Bruzek

A Marian

S Holé

N Lallouet

E Marzahn

C E Bruzek

Demonstration tests of a 320-kV-class DC superconducting cable for transmission of high powers

Demonstration tests of a 320-kV-class DC superconducting cable for transmission of high powers

Introduction

Best Paths, the largest research project in the field of energy financed under the European Union's 7th Framework Programme for Research, Technological Development and Demonstration, explored new avenues for delivering reliable power in Europe through the large-scale integration of renewable energy. The project ended in September 2018, having united 38 partners from 11 countries around five largescale demonstrations to validate the technical feasibility, costs, impacts and benefits of various grid technologies [1]. One of these five demonstration areas focused on validating high-voltage direct-current (HVDC) superconducting links capable of transporting large amounts of electricity -on the multi-gigawatt scale [START_REF] Ballarino | The BEST PATHS project on MgB 2 superconducting cables for very high power transmission[END_REF].

The high-power superconducting cable system sketched in Figure 1 was presented in a previous EIM article [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF], which summarized the main outcomes achieved in the first half of the project. The key components of the cable system, their conceptual design and specifications were first introduced in the article, followed by detailed presentations of the cable conductor consisting of superconducting magnesium diboride (MgB 2 ) wires and the novel high-voltage lapped insulation operating at cryogenic temperatures (-200°C).

Figure 1: Sketch of the cable system highlighting the key components. The superconducting conductor is maintained at about 20 K in the inner cryogenic envelope, whereas the electrical insulation is housed in the outer cryogenic envelope and maintained at liquid nitrogen temperature. Due to their large size, the electrical terminations used to provide the connection to the grid are not represented here.

Both polypropylene lapped paper (PPLP) and Kraft paper immersed in liquid nitrogen were tested as insulation materials under hydrostatic pressure up to 5 bar, in particular with regard to detecting and analysing the space-charge distributions in the insulator. Since charge effects are often thermally triggered, it was expected that few charge effects would be detected at such low temperature. As space charge build-up can lead to undesired effects in high-voltage insulation, including its breakdown, development of detailed testing procedures was mandatory for checking the superconducting cable design. While no significant difference was found between the two liquid-nitrogenimpregnated insulation materials, charge injection was revealed in both cases at the interface between the insulation and the carbon loaded paper electrodes. The detected charges were attributed to trapping by isolated carbon particles detached from the electrodes. At the same time, no accumulation of charge over time was measured in the bulk of the insulation for electric fields up to 30 kV/mm [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF].

Based on these promising results, a full-size cable system demonstrator was manufactured and tested in 2018, including two terminations for connecting it to the electricity grid. The testing program was inspired by recommendations and standards for HVDC cables and accessories. As a first demonstration of a complete cable system operating at 320 kV DC class, this constitutes an important step toward the new standard required for testing superconducting HVDC cables before their installation in the grid. This article reports on the research and testing activities from the final year of the project and is divided into three sections. The first section "Insulation assessment" presents new experimental evidence confirming the mechanism of charge injection and afterwards focuses on the insulation properties of Kraft paper. The second section entitled "New results on the cable system" includes a brief update on the superconducting wires and cable conductor, a presentation of the developed termination concept, and details of the HVDC testing protocol and demonstration tests. The last section preceding the conclusion is dedicated to considerations on long-length superconducting cable systems and challenges regarding their implementation.

Insulation assessment

The insulation between the energized conductor and ground is housed inside the outer cryogenic envelope of the cable, in liquid nitrogen. As already mentioned, lapped Kraft paper and PPLP were tested in a dedicated space-charge measurement setup by means of the pressure-wave-propagation method [START_REF] Holé | Non-destructive methods for space charge distribution measurements: What are the differences?[END_REF]. In the experiments, carbon loaded paper was used as a surface and electric field smoother between the metallic electrodes and the insulation. It was found that charges were rapidly injected under voltage at both carbon loaded paper electrodes, but the amount of injected charges was smaller in the case of Kraft paper insulation [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF].

