Brief Review of the Field Test and Application of a Superconducting Fault Current Limiter

Home , Fault current limiter

pISSN 1229-3008 eISSN 2287-6251 Progress in Superconductivity and Cryogenics Vol.19, No.4, (2017), pp.1~11 https://doi.org/10.9714/psac.2017.19.4.001

Brief review of the field test and application of a superconducting fault current limiter

Ok-Bae Hyun*

SuperGenics, Changwon 51543, Korea

(Received 5 December 2017; revised or reviewed 14 December 2017; accepted 15 December 2017)

Abstract

This article reviews the recent activities of field testing and application of superconducting fault current limiters (SFCL) based on high-temperature superconductors (HTS). The review particularly focuses on the trends in the field tests in terms of the technical aspects and commercial activities of the SFCLs. Stimulated by the discovery of HTS, numerous research and development activities have been conducted worldwide for SFCLs operating from distribution voltages to transmission voltages. Different types of SFCLs have been developed and field-tested. Consequently, more than 20 field tests and applications have been performed on real grids worldwide while supplying electric power to the customers. These field tests have not only provided the track records of the operation experiences including the problems and maintenance during operation, but also proved their current limiting capabilities against real faults, rendering this new technology highly viable. Through these activities, the following trends in the status of field testing and application are observed. Resistive-type SFCLs with HTS-coated conductors were dominantly used in the most recent field tests. This implies that the resistive type is technically more mature than the other types. Bus-bar coupling and transformer feeders were the major application locations. It is of importance that most of the field applications were conducted as R&D projects. A relevant change from the R&D stage to the application stage is shown as recently deployed SFCLs are expected to be under long-term operation and commercial service. Here, we review the installation of these SFCLs by substation. This review also discusses the recent activities for their commercial applications.

Keywords: superconducting fault current limiter, SFCL, field test, resistive type, bus-bar coupling, fault current

``` 1. INTRODUCTION tests provided instances of long-term operation as well as successful limitation of the real fault currents upon faults in A superconducting fault current limiter (SFCL) is a the grids. The tests also went through various trouble power machine that limits the fault currents in a power grid. shooting and maintenance, yielding valuable lessons in SFCLs are considered as one of the most promising handling the new machine. Even with the track record of candidates for power application based on high SFCL operation in the last 20 years, they are still not temperature superconductors (HTS). Extensive research considered as fully commercialized till date because the and development on HTS SFCL has been conducted field applications were performed in the research and worldwide [1-8]. Researchers have explored various SFCL development (R&D) environment. However, the recent models and produced more than 20 units operating at activities in the private sector could be an indicator of their medium as well as transmission voltages. Most of them commercial application in the electric utility field. were installed in real grids and field-tested while supplying In this review, we will focus on the field tests of SFCLs, electrical power to customers. For these systems, a field test rather than either researches or technological treatments of involves performing the proposed operation and an SFCL. Consequently, of major interest is the status of commercial service in a real grid for a significant period of more than 20 field test projects and applications of the time. A field test is necessary to validate the feasibility of SFCLs worldwide. Based on those tests, we will briefly an SFCL. It includes installation and operation, trouble discuss the trends in the SFCL types and the technical shooting, and maintenance, and fault current limitation issues inherent to the SFCL technology associated with upon fault in the real grid. general applications. This review is based on public The first successful field test of an SFCL based on HTS documents such as papers, articles, project reports, and was pioneered by ABB in 1996, who developed a 6.6-kV documents available online. SFCL [9, 10]. The SFCL was installed in the auxiliary line of a hydroelectric power plant, and field-tested for one year. Since then more than 20 SFCLs of various models have 2. SUPERCONDUCTING FAULT CURRENT been built and installed at various locations in distribution LIMITER TECHNOLOGY and transmission grids. They were field-tested to prove their feasibility and current limiting capabilities. The field 2.1. SFCL types * Corresponding author: [email protected] Various types of SFCLs have undergone the R&D stage.

