K Shinoda

A Benchaib

X Guillaud

S Bacha

B Francois

A preliminary and reduced paper has been presented at the CISTEM

Keywords: HVDC grid, Voltage source converter (VSC), MTDC Grid, DC Cable, Primary Control, Voltage droop control, Wind power generation

Purpose

Self-commuted Voltage Source Converter (VSC) can significantly extend the flexibility and operability of HVDC system and be used to implement the concept of Multi-Terminal HVDC (MTDC) grid. In order to take full advantage of MTDC systems, its overall behavior must be characterized in quasi static and dynamic states. Based on the numerous literatures, a dedicated 2-level VSC model and its local controllers as well as DC grid voltage regulators are developed for this purpose. Furthermore, the requirement of the system to guarantee all the physical constrains must be well-assessed and concrete demonstrations must be provided by numerical simulations.

• Design/methodology/approach Firstly, a 2-level VSC model and its local controllers as well as DC grid voltage regulators are developed. Then, various DC cable models are investigated and their characteristics are assessed in the frequency domain. Those developed models are combined to form a 3-Terminal HVDC grid system on Matlab/Simulink platform. To analyse the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed.

• Findings

To analyse the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. The differences in the DC grid voltage dynamics and the power flow of the converter stations coming from the embedded primary controls are analysed and the technical requirements for both cases are assessed.

• Originality/value

In this paper, the dynamic stability of a MTDC system has been analysed and assessed through an adequate simulation model including its control scheme and the cable models. The interests of the improved PI model for cables are highlighted.

INTRODUCTION

Over the last decade, integration of renewable energies has been a serious challenge around the world due to the sustainable development concerns (Davigny at al. 2014). In Europe, EU countries have agreed an ambitious development plan of renewable energy integration which targets 27 [%] share of final energy consumption by 2030 towards the complete CO2 abatement by 2050 (European Commission 2014). Wind energy is considered as a keystone in the policy of European countries to reach those targets (Z.Q. Zhu, Jiabing Hu 2013). Due to the lack of remaining onshore sites, offshore wind farms are prospected to rapidly increase its share in the next decades [START_REF] Bell | Economic and technical criteria for designing future off-shore HVDC grids[END_REF]. Many of large offshore wind farm projects have been already launched and started their commercial operations [START_REF] Barnes | Voltage Source Converter HVDC Links -The State of the Art and Issues Going Forward[END_REF]. However, offshore wind farms are mostly located far away from the electric coupling point of the national grid, and so a largescale power transfer implies a large amount of reactive power from submarine cables in HVAC. Hence, HVDC technology appears as a better economic solution [START_REF] Asplund | HVDC Grid Feasibility Study[END_REF]) [START_REF] Ackermann | Evaluation of Electrical Transmission Concepts for Large Offshore Windfarms[END_REF] [START_REF] Schifreen | Charging Current Limitations in Operation or High-Voltage Cable Lines[END_REF].

HVDC technology has been led by remarkable advances in power electronic devices. Voltage Source Converter (VSC) is capable of bi-directional power flow and can significantly extend the flexibility and operability of the HVDC technology, compared to conventional Line-Commuted Converter (LCC) based HVDC. The radial or meshed connection of HVDC configuration, so-called Multi-Terminal Direct Current (MTDC) grid, has become an attractive solution to acquire further reliability and flexibility to achieve the EU's renewable energy integration target, and moreover, to enhance energy security and economic efficiency of electric power network. This concept is also known as "SuperGrid" [START_REF] Asplund | HVDC Grid Feasibility Study[END_REF].

In order to take full advantage of MTDC systems, its overall behavior must be characterized in quasi static and dynamic states. Furthermore, the requirement of the system to guarantee all physical constrains must be well-assessed, and concrete demonstrations must be provided by numerical simulations.

Main contributions of this paper are: -The development of appropriate MTDC network simulation models including power electronic controllers, -The analysis on power transfer capability of the grid connected converter, -The stability analysis of MTDC network in quasi static and dynamic states [START_REF] Broutin | Influence of Medium Voltage Distribution System Models: Application to the study of the Harmonic Propagation[END_REF], -The assessment of required dynamic response of embedded controllers.

To perform a stability analysis, a compromise must be chosen between the simplification of used models and the required accuracy. In this paper, the followed method is to design linear models of each device. This method enables an acceptable computation time while taking into account behavior of all equipment and also effects of the control system Firstly, a 2-level VSC model and its modulation/regulation controls are recalled. Then, various DC cable models are investigated, and their behaviors are checked in the frequency domain. To get a more accurate model, an improved PI model, is developed and based on the chain connection of PI lumped parameterized cells. Those developed models are combined to form a MTDC grid system. Several DC grid voltage controllers are developed, and transmission capability is quantified according to the dc voltage variations. Then, achievable performances in dynamic stability are analyzed.

II. TEST SYSTEM CONFIGURATION

VSCs are intensively used to perform required power electrical conversions under the intermittency of renewable resources and also under prescribed technical constrains from the main AC transmission system. In case of fault, the balancing between the generated power and exported power is timely broken. Primary DC voltage controllers are needed in practice to damp DC voltage oscillations into security margins within a certain time. To study and understand the behavior of these power systems in quasi static and dynamic states, a HVDC grid topology has been considered. The proposed HVDC grid model consists of two onshore terminals (VSC 1 and VSC2) and one offshore terminal (VSC3) (Fig. 1). All 1000 MW VSC stations are associated with 75µF DC capacitors and connected through 100 [km] cables. • Independent control of active and reactive power within the rating of the equipment, • Bi-directional power flow control, • Reduction of the harmonics content by fast switching, • Requirement of smaller filters. Disadvantages of 2-levels VSC are the slightly higher power losses and the smaller power rating.

VSC consists of several Insulated Gate Bipolar Transistors, IGBT, which allows switching on and off large currents by controlling the gate signals (Fig. 2). As high frequency switching is possible, the filtering of harmonic components is easily performed by chokes, and so that nearly ideal AC current outputs ( , , ) are obtained. Then an average model of the power converter can be considered for the control design of power conversion between DC and AC grid.

B. Principle of VSC Control

A Phase Lock Loop (PLL) is used to track the AC grid voltage frequency and phase angle. A synchronised rotating dq frame with AC voltages is considered for the modelling and control design [START_REF] Haileselassie | Multi-Terminal VSC-HVDC System for Integration of Offshore Wind Farms and Green Electrification of Platforms in the North Sea[END_REF], [START_REF] Lee | Grid synchronization PLL robust to frequency variation, unbalanced and distorted voltage[END_REF]. Then direct and quadrature coordinates of electrical quantities are calculated with the Park transform.

The designed control system is divided into two main parts: the inner control and the outer control (Fig. 3) [START_REF] Rault | Modélisation Dynamique et Commande des Réseaux à Courant Continu Multi-Terminaux Haute Tension[END_REF]. The role of inner control is to control the currents and by regulating d and q coordinates of modulated voltages ( * and * ) in the Park frame. The outer control generates: -The d-axis current reference, which can be either given by the active power controller or the DC voltage controller -The q-axis current reference can be either given by the reactive power controller or the AC voltage controller

C. Design of the Current Controller

In a dq rotating frame, AC currents are expressed by following dynamic equations with d and q coordinates of grid voltages ( and ) and modulated voltages ( and ):

= 1 (1) = 1 .
(2)

As coupled terms appear, the current controller includes a decoupling action, and a PI or IP controller is used to avoid the residual steady state error between the reference and the output current (Fig. 4). Parameters of the corrector affect the response time and stabilization characteristics of the overall system. Therefore, they must be determined carefully [START_REF] Robyns | Equivalent Continuous Dynamic Model of Renewable Energy Systems[END_REF].

