pISSN 1229-3008 eISSN 2287-6251 Progress in Superconductivity and Cryogenics Vol.22, No.1, (2020), pp.7~11 https://doi.org/10.9714/psac.2020.22.1.007
Design of HTS power cable with fault current limiting function
Dongmin Kim a, Sungkyu Kimb, Jeonwook Chob, and Seokho Kim *, a
a Changwon University, Changwon, Korea b Korea Electrotechnology Research Institute (KERI), Changwon, Korea
(Received 19 February 2020; revised or reviewed 27 March 2020; accepted 28 March 2020)
Abstract
As demand for electricity in urban areas increases, it is necessary to improve electric power stability by interconnecting neighboring substations and high temperature superconductor (HTS) power cables are considered as a promising option due to its large power capacity. However, the interconnection of substations reduces grid impedance and expected fault current is over 45 kA, which exceeds the capacity of a circuit breaker in Korean grid. To reduce the fault current below 45 kA, a HTS power cable having a fault current limiting (FCL) function is considered by as a feasible solution for the interconnection of substations. In this study, a FCL HTS power cable of 600 MVA/154 kV, transmission level class, is considered to reduce the fault current from 63 kA to less than 45 kA by generating an impedance over 1 Ωwhen the fault current is induced. For the thermal design of FCL HTS power cable, a parametric study is conducted to meet a required temperature limit and impedance by modifying the cable core from usual HTS power cables which are designed to bypass the fault current through cable former. The analysis results give a minimum cable length and an area of stainless steel former to suppress the temperature of cable below a design limit.
Keywords: impedance, fault current, former, HTS power cable, quench
``` 1. INTRODUCTION nitrogen to ensure its dielectric strength. The resistive impedance is related material and cross section area of Annual power demand is continuously increasing and cable former and resistance of HTS wire itself over the interconnections of substations are considered in an urban critical current at quench condition. The temperature rise area to enhance grid stability. The interconnection of of the FCL HTS power cable is related to the heat capacity substations however is expected to increase the fault and the generated Joule heating. Therefore, the heat current in the event of a short circuit. This is meaning capacity should be increased while the resistance should be exceeding the maximum capacity of the existing circuit larger than the design value to reduce the fault current. breaker. [1, 2]. This paper describes a design of FCL HTS power cable To reduce the fault current, many kinds of of 600 MVA, 154 kV class with developed HTS wires. The superconducting fault current limiters (SFCL) have been target FCL function of the cable is reducing the fault developed and some are operating in commercial grids current from 63 kA to less than 45 kA considering [3–12]. A standalone SFCL usually is installed in the transmission level in South. To meet the allowable substation and connected to power cables. It requires temperature rise and the fault current reduction, parametric cryogenic refrigeration system and cryostat as in high studies were conducted for the resistance characteristics of temperature superconductor (HTS) power cables. Since the HTS wire, cross sectional area of former which is made the SFCL operating in case of fault conditions, it just by stainless steel and the cable length. The design process consumes electric power and occupies an installation space involved a thermal analysis model incorporated with an for normal operation. To solve this inconvenience, several electric circuit that includes electric characteristics of FCL HTS power cable. research efforts have been made to combine the SFCL with the HTS power cable, so called fault current limiting
(FCL) HTS power cable [12, 13]. 2. TEMPERATURE RISE OF FCL HTS POWER The general of HTS power cables are designed to bypass CABLE BY LUMPED ANALYSIS the fault current using a low resistance of copper former, where the maximum fault current is limited by an internal 2.1. Characteristics of HTS wire impedance in a transformer [14, 15] The FCL HTS power cable utilizes the increase of However, the FCL HTS power cable should be designed resistance caused by quench of HTS wires when the fault to generate appropriated resistive impedance when the current is introduced. Thus, designing the FCL HTS power fault current is introduced while temperature of the HTS cable is related with the resistance of HTS wire during cable should be below the saturation temperature of liquid quench status at operating temperature. * Corresponding author: [email protected]
8 Design of HTS power cable with fault current limiting function
Fig. 1. Circuit diagram of HTS wire.
