Superconducting Fault Current Limiters as New Devices for Distribution Installations

Transcription

1 1/6 Title Superconducting Fault Current Limiters as New Devices for Distribution Installations Registration Nº: (Abstract) Summary Superconducting fault current limiters (SFCLs) as versatile devices are excellent means to overcome the existing problems resulting from high short circuit currents. Due to the strong increase in resistance of a SFCL initiated by a fault the technology is well suited to protect today s electricity grids. The very low reactance under all operating conditions is a further advantage of these superconducting devices. Nexans SuperConductors has for the first time transferred such devices from the R&D phase into commercial production. Several SFCLs have been designed, built, tested, and commissioned and are operated by DNOs or power plant operators. One of the systems built has been especially designed for multiple applications at different installation locations. A recent SFCL system is installed together with a HTS cable to supply up to 40 MVA on 10 kv medium voltage in the inner city of Essen, Germany, an installation which has become known as AmpaCity. The SFCL is protecting the HTS cable and the grid lying behind and is at the same time enabling a very compact cross section for the concentric HTS cable. Further systems under construction will be installed in the UK grid to enable improve network short-circuit strength and enable increased power generation with low CO 2 footprint. The latest development achievements as well as test and operation results are presented. Authors of the paper Name Country Joachim Bock* Germany joachim.bock@nexans.com Achim Hobl Germany achim.hobl@nexans.com Judith Schramm Germany judith.schramm@nexans.com Christian Jänke Germany christian.jaenke@nexans.com Simon Krämer Germany simon.kraemer@nexans.com *corresponding author Name of company and address Nexans SuperConductors GmbH Chemiepark Knapsack, Huerth, Germany Tel.: FAX: Key words Growing electricity demand, distributed generation, short circuit power increase, Superconducting Fault Current Limiter (SFCL)

2 2/6 Superconducting Fault Current Limiters as New Devices for Distribution Installations 1. INTRODUCTION While on the one hand the demand for electricity and system reliability continues to increase, on the other hand utilities around the world are adding more and more distributed generation and are building new interconnections resulting in evermore tightly meshed networks. These tendencies result in higher fault current levels challenging the ability of substation equipment to withstand mechanical and thermal loads associated with these high fault levels. Utilities must react by adapting their equipment to meet the challenges of the higher level of fault currents. Electric utilities around the world have long employed a variety of fault current mitigation techniques such as fault current limiting reactors like air cores, selective tripping schemes, and so forth. Each of these measures has distinct drawbacks. An ideal solution would be a re-usable, automatic device that does not affect the operation of the power system during normal operation, but yet would limit fault currents already during the first cycle peak. Utilities in Europe have recently begun to employ Superconductor Fault Current Limiters (SFCLs). Resisitive type stand-alone fault current limiters based on high temperature superconductors (HTS) technology have been tested in the grid since a full decade now. Nexans SuperConductors has for the first time transfered such a device from the R&D phase into commercial production. Characteristics and typical applications of this type of SFCL are presented along with the operational advantages and their potential role in the deployment of future electrical networks such as the so called Smart Grids. 2. FUNCTIONALITY AND APPLICATIONS CASES Working Principle of resistive SFCL SFCLs have the unique characteristics of almost zero impedance under normal operating conditions and high impedance at fault conditions. An excellent overview about all aspects of development and application of SFCLs is given in [1]. Several principles have been developed, inductive types, saturated iron core or shielded iron core, which typically require a device like a substation transformer with similar footprint. Compact and efficient SFCLs based upon the resistive type have been demonstrated in several real grid installations. The working principle is based on the nonlinear voltagecurrent-characteristic in the superconducting material of the form U~ ( I Ic ) n, where I c is the critical current of the superconductor, defined as the current, at which 1 µv voltage drop per cm superconductor length is reached. The exponent n is usually between 5 and 30 for typical high temperature superconductors (HTS) and differs with the material type. Figure 1: current voltage characteristics of a superconductor compared to a normal conductor With the short circuit event the current increases and the voltage follows this power law. In that way the resistance of the material rises about several orders of magnitude until the material completely leaves the superconductor state and behaves like a normal resistor with a proportional current-voltage-law (Fig. 1). Within a few ms the associated heating brings the superconductor above its critical temperature, whereby the transition is reached even faster. After the first half wave a further heating of the material during the short circuit also in the normal conducting state leads to further increasing resistance and a decrease in the limited current (Fig. 2). This is associated with a phase change of the material and the transition from the superconducting to the normal conducting regime is called a quench. SFCLs make use 2 / 6