D. Active and Reactive Power Controller

As long as the dq frame is aligned with the d component of the grid voltage, the quadrature component of the voltage is equal to zero. Hence, the active and reactive grid powers are simply obtained by following expressions:

= = (3) = = (4)
Then any active and reactive power references can be easily converted into the current references [START_REF] Haileselassie | Multi-Terminal VSC-HVDC System for Integration of Offshore Wind Farms and Green Electrification of Platforms in the North Sea[END_REF]

): * = * (5) * = * (6)

E. DC Voltage Controller

As the DC voltage is issued from a capacitor, it is modelled by the following linear differential equation:

= 1 . ( 7
)
By considering the input current ( ) as a disturbance, the voltage can be regulated by decreasing the error between the If losses are small in the power electronic converter, then the balancing of powers shows that the full DC power is converted to the active power in the AC side (fig. 2): . = (9)

Then, the corresponding power reference is deduced by assuming that the input DC current ( ) is a disturbance and corrected by the PI controller. * = * .

(10)

According to this scheme, the DC voltage can be controlled by the current injection and extraction with the AC grid (equ 5).

IV. HVDC CABLE

To obtain the accurate dynamics of the MVDC system, the dynamics of cable has to be considered, and now various models of cables are examined.

A. Wide Band Cable Modelling

The transmission line or cable systems can be characterized by two main matrices of the characteristic admittance matrix and the propagation function [START_REF] Broutin | Influence of Medium Voltage Distribution System Models: Application to the study of the Harmonic Propagation[END_REF]. Wide Band model is the most sophisticated model available and is used for a wide range of frequency for both overhead lines and underground cables. WB model is formulated in the phase domain taking into account the full-frequency-dependence of the line parameter. A Wide Band cable model is developed by using the cable parameters studied for the INELFE link project [START_REF] Descloux | Protection contre les courts-circuits des réseaux à courant continu de forte puissance[END_REF]. The required information of the cable arrangement and the fundamental parameters, such as the radius, the resistivity of the core and the screen, the permittivity of the insulator, are detailed in Table 1 andTable 2. Then the admittance and impedance matrices are fitted into the cable for each frequency by the vector fitting tool [START_REF] Kocar | Weighting method for transient analysis of underground cables[END_REF].

Capacitance C [µF/km] 1 2 1 2.42e-1 -2.42e-1 2 -2.42e-1 1.76e0

B. Frequency Analysis of Simple PI Electrical Circuit Model

The impedance matrix at 10 [&'(], which is derived in the process of the Wide Band cable modelling, is chosen as a reference value to be fitted on a PI electrical circuit model. The obtained impedance matrix at 10 [&'(] is shown in Table 3. Then frequency analyses are carried out, and results are compared with that of Wide Band Model calculated with EMTP-RV software.

The obtained impedance matrix at 10 [&'(] is multiplied by 100 to simulate 100 [*+] cable connection, and then assigned to the PI model (shown in Fig. 6).

To fit it in PI model, only the parameters of core conductor and core to ground are taken into account. To get a more accurate model, the chain connection of ten PI lumped parameterized cells (representing 10 km cable) is considered to have a 100 [km] line model.

In Fig. 7, the obtained impedance and phase shift of the PI models for various frequencies are compared with that of Wide Band model. It can be seen that the simple PI model does not represent the actual behavior of the cable, and so the PI model must be improved.

C. Improved PI Model

The existence of coupling inductance on a simple PI model can have a large impact on the accuracy of the cable behaviour. An improved PI model is proposed by P. Rault [START_REF] Rault | Modélisation Dynamique et Commande des Réseaux à Courant Continu Multi-Terminaux Haute Tension[END_REF]. This model takes into account the mutual inductance between the core and the screen conductor of the cable, as shown in Fig. 8.

D. Frequency Analysis of Multi Cable Connection

To consider practical conditions, multi-phase cable models are developed. Similarly to the previous section, a multi-phase

V. PRIMARY VOLTAGE CONTROL OF MTDC SYSTEM

A. Primary Control Methods

Due to impedances (coming from cables), voltage drops appear between power stations. The DC grid voltage control is the most important role for the VSC stations to accomplish the secure operation of a MTDC system. The DC grid voltage has to be maintained in a certain range against any power variations; otherwise a large DC voltage drop can temporarily limit the capability of the AC voltage controller and affect the linear stability of the control systems (Zhang et al. 2011). For this matter, several strategies, often called Primary Control Methods, exist. The primary control must guarantee the power balance in the MTDC grid. Many primary control methods, such as Voltage Margin Method (Nakajima and Irokawa 1999), Ratio Control (Bazargan et al. 2009), etc., have been proposed in past decades [START_REF] Pinto | Comparison of direct voltage control methods of multi-terminal DC (MTDC) networks through modular dynamic models[END_REF][START_REF] Gonzalez-Longatt | Impact of DC control strategies on dynamic behaviour of multi-terminal voltagesource converter-based HVDC after sudden disconnection of a converter station[END_REF]) and [START_REF] Bacha | Power electronic converters modeling and control[END_REF]). However, most of them are based on two main methods called Master Slave Method and Voltage Droop Method or a combination of both of them (Wang and Barnes 2014).

B. Master Slave Method

Master Slave Method is an extension of the method adapted for the control of a point to point HVDC system [START_REF] Barker | Autonomous converter control in a multi-terminal HVDC system[END_REF]. Only one station in the MTDC system, which is called "Master" station, controls the DC grid voltage to keep it in a certain range while other stations, which are called "Slave" stations (Fig. 11). "Slave" stations control their power injection or absorption to/from the DC grid. All power variances in the DC grid are compensated by the Master station and balanced. Thus, the Master Station must be connected with a strong AC grid, which must keep continuously large power reserves.

C. Voltage Droop Method

In contrast to the Master Slave Method, several power stations can contribute to the voltage regulation with the Voltage Droop Method. Classically, the power contribution (∆-) of these stations is linearly deduced from the voltage deviation (∆ ) as (Fig. 12):

∆-= 1 * ∆ (11)
The droop value of the stations can largely affect the dynamic response of the system. Hence, the droop values of each station must be carefully chosen. Since the DC grid balance is achieved by several stations, this method is more reliable and suitable for multi-terminal HVDC system [START_REF] Akkari | Small-Signal Modelling for In-Depth Modal Analysis of an MTDC System[END_REF]. The main drawback of this method is the linear behavior of the controller. Hence the DC grid voltage is not strictly constant (Fig. 13) [START_REF] Dragon | Effects of DC voltage droop characteristics on contingency behaviour of AC/DC systems[END_REF]. converter must be properly sized to be capable to transfer a certain power. By neglecting the filter resistance (in Fig. 2), the power flow between the converter and the AC grid is expressed by the vector diagram (Fig. 14). The vector . / corresponds to the AC side voltage of the converter. When a constant switching frequency and the Space Vector Modulation technique are used, the rms value of the voltage output . is expressed by using the conversion ratio + (0 0 + 1 1) and the DC grid voltage [START_REF] Bouhali | Power sizing and control of a three-level NPC converter for grid connection of wind generator[END_REF]:

. = 2 √6 + . ( 12
)
As it is seen, the AC voltage output of the converter is limited by the DC grid voltage. Considering the maximum conversion ratio (+ = 1), the maximum AC voltage is given as:

.

5 6 = 2 √6 . ( 13
)
The active and reactive powers fed to the AC grid are given as follows:

-= 3 . . # 89 :

; = 3 . . <=#9 . > : (14)
with : = . Combining both equations, a circle equation is obtained:

-> ?; ; @ A > 1 B3 . C . + : # D 2 (16)
with ; @ = 3. > : .

From ( 14), the maximum active power -5 6 is obtained with the reactive power ; = ; @ and the modulation index at maximum value (+ = 1):

-

C FGH = 3 . : s √6 . ( 17
)
When disturbance such as a power variation at one of the stations or sudden loss of a power station occurs, the DC grid voltage starts to decrease or increase until the disturbance is compensated by the appropriate actions taken by the other power stations. If there are some operational constrains related to the variation of the DC voltage, those can interfere with the actions to bring back the DC grid voltage to a steady state and cause instability of the system. However, the control of VSC is often designed in a dq rotating frame. To maintain the operating point within an admissible region which satisfies all the operational constrains, those constrains have to be interpreted in a comprehensive manner for the controllers. Hence, the possible operational constrains of the 2-level VSC are investigated and analyzed in the dq frame, then the impacts of the DC grid voltage variation on those constrains are analyzed.

