for detecting ozone gener-

ated

Introduction

This Chapter gathers the experimental protocols and results of tests conducted on previously described benches followed by statistical analysis and main observations.

Breakdown voltage tests

The experimental characterization of electrical insulation materials may be per-

formed

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Table IV The Normal (or Gaussian) distribution is probably the best known distribution which often gives a good approximation of breakdown voltage of solid dielectrics [1] and [3]. The Normal distribution is described in terms of the mean µ and standard deviation σ measuring the dispersion of the data points relative to the mean. A continuous random variable X (e.g., breakdown voltage data) with probability density function

f (x) = 1 2πσ e -(x -µ) 2 2σ 2 -∞ < µ < ∞ (IV.1)
is a normal random variable with parameters µ, where -∞ < µ < ∞ and σ > 0. The value of the random variable measured with n repetitions is denoted by x.

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Figure IV.2: Comparison among different PDF plots of breakdown voltage data.

Also, the expected value E(X) and the variance V(X) figured by (IV.2) and (IV.3) , the standard deviation σ = σ 2 and the notation N(µ, σ 2 ) is used to denote the distribution [2]. The estimated parameters μ and σ are given in Table IV.2.

µ = E(x) μ = 1 n n Σ i =1 x i (IV.2) V (x) = σ 2 σ2 = 1 n -1 n Σ i =1 (x i -μ) 2 (IV.3)

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Table IV In order to estimate the effect of winding wire's type (insulation material) on the obtained breakdown voltage, a one-way analysis of variance (ANOVA) has been performed.

It shown that the experimental data do not go against the equivalence between PETD-180 and APX-1 with regard to breakdown voltage. At the same time, these wires shows significant difference from the other (PETD2-K-180 and APX-2).

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

The values of breakdown voltage given in specifications are lower because it may be multiplied by reduction factor with regard to provide the minimal guaranteed limits of insulation electrical strength taking into account possible defects. Furthermore, the value of dielectric strength E spec given in specification also depends on measurement method, test equipment and environmental conditions. The dielectric strength of an enamel could be tested on enamel coated steel plates and not on wires. It should be noted, that higher electric strength of rectangular section wire APX-2 comparing to other winding wires for given test conditions might be explained by higher insulation layer thickness.

Basing on measured breakdown voltages of winding wires, the voltage values for further lifetime tests under sinusoidal voltage are then chosen.

Endurance tests

In the present work, the insulation endurance tests are conducted under constant stress choosing as factors voltage waveforms parameters and temperature levels. These tests are also called accelerated life tests; times to breakdown are measured in specified test conditions accelerating the insulation ageing and failure. At least five repetitions (winding wire specimens tested in the same conditions) are made for the sake of statistical analysis of experimental variability.

Endurance tests under sinusoidal voltage

Experimental protocol

The endurance tests are started by applying AC sinusoidal voltage (50 Hz). Both electrodes systems are used: (i)"shot-wire" test system and (ii) two wires connection (twisted pairs for round section wires and coils for rectangular section wires). The test are performed on unimpregnated specimens. Table IV

Experimental results and analysis

Lifetime data obtained corresponding to the experimental protocol and estimated parameters μ (IV. The experimental results analysis does not show a perceptible difference between corona resistant and conventional wires in these given test conditions. Considering the lognormal distribution, a two-way ANOVA has been performed on logarithms of experimental data. This test show that the effect of winding wire' type is not significant. On the other hand, the time to breakdown logarithmic means are significantly different when changing the voltage level. Indeed, the rise in voltage shorten the wires' lifetime.

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

This observation correlates with the results of Weibull distribution analysis. The expression for the probability density function of the two-parameter Weibull distribution according to [4] is given by IV.6:

f (t ) = β t β-1 η β e -1 η β (IV.5)
where t is the measured variable (time to breakdown in our case); η is the scale parameter or characteristic life, η > 0, and β is the shape parameter, β > 0. The scale parameter η represents the time for which the failure probability is 0.632, and the shape parameter β is a measure of the range of the failure times. The larger shape parameter is, the smaller is the failure time range. The shape parameter has three ranges of values: for β < 1 the failure rate decreases with time, for β = 1 the failure rate is constant (consistent with the exponential distribution), and for β > 1 the failure rate increases with time. Experimental results analysis shows that the estimated scale parameter α decreases with the increasing of voltage applied to tested specimens and the shape parameter β ≥ 2.

A ratio of mean time to breakdown K is also used as an indicator for estimating the effect of test conditions (voltage, temperature, type of specimen) as suggested in [5]. For the described above endurance tests under sinusoidal voltage, the ratio K RMSsin is calculated using mean times to breakdown t 4si n and t 5si n of two wires tested at 4 and 5 kV RMS respectively.

K RMSsin = t 4si n t 5si n (IV.6)
For the studied winding wires the ratios are the following: Moreover, the mean time to breakdown ratio K Tsin is calculated for the twisted pairs and coils tested under sinusoidal voltage at two temperature levels ( t 25si n at 25 °C and t 180si n at 180 °C):

K RMSsin (PETD2 -K -180) = 2.
K Tsin = t 25si n t 180si n (IV.7)
Respectively to studied winding wires the ratios are the following:

K Tsin (PETD -180) = 1.60, K Tsin (APX -1) = 3.00 and K Tsin (APX -2) = 22.01. These ratios highlight the temperature effect, especially significant for the rectangular wire APX-2.

Long duration tests (especially in case of rectangular wire) at 25 °C provoked an ozone accumulation in the enclosed test chamber. Visible light blue deposits (products of decomposition of solid dielectric) called "frost-effect" in [6] and [7] The simultaneous heating in the thermal chamber at 180 °C during endurance tests may cause ozone decomposition, weakening its impact on the insulation destruction. The "frost-effect" is less marked on the insulation surface. The impact of chemical attack could be less important in this case. Moreover, as reported in [8], increasing in temperature decreases the total energy of partial discharges weakening their effect. On the other hand, the rise in temperature leads to thermal ageing of insulation materials.

According to Zhurkov's thermo fluctuational theory [9], the breaking of chemical bonds in material occurs due to the energy of thermal fluctuations of atoms creating these bonds.

Electrical and thermal stresses decrease the potential barrier and facilitate the bonds scissions.

As it can be seen from previous charts, both bent wires and twisted pairs of wire PETD-180 have been tested under sinusoidal voltage at 5 kV RMS while heating at 180 °C.

That allows to compare the breakdown data as shown in Figure IV.8.

The mean time to breakdown ratio between two types of round sections wires' specimens under sinusoidal voltage K SR demonstrating their difference is equal to:

K SR = t t wi st t bent = 4.33 (IV.8)
where t twist and t bent are the mean times to breakdown of twisted pairs and bent specimens respectively. The time to breakdown of bent specimens is lower than the time demonstrated by twisted pairs. In the electrode system "wire-shot" the partial discharges occur between the enamel insulation layer and the grounded metal shot, whereas in twisted pair two enamel layers separate copper conductors slowing the destruction process. The twisted pair specimen's type is kept for further tests.

Endurance tests under PWM impulse voltage: low dV/dt

First of all, this section presents endurance test results obtained on unimpregnated specimens (twisted pairs and coils) made of four studied winding wires. The effect of impregnating varnish was neglected as in previously described tests. The influence of thermal stresses and impulse voltage waveform is investigated. Then, we compare times to breakdown of two constructions of rectangular section winding wire specimens proposed in Chapter III.

The second part of this section gives an analysis of a "healing" of defects in the winding wires insulation by the impregnation. The time to breakdown is measured on different tested insulation systems "winding wire -impregnating varnish". In this experiment, twisted pairs made of corona resistant wire having artificial defects impregnated by different compositions are exposed to PWM impulse voltage and thermal stress. The study is intended to reveal the effect of dielectric materials compatibility in the insulation system (turn insulation of electrical machine winding).

Tests on unimpregnated specimens

Experimental protocol

By analogy with previous tests performed under sinusoidal voltage, the endurance tests of winding wires subjected to electro-thermal stresses have been carried out under impulse PWM voltage.

Experimental results and analysis

Experimental results corresponding to the experimental protocol given above are reported in Appendix IV-B and graphically shown in figures below.

K T µs = t 25µs t 180µs (IV.9)
Respectively to studied winding wires the ratios are the following:

K T µs (PETD2-K-180) = 15
.99, K T µs (PETD-180) = 6.66, K T µs (APX-1) = 2.40, and K T µs (APX-2) = 10.18. As can be seen from the boxplot and the calculated ratios, the temperature effect is stronger marked for wires having better endurance comparing to others. The in-

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

sulation with good resistance to electrical stresses and partial discharges will be less performant at increasing environmental temperature; the thermal ageing might be a critical factor accelerating the final insulation breakdown. As expected, the decrease in the overvoltage and dV/dt increase the insulation lifetime; this factor causes the significant difference in times to breakdown according to ANOVA results. The mean time to breakdown ratio K osµs is equal to:

K osµs = t 300µs t 900µs , (IV.10)
where t 300µs and t 900µs correspond to mean time to breakdown at lower overshoot (V os = 300 V) and at higher overshoot (V os = 900 V) respectively. The ratios are the following:

K osµs (PETD -180) = 1.21 and K osµs (APX -1) = 1.48.

As described in Chapter III Section 3.2, two specimen's types are proposed for testing the rectangular section wire. The first specimen's configuration called single coil (or simply "coil"), represents a small coil made of two rectangular wire's pieces wound in parallel. In the second configuration called double coil, two pieces of wire are superimposed while winding creating contact on wires' wide sides. The mean time to breakdown ratio between two types of rectangular section wire's specimens K SC is:

K SCµs = t d oubl e t si ng l e = 2. 03, (IV.11)
where t d oubl e and t si ng l e are the mean times to breakdown of twisted pairs and bent specimens respectively. The double coil specimens demonstrated higher endurance compared to single coils, that confirms by ANOVA results. In the double coil the contact areas (PDs activity zone) are larger, the electric field distribution is more uniform, that decreases the electric stress effect. Depending on a winding turns arrangement of studied electrical machine, the corresponding specimen type may be used to test the turn insulation. In this work, the single coil specimen type is selected for endurance tests.

Influence of defects on turn insulation endurance

As shown in Chapter I, electrical machines failures generally occur because of winding insulation damages. One of the failure criterion may be the presence of defects within the insulation, such as impurities, voids and cuts reducing the insulation performance and service life. All things being equal, the greater the number of existing defects ("weakest-links"), the higher the probability of insulation failure. If there is a weak point (defect), which cannot carry the applied electric field, a complete breakdown will be initiated.

The term defectiveness λ (1/mm) is commonly used for a quantitative assessment of the insulation damage [10]. This characteristic defines the number of defects per unit of the winding wire length. These defects can exist in winding wires insulation in the delivered condition (defects in the delivered condition λ D ), occur during the production technology process (technology process defects λ TP ) and electrical machines operation (operational defects λ OP ) [10]. The total defectiveness for the entire period of the electrical machine service life can be determined as the sum:

λ = λ D + λ TP + λ OP (IV.12)
Defects existing in the delivery condition and appearing during the windings manufacturing process may be "healed" due to the high quality impregnation of machine's winding, e.g. VPI. In this case, the dielectric strength of the insulation's damaged parts

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

recovers and provides the required endurance under operational factors. It is therefore very important to provide the most complete "healing" of such defects by impregnation.

A relevant question is how effective is the defects "healing" by the impregnation when stressed by impulse voltage?

To answer this question, first of all, notion of turn insulation's defects should be given more in detail. The defects "healing" quality depends on their type (point, ring, extended cut) and their location in the winding. Two types of defects, dependent and independent, are distinguished in turn insulation [11]. Dependent defects penetrating simultaneously in the structure "enamel insulation -impregnating varnish -enamel insulation" are considered as critical defects (see Figure Closely located defects within adjacent winding turns may be also critical for insulation system endurance. In this case, a short circuit associated with the effect of spark discharges in defective areas during switching operations (electrical machine's run and stop) may probably occur due to switching overvoltage pulses. These spark discharges and insulation impurities in defective areas provoke an increase in leakage current and local overheating. As a result, the insulation flashover voltage decreases in the defective areas. Finally, it leads to a sustainable spark discharge appearing under operating voltage, i.e., the insulation system failure.

The described failure mechanism is characteristic for sinusoidal supply voltage.

The endurance tests have been performed with the objective to examine the failure mech-

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

anism and the effectiveness of defects "healing" by the impregnation when stressed by impulse voltage.

Experimental protocol

The experimental procedure includes a particular specimens' preparation. Speci- Then, the specimens are stressed under PWM impulse voltage with surges at 180 °C till the breakdown. The partial discharge activity is observed during the tests. In order to evaluate the effect of defects "healing" the mean time to breakdown ratio for studied impregnating composition K healµs is calculated:

Experimental results and analysis

K healµs = t i

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

As discussed in Chapter I Section 4.3, the compatibility of impregnating compositions and winding wires enamel insulation plays an important role in insulation system reliability. In this regard, the quality of defects "healing" will also depend on the compatibility of insulating materials.

To sum up, it should be noted that even mismatched (independent) defects of winding wires critically affect the turn insulation lifetime in inverter-fed motors. The defects within the insulation create weak spots facilitating the PDs inception which is detrimental to low voltage insulation. The winding wires having low defectiveness as well as impregnating compositions having compatible chemical and physico-mechanical properties, should be used in order to obtain the insulation system with minimum defects.

Endurance tests under rectangular impulse voltage: high dV/dt

With the objective to investigate the effect of high dV/dt impulse voltage, winding wires endurance tests have been carried out using the designed SiC-based pulse generator. The full bridge inverter configuration is used to apply bipolar impulse voltage on the tested specimens. As it was noted in Chapter III, the parameters of the generated voltage pulses such as the impulse amplitude, the switching frequency and the duty cycle are adjustable. Therefore, the effects of impulse voltage amplitude and frequency on the insulation lifetime are studied in present endurance tests. Moreover, the winding wires were also subjected to thermal stresses simultaneously with the electrical stresses.

Experimental protocol

Table IV.5 gives the tested specimen's types and details experimental conditions.

The

Experimental results and analysis

The time to breakdown data of studied winding wires obtained in different com-

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

For better understanding of acquired data, the time to breakdown ratios were calculated by analogy with the previous tests. The mean time to breakdown ratio K Tns of twisted pairs and coils tested under impulse voltage with ns -level rise time (V dc = 1.2 kV, f s = 20 kHz) at two temperature levels: at 25 °C t 25ns and at 180 °C t 180ns ) is equal to:

K Tns = t 25ns t 180ns (IV.15)
Respectively to studied winding wires the ratios are the following:

K Tns (PETD -180) = 1.42, K Tns (APX -1) = 6.84, and K Tns (APX -2) = 0.72.

