1LB02 1 System Technology and Test of CURL 10, a 10 kv, 10 MVA Resistive High-Tc Superconducting Fault Current Limiter Ronald Kreutz, Joachim Bock, Frank Breuer, Klaus-Peter Juengst, Martin Kleimaier, Hans-Udo Klein, Detlef Krischel, Mathias Noe, Ralph Steingass, Karl-Heinz Weck Abstract A full scale three-phase resistive high-tc superconducting fault current limiter (SCFCL) designed for 10kV, 10 MVA, has been developed, manufactured, and tested within a publicly funded German project called CURL 10. The device is based on 90 bifilar coils of MCP BSCCO-2212 bulk material. The operating temperature of 66 K is achieved by subcooling of liquid nitrogen using two Stirling cryocoolers. Until today, this is the largest HTS fault current limiter world wide. We report on the design features, the composition, and the operation parameters of the SCFCL system. From April 2004 on a field test of the system within the network of the utility RWE at Netphen near the city of Siegen, Germany is performed. The results of the laboratory test and the field test are given. Index Terms fault current limiter, high-tc bulk material, high-tc superconductor, SCFCL S I. INTRODUCTION EVERAL investigations using different approaches have been made for applying high-tc superconductng (HTS) bulk material as a resistive element in superconducting fault current limiters (SCFCL) [1]-[3]. In recent years considerable progress has been made towards the development of a power Manuscript received November 3, 2004. This work was supported by the German Ministry of Education and Research (BMBF) under contract No. FKZ: 13N7568/0 R. Kreutz, D. Krischel, H.-U. Klein, and R. Steingass are with ACCEL Instruments GmbH, D-51429 Bergisch Gladbach, Germany (phone: +49+2204-842368; fax: +49+2204-842501; e-mail: kreutz@accel.de). J. Bock and F. Breuer are with Nexans SuperConductors, D 50351 Hürth, Germany (e-mail: joachim.bock@nexans.com). M. Noe and K.-P. Juengst are with the Institut für Technische Physik, Forschungszentrum Karlsruhe, D 76021 Karlsruhe Germany (phone: +49 +7247-825993; fax: +49 +7247 82 5993; e-mail: mathias.noe@itp.fzk.de). M. Kleimaier is with RWE Energie AG, D 44047 Dortmund, Germany (phone: +49 +231-438 5565; fax: +49 +231 438 1207; e-mail: martin.kleimaier@rwe.com). K.-H. Weck was with FGH Engineering & Test GmbH, D 68219 Mannheim Germany and is now consultant (telephone: +49 +621-8047 200; fax: +49 +621 8047 113; e-mail: weck@fgh-ma.de). system demonstrator [4], [5]. This paper reports about the design features and system parameters of the three-phase 10 kv/ 10 MVA resistive fault current limiter CURL 10 based on high-tc superconducting BSCCO-2212 bulk material, the first of its kind for this voltage and power level; and about the results which have been obtained in a laboratory test and in a field test in the electrical network of the German utility RWE. CURL 10 was funded by the German Ministry of Education and Research. The system was designed, built, and tested under leadership of ACCEL Instruments GmbH, Bergisch Gladbach, in cooperation with ACCESS e.v., Aachen, ATZ Adelwitz Technologiezentrum GmbH, Adelwitz, E.ON Energie AG, München, EUS GmbH, Dortmund, Forschungszentrum Karlsruhe GmbH, Nexans SuperConductors, Hürth and RWE Energie AG, Dortmund. II. SYSTEM COMPONENTS AND PERFORMANCE PARAMETERS A. Main Components The main components of CURL 10 are, see Fig. 1 and Fig. 2: - cryostat with liquid nitrogen (LN 2 ), - 90 MCP BSCCO-2212 elements (bifilar coils) - six conventional current leads (two for each phase), - safety and controlling instrumentation (relief valve, rupture disc, pressure sensor, liquid nitrogen level probe), - two cryocoolers, - nitrogen transfer lines, - electrical power supplies and electronic control of the cryocoolers, - chiller for heat dissipation. The design, development and tests of the MCP BSCCO- 2212 bifilar coils is described in [6]-[9]. The system components are assembled in a compact way on a chassis which enables the transport of the SCFCL as a whole unit. In particular, the cryocoolers are mounted at the height of the top plate of the cryostat in order to keep the nitrogen transfer lines short. Not shown here is the air-cooled chiller which removes the heat from the cryocoolers.