B. Admissible AC Current

The admissible AC current fed by the power electronic converter is restricted by the thermal capability of the grid components such as the AC grid cables and the phase reactor. For a given AC grid voltage . , the maximum AC current J 5 6 is deduced from the following expressions:

K = L-> ; > 0 K 5 6 (18)
K 5 6 = 3. J 5 6 ( 19
)
where the admissible AC current in a dq frame is given in a form of a circle equation (Fig. 15):

> > 0 J 5 6 > . ( 20
)
Then the limitation of the converted AC power can vary along with the variation of the AC grid voltage. It is notable that the variations of the DC grid voltage have no impact on this limitation.

C. Maximum AC Voltage

By neglecting the filter resistance in equations ( 1) and (2), and considering the steady state, we obtain:

= (21) = . ( 22
)
When the rotating dq frame is synchronized with AC voltages, the quadrature component is equal to zero. The magnitude of the modulated voltage is expressed as:

> = > > . ( 23
)
Then a second circle equation is obtained:

# > M # N > = > # > . (24)
As the AC voltage has a maximum magnitude, . 5 6 , the operating point of grid currents, and , must be within the circle with the radius equals to . 5 6 / . According to the equation ( 12), it is obvious that this radius can vary along with the variation of the DC grid voltage. Hence the admissible operating range which the converter will also change its size.

D. Admissible DC Current

The DC current is limited by the thermal capability of the cable. By neglecting the switching and phase reactor losses, the limitation is then expressed as: P-C P # 0 PJ Q 5 6 P .

(26)

E. Capability operating domain

The aforementioned physical limitations with various DC grid voltages ( ) are drawn in Fig. 15. The allowable operation area is given by the overlapped area which satisfies all the limitations. This allowable operation area depends on the connected grid conditions and so can vary. Especially, when a DC grid voltage drop occurs, the allowable operation area can be largely reduced due to the Maximum AC Voltage (equ. 24) and the Admissible DC Current limitations (equ. 25). Therefore, the DC voltage must be maintained within a certain range, and the set points of the converter controllers ( * , * ) must be adjusted by taking into account the new operating domain. To assess the modelling equations, a simulation set up has been developed with the Matlab/Simulink software. Table 5 shows the calculated control parameters. For the Master Slave method, the station VSC 1 is chosen as the Master station (fig. 1). For the Voltage Droop method, the stations VSC 1 and VSC 2 are chosen to endow for the DC voltage control. The controller parameters are calculated to obtain a 100 [ms] response time. The tuning techniques of the primary voltage control is detailed in [START_REF] Rault | Modélisation Dynamique et Commande des Réseaux à Courant Continu Multi-Terminaux Haute Tension[END_REF]. Table 6 shows the set points of the controllers. Power variations are caused set at t =3 and 4 [s] at VSC 3.

In Fig. 16 and Fig. 17, the dynamic responses of the DC grid voltage and active power controls are depicted. With the Master Slave method, only VSC 1 contributes for the voltage stabilization. In the case of the Droop voltage method, VSC 1 and 2 adjust their power output to compensate the power imbalance.

Since the same droop values are assigned, the contribution of the power given by the both stations are the same. With the Master Slave method, the DC grid voltage comes back to the nominal value in certain response time. With the Droop control case, the voltage is stabilized at lower or higher levels than the nominal value. Thus, it should be pointed out that the parameters of the droop controller must be carefully chosen taking into account the maximum power variation to cope, the required response time and the range of voltage margin. In this paper, the dynamic stability of a MTDC system has been analysed and assessed through an adequate simulation model including its control scheme. Firstly, a 2-level VSC model and its local controls were recalled. The inner control, which implements the modulation of VSC and regulation of the PCC voltage, and the outer controls, which are in charge of the output power regulation or the DC grid voltage, have been developed.

Secondly appropriate DC cable models have been developed. It was clarified that the simple PI electrical circuit model does not represent the actual HVDC cable behavior. Hence, the improvement of the cable model was carried out by taking into account the mutual coupling of the cable core and the sheath. The characteristics in frequency domain are compared with the Wide Band model and the validity of the developed cable model was confirmed. Thirdly, those developed models are combined to form a MTDC grid system. In addition, main primary voltage controls were developed and embedded. The dynamic stability on the developed MTDC model was observed, and the differences in dynamics coming from the embedded primary controls were analysed.

In future works, the use of a Modular Multilevel Converter (MMC) instead of a PWM based 2-level VSC will be considered. The difference in voltage and current control can have an impact on the dynamics. It is also important for the design of MTDC network to investigate the fault ride through capability in case of the loss of one power equipment (N-1 situation). In this case, the design of automatic reconfiguration methods taking into account the physical limitations of the MTDC grid should be concerned. Institute of Electrical Engineers. Part III: Power Apparatus and Systems, 75(3), pp.803-817. Bazargan, M. Xu, L., Yao, L. and (2009), "DC grid management of a multi-terminal HVDC transmission system for large offshore wind farms. International Conference on Sustainable Power Generation and Supply, pp.1-7. Zhang, L., Harnefors, L. and Nee, H.P. (2011), "Interconnection of two very weak AC systems by VSC-HVDC links using power-synchronization control", IEEE Transactions on Power Systems, 26(1), pp.344-355. Z.Q. Zhu, Jiabing Hu, (2013) "Electrical machines and power-electronic systems for high-power wind energy generation applications: Part IIpower electronics and control systems", COMPEL -The international journal for computation and mathematics in electrical and electronic engineering, Vol. 32 Iss: 1, pp.34 -71. Wang, W. and Barnes, M. (2014), "Power flow algorithms for multi-terminal VSC-HVDC with droop control," IEEE Trans. Power Syst., vol. 29, no. 4, pp. 1721-1730

Fig. 1 .

 1 Fig. 1. Architecture of the studied 3 terminal HVDC system III. VSC CONTROL SYSTEM A. Introduction of VSC and Modelling Method VSC transmission can offer several advantages in addition to LCC (Andersen et al. 2005) (Jones and Davidson 2013):• Independent control of active and reactive power within the rating of the equipment, • Bi-directional power flow control, • Reduction of the harmonics content by fast switching, • Requirement of smaller filters. Disadvantages of 2-levels VSC are the slightly higher power losses and the smaller power rating.VSC consists of several Insulated Gate Bipolar Transistors,

Fig. 2 .

 2 Fig. 2. Structure of a 2-Level VSC

Fig. 3 .

 3 Fig. 3. Structure of the 2 Level Control System in a dq rotating frame

Fig. 4 .

 4 Fig. 4. Current Controller in a dq frame

  Fig. 5. DC Voltage Controller

Fig. 6 .

 6 Fig. 6. Simple PI Model

Fig. 8 .

 8 Fig. 8. Improved PI Model

  model is developed on EMTP-RV. Then calculated impedance matrix is applied to the improved PI models, and the results of the frequency analysis are compared. To develop multi-phase cable model, the geometrical arrangements of the cables are modified to 1.33 [m] vertical distance, 0.5 [m] horizontal distance. The other parameters such as the outer insulation radius and the dielectric properties remain unchanged. The same values for the conductor/insulator data (as in previous subsection) are used. The cable models are connected by a 50 [µF] capacitor, which is adequate as a station capacitance.As it is observed in Fig.10, the frequency scan responses of the improved PI models are very close to those of the Wide Band model. The ten cell based model has the first 10 line modes at almost the same frequency of the Wide Band model. Thus, the proposed cable models are validated.

Fig. 10 .

 10 Fig.10. Comparison of Bode Plots in Multi Cable Connection

Fig. 11 .

 11 Fig. 11. Concept of the Master Slave method

Fig. 12 .