In order to check the assumption that the injected charges accumulate at isolated carbon particles detached from the electrodes, the outer carbon loaded paper electrode was removed from the sample. As a consequence, the outer surface of the insulation was in direct contact through liquid nitrogen with the aluminium waveguide, which became the outer electrode of the sample. The inner electrode continued to be made out of carbon loaded paper. A photograph of the original sample is shown in Figure 2(a). The sample is 2-mm thick and the tested area is located in the middle, where the outer electrode is indicated. Note that the Kapton® tape at the right of the outer electrode serves just for holding the carbon loaded paper, and there is no other polymer in the sample. The lateral Kraft paper parts are used to avoid the flashover between the electrodes when a high voltage is applied. A sketch of the sample cross section in the tested area is presented in Figure 2(b). The cross section of the sample with the outer carbon paper electrode removed is shown in Figure 2(c), along with the direction and area of the measurement. Measurements were carried out first under short-circuit to check that no charges are present in the sample, then the sample was stressed under voltage at 40 kV and, finally, back to short-circuit to estimate the amount of charges injected under the voltage stress. Figures 2(d In Figure 2(d), the peak at the front electrode is much sharper in the absence of the carbon loaded paper electrode than in its presence (see for instance Figure 8 in [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF]). When the carbon loaded paper electrode is used, the pressure wave generated by the measurement system gets weakened upon entering the insulator and results in lower sensitivity and decreased spatial resolution of the measurement. Without the carbon loaded paper electrode, however, any charges at the outer interface should be particularly well detected thanks to the high sensitivity. In Figure 2(e), the signal at the outer electrode is clearly nil, in contrast to the inner electrode where a clear signal is detected due to the charges trapped in the isolated carbon particles. This experiment has therefore provided proof that the presence of carbon loaded paper is responsible for the charge injection. This is schematically illustrated in Figure 3. At short time scales, the presence of these isolated carbon particles holding charges is not dangerous. However, with time, the charged carbon particles may migrate under voltage stress, due to electrophoresis, and build up a space charge distribution detrimental to the insulation. The process can worsen if the insulation prevents the charges from easily moving, as in the case of PPLP where the carbon particles are more deeply trapped. As a consequence, the carbon loaded paper used to smooth the electric field close to metallic electrodes should be carefully chosen to prevent the formation of isolated particles at cryogenic temperatures.

As copper is foreseen to cover the carbon loaded paper in the final cable structure to increase outer electrode conductivity, it is worth testing the sample structure with copper directly as the outer electrode. Various arrangements of copper electrodes were used in place of the carbon loaded paper (under the aluminium contact) at the outer interface, as illustrated in Figure 4(a). The gap introduced between lapped copper tapes is necessary to allow for sufficient cable-bending capability. [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF]), the signal at the outer copper electrode is unipolar. This means that charges are present only on the electrode, but not inside the material. Their presence is due to the electric field generated in the insulation by the charges at the inner electrode. As a consequence, image charges appear on both electrodes, predominantly on the inner electrode, as the charges are located in its immediate proximity. Unlike the case of the aluminium electrode (see Figure 2(d)), the appearance of a peak for the copper electrode is owed to the presence of electrode edges at the gaps, which create a divergent field that generates a signal independently from the presence of charges [START_REF] Holé | Influence of divergent fields on space charge distribution measurements using elastic methods[END_REF]. As seen in Figure 4(d), the peak size increases with the gap opening, since the electric field becomes more divergent as the gap widens. This experiment shows that using directly metallic tape prevents charge injection, but generates divergent fields that may eventually be detrimental to the insulation. It can be concluded that carbon paper remains necessary, but a more compact carbon loaded paper that does not allow the formation of isolated particles at cryogenic temperatures should be employed.

Figure 4: (a) Various copper configurations on Kraft paper samples, (b) Typical signal obtained under voltage stress, (c) Typical signal obtained under short-circuit condition (directly after applying the voltage), (d) Comparison of the ratio between the peak amplitude under short-circuit and the peak amplitude under voltage for the various gap cases; note that in the case of carbon loaded paper, the amplitude of the ratio is not to scale at the outer electrode, as it actually exceeds 20%.

As voltage withstand is one of the major properties of insulation, the insulation thickness of the coaxial samples was reduced to 1 mm (8 layers of Kraft paper) in order to carry out space charge distribution measurements under larger electric fields, up to 60 kV/mm. The samples had metallic or carbon loaded paper as electrodes and were subjected to voltage from 10 kV up to 60 kV. The voltage was increased in steps of 5 kV every 15 minutes and was maintained at 60 kV for 5 hours. None of the samples broke down under these conditions. The amplitude of the outer electrode peak in the space charge distribution measurements is reported in Figure 5 as a function of the applied voltage for two samples, one with carbon loaded paper electrodes and the other with metallic electrodes. As the peak amplitude is proportional to the interface electric field inside the sample, a linear shape is a good indication of the absence of any charge effect. Indeed, charge injection (homocharges) would have bent down the shape by reducing the peak amplitude with voltage, and charge accumulation of opposite polarity (heterocharges) would have bent up the shape. Here the shape is clearly linear, thus no charge effects are detected during these experiments. Note that the slope with metallic electrodes is larger than the one with carbon loaded paper electrodes because the sensitivity is smaller in the latter case, as previously discussed.

Figure 5: Amplitude of the outer electrode peak in the pressure-pulse-propagation signal as a function of the applied voltage for a sample with metallic electrodes on both sides (blue circles), and for a sample with carbon loaded paper electrodes on both sides (green ovals). Insets: photos of defects existing in the tested samples, such as scratches (top) or pinches (bottom) during the tests.