Brief review of the field test and application of a superconducting fault current limiter

hydroelectric power plant in Switzerland [9]. Since then more than 20 field tests have been performed. In this section, we will review the tests by country and chronologically.

3.1. Switzerland • Löntsch, NOK power plant The first of field test of an SFCL based on an HTS was pioneered by ABB in cooperation with the Swiss utility NOK and with financial support from the Swiss Utility Study Fund (PSEL) [9-11]. ABB successfully developed a 6.6-kV magnetic shield type SFCL (1.2 MVA), which utilized stacked Bi2212 bulk tubes. The machine was installed to protect the auxiliary line of an NOK hydroelectric-power plant in Löntsch, Switzerland, for which a one-year endurance test was performed from November 1996. The test was expected to provide insight on the cooling system and possible fatigue aging of the HTS. After six months of testing, no major problems were encountered, and no fault occurred during the test.

3.2. United States of America Four field tests were performed in the USA: two were sponsored by the Department of Energy (DOE), one by the New York State Energy Research and Development Authority (NYSERDA), and one by a manufacturer. Fig. 1. Basic circuits of the SFCL types: (a) Magnetic shield type (shielded core type), (b) bridge type (electronic • General Atomics and Southern California Edison inductive, DC reactor, or rectifier type), (c) saturated The first pre-commercial SFCL in the USA was iron-core type, and (d) resistive type. The grey and blue developed by General Atomics (GA) and the Los Alamos areas denote the SFCL system and superconducting parts, Nation al Laboratory (LANL) with support from the DOE. respectively. The diagrams represent single phase It was a bridge type with a rated voltage and current of 15 structures, although multiple variations exist for each type. kV and 1200 A, respectively [12-14]. The SFCL unit was equipped with three of the largest Bi-2223 coils of the Among them, four types of SFCL structures have been world at that time. This machine was installed in June 1999 successfully developed to be field-tested and applied: (1) at the Center substation of the Southern California Edison magnetic shield type, (2) bridge type, (3) saturated (SCE) grid. During high-voltage testing, each of the three iron-core type, and (4) resistive type. Figure 1 shows the single-phase units experienced a voltage breakdown, one basic circuits of the four types of SFCLs. Multiple externally and two internally [14]. After redesigning the variations of these four types are under research and structure, a high-voltage test as well as load and development. short-circuit tests were performed for the single-phase unit Details of the working principles of these types of SFCLs operating at the LANL 13.7-kV substation. are widely reported in articles. Each of these SFCLs has its own merits and de-merits. We will discuss the technical • Avanti circuit and Shandin substation, SCE issues associated with the application of the different SFCL The second pre-commercial SFCL of the saturated types later in another section of this article. iron-core type was developed by Zenergy Power. The Early successes in the SFCL developments were voltage and current ratings were 15 kV and 1200 A, obtained with the magnetic shield type and bridge type. respectively. The SFCL used one HTS coil for three phases. More successful developments and field tests were It generated a strong magnetic field to saturate the six iron achieved with the saturated iron-core type. However, most cores, each of which carried one AC line coil. The SFCL of the recent trials of SFCL field test have been performed enabled the first successful field test in the USA at Avanti with the resistive-type SFCL. This is indicative of the circuit, Shandin substation of the SCE grid in March 2009 technological advantages of the resistive type over the [15]-[20]. This SFCL experienced multiple fault events in other types in terms of the structure and effectiveness. the grid during operation, and as designed, successfully limited the fault currents.