 12 Fig. 12. Voltage droop controller

Fig. 13 .

 13 Fig. 13. Concept of the Voltage droop controller

Fig. 14 .

 14 Fig. 14. Vector diagram of the grid connection

Fig. 15 .

 15 Fig. 15. Variation domain of currents for a stable operation of a VSC VII. COMPARISON OF PRIMARY CONTROL METHODS IN A MTDC NETWORK

  Fig. 16. Dynamic response of DC grid voltages

Table 1 .

 1 Arrangement of the Cable

	Vertical	Horizontal	Outer Insulation
	Distance [m]	Distance [m]	Radius [m]
	1.33	0	63.8e-3

Table 2 .

 2 Conductor/Insulator Data

	Radiu s	Resistivit y [Ωm]	Cor e			Insulator
		Inside [m]	Outsid e [m]		Relative Permeabilit y	Relative Permeabilit y	Relative Permittivit y	Loss Factor
	Core		0				32e-3	1.72e -8	1 1	2. 5	0.000 4
	Scree n		56.9e-3			58.2e-3	2.83e -8	1 1	2. 5	0.000 4
			Table 3. Impedance Matrix at 10µHz
		Impedance			Matrix
		Series			1	2
		Resistance	1	5.35e-3	9.87e-9
		R [Ω/km]	2	9.87e-9	6.02e-2
		Series			1	2
		Inductance	1	3.65e0	3.48e0
		L [mH/km]	2	3.48e0	3.48e0
		Shunt			1	2
		Conductance	1	6.07e-8	-6.07e-8
		G [S/km]	2	-6.07e-8	4.41e-7
		Shunt

Table 5 .

 5 Control parameters

				Method
	Station	Master/Slave		Droop
		Control	Kp [p.u.]	Ki [p.u.]	Control	k [p.u.]
	VSC 1	Master	3.91	82.9	Droop	0.723
	VSC 2	Slave	Not used	Not used	Droop	0.723
	VSC 3	Slave	Not used	Not used	Power Control	Non
		Table 6. Set points of the stations
	Reference	t [s]		Value


Enrique Romero-Cadaval

Bruno Francois

Mariusz Malinowski

Qing-Chang Zhong

Grid-Connected Photovoltaic Generation Plants as Alternative Energy Sources

Keywords: Photovoltaic Plants, Grid Integration, Alternative Renewable Energy Sources. I

Large photovoltaic (PV) plants are being built all over the world, commonly having ratings up to 1 MW and reaching the level of 250 MW. These plants could be an alternative energy source replacing conventional ones and solving the energy dilemma of humanity. However, the associated electronic systems, mainly inverters used for interfacing with the grid, have to solve some problems related to the operation in the electric distribution system. They should be reliable, robust and manageable. This paper presents and discusses the most critical points existing in actual inverters, summarizing, explaining and proposing approaches to solve or mitigate them.

INTRODUCTION

Governments and public organizations are nowadays concerned about the production of energy with technologies as clean as possible [START_REF] Zhong | Control of Power Inverters in Renewable Energy and Smart Grid Integration[END_REF]. As a consequence, the guidelines for future energy production are established according to the Kyoto protocol [START_REF]Kyoto protocol[END_REF], which for European countries inspires the "20-20-20" target [START_REF]The EU climate and energy package[END_REF]. The energy production technologies based on hydro, wind, photovoltaic and geothermal energies can be considered to be clean and renewable alternatives to the non-clean conventional technologies based on fossil fuels and nuclear fission. Among the clean technologies, photovoltaic (PV) is one that has experienced a great growth in the last years, close to 60% in Europe.

PV Generation Plants (PVGP) are becoming economically viable, even without government subsidies. The PV plant capacity is increasing continuously all Controlling frequency and voltage has always been an essential part of operating a power system. However, since the liberalization of the electricity supply industry, the resources required to achieve this control have been treated as services that the system operator has to obtain from network stakeholders, usually known as ancillary services. Large grid-connected PVGPs should contribute to deliver this service, especially if they have to operate in the isolated mode.

Ancillary services are components of electric services, required to support the reliable delivery of electricity and operation of transmission systems [START_REF] Chuang | Integration of Wide-Scale Renewable Resources into the Power Delivery System[END_REF]. These services contribute to frequency stability (e.g., frequency regulation and operating reserves, in different time scales Fig. 3), power balance (e.g., scheduling and dispatch of balancing energy), voltage control (e.g., tap changer control and reactive power control), restoration of supply (e.g., black start capability and island operation), or system management (e.g., power quality assurance, operation and asset management). In order to maintain power system reliability, the natural intermittent characteristics of primary renewable energy must be taken into account, requiring a precise estimation of their ancillary service provision in advance. If necessary, other utilities, such as Energy Storage Systems (ESS) or fast generators, must be planned. Hence, the integration of PVGPs into an electrical system implies the extension of the actual hierarchical power management system that takes into account different time-scales and economic dispatch. It has to pave the way to develop new interactions between real-time pricing, physical constraints and demand response-based loads.

At the present, and even more in a future Smart Grid scenario [START_REF] Calderaro | A Smart Strategy for Voltage Control Ancillary Service in Distribution Networks[END_REF], PVGPs can receive operation references, mainly active and reactive power references to generate/consume energy for delivering ancillary services, from the corresponding Electric System Agent. PVGP central control systems will have to translate these global references into individual operation references for each inverter within the plant, and taking into account the actual state of all the inverters and weather forecast. Therefore, inverters will be the main device that must be able to interact with the grid and with the ESS for providing these services in the required way.

This paper presents new solutions that enable PVGPs inverters to provide these ancillary services with the right topology, additional systems, power or current ratios, control algorithms and communication systems. The paper starts with a general overview of the current situation in Section I and continues by presenting in Section II the control structures that are most commonly used for controlling the converters that inject the energy produced by these plants into the grid. In Section III Active PVGPs with embedded ESS and the associated control strategies are discussed. Section IV presents advanced functions that these PVGPs can implement in the near future, in the context of the fore coming Active Distribution Grids, providing ancillary services. In Section V, synchronverters are presented as an alternative to the conventional control structures discussed in previous sections, emulating the operation of conventional synchronous generators.

Finally, this paper ends with conclusions.

II. Control of Grid-connected PVGP Inverters

During the last years, PWM converters have drastically increased their importance on the market of energy conversion for PV applications. Two technology breakthroughs enabled these remarkable developments:

• innovations in the field of power electronics, which brings improvements in efficiency (reduction of switching losses), power density, power quality, common mode voltage and electromagnetic interference, etc.;

• innovations in the field of control schemes, which contain now many sophisticated control functions (Fig. 4) e.g. inner current/power and outer dc-link control [START_REF] Kazmierkowski | Control in Power Electronics[END_REF][START_REF] Wilamowski | Industrial Electronics Handbook[END_REF][START_REF] Malinowski | Sensorless control strategies for three-phase PWM rectifiers[END_REF], maximum power point tracking (MPPT) [START_REF] Romero-Cadaval | Grid-Connected Photovoltaic Generation Plants[END_REF],

monitoring and grid synchronization [START_REF] Teodorescu | Grid Converters for Photovoltaic and Wind Power Systems[END_REF], proper operation at significantly distorted grid [START_REF] Kazmierkowski | DSP-Based Control of Grid-Connected Power Converters Operating Under Grid Distortions[END_REF], islanding/anti-islanding operation [START_REF] Romero-Cadaval | Grid-Connected Photovoltaic Generation Plants[END_REF],