These results have been repeated with various samples, some of which had small defects such as scratches (Figure 5 upper inset) or pinches (Figure 5 lower inset), but no significant differences were observed among the samples. In order to estimate the insulation resistance to larger defects, the Kraft paper was totally removed all around the sample over a length of 5 mm, thus leaving only liquid nitrogen between the electrodes in this gap. In that situation, breakdown appeared above 25 kV/mm indicating that below that voltage level the insulation was safe, even in presence of severe damages of the lapped paper.

New results on the cable system

Assembly and testing of the cable conductor

In addition to the advances in high-voltage insulation materials presented above, multiple other components have been designed, manufactured and validated in order to experimentally demonstrate the developed superconducting cable system. Moreover, to validate some of the more innovative concepts, specific testing strategies have been devised.

One such instance concerns the MgB 2 wires and cables, which are now ready for largescale deployment. A cable conductor with a diameter smaller than 1 cm has been designed based on 1 to 1.3 mm diameter MgB 2 wires. The characteristics of this cable conductor were reported elsewhere [START_REF] Bruzek | Cable conductor design for the high-power MgB 2 DC superconducting cable project of BEST PATHS[END_REF]. The operating current was set at 10 kA in the demonstration project, but this level is adjustable to grid requirements by simply adding or removing one or several wires.

For the future electricity grids, we have imagined installing hundreds of kilometres of HVDC cables, which will require a large quantity of superconducting wires. The MgB 2 strands represent a promising solution, as they are based on abundant and low-cost constituents. Consequently, special attention was paid to improving and validating the production capacity of large quantities of wires while maintaining their high-quality level. During the last phases of the project, the length of a single piece was constantly increased to finally reach 2 km, with reproducible level of the critical current (larger than 500 A @ 20 K and 1 T) and a limited variation of its outer diameter (lower than 2%). This was achieved not only by increasing the size of the starting billets, but also by improving the swaging tools employed in the last steps of the manufacturing process.

A 2 km piece length is well suited both for the industrial cabling process and for the grid installation requirements that can only accommodate a limited number of cable joints. As the wires also have to be handled, wound and assembled on industrial cabling machines (see Figure 6), they should be tolerant to complex multiaxial stresses. Consequently, in addition to standard tensile and bending laboratory testing, a test procedure has been devised that combines both these stresses and is more representative of the cabling processes. No critical current degradation was detected on the wires when simultaneously pulling them by 100 N and bending on 200 mm diameter. To validate the cabling process, critical current measurements were carried out on the full conductor as well as on extracted wires [START_REF] Tropeano | MgB 2 round wires and cables for the high-power superconducting demonstrator in the Best Paths project[END_REF]. At the end of the optimization activities, several hundreds of meters of cable conductor have been produced, demonstrating the high reliability of the MgB 2 wire and cable manufacturing processes, which are now ready for large-scale production.

High-voltage terminations

The main challenge in the development of cable terminations was to demonstrate the 320 kV voltage class in DC while accommodating the MgB 2 superconductor, which is promising for carrying ultrahigh current. Such terminations need to fulfil two functions: (i) secure the transmission of the high current and high voltage from the grid to the cable, and (ii) allow the transmission of pressurized cryogenic fluids from the cooling system to the cable. As the electric current coming from the grid is injected at ambient temperature while the MgB 2 cable conductor operates at 20 K, the termination includes a special electrical connection zone between resistive and superconducting materials. This connection zone is optimized to maintain low thermal conduction losses in the cryogenic fluids of the system despite the significant Joule losses from high current flow in the resistive part. The termination also manages the electric field stress between the high voltage at its top and the ground potential at its bottom.

The design of the termination is innovative, as it effectively separates the current and voltage functionalities, resulting in a strong modularity of this accessory. As shown in Figure 7, the termination is split into two independent parts: in the upper part the current is injected through a special lead connected to the cable conductor, while the high-voltage gradient is managed in the lower part. With this design, the performance of the superconducting cable system can easily be adapted to various voltage and current values, without any need for additional development work. For instance, if the operating current of the system changes, only the upper part of the termination has to be modified, whereas the lower part remains unaffected, and vice versa. Moreover, as in the cable design, the termination concept clearly separates the functions of the two cryogenic fluids. Thus, the gaseous helium at 20 K is exclusively used for cooling the MgB 2 material down to its superconducting state without any applied electric field. On the other hand, the liquid nitrogen at 70-80 K is employed for thermal screening and for electric field management of the high-voltage insulation. In this fashion, the termination also ensures the continuity of the modularity present in the cable, as the high flowing current is found only in gaseous helium and the electric field acts only in liquid nitrogen.