1LB02 2 property requires minimum distances between the cryostatinternal components and between these components and mass potential. This requirement is contrary to the cryogenic requirements which demand for an as far as possible compact design. According to electrical insulation coordination standards [10] the system is designed to withstand an AC voltage of 28 kv for a hold time of 1 minute and a pulse with 75 kv peak voltage and a rise time of 0.1 µs, both between phase to phase and phase to ground. Table I summarizes the main design parameters of the cryostat of CURL 10. TABLE I MAIN DESIGN PARAMETERS OF CURL 10 Quantity Unit Value LN 2 operating temperature K 66 LN 2 operating pressure mbar 206 LN 2 volume l ~ 600 Design pressure of the cryostat bar 10 HV insulation resistivity: Voltage with hold time of 1 min kv 28 Peak value for pulse with rise time 0.1 µs kv 75 Cooling power of the cryocoolers at 66 K W ~ 1450 Fig. 1. Superconducting fault current limiter CURL 10 B. Design Features and System Parameters The cryostat vessel and the nitrogen transfer lines are designed for a pressure of 10 bar. This requirement is set in order to account for an accidental pressure excursion by an internal lightning arc. The cryostat vessel satisfies the corresponding German pressure vessel rules. Previous simulations of such excursions with a test cryostat have shown that the building of a lightning arc will not be suppressed by the liquid nitrogen but could cause a shock wave inside the cryostat. In case of such thermal power excursion the high pressurized nitrogen gas of the cryostat would be released by a rupture disc. The operating temperature of the fault current limiter is 66 K which corresponds to a pressure of the liquid nitrogen inside the cryostat of 206 mbar. The LN 2 pressure is monitored by a pressure sensor on top of the cryostat vessel and kept via the software-controlled electronics of the cryocoolers. In a loop process, the gaseous nitrogen is sucked by the cold heads, subcooled and liquefied there, and returned to the cryostat vessel. A temperature monitoring inside the cold heads is provided which in particular should prevent a cool down of the nitrogen below 64 K (triple point of nitrogen: 63.4 K). The total cooling power of the cryocoolers at 66 K is about 1450 W. This power has to compensate: - heat losses of the cryostat, - AC losses of the HTS elements, - ohmic losses of the copper joints connecting the HTS elements. According to the operation of the system at 10 kv the cryostat and the currents leads are designed for a corresponding high-voltage insulation capability. This Fig. 2 shows the assembly of the top plate of the cryostat with the currents leads, the nitrogen transfer lines, and the block of the 90 BSCCO 2212 elements (bifilar coils); for each electrical phase 30 elements connected in series are mounted in a space sector of 120. To save space half of the elements is hanging at the upper plate, the other half is mounted on the bottom plate; nevertheless, the minimum distances between the terminals and the connections of the HTS elements and their distance to the ground material, required for the HV insulation, are kept. The details of the high voltage design, requirements and verification by tests for the HTS elements is described in [11]. Fig. 2. Cryostat insert of CURL 10 with 90 BSCCO 2212 elements (bifilar coils) The design for a low ohmic resistance of the copper