3. FIELD TESTS AND APPLICATIONS OF SFCL • Knapps Corners substation, Central Hudson Gas & Electric The first field test of an SFCL based on an HTS was The third SFCL field test was performed by Applied conducted in 1996 at the auxiliary power grid of a Materials (AMAT). AMAT developed a resistive type

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SFCL (ratings 13.8 kV and 1000 A), and installed it in its installed at the Hercules substation in the inner city of Silicon Valley corporate grid for a one-year service starting Essen [37-41]. The SFCL was connected in series with the from July 2013 [21]. HTS cable, having the same current and voltage ratings of Next, AMAT built and installed a resistive type SFCL in 10 kV and 2300 A, respectively. Nexans Superconductors the Knapps Corners substation of Central Hudson Gas & was assigned with the task of manufacturing, installing, and Electric grid, Poughkeepsie, NY. In contrast with other commissioning the HTS cable and SFCL combined system. field tests, this SFCL bypassed the neutral grounding This system was installed in October 2013 and put into reactor (NGR) of a main transformer (115 kV, 14.4 kV), operation from April 2014 for a two-year field test. In the treating the line-to-ground faults [22, 23]. This project was project, the official target for the non-operational time was sponsored by NYSERDA. four hours or less. After half a year of operation, the system The SFCL system was was operated for more than three was found to be highly reliable with only two or three hours years under various environmental conditions including of non-operational time. The system operation was temperature extremes and heavy snow [24]. Late 2017, extended beyond the initial project period, aiming for a when the machine was shut down, the system was reported long-term operation to have responded to 31 fault currents [25]. • Lechhausen substation, Statwerke-Augsburg 3.3. Germany The most recent progress in SFCL field testing has been Germany is active in both the manufacturing and initiated by Statwerke-Augsburg and Siemens under the application of SFCLs. Two manufacturers, Nexans project ASSiST. This project shows how decentralized Superconductors and Siemens, have supplied five SFCLs feed-in systems can be safely integrated into a for the field tests in Germany. medium-voltage network [42]. Here, a 10-kV SFCL protects the distribution grid between Augsburg and a • Netphen substation, RWE with CURL10 municipal utility company, MTU Onsite Energy The first field test was the CURL10 project. Nexans manufacturing a combined heat and power (CHP) plant Superconductors developed a 10-kV, 10-MVA resistive [43]. Siemens developed [44-46] and installed the SFCL at type SFCL using Bi2212 bulk. It was installed in a bus-tie the Lechhausen substation of the Statwerke-Augsburg grid position in the RWE grid of the Netphen substation and that started operating from March 2016 [42]. field-tested for one year, starting from April 2004 [26-30]. The test was to confirm not only the functionality and No fault conditions were reported during the testing period, reliability, but also the operational experience in regard to the economic aspect. The field test may continue until 2017. but a low-magnitude single-phase fault occurred, which However, this SFCL is expected to be in long term could not quench the superconducting elements. operation as contracts are arranged for permanent

installation [46]. Boxberg with Vattenfall and ENSYSTROB • Subsequent SFCL tests were performed at the Boxberg 3.4. United Kingdom power plant of Vattenfall. Nexans Superconductors The United Kingdom (UK) is very active in applying supplied an SFCL with ratings of 12 kV and 800 A. It was SFCLs, owing to its shift toward a low carbon-based installed in a feeder to a rebound hammer mill and economy. Here, we discuss the five field tests of SFCLs underwent testing by daily routine operation from November 2009 to October 2010 [31-34]. During operation, this system encountered two occurrences of power outages caused by ancillary part issues, the LN2 level sensing and temperature of the vacuum pumps. In both cases, the system was disconnected from the grid and brought back to operation shortly after resolving the problems [33]. The objective of ENSYSTROB, a government-funded project, was to substitute the Bi2212 superconductors of the SFCL of Vattenfall in Boxberg with components based on HTS coated conductors, to achieve low AC loss and faster fault current limitation [35,36]. All the non-superconducting parts such as the cryogenic system and ancillary equipment kept unchanged. A successful factory test was completed in September 2011, followed by implementation in the SFCL system of Vattenfall for a one-year field test.

• AmpaCity and Hercules substation, RWE Another SFCL field test program is the Ampacity project, Fig. 2. Main part of the SFCL at Statwerke-Augsburg [43]. in which an HTS cable and SFCL combined system was (Courtesy by Siemens.)