Figure 8 shows the two terminations connected to a 30-meter-long superconducting cable prototype, which was built using 27-mm-thick PPLP insulation and carbon paper electrodes around the metallic elements, as described in [START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF]. The assembly of the highvoltage testing loop was completed on the premises of Nexans Germany in Hanover at the end of the summer 2018. This testing loop aimed to not only validate all the proposed modular designs, but also the manufacturing processes of the dielectric components. Moreover, it constituted a demonstration case for the installation and connecting methods of the cable on a one-to-one scale.

The completed loop was slowly cooled down from room temperature to cryogenic temperature, reaching 76 K after five days. The liquid nitrogen pressure was set at 5 bar. No thermo-mechanical damage was found during the cool-down stage, confirming the tightness of the overall system for pressurized cryogenic fluids and vacuum. The loop was thus considered commissioned and ready for the HVDC testing program.

HVDC testing protocol and results

As no reference is available for testing HVDC superconducting cable systems, the testing program was inspired by the Cigré technical brochure no. 496 [START_REF][END_REF], which provides recommendations for testing DC extruded cables at voltages up to 500 kV. Another helpful reference was the IEC standard 62895 (ed. 2018) [START_REF]High voltage direct current (HVDC) power transmission -Cables with extruded insulation and their accessories for rated voltages up to 320 kV for land applications -Test methods and requirements[END_REF], which presents test methods and requirements for HVDC extruded cables and accessories up to 320 kV. Consequently, the protocol adopted in this project represents the first step towards developing a new standard for testing superconducting HVDC cables before their installation.

Apart from the pure DC high voltage tests, several superimposed impulse voltage tests were required for the loop validation, corresponding to the two different converter technologies in use (line-commutated converters and voltage-sourced converters). The tests aimed at simulating as thoroughly as possible the various grid events that can be encountered by a DC cable system. The testing protocol was therefore shared with and accepted by the transmission system operators who were partners of the Best Paths project. The test plan consisted of overvoltage steps (1.85 times the class voltage), of polarity reversals, and several sequences of superimposed switching and lightning impulses up to 680 kV, as detailed in Table 1.

It was decided to proceed with caution for this very new technology by first validating the cable for 200 kV HVDC class and then carrying out the test program for the 320 kV class (see Table 1).

During the DC voltage tests, the HVDC generator shown on the right-hand side of Figure 9 was connected to the cable system via a flexible metallic hose. To perform the superimposed impulse tests, the impulse voltage generator shown on the left-hand side of Figure 9 was added to the circuit. An uncoupling resistor R U and a surge capacitor C F acted as a filter protecting the HVDC generator from the signals of the impulse generator. A sphere gap (SG 2 ) was adjusted between the impulse generator and the testing loop to withstand the DC voltage and to trigger the impulses. Resistors R P and R S determine the shape of the impulses. The impulse parameters used for the high-voltage tests were chosen in accordance with the relevant IEC standard [START_REF]Impulse tests on cables and their accessories[END_REF] as follows:

1.

For the switching impulses, the peak value was reached in 260 µs and the duration to half the peak value was approximately 3000 µs. These values were within the required boundaries of 250 µs ± 50 µs and 2500 µs ± 1500 µs, respectively [START_REF]Impulse tests on cables and their accessories[END_REF].

2.

For the lightning impulses, the peak value was reached in 3 µs and the duration to half the peak value was 54 µs. These values were also within the required boundaries of < 5 µs and 50 µs ± 10 µs, respectively [START_REF]Impulse tests on cables and their accessories[END_REF].

Impulse voltage generator

HVDC generator Table 1: Demonstrator testing program including details of all the conducted voltage tests as well as sketches of the pulse shapes used [START_REF][END_REF].

S T V C R S C T R P C S D 1 C F R U C G D 2 S RT HT V R SG 2 SG 1
All the relevant system parameters that can impact the insulation properties of the liquid nitrogen such as its temperature, pressure and mass flow were carefully recorded during the test sequences. A typical example of the data recorded over a 24hour period is presented in Figure 10.

The cable system was maintained at cryogenic temperature (76 K) and under pressure (5 bar) during the 1.5 months required to complete the full testing program. Only the voltage was adjusted according to the test step. The values of the inlet and outlet temperatures, the pressure and mass flow were found to be very stable, as reported in Table 2. The cable system successfully passed all tests without any breakdown during the entire testing period, thus confirming the viability of this technology for both 200 kV and 320 kV HVDC systems. The latter is the first time that a superconducting cable system was demonstrated to operate at such a high DC voltage level, proving that the adopted modular design along with the manufacturing and assembling solutions secure feasibility, robustness, and practical usefulness. These innovations also open new possibilities to design superconducting DC cable systems for the future.