The conventional wire APX-1 demonstrated the decrease of lifetime with the temperature increasing that is in line with expectations. However, the other conventional wires, PETD-180 and APX-1, did not followed the same trend showing the mean time to breakdown at 100 °C lower than at 180 °C. In case of wires having better endurance and thermal stability, the environmental temperature may be not the determining factor limited the insulation lifetime when subjected to high dV/dt impulse voltage. Electrical stresses and partial discharges become the detrimental factors. The breakdown time data of corona resistant wire had not been acquired in these conditions because the time span required to have a complete data set exceeded the day duration. Among tested winding wires, the corona resistant one shown the best endurance with the highest times to breakdown, especially at lower voltage amplitude. However, the performance of this wire is limited and decreases remarkably when the voltage amplitude and the switching frequency increase. The rectangular winding wire APX-2 demonstrates a good endurance comparing to other conventional wires, especially when the stresses are increased. As to the conventional wire APX-1, its lifetime is very low under high dV/dt impulse voltage, that complicates the analysis of factors effect, but it also means that this wire (insulation composition) should not be used for electrical machines operating in such conditions. In general, the box plot charts and the time to breakdown ratios highlight the significance of impulse The analysis of probability plots shows that the lognormal and Weibull distributions may be appropriate for fitting the time to breakdown distribution for all tested wires and experimental conditions. The estimated shape parameter β > 1 for all factors combinations, that means that the failure rate increases with time. The analysis of variance

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Table IV.6: Time to breakdown ratios of studied wires depending on test conditions. and it has a thermal breakdown nature. The insulation thermal run away issued not from external heating but under high frequency pulses. The insulation destruction process will be discussed more in details in Chapter V. It should be noted, that in this case, the insulation defects might not play such an important role in insulation breakdown as during the tests under low dV/dt impulse voltage.

Winding wires Test conditions Time to breakdown ratio V

Considering the drastic destruction in such short terms, dielectrics ageing might be caused by both the partial discharges and the fast rising of dielectric losses. The high ozone concentration, high visible and UV range radiation have been observed during the endurance tests indicating a high level of PD activity. As far as, the effect of significant temperature increase of tested specimens has been revealed in these conditions, the temperature measurements were performed.

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Corona resistant wire PETD2-K-180

Conventional wire PETD-180

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Thermal imaging study

The thermal imaging method can be used as PD sensing method showing local heating (hot spots) of electrical equipment resulted from partial discharges currents [12].

However, this method has a limitation due to relatively low temperatures of PDs. In the present study an infrared camera is used with the objective to compare the thermal radiation of specimens subjected to sinusoidal and impulse voltage. high electrical stress levels, the measurements correspond to the temperature immediately prior to the specimens' insulation breakdown. It should de noted that the partial discharges activity was detected immediately after application of voltage.

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

A slight temperature rise of twisted pairs and coils was observed during the tests at sinusoidal voltage, despite the partial discharge activity. While the impulse voltage test shows the more significant increase in temperature. It is particularly remarkable for rectangular wire specimens, where discharges are concentrated in a small volume and heat transfer is worse than in twisted pairs. In high dV/dt tests, the rise in voltage amplitude and especially in switching frequency had a significant effect on the tested specimens' rapid heating. The increase in frequency leads to an increase in the number of partial discharge repetition rate, and the total energy thereof is increased. The time to enamel insulation breakdown is also significantly reduced.

In this experiment, dielectric heating occurs as a result of PDs activity and depends on their intensity and energy, which in turn is determined by the applied voltage parameters. Moreover, partial discharges lead to the increase of leakage currents and the dielectric losses provoking an extra heating. Thus, the interaction and mutual intensification of both electrical and thermal stresses accelerate the insulation ageing and destruction, leading to a thermal runaway and thermal insulation breakdown.

Conclusion

With the objective to investigate winding wires endurance, tests have been con- The endurance tests conducted on impregnated wires allowed to study the impregnation "healing" effect and failure mechanism in presence of insulation defects. It was shown, that even independent defect leads to insulation weakening that accelerates the destruction. The experimental results highlight the important role of impregnating compositions and enamel insulation compatibility.

Among studied wires, the tested corona resistant round section wire and rectangular section wire demonstrate the best endurance having the highest time to breakdown.

The better endurance of rectangular winding wire comparing to other conventional wires may be related to its enamel composition and thicker insulation, as well as the better thermal stability -its thermal index is 200 °C. However, the time to breakdown decreased significantly when heated at 180 °C or with the rise of voltage amplitude and switching frequency.

CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS

Endurance tests results and insulation destruction analysis confirm the significant influence of voltage and temperature interaction on insulation deterioration. Higher temperature accelerates the electrical ageing, leading to electro-thermal failure mechanism.

PD would be the main factor causing the final breakdown at low temperatures due to me- Thus, the fast-rising impulse voltage combined with high switching frequency can be a limiting factor that determines the applicability of insulating materials in inverter fed motors. Conducting insulation endurance tests on winding wires' specimens gives an opportunity, at machine design stage, to estimate and compare the insulation materials ability to tolerate applied stresses and thus, to minimize the risk of unexpected machine's failure. The analysis of breakdown data acquired on wire's specimens cannot be straightforwardly used for machine's winding design or life time prognosis. Nevertheless, based on this experiment the following recommendations may be given for improving winding reliability:

• using corona resistant wires;

• improving temperature stability by using winding wires with higher operating temperature and short-time permissible temperature considering the electro-thermal failure mechanism;

• ensure the manufacturing quality of winding wire and impregnation (defects free insulation, dielectric material compatibility, impregnation quality); With the objective to supplement these observations, a FEM modelling is performed to calculate the electric field distribution in the twisted pair in order to correlate areas of high electric field and damaged insulation areas.

Furthermore, a synthetic test bench equipped with two different voltage sources and a vacuum pump is proposed. This synthetic test bench allows to control experimental conditions as temperature, pressure, and gas media of tested specimen in the absence of climate chamber. Finally, a brief study of low pressure effect is conducted. In the absence of vacuum chamber, the twisted pair is placed in a hermetical glass tube where the pressure is lowed using the vacuum pump. Consequently, a method to detect defects in the insulation under low pressure and partial discharge activity is proposed. winding. This is the specimen recommended by international standards for testing and qualifying winding wires insulation in terms of electrical [1], thermal [2], chemical [3] and mechanical properties [4]. Thermal life evaluation of winding wire [5], dielectric breakdown at rated temperatures, and retained dielectric after refrigerant conditioning [6] tests are also conducted on twisted pairs. Moreover, as mentioned in Chapter III, twisted pairs are often used in studies focused on partial discharges in inverter-fed motors. Authors of papers [7], [8], [9], [10], [11], [12], [13] and [14] investigate lifetime of insulation subjected to partial discharges, and PD statistical features as PDIV, PD magnitude and repetition rate. Thus, the twisted pair can serve as physical model and instrument to investigate the phenomenon of electrical discharges occurred in air gaps between two twisted wires as well as to examine the insulation material degradation under these discharges. The twisted pair can be characterized by its geometrical factors such as wire's diameter, insulation thickness, length, and electrical properties such as relative permittivity ε r and RLC-parameters.

Evaluation of overvoltage at twisted pair resulting from high dV/dt and cable connections

Considering that the designed SiC-inverter described in Chapter III Section 3.4. Relative permittivity ε r values are determined using references [16] and [17] for cable and enamelled wire respectively. Electrical and geometrical parameters are given in Table V.1.

For enamelled wire, the value ε r = 3 is used in calculations. The transmission line's low-loss approximation makes it possible to express the phase velocity V p as follows:

V l ≈ 1 L l C l = 1 µ 0 ε 0 ε r (V.2)
Therefore, the self inductance per unit length L l can be written as

L l = µ 0 2π ln R out R in (V.3)
Then, the capacitance and inductance of enamelled wire and cable can be calculated by multiplying C l and L l by their length l given in Table V.

L = µ 0 π ln d -a a l (V.7)
where a = R out , d = 2R out + x and ε r is the complex dielectric permittivity of composed dielectric media (cable insulation and air), also known as effective relative permittivity. To estimate ε r of two-component mixture different mixing rules can be used.

The proposed by Lichtenecker logarithmic mixing rule [18] and [19] is

log(ε r ) = m i =1 y i 3 ε i (V.8)
The equation of Landau-Lifshitz to calculate the effective relative permittivity of mixed components [18] is

3 ε r = m i =1 y i 3 ε i (V.9)
The proposed by Beer mixing rule [18] is

ε r = m i =1 y i ε i (V.10)
In equations (V.8), (V.9) and (V.10) m is the number of mixture's components, y i designates the volume concentrations of these components and ε i corresponds to relative permittivities of mixed dielectrics.

Volume concentrations can be determined considering the distance x between the two cables and the cable insulation thickness δ (calculated using equation III.1). For the distance x = 50 mm and the insulation thickness δ = 0.18 mm, the volume concentrations of dielectrics are the following:

• air: y air =99.64%,

• TFE: y TFE =0.36%.

In this case, the effective relative permittivity calculated using (V.8), (V.9) and (V. effective relative permittivity with distance between two cables calculated using the three mixing rules. This plot shows that the used mixing rules give similar results and ε r value tends to be equal to ε ai r with increase in distance between the cables. Therefore, then x δ, the effective relative permittivity can be defined as ε r ≈ ε ai r = 1.0006. Consequently, the capacitance of cable connections will decrease with distance increasing. voltage rate lead to increasing of cable and twisted pair resistance due to skin effect. In consequence of increased resistance, the damping characteristic of the overvoltage transient is affected. The skin depth δ is defined in accordance with (V.12) [20].

δ = 1 π f µσ (V.12)
where f is the frequency of voltage oscillations estimated at 23 MHz basing on the voltage waveform shown in FigureV.1d; µ and σ are the copper permeability and conductivity.

Thus the effective resistance of cable and twisted pair are simply calculated according to (V.13) and are equal to 0.5484 Ω and 0.0624 Ω respectively:

R = ρl π(2r -δ)δ (V.13)
where ρ is the copper resistivity.

Therefore, the test bench is modelled using LTSpice (SPICE simulation software)

and MATLAB software to control the simulations and to analyse the obtained results. This proposed equivalent circuit gives an approximate model of the laboratory designed SiC-inverter with the output cable used to connect the twisted pairs. It allows to analyse the voltage transients resulted from very fast transistors switching. We have observed the importance to minimize the cable line length when the voltages oscillations and surges should be limited. This objective can be achieved using a low inductive damping resistor. On the hand, it will decrease output voltage edge rate that can be also seen from the proposed model.

Barrier discharges in the twisted pair and in the turn insulation

For better understanding of physical processes in turn insulation under PD activity, the type of partial discharges should be defined. In earlier researches e.g., in [21] and [22], the partial discharges in inverter-fed motor were characterized as corona discharges. Thus, the wire having greater withstanding capability to these discharges is named "corona-resistant" winding wire [22] and [23]. Corona is a form of partial discharges, and after Stone [24], "the term corona is reserved for the visible PDs that can occur on bare metal conductors operating at high voltage, which ionize the surrounding air". Thus, this term should not be used for PDs within the machine's winding.

Regarding to the twisted pair construction the arising partial discharges may be characterized as Dielectric Barrier Discharges (DBD) in accordance with their definition:

"DBD are electric discharges between two metallic electrodes, with at least one of them covered by a dielectric material" [25]. We suggest, that PDs within the turn insulation occurs in air gaps between barriers composed of impregnation and enamel insulation.

Barrier discharges based generators are widely investigated and used for ozone CHAPTER V. DETERIORATION OF TURN INSULATION UNDER PARTIAL DISCHARGES AT HIGH DV/DT IMPULSE VOLTAGE electrochemical synthesis, UV light generation and surface treatment [25], [26], [27], [28],

[29] and [30]. Basing on the accumulated knowledge and experience in this domain, the new approaches to study partial discharges in electrical machine windings may be proposed. The using of these approaches will be discussed in the following sections.

Modelling of dielectric barriers in twisted pair

As shown in the Section 2.1, the twisted pair can be represented as a part of electric circuit and the parameters of the equivalent circuit can be calculated using the transmission line approach. With the objective to study partial discharges in the twisted pair we propose to model it's insulation as a solid dielectric with air voids where PDs will occur.

From the point of view of classical partial discharges theory [31], the solid dielectric having an inclusion (e.g., an air void or insulation defect) with lower electric strength can be represented by its capacitance C x . This capacitance is defined by (V.14) and schematically shown in Figure V.9. The variable resistance is used in order to take into account the changing of electric circuit parameters due to PD inception. This model is proposed basing on the scheme used for barrier discharge study presented in [25], [26], [27], [28] and [32].

C x = C a + C in C d C in + C d (V.
C b = C b1 2 = C b2 2 (V.15) C tp = C b C air C b + C air (V.16)
Parameters of the proposed model are derived from capacitance measurements performed using a LCR meter E7-13 (see Appendix V-A). To this purpose, twisted pairs are made of the conventional enamelled wire PETD-180. The geometry is given in Table V.1.

The measured capacitance of twisted pair C tp = 14 nF. This value is close to the capacitance calculated in Section 2.1 (see Table V

Current-voltage characteristics of twisted pair 2.4.1 Theoretical current-voltage characteristics

The study of current-voltage characteristics of twisted pair is based on the method described in [32]. The discharge current in the air gap between the dielectric barriers of twisted pair is equal to the current of the external circuit according to the condition of the total current continuity. In air gaps, the current is transported by electrons and ions, and the current through dielectric barriers flows to the external circuit due to a displacement current I d . According to [35] the displacement current density is written by:

j d = ε 0 ε r ∂E ∂t = ε 0 ∂E ∂t + ∂P ∂t (V.18)
where E is the electric field strength, and P is the dielectric polarization. In this equation, the first term corresponds to displacement current in vacuum and the second term corresponds to displacement current in dielectric caused by displacement rate of charges in dielectric. In addition to the displacement (capacitive) current, a conduction current I cond flows through dielectric barriers, thus the total current is defined by V.19. In our case, the conduction current may be neglected.

I tot = I cond + I d (V.19)
In

I tp = I 2 a + I 2 r (V.20)
If the impulse voltage is applied to the twisted pair, when the maximum capacitive current i c max will be defined by (V.21). In full-bridge inverter topology, the tested specimen is connected to inverter's midpoints that complicates the current measurements. Therefore, to measure currents on twisted pair the oscilloscope was powered via an isolation transformer having a shielded secondary winding. Thus, a current shunt can be placed in one of the inverter's leg. This shunt is made with low inductive ceramic encased resistors placed on round copper plate.

It is connected to the oscilloscope Tektronix TDS 2014C via a coaxial cable and impedance match resistors. One of the ends of the twisted pair is connected to the pulse generator output via this shunt that allows to measure its current. During the current measurements the voltage on the twisted pair was recorded using the designed RC-divider with ratio 280:1.

Thermal study

Basing on consideration that an important part of PD energy transforms into heat, the discharges intensity can be evaluated by temperature monitoring during the tests. The The rapid insulation breakdown at high voltage amplitude complicates the measurements. Despite this, the obtained results put into evidence the influence of both voltage and frequency variations on the twisted pair heating by discharges, considering the fact that this heating started in the presence of PDs.

As mentioned above, the main part of discharge energy transforms into heat, the rest of this energy consumes for chemical reactions, e.g., for nitrous oxide and ozone generation, and dissipates in light and sound. Basing on that, one of the methods used for study DBDs characteristics is the optical emission spectroscopy.

Optical emission study

In

Microscopic study

With the purpose to study the structure of barrier discharges, photographs are taken through an optical microscope.