active power feed-forward [START_REF] Jasinski | Direct power and torque control of AC-DC-AC converter-fed induction motor drives[END_REF][START_REF] Stynski | Analysis and Control of Multilevel AC-DC-AC Flying Capacitor Converter Fed from Single-Phase Grid[END_REF], outer control loop for active/reactive power to support power system etc. [START_REF]Power Plant Control for Utility-Scale Photovoltaic Installations[END_REF][START_REF] Guerrero | Advanced Control Architectures for Intelligent Microgrids -Part I: Decentralized and Hierarchical Control[END_REF], energy storage, harmonic compensation or synchronous generator emulation. The conventional VOC uses closed-loop current control in the rotating reference frame. A characteristic feature for this current controller is the processing of signals in two coordinate systems (Fig. 6), after converting three phase measured values to the equivalent two-phase system α-β and then to the rotating d-q coordinate system via a α-β/d-q block [START_REF] Malinowski | A Comparative Study of Control Techniques for PWM Rectifiers in AC Adjustable Speed Drives[END_REF][START_REF] Ooi | An integrated AC drive system using a controlled current PWM rectifier/inverter link[END_REF]. As the d-q frame rotates with the same speed as the electrical quantities, the projected coordinates are DC signals. In the voltage oriented d-q frame, the AC grid current vector i is split into two rectangular components i = [id, iq]. The component iq determines indirectly the reactive power, whereas id decides indirectly the active power flow. Thus the reactive power and the active power can be controlled independently. The Unity Power Flow (UPF) condition is met when the grid current vector, i, is aligned with the grid voltage vector u, which means that the q-axis current should be set to zero in all cases while the reference current id is set by the DC-link voltage controller and controls the active power flow. The output signals from Proportional-Integral (PI) controllers after a dq/αβ transformation are used to generate switching signals via a Space Vector Modulator (Fig. 7a).

(a) (b) An important drawback of VOC is the high algorithm complexity (many coordinate transformations are needed) as well as the sensitivity to grid voltage distortion. The last drawback can be eliminated [START_REF] Kazmierkowski | DSP-Based Control of Grid-Connected Power Converters Operating Under Grid Distortions[END_REF], but it significantly complicates the algorithm.

p ref p ref i i i i _ _ _ _ J β β α α - + - = [ ] ) ( ) ( ) ( i ) 1 ( i , , , , k u k u L T k k S S β α β α β α β α - + = + (c) (d)
The VOC can also be realized in the stationary α-β coordinate system but PI controllers cannot be used for current regulation because they result in a steady state error. However, this can be solved by replacing PI current regulators with Proportional/Resonant controllers (PR), which consist of a proportional gain and a resonant integrator. The transfer function of a PR controller contains a pair of conjugate poles tuned at the fundamental grid frequency ω, which allows perfect tracking of sinusoidal signals without any error [START_REF] Stynski | Analysis and Control of Multilevel AC-DC-AC Flying Capacitor Converter Fed from Single-Phase Grid[END_REF].

A basic block diagram of the VOC in the stationary coordinate system is shown in Fig. 7b and it is based on an inner current control loop with PR and an outer voltage control loop with PI. The commanded dc-link voltage udc * is compared with the measured udc voltage. The error is delivered to the PI controller, which generates the amplitude of the commanded current idc_ref. Next, this current is multiplied by the angle of the line voltage uL as cos( ) and sin( ) to get iα_ref and iβ_ref. An interesting feature of this control is the proper operation under distorted grid voltages by connecting multiple PR compensators, which are tuned at specific high order harmonics, e.g. 5 th , 7 th , in parallel [START_REF] Stynski | Analysis and Control of Multilevel AC-DC-AC Flying Capacitor Converter Fed from Single-Phase Grid[END_REF][START_REF] Abu-Rub | Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications[END_REF][START_REF] Teodorescu | A new breed of proportionalresonant controllers and filters for grid-connected voltage-source converters[END_REF].

Another less known method is direct power control -space vector modulated (DPC-SVM) which uses inner loops of active and reactive power control (Fig. 7c) [START_REF] Malinowski | Simple Direct Power Control of Three-Phase PWM Rectifier Using Space-Vector Modulation (DPC-SVM)[END_REF]. Recently, a very popular control scheme called Model Predictive Control (MPC) (Fig. 7d) has been applied to control inverters. It is based on the mathematical model of the controlled system and the calculation (prediction) of future values of the state variables. It is assumed that the system can represent a finite number of states in every time period [START_REF] Rodriguez | Predictive Control of Power Converters and Electrical Drives[END_REF][START_REF] Rodriguez | Predictive Current Control of a Voltage Source Inverter[END_REF][START_REF] Kazmierkowski | Current control techniques for three-phase voltage-source PWM converters: A survey[END_REF][START_REF] Cortes | Predictive Control In Power Electronics and Drives[END_REF][START_REF] Kouro | Model Predictive Control -A Simple and Powerful Method to Control Power Converters[END_REF]. Thus, first of all, the model must be as accurate as possible, because the control performance is highly dependent on the parameters. Using measured values of grid currents, their values in the forthcoming sampling can be calculated, with respect to available control states. It must be done for all switching states in each iteration of the algorithm. Some features of predictive control are very interesting, e.g. fast dynamics and the capability of dealing with multiple constrains, which is very attractive especially for high power converters operating at very low switching frequency (500Hz-1000Hz). Unfortunately, MPC has some drawbacks, e.g. variable switching frequency (causing difficulties for the design of the LC input EMI filter), high sampling frequency needed for digital implementation (demand of fast microprocessor) and sensitivity to parameter variations of the grid filter, etc, which cause difficulties in implementing MPC in industry. All the above drawbacks can be eliminated when a constant switching frequency is applied but it significantly complicates the control algorithm [START_REF] Antoniewicz | Predictive Control of Three-Phase AC/DC Converters[END_REF].

The advantages and features of the control schemes described above are summarized in Table I.

IV. Active PV Generators with Embedded Energy Storage Systems

Electrical systems must ensure a balance between production and consumption at all times, while maintaining a satisfactory voltage. Historically, grid reliability was mainly ensured by having excessive capacity in the system with unidirectional flow from centrally dispatched large power plants to dispersed consumers. Dispatched production refers to sources of electricity that can be dispatched at the request of system operators. They are able to change their power production upon demand.

The large scale development of intermittent PV sources causes large amount of variable power [START_REF] Robyns | Electricity Production from Renewables Energies[END_REF]. Renewable energy based generators with intermittency decrease the reliability of a power system. As the percentage of intermittent generation capacity increases and becomes more significant, an additional uncertainty is appearing in the real time management of the electrical system balance between demand and generation. This requires increasing amounts of conventional power reserve capacity that can be available immediately (spinning reserve) and of plants capable of providing ancillary services (frequency response and voltage control as example), which are required to manage the electrical power system securely [START_REF] Peacock | Micro wind turbines in the UK domestic sector[END_REF].

However, an electrical generation system depending entirely on the renewable energy sources is not reliable because the availability of the renewable energy sources cannot be constantly ensured.

Because of the intermittency of PV power generation, PVGP cannot be used as a stable, reliable and controllable power source and cannot provide ancillary services like conventional generators. One solution is to upgrade PVGP with an embedded Energy Storage System (ESS) and a Local Energy Management System (LEMS) for the coordination of inner power flows. Storage technologies are varying and first it is essential to characterize the required need and complementary performances they must offer. Typically, an energy reserve must be provided to the electrical system and can be implemented by long-term energy storages. Moreover, the supply of power with fast dynamics is also mandatory to smooth the generated PV power, compensate the power gap and absorb instantaneous high power peaks.

At the present state of art, a high energy storage unit, delivering high power with fast dynamics in a reduced volume, does not exist and a mix of storage technologies has to be considered [START_REF] Yu | Combined long-term and short-term access storage for sustainable energy system[END_REF]. Lead acid batteries for long term energy storage device and ultracapacitors for fast dynamic power regulation are today realistic and economic choices [30], but other technologies can also be considered [START_REF] Guerrero | Overview of medium scale energy storage systems[END_REF], as Fuel Cells, REDOX batteries [START_REF] Wang | Minimising output power fluctuation of large photovoltaic plant using Vanadium Redox Battery storage[END_REF][START_REF]Intelligent storage systems based on vanadium redox flow technology[END_REF] or Flywheels [START_REF]Fiber Carbon Flywheels[END_REF].