Fig. 4. Circuit diagram of FCL HTS power cable. The resistance of Rare earth (RE) Ba2Cu3O7-x (REBCO) HTS wire is mainly affected by a substrate, stabilizer layers, silver layer, and other additional lamination layers. The fabricated HTS wires were installed in a resistance Then, the equivalent electric circuit can be express as in measurement facility, which uses a G-M cryocooler, and Fig. 1 including equivalent matrix resistance of metal Rm of each wire was measured depending on temperature. layers (Rm) parallel to that of REBCO HTS layer (Rs). Fig. 2 shows the measured temperature resistance curve for To generate the large resistance when the fault current is each test sample with transport current of 1 A. introduced, Rm should be large and it depends on the According to the experimental results, the sample No. 2 configuration of substrate, stabilizer layer and other showed the largest resistance, where the thicknesses of additional lamination layers. In this study, we considered 3 silver and copper layer were minimized. In addition, the kinds of HTS wire, modified for the FCL HTS power cable, critical current of each HTS wire was also measured in a as shown in Table I. Ordinary copper stabilizer by liquid nitrogen bath as in Fig. 3. electroplating outside the HTS wire was excluded to 2.2 Lumped thermal analysis model of FCL HTS power increase the resistance. cable. TABLE I Prior to detail design and analysis on the thermal and SPECIFICATIONS OF HTS WIRE. electric behavior of FCL HTS power cable, a lumped analysis, which assumes uniform temperature in the cross Substrate Stabilizer Overall Sample thickness configurations Thickness / section of cable, was conducted to investigate the effect of number (m) thickness (m) width (mm) design parameters such as the cable length and the cross STS310s Ag/Cu/Sn sectional area of former. No. 1 103.0 5.7/2.9/29 0.143/4.05 The magnitude of the fault current is determined by STS310s Ag/Cu/Sn/STS No. 2 0.161/4.38 100.3 3.3/1.6/13.1/42.3 equivalent cable resistance (Rcable) and the internal STS310s Ag/Sn/STS resistance of the transformer (Ri). Fig. 4 shows a simplified No. 3 0.233/4.46 102.0 4.1/18.4/104.0 circuit diagram to calculate the fault current. The equivalent cable resistance is calculated by the parallel resistances: Rs, Rm and resistance of former (Rf). Fig. 4 illustrates the circuit diagram for the fault current when the load resistance (RL) becomes zero by short circuit. The fault current (If) for the FCL HTS power cable is represented by the sum of the critical current flowing through HTS layers (Ic(T)) and the current thourgh parallel resistance (Rm,f) of Rm and Rf with the voltage drop across the cable (Vcable) as equation (1) [16].
Vcable IITfc=+() (1) Fig. 2. Temperature dependent resistance of REBCO Rmf, wires. The cable critical current is the sum of the critical current of all HTS wire and its temperature dependency can be simply represented by equation (2) considering the critical current at 77 K (Ic,77K) and the critical temperature (Tc) [16].
Ic,77 ITTTcc()()=− (2) Tc − 77 The voltage drop across the cable in (1) is obtained by subtracting voltage drop across Ri from Vs as in equation (3). Fig. 3. Measurement of critical current at 77 K.
Vcable =Vs − Ri I f (3)
Dongmin Kim, Sungkyu Kim, Jeonwook Cho, and Seokho Kim 9
Combining equation (3) with (1), If is expressed as and areas of the former. To increase the resistance while keeping the large heat capacity, stainless steel was Vs + Ic(T )Rm, f considered for the former material. I = (4) f The specifications and operation requirements of the Rm, f + Ri FCL HTS power cable are described in Table II and Fig. 5
According to equation (4), If depends on Rm,f and Ic(T). shows the schematic diagram of its cross section. The As Rm,f decreases, If approaches the maximum value, which internal resistance was determined to be 1.411 to make is the case of usual HTS power cables. On the other hand, the maximum fault current (63 kA) in transmission level in Rm,f should be increased to reduce If, Yet, the temperature South Korea. At the instance of fault, the RL is assumed as rise should meet the maximum allowable temperature, 0 . Considering the capacity of a circuit breaker, If should which is an operation constraint. Combining equation (3) be decreased below 45 kA, while keeping the maximum and (4), the heat generation (Gcable) is given by temperature below 95 K to avoid dielectric problem. The fault currents and allowable cycles were calculated Gcable = Vcable I f (5) using (4) and (6). Fig. 6 and Fig. 7 show the allowable fault Vs + Ic (T )Rm,f Vs + Ic (T )Rm,f current cycles calculated from the time to reach the = V − R m C s i maximum operation temperature limit and the fault current Rm,f + Ri Rm,f + Ri in accordance with the cable length, the types of HTS wire, Where mass(m)ⅹ specific heat(C) is the total heat and the area of former. In transmission level in South capacity of the whole cable components at 80 K, and is Korea, the cable should transport at least 12 cycles of the proportional to the cross sectional area and the length of fault current. According to the analysis results, the cable the cable (L). length should be longer than 6 km with the stainless steel Considering the energy balance in the cable, the cross sectional area of former of 1,200 mm2 to meet the temperature rate (dT/dt) is given by constraints of fault current(below 45 kA) and dT temperature(blower 95 K). G = mC Although No. 3 HTS wire showed the least temperature cable dt (6) rise, it has no significant difference in the magnitude of the dT V + I (T )R V + I (T )R = V − R s c m,f s c m,f m C fault current because resistance among the parallel s i connection of former and HTS wire. dt Rm,f + Ri Rm,f + Ri
2.3 Lumped analysis results of FCL HTS power cable The temperature rise and the fault current were analyzed in accordance with the length of cable, types of HTS wire,
Fig. 6. Calculated allowable cycles of FCL HTS power cable by lumped analysis.