Finite element modelling

Electrostatic simulations using FEM finds application in works dedicated to partial discharges in inverter-fed motors, e.g., for evaluate PDIV within two parallel winding wires [14] or to localize the maximum electric field strength in air gaps of twisted pair [36]. In this thesis, the FEM modelling was performed with the purpose to link the areas of high electric field between wires' turns giving rise to barrier discharges and damaged insulation areas.

The electric field distribution in the twisted pair is calculated using ANSYS Maxwell The experimental study at low pressure is a relevant task for characterization of partial discharges and insulation destruction of electrical equipment in aerospace applications.

If a vacuum chamber is not available, the proposed setup may be used. It also allows to keep easy access to the tested specimen for observations. tube with the tested twisted pair is decreased to 9 × 10 -2 mm Hg. This pressure level excludes the ignition of partial discharges at voltage level up to 1400 V (high dV/dt test bench). Then, the atmospheric air is injected into the tube, the pressure is measured, and the impulse voltage applied to the twist pair is progressively increased from 0 to 1400 V, f = 50 kHz, at a fixed pressure. The partial discharge ignition is determined observing the twisted pair current and the optical emission. Therefore, PDs were observed at minimum voltage level of 370 V when the pressure in the glass tube was equal to 38 mm Hg.

In the absence of PD, the heating of tested specimen (observed by the infrared

Conclusion

To deal effectively with partial discharges in the turn insulation and to limit their consequences, these phenomena should be thoroughly investigated considering the characteristics of applied electric stresses, specifically for high dV/dt impulses, and studied specimens. For this purpose, the experimental test bench and the twisted pair are modelled using different approaches. Firstly, the impact of high voltage rate on the voltage transients even with short cable connections is analysed. To limit the transient overvoltage a damping resistor is added in the scheme for the following experiments.

On the other hand, the modelling of the twisted pair using the barrier discharge approach offers an opportunity to apply the methods developed for plasma and ozone generators in our study revealing common aspects. Thus, the current-voltage characteristics of twisted pair in the absence and presence of barrier discharges are given. These characteristics can be used for the discharges energetic aspects study. However, the current measurements method should be improved. The proposed equivalent circuit of twisted pair with barrier discharges allows to separate discharge current and capacitive current, especially significant at high dV/dt. This model can be refined by additional series and parallel resistors taking into account conductivity and relaxation losses.

The performed current, temperature and spectra measurements show high energy of barrier discharges increasing with the electric filed strength and the switching frequency. The total energy of barrier discharges is mainly divided between generation of ozone and NOx, heating of tested specimen and heat transfer by convection, and UV radiation. Higher is this energy, faster will be the insulation degradation. Depending on energy distribution between different processes, the electro-chemical or the electro-thermal destruction will predominate. For the first time, the filamentary structure of barrier discharges in the twisted pair subjected to high dV/dt is put into evidence that allows to relate the enamel insulation destruction with chemical etching by repetitive filaments. Moreover, the rise in switching frequency and voltage amplitude increase filaments propagation zone enlarging the deteriorated insulation surface.

To go further in the experimental study, the synthetic test bench is proposed. It provides a possibility to combine different electrical stresses using two voltage sources, and to control the media of tested specimen, changing the gas or adding a liquid, varying the pressure. Furthermore, the possibility of the insulation weakest spots visualization by barrier discharges at low pressure is demonstrated.

The observations made on the twisted pair may be used for understanding of turn insulation destructions mechanisms as long as this insulation has defects and air gaps.

In this case, layers of enamel insulation and impregnating varnish compose the dielectric barriers and the air gap ionization between these barriers may occur depending on the machine's operation conditions.

Conclusion and future work

The experimental investigation and numerical simulation study performed in this work are essentially focused on the effects of impulse voltage with high dV/dt and high switching frequency on winding insulation robustness of inverter-fed electrical machines.

Therefore, the work covers a number of issues related to electric drives, power electronics, electrical machines, dielectric materials and partial discharges. Its relevance is due to the implementation of wide bandgap semiconductors, such as Silicone Carbide, in power electronic inverters that may intensify the electric stresses in motor drives, accelerating motor's winding insulation failure.

Chapter I provides a literature survey devoted to low voltage electrical machines impulse with high dV/dt, and to choose the optimal insulation type for inverter-fed motors. The mean time to breakdown has been used as the end-point criterion for the insulation endurance evaluation. In line with expectations, the corona resistant wire mainly shown better endurance comparing with the conventional wires. The rectangular section wire also demonstrated higher mean time to breakdown than other conventional wires that may be related to its higher thermal index and thicker insulation.

The study of influence of defects on turn insulation endurance has been carried out. For this purpose, non-defective unimpregnated, defective unimpregnated and defective impregnated by different compositions specimens were tested under impulse volt-age with low dV/dt. Comparing the obtained mean times to breakdown, we noticed that, firstly, independent defects critically affect the turn insulation when subjected to PDs and, secondly, the impregnation may not provide sufficient defects "healing" that highlights the importance of compatibility of enamel insulation and impregnating composition.

The experimental results presented in this Chapter illustrate the following:

• heating of specimens in the thermal chamber at temperature corresponding to their thermal index significantly accelerates the insulation destruction when subjected to PDs (electro-thermal failure mechanism);

• in specimens tested under sinusoidal voltage with frequency of 50 Hz and under low dV/dt PWM-like impulse voltage with carrier frequency of 5 kHz, breakdown mainly occurred at defective area; the "frost effect" and localized damages were observed on the insulation surface (domination of electrical and chemical failure mechanisms);

• in specimens tested under high dV/dt voltage with switching frequency of 50 kHz the effect of carbonization and sticking of turns were observed (domination of thermal destruction mechanism).

These observations put into evidence the effect of impulse voltage having short rise time and high switching frequency on partial discharges leading to higher ozone concentration and significant temperature rise extremely shortening the insulation life time.

Unfortunately, the effects of high dV/dt and high switching frequency could not be separated that complicates the analysis of dV/dt effect. On the other hand, the interest of higher frequency operation is linked to fast switching SiC transistors enabling the conduction losses reduction.

The presented above experimental results motivated me to examine more thoroughly deterioration processes in turn insulation subjected to high dV/dt impulse voltage as well as the features of arising partial discharges as described in Chapter V. The twisted pair specimens have been used for conducting this phenomenological study. The main results and contributions of the present thesis:

• high frequency model intended to study of transient overvoltage in inverter-fed switched reluctance machine's winding has been proposed;

• test bench containing SiC-based pulse generator providing high dV/dt impulse voltage (up to 60 kV/µs applied to tested specimen), voltage amplitude up to 1700 V and maximum switching frequency up to 125 kHz has been designed;

• novel specimen's shape for testing rectangular section wires has been designed;

• synthetic test bench containing two voltage sources offering a possibility to apply desired impulse voltage waveform to the tested specimen and at the same time to control its temperature, pressure and media has been proposed;

• estimation of insulation defectiveness using defects visualization when tested specimens subjected to high dV/dt impulse voltage exceeding the partial discharge inception level and low pressure has been suggested;

• partial discharges in air gaps between turns of tested twisted pairs have been characterized as the dielectric barrier discharges; having regard that the twisted pair represents a physical model of machine's winding part, the PDs within the winding may be also considered and studied as dielectric barrier discharges;

• dielectric barrier discharges in twisted pairs tested under high dV/dt impulse voltage with high switching frequency have a filamentary structure creating periodical erosion tracks on the enamel surface;

• in case of barrier discharges occurred on twisted pairs subjected to high dV/dt impulse voltage with high switching frequency, the electro-thermal insulation destruction may predominate the electro-chemical one even in the absence of the external heating.

Regarding to the future work, the following points would be advisable thereafter:

• to improve the accuracy of the proposed high frequency model of the inverter-fed switched reluctance machine and the feeding cable extending the frequency range considering the possible very short impulse voltage rise time up to 10 ns;

• to develop a model to simulate the turn-to-turn voltage distribution of electrical machine's winding (distributed and concentrated);

• to study the probable distribution of defects in winding insulation which in association with the voltage distribution will make it possible to more accurately predict the zones with risk of partial discharges inception;

• using the high dV/dt test bench to establish the test procedure allowing to investigate simultaneously and separately the effects of thermal and electrical stresses;

• to enlarge this experimental study of turn insulation endurance and destruction mechanism under low pressure using a vacuum chamber or proposed in this work setup with hermetical tube and vacuum pump, in order to simulate altitude conditions of aeronautic motors operation;

• to suggest a corona resistance criteria basing on endurance tests results: e.g., maximal time to breakdown or without breakdown, or number of ageing cycles;

• to develop a mathematical model of insulation life time considering high dV/dt and high switching frequency as well as electro-thermal and electro-chemical ageing processes in turn insulation in presence of dielectric barrier discharges;

• regarding the obtained experimental results under high frequency impulse voltage, to propose new nanostructured corona-resistant enamels with higher thermal index that may improve wires temperature stability and extend the winding life time;

• to enlarge the performed study of dielectric barrier discharges in turn insulation determining a balance of discharges power evaluating the power for heating and UV radiation, the power needed to ionization processes and ozone generation, as well as dielectric losses;

• to examine the effect of relative permittivity and wires' capacitance on barrier discharges inception;

• to improve the dielectric barrier model of twisted pair considering a dynamic capacitance changing in presence of discharges as well as variable resistance characterizing the air ionization.

RC-divider

The RC-divider has been designed with the purpose to measure high dV/dt impulse voltage produced by the SiC-based generator as the differential probe Tektronix THDP0100 was not always available. The RC-divider can be used to measure the impulse voltage on one of the inverter's legs and the use of two identical dividers allows to conduct differential measurements.

The construction of the RC-divider is shown in Figure 7 and Figure 8. In the scheme:

  love of my family. My sincere gratitude goes to my boyfriend, Guillaume Valentin, for your love and faith in me, for being by my side, caring about me. Thank you for keep me moving forward. iii Résumé détaillé en français Résumé de thèse intitulée : Caractérisation des isolants de bobinage des machines électriques alimentées par des ondes de tension à forts dV/dt La présente thèse est le fruit d'une collaboration entre l'Université Polytechnique de Tomsk (TPU), en Russie, et l'Université Paris-Saclay, en France. Dans le cadre d'une convention de cotutelle internationale, les travaux de recherche et la formation complémentaire ont été menés au laboratoire Systèmes et Applications des Technologies de l'Information et de l'Énergie (SATIE) de l'ENS Paris-Saclay -France et de la division Power and Electrical Engineering de la TPU -Russie.

FIGURE 1 :

 1 FIGURE 1: Banc expérimental de mesure d'impédances de la machine à réluctance variable.

FIGURE 2 :

 2 FIGURE 2: Comparaison des impédances mesurées et simulées : (a) mode commun et (b) mode différentiel.

FIGURE 3 :FIGURE 4 :FIGURE 5 :

 345 FIGURE 3: Formes d'ondes de tension simulées pour différents temps de montée avec une longueur de câble de 1 m.

FIGURE 6 :

 6 FIGURE 6: Aperçu d'ensemble du montage expérimental (a) et SiC onduleur (b).

FIGURE 7 :FIGURE 8 :

 78 FIGURE 7: Aperçu d'ensemble du montage expérimental (a), SiC onduleur (b), éprouvette testée (c).

FIGURE 9 :

 9 FIGURE 9: Formes d'ondes de tension mesurées sur l'éprouvette mise dans la chambre thermique et modélisées par le circuit équivalent RLC sous LTSpice.

FIGURE 10 :FIGURE 11 :

 1011 FIGURE 10: Formes d'ondes de tension avec le temps de montée de 27 ns modélisées pour deux longueurs de câble.

FIGURE 12 :

 12 FIGURE 12: Images microscopiques : (a) décharges à barrière avec structure filamentaire et (b) érosion de l'isolant par des attaques chimiques due aux filaments.

FIGURE 13 :FIGURE 14 :

 1314 FIGURE 13: Synoptique du banc d'essai synthétique.

2 2 . 1 3 3 . 1 5 . 1

 213151 Reluctance Motor (3-phase, 6-slot, and 4-pole). The high frequency model of the studied motor represents an equivalent circuit model with RLC lumped parameters extracted from common mode and differential mode impedance measurements. The supply cable model existing in literature is used. In the proposed electric drive model, the study of a single commutation is performed. In this regard, the inverter is modelled by an ideal pulse generator. The third chapter presents the experimental study of winding wires endurance when subjected to electrical and thermal stresses provoking partial discharges activity. At the beginning of this chapter, the brief review of existing testing methods aimed to investigate partial discharges in electric motors and winding wires, as well as to determine wires insulation endurance is provided. Then the proposed methods and experimental benches allowing to perform insulation electro-thermal accelerated ageing tests are presented. The original test bench with the objective to study the effect of electrical stresses on motor winding insulation engendering by SiC high-speed switches has been designed. The fourth chapter gathers the experimental protocols and obtained results followed by statistical analysis and main observations. Firstly, the results of breakdown voltage tests carried out under sinusoidal voltage are given. Then, the results of endurance tests conducted on studied winding wires under sinusoidal voltage, Pulse Width Modulation (PWM) low dV/dt impulse voltage (µs-level of rise time) and rectangular high dV/dt GENERAL INTRODUCTION impulse voltage (ns-level of rise time) are presented. The influence of voltage waveform on turn insulation ageing and endurance, as well as the effect of thermal stresses are discussed.The fifth chapter presents a phenomenological study of deterioration processes in turn insulation under PDs when subjected to high dV/dt impulse voltage. The high dV/dt bench and tested twisted pairs are modelled using different approaches. The partial discharges in twisted pair have been characterized as dielectric barrier discharges that offers an opportunity to apply the methods developed for ozone generators to our study using common aspects. The characterization of insulation deterioration processes in twisted pairs subjected to high frequency impulse voltage with high dV/dt allows a better understanding of winding insulation destruction of motors powered by SiC-based power converters.GENERAL INTRODUCTIONThe present thesis is a result of collaboration between the National Research Tomsk Polytechnic University (TPU), in Russia, and the Paris-Saclay University, in France. In accordance with the agreement on joint international supervision the research work and complementary training have been conducted at the laboratory of Systems & Applications of Information & Energy Technologies (SATIE) of ENS Paris-Saclay -France and the School of Energy & Power Engineering of TPU -Russia. Electrical machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Induction machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Switched reluctance machine . . . . . . . . . . . . . . . . . . . . . . . . . Electrical machine winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distributed winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Concentrated winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Electrical machine winding insulation system . . . . . . . . . . . . . . . . . . 4.1 Winding wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Winding impregnation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Impregnating compositions and wires enamel insulation compatibility 5 Operational factors affecting winding insulation . . . . . . . . . . . . . . . . Failures in motor windings . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Effect of impulse supply voltage on winding insulation . . . . . . . . . . 6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure I. 2 :Figure I. 3 :

 23 Figure I.2: Traction and charging power trains on-board of Renault's electric vehicle: ZOË [39].

  Due to these features, SiC power semiconductor devices are especially attractive for motor drives in automotive, aerospace, and industrial applications where power density and weight are the main factors for power electronics sizing (see Figure I.4) [43].