1LB02 3 connections soldered to the terminals of the HTS elements was verified by mock-ups. The resistance of a connection between two HTS elements was measured as 3.5 µω. The design of the connectors and the soldering procedure had to guarantee that the low-ohmic quality of the connections was maintained even under many thermal cycles and the corresponding thermal stress and displacement impacts. C. System Assembly Several test of pre-series BSCOO elements were performed before starting the series production of the BSCCO elements by Nexans to verify their fault current limiting performance parameters [7],[8]. Each of the series elements was tested by Forschungszentrum Karlsruhe in their testing facility. The ninety series BSCCO elements were mounted and connected in a compact block which was fastened to the top plate of the cryostat, and connected with the current leads; see Fig.2. Before the top plate with BSCCO elements was inserted in the cryostat vessel, the cryogenic operation of the cryocoolers was tested with the cryostat vessel only. The heat load in the liquid nitrogen was simulated by a heater up to a power of 1100 W and the operation parameters of the cryocoolers and the water loop of the chiller were optimized by Stirling. After the complete assembly of the fault current limiter the system underwent final electrical tests at the cryogenic operation at 66 K: - high voltage loads according to the design requirement in Table I, - AC current loads up to 600 A with AC loss measurements. from [9]. B. Field Test The field test of the fault current limiter CURL 10 was performed at a substation with a busbar coupling on the 10 kv network level. The principle scheme of such coupling is shown in Fig. 4. Fig. 4. FCL in busbar coupling substation of 10 kv network level. In March 2004 CURL 10 was installed at the 10 kv busbar coupling substation of RWE at Netphen; see Fig. 5. After the HV insulation tests AC loss measurements were performed. A balance of the available cooling power of 1450 W is reached by an AC current of 400 A with AC losses of 1100 W and losses of 350 W due to the heat losses of cryostat and the ohmic losses of the element connections. III. TESTS OF CURL 10 A. Laboratory Test After these test CURL 10 was shipped to the HV test laboratory of FGH Engineering and Test GmbH at Mannheim, Germany, where short-circuit currents simulating the specified fault current loads were applied. The required fault limiting behavior was verified for each single phase and for the 3phase system at the operation temperature of 66 K [12]. For a shortcircuit of 60 ms a prospective peak short-circuit current of 18 ka was effectively limited to 7.2 ka, a value which was considerably lower than the prospected value of 8.7 ka; see Fig. 3. Fig. 5. CURL 10 at the 10 kv busbar coupling substation of RWE network at Netphen. Fig. 3. 3phase fault current limiting by CURL 10 in laboratory test at FGH. More results obtained in this laboratory test can be taken From April to June 2004 a first field test period was
1LB02 4 performed. In a first phase without having coupled CURL 10 to the net, it turned out that the cryogenic operation became unstable after about ten days, i.e. the temperature of the liquid nitrogen increased and decreased periodically. This behavior was due to a freezing of the nitrogen in the cold heads. This problem was solved by Stirling introducing a periodical heating /cooling cycle of the cold heads in addition to the basic temperature controlling. After this change CURL 10 was coupled to the net and the cryogenic operation of the system worked without any problem. The coupling of CURL 10 to the net is done via fast power switches between the fault current limiter and each busbar of the 10 kv substation. In normal operation, the current going through the fault current limiter is the difference of the currents from both sides of the coupling,; this current does not exceed about 100 A. At the fast power switches a limit was set to 200 A above which the fault current limiter was taken from the net in any case. During this first period of operation at the net no fault current event occurred. Only one electrical disturbance occurred which triggered the decoupling of CURL 10 from the net: an irregularity of the transformer operation caused a current of higher than 200 A through the fault current limiter, so that the fast power switches were triggered for decoupling the CURL 10 from the net. The decoupling event triggered the data recording of the measurement equipment for monitoring the electrical data on a sub-millisecond time scale as it should do just in case of a fault current event; thus, the data recording showed its desired functioning. [5] D. Krischel, H. Salbert, P. Behrens, F. Breuer, A. Cieleit, S. Elschner, K.