FIELD DEMONSTRATION OF WORLD WIDE LARGEST SUPERCONDUCTING FAULT CURRENT LIMITER AND NOVEL CONCEPTS

FIELD DEMONSTRATION OF WORLD WIDE LARGEST SUPERCONDUCTING FAULT CURRENT LIMITER AND NOVEL CONCEPTS J. Bock*, S. Elschner, F. Breuer, H. Walter, M. Noe, M. Kleimaier, R. Kreutz, K.H. Weck, X. Yuan, K. Tekletsadik *Nexans SuperConductors GmbH, Hürth Germany ABSTRACT Within the German project CURL 10 a full scale threephase resistive current limiter was developed and successfully tested up to the nominal voltage and power (10 kv, 10 MVA). This is up today the largest HTS (high temperature superconducting) current limiter world wide. The device is based on bifilar coils of MCP-BSCCO 2212 bulk material, protected with a normal conducting shunt, and operates at T = 66 K. Since April 2004 the demonstrator has been installed within a field test in the grid of RWE. Although technically feasible, an upscale of the same concept to the 110 kv level seems not to be economically viable. Therefore very promising novel concepts based on a magnetically switching of superconducting properties are actually under development and presented in this contribution. INTRODUCTION Superconducting fault current limiters (SFCL) are one of the most exciting potential applications of superconductors in the electrical grids of the future. Already the pioneering work of Alcatel [1] on the basis of low Tc material has shown that a device based on superconductors combines the advantages of vanishing resistance in the normal operating state, and self triggered resistance with current limitation in the case of fault current events. The discovery of high T c material drastically increased the chances to develop a system which also meets the economic requirements. Consequently, a number of physical concepts based on various superconducting materials were developed and tested at different scales. However, in the last five years the resistive concepts [2,3] seem to have the most promising prospects for technical and economical viability. APPLICATION CASES Future networks will be characterised by increased application of power electronics and by enlarged number of dispersed generation facilities. To ensure a satisfactory power quality a sufficient short circuit power has to be assured. Normally, these efforts are limited by the short circuit strength of the substations involved. If a high short circuit power can be achieved in the network, the connection of powerful consumers and a reduction of system perturbation by power electronics can be realised. Additional short circuit current contributions by dispersed generation facilities often lead to an exceeding of the short circuit strength of the substation in question and to an insufficient selectivity of the protection devices. An effective short circuit current limitation can solve this problem. There is a number of possible application cases for superconducting fault current limiters [4], as a connection in series with a low impedance transformer, bus bar couplings in the medium voltage (MV) range or direct connection of distributed generation and wind turbines to the MV grid. A particular interesting example, part of the RWE-grid in Germany, is shown in Fig. 1. A 110 kv system is subdivided into different sub-grids to cope with a rated shortcircuit current of 31.5 ka. Today, in order to fulfil the (n-1)-criterion and to take into account the outage of one transformer two feedings have to be foreseen for each sub grid. Nordl Thyssen SFCL Bp Rosend Karnap Fig.1: Two 110 kv sub grids fed by 380/110 kv, situation of today and coupled via SFCL. The redundancy of transformer capacity can considerably be reduced by coupling the two 110 kv sub-grids and by installing SFCLs into the bus coupler bay of the two 110 kv stations in question (Fig.1). In case of a transformer outage in one sub-grid the other sub-grid is able to deliver the reserve power and in case of a short-circuit the SFCL limits the short-circuit current to admissible values. Assuming a short circuit in sub-grid Bp the full shortcircuit current occurs in Bp, whereas in sub-grid Ge the short-current is limited by the SFCL. Due to the resistive behaviour of the SFCL in the non-conductive state a phase shift appears. Thus the superposition of the unlimited shortcircuit current of sub-grid Bp and the limited short-circuit current of sub-grid Ge leads to a maximum short-circuit current only a little bit higher than that of sub-grid Bp. For this application cost savings for a 380/110 kv transformer and for two transformer feeders ( and 110 kv) can be obtained. Thus the use of a SFCL makes available a very economical solution. Ge Huellen Gelsenb Eiberg

THE PROJECT CURL 10 The consortium The German government funded fault current limiter project CURL 10 aimed at the development and construction of a three phase 10 MVA demonstrator for the distribution level (10 kv). The consortium of partners from industry and universities covered all relevant domains from material development up to the field test. Partners and roles within the consortium were as follows: The two largest German utilities, RWE and E.ON defined a set of specifications, organised the preliminary tests at FGH Mannheim and installed the demonstrator in the grid, Nexans SuperConductors (NSC) was responsible for the development and manufacturing of the superconducting components (BSCCO 2212). ATZ Adelwitz developed components with respect to an alternative material option YBCO. Forschungszentrum Karlsruhe supported the component development by testing and characterising material and single components and contributed to the important electrical insulation. ACCESS and EUS did simulation work concerning