Fig. 5. Schematic diagram for the cross section of FCL HTS power cable. TABLE II SPECIFICATIONS AND ANALYSIS PARAMETERS OF FCL HTS POWER CABLE. Parameter Value
Rated capacity (MVA) 600 Rated voltage (kV) 154 Number of HTS wire (ea) 24 Internal resistance () 1.411 Fault current (kA) 63 Fault current input cycle 12 (Zone-1 of Korean grid) Fig. 7. Calculated fault current of FCL HTS power cable Former (mm2) 500, 900, 1,200 by lumped analysis. Length of HTS Power Cable (km) 1 ~ 10
10 Design of HTS power cable with fault current limiting function
3. FEM ANALYSIS OF FCL HTS POWER CABLE 4. SUMMARY AND CONCLUSION
3.1 FEM analysis model of FCL HTS power cable In this study, we developed a simplified lumped analysis The lump analysis assumes the same temperature in the model to design of a FCL HTS power cable. For the cross section of cable, but the actual temperature precise modelling, the resistances of manufactured HTS distribution in the cross section varies with the radial wires were measured and taken into the analysis model. distance, depending on the heat capacity and the heating Through the lumped analysis model, it was confirmed that power of individual components such as the stainless steel the cable length should be longer than 6 km with a stainless former and the HTS layer. To consider the detail steel cross sectional area of former of 1,200 mm2 to limit temperature distribution, 2D FEM analysis was conducted the maximum temperature below 95 K while reducing the the radial heat transfer between components in Fig. 5. fault current below 45 kA from 63 kA. For more precise In order to distinguish the local heat generations in the simulation, FEM analysis was conducted with the cross former and the HTS layer, the current in each layer and section of the cable using a fully coupled heat transfer and resultant heat generations should be separated as in the electric circuit model. Due to the localized heat generation circuit diagram of Fig. 4. The voltage drop of cable can be and poor heat transfer between the HTS layer and stainless obtained combining (1) and (3) as in (7). Then, Iformer and steel former, the temperature of HTS layer increased more Iwire can be determined by (8). than the lumped model. In this case, the required cable length was increased to 9 km. VRIT− () V = s i c (7) Through the lumped analysis and FEM analysis, it was cable R 1+ i possible to obtain the required cable length and the cross R mf, sectional area of former by increasing both the resistance and heat capacity. However, it is not thought to be easy to V meet our design requirements with only resistive IIII=cable , = − (8) formerR wire f former impedance due to the large fault current and the grid f voltage in Korea transmission grid. To meet the design To calculate the transient heat transfer and the variation requirements with a short length of the cable, other of fault current, a commercial FEM analysis software, impedance generation mechanism, such as inductive comsolmultiphysics ver. 5.2, was adapted using a fully impedance, should be incorporated. Whereas it may be coupled heat transfer and electric circuit module. The Joule possible to obtain an appropriate design results for the heating of each layer is calculated by Vcable, Iformer and If to distribution level grid, where the grid voltage is 22.9 kV be used in the heat transfer module. The low thermal and maximum fault current is 25 kA. conductivity of insulation layer of PPLP (polypropylene laminated paper) and temperature boundary condition were considered [17]. The resultant temperature is reflected in the calculation of Ic(T) and temperature dependent electric resistivity. 3.2 Analysis results of FCL HTS power cable In the analysis model, the sample No. 3 was considered due to its largest resistance. Table III shows the analysis parameters. According to the FEM analysis, the concentrated heat generation in the thin HTS layer increased the temperature more and the resultant allowable fault cycles decreased at the same cable length compared with the lumped analysis. Fig. 8. Comparison of FEM and lumped analysis for the And the fault current was decreased from FEM analysis allowable cycles and the fault current. due to the increased temperature of HTS wire. Fig. 8 compares the FEM analysis with the lumped analysis. Fig. 9 shows a representative FEM analysis result of 9 km that meets the temperature and fault current constraint. In this case, the allowable fault cycle was 13 cycles and the fault current was 17 kA.