Figure I. 4 :

 4 Figure I.4: SiC Application areas presented in AgileSwitch SiC Product Overview [43].

Figure I. 5

 5 Figure I.5 shows inverters developed by ROHM demonstrating the power devices technology evolution in sport electric vehicles of Formula E, all-electric-powered FIA (Fédération Internationale de l'Automobile). The new embedded Full SiC Power module makes 200 kW inverters 43% smaller and 6 kg lighter [44] compared with Silicon (Si)-based converters.

Figure I. 6 :

 6 Figure I.6: Chart of possible motor choices for HEVs and EVs [45].

Figure I. 7 :

 7 Figure I.7: Machine options for the MEA drive system [46].

Figure I. 9 :

 9 Figure I.9: Construction of an induction motor (dissected view) [8].

Figure I. 10 :

 10 Figure I.10: Magnetic circuit of induction motor's stator and rotor [8].

Figure I. 11 :

 11 Figure I.11: Three-phase SR Drive®stator and rotor (graphic of NIDEC Motors) [54]: (a) SRM construction; (b) principle of operation; (c) magnetic circuit.

Figure I. 13 :

 13 Figure I.12: Three-phase SRM driven by asymmetric half-bridge converter [57].

3. 1

 1 Distributed winding Like most low voltage motors, the cage induction motor has a randomly distributed stator winding. The construction of a random wound winding is given in International Electrotechnical Commission (IEC) standard 60034-18-41:2014 [63] and depicted in Figure I.14; the stator winding photograph is shown in Figure I.15 [64].

Figure I. 14 :

 14 Figure I.14: Section of stator winding [63].

Figure I. 15 :

 15 Figure I.15: Distributed stator winding [64].

  By contrast with induction motors, switched reluctance motors usually have a concentrated coil winding configuration. Comparison between distributed and concentrated winding design is demonstrated in Figure I.16 by the example of synchronous permanent magnet machine stator structures given in [65]. As can be seen, overlaps between coils referring to the different phases are eliminated in concentrated winding. The photograph of concentrated stator winding is shown in Figure I.17

  .

Figure I. 16 :

 16 Figure I.16: Distributed (a) and concentrated (b) winding configurations [65].

Figure I. 17 :Figure I. 18 :

 1718 Figure I.17: Concentrated winding configuration of a Hyundai Sonata 2011 motor [66].

Figure I. 19 :

 19 Figure I.19: Enamelled wire with double layer insulation: 1 -copper conductor; 2 -inner insulating layer (base), 3 -outer insulating layer (top).

Figure I. 20 :

 20 Figure I.20: Scanning electron microscopy image of enamelled wire, chemical composition PAI and PEI, 13 layers, total thickness 50 µm [75].

Figure I. 21 :

 21 Figure I.21: Turn insulation microstructures microscopy images: (a) monolithic microstructure; (b) cellular microstructure; 1 -winding wires' conductors, 2 -impregnating material, 3 -enamel insulation film [71].

Figure I. 23 :

 23 Figures I.23a and I.23b [71].

Mechanical and thermomechanical factors

  Figure I.24: Main factors affecting winding insulation during electrical machines manufacturing and operational processes [80].

Figure I. 25 :

 25 Figure I.25: Expected insulation life vs. operating temperature for different thermal classes [83].

Figure I. 26 :

 26 Figure I.26: Insulation damage caused by inter-turn short circuit of the stator windings in three-phase induction motors [8].

  put into evidence that the use of PWM based inverters increases electrical stresses in low voltage electrical CHAPTER I. FACTORS AFFECTING LOW VOLTAGE ELECTRICAL MACHINES FED BY INVERTERS: LITERATURE SURVEY machines (see Figure I.27). Voltage inverters are made on the basis of power semiconductor devices -electronic switches, e.g., Gate Turn-Off thyristor (GTO), Bipolar Junction Transistor (BJT), Insulated Gate Bipolar Transistor (IGBT) and Metal-oxide-semiconductor Field-effect transistor (MOSFET). Depending on type and characteristics of used semiconductor devices the switching frequency could reach dozens of kHz and the impulse voltage front duration can be from tens of nanoseconds to several microseconds [89] and [90] creating high dV/dt slopes and oscillating frequencies up to several megahertz.

Figure I. 27 :

 27 Figure I.27: Principle of induction motor adjustable speed drive with its schematic voltage characteristics [88].

Figure I. 28 :

 28 Figure I.28: Voltage impulse waveform parameters [63].

Figure I. 30 :

 30 Figure I.30: Inverter fed motor pu overvoltage vs. cable length and impulse voltage risetime [90].

Figure I. 31 :

 31 Figure I.31: Phase-to-phase voltage waveforms during motor operation at the motor input with a 30 m, 25 m and 15 m length connecting cable [84].

  partial discharges comprise local discharges on surface or inside insulation such as a corona, a creeping discharge, or an insulation element local breakdown, which shunts a part of insulation between electrodes having different potentials. The corona, local internal and surface discharges are partial discharges that occurs at the threshold applied voltage caused ionization of the air around conductors. These partial discharges lead to failures in local areas with high voltage intensity and weakened insulation. The term "corona" here is used for a surface discharge in electrical machine's winding insulation (Figure I.32 [95] and [96]).

Figure I. 32 :

 32 Figure I.32: Electrical discharges in stator windings [95] and [96].

Figure I. 33 :

 33 Figure I.33: Microscopy images of turn insulation aged by PDs [102].

  surges and inverter voltage front steepness, and can also decrease the voltage distribution non-linearity in motor winding. Another way is to use a multi-level inverter topology.The use of multi-level inverter creates a higher number of voltage levels and, therefore, increases the number of impulse voltage fronts, but decreases their amplitude, which reduces overvoltage and extends insulation lifetime[63] and[START_REF] Carlo Montanari | Partial Discharge and life behavior of rotating machine wire insulation under PWM waveforms: The influence of inverter characteristics[END_REF] (Figure I.34). However, this solution leads to a rise in electric drive cost and size.

Figure I. 34 :

 34 Figure I.34: Five step phase to phase voltage at the terminals of a machine fed by a 3-level converter [63].

Figure I. 35 :

 35 Figure I.35: Electrical breakdown path through inorganic filler spherical particles [115].

  and analysed in frequency and time domains. The model of a random wound winding consists in a cascade connection of single-coil models (lumped equivalent circuits) and inductive couplings among the phases as shown Figure II.1.

Figure II. 1 :

 1 Figure II.1: HF equivalent circuit of a random wound coil [14].

Figure II. 2 :

 2 Figure II.2: Single-phase HF equivalent circuits: (a) model without the skin effect [6], (b) model taking into account the skin effect [15].

Figure II. 4 :

 4 Figure II.3: Induction motor HF model: (a) per phase circuit, (b) three phase model [9].

Figure II. 5 :

 5 Figure II.5: Induction motor HF model: (a) HF per phase equivalent circuit, (b) measured and fitted impedances [11].

Figure II. 6 :

 6 Figure II.6: Measured and simulated voltage waveforms at the motor terminals for 70-m cable [11].

Figure II. 7 :

 7 Figure II.7: Overvoltage evolution as function of the inverter's rise time for different cable lengths [4].

  Figure II.8: High frequency per phase equivalent circuits:(a) simplified model defined in[9],(b) extended model defined in[11].

  Figure II.9a provides the front-view of this motor demonstrating a simple access to stator winding coils terminals. During the impedance measurements, the winding is star-connected.

Figure

  Figure II.9 shows the measurement setup equipped with the network analyzer Agilent Technologies E5061B having a frequency range from 5 Hz to 30 MHz of gain-phase test port. In order to minimize the influence of external parasitic inductances during measurements, the shortest cable connections were used. These cables were installed before the analyzer calibration procedure.

Figure II. 11 :

 11 Figure II.11: Differential mode impedance measurement: (a) per phase circuit connections, (b) winding coils connections.

  and II.13. The characteristic points used for model parameters calculation described in the following section are shown in these figures.

Figure II. 12 :

 12 Figure II.12: Measured CM impedance of SRM.

3 )

 3 Using the measured DM impedance and basing on the equation L = Z 2π f , the DM inductance L DM (point DM 1 ) is calculated. The impedance magnitude Z max is identified on the global maximum (point DM 2 ). Moreover, the impedance magnitudes Z mi n2 and Z mi n3 on global minima (points DM 3 and DM 4 ), the respective resonance frequencies f res2 and f res3 , as well as the phase θ Z3 at point DM 4 , are extracted from the DM impedance. The copper losses R cu according to [9] can be estimated by a value less than 5 Ω for this model. The iron losses R e are estimated by II.4. The equation II.5 gives the stray inductance of measuring cable connections and motor internal feed lines L zu and the equation II.6 gives the parasitic resistance R g1 .

  Figure II.14 shows the studied per phase circuit. The three-phase equivalent circuit model is obtained assuming a star-connection of three per phase circuits as during the impedance measurements. Figure II.15 provides the obtained simulation results in common and differential modes superimposed with the measurements.

Figure II. 14 :

 14 Figure II.14: Simplified HF per phase circuit motor model basing on [9].

Figure II. 17

 17 Figure II.17 demonstrates the obtained results.

Figure II. 16 :

 16 Figure II.16: Extended HF per phase circuit motor model basing on [11].

Figure

  Figure II.17: Comparison of measured and simulated by extended HF model motor impedances: (a) CM and (b) DM.

Figure II. 18

 18 Figure II.18 provides the equivalent circuit (or cell) modelling the cable having two symmetrical cores (phases) and metal shield.

Figure II. 18 :

 18 Figure II.18: Two-wire shielded cable: (a) cable's structure, (b) HF equivalent circuit model basing on [12].

  obtained waveforms are shown in Figure II.19. Taking into account the rated voltage of the studied SRM, the voltage of the ideal pulse generator is set at 300 V, the pulse rise time is equal to t R = 30 ns, and the single phase cable model corresponds to cable length L = 1 m. The similar results are obtained using the simplified and the extended models.The extended motor model is retained for the further simulations.

Figure II. 19 : 90 2 90 3 94 3. 1 94 3. 2 98 3. 4 1 Introduction

 1990909419429841 Figure II.19: Voltage waveforms for pulse generator voltage rise time t R = 30 ns (10 kV/µs) and cable length L = 1 m simulated with two proposed motor models.

  specimens represent two enamelled wires having one contact point where partial discharges are generated (Figure III.4), what allows to investigate partial discharges phenomenon.

Figure III. 1 :

 1 Figure III.1: Examples of motorette test setup configurations: (a) phase-to-phase motorette setup and (b) phase-to-ground motorette setup [8].

Figure III. 2 :

 2 Figure III.2: Samples for hairpin motors insulation qualification tests [10].

Figure III. 3 :

 3 Figure III.3: Device for twisting the specimen for breakdown voltage test: 1 -spacer; 2 -rotary hook; 3 -specimen [29].

Figure III. 4 :

 4 Figure III.4: Two crossed enamelled wires creating one-point contact specimen [30].

Type I and

 and Type II are distinguished according to their rated voltage level, partial discharges withstanding capability, used dielectric materials and winding constructions. The insulation of Type I is generally used in machines rated at 700 V RMS or less (low voltage electric machines) and should not be subjected to PDs contrary to the Type II insulation which is generally used above 700 V RMS. However, the previous studies mentioned in the present work show that partial discharges may occur in low voltage machine's winding when supplying by PWM converters. Partial discharges activity and insulation destruction depend on operation conditions (humidity, pressure) and thermal, electrical and mechanical stresses. PDs appearance can also be a symptom of the insulation degradation process, which can lead to a final failure.

Figure III. 5 :

 5 Figure III.5: Geometry of winding wire: (a) round section wire and (b) rectangular section wire.

Firstly, the breakdown

  voltage tests are conducted on bent wire specimens shown in Figure III.6 according to the standard IEC TS 60851-5 [29]. The experimental setup and testing procedure will be given hereinafter. Specimens are made of straight pieces of round and rectangular sections wires of 200 mm length, where tested area is equal to 125 mm.

Figure III. 6 :Figure III. 7 :

 67 Figure III.6: Bent wire specimen for breakdown voltage tests.

  and III.8d respectively. The coupled bar specimens had a limitation consisting in nonuniformity of contact area where partial discharges occur. It leads to different enamel degradation along specimens' length and considerable deviation in time to breakdown.

Figure III. 8 :

 8 Figure III.8: Coupled bar specimen: (a) general view, (b) schematic view, (c) and (d) cross-sections at the contact area.

Figure III. 9 :

 9 Figure III.9: Coil specimen: (a) single layer coil general view and (b) single layer coil cross-section at the contact area,(c) double layer coil general view and (d) double layer coil cross-section at the contact area.

  eties. With the objective to complete the specifications data by experimental results, AC sinusoidal voltage is applied to studied wires. This experiment allows to compare wires breakdown voltages under sinusoidal waveform before conducting endurance tests at impulse voltage. To obtain experimental breakdown voltage values, bent wires specimens (see Figure III.6) are exposed to AC sinusoidal voltage of 50 Hz nominal frequency at normal am-CHAPTER III. ENDURANCE TESTING METHODS FOR TURN INSULATION OF MOTORS FED BY INVERTERS bient temperature (approximately 25 °C) and atmospheric pressure. The test voltage is applied at zero and increased at a uniform rate until reaching insulation breakdown voltage level. The experimental bench contains a step up transformer, a variable autotransformer providing sinusoidal voltage level adjustment, a fault detection circuit, and a voltmeter (RMS breakdown voltage is measured).

(Figure III. 10 :

 10 Figure III.10: Sketch (a) and photograph (b) of the specimen placed in "wire -shot" test. Developed and installed in TPU, Russia

  activity has been detected. The partial discharges detection methods and utilized equipment are detailed in Section 3.5. The electrical ageing of insulation is done at ambient temperature equal to 25 °C. Furthermore, combined stresses (electrical and thermal), are applied simultaneously with the purpose to enlarge the study considering the interaction of temperature and impulse voltage effects. In this case, the temperature in the thermal chamber is set at 180 °C. It corresponds to wires' insulation thermal class H. In the context of the electrical machine operation, it indicates the peak temperature into a stator winding which winding wire can tolerate.The endurance tests have been performed under PDs activity until enamel insulation breakdown occurred. Thus, the times to breakdown are measured for a number of test specimens conforming to IEC standards[53] and[54]. The tests parameters sequences (ageing factors levels) are randomized to avoid false trends due to possible interactions with uncontrollable environmental factors.

  corona (surface) discharges is estimated. Sinusoidal voltage is applied as shown in Figure III.11a. Electrical discharges are observed on the insulation surface contacting with the metal shot when voltage level exceeds the ionization voltage in this system of electrodes. The methods used to detect electrical discharges are described in Section 3.5. The CHAPTER III. ENDURANCE TESTING METHODS FOR TURN INSULATION OF MOTORS FED BY INVERTERS chosen test sinusoidal voltage levels are higher than discharge inception voltage but lower than immediate insulation breakdown threshold. Thus, the enamel insulation is gradually aged and degraded by electrical discharges up to the final breakdown. The mean time to breakdown, obtained by calculating the arithmetical mean of at least five tested winding wire specimens, is used as the endurance criterion for further comparative analysis. Furthermore, endurance test under sinusoidal voltage are carried out on twisted pairs and coils specimens (see Section 3.2). In this case, one wire of the tested twisted pair or coil is connected to the ground and to the other wire the sinusoidal voltage is applied as shown in Figures III.11b, III.11c and III.11d. The wires' ends are in the air. To study the electro-thermal stresses effect, the test bench is equipped by a thermal chamber: oven with air-forced circulation, maximum temperature 200 °C, deviation ± 5 °C. Partial discharges endurance tests are performed at two temperature levels: 25 and 180 °C.