Brief review of the field test and application of a superconducting fault current limiter

conducted in the UK. All of them were supported by the 3.5. Italy, Spain, and other parts of Europe Low Carbon Network Fund (LCNF) through the • San Diogini, A2A Reti Elettriche, Italy Innovation Funding Incentive [47-50]. A consortium The Italian SFCL program was conducted by Ricerca sul comprising of Applied Superconductor Limited (ASL) and Sistema Energetico (RSE). A 9-kV, 3.4-MVA SFCL was three distribution network operators deployed three pilot developed and installed to protect a distribution feeder in a SFCLs, which were installed at the Bamber Bridge, medium voltage substation, San Dionigi, in the Milan Ainworth Lane, and Scunthorpe substations. In addition, distribution grid belonging to the utility company, A2A Western Power Distribution (WPD) supported two SFCLs Reti Elettriche S.p.A. The SFCL was commissioned on for field application at the Chester Street and Bournville March 2012 and field-tested until June 2014. This SFCL substations in the Birmingham area [51, 52]. experienced an artificial three-phase fault, with a prompt limitation of the fault current by a factor of approximately 2. • Bamber Bridge substation, Electricity North West The second phase test with an upgraded 15.6-MVA unit is (ENW) underway [60-63]. The first of the three SFCLs by ASL and consortium was a resistive type SFCL, with 12 kV and 100 A ratings. • ECCOFLOW project (Endesa & VSA) Nexans Superconductors supplied the SFCL, which This was an EU project for the development and testing consisted of Bi2212 tubes in a bifilar design. It was of a 24-kV, 1-kA SFCL based on HTS coated conductors installed in October 2009 in the 11-kV bus section of the [64, 65]. It aimed to becoming the first permanent grid Bamber Bridge substation of the ENW network near installation. Two installation sites were proposed: the Lancashire. It was energized for one year and carried load Endesa grid in Spain, and the Kosice of the VAS grid in currents for approximately four months [47, 53, 54]. Slovakia. The machine was successfully designed and manufactured by Nexans Superconductors [66]. • Ainworth Lane substation, Scottish Power The SFCL was installed in the bus-bar coupling location The second resistive SFCL, rated 11 kV and 400 A, was of the San Juan de Dios substation in the Endesa grid, commissioned at the Ainsworth Lane substation in Palma de Mallorca, Spain. The live grid operation was Liverpool in the Scottish Power grid in August 2012. The scheduled from October 2013 for six months [67]. SFCL was installed in the bus-section location [47]. This Thereafter, it was to be shifted to the VSA network in SFCL, however, was only in normal operation for Košice, Slovakia [68, 69]. approximately 12 months, whereas it was operational in situ for approximately 20 months, due to the technical 3.6. China problems associated with the cryogenic system [55]. • Gaoxi and Baiyin The Chinese Academy of Sciences (CAS) initiated the • Scunthorpe, Northern Power Grid (Formally CE first SFCL development. The SFCL was the bridge type, Electric) with ratings 10.5 kV and 1500 A and three phases. It was The third trial of SFCL in the UK was a saturated first installed in the Gaoxi substation, Hunan, China, and iron-core type SFCL developed by ASL. It was installed in operated for 11000 hours. Thereafter, it was shifted to the the bus-section location of the Scunthorpe substation of the HTS power substation to be integrated with an HTS cable Northern Power Grid in Lincolnshire. The SFCL, rated 11 and a superconducting magnetic energy storage in Bayin, kV and 1250 A, was in a live-grid service from July 2012, Gansu for further operation [70-72]. and it operated for two years, which ended in 2014 [47]. The SFCL experienced a three-phase fault with a successful • Puji substation, China Southern Power Grid current limitation [53]. Innopower successfully developed a saturated iron-core type SFCL, operating at 35 kV. The characteristic features • WPD – Chester Street and Bournville, Birmingham of the SFCL included a power electronic switch and a piezo Two more trials of SFCL applications were initiated by resistor, which are essential parts to de-energize an HTS WPD. Both were supplied by Nexans Superconductors, coil. The SFCL was installed to protect a feeder line at the and installed in the bus-section location of the WPD grid. Puji substation in the China Southern Power Grid, and was [51], [56-58]. commissioned on December 2007 for four years of One SFCL unit, rated 12 kV and 1600 A, was installed in operation [73-77]. Since being commissioned, one minor the Chester Street substation. Another unit, rated12 kV and short circuit