Outlook: Considerations on long-length systems

This project has demonstrated operation of gigawatt-scale superconducting cable systems on industrial test platforms, paving the way for realizing very long superconducting links. If the same cooling concept is maintained, some dimension adjustments for the cryogenic fluid systems will be necessary. The final design will represent a compromise between the three following requirements:

1. The pressure of both cooling fluids at any point along the cable should be kept above 5 bar to allow for sufficient insulation and cooling capacities. As a consequence, the pressure drop of the two cryogenic fluids needs to be kept at an acceptable level, below 15 bar. To this end, fluid dynamics laws require that the diameter of both the inner and outer cryogenic lines be significantly increased, by a factor of two to three compared to the demonstrator;

2. The thermal losses should be kept at a reasonable level, as well. To attain this, the cryogenic envelope should, however, be as small as possible;

3. Finally, the "flexible" cryogenic envelope needs to be transported on drums from the production plant to the installation site. This limits the outer diameter of the cryogenic envelope to not more than 30 cm.

These requirements still need to be thoroughly evaluated, along with additional challenges related, for instance, to elevation changes along the transmission path as well as qualifying field joints for both the cable conductor and the high-voltage insulation.

Apart from the testing activities and the elaboration of a long-distance concept, the project also assessed the environmental impact and economic viability of the cable system. Concerning the former, the advantages in terms of compact size and reduced footprint are clearly highlighted in Figure 11. As seen here, one pair of high-power superconducting cables has the same transmission capacity as eight resistive cables, leading to a reduction of the footprint by a factor of 10. As far as the economic viability is concerned, the small footprint of the superconducting link results in a reduction of the expenditure on engineering and right-of-way by a factor of 2.5. Our calculations for 6.4 GW resistive and superconducting links of 500 km length show that they are very similar in terms of overall capital costs. This assessment is, however, based on existing production lines for the cryogenic components and is expected to decrease, as more efficient production lines will be required to install links that are several hundreds of kilometers long. The length of 500 km was chosen according to the present vision for the future European grid [START_REF]Europe's future secure and sustainable electricity infrastructure[END_REF].

Conclusions and next steps

Within the Best Paths project, a new 3-gigawatt-class superconducting DC cable system was demonstrated, based on the MgB 2 superconductor. First of all, robust and reproducible manufacturing technologies for the superconducting wires and cables were developed and optimized at industrial scale.

Furthermore, a measuring system has been designed to test lapped insulations under high voltage at liquid nitrogen temperature. No charges were found in the bulk of the insulation up to a stress level of 30 kV/mm, but resilient charges were detected in the isolated conductive particles of the carbon loaded paper used to smooth the electric field between the metallic electrodes and the insulation. Hence, carbon paper with a compact structure preventing the formation of isolated particles at cryogenic temperatures should be used as an electric field smoother. The direct use of metallic stripes generates a divergent electric field at their edge, which is detrimental to the overall breakdown strength.

We have also shown that simple Kraft paper can withstand electric fields as high as 60 kV/mm without evidence of charge injection even in the presence of small imperfections. In the case of larger damages, the insulation can still withstand at least 25 kV/mm thanks to the liquid nitrogen breakdown strength. In future experiments, it is envisaged to implement Kraft paper insulation instead of the more expensive and less permeable PPLP for the cable system and subsequently confirm its promising properties during HVDC tests.

Lastly, an innovative design was devised for the electrical terminations, which separates the current and voltage functionalities and thus leads to a strong modularity of this component. Hence, no new development work will be required in the future to configure the system for different operating voltage and current levels.

Upon completion, the cable system was subjected to an HVDC testing program for the 200 kV and 320 kV voltage levels, successfully passing them both. This represents the first instance of a superconducting cable system operating in the 320 kV DC voltage class.

An important next step will be to develop testing guidelines for HVDC superconducting cables to guarantee safety and quality standards. The knowledge gained within the project has already been integrated into the activities of Cigré Working Group D1.64 (Cryogenic electrical insulation) and various standardization technical committees, paving the way for the elaboration of relevant standards.

Finally, only the operation of such a high-power cable in real-life grid conditions will confirm the potential of this technology in a definitive manner. In this respect, the involved partners recommended at the end of the project that appropriate de-risking instruments for the transmission system operators be put in place within the framework of European energy-climate policies.

  ) and 2(e) respectively show the signals obtained under voltage and under short-circuit directly after the voltage stress.

Figure 2 :

 2 Figure 2: (a) Photo and (b) cross section of the sample before removing the outer carbon loaded paper electrode. (c) Cross section of the sample after removing the outer carbon loaded electrode. The green arrow indicates the area and direction of the measurement. (d) Measurement under voltage (e) Measurement under short-circuit directly after applying the voltage.