the system and the components, ACCEL (project coordinator) was responsible for the cryostat, the cooling device and system integration. The project started in Nov. 1999 and was accomplished with the successful installation of the demonstrator for a field test in the grid of RWE in April 2004. The superconducting material Initially two different superconducting material options were considered, melt textured polycrystalline YBCO and melt cast processed MCP-BSCCO 2212. However, although YBCO showed excellent limiting properties, the weak link issues could not be overcome, and this option had to be abandoned. The Melt Cast Process (MCP) of BSCCO 2212 is an optimised and well established succession of melting, centrifugal casting, heat treatment and oxygenation processes and has been described in detail [5]. It allows the production of superconducting tubes with diameters between 20 and 200 mm and lengths up to 50 cm with integrated Ag contacts. Within the limiter project we started with 10cm long rods (diameter 5mm). In a large number of successive steps, the process could be adapted with respect to a manufacturing of the finally used bifilar coils. The superconducting components The device is based on bifilar coils of MCP-BSCCO 2212 bulk material (length: 30cm, outer diameter: 50 mm). With a superconducting cross section of 6 x 4 mm 2, the resulting superconducting length per component is about 540 cm. The critical current density at T=66K was 3600 A/cm². The manufacturing of the bifilar coils is a multistep procedure, which includes material, mechanical and electrical issues and is described in detail in [6]. Fig. 2: MCP- BSCCO 2212 superconducting bifilar coil with contacts and insulation The central problem of the component development were the inevitable small inhomogeneities of the critical current density causing locally different heating rates. As a consequence small parts of the material are already in the normal conducting state while the system as a whole remains in the current driven regime. The unavoidable avalanche type heating leads to a subsequent burning out (hot spot). As a protection against hot spot formation the thermal and electrical stabilisation is based on a metallic shunt contacted continuously to the superconductor. Its low resistivity in the order of 40µOhmcm limits the applicable electrical field and defines a lower limit for the overall superconducting length. The shunt protected components allow an electrical field of up to 0.6 V/cm. The technical lay out As a consequence of this maximum electrical field an overall superconducting length of about 150 m per phase is required for each phase of the 10 kv-limiter. To obtain this long length 30 components per phase are connected in series in order to obtain the required resistance. The bifilar coils are mounted on a FRP-base and joined with Cu-braid connectors. The six current leads are a commercially available product and yield a total thermal input of about 150 W. The AC-losses according to sect. IV were extrapolated to a maximum load of 1500 W. The operation temperature of 66 K is maintained by two Stirling-machines with a total power input of 24 kw, only 0.2 % of the rated power. The main reason for the choice of this temperature, somewhat lower than boiling nitrogen, is the strongly enhanced critical current density of the superconducting material.. For the high voltage insulation the lower pressure, due to Paschens law, is a strong disadvantage. High voltage tests For the first time world-wide, also the withstanding of the concept with respect to high voltage lightning events was tested up to 75 kv. A well defined high voltage peak impulse, which imitates a typical lightning event to a

Fig 3: Impulse transient voltage simulating a lightning event Fig.4: three-phase limitation test of SFCL, prospective current: 18kApeak 8 23 kv peak 1 µs/div Current (ka) 6 4 2 0-2 -4 i L1 (ka) i L2 (ka) i L3 (ka) 6.7 kv/div -6 distribution line during a limitation case, was applied by controlled discharge of a capacitor through a resistor [7]. This event could be triggered at an arbitrary time with respect to the short circuit current and was set on the 4th maximum of the fault current. The ramp of the high voltage peak was set to 0.1 µs up to a prospective value of 75kV. The ramp propagates along the superconductor according to the wave resistance of the component. Due to the bifilar design the conductor length between both contacts of each component is about 5m and the ramp creates a voltage up to 10 kv between the first and last winding and both contacts. This definitely is a weakness of the bifilar geometry. The multicomponent test was performed with nine elements because this is the minimum number which makes sure, that the full voltage is built up between the current leads of the set up, as in the planned prototype. Definitely no discharge and no voltage breakdown was observed. This confirms, that the insulation concept of current leads and components [10] is appropriate. To our knowledge the electrical stability of a SFCL against lightning surges has been demonstrated for the first time. Preliminary tests The electric insulation with respect to the nominal voltage between the grounded cryostat and the different phases as well as between the phases was tested at room temperature and at 66 K, i.e. at low pressure, up to a value of 28 kv. In a series of preliminary small scale tests at different temperatures reliable current limitation could be demonstrated in the full range of prospective fault currents, also in the particular dangerous small load regime. The mounted demonstrator was tested at FGH Mannheim. Although the variations of critical