Figure III. 11 :Figure III. 12 :

 1112 Figure III.11: Voltage on specimens while insulation endurance testing: (a) in "wire -shot" test, (b) twisted pair test, (c) single coil test and (d) double coil test.

Figure III. 13 :

 13 Figure III.13: Simplified electric scheme and voltage waveform measured on the tested specimen, V rms = 5 kV, f = 50 Hz

  of reproducing a voltage waveform generating by conventional inverter based on Silicone semiconductor technology. The main component of this setup is a pulse generator having a single-phase half-bridge inverter topology. This inverter is built using 600 V Si IGBT (IRGP6630DPBF) transistors which are controlled by ACPL-333J gate drivers. The 103 CHAPTER III. ENDURANCE TESTING METHODS FOR TURN INSULATION OF MOTORS FED BY INVERTERS schematic structure of the pulse generator is detailed in Figure III.14. The inverter provides low voltage bipolar impulses which are increased using a step-up transformer ÌÏ2000ÍÌ (primary windings w1 = w2 = 105 wire turns, secondary winding w3 = 2800 wire turns).Thus, the obtained bipolar PWM-like voltage waveform having µs-level rise time is applied on enamelled wires' specimens. A time counter is used for recording a time to samples' insulation breakdown. This test bench is designed and installed in TPU laboratory (Tomsk, Russia); to have more details on test bench conception, please refer to[57].

Figure III. 14 :

 14 Figure III.14: Scheme of the low dV/dt pulse generator: Si IGBT inverter.

Figure III. 15 :

 15 Figure III.15: Sketch of the low dV/dt experimental bench.

Figure III. 16 :

 16 Figure III.16: Low dV/dt experimental bench photographs: (a) general view, (b) testing twisted pairs' installation and visible effect of PDs (purple light on the twists), (c) pulse generator view.Developed and installed at TPU, Russia

Figure III. 17 :

 17 Figure III.17: Bipolar impulse voltage waveform parameters.

Figure III. 18 :Figure III. 19 :

 1819 Figure III.18: Electrical scheme of the high-frequency RC-divider 1000:1.

Figure III. 20 :Figure III. 21 : 3 . 4 . 3

 2021343 Figure III.20: Enlarged plot showing impulse voltage parameters.

Figure III. 22 :

 22 Figure III.22: General view of the high dV/dt experimental bench and SiC inverter.Developed and installed at SATIE, France

  main components of this setup are shown in Figure III.25 and detailed in Table III.3.

Figure III. 25 :

 25 Figure III.25: Scheme of experimental setup for high dV/dt tests: SiC full bridge inverter.

  waveform is generated by means of a function generator connected to drivers' inputs. Comparing to the experimental bench described in Section 3.4.2, the advantage of this setup is in the possibility to vary voltage waveform parameters, such as the amplitude, the switching frequency, the duty cycle and dV/dt. It makes possible to examine their specific effects on the partial discharges and insulation degradation. Tested specimen connection points are shown in Figure III.25 and Figure III.26: two wires of tested twisted pair or coil are connected via cables to midpoints of the inverter. This connection allows to apply the impulse voltage to the both of twisted wires alternately as differentiated from previous test benches where only one of tested wires is under voltage.

Figure III. 26 :

 26 Figure III.26: General view of the high dV/dt experimental bench (a), SiC inverter (b) and tested specimen installation (c). The SiC inverter is developed at SATIE, France, and experimental assembly is developed and installed at TPU, Russia.

  Figure III.27 gives the measured waveform corresponding to pulse generator output voltage for a 1 kV DC bus voltage level. The obtained voltage waveform represents the RC-divider response to the signal generated by the pulse generator with high frequency oscillations during the first few hundred nanoseconds after switching. Characteristics of the used RC-divider are detailed in Appendix III-B.

Figure III. 27 :

 27 Figure III.27: High dV/dt impulse voltage scopes.

Figure III. 28 :

 28 Figure III.28: Voltage measured at the pulse generator output without specimen (a), voltage measured on the tested specimen in the thermal chamber (b).

Figure III. 29 .

 29 This device allows to record the electromagnetic radiation in the UV spectrum band if surface partial discharge and corona discharges appear. This monitoring procedure is often used for detecting defects and failures in high voltage electrical equipment (power transmission lines, insulators).

Figure III. 29 :

 29 Figure III.29: Electron-optical defectoscope Filin-7.

Figure III. 31 :Figure III. 32 :

 3132 Figure III.31: Test under low dV/dt PWM impulse voltage V a = 1.2 kV, f c = 5 kHz: twisted pairs' installation (a), PDs photograph (b) and PDs optical image (c).

Figure III. 34 :Figure III. 35 :

 3435 Figure III.34: Photograph of antenna detecting PD on the coil tested under high dV/dt impulse voltage V dc = 1.4 kV, f = 50 kHz.

Figure III. 36 :

 36 Figure III.36: Voltage waveforms measured at high dV/dt test bench above the PDIV: rising and falling rates at V dc = 1200 V.

  Figure III.37: Experimental setup for ozone detection.

Figure III. 38 :

 38 Figure III.38: Photographs of chemical reactions resulting from PDs on tested specimen: changing of solution color with the start of partial discharges.

4 Conclusion23 3 . 1

 431 With regard to minimize the risk of unexpected motor failures it is indispensable to analyse comprehensively voltage parameters influence, such rising and falling times, dV/dt and frequency, combined with thermal stresses. In this regard, it is essential to design experimental equipment to reproduce the constraints imposed by converters controlling electrical machines.The presented there experimental benches allow to evaluate the endurance of insulating materials considering the changing in the process of ageing and the mechanism of enamel insulation destruction due to changes in controlling power electronics. The relevance of new testing methods and test benches is well found by remarkable modern advances in semiconductors technology. Testing methods for low voltage motors winding wires insulation proposed in this work permit to evaluate the withstand capability of insulation materials applying firstly classic sinusoidal voltage of 50 Hz; secondly, the PWM signals representative for traditional Silicone-based converters supplying (µs-level of impulse rise/fall times); and finally, the voltage waveforms with steep dV/dt, very fast rise and fall times (ns-level of impulse rise/fall times) and high switching frequency, engendered by new semiconductor devices based on wide bandgap materials. Furthermore, the proposed test benches make possible an insulation ageing investigation incorporating both electrical and thermal stresses as they are significant ageing factors in inverter fed motors. Thus, voltage parameters of designed pulse generator based on SiC inverter, as well as temperature levels in the thermal chamber, can be varied. In this chapter the testing methodology and experimental benches are detailed and the following chapter gathers the obtained experimental data and analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Breakdown voltage tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endurance tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endurance tests under sinusoidal voltage . . . . . . . . . . . . . . . . . . 3.2 Endurance tests under PWM impulse voltage: low dV/dt . . . . . . . . . 3.2.1 Tests on unimpregnated specimens . . . . . . . . . . . . . . . . 3.2.2 Influence of defects on turn insulation endurance . . . . . . . 3.3 Endurance tests under rectangular impulse voltage: high dV/dt . . . . 4 Thermal imaging study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Firstly, the results

  of breakdown voltage tests carried out under sinusoidal voltage are given. Then, the results of endurance tests conducted on studied winding wires under sinusoidal voltage, PWM impulse voltage (µm-level of rise time) and rectangular impulse voltage (ns-level of rise time) are presented. The influence of voltage waveforms on turn insulation ageing and endurance, as well as the effect of thermal stresses are discussed.

  using progressive stress tests in which breakdown voltage values are measured, and constant stress tests in which times to breakdown are observed. Data obtained for a number of identical specimens for given test conditions may be represented by a statistical distribution [1]. Firstly, the breakdown voltage tests are conducted on all studied winding wires under AC sinusoidal voltage of 50 Hz nominal frequency at normal ambient temperature (approximately 25 °C) and atmospheric pressure. The tests are performed in "wire-shot" test configuration in accordance with the method described in Chapter III Section 3.3. The broken down specimens exhibit insulation punctures as shown in Figure IV.1. The experimental results constituting of measured breakdown voltages are given in Table IV.1. In tables and figures herein after, the following abbreviations are used to indicate the type of tested winding wires: corona resistant (CR) wire, and conventional (CV) wires.

Figure IV. 1 :

 1 Figure IV.1: Photograph of a broken specimen made of APX-1 (CV).

  as complete. In order to identify the statistical distribution which best fits the recorded breakdown voltage data, probability distribution plots are compared (see Figure IV.2). Adequacy tests of Weibull, Normal, lognormal and exponential distributions are performed with 95% confidence interval using the MATLAB Distribution Fitting Tool. Based on Probability Distribution Function (PDF) [2] plots, the Normal, lognormal and Weibull distributions seem appropriate for fitting the breakdown voltage data distribution. The hypothesis of the exponential distribution is rejected for all wires tested under sinusoidal voltage, even under the weaker conditions induced by the rounding and the estimation of the parameter of the exponential distribution.

Figure IV. 3 Figure IV. 3 :

 33 Figure IV.3 sums up experimental mean breakdown voltages and breakdown voltage values given in wires' specifications. For wires APX-1 and APX-2 the values of dielectric strength E spec (kV/mm) are given in the specifications. To compare with other winding wires the breakdown voltage V spec is calculated by IV.4 using averaged measured insulation thickness δ : δ r ad for the round section wire, δ a and δ b for the rectangular section wire [see Chapter III Section 3.1].

2 )Figure IV. 4 :

 24 figures below. In order to provide a clear visual insight, boxplots are used to present the experimental data. The time to breakdown is given in logarithmic scale.

Figure IV. 5 :

 5 Figure IV.5: Comparison among different probability distribution plots of time to breakdown data of PETD2-K-180 and PETD-180

Figure IV. 6 :

 6 Figures IV.6a and IV.6b show boxplots of times to breakdown for twisted pairs and coils tested under sinusoidal voltage at 5 kV RMS at two temperature levels: 25 and 180 °C. The corona resistant wire PETD2-K-180 is not displayed on this chart. Without heating (lifetime tests at 25 °C) the applied voltage had insufficient capability to break down the specimens made of corona resistant wire during a day. The specimens made of rectangular wire APX-2 also shown very high endurance at 25 °C, which decreased significantly when heated at 180 °C as can be seen from Figure IV.6b).

  have appeared on the enamel surface resulted from chemical reactions between the insulation and the aggressive ionized substances (ozone O 3 and nitrous oxide NO x gases) produced by the discharges. This effect is shown in Figure IV.7. The longer was the test duration, the more marked was the "frost-effect". In this case, the insulation deterioration may be caused by mainly two mechanisms: chemical reactions and direct physical attack by ion and elec-CHAPTER IV. ANALYSIS OF TURN INSULATION ENDURANCE EXPERIMENTAL RESULTS tron bombardment provoking C-H bond scissions in the dielectric material.

Figure IV. 7 :

 7 Figure IV.7: Photograph of the coil specimen after being tested at 5 kV RMS, T = 25 °C.

Figure IV. 8 :

 8 Figure IV.8: Time to breakdown of two specimens' types made of PETD-180 tested at 5 kV RMS, 180 °C.

  Figure IV.9: PWM impulse voltage waveform: (a) with surges V os = 900V, (b) surges are limited V os = 300V.

Figure IV. 10

 10 Figure IV.10 gives the lifetime boxplot of four wires tested under impulse voltage with repetitive surges at five levels of temperature (five test repetitions). The corona resistant wire demonstrated outstanding endurance at 25 °C; tests were continued during several days. Thereby, only three repetitions were effectuated at this temperature level on the corona resistant winding wire. According to the probability distribution functions analysis, the lognormal and Weibull distributions seem appropriate for fitting the time to breakdown data distribution. The two-way ANOVA conducted on logarithms of times to breakdown shows the significance of temperature effect. In these experimental conditions, two conventional winding wires PETD-180 and APX-2, demonstrate the insignificant difference in times to breakdown. The analysis of the Weibull distribution parameters show that increase in the heating of specimens accelerates the insulation deterioration. It should be noticed that the values of time to breakdown are lower and shape parameters are higher for wires tested under the PWM impulse voltage compared to the results of endurance tests under sinusoidal voltage. The estimated shape parameter for and the wire APX-1: β = 13.91 at 25 °C, and β = 12.00 at 180 °C.

Figure IV. 11

 11 Figure IV.11 shows the lifetime box charts of wires tested under impulse voltage having overshoot and voltage edge V os = 900 V, dV/dt = 0.40 kV/µs, compared to the same wires subjected to impulse voltage with lower parameters V os = 300 V, dV/dt = 0.18 kV/µs.In order to accelerate the endurance tests, two conventional winding wires, PETD-180 and APX-1, have been subjected to the electro-thermal stresses, the temperature in the thermal chamber was set to 180 °C.

Figure IV. 11 :

 11 Figure IV.11: Time to breakdown of twisted pairs under PWM impulse voltage V a = 1.2 kV, f m = 400 Hz, f c = 5 kHz with surges V os = 900 V and when surges are limited V os = 300 V: surge effect, T = 180 °C.

  Figure IV.12 reports the lifetime boxplot of two specimens' configurations tested under PWM impulse voltage with surges and simultaneous heating at 180 °C.

Figure IV. 12 :

 12 Figure IV.12: Time to breakdown of two specimens' types made of rectangular wire APX-2 under PWM impulse voltage with surges V os = 900 V, V a = 1.2 kV, f m = 400 Hz, f c = 5 kHz, T = 180 °C.

Figure IV. 13

 13 Figure IV.13 gives photographs and microscopic images (taken through an optical microscope) of corona resistant wire's and conventional rectangular section wire's specimens. The "frost-effect" was observed on the enamel insulation surface after being tested under impulse voltage.

Figure IV. 13 :

 13 Figure IV.13: Views of specimens made of corona resistant wire PETD2-K-180 (a) and conventional wire APX-2 (b) after being tested under PWM impulse voltage with surges V os = 900 V, V a = 1.2 kV, f m = 400 Hz, f c = 5 kHz, T = 180 °C.

  .14).

Figure IV. 14 :

 14 Figure IV.14: Possible defects in turn insulation system: 1 -copper conductors, 2 -enamel insulation, 3 -impregnating varnish, 4 -independent defects, 5 -dependent defect.

  mens with artificial defects were made of corona resistant wire PETD2-K-180. The enamel insulation was damaged by an independent ring defect on one of the turns (ring cutting of enamel insulation until reaching a conductor) as shown in Figure IV.15.

Figure IV. 15 :

 15 Figure IV.15: Twisted pair having a ring defect on the one turn.

Figure IV. 16

 16 Figure IV.16 shows the lifetime boxplot of defective specimens impregnated by different varnishes compared to non-defective specimens of corona resistant wire tested before. The time to breakdown data is also given in Appendix IV-C. The presence of defects in the enamel insulation, even on one turn (independent defect), leads to the endurance reduction (Figure IV.16 B and C).The mean time to breakdown ratio showing the influence of defects on insulation lifetime K def µs is equal to:

  Figure IV.17: Views of twisted pairs' insulation breakdown in defective area.