Figure 3 :

 3 Figure 3: Charges on sample electrodes (a) under applied voltage, negative charges enter from the outer aluminium electrode and leave at the carbon loaded paper electrode, leaving even the isolated particles by small discharges (b) back to short-circuit, the charges on the electrodes are neutralized, except for those on the isolated particles, which results in accumulation of negative charges on the carbon loaded electrode and residual positive charges on the isolated carbon particles.

Figures 4 (

 4 Figures 4(b) and 4(c) show typical signals obtained under voltage stress and under short-circuit conditions. Compared to the sample with two carbon loaded paper electrodes (see for instance Figure 10(d) in[START_REF] Marian | Validation of the superconducting and insulating components of a high-power HVDC cable[END_REF]), the signal at the outer copper electrode is unipolar. This means that charges are present only on the electrode, but not inside the material. Their presence is due to the electric field generated in the insulation by the charges at the inner electrode. As a consequence, image charges appear on both electrodes, predominantly on the inner electrode, as the charges are located in its immediate proximity. Unlike the case of the aluminium electrode (see Figure2(d)), the appearance of a peak for the copper electrode is owed to the presence of electrode edges at the gaps, which create a divergent field that generates a signal independently from the presence of charges[START_REF] Holé | Influence of divergent fields on space charge distribution measurements using elastic methods[END_REF]. As seen in Figure4(d), the peak size increases with the gap opening, since the electric field becomes more divergent as the gap widens. This experiment shows that using directly metallic tape prevents charge injection, but generates divergent fields that may eventually be detrimental to the insulation. It can be concluded that carbon paper remains necessary, but a more compact carbon

Figure 6 :

 6 Figure 6: Photograph of the industrial cabling machine used to manufacture the cable conductor tested in the project.

Figure 7 :

 7 Figure 7: Conceptual design of the high-voltage termination showing its main components and highlighting its modular functionality.

Figure 8 :

 8 Figure 8: Final testing loop together with the 800 kV DC generator.

Figure 9 :

 9 Figure 9: Schematic of the electric circuit for the superimposed DC and impulse voltage tests.

  kV DC and 10 shots at +425 kV Negative: -200 kV DC and 10 shots at -425 kV Opposite polarity: Positive: -200 kV DC and 10 shots at +241 kV Negative: +200 kV DC and 10 shots at -241 kV Same polarity: Positive: +320 kV DC and 10 shots at +680 kV Negative: -320 kV DC and 10 shots at -680 kV Opposite polarity: Positive: -320 kV DC and 10 shots at +385 kV Negative: +320 kV DC and 10 shots at -385 kV 10 hours under DC voltage before impulse sequence Superimposed lightning impulse Opposite polarity: Positive: -200 kV DC and 10 shots at +425 kV Negative: +200 kV DC and 10 shots at -425 kV Opposite polarity: Positive: -320 kV DC and 10 shots at +680 kV Negative: +320 kV DC and 10 shots at -680 kV 10 hours under DC voltage before impulse sequence

Figure 10 :

 10 Figure 10: Cooling system data recorded during superimposed DC and switching impulse sequence at +320 kV DC.

Figure 11 :

 11 Figure 11: Comparison of the environmental footprint of resistive XLPE cables and MgB 2 superconducting cables for a 6.4 GW DC power link.

Table 2 :

 2 Liquid nitrogen data during the testing program

Acknowledgements

We are very grateful to all the DEMO 5 partners who contributed over the years to the success of the project. This work was supported in part by the European Commission within the Seventh Research Framework Programme under Grant 612748. The Institute for Advanced Sustainability Studies is funded by the German Federal Ministry of Education and Research (BMBF) and the Brandenburg State Ministry of Science, Research and Culture (MWFK).

Nicolas Lallouet has 20 years of experience in superconductivity at Nexans. Since 2003 he has been responsible for the development of high-voltage and medium-voltage accessories for superconducting cable systems. He is also in charge of developing HVDC cable system accessories for standard extruded cables. Previously, he was in charge of developing high-temperature superconducting tapes within Nexans. He has about 20 publications and 27 patents in the field of superconductivity. E-mail address: nicolas.lallouet@nexans.com Erik Marzahn studied electrical engineering at the Technical University in Braunschweig, Germany. He obtained his PhD at the Institute for Electrical Energy Systems of TU Braunschweig on the topic of fault current limiting behavior of arcs in circuit breakers. In 2004 he started working at Nexans Deutschland in Hannover, focusing on the development of superconducting cables and fault current limiters. He is responsible for the high-voltage field test and all measurements related to superconductivity. E-mail address: erik.marzahn@nexans.com Christian-eric Bruzek received a PHD and engineering diploma in material sciences at Lille University, France.