event has occurred, for which the DC 1050 A, was installed in the Bournville substation in magnetization switch was safely switched as designed. Birmingham. The Chester Street SFCL was put into service However, a complete performance evaluation was not from November 2015, whereas the Bournville SFCL available due to small fault currents. started service from March 2016. Both machines encountered temporary stops owing to the problems of the • Schegezhuang substation, State Grid of China cooling system in the early stage of operation. They were Following the 35-kV SFCL, Innopower continued to re-connected after repairs [59]. develop a 220-kV (300 MVA) saturated iron-core type Both are expected to complete a two-year field testing. SFCL under the government support through National 863 They are projected to be operated for commercial service Plan. After completing the factory tests, the SFCL was after the test. installed at the Shigezhuang substation in the Chinese State

Ok-Bae Hyun

grid in Tianjin, China. It began operation since June 2012, 115-kV SCE power network near Riverside, California, and was planned for a three-year live grid operation to test where it was supposed to service a bus tie. its feasibility [78-81]. Another case is the 138-kV saturated iron-core type SFCL of Zenergy, which was eventually not completely 3.7. Korea developed. It was to be installed at the Tidd substation of Supported by 10 years of SFCL development efforts [82], the American Electric Powers located near Steubenville, KEPCO Research Institute (KEPRI), Korea Electric Power Ohio in 2012. The installation was planned to be on the low Corporation (KEPCO), launched a project for SFCL field side of the 345-kV to-138 kV transformer to protect the testing. LS Industrial Systems (LSIS) manufactured a 138-kV feeder [16]. resistive type SFCL rated 22.9 kV and 630 A [83-88]. It was installed to protect a feeder line at the Icheon 3.10. Commercial application, Glow Energy, Thailand substation of the KEPCO grid. Prior to energizing, the In April 2015, AMAT announced that it received orders SFCL was pre-tested for more than five months. It was for two transmission-class SFCLs from Glow Energy, an energized on August 2011, and started its live-grid service independent power producer (IPP) in Thailand. As the for more than 1.5 years. Two fault events occurred on the utility company planned to connect the additional line, but the SFCL limited the fault currents successfully as generation capacity to the 115-kV line, the increased fault designed [87]. current was to be reduced prior to coupling with the power grid [25]. 3.8. Tokyo Gas AMAT manufactured two sets of resistive type SFCLs Toshiba developed a 6.6 kV resistive type SFCL. The with ratings of 115 kV and 900 A [93-95]. They were SFCL was primarily comprised of sets of current limiting installed at the secondary side of the transformers at Map coils of coated conductors operating in subcooled liquid Ta Phut Industrial Estate of Glow Energy grid in Rayong, nitrogen. The SFCL was installed between a gas engine Thailand, and commissioned on July 2016. This may be the generator and utility grid at the Senju Techno station, first commercial activity of an SFCL application, as the Tokyo Gas in early 2008, followed by a consecutive SFCL installation and operation were conducted in the operation. It was operated on a daily start-and-stop basis private sector. [89-91]. 3.11. Ongoing projects and further activities 3.9. Un-completed trials of field testing One of the trials of SFCL in the UK sponsored by the There were projects aimed at development and field tests, LCNF is to apply a 33-kV unit at the Jordanthorpe but did not result in live-grid operation. These are substation of the Northern Power Grid in South Yorkshire. summarized by the location of the SFCL installation in the After ASL, ASG Power Systems assumed the development grid. of a 35 kV saturated iron-core type SFCL. It was planned to An example is the DOE-sponsored 138-kV SFCL be installed at the tail side of a 275-kV, 33-kV transformer project led by American Superconductors and Siemens. As at the substation in 2017 [96, 97]. of 2011, when the project was stopped [1], a single-phase KEPRI has developed a single-phase prototype of the prototype of a resistive SFCL operating at 138 kV was resistive SFCL with ratings of 154 kV and 2000 A [98]. successfully built and tested [92]. A three-phase unit was The SFCL was installed at the Gochang Power Testing planned to be installed at the Devers substation in the Center, KEPCO, for various performance tests including a short-term connection to the 154 kV line [99]. Manufacturing and field test of a three-phase unit are under planning.