Figure IV. 18 :

 18 Figure IV.18: Model of turn insulation destruction mechanism in presence of partial discharges: 1 -copper conductors, 2 -enamel insulation, 3 -impregnating varnish, 4 -PDs in defective area .

Figure IV. 16

 16 Figure IV.16 as well as calculated K def µs and K healµs show that in most cases, the mean time to breakdown of non-defective specimens was higher than if the specimen had a defect despite the impregnation. The utilization of the organic-silicone varnish does not provide sufficient defects "healing"; the lifetime almost does not change compared to unimpregnated defective specimens (Figure IV.16 C). According to ANOVA results, the two groups of specimens B and C shown in Figure IV.16 have not significant difference in times to breakdown. Thus the winding impregnation may not provide sufficient "healing" of defects when insulation subjected to electro-thermal stresses, typical for inverter-fed motors. On the other hand, a significant lifetime raise is provided by the impregnation using compounds (Figure IV.16 D and E). Therefore, among the investigated impregnating compositions, the "healing" effect of defects within enamel insulation may be provided by the impregnating compound based on oligomers. The groups of specimens A and E shown in Figure IV.16 have not significant difference in times to breakdown. The solidified compound's film has lower shrinkage and greater thickness compared to varnishes that provide better resistance to electro-thermal stresses, increasing the insulation system lifetime. Taking into account that the endurance tests have been performed at T = 180 °C, it can be noted that this compound may provide a good impregnating quality with thermal ageing.

  tests are conducted at normal atmospheric pressure and humidity φ = [15 ÷ 30]% slightly varies during tests. Figure IV.19 recalls the voltage waveform applied to the specimens (measured on the twisted pair put into the thermal chamber). The use of cable between inverter output and specimen input creates surges up to 500 V.

Figure IV. 19 :•

 19 Figure IV.19: Rectangular impulse voltage waveform having high dV/dt.

  binations of electrical and thermal stresses are reported in Appendix IV-D and shown by box charts below.

Figure IV. 20

 20 Figure IV.20 shows the effect of simultaneous heating of specimens lifetime tested under impulse voltage with high dV/dt, DC bus voltage V dc = 1.2 kV and switching frequency f s = 20 kHz at three levels of temperature.

Figure IV. 20 :

 20 Figure IV.20: Time to breakdown of twisted pairs and coils under impulse voltage with high dV/dt, V dc = 1.2 kV, f s = 20 kHz: effect of temperature.

Figures IV. 21 ,

 21 Figures IV.21, IV.22, IV.23 and IV.24 report the lifetime of each studied winding wire tested at different combinations of electrical and thermal stresses. These combinations are obtained by varying the following parameters: three levels of DC bus voltage V dc , two levels of switching frequency f s , and two levels of temperature T. The time to breakdown ratios of studied wires showing effects of voltage K Vns , temperature K Tns and switching frequency K fsns , are calculated according to (IV.15, IV.16 and IV.17) and reported in Table IV.6.

  ) and reported in

Figure IV. 21 :

 21 Figure IV.21: Time to breakdown of twisted pairs made of PETD2-K-180 (CR) under impulse voltage with high dV/dt: (a) f s = 20 kHz, T = 25 °C; (b) f s = 20 kHz, T = 180 °C; (c) f s = 50 kHz, T = 25 °C.

Figure IV. 22 :

 22 Figure IV.22: Time to breakdown of twisted pairs made of PETD-180 (CV) under impulse voltage with high dV/dt: (a) f s = 20 kHz, T = 25 °C; (b) f s = 20 kHz, T = 180 °C; (c) f s = 50 kHz, T = 25 °C.

Figure IV. 23 :

 23 Figure IV.23: Time to breakdown of twisted pairs made of APX-1 (CV) under impulse voltage with high dV/dt: (a) f s = 20 kHz, T = 25 °C; (b) f s = 20 kHz, T = 180 °C; (c) f s = 50 kHz, T = 25 °C.

Figure IV. 24 :

 24 Figure IV.24: Time to breakdown of coils made of APX-2 (CV) under impulse voltage with high dV/dt: (a) f s = 20 kHz, T = 25 °C; (b) f s = 20 kHz, T = 180 °C; (c) f s = 50 kHz, T = 25 °C.

  of all the factors. More tests with greater test repetition number should be done in order to determine which of these factors has the most significant effect on ageing acceleration. This analysis is complicated by the electrical and thermal stresses interaction.Microscopic images of broken specimens in Figures IV.25, IV.26 and IV.27 show the impact of thermal and electrical stresses on enamel insulation degradation depending on test conditions. The "frost effect" can be observed on the enamel surface at lower voltage amplitude (V dc = 1.0 kV and V dc = 1.2 kV, f s = 50 kHz). However, when electrical stress reached critical values (V dc = 1.4 kV, f s = 50 kHz), the enamel insulation was severely damaged, effects of carbonization and sticking of turns is observed. This destruction process in most cases was very fast and lasted 30-50 s before the breakdown occurred. As noted below, the damage is more severe in the central part of the broken specimen(Figure IV.25)

Figure IV. 25 :Figure IV. 26 :

 2526 Figure IV.25: Winding wires' photograph and microscopic images after testing under high dV/dt impulse voltage, T =25 °C, f s = 50 kHz: (a) V dc = 1 kV; (b) V dc = 1.2 kV and (c) V dc = 1.4 kV.

Figure IV. 27 :

 27 Figure IV.27: Microscopic images of rectangular winding wire after testing under high dV/dt impulse voltage, T =180 °C, f s = 20 kHz, V dc = 1.2 kV.

  photographs have been obtained as shown in Figure IV.28 and Figure IV.29. The results of temperature measurements (five repetitions), the mean value and standard deviation of the hot spots on the surface of tested twisted pairs and coils are given in Appendix IV-E and mean values are shown in Figure IV.30.

Figure IV. 28 :

 28 Figure IV.28: Photographs (a) and thermal images (b) of twisted pair and coil tested under PWM-like low dV/dt impulse voltage at V a = 1.2 kV and f c = 5 kHz

  ducted on standard twisted pairs and proposed coil-type specimens under electro-thermal stresses till an enamel insulation breakdown. Most of the time, tests have been performed on unimpregnated wires' specimens in order to evaluate their endurance to partial discharges in motor winding. It is based on consideration that partial discharges can occur in voids and cracks of winding impregnation. Testing winding wires with different enamel composition allows to investigate enamel insulation failure mechanisms and to choose the optimal insulation type for motors fed by inverters.

  chanical and chemical erosion. At higher temperatures, thermal ageing becomes more and more important.The results of endurance tests conducted at PWM-like pulses with low voltage edge shown that breakdown mainly occurred at defective area forming weak point for partial discharges. The additional thermal stress reduces the lifetime. The tests performed under high dV/dt impulse voltage with high switching frequency show that damages (rapid irreversible deterioration, enamel melting, burning) tend to be more significant at the central part of the specimens indicating a thermal failure nature. At higher frequency the increasing of ultraviolet radiation was observed that indicates the rise of PDs currents and intensity. The rapid dielectric destruction is a result of ionization in gaps between wires' turns, photoionization, high ozone concentration and local temperature raise. Therefore, it can be assumed that electro-thermal ageing and the related deterioration of the dielectric's physico-chemical properties due to PDs will eventually lead to a thermal failure of dielectric, lowering the threshold breakdown voltage.

•2 2 . 1 3 3 . 1 4 1 Introduction

 21311 limiting voltage surges and dV/dt. Characterization and modelling of twisted pair . . . . . . . . . . . . . . . . . Evaluation of overvoltage at twisted pair resulting from high dV/dt and cable connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Barrier discharges in the twisted pair and in the turn insulation . . . . 2.3 Modelling of dielectric barriers in twisted pair . . . . . . . . . . . . . . . 2.4 Current-voltage characteristics of twisted pair . . . . . . . . . . . . . . . 2.4.1 Theoretical current-voltage characteristics . . . . . . . . . . . . 2.4.2 Empirical current-voltage characteristics . . . . . . . . . . . . . Study of turn insulation deterioration . . . . . . . . . . . . . . . . . . . . . . Experimental study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Finite element modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic experimental setup for turn insulation testing . . . . . . . . . . . 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reliability of electrical machine depends on reliability of electric insulation sys-tem. The winding turn insulation is the weakest element in this system. The literature review and the performed endurance tests put into evidence the detrimental effect of partial discharges on turn insulation of inverter-fed motors. In this regard, the phenomenological investigation of turn insulation deterioration when subjected to high dV/dt impulse voltages and high switching frequency is a relevant task. Thus, Chapter V is devoted to study of enamel insulation destruction mechanisms using numerical simulations and experimental failure analysis methods.Firstly, the twisted pair is modelled as a part of electric circuit with RLC-parameters.It allows to simulate and evaluate overvoltage caused by cable connections between the twisted pair and the SiC inverter generating high dV/dt impulse voltage.Secondly, the dielectric barriers composed by enamel film and air voids between turns of twisted pair are modelled by corresponded capacitances. A variable resistance is used in order to take into account the changing of electric circuit parameters due to PD inception. This model is used to analyse current-voltage characteristic of twisted pair and partial discharges comparing it with experimental observations. Experiments are conducted on twisted pairs under different impulse voltage amplitude and switching frequencies using previously described test benches. Specimens' temperatures, discharge currents and light emission spectra are measured. The discharge filamentary structure and periodical traces of erosion on twisted pair's turns have been observed by optical microscopy.

  CHAPTER V. DETERIORATION OF TURN INSULATION UNDER PARTIAL DISCHARGES AT HIGH DV/DT IMPULSE VOLTAGE 2 Characterization and modelling of twisted pair In order to investigate the deterioration of turn insulation under partial discharges we have used unimpregnated twisted pair specimens. The twisted pair represents a physical model of turn insulation for low voltage electrical machines having random wound

  Figure V.2: Scheme of experimental setup presented measurement points.

Figure V. 3 :

 3 Figure V.3: Test setup modelled by an equivalent circuit.

Figure V. 4 :

 4 Figure V.4: Coaxial strictures corresponding to enamelled wires and cables configurations: (a) single wire or cable; (b) twisted pair; (c) two cables .

1 .

 1 With regard to twisted pair construction, it can be approximatively represented by two parallel coaxial cylinders having geometrical parameters as shown in Figure V.4b. In this case, the mutual capacitance and inductance of twisted pair are given by (V.4) and (V.5). C = πε 0 ε r l ln d a (V.4) CHAPTER V. DETERIORATION OF TURN INSULATION UNDER PARTIAL DISCHARGES AT HIGH DV/DT IMPULSE VOLTAGE a = R out and d = 2R out . Two cables connecting the inverter output and twisted pair are placed at the distance x from each other as shown in Figure V.4c. Thus, the capacitance and the inductance of two parallel cables are given by (V.6) and (V.7) respectively.

  10) is equal to 1.0033, 1.0036 and 1.0038 respectively. Figure V.5 demonstrates the variation of CHAPTER V. DETERIORATION OF TURN INSULATION UNDER PARTIAL DISCHARGES AT HIGH DV/DT IMPULSE VOLTAGE

Figure V. 5 :

 5 Figure V.5: Variation of effective relative permittivity with distance between two cables.

  Simulated and measured voltages are superimposed in Figure V.6. The simulated voltage source generates bipolar voltage V = ± 600 V and rise and fall times are set equal to 27 ns.The electrical circuit parameters such as an inverter output impedance (parasitic resistance and capacitance contributing to the inverter ringing), a proximity effect and additional losses were not defined here. Therefore, the damping resistor R d =12 Ω is adjusted to limit oscillations in the LC-circuit and match the damping time to the experimental one, that allows to obtain the good fitting of simulation results to measurements.

Figure V. 6 :

 6 Figure V.6: Inverter's voltage waveforms measured at contacts in the thermal chamber (twisted pair connected) and simulated by equivalent RLC-circuit using LTSpice.

  14) where C in is the inclusion capacitance, C d is the capacitance of a part of solid dielectric containing the inclusion (C d is connected in series with the C in ), and C a is the capacitance of another part of solid dielectric without inclusions. Partial discharge will occur when the voltage applied at the inclusion reaches the breakdown voltage of this inclusion, which is defined as the partial discharge inception voltage.

Figure V. 9 :

 9 Figure V.9: Equivalent circuit when studying partial discharges in solid dielectric [20].

Figure V. 10 :

 10 Figure V.10: Twisted pair structure (a) and corresponded equivalent circuit (b) when studying partial discharges in twisted pair.

. 2 )

 2 . To determine the capacitance of dielectric barriers C b , wires forming the twisted pair are metallized -covered by conducting paste (liquid metal composition "indium -gallium"). The metalized twisted pair is shown Figure V.11. It should be noted, that only the "active" (twisted) length l = 0.125 m of specimen is metallized. In this case, the capacitor C air is short-circuited and the effect of air gaps between turns can be neglected, while the dielectric barriers capacitances C b1 and C b2 are connected in series. The measured capacitance of dielectric barriers C b = 96 pF. Thus, the capacitance of air voids can be derived from (V.16): C air =16.39 pF. CHAPTER V. DETERIORATION OF TURN INSULATION UNDER PARTIAL DISCHARGES AT HIGH DV/DT IMPULSE VOLTAGE

  the absence of barrier discharges the current flows through the capacitance of air gaps and dielectric barriers as shown in Figure V.13a. When the electric field is strong enough to start the air ionization, the active resistance is connected in parallel to the air gaps capacitance. Corresponding to the scheme given in Figure V.13b, the current through dielectric barriers I b is equal to the current in the air gaps I air , and is equal to the vector sum of active I a (discharge active current) and reactive I r (capacitive current in the air gaps) current components as written in (V.20).

Figure V. 13 :

 13 Figure V.13: Equivalent circuits of twisted pair indicating the currents: (a) without barrier discharges and (b) with barrier discharges.

Figure V.14bFigure V. 14 :

 14 Figure V.14b and the obtained characteristic corresponds to the line AC in Figure V.15. Table V.4: Theoretical points of twisted pair's current-voltage characteristic.

Figure V. 15 : 2 . 4 . 2

 15242 Figure V.15: Current-voltage characteristic of twisted pair: AB -current-voltage characteristic of dielectric barriers, AC -current-voltage characteristic of twisted pair without discharges, DE -current-voltage characteristic of twisted pair with discharges, AF -current-voltage characteristic of air gaps without discharges and MN -current-voltage characteristic of discharges in air gaps

Figure V. 16 :Figure V. 17 :

 1617 Figure V.16: Sketch of the test setup for current measurements.

  Figure V.18 and Figure V.19 show the installation and connections of measurement means. Appendix V-B provides more details about the construction of the current shunt and RC-divider.

Figure V. 18 :

 18 Figure V.18: Photograph of used measurement means.

Figure V. 25 :

 25 Figure V.25: Current peak values at different voltage levels and switching frequencies.

Figure V. 26

 26 Figure V.26 gives the temperature values measured on twisted pair depending on voltage amplitudes and switching frequencies. These measurements are performed with cooling fan in order to limit it below a critical value that increase the time to breakdown.