3 3/6 of this transition, when the normal amplitude of the rated current is below the critical current I c and the quench is triggered by the high current occurring during a short circuit event. Behaviour in the Grid The SFCL technology provides the unique possibility to design new innovative grid structures: The grid can have very low impedance, e.g. through grid coupling or due to low impedance equipment, but without the risks associated to high short circuit power. With a resistive SFCL in the line, the limiter also minimises the phase shift between current and voltage during a short circuit. This effect strongly reduces the stress and thus the requirements on the circuit breakers in line, because the current and voltage are almost in phase, which also means zero crossing is occurring simultaneously. In any case, all circuit breakers, busses and cables downstream of a limiter can have much lower ratings and significant equipment cost can be saved. Especially high savings are expected for power plant installations in the case of building new blocks or expanding existing equipment. An interesting aspect of using SFCLs is also that equipment can be operated closer to its limits. With this an SFCL is providing more headroom at the substation e.g. for further integration of distributed generation. All in all investment can be avoided or shifted by temporarily installing a SFCL. Figure 2: Prospective and limited current during a shortcircuit event The red line in Fig. 2 marks the prospective current flowing without limiter. The green line denotes the limited current and shows effective and reliable limitation already in the first half wave. The extensively symmetrical curve progression in the following half waves, the so called follow current, comes out of the nearly pure resistive behavior of the limiter. This follow current can be adapted in amplitude by selection of appropriate materials. Grid Type Applications SFCL as versatile devices for fault current management can be applied at different positions within a typical grid, as shown in Fig. 3. Figure 3: grid installations of SFCLs Feeder Application Depending on the protective function, the SFCL can be used either in incoming feeders, e.g. as transformer feeder, or in the outgoing feeders. This in-line application protects all elements downstream of the point of installation. Busbar Coupling The SFCL is especially advantageous for busbar couplings, as fully redundant feed-in is possible without a normally associated increase in short-circuit currents. In case of a fault, the limiter ensures that the short-circuit contribution from the un-faulted bus is strongly reduced. Even more, the un-faulted side can maintain almost stable voltage and operation. An additional advantage for busbar coupling is that under normal operating conditions perfect load levelling can be reached resulting in lower operational losses and less stress for the equipment. FIRST INSTALLATIONS Following successful SFCL field tests in a distribution level substation in 2005 [2], NSC installed the world s first HTS-based Fault Current Limiter in a power station in 2009 [5]. NSC has since implemented several SFCLs in distribution level networks. 3 / 6