currents between components was small, the 90 elements were grouped in three sections according to their critical current density, i.e. the thirty best and so on. This further improves the homogeneity within each phase. Since in all possible short circuit cases (one phase, three phases etc.) the different phases see different currents, this approach is justified. In the main three-phase test, the phases of the current limiter were with one pole connected to the phases L1, L2, L3 of the test facility. The other poles were connected to -8 Time (ms) each other and grounded (Y-connection). The short circuit time was 60 ms as specified. An experiment, for comparison without current limiter, showed the three prospective currents of 10kA, 17kA, 18kA peak for the three phases. Fig. 4 shows the transients for the three phases with current limiter. The currents in the first peak did not exceed 7.2 ka, which is considerably less than the specified 8.75 ka. In the last half period a current of 2.1 ka rms at a voltage of 6.7 kv rms was measured for the three phases. This experiment demonstrated the expected limitation of current and the reliable protection of a load of 10 MVA, until today the world wide highest level. However, after these tests the AC-losses as well as the overall DC resistance of the whole system were slightly enhanced. As verified after the warming up, this is due to a degradation of one or two samples in each phase. There are strong hints that an improvement of the soldering procedure will avoid this type of degradation in the future. As a side effect it was proven, that the shunt concept clearly avoids any arcing and thus remains safe also in the case of damaged superconductors. In fact, this type of small damage did not all spoil the functionality of the device, yet slightly increased the AC-losses. Since the remaining 28 samples in each phase definitely had seen the full load without any degradation, the utilities within the consortium agreed to simply bridge the broken components before installing the device in the field test. The field test 60 70 80 90 100 110 120 In April 2004 the prototype was installed for a long term field test in the grid of RWE in Netphen, Germany [8]. The limiter is in a location where it couples two 10kV/15MVA grids in a bus tie in order to level out asymmetric loads. The normal operation current usually is small (<200 A) but can reach values up to 600 A. At this special location the withstanding power with respect to short circuits is 125 MVA and the short circuit current has to be limited to an Ik =7kA rms. Limited short circuit currents between 2.75 ka rms and 7 ka rms are expected dependent on the location

of the short circuit event. The maximum prospective current of 14 ka rms is expected if the short circuit is near the bus tie or in the limiter itself. The acceptable effective currents on the different time scales are 8.7 ka p (<3.4 ms) and 5kA rms (<0.5s). The limitation time is specified with 60ms. After this time a conventional breaker opens the circuit in a zero transition of current. This breaker also operates in the cases of small loads not sufficient to heat up the system fast enough. The fault current limiter meets all the requirements of the European norm IEC 50056. After an optimisation phase of the cooling system the limiter now works reliably. Unfortunately up to now no short circuit could be recorded. It is planned to keep the limiter in place until the system has proven its limiting behaviour. After this operation in the field a further thorough testing at FGH in Mannheim is foreseen. Fig. 5: field dependence of critical current density MAGNETICALLY TRIGGERED CONCEPTS However, although the system seems to meet all the requirements with respect to the distribution level, the desired upgrading for the high voltages of the transmission level is not straightforward. The bifilar geometry is critical with respect to high voltages between adjacent windings near the current feeds of each component. Moreover, the concept of a normal conducting shunt in parallel to the superconductor limits the maximum electric field in the short circuit case to a value below 1V/cm. Higher fields would heat up the shunt to unacceptable temperatures. This implies superconducting lengths of more than 1000m. Therefore a completely new concept for the protection of the superconductor against hot spots is currently under development. Since a metallic shunt is completely abandoned, electric fields up to 10V/cm make possible to fully exploit the industry properties of the superconductor and the full potential of the material. Field dependence of jc The new concept is based on the strong dependence of the critical current density of BSCCO 2212 from an external magnetic field. As shown in Fig. 5 the critical current density is strongly affected by a magnetic field. This effect can be used to trigger a homogeneous quench all over the length of the superconductor if in the fault event a part of the fault current is shunted on a coil wound around the superconducting tube. The major improvement of this concept is the higher E- field in comparison with the bifilar coils. No metallic shunt is needed and only the BSCCO-2212 material with its high resistivity and the parallel connected trigger coil in the normal conducting state is connected to the circuit, which results in low heating rates during short circuit conditions. The E-field can be increased by a factor of ten to create the same heating as in a bifilar coil. To decide which amount of magnetic field is needed for a homogeneous quench it is useful to compare with the magnetic self field. A prospective short circuit current of 31.4 ka as discussed