Figure V. 26 :

 26 Figure V.26: Temperature measurement results at different impulse voltage amplitudes and switching frequencies.

  the light of the information provided below, this experimental study includes the analysis of light emission spectra of barrier discharges in the tested twisted pair. As in the previous tests, light spectra are measured on different voltage levels and switching frequencies. In obtained spectra the same peaks (on the same corresponding wavelengths), but of different intensities were observed. Figure V.27 reports the spectrum obtained at V dc = 1400 V and f = 50 kHz. Figure V.28 summarizes the values of peak corresponded to 336.51 nm of wavelength depending on voltage amplitudes and switching frequencies. Asis the case with tested specimen's temperatures, the higher frequency leads to the higher emission intensity.

Figure V. 27 :

 27 Figure V.27: Emission spectrum of barrier discharges in twisted pair at V dc = 1400 V and f = 50 kHz.

  Figure V.29 and Figure V.30 demonstrate DBDs between two enamelled wires in the tested twisted pair under impulse voltage with recorded voltage of DC bus equal to 1200 V and 1400 V and switching frequency f = 50 kHz. A large number of microdischarges (filaments) distributed on the dielectric surface is observed in the air gap between enamelled wires. The filaments build up surface charges which create electric field opposite to that of the applied voltage leading to the discharge extinction till the following voltage rate. The obtained images reveal the presence of repetitive filamentary structure of barrier discharges, the rise of light intensity and the propagation of discharges from the central axe between wires' turns to the wires' surface with the increasing of the applied voltage.

Figure V. 29 :

 29 Figure V.29: Microscopic images of twisted pair turns tested at V dc = 1200 V, f = 50 kHz showing filamentary structure of DBDs (twisted pair is exposed to light at left figure).

Figure V. 31

 31 Figure V.31 demonstrates the wires surface after testing at 1200 V peak amplitude of impulse voltage with switching frequency f = 50 kHz during 4 minutes (with cooling). The enamel erosion pattern is deep and clear (up to tenths of a millimetre), craters corresponds to tracks of observed DBDs filaments. It put into evidence a memory effect of filament and shows their role in the insulation destruction which seems to be due to the chemical reactions with the ions and radicals created by discharges. The residual electron concentration in the filament region is high enough for the next air gap breakdown followed by filament production occurring again in this region.

Figure V. 31 :

 31 Figure V.31: Microscopic images of insulation surface with filaments track erosion.

2DFigure V. 33 :Figure V. 34 :

 3334 Figure V.33: Geometry of twisted pair turns in electrostatic calculations: (a) two wires touched one another, (b) microgap between wires.

Figure V. 35 :

 35 Figure V.35: Electrical intensity distribution in air along the line 1-2 when d gap = 0.

Figure V. 39 :

 39 Figure V.39: Partial discharges on twisted pair tested under low pressure.

  camera) is only due to its dielectric losses: conductivity and relaxation losses, while the ionization losses are omitted. The temperature measurements of specimen tested under 1300 V and 50 kHz at 38 mm Hg shows very slight heating without PD. It indirectly indicates that the observed high temperature rise of specimen in previous tests is mainly caused by partial discharges. At the same time, the temperature increase caused by PD leads to higher dielectric material conductivity and dielectric losses. The conducted experimental study of PDs in twisted pairs under low pressure also shows that insulation defects can be put into evidence in certain conditions. Figure V.40 demonstrate the visualization of the point defects by partial discharges as a bright glow at the defective areas.

Figure V. 40 :

 40 Figure V.40: Visualization of insulation defects in twisted pair tested under low pressure.

  and their insulation system. It synthesizes the factors affecting electrical machines winding insulation during their fabrication (compatibility of winding wires and impregnating composition) and operation (thermal, electrical, ambient and mechanical stresses). Particular attention should be paid to electrical stresses due to impulse supply voltage in inverter-fed motors. As can be seen from the literature, the electrical machines driven by power converters with pulse width modulation are subjected to repetitive surges due to fast changing voltage pulses and reflection phenomena in supply cable, as well as non-uniform voltage distribution in the stator winding. The overvoltage at motor terminals leads to partial discharge inception accelerating winding insulation degradation and causing its premature failure, that is especially critical for Type I insulation used in low voltage machines (up to 700 V RMS rated voltage).At the same time, power electronics technology does not stand still. The development of Silicon Carbide technology for power electronics offers better performances than demonstrated by Silicon-based power semiconductor devices. SiC-based power converters show high power density, fast and high frequency switching. However, the use of SiCbased power electronics in the electric drive increases electrical stresses in controlled electrical machine. Machine's windings may experience higher overvoltage caused by very short impulse rising and falling times (ns-level), electrical insulation is subjected to steep voltage changing rates, that may increase a risk of PD and accelerate insulation ageing and destruction. In this regard, constraints related to high dV/dt impulse voltage waveform generated by SiC-based inverters and their effects on winding insulation should be thoroughly investigated.Following up upon the literature survey, Chapter II deals with the study of transient overvoltage in motor fed by inverter that allows to estimate the electrical stresses causing the motor winding insulation degradation. For this purpose, a high frequency model from the literature has been used for electric drive simulations. High frequency models found in literature are typically intended to examine and predict transient phenomena in induction motors being the most widely used electrical machines. At the same time, other machines' types, which are used in electric drives, also need to be considered. Therefore, our study focuses on the modelling of the switched reluctance machine which is particularly interesting due to its redundant and fault tolerant construction, particular control using asymmetric bridge converter and concentrated winding construction by contrast with induction motors. Moreover, an absence in literature of SRM models for analysis of high frequency behaviour makes it a crucial task.Basing on the existing models developed for three-phase induction motors, the studied switched reluctance motor (3-phase, 6-slot, and 4-pole, V dc = 300 V) has been modelled by equivalent circuits with lumped parameters. Model parameters have been derived from impedance measurements in frequency range from 30 kHz to 30 MHz. Performing a model validation in frequency-domain, we have observed a good correlation between measured and simulated results. We have studied the transient overvoltage in motor winding combining the suggested motor equivalent circuit and a multi-cell model of feeding cable proposed in the literature. Owing to the fact, that the phases of SRM may be energized independently, single-phase and single commutation simulations have been performed approximating the inverter voltage by an ideal pulse generator with specified rise/fall times and dV/dt. Simulation results confirm that the steeper voltage rates leads to the greater overvoltage. It should be noted, that in case of extremely short rise time up to 10 ns, even one meter of power cable represents a critical length. Furthermore, the increasing in cable length intensifies the transient overvoltage, affects the oscillation frequency and makes the transient oscillations longer. However, the overvoltage value do not reach two times of pulse generator voltage that may be related to cable losses presented in the model. The performed study correlates with the previous works and highlights the importance of high dV/dt effects characterisation.In order to investigate the effects of voltage waveform parameters, such as dV/dt and switching frequency, on winding insulation, different testing methods are proposed in Chapter III. Considering the fact, that the turn insulation is the weakest element of electrical machine winding, endurance tests have been carried out on standard twisted pair specimens made by conventional and corona resistant enamelled wires for random wound winding type. As for the concentrated winding made with rectangular wire, a novel specimen's shape representing a coil is proposed for testing enamelled rectangular section winding wires. First of all, the breakdown voltage and endurance testing under sinusoidal voltage of 50 Hz nominal frequency is described. Then, the endurance tests under PWM-like impulse voltage having considerably low dV/dt up to 0.4 kV/µs with rise time of 4 µs is presented. This experimental bench previously developed and installed at TPU (Russia) contains a pulse generator allowing an adequate reproducing of stresses typical for inverter-fed motors with electronic devices based on Silicon technology. Associating this pulse generator with a thermal chamber makes it possible to combine electrical and thermal stresses for endurance tests. With the purpose to apply the electrical stresses on turn insulation engendering by Silicone Carbide high-speed switches, we have developed an experimental bench integrating a SiC-based pulse generator at SATIE laboratory (France). Firstly, a half-bridge inverter have been designed with modern SiC MOSFET transistors having ns-level rise time with maximum voltage amplitude up to 1200 V and switching frequency up to 64 kHz. It already allowed us to apply very steep voltage rates and served us as a first prototype for the high dV/dt test bench. Consequently, a full bridge topology SiC-based pulse generator has been designed at SATIE laboratory, and the high dV/dt test bench containing the thermal chamber and measuring tools has been assembled in TPU. The pulse generator provides high dV/dt impulse voltage (up to 60 kV/µs applied to tested specimen), voltage amplitude up to 1700 V and maximum switching frequency up to 125 kHz. Adjusting the voltage waveform parameters, we can study their effect on enamel insulation degradation. Considering the fact that the overvoltage in inverter-fed motor may exceed the value of partial discharge inception voltage, all the insulation endurance tests have been conducted in presence of partial discharges representing the main ageing factor. The PDs activity has been detected using optical, electromagnetic and chemical methods. The weakest partial discharges have been observed when applying sinusoidal voltage and the high dV/dt impulse voltage given rise to intensive, bright PDs. The experimental protocols followed by the obtained results and their analysis are provided in Chapter IV. Tests have been conducted on unimpregnated specimens made of winding wires different in enamel compositions in order to evaluate and compare their resistance to PDs applying various voltage waveforms: sinusoidal, impulse with low dV/dt,

Firstly, the overvoltage

  caused by cable connections between the laboratory-designed SiC-based pulse generator and the tested twisted pair has been modelled and analysed basing on electromagnetic approach and low-loss transmission line approximation. The obtained results highlight the importance to minimize the cable line length to limit the voltage oscillations and surges that correlates with the observations described in Chapter II. Secondly, considering partial discharges arising in air gaps between twisted pair turns as barrier discharges, the dielectric barriers in twisted pairs have been modelled. It allowed us to analyse numerically and experimentally obtained current-voltage characteristics, linking the specimen geometry and relative permittivity with the inception voltage, and separating barrier discharges current and capacitive current, especially significant at high dV/dt. However, the current and voltage measurements when applying the impulse voltage with very short rising time is a challenge to overcome because it requires an accuracy which may be difficult to achieve with the available and laboratory-designed probes.The performed current, temperature and emission spectra measurements show high energy of barrier discharges increasing with the switching frequency rise and the electric field strength (also demonstrated by performed electrostatic calculations using finite element modelling). The obtained microscopic images of barrier discharges and enamel insulation surface erosion put into evidence the filamentary structure of discharges relating to the insulation destruction with chemical etching by repetitive filaments. Thus, the destruction mechanism of turn insulation subjected to high dV/dt impulse voltage corresponds to ionization breakdown. The discharge between the dielectric barriers produces heat and UV radiation, generates ozone and NOx, leads to accelerated charged electrons and ions bombarding and additional heating of the dielectric surface (dielectric barriers). Depending on energy distribution between different processes the electrochemical breakdown or the electro-thermal breakdown will occur. To go further in the experimental study, we propose a synthetic test bench containing two voltage sources (the sinusoidal voltage source providing the specimen heating and the pulse generator to apply the impulse voltage) and providing a possibility to control the pressure and the media (gas, liquid) of tested specimens. The tests performed on twisted pairs under impulse voltage and low pressure allowed us to visualize the weakest insulation spots by partial discharges that could be used for insulation defectiveness estimation.

Table 4 :

 4 Time to breakdown of twisted pairs and coils under sinusoidal voltage. VII The shunt is connected to the oscilloscope via a RF coaxial cable 50 Ω with impedance matching resistors R 1 = 50 -R sh = 49 Ω and R 2 = 50 Ω. It is necessary to match the coaxial cable and the shunt impedances in order to damp oscillations and reflected waves due to steep edge pulses. The matching resistor R 1 connected in series with the central core of coaxial cable and the resistor R 2 at the end of the cable connected to the cable shield, form an active voltage divider for the signal of the shunt. Since the resistors are almost identical, the divisor coefficient is 2:1. Before starting partial discharges measurements, the shunt characteristics were tested. Firstly, the shunt resistance was measured using the DC Milli-Ohm Meter Instek GOM -802. The resistance measured on shunt terminals is equal to 1.03 Ω. Then, this shunt was compared with a reference resistor (low inductive resistor R ref = 0.86 Ω) under pulse voltage. The measured amplitude and edges were compared. The standard pulse generator produced impulse peak-to-peak voltage of 10 V, 50 kHz and 10 µs pulse duration. The scheme of the calibration test bench and scopes are provided in and respectively.

Figure 5 :

 5 Figure 5: Scheme of shunt calibration test bench.

Figure 6 :

 6 Figure 6: Calibration test scopes CH1 -current shunt, CH3 reference resistor: (a) measurement without matching resistors R 1 and R 2 , (b) and (c) measurement with matching resistors R 1 and R 2 at different time-base.

C 1 =

 1 1 pF is a vacuum capacitor in upper arm of the divider, R 1 = 4 Ω is a damping lowinductive resistor, C 2 = 56 pF is a ceramic capacitor, C 3 = 5 ÷ 25 pF is an adjustable capacitor, R 2 = 0 ÷ 100 Ω is an adjustable matching resistor, R 2 = ρ -R 1 , where ρ is equal to the coaxial cable impedance, U 1 is an input signal, and U 2 is an output signal of the RCdivider. The terminal of upper divider's arm is connected directly to the tested specimen, and the terminal of the lower arm is connected to the oscilloscope via the coaxial cable.

Figure 7 :Figure 8 :

 78 Figure 7: RC-divider scheme.

Figure 9 :

 9 Figure 9: Scopes of calibration and dividers ratios estimation: (a) and (b) correspond to the RC-divider #1, and (c) corresponds to the RC-divider #2.