Storage devices can be used to store or to release electrical power like an energy buffer. Therefore, they can help to solve problems due to renewable energies' intermittent availabilities and fast transients. A PVGP combined with ESS can be considered as an Active PV Generator for the grid since it can supply ancillary services as conventional generators. Moreover it can be dispatched and then provide a power reference that is demanded by the grid operator [START_REF] Li | Implementation of interactivity across a resilient microgrid for power supply and exchange with an active distribution network[END_REF].

A general structure that has been widely used in isolated power systems for integrating ESS is based on the direct connection of a battery bank to the DC-bus of the grid-connected inverter. A PV controller is used to extract the Maximum Power from PV panels and send it to the battery bank. However, the stochastic nature of the PV power output and power demand leads to fast charge/discharge actions of batteries and a fast battery ageing.

To enable a more efficient use of batteries, AC-coupled and DC-coupled power electronic converters can be considered in order to have control abilities of the exchanged powers with the batteries thanks to the development of power electronics. In an AC-coupled Hybrid Power Generators (HPG) structure, all sources are connected to the main AC network (Fig. 8a) [START_REF] Li | Implementation of interactivity across a resilient microgrid for power supply and exchange with an active distribution network[END_REF]. A communication network is required to implement the coordination of this set. In a DC-coupled HPG, all sources are connected to a common inner DC-bus before being connected to the grid through a main inverter (Fig. 8b) [START_REF] Fakham | Power Control Design of a battery charger in a Hybrid Active PV generator for load-following applications[END_REF]. One advantage is that the battery bank is connected to the dc-bus via a DC/DC converter, which can be used to implement an optimized charge/discharge operation mode. The second advantage is that a super capacitor bank is added and is also connected to the DC bus via another DC/DC converter.

Hence fast power compensation can be performed. The PV array is connected to the DC bus via a PV converter. Apart from these two coupling structures, a mixed structure can be also used (Fig. 8c) with some advantages taken from both of DC and AC coupled structures. In order to ensure an optimal operation and coordination, a LEMS of the active PV generator must calculate and send control signals to each power electronic converter in order to enable

• the production of power demand for the grid operator,

• the management of renewable energy intermittency,

• the management of storage State of Charges,

• the power system protection,

• the provision of grid ancillary services.

For example, a hierarchical structure of the control system is shown in Fig. 10 and has four stages:

• the Switching Control (SC),

• the Automatic Control (AC),

• the Power Control (PC),

• the Mode Control (MC).

The SC stage implements the modulation technique to each converter and generates the switching signals. The AC stage implements control algorithms in order to meet the current or voltage references. The PC stage calculates these references according to power references from the MC level and measured values.

The MC level decides the operating mode for the whole hybrid generator according to the availability of the PV production, the state of each storage unit and the actual power demand from the grid.

- + + =
(1) With a "Grid following strategy" (Fig. 11a), the reference for the power to be generated by the inverter is deduced by using the measured power and contributes to the regulation of the DC bus:

) ( ) ( ) ( ) ( ) ( _ _ t p
- + + = ) ) ) (2)
Then the required power reference from the grid operator is provided by the storage units by taking into account the available PV power.

) ( ) ( ) ( _ _ t p t p t p PV ref gc ref sto ) - =
(3) A simple method to dispatch the power between batteries and ultracapacitors is used with a low-pass filter [START_REF] Lu | Application of Petri Nets for the energy management of a photovoltaic based power station including storage units[END_REF]. Ultracapacitors are controlled to supply the required transient power from storage units. With a "Source supplying strategy" (Fig. 11b), the power from the storage units is adjusted according to the sensed PV power to regulate the DC bus. The required power reference fixed by the grid operator is directly provided by the inverter.

With both strategies, the PV generator can be dispatched by the grid operator since it is now able to deliver the prescribed active and reactive powers (pgc_ref(t), qgc_ref(t)), but only "Source supplying strategy" is able to work without a connection to the grid because the DC bus is regulated by the inner power. This strategy is preferred since autonomous, or isolated, operation is possible and may be used to store PV energy without grid connection. Another advantage is the possibility to stay connected in case of grid AC undervoltage because the dc bus voltage is autonomously regulated by inner sources (PV panels and storage units). Hence, real and reactive power generation is possible to help the electrical network in this situation.

An experimental test is presented on Fig. 12 using this strategy, having a step change in the reference power to be injected into the grid (pgc_ref). During the test, the batteries cannot immediately supply all the surplus production when the PV power production changes or when the PV production decreases fast, so the ultracapacitors help to perform the power balancing.

V. Ancillary Services: Active, Reactive and Harmonic Control

As previously discussed, most PVGPs operate as current sources and inject into the grid the energy produced by photovoltaic cells tracking the Maximum Power Point (MPP) [START_REF] Romero-Cadaval | Grid-Connected Photovoltaic Generation Plants[END_REF]. The electronic converters generate a reference current that is sinusoidal, in phase with the grid voltage, and with the required root mean squared (RMS) value to achieve the operation in the MPP.

If the PVGPs want to become an alternative to conventional energy sources they also have to provide ancillary services [START_REF] Zhong | Control of Power Inverters in Renewable Energy and Smart Grid Integration[END_REF][START_REF] Blaabjerg | Overview of Control and Grid Synchronization for Distributed Power Generation Systems[END_REF][START_REF] Altin | Overview of recent grid codes for wind power integration[END_REF] including the control of active power [START_REF] Blaabjerg | Overview of Control and Grid Synchronization for Distributed Power Generation Systems[END_REF], reactive power [START_REF] Blaabjerg | Overview of Control and Grid Synchronization for Distributed Power Generation Systems[END_REF][START_REF] Bialasiewicz | Renewable Energy Systems With Photovoltaic Power Generators: Operation and Modeling[END_REF] and compensation of harmonics [START_REF] Lo | Grid-Connected Photovoltaic System With Power Factor Correction[END_REF][START_REF] Minambres-Marcos | Three-phase single stage photovoltaic inverter with active filtering capabilities[END_REF]. In order to achieve this, most of the Distribution System Operators (DSO) are establishing various codes to regulate how these plants should operate.

If it is assumed that the distribution grid voltage is sinusoidal, then the following current components can be defined, according to power definitions given by IEEE-Standard 519 [START_REF]IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions[END_REF]:

• Active or Direct current component, iP, that is related to the average active power (P).

• Reactive or in-Quadrature component, iQ, related to the fundamental reactive power (Q)

• Harmonic component, iH, related to the Harmonic Power (H).

Usually only the active current component is controlled in conventional

PVGPs and the others are set to zero.

In a Smart Grid context, where Active Distribution Grids are an strategic priority [START_REF]Smart Grids European Technology Platform[END_REF], PVGPs offer a unique opportunity to increase the operation possibilities of energy generators as they are distributed sources that have free capacity (because most of the time they operate under its nominal power, mainly during nights) to be used to help grid management.

If we consider an ideal photovoltaic generation day where the maximum power reaches the 90% of the nominal power ratio of the Inverter, we can discount the active component from the nominal current obtaining the available current (non-active current), INA, given by (4)

2 2 2 2 2 NA N P Q H I I I I I = -= + (4)
In the proposed case of Fig. 13a, INA (Fig. 13b) could reach values that, depending on the time of the day, vary from 40% to 100% of the nominal current. In the previous section it has been discussed how the active component can be controlled by using ESS, in this section the attention is focused on how this non-active current component availability can be used to generate other current components, such as reactive or harmonic ones.

(a) (b) Fig. 13. Non-active current (p.u.) (b) that can be produced depending on the power generated by the PVGP, PG (p.u.) in relation with its nominal power, PN (a).