in the example (see above) creates a self-field of 0.6T. The additional trigger-field probably has to have a similar magnitude. Actually different concepts are studied how to induce the desired current into the field coil. In principle there are three possibilities: The field coils can be connected in series, in parallel or an externally driven source can be used. The latter solution has the disadvantage that it is not self triggering and therefore not fail safe. The trigger matrix concept In the frame of a large US-DOE project IGC SuperPower develops this new concept (MFCL) with respect to an application at 138 kv. The trigger concept is basically serial with a sophisticated arrangement of trigger and limiting elements. It is described in detail elsewhere [9]. NSC develops and supplies the HTS elements based on its MCP-BSCCO 2212 customized for this concept. The MFCL-limiter is based on superconducting tubes connected in series. It only needs 10% of the superconducting length in comparison to the bifilar coil concept. This cuts off AC losses by more than 90%, saves volume and significantly reduces the costs. In the new design the superconducting ceramic is not packed and is in direct contact with the liquid nitrogen bath with its inner and outer surface. Thus, and because the wall thickness of the HTS tubes is reduces, the recooling time is significantly shortened. The MFCL was tested in two stages. A scaled mock-up of the device was initially tested in late 2003 at the Center for Advanced Power Systems (CAPS) at Florida State University. The CAPS testing demonstrated the excellent material performance, and consistently achieved 6Vpeak/cm electric field strength on the material. The CAPS test also validated the desired dynamic characteristics of the matrix concept, i.e. equal current sharing, no interference from magnetic fields of adjacent elements and rapid HTS dynamic resistance development and current transfer to the current limiting impedance. After the CAPS tests, a single-phase pre-prototype device was fabricated and testing of the MFCL was conducted at

Characteristics Bifilar Coil MFCL E-Field 0.6 Vpeak/cm 6Vpeak/cm Current @66K 600A 2500A 300mm Tube 212Vrms 180Vrms Re-cooling > 30sec < 10sec AC Losses 100% < 10% Volume Per 1000cm 3 200cm 3 Element Power Per Element 130 kva 450 kva Tab. 1: Comparison of superconducting components of both concepts, electrically shunted (CURL 10) and magnetically triggered (MFCL). KEMA PowerTest, which is the largest short circuit test facility in the United States. The test system consists of the short circuit power source capable of providing a prospective symmetrical rms current of 10kA (asymmetrical peak fault current around 25kA), at voltages up to 8660Vrms. The supply has the capability to vary both voltage and system source impedance to control the voltage and short circuit current of the MFCL assembly. Pre-Prototype - 36 HTS elements at 4160 V, with peak prospective currents of 23.4 ka, first peak limited to 19.4 ka ( 82.9 % of peak prospective current) 25 17.5 23.4 ka Prospective Current Current [ka] Voltage across MFCL [kv] 20 Limited to 19.4 ka by 15.0 MFCL 15 10 5 0-5 -10 Voltage Across MFCL Circuit Breaker Opens -15-2.5 0 10 20 30 40 50 60 70 80 Time [ms] Fig.6: magnetic field triggered current limitation with an assembly of 36 superconducting components Pre-qualification tests of the matrix assembly were conducted at 480Vrms and 2400Vrms in the open LN2 bath. The main purpose of this test was to qualify the matrix assembly before it was installed in the cryostat by ensuring similar results to that achieved at CAPS. The first set of tests at 480VAC was conducted with a partially populated matrix assembly of six HTS elements out of 36 possible elements. Twenty-two faults were applied with fault durations ranging from.5 to 3 cycles and at prospective fault levels ranging from 20kA to 27kA asymmetrical first peak. This was followed by a series of tests with a fully populated matrix of 36 elements. Fig. 6 shows an example waveform from these tests. It shows current limiting before the first peak of the prospective current and shows significant current limiting in effect by the time of the 3rd cycle. 12.5 10.0 7.5 5.0 2.5 0.0 The parallel concept Another field based concept has been proposed by Nexans SuperConductors. A coil is wound around and directly connected in parallel to the superconducting tube. In the beginning of the fault event a small voltage occurs between the ends of the coil and causes current flow. The occurring magnetic field destroys the superconducting properties and in an avalanche type process, the tube quenches. This concept has so far been tested in the lab scale on single components and it has been shown, that all type of short-circuit situations, also the particularly dangerous small load events are reliably mastered. CONCLUSION The development of superconducting fault current limiters definitely approaches the commercialisation on the market place. An excellent technical solution based on bifilar coils equipped with a metallic shunt has been developed for the medium voltage. However up to 10 kv, also because of existing technical alternatives, the applications for this concept will be restricted to niche markets only. Therefore alternative concepts, also with the perspective to be used on highest voltage levels are requested. Different magnetically triggered concepts actually are under development and will pave the way to fully exploit the excellent limiting properties of the bulk material. Acknowledgements Acknowledgement: The work was partly funded by BMBF/VDI under grant No 13N 7877. 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