Fig. 3. SFCL of 22.9 kV at the Icheon substation, which performed a continuous operation for two years [86]. Fig. 4. SFCL of 115 kV under commercial service at Glow (Courtesy by KEPRI.) Energy. (Source: Applied Materials, Inc.)

Brief review of the field test and application of a superconducting fault current limiter

SuperOx project, through which UNECO, a utility company in Moscow, Russia, is aiming for a commercial application of an SFCL. In this project, a 220 kV SFCL is used to protect the network coupling between the substations in the UNECO grid. A single phase prototype is under development by SuperOx. A three-phase unit is planned to be installed at the Mnevniki substation for in-grid operation in 2018 [100, 101]. This project deserves attention in terms of the commercialization since it was planned and carried out in the private sector.

4. APPLICATION TREND and DISCUSSION

Fig. 5. Prototype of the 154 kV SFCL by KEPRI [99]. As described in the previous section, since 1996, there (Courtesy by KEPRI.) have been more than 20 field applications. These are summarized in Table 1 chronologically, instead of by The most recent activity in the SFCL application is the country.

TABLE 1 SUMMARY OF THE SFCL FIELD TESTS AND APPLICATIONS. Project or Financial Manufacturer Ratings Field test (FT) or live-grid operation period Installation site Support (Utility) (Type) V(kV) I(A) Löntsch, Switzerland PSEL (NOK) ABB (MS) 10.5 66 Power plant grid (1996 - 1997, six months) SCE Center SS, LA,USA SPI-DOE (SCE) GA (BG) 15 1200 SCE grid (operation test N/A) Nephen SS, Germany CURL10 (RWE) Nexans (R) 10 800 Bus-tie (Apr. 2004 - Mar. 2005) Baiyin SS, Gansu, China MOST, China (N/A) CAS (BG) 10.5 1500 Transformer (secondary) (Feb. 2011 - Mar. 2015) Tokyo Gas, Japan NEDO (Tokyo Gas) Toshiba (R) 6.6 72 IPP connection (Jan. 2008 -, N/A) CMST (Southern China Innopower Puji SS, Kunming, China 35 1500 Outgoing feeder (Feb. 2008 - more than four years) Power Grid) (SI) CEC Avanti & DOE Shandin SS, LA, USA Zenergy (SI) 15 2000 Outgoing Feeder (Mar. 2009 - Oct. 2010) (SCE) Bamber Bridge, IFI-LCNF (ENW) Nexans (R) 11 100 Busbar coupling (Oct. 2009 - Jun. 2010) Lancashire, UK Boxberg power plant, Outgoing feeder (power plant grid) Vattenfall (Vattenfall) Nexans (R) 12 800 Germany (Nov. 2009 - Oct. 2010) Boxberg power plant, ENSYSTROB Outgoing feeder (power plant grid) Nexans (R) 12 800 Germany (Vattenfall) (Oct. 2011 -, one year) Icheon SS, Korea GENI (KEPCO) LSIS (R) 22.9 630 Outgoing feeder (Aug. 2011 - Jan. 2013) Ainsworth Lane, IFI-LCNF (Scottish Nexans (R) 11 400 Busbar coupling (Aug. 2012 - Mar. 