4 4/6 First installations have been realized in the distribution network of different DNOs in UK. The SFCLs are based upon BSCCO-2212 bulk superconductor modules which are manufactured by NSC by a melt casting process [4]. Using mono- or bifilar components the latter ones for reduced AC losses NSC has built and integrated several complete SFCL systems based on this material type. Bamber Bridge 33/11 kv Substation (ENW, UK) The existing switchgear at the substation was rated at 11 kv with a short circuit capability of 150 MVA equating 19.7 ka making capacity and 7.87 ka breaking capacity. Upgrading two 33/11 kv transformers increased fault current contributions of each unit to 11 ka peak, 4.2 ka at the substation. A Nexans SFCL deployed in a bus section configuration limits the fault contribution from healthy to faulted busbar in case of a fault on an outgoing circuit. The total of fault contribution and contribution from the transformer directly feeding the busbar are now limited to 95% of the switchgear s rating. SFCL implementation saves upgrading the substation with a new 11 kv switchboard of primary distribution circuit-breakers [3]. Ainsworth Lane 33/11 kv Substation (Scottish Power, UK) The installation of a new transformer increased the fault level at the substation to 270 MVA. This would have exceeded UK DNO 250 MVA design fault level in a region where 33 kv and 11 kv networks are meshconnected as opposed to the radial connecting prevalent in the UK. The SFCL is installed in the substation s bus section and reduces fault currents passing through it from a prospective ~7.5 ka to less than 3.2 ka, thus limiting the busbar fault level to around 200 MVA, equalling 80% of the capability of the switchgear in place. Boxberg Power Plant (I), (Vattenfall, D) In 2009, a Nexans SFCL based upon BSCCO-2212 bulk superconductor modules was deployed at the power plant for short circuit protection of the auxiliary supply including lignite crushers [5]. The device in a 12 kv feeder bar limits a 63 ka peak prospective short circuit current to less than 30 ka peak immediately and to about 7 ka after 10 milliseconds. With a nominal current rating of 800 A, its design allows inrush currents of up to 4.1 ka during coal crusher start up and of 1.8 ka for subsequent 15 seconds without tripping the limiting function. Field testing of the device was successfully completed. Boxberg Power Plant (II), (Vattenfall, D) In 2011, a Nexans SFCL based upon YBCO coated conductor modules replaced the first device and was commissioned after successful testing [6]. The YBCO based SFCL combines further improved energy efficiency with a faster response to fault currents and stronger initial limitation: 63 ka peak prospective short circuit current are limited to 16 ka peak. Essentially, SFCL deployment permits internal MV supply switchgear to be downsized to standard component level and thus affords a substantially lower investment requirement. TOWARDS A STANDARD SOLUTION The superconducting part of Nexans SFCLs consists of series- and parallel-connected modules for each current phase. Protecting Power Plant Auxiliary System The resistive SFCLs are attractive devices for short circuit protection of the auxiliary supply in power stations. Special need is seen for coal power stations due to high power drives for coal crushing and induced draught fans. Two unique features of the resistive SFCL were most relevant for the customer to have a device installed: The fail safe behaviour and the fact that the grid is not dark after the short circuit event, the device was designed to fit into the existing grid protection scheme. Figure 4: Installation of SFCL at Boxberg Stacking these units provides the system with the specific fault current limiting capacities, durations, and reaction rates required in individual grid environments. Stacked modules are immersed in liquid nitrogen within a cryogenic vessel equipped with a commercial cooling 4 / 6

5 5/6 system. The modules operate maintenance-free at temperatures of -200 C. Palma de Mallorca 16 kv Substation (Endesa, E) Košice 110/24 kv Substation (VSE, SK) The EU-funded ECCOFLOW project aimed at developing a multi-purpose SFCL providing flexibility for a range of power grid applications. Nexans acted as coordinator and device manufacturer in the ECCOFLOW consortium which involved five European power utilities and eight scientific and industrial partners [7]. The ECCOFLOW design is based upon YBCO coated conductor modules. The SFCL s power rating is 1 ka at 24 kv. Combined with inductive or resistive shunts, the SFCL limits fault currents for extended intervals of up to 1,000 ms. Figure 5: Installed SFCL system ENABLING NEW MV METROPOLITAN CABLES AmpaCity 10 kv Superconducting Cable (RWE, D) End of 2013, a superconductor power transmission system by Nexans has been integrated in the inner city power grid of Essen, Germany [8]. It comprises a 1 km long 10 kv three-phase concentric HTS cable overloadprotected by a SFCL. The superconductor cable has a rating of 40 MVA and will replace an entire conventional 110 kv installation of equal capacity by simultaneously providing low-loss transmission. The AmpaCity project thus provides a blueprint for removing high-voltage installations and a good part of today s substations from inner cities in the course of rebuilding metropolitan grids for future requirements. Overload protection by a Nexans SFCL enables a compact cable design, as shunts in parallel to the superconductors become expendable [9]. Figure 6: SFCL for the AmpaCity Project A superconductor cable can be laid in existing ducts which affords substantially lower construction costs. In total the superconductor system facilitates the refurbishment of inner city power infrastructures by offering high performance and savings of space and cost. AmpaCity at the RWE grid is an international lighthouse project and the HTS system is now constantly supplying the inner city of Essen since April CONCLUSIONS Two different superconducting material options are currently available for manufacturing resistive SFCL systems. For the BSCCO-2212 bulk material, NSC has proven the viability of SFCLs from the basic superconducting material to the complete system. SFCLs using YBCO tape with stronger limitation also have been produced, type- and field tested. Both technologies provide the exceptional possibility to design new innovative grid structures: The grid can have very low impedance, e.g. by grid coupling or low impedance equipment, but without the risk associated to high short circuit power. With a resistive SFCL in the line, the limiter minimises the phase shift between current and voltage during a short circuit, an effect which strongly reduces the stress and requirements on the circuit breakers in line, because the current and voltage are zero almost simultaneously. All circuit breakers, busses and cables downstream of a limiter can have much lower ratings and significant equipment cost can be saved. For e.g. the power plant installation this would be attractive in the case of building new blocks or extending the existing ones. An interesting aspect of using SFCLs is also that in case of equipment loaded close to its operating limits, investment can be shifted by temporarily installing an SFCL. 5 / 6