By controlling the reactive component it is possible to stabilize the grid via controlling the grid frequency [START_REF] Bialasiewicz | Renewable Energy Systems With Photovoltaic Power Generators: Operation and Modeling[END_REF] or the voltage [START_REF] Camacho | Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault[END_REF] at the Point of Common Coupling (PCC) where the PVGP is connected. By controlling the voltage at this point, it is also possible to prevent over-voltages and reduce grid losses [START_REF] Ueda | Analysis Results of Output Power Loss Due to the Grid Voltage Rise in Grid-Connected Photovoltaic Power Generation Systems[END_REF]. Grid losses can be reduced further if an optimization strategy is applied in multi-bus systems [START_REF] Cagnano | Online Optimal Reactive Power Control Strategy of PV Inverters[END_REF].

Therefore, the reactive component can be used during abnormal grid or transient states, mainly during faults or after-faults, in order to compensate voltage sags [START_REF] Vasquez | Voltage Support Provided by a Droop-Controlled Multifunctional Inverter[END_REF]. In three-phase balanced voltage sags, the PVGP should inject reactive power in order to raise the voltage in all phases. In one-or two-phase faults, the

PN +INA -INA
PG main concern of the PVGP is to equalize voltages by reducing the symmetric negative and, if possible, the homopolar (or zero) sequences [START_REF] Camacho | Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault[END_REF].

PVGPs can generate harmonic component to prevent the effects of the current harmonic produced by non-linear loads connected to the grid, operating as integrated Active Power Filters [START_REF] Lo | Grid-Connected Photovoltaic System With Power Factor Correction[END_REF][START_REF] Minambres-Marcos | Three-phase single stage photovoltaic inverter with active filtering capabilities[END_REF][START_REF] Wang | Modeling and Control of Dual-Stage High-Power Multifunctional PV System in d-q-o Coordinate[END_REF][START_REF] Romero-Cadaval | Power Injection System for Grid-Connected Photovoltaic Generation Systems Based on Two Collaborative Voltage Source Inverters[END_REF]. PVGPs could even compensate current unbalance [START_REF] Wang | Modeling and Control of Dual-Stage High-Power Multifunctional PV System in d-q-o Coordinate[END_REF] or control the leakage or earth current [START_REF] Barater | Active Common-Mode Filter for Ground Leakage Current Reduction in Grid-Connected PV Converters Operating With Arbitrary Power Factor[END_REF] avoiding the propagation into the grid. For example, in the case presented in Fig. 14, the current produced by the PVGP is not sinusoidal because it contains the harmonic components demanded by the load achieving in this way a sinusoidal current flowing into the grid (and so a harmonic compensation). In some cases, dual or collaborative topologies are used, associating the different current components under control to different converters [START_REF] Wang | Modeling and Control of Dual-Stage High-Power Multifunctional PV System in d-q-o Coordinate[END_REF][START_REF] Romero-Cadaval | Power Injection System for Grid-Connected Photovoltaic Generation Systems Based on Two Collaborative Voltage Source Inverters[END_REF].

When the active, reactive and harmonics components that PVGP should generate are greater than the available non-active components, some kind of priority has to be established. This operation priority could be dependent on the status of the grid, being different in normal and abnormal states [START_REF] Romero-Cadaval | Active Functions Implementation in Smart Inverters for Distributed Energy Resources[END_REF].

The possibilities of PVGP, as a manageable distributed generation, are much more interesting than conventional centralized energy resources. For example, in the proposed initial system of Fig. 2, PVGP could generate the reactive power for controlling the voltage at the PCC or to compensate the reactive power consumed by connected loads. PVGP can also generate the harmonic component needed to compensate the harmonics components of current iL1 and iL2, which prevent these harmonic currents from flowing from the substation. By controlling properly the PVGP we can assure that the current demanded from the grid, iGrid, will be sinusoidal and in phase with the substation voltage, achieving a unity power factor, reducing distribution losses and preventing any undesirable effect on the substation transformers.

VI.

Grid-friendly connection of PVGP through inverters that mimic synchronous generators (Synchronverters)

As previously discussed, most of PVGP operate as current sources to inject the energy produced into the grid. When the penetration level of PVGP reaches a certain level, PVGPs should also provide ancillary services. As demonstrated in the previous section, these can be achieved by controlling the current injected into the grid. Another way is to control the grid-connected inverters to behave like conventional synchronous generators so that PVGPs can interact with the grid in a friendly way [START_REF] Zhong | Control of Power Inverters in Renewable Energy and Smart Grid Integration[END_REF][START_REF] Zhong | Static synchronous generators for distributed generation and renewable energy[END_REF][START_REF] Zhang | Power-Synchronization Control of Grid-Connected Voltage-Source Converters[END_REF][START_REF] Zhong | Synchronverters: Inverters that mimic synchronous generators[END_REF][START_REF] Beck | Virtual synchronous machine[END_REF][START_REF] Driesen | Virtual synchronous generators[END_REF][START_REF] Visscher | Virtual synchronous machines (VSG's) for frequency stabilisation in future grids with a significant share of decentralized generation[END_REF][START_REF] Sao | Autonomous load sharing of voltage source converters[END_REF][START_REF] Vasquez | Adaptive droop control applied to voltage-source inverters operating in grid-connected and islanded modes[END_REF][START_REF] Zhong | Robust droop controller for accurate proportional load sharing among inverters operated in parallel[END_REF][START_REF] Ashabani | Novel Comprehensive Control Framework for Incorporating VSCs to Smart Power Grids Using Bidirectional Synchronous-VSC[END_REF].

Synchronverters [START_REF] Zhong | Static synchronous generators for distributed generation and renewable energy[END_REF][START_REF] Zhong | Synchronverters: Inverters that mimic synchronous generators[END_REF] are inverters that mimic synchronous generators.

As shown in Fig. 15a, the basic idea is to embed the mathematical model of a synchronous generator [START_REF] Fitzgerald | Electric Machinery[END_REF][START_REF] Walker | Large Synchronous Machines: Design, Manufacture and Operation[END_REF][START_REF] Kundur | Power System Stability and Control[END_REF] given below as the core of the controller for the inverter in the general scheme (Fig. 4):

= ( -- ) = 〈 , sin 〉 " = sin # = - 〈 , cos $ 〉 (5)
where Tm is the mechanical torque applied to the rotor; Te is the electromagnetic torque; e is the three-phase generated voltage; θ is the rotor angle; Q is the reactive power; J is the moment of inertia of all the parts rotating with the rotor; Dp is the friction coefficient, p is the number of pairs of poles of the magnetic field, i is the stator current; if is the field excitation current; Mf is the maximum mutual inductance between the stator windings and the field winding and is the angular speed of the machine. The vectors sin and cos $ are defined, respectively, as The generated voltage e is converted into PWM pulses to turn on/off the switches of the inverter in the general scheme (Fig. 5). The inductor current is measured and fed back to the mathematical model as the stator current, as shown in Fig. 15a. This closes the loop between the controller and the power part of the inverter. On top of this, the matured frequency droop and voltage droop control strategies are applied to regulate the frequency and voltage of the synchronverter through the frequency droop coefficient and the voltage droop coefficient + , which are defined, respectively, as

= ∆- ∆. , + = ∆/ ∆0 , (7)
where 8 is the nominal angular frequency. Hence, the frequency control, voltage control, real power control, and reactive power control are all integrated in one compact controller with only four parameters. Fig. 16 demonstrates the frequency regulation capability of a synchronverter tested in the lab, where the real power output of the syncrhonverter automatically changed according to the change of the frequency.

Fig. 16. Frequency regulation of a synchronverter.

A phase-locked loop (PLL) is adopted to obtain the grid frequency and phase so that the synchronverter is able to synchronize with the grid. However, PLLs are difficult to tune and often cause problems [START_REF] Harnefors | Input-admittance calculation and shaping for controlled voltage-source converters[END_REF][START_REF] Jovcic | VSC transmission model for analytical studies[END_REF]. The PLL in a synchronverter can be removed to achieve a self-synchronised synchronverter [START_REF] Zhong | Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit[END_REF], as shown in Fig. 15b, after making some changes (highlighted in red) to the original synchronverter. This has taken advantage of the inherent synchronization property of synchronous generators. As a result, a self-synchronized synchronverter can synchronize with the grid before it is connected to the grid and keep synchronized with the grid after it is connected to the grid, without the need of a dedicated synchronization unit.