He worked at ALSTOM in superconducting department as Technical manager before joining Nexans as manager of "HTS tapes" division. Then he directed the Material division within the LNE testing laboratory. Today he is project director at Nexans. He is chairing the International Superconductivity Standardization Committee. His current interests are super grids but also high current cables for industrial applications. E-mail address: christian_eric.bruzek@nexans.com


ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Transnational evidence-gathering and the extraterritorial application of criminal procedure

Prof. Laurent Desessard, Poitiers University (France)

1.1. Principle locus regit actum. The locus regit actum principle conventionally governs the execution of mutual assistance requests whom the judicial authorities of a State refer when they need to collect evidence in the territory of another State.

1.2. Formulation. In Europe, this principle is formulated by the European Convention on Mutual Assistance in Criminal Matters of 20 April 1959. Article 3 provides that "the requested Party shall execute, in the manner provided for by its law any letters rogatory relating to a criminal matter and addressed to it by the judicial authorities of the requesting Party (…)".

Where the requested act consists of the hearing of a witness or an expert, the 1959 Convention, however, allows the requesting Party to ask the requested Party that the witness or expert file under oath. In this case, the requested Party must comply with the request if the law of its country does not prohibit it. This rule can then lead to extraterritorial application of a criminal procedure : the requested State may have to apply a rule which is not provided by its domestic law, but which is imposed by the criminal procedure of the requesting State. However, this derogation from the locus regit actum principle has a very limited scope of application. The oath, like the hearing, must then be administered in accordance with the rules of the requested Party.

Justifications.

Respect for the criminal sovereignty of States is the main justification given to the locus regit actum principle. But practical considerations also justify it, in connection with another principle of execution of requests for mutual assistance. It is a matter of principle that the acts requested in this context may be executed only by the competent authorities of the requested State. Police authorities and the judicial authorities of one State can't indeed perform acts in the territory of another State.

However, this second principle of execution by local authorities reinforces, in a certain way, the locus regis actum principle. The competent authorities of the requested State are in a better position to apply their own criminal procedure -a criminal procedure which they know -than the one applicable in the requesting State -a criminal procedure which they are not used to.

1.4. Disadvantages. The execution of a request for assistance by the competent authorities of the requested State and according to the law of that State presents, however, disadvantages.

Relevance of investigations.

The execution of a request for assistance by the competent authorities of the requested State may affect the relevance of the investigations requested. For example, when a request for assistance is for the purpose of hearing a person, the authorities of the requested State do not know the file and therefore may not ask all the questions that would be useful to the judicial authority of the requesting State.

To avoid such disadvantages, article 4 of the 1959 Convention allows the authorities of the requesting State to be present at the execution of the request. Such presence implies, however, an express request in this sense and remains subject to the goodwill of the authorities of the requested State. If the requested State consents to the presence of the authorities of the requesting State, it must be seen, moreover, that these authorities may only to be present at the execution of the request, because only the local authorities can perform the requested act. This presence may nevertheless be particularly useful in making investigations relevant. In our example of a request for hearing, it can be imagined that the authorities of the requesting State may then inform the authorities of the requested State the questions to be asked to the auditioned person.

Value of investigations.

The execution of a request for assistance in accordance with the procedural rules of the requested State may also have disadvantages. It is quite possible that a mutual assistance act carried out in accordance with the law of the requested State has a limited probative value or even is inadmissible in the requesting State, from the time when that act was not carried out in accordance with the procedural rules under the law of that State. This is why modern instruments on mutual legal assistance in criminal matters tend to question the locus regit actum principle, particularly within the European Union.

Forum regit actum.

In order to give investigations carried out in the territory of another State than the State of prosecution a full probative value in that State, the European Union has indeed wanted to make the rule forum regit actum the principle of execution of requests for mutual assistance within the Union.

Formulation. This rule was originally formulated by the Convention of 29 May 2000 on

Mutual Assistance in Criminal Matters between the Member States of the European Union. Article 4 of the Convention provides that "Where mutual assistance is afforded, the requested Member State shall comply with the formalities and procedures expressly indicated by the requesting Member State". In the 2000 Convention, the requested State must therefore apply the law of the Forum and not its own law, in order to, indicates its explanatory report, "facilitate the use of the information gathered by mutual assistance as evidence in the subsequent proceedings in the requesting Member State". A report which further states that compliance with the formalities and procedures indicated by the requesting Member State also implies for the requested Member State to authorise a representative of the judicial authorities of the requesting State or a representative of the defence to attend the testimony of a witness, in order to ensure the relevance of the investigations carried out.