- P. Jüngst, A. Kemnitzer, M. Kleinmaier, M. Noe, T. Rettelbach, G.-J. Schmitz, K.-H. Weck, F. Werfel, A. Wolf, Praxisgerechte Entwicklung eines resistiven 15 MVA HTSL-Strombegrenzers auf Massivmaterialbasis, Energietechnik für die Zukunft: Internat. ETG-Kongress, Nürnberg, 23.- 24.Okt. 2001 Berlin: VDE-Verlag, (mit CD-ROM) (ETG-Fachbericht ; 85), pp. 361-368 [6] S. Elschner, F. Breuer, M. Noe, T. Rettelbach, H. Walter and J. Bock, Manufacturing and Testing of MCP2212 Bifilar Coils in a 10 MVA Fault Current Limiter, Applied Superconductivity Conf., Virginia Beach, Va., September 17-22, 2000, IEEE Transactions on Applied Superconductivity, 11, 2001, pp.2507-10. [7] J. Bock, F. Breuer, H. Walter, M. Noe, K.-H. Weck, R. Kreutz, S. Elschner, Design, manufacturing and testing of robust HTS components based on MCP-BSCCO 2212 bifilar coils for use in a 10-MVA fault current limiter CIRED 2003, Barcelona, 12-15 May 2003. [8] J. Bock, F. Breuer, H. Walter, M. Noe, R. Kreutz, M. Kleimaier, R. Weck, S. Elschner, Development and successful testing of MCP BSCCO-2212 components for a 10 MVA resistive superconducting Fault Current Limiter, EUCAS 2003, Sorrento, 14-18 Sept. 2003. [9] J. Bock, F. Breuer, H. Walter, S. Elschner, M. Kleimaier, R. Kreutz, M. Noe, CURL 10: development and field test of a 10 kv / 10 MVA resistive current limiter based on bulk MCP BSCCO-2212, to be presented at ASC 2004, 1LB01, 3-8 Oct. 2004, Jacksonville, USA. [10] International Standard, IEC 71-1, Insulation co-ordination-part 1: Definition, priprincuples, rules, seventh edition, 12-1993. [11] M. Noe, K.-P. Juengst, S. Elschner, J. Bock, F. Breuer, R. Kreutz, M. Kleimaier, K.-H. Weck, N. Hayakawa, High voltage design, requirements and tests of a 10 MVA superconducting fault current limiter, to be presented at ASC 2004, 4LG18, 3-8 Oct. 2004, Jacksonville, USA. [12] R. Kreutz, D. Krischel, H.-U. Klein, HTSL- Massivmaterial- Strombegrenzer - Aufbau, Wirkungsweise und Anwendung, DKV- Tagungsbericht 2003, Bonn, 19.-21. Nov. 2003, pp. 189-206. IV. CONCLUSION The functioning of CURL 10 with respect to its required fault current limiting capabilities was demonstrated in a laboratory test. In the first field test period no fault current event occurred, but the field test is continuing in order to have the possibility of verifying the functional ability of CURL 10 with a fault current event in the net. The results obtained with CURL 10 are a promising basis for the development of a fault current limiter for a higher voltage level of 110 kv. This needs further efforts in particular for developing the HTS material and for the upgrade of the system technology. REFERENCES [1] W. Paul, M. Chen, M. Lakner, J. Rhyner, D. Braun and W. Lanz, Fault current limiter based on high temperature superconductors different concepts test results, simulations, applications, Physica C 354, 2001, pp. 27-34. [2] P. Tixador, L. Porcar, E. Floch, D. Buzon, D. Isfort, D. Bourgault, X. Chaud and R. Tournier, Current limitation with bulk Y-Ba-Cu-O, IEEE Transactions on Applied Superconductivity, Vol. 11, No. 1, March 2001, pp. 2034-2037. [3] T. Verhaege, et. al., HTS materials for ac current transport and fault current limitation, IEEE Transactions on Applied Superconductivity, Vol. 11, No. 1, March 2001, pp. 2503-2506. [4] M. Chen, W. Paul, M. Lakner, L. Donzel, M. Hoidis, P. Unternaehrer, R. Weder and M. Mendik, 6.4 MVA Resistive Fault Current Limiter Based on Bi-2212 Superconductor, Proceedings of the 5th European Conf. on Applied Superconductivity, (EUCAS 2001), Copenhagen; Physica C.
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