6 6/6 The present movement in the power generation towards distributed and renewable energies leads to a fundamental downsizing of average power generation units. Where the integration of distributed generation meets existing grids and leads to excess short circuit overloading, SFCLs can be a very promising solution: Most generation units are able to supply directly into the medium voltage grid, but that would need a higher short-circuit capability than in many cases available. The typical solution is the connection to the high voltage grid with the need for a separate and expensive transformer with considerable additional cost for breakers, cables, and civil works. An alternative solution would be the integration via SFCL, as shown in Figure 7. All installations realized today are on the medium voltage level. However, due to the performance of SFCLs, they will also be large interest to integrate such devices into high voltage grids. [5] J. Bock, M. Bludau, R. Dommerque, A. Hobl, S. Kraemer, O.M. Rikel; S. Elschner HTS Fault Current Limiters First Commercial Devices for Distribution Level Grids in Europe IEEE Trans. Appl. Supercond., vol. 21, No. 2, pp , 2011 [6] S. Elschner, A. Kudymow, S. Fink, W. Goldacker, F. Grilli, C. Schacherer, A. Hobl, J. Bock, M. Noe, ENSYSTROB - Resistive Fault Current Limiter based on Coated Conductors for Medium Voltage Application, IEEE Trans. Appl. Supercond., (Proc. ASC 2010), in press [7] J. Bock, A. Hobl, S. Krämer, M. Bludau, J. Schramm, C. Jänke, M. Rikel, S. Elschner Nexans Superconducting Fault Current Limiters for Medium Voltage Applications Status and prospects CIRED 2011, Frankfurt [8] A. Hobl, S. Elschner, J. Bock, S. Krämer, C. Jänke, J. Schramm, Superconducting fault current limiter a new tool for the grid of the future, CIRED 2012, Lisbon [9] J. Bock, A. Hobl, J. Schramm, Superconducting Fault Current Limiters a New Device for Future Smart Grids, CICED 2012, Shanghai Figure 7: grid connection of DG with a SFCL BIBLIOGRAPHY [1] M. Noe, M. Steurer, High-temperature superconductor fault current limiters: concepts, applications, and development status Supercond. Sci.Tech, 20 R15, 2007 [2] J. Bock, F. Breuer, H. Walter, S. Elschner, M. Kleimaier, R. Kreutz, M. Noe, IEEE Transact. Appl. Supercond., Vol. 15, pp , 2005 [3] D. Klaus, A. Wilson, R. Dommerque, J. Bock, D. Jones, J. McWilliam, A. Creighton Fault Limiting technology trials in distribution networks CIRED 2009, Prague [4] J. Bock, S. Elschner, P.F. Herrmann, Melt-cast processed MCP-BSCCO 2212 tubes for power applications up to 10 ka, IEEE Trans. Appl. Supercond., vol. 5, No. 2, pp , / 6