TABLE III FEM ANALYSIS PARAMETERS. Parameter Value Length of HTS power cable (km) 5 ~ 9 Cross sectional area of former (mm2) 1,200 HTS wire No.3 Fig. 9. Representative FEM analysis results for the variation of Initial temperature (K) 73.5 temperature and fault current when the cable length is 9 km.
Dongmin Kim, Sungkyu Kim, Jeonwook Cho, and Seokho Kim 11
ACKNOWLEDGMENT [6] B. Li, F. Guo, J. Wang, and C. Li, “Electromagnetic transient analysis of the saturated iron-core superconductor fault current limiter,” IEEE Trans. Appl. Supercond., vol. 25, no. 3, pp. 1-5, This supported by the Korea Institute of Energy 2015. Technology Evaluation and Planning (KETEP) and the [7] M. Chen et al., “6.4 MVA resitive fault current limiter based on Ministry of Trade, Industry & Energy (MOTIE) of the Bi-2212 superconductor,” Phys. C Supercond., vol. 372–376, no. Republic of Korea (No. 20171220100400), and the PART 3, pp. 1657–1663, 2002. [8] S. Srilatha and K. V. Reddy, “Analysis of positioning of National Research Foundation of Korea (NRF) grant superconducting fault current limiter in smart grid application,” funded by the Korea government (MSIT) (No. INTERNATIONAL JOURNAL OF ADVANCED AND 2019R1A5A808320111) INNOVATIVE RESEARCH, no. January 2012, pp. 24-31, 2016. [9] R. Dommerque et al., “First commercial medium voltage superconducting fault-current limiters: Production, test and installation,” Supercond. Sci. Technol., vol. 23, no. 3, 2010. REFERENCES [10] J. G. Kim et al., “HTS power cable model component development for PSCAD/EMTDC considering conducting and shield layers,” [1] L. Ye, M. Majoros, T. Coombs, and A. M. Campbell, “System IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 1785–1788, 2009. studies of the superconducting fault current limiter in electrical [11] C. Neumann, “Superconducting fault current limiter (SFCL) in the distribution grids,” IEEE Trans. Appl. Supercond., vol. 17, no. 2, pp. medium and high voltage grid,” 2006 IEEE Power Eng. Soc. Gen. 2339–2342, 2007. Meet., pp. 1–6, 2006. [2] Y. Jia, J. Yuan, Z. Shi, H. Zhu, Y. Geng, and J. Zou, “Simulation [12] C. M. Rey et al., “Test results for a 25 meter prototype fault current method for current-limiting effect of saturated-core limiting HTS cable for project hydra,” AIP Conf. Proc., vol. 1218, superconducting fault current limiter,” IEEE Trans. Appl. pp. 453–460, 2010. Supercond.,vol. 26, no.4, pp. 1-4. 2016. [13] J. Maguire et al., “Status and progress of a fault current limiting [3] M. C. H. V. M. R, “A review on super conducting fault current HTS cable to be installed in the con Edison grid,” AIP Conf. Proc., limiter ( SFCL ) in power system,” Int. J. of Eng. Research and vol. 1218, no. 3, pp. 450 2010. General Science, vol. 3, no. 2, pp. 485–489, 2015. [14] J. H. Kim et al., “Investigation of the over current characteristics of [4] S. Yadav, G. K. Choudhary, and R. K. Mandal, “Review on fault HTS tapes considering the application for HTS power devices,” current limiters,” Int. J. Eng. Res. Technol., vol. 3, no. 4, pp. IEEE Trans. Appl. Supercond., vol. 18, no. 2, pp. 1139–1142, 2008. 1595–1604, 2014. [15] M. Yagi et al., “Design and evaluation of 275 kV-3 kA HTS power [5] S. H. Lim, “Comparative study on current limiting characteristics of cable,” Phys. Procedia, vol. 45, pp. 277–280, 2013. flux-lock type SFCL with series or parallel connection of two [16] Y. Iwasa, Case studies in superconducting magnets: Design and coils,” Phys. C Supercond., vol. 468, no. 15–20, pp. 2076–2080, operational issues: Second edition, Springer Science & Business 2008. Media, 2009. [17] Y. S. Choi, D. L. Kim, D. W. Shin, and S. D. Hwang, “Thermal property of insulation material for HTS power cable,” AIP Conf. Proc., vol. 1434, no. 57, pp. 1305–1312, 2012.