. . 74 Simulation of transient overvoltage in inverter-fed motor drive . . . . . . . 76 Conclusion

	Chapter II
	High-frequency modelling of inverter-fed
	motor drive

The main goal is to study voltage transients and surges at motor terminals caused by high dV/dt and impedance mismatch between the cable and the motor fed by the SiC-based inverter. age Motors Driven by Voltage Inverters. IEEE Transactions on Dielectrics and Electrical Insulation, 14(6):1516-1530, dec 2007. [96] Thibaut Billard, Cedric Abadie, and Bouazza Taghia. Non-Intrusive Partial Discharges Investigations on Aeronautic Motors. In SAE 2016 Aerospace Systems and Technology Conference, volume 1. SAE International, sep 2016. [97] Veronika Bolgova, Andrey Leonov, and Dmitry Charkov. Influence of VFD parameters on voltage stresses in low voltage windings. In 2016 57th International Scientific Conference on Power and Electrical Engineering of Riga Technical University (RTU-CON), pages 1-5. IEEE, oct 2016. [98] A. Cavallini, G. C. Montanari, D. Fabiani, and M. Tozzi. The influence of PWM voltage waveforms on induction motor insulation systems: Perspectives for the end user. In 8th IEEE Symposium on Diagnostics for Electrical Machines, Power Electronics & Drives, pages 288-293. IEEE, sep 2011. [99] J.D. Kueck, H.D. Haynes, and R.H. Staunton. Stator insulation degradation test uses ASD switching frequency. IEEE Power Engineering Review, 22(1):7-11, 2002. [100] Veronika Bolgova, Stephane Lefebvre, Sami Hlioui, Nidhal Boucenna, Francois Costa, and Andrey Leonov. Development of testing methods for winding turn-toturn insulation of low voltage motors fed by PWM converters. In 2017 19th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe), pages P.1-P.10, Warsaw, sep 2017. IEEE. [101] Nadine Lahoud, Manh Quan Nguyen, Pascal Maussion, David Malec, and Dominique Mary. Lifetime model of the inverter-fed motors secondary insulation by using a design of experiments. IEEE Transactions on Dielectrics and Electrical Insulation, 22(6):3170-3176, dec 2015. [102] Daniel Roger, Sonia Ait-Amar, Ewa Napieralska, and Piotr Napieralski. A Proposition for Improving the Design of Motor Windings for low-Pressure Environment. In 2018 IEEE Transportation Electrification Conference and Expo (ITEC), pages 424-429. IEEE, jun 2018. [103] F. Koliatene, Th Lebey, J.P. Cambronne, and S. Dinculescu. Impact of the aeronautic environment on the Partial Discharges Ignition: A basic study. In Conference Record of the 2008 IEEE International Symposium on Electrical Insulation, pages 603-606. IEEE, jun 2008. [104] Peng Wang, Hongying Xu, Jian Wang, Andrea Cavallini, and Gian Carlo Montanari. Temperature effects on PD statistics and endurance of inverter-fed motor insula-Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 High frequency electrical machine modelling . . . . . . . . . . . . . . . . . . 58 2.1 High frequency motor models: literature review . . . . . . . . . . . . . . 58 2.2 Switched reluctance motor modelling . . . . . . . . . . . . . . . . . . . . 64 2.2.1 Proposed motor models . . . . . . . . . . . . . . . . . . . . . . . 64 2.2.2 Motor impedance measurements . . . . . . . . . . . . . . . . . 65 2.2.3 Model parameters identification . . . . . . . . . . . . . . . . . . 68 2.2.4 Comparison of measured and simulated motor impedances . 70 High frequency cable modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Table II .

 II 2: Influence of model parameters on the CM and DM impedance magnitude curves.

	Model parameter	Effect
		CM: modify the low-frequency slope (having point CM 2 ), resonance
	C g1 at point CM L d DM: modify the low-frequency slope (having point DM 1 ) and the CM: modify the shape near to point CM 3
		resonance at point DM 2
	L zu	CM: modify the last resonance amplitude and frequency DM: modify the last resonance amplitude and frequency
	C t , R t , L t	CM: modify the resonance at point CM 3 DM: modify the resonances at points DM 2 and DM 4

3 , high-frequency slope (having point CM 1 ), and shift the frequency of the last resonance DM: shift the resonance frequency at point DM 2 , high-frequency slope, and shift the frequency of the last resonance C g2 CM: modify the low-frequency slope (having point CM 2 ) and the shape near to point CM 3 DM: no effect R g1 CM: modify the last resonance amplitude DM: modify the last resonance amplitude R g2 CM: modify the shape near to point CM 3 DM: no effect R cu CM: no effect DM: slightly modify the low-frequency slope (having point DM 1 ) R e CM: modify the shape near to point CM 3 DM: modify the amplitude of resonance at point DM 2

  PETD-180 (conventional wire) are chosen. Furthermore, two wires, named herein APX-1 and APX-2, issued by the cable supplier APX (France) are tested. The studied wires have similar applications: electrical machines, apparatus and appliances production. The wire APX-2 has a rectangular section; other wires have round section. Rectangular enamelled wires can be used in motors with concentrated or coil stator windings (e.g., switched re-The insulation thickness of tested wires is determined experimentally using a micrometer MK-25 0.01 SHAN 123738 having accuracy class 1. The obtained results are given in TableIII.1 and Table III.2. Table III.1: The enamel insulation thickness measurement results for round section wires.

			PETD2-K-180	PETD-180	APX-1
			D, mm d, mm D, mm d, mm D, mm d, mm
		#1	1.21	1.13	1.13	1.06	1.08	1.01
		#2	1.19	1.11	1.14	1.06	1.09	1.03
		#3	1.19	1.12	1.13	1.08	1.08	1.04
	Specimens	#4 #5 #6 #7	1.19 1.19 1.20 1.19	1.12 1.11 1.12 1.13	1.13 1.13 1.13 1.13	1.06 1.07 1.06 1.11	1.08 1.08 1.09 1.09	1.02 1.02 1.02 1.03
		#8	1.19	1.12	1.13	1.09	1.08	1.02
		#9	1.21	1.11	1.13	1.06	1.08	1.03
		#10	1.21	1.11	1.13	1.06	1.08	1.02
	D and d , mm 1.1970 1.1180 1.1310 1.0710 1.0830 1.0240
		σ, mm	0.0095 0.0079 0.0032 0.0173 0.0048 0.0084
	δ r ad , mm	0.0395	0.0300	0.0295

luctance machines), and round section wires are usually used for distributed windings (e.g., induction machines). For more details on studied wires, please refer to Appendix III-A.

In Table

III

.1, D is the insulated wire's diameter, d is the conductor's diameter, D

  TableIII.2: The enamel insulation thickness measurement results for rectangular section wire.

				APX-2
			A, mm a, mm B, mm b, mm
		#1	2.96	2.82	1.15	1.05
		#2	2.94	2.83	1.16	1.06
		#3	2.95	2.82	1.15	1.06
	Specimens	#4 #5 #6 #7	2.94 2.95 2.96 2.95	2.82 2.80 2.80 2.81	1.16 1.15 1.15 1.15	1.05 1.03 1.03 1.06
		#8	2.96	2.82	1.16	1.03
		#9	2.96	2.82	1.16	1.03
		#10	2.96	2.82	1.15	1.05
		Mean, mm	2.9530 2.8160 1.1540 1.0450
	Standard deviation, mm 0.0082 0.0097 0.0052 0.0135
		δ a , mm	0.0685	-
		δ b , mm	-		0.0545

In Table

III

.2, A is the insulated wire's width, a is the conductor's width, B is the insulated wire's thickness, b is the conductor's thickness, δ a and δ b are the insulation thickness on wide and narrow wire's sides respectively (see Figure III.5b). The insulation thickness is find by analogy with (III.1).

Table III .

 III 3: Main components of high dV/dt experimental bench with pulse generator having H-bridge topology.

	Component	Characteristics
		SiC inverter
	Switches, S 1 -S 4	MOSFET SiC Wolfspeed transistors with built-in SiC body diode, ref. C2M1000170D, V DS = 1700 V, R DS(on) = 1 Ω at 25	°C
	Decoupling	TDK DC Link Film Capacitors, ref. B32776G, 8 µF, and
	capacitance, C 1 and C 2	Multilayer Ceramic Capacitors MLCC -SMD, 100 nF
	Gate resistance, R G	3.5 Ω
	Drivers	Dual 1700 V SIC MOSFET Driver, ref. PT62SCMD17
		Measuring tools
	Oscilloscope	Keysight Technologies DSOX2014A 100 MHz, Tektronix MDO3024 200 MHz
	Kilovoltmeter	AC/DC, 0-120 kV, precision 0.25 %
	Divider	RC voltage divider, 3000:1
	Antenna	150 Ω coaxial radio frequency cable PK 150-7-11
	Differential probe	Tektronix THDP0100, 100 MHz, up to 6000 V differential

  .1: Breakdown voltage under progressive voltage stress on winding wires.

		Breakdown voltage V BD , kV
	Test (repetition)	PETD2-K-180 PETD-180 APX-1 APX-2 1
		(CR)	(CV)	(CV)	(CV)
	#1	7.0	6.0	5.5	9.0
	#2	7.0	6.0	5.8	10.5
	#3	7.5	6.0	6.1	11.0
	#4	8.0	6.0	6.5	12.0
	#5	8.5	6.0	6.7	12.0
	#6	8.5	6.7	6.7	-
	#7	8.5	7.0	6.8	-
	#8	9.0	7.5	6.9	-
	#9	9.0	8.3	7.1	-
	#10	10.0	8.5	7.3	-

All the tested specimens are broken down in this test, so the data is referred to

  .2: Normal distribution parameters of breakdown voltage data.

	Parameter	PETD2-K-180 PETD-180 APX-1 APX-2
		(CR)	(CV)	(CV)	(CV)
	Mean μ, kV	8.30	6.80	6.54	10.90
	Standard deviation σ, kV	0.949	0.993	0.574	1.245

  Table IV.6. Where t 1.0ns and t 1.4ns are mean times to breakdown corresponding to DC bus voltage of 1.0 and 1.4 kV; t 20ns and t 50ns are mean times to breakdown corresponding to switching frequencies of 20 and 50 kHz respectively. The test conditions are detailed in Table IV.6.

	K Vns =	t 1.0ns t 1.4ns	(IV.16)
	K fsns =	t 20ns t 50ns	(IV.17)

Table V . 1 :

 V1 Electrical and geometrical parameters of cable connections and tested winding wire.

	Type	Insulation	ε r	D out , mm D in , mm	l, m
	unshielded cable MGTF	tetrafluoroethylene (TFE)	2.1	1.090	0.730	1.000
	conventional	polyesterimide (PEI)
	enamelled wire	and polyamide-imide	2.9 -4.3	0.885	0.800	0.125
	PETD-180	(PAI)

  Table V.2. Thus, the propagation delay t d of 1 m of cable connections is equal to 3.34 ns, t d of 1 m of twisted pair is equal to 5.78 ns and t d of 0.125 m of twisted pair is equal to 0.72 ns. Rise time of the impulse voltage produced by SiC pulse generator is about t r = 25 ∼ 30 ns, so t d < t r and the proposed equivalent circuit model given in Figure V.3 can be used for high dV/dt pulses. AT HIGH DV/DT IMPULSE VOLTAGE The high frequency of voltage and current oscillations caused by steep impulse

	Table V.2: Calculated electrical parameters for electrical circuit simulations
	(twisted pair length l = 0.125 m and cable length l = 1 m).
	Type	C l , pF/m	L l , nH/m	C, pF	L, nH
	Twisted pair	120.388	277.259	15.049	34.657
	Cable connections	6.130	1816.223	6.130	1816.223
	Additionally, using the low-loss approximation the propagation delay t d is esti-
	mated for the twisted pair and cable connections according to (V.11).
			t d = L l C l		(V.11)

Table V .

 V 3: Theoretical points of dielectric barriers' current-voltage characteristic.

	Voltage, V	0	600	900	1400
	Current, A	0	2.304	3.456	5.376
	The current-voltage characteristic of twisted pair without discharges is described
	by I

tp = C tp • dV dt; points are provided in Table V.4. The equivalent circuit is shown in

Table 14 :

 14 Parameters of RC-divider #1.

	U g , V(CH1)	18.6	18.6	18.6
	U 2 , mV(CH2)	60.8	65.6	63.2
	k 1	k 1max = 306	k 1min = 283	k 1op = 294

Table 15 :

 15 Parameters of RC-divider #2.

	U g , V(CH1)	18.6	18.6	18.6
	U 2 , mV(CH2)	68.8	71.2	70.4
	k 1	k 1max = 270	k 1min = 261	k 1op = 264
				XXI

According to IEC classification, low-voltage refers to motors that operate at voltages less than 1000 V

(a) (b)

(a) (b)

(a) (b) (c)

The tested specimens' number is reduced to five for rectangular section wire APX-2 by the reason of difficulty to avoid flashovers between wires ends and the grounded metal shot.

Acknowledgements

I would like to express my gratitude to my committee chair, Professor Emmanuel Odic, who presided the thesis defence conducted via video conference and should adapt to these unusual circumstances.

The obtained equivalent circuit allows to reproduce the test setup making it possible to conduct an analysis of voltage transients depending on model parameters. Therefore, the influence of cable length and impulse voltage rise time on voltage oscillations and surges can be investigated. Firstly, the cable length is kept equal to 1 m and the pulse rise time t R is taken as variable. Voltage waveforms obtained at three t R levels are given in dV/dt pulse generator and its load leads to increasing of maximum voltage at the load.

Therefore, the higher dV/dt will give rise to higher overvoltage and stronger electric field, which will increase the probability of partial discharge ignition in inverter-fed motor. Voltage divider ratio k is defined by (V.17) [33] and equal to 1.171.

Current of twisted pair

Then, the current is measured on twisted pair (without metallization) also tested at high dV/dt impulse voltage with switching frequency f = 50 kHz. The conducted study shows that the method based on the barrier discharge theory may be applied to the twisted pair in further investigations.

Study of turn insulation deterioration

The previous sections show that enamel destruction mechanism can be investigated basing on the barrier discharges approach. In this regard, the further study of the destruction mechanism of enamel insulation when subjected to fast and high switching frequency voltage pulses is proposed. The twisted pair specimens have been tested un- Specimens' temperatures, currents and light emission spectra are measured. In addition, microscopic images and electrostatic simulations were made with the purpose to link the discharges arising between enamelled wires and erosion on their surface.

Experimental study

Experimental The characteristics of enameled wires given in Table 1 are issued from data sheets available in products catalogues of cables manufacturers and suppliers.

Table 1: Tested wires characteristics.

IV

APPENDICES

Appendix III-B. RC-divider characteristics

The RC-divider used for voltage measuring in the high dV/dt test bench is commercially available probe designed to measure and control parameters in high voltage electrophysical and technological installations. Its technical characteristics are given in Table 2; the photographs are shown in Figure 1.

Table 2: Tested wires characteristics. T 0 is the ambient temperature.

XVI Appendix V-A. LCR meter E7-13

LCR meter E7-13 is intended to measure capacitance, inductance and resistance of the alternating current, as well as the parameters of the complex impedance of capacitive and inductive nature in a wide range of measurement of phase angle. Technical characteristics:

• Capacity -from 0.1 pF to 10 µF;

• Inductance -from 1 µH to 100 H;

• Conductance -from 10 nS to 100 mS;

• Active resistance -from 0.01 Ω to 1 MΩ;

• Test frequency -1 kHz.

XVII

APPENDICES

Appendix V-B. Current shunt and RC-divider

Current shunt

The current shunt having equivalent resistance R sh = 1.03 Ω is made with low inductive ceramic encased resistors placed on round copper plate as shown in Figure 3 and Abstract : The insulation of electrical machines driven by power converters with pulse width modulation are subjected to repetitive surges due to fast changing voltage pulses and reflection phenomena in supply cable, as well as non-uniform voltage distribution in the stator winding. The overvoltage at motor terminals may lead to partial discharge inception accelerating winding insulation degradation and causing its premature failure. The implementation of wide bandgap power semiconductors as Silicon Carbide allows to create converters with high power density for variable frequency drive applications. The fast and high frequency switching of electronic devices based on wide bandgap semiconductors increase electrical stresses caused by steep voltage changing rates in controlled electrical machine. It may increase a risk of partial discharges and accelerate insulation aging and destruction. The experimental investigation and numerical simulation study performed in this work is essentially focused on the effects of impulse voltage with high dV/dt and high switching frequency on winding insulation robustness of inverter-fed electrical machines. Therefore, the work covers a number of issues related to electric drives, power electronics, electrical machines, dielectric materials and partial discharges.

XVIII

List of Figures