Today, the rule forum regit actum is formulated by the directive of 3 April 2014 regarding the European Investigation Order in criminal matters. The European Investigation Order is the instrument to which the judicial authorities of a Member State of the European Union must now resort to,when they want to carry out an investigative measure in another Member State to obtain evidence. However, in order to guarantee the value of the evidence thus collected, article 9 (2) of that directive provides that "the executing authority [of a European Investigation Ordre must] comply with the formalities and procedures expressly indicated by the issuing authority". And, in order to ensure the relevance of the investigations carried out, the same article allows, in paragraph 4, the authorities of the issuing State to be present during the execution of the European Investigation Order.

Refusal of application.

The application of the law of the forum by the executing authorities of a European Investigation Order may, however, be refused in certain cases. Article 9, paragraph 2, of the 2014 directive specifies that "The executing authority shall comply with the formalities and procedures expressly indicated by the issuing authority (…) provided that such formalities and procedures are not contrary to the fundamental principles of law of the executing State". Fundamental principles may also lead the executing authority to refuse to allow a representative of the issuing authority to attend the execution of the investigation order, as pointed out in paragraph 4 of this article 9.

Difficulties.

The application of these provisions is not without problems. The main question is when the executing authority of a European Investigation Order may refuse to apply the formalities and procedures indicated by the issuing authority, on behalf of the fundamental principles of its State.

Determination of the fundamental principles of the executing State.

Article 9 of the 2014 Directive, as previously Article 4 of the 2000 Convention, refers to the fundamental principles of the law of the executing State, without further precision. An imprecision may lead an executing authority, unwilling to cooperate, to refuse to apply the formalities and procedures indicated by the issuing authority when they are not requested by its own law.

The French transposition provisions of the European Investigation Order are more specific on this point. Article 694-36 of the Code of Criminal Procedure allows the French authorities to refuse to carry out the formalities and procedures indicated by the issuing authority if these rules reduce "the rights of the parties and the procedural safeguards applying the fundamental principles provided for in the preliminary article" of the same Code. In the context of a European Investigation Order, the French authorities may therefore refuse to apply the procedural rules of the issuing State only if they infringe the rights of the parties and the procedural safeguards resulting from the fundamental principles laid down in this preliminary article. Which is however wide. The preliminary article of the Code of Criminal Procedure makes very numerous principles, such as the fairness and adversarial nature of the proceedings or the observance of the presumption of innocence. So many principles that are roughly those formulated by article 6 of the European Convention on Human Rights. Thus, by allowing the French authorities to refuse to execute the procedural rules of the issuing State which would be contrary to the fundamental principles formulated by the preliminary article of its Code of Criminal Procedure, article 694-36 allows the French authorities to refuse to apply a criminal proceedings which would be contrary to the European Convention on Human Rights.

Assessment of the quality of the criminal procedure of the issuing State.

The provisions of the directive of 2014 on the application of the law of the Forum therefore raise another difficulty, which can lead the authorities of the executing state of a European Investigation Order to assess the quality of the criminal procedure of another State. One can indeed ask whether it is for the executing authority of a European Investigation Order to make such an assessment. If the criminal procedure of the issuing State does not conform to the fundamental principles of the European Convention on Human Rights, it may not be for a foreign judicial authority to say so, even implicitly by refusing to execute a European Investigation Order. Such an assessment could even lead to paradoxical situations where an executing authority would refuse the application of a procedural rule validated by the European Court of Human Rights in the name of the fundamental principles laid down by the European Convention on Human Rights. 4.1. Solutions. It may therefore be asked whether the forum regit actum rule is the solution to the difficulties of gathering evidence in the territory of another State that the prosecuting State. This rule is based on the assumption that there are significant divergences between the criminal procedures of States. In order to prevent these divergences rendering inadmissible or reducing the value of the evidence collected in the territory of another State, the authorities of that State are asked to apply another procedure than his own. Such an application may appear more appropriate if the criminal procedure of the executing State is similar to that of the issuing State or, at the very least, respects common principles.

It is to such a rapprochement that we have been witnessing in the European Union since the entry into force of the Lisbon Treaty. In order to facilitate mutual recognition of judgments and judicial decisions -the principle on which the European Investigation Order is based -article 82 (2) Treaty on the Functioning of the European Union allows the Union to establish minimum rules concerning the rights of individuals in criminal procedure. And several directives have been adopted since by the European Union -on the right to interpretation and translation (2010), on the right to information (2012), on the right of access to a lawyer (2013), or on the presumption of innocence (2016) -which participate in the establishment of a common base of procedural rights and safeguards within the European Union. A common base which may therefore render moot the provisions which allow the executing authority of a European Investigation Order to refuse to apply the formalities and procedures indicated by the issuing authority in the name of its fundamental principles. A common base which may also render unnecessary the execution of a European Investigation Order in application of the procedural rules of the issuing State, in other words to render unnecessary the extraterritorial application of the criminal procedure in transnational evidence-gathering.