2014) Liverpool, UK Power) San Dionigi SS, Milano, RSE (A2A) RSE (R) 9 (3.4 MVA) Feeder protection (Mar. 2012 - Jun. 2014) Italy Scunthorpe SS, North IFI-LCNF (Northern Zenergy, ASL 11 1250 Transformer (secondary) (Aug. 2012 - 2014) Lincolnshire, UK Power Grid) (SI) Shegezhuang SS, Tianjin, 863 plan, China Innopower Transformer (primary) or Network coupling 220 800 China (Chinese State Grid) (SI) (Jun. 2012 -, three year operation planned.) San Juan de Dios, Palma ECCOFLOW (Endesa) Nexans (R) 24 1005 Bus-tie (N/A, Planned Oct. 2013 -, for six months) de Mallorca, Spain AMAT, Santa Clara, USA AM AT (AM AT) AM AT (R ) 15 1000 Local grid protection (Jul. 2013 - N/A) Hercules SS, Essen, HTS cable protection (Mar. 2014 -, two year field test, Ampacity (RWE) Nexans (R) 12 2300 Germany operation term extended) Knapps Corner SS, NYSERDA (Central AM AT (R ) 15 400 NGR (Jun. 2014 - Fall 2017) Poughkeepsie, NY, USA Hudson G&E) Chester street SS, Busbar coupling (Nov. 2015 -, two year FT & long term FlexDGrid (WPD) Nexans (R) 12 1600 Birmingham, UK operation planned) Bournville, Birmingham Busbar coupling (Mar. 2016 -, two year FT & long term FlexDGrid (WPD) Nexans (R) 12 1050 SS, UK operation planned) Lechhausen SS, ASSiST IPP (CHP) connection Siemens (R) 10 815 Augsburg, Germany (Stadtwerke-Augsburg) (Mar. 2016 - 2017 & long term operation planned) Map Ta Phut, Rayong, Transformer (secondary), two units N/A (Glow Energy) AM AT (R ) 115 800 Thailand (Jul. 2016 -, commercial service) ※ MS = Magnetic shield type, BG = Bridge type, SI = Saturated iron-core type, R = Resistive type, N/A = Not Available.

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4.1. Application trend locations in Fig. 6. Un-completed trials of the field tests are Table 1 shows a clear trend in the SFCL types. It reveals included to examine the potential application sites. Table 2 that the resistive type is more preferred in the SFCL field reveals that the major applications of the SFCLs in the grid applications than any other types. In particular, all the are for the protection of the bus-tie section and transformer recent trials of the field tests in the last five years have been feeder. Various other locations such as network coupling, performed using the resistive type. Furthermore, as of 2017, IPP coupling, outgoing feeder, NGR, as well as those under operation are all resistive types. This implies combination with other HTS equipment such as an HTS that the resistive type has more technological advantages cable are also feasible, as proven by the field tests and than the other SFCL types. applications.