VII. CONCLUSIONS

PVGPs connected to the grid are increasing both in the number of installations and also in the rated power of each plant, becoming a significant part of the electric generation mix, and offering an important opportunity to change the present distribution grid toward a more intelligent and efficient operation.

In this paper, a comprehensive overview of control structures and their abilities to implement active distribution grid functions have been presented, with the aim of giving ideas for future improvements and research activities in the field.

Some of these methods and functions described in this paper are already used by commercial MW rated PVGP converters, as shown in Table II. Other functions are being considered for next generation of Smart Inverters, opening for Industrial Electronic Researchers an interesting working field that will change in the near future the electric grid from the one we know at the present.

The current research and industry trends for power electronics converters used in PVGPs focus on several important aspects including costs, losses, failure rate, weight and volume [START_REF] Kolar | Performance Trends and Limitations of Power Electronic Systems[END_REF].

The design of power electronic converters and passive elements must be optimized to decrease losses and increase efficiency. Possible solutions are based on new topologies [START_REF] Kouro | Recent Advances and Industrial Applications of Multilevel Converters[END_REF] and new semiconductor components such as silicon carbide (SiC) components [START_REF] Abu-Rub | Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications[END_REF][START_REF] Burkart | Comparative Evaluation of SiC and Si PV Inverter Systems Based on Power Density and Efficiency as Indicators of Initial Cost and Operating Revenue[END_REF], the elimination of transformer and direct operation in MV grid, the replacement of low frequency passive components by high frequency transformers and inductances [START_REF] Malinowski | A Comparative Study of Control Techniques for PWM Rectifiers in AC Adjustable Speed Drives[END_REF], the fast and precise GMPPT [START_REF] Bidram | Control and Circuit Techniques to Mitigate Partial Shading Effects in Photovoltaic Arrays[END_REF] and new modulation methods [START_REF] Asiminoaei | Reduction of Switching Losses in Active Power Filters With a New Generalized Discontinuous-PWM Strategy[END_REF].

Other very important trends especially in industry include modular and redundant systems for proper post fault operation, highly reliable and commercially available topologies [START_REF] Blaaabjerg | Power Electronics in Renewables -Status and Perspective[END_REF], enhanced energy storage technologies and the integration of photovoltaic plants into smart grids including additional functions such as grid support (V, f, Q control), fault ride through and harmonic compensation [START_REF] Blaaabjerg | Power Electronics in Renewables -Status and Perspective[END_REF][START_REF] Vezzini | Latest in PV Inverter & Trends[END_REF].

Fig. 1

 1 Fig. 1 Nominal Power (in MW) of PVGPs classified by the continent where they are

Fig. 2 .

 2 Fig. 2. Example of distribution grid where a Photovoltaic Generation Plant is connected.

Fig. 3 .

 3 Fig. 3. Time Association of Frequency Regulation and Operating Reserves after a Contingency Occurs

Fig. 4 .

 4 Fig. 4. General control structure of grid-connected PV plant.

Fig. 5 .

 5 Fig. 5. Simplified DC/AC PWM converter scheme to facilitate the discussion of control algorithms.

Fig. 6 .

 6 Fig. 6. Coordinate transformation of line current and line voltage from the stationary α-β coordinates to the rotating d-q coordinates.

Fig. 7 .

 7 Control System schemes: (a) VOC in the synchronous rotating coordinate system, (b) VOC in the stationary coordinate system, (c) Direct power control with the space vector modulator (DPC-SVM), (d) Model Predictive Control (MPC).

  These reference current signals in the stationary coordinate system are compared with the measured grid currents iα and iβ and the errors are delivered to the PR controller. The outputs of the PR controller are directly (without transformation) used to generate switching signals by a Space Vector Modulator (SVM).

  The commanded reactive power qref (set to zero for UPF operation) and active power pref values (delivered from the outer PI-DC voltage controller) are compared with the estimated q and p values, respectively. The errors are delivered to PI controllers to eliminate steady-state errors because they are DC quantities. The output signals from the PI controllers after transformation from the stationary to synchronous rotating coordinates are used to generate switching signals by a SVM.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Various structures of Hybrid ESS embedded in PVGPs: (a) AC-coupled, (b) DCcoupled, and (c) Mixed structure.Fig. 9 outlines the main required devices to design a DC-coupled Active PV

Fig. 11 .

 11 Fig. 11. Operation strategies: (a) Grid following strategy, (b) Source supplying strategy.

Fig. 12 .

 12 Fig. 12. Experimental performances of an active based generator.

Fig. 14 .

 14 Example of injecting the harmonic component: (a) harmonic injection turned OFF, and (b) harmonic injection turned ON. Curves from top to bottom: Phase-to-neutral grid voltage, Inverter current, Local load current, and Current flowing to the substation.

Fig. 15 .

 15 Controllers for synchronverters (a) the original version [54,56], and (b) the improved version without the need of a dedicated synchronization unit [62].

8 )

 8 according to the desired change of real power (torque ∆ ) with respect to the change of the frequency ∆ and the desired change of reactive power ∆# with respect to the change of the voltage ∆2. Note that the friction coefficient actually performs the function of frequency droop control so there is no need to introduce an extra loop to regulate the frequency, which simplifies the control structure and the associated parameter tuning process. The controller of a synchronverter shown in Fig. 15a has two channels. The upper channel has a cascaded structure, where the inner loop is the frequency loop (with the feedback gain Dp) and the outer loop is the real power loop (with the feedback coming from the current i via the torque Te). For a given time constant of the frequency loop 3 , the virtual moment of inertia J is 4 = 3 (This channel regulates the (imaginary) frequency of the generated voltage e and creates the phase angle θ for the control signal e. The lower channel has a cascaded structure, with the inner loop to regulate the voltage and the outer loop to regulate the reactive power. This channel regulates the field excitation to control the amplitude of the generated voltage. For a given time constant of the voltage loop 3 5 , the parameter K can be chosen as [57-59] 6 = 7 3 5 + ,

Table I -

 I Advantages and features of control schemes for DC/AC converter in PV

	applications
			VOC	VOC	DPC-SVM	MPC
			(Fig. 7a)	(Fig. 7b)	(Fig. 7c)	(Fig. 7d)
	Operation in stationary coordinate system		NO	YES	YES/NO	YES
	Power control -indirect		YES	YES	NO	YES
	Power control -direct		NO	NO	YES	NO
	Constant switching frequency		YES	YES	YES	NO
	Low algorithm complexity		NO	YES/NO	YES/NO	YES
	Low computation intensity		YES	YES	YES	NO
	Low sensitivity to line inductance variation		YES	YES	YES	NO
	Low sensitivity to line voltage distortion	THD of line current Power factor	NO YES	YES NO	YES NO	NO YES

Table II -

 II Examples of commercial high power DC/AC converters for PVGPs[START_REF] Abu-Rub | Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications[END_REF].

		PVS800-IS*	Inverters*	Prism	PVS 600 series
				Platform	to PVS2400*
				Equinox*,
	Power	up to 1,25MW up to 2MW*	up to 1,5MW	up to 1,25MW	up to 2,4MW	up to 1,6MW
	dc voltage range	525-825V 525-850V*	455-950V	525-825V	570-750V	583-820V
	Max. dc voltage	1000V 1100V*	1000V	1000V	820/1000V	1000/1100 V
	Output AC voltage	20kV 400V*	430V*	320V*	370V*	20kV
	Max. Efficiency	97.8% 98.8%*	98%*	98.5%*	98.6%*	98.2%
	Harmonic distorion	<3%	<3%	<3%	-	<3%
	Independent MPPT	2	-	2	4	2
	*-converter without MV separation transformer
	Parameter	ABB PVS800-MWS,	Danfoss Central PV	Satcon	Siemens Sinvert	SMA Sunny Central

Draft version, IEEE Industrial Electronics Magazine , March 2015, Vol. 9, Iss. 1, pp18 -32