4.2. Technical issues Early trials of the SFCL field tests include the magnetic shield type and bridge type SFCLs. The former was applied once, as can be noted from Table 1. Producing large-sized superconducting rings may be difficult. The latter was applied twice. The SFCL by GA had troubles not only in the large power electronic switches, but also in electrical insulation [13, 14]. These issues might deter further development of the SFCL types. Saturated iron-core type SFCLs have been actively researched and developed, as summarized in Table 1. This type is particularly attractive because no line currents are carried to the superconductors. Here, it is important to protect the HTS coil from the induced current due to the abrupt change in the magnetic flux in the iron core upon fault. There are two approaches to prevent the HTS coil damage: (i) use a protection circuit that bypasses the induced current to external circuits and (ii) discharge the Fig. 6. Schematic of the potential SFCL locations [3]. HTS coil in a few milliseconds. The former was used by

Zenergy, ASL, and currently, ASG power Systems. The TABLE 2 latter was developed by Innopower. APPLICATION LOCATIONS OF THE SFCLS. (NUMBERS IN BRACKETS ARE The protection circuit of ASG has not proven sufficiently INSTALLATION LOCATIONS BASED ON FIG. 6.) [96, 97]. In fact, no further successful development and Application Locations Installation site Generator feeder (1) field-test was publicly reported after the 12-kV SFCL of - Löntsch Power station auxiliaries (2) ASL was installed at the Scunthorpe substation. However, - Boxberg the design of Innopower was technically viable, as proved - Puji by the 35-kV and 220-kV SFCLs as well as the recent - Netphen validation test with a single-phase prototype operating at - Bamber Bridge - Ainsworth Lane Network coupling (3), 500 kV [102]. This design requires actively-controlled - San Juan de Dios Busbar coupling (4) circuits to discharge the HTS coil in a few milliseconds on - Chester Street encountering a fault [80]. - Bournville Major technical complications in a resistive-type SFCL - Devers, SCE (115 kV)* - Mnevniki (220 kV)** design are due to the structure that carries the line current Protection of superconducting - Hercules directly through the superconductors. This causes technical equipment (6) Loop - Closing ring circuit (7) problems in the electrical insulation at cryogenic temperatures, and requires bulky cryostats and a high Transformer (primary) (5) - Shigezhuang (220 kV)*** cryogenic load. However, these technical issues can be - Gaoxi–Loudi & Baiyin - Shandin solved, in principle, by adapting an insulation distance - VSA (Eccowflow) under a high pressure, a live-tank cryostat, or high-capacity Transformer (secondary) (8) - Scunthorpe cryocoolers. These lend the resistive type SFCL a - Map Ta Phut technological maturity, leading to further field applications - Jordanthorpe (33 kV)** - Tidd, AEP (138 kV)* [103]. However, further technological development is - Icheon Outgoing feeder (9) necessary for cost reduction. - San Dionigi Shunting CLR / NGR (10) - Knapps Corner 4.3. Field test locations Coupling local generating units - Tokyo Gas (IPP) Figure 6 is a schematic of the potential locations for an (11) - Lechhausen, Augsburg (CHP) * Planned, but did not conclude with field test. SFCL. It is applicable wherever fault currents need to be ** On-going project. reduced. Table 2 lists the installation locations of the ** The SFCL is presumably at the primary side of a transformer (5) or SFCLs in Table 1. The numbers in brackets denote the network coupling location (3).

Brief review of the field test and application of a superconducting fault current limiter

Early field tests were conducted primarily to prove the locations such as network coupling, IPP coupling, SFCL performance, as they were often installed at locations outgoing feeder, NGR, as well as combination with other where the SFCLs were not entirely needed. This was a HTS equipment were also feasible. useful approach because the newly developed power • The current limitation capability was proven by their device was not sufficiently reliable, therefore, could be limiting the currents of real faults in the grids during the disconnected safely from the grid in case of emergency. tests. For the recent SFCL installations in the last several years, • A high reliability with a cryogenic system is desired for the SFCL locations were carefully selected based on the long-time operation. Pre-operation before live-grid fault current analysis. All the five substations in the UK operation or redundant cryocoolers may be practical. trials were under excessive fault currents, so that fault • Most of the field tests were conducted as R&D projects current mitigation was demanded anyway [58]. In under the support of public funds. This implied that an particular, fault current reduction was necessary for both SFCL is not economically feasible under the current Stadtwerke-Augsburg and Glow Energy due to the technology, and efforts for reducing the cost are crucial increased fault currents when additional generations were for its general-purpose applications. connected to the grid. The SFCLs may be a vital technology • A relevant change from the R&D stage to the application for current limitation in the next stage. stage was shown, as recently deployed SFCLs are expected to be under long-term operation beyond the 4.4. Economic feasibility initial project period. Furthermore, as in Glow Energy Early trials of a new technology used to depend on the and UNECO, the recent activities in the private sector technological completeness. In this view, it is logical that could be indicators of the commercial application from the technical maturity of the resistive type leads to the the perspective of electric utility.

SFCL tests.

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