Superconducting Fault Current Limiter Coil Research Articles

Integration of superconducting fault current limiter with solid-state DC circuit breaker

The onboard direct current (DC) system in electric aircraft could face a severe fault current, which is tens of times the nominal current in a short-circuit fault. It is critical to limit the fault current and clear the fault within a few milliseconds to prevent any damage to the DC system. A protection method using a resistive superconducting fault current limiter (SFCL) with a solid-state DC circuit breaker (SSCB) to manage the DC short-circuit fault is proposed and experimentally verified. A bifilar SFCL coil prototype with two types of connection to achieve low and high inductance is designed and tested, which reduces the fault current considerably from 2000 A to below 1000 A. The performance when integrating the low and high inductance SFCL with a solid-state DC circuit breaker are investigated. It is found that when integrating the SFCL with the SSCB, a high voltage is induced across the high inductance SFCL during current interruption tests. In terms of reliability and durability, the low inductance SFCL is preferred to integrate with the SSCB. The experimental results show that the low inductance SFCL can be an effective solution to protect the DC system from severe fault currents and then SSCB can rapidly and reliably interrupt the fault current at 1000 A.© 2017 Elsevier Inc. All rights reserved.

Quench recovery time of an optical fiber encapsulated resistance-type DC superconducting fault current limiter coil determined by a Raman-based distributed temperature sensing system

The superconducting fault current limiter (SFCL), as a very important power facility for high voltage DC system, can consume fault current energy and make DC breakers cut off fault transmission lines successfully. For the moment of reclosing DC breakers, the quench recovery time of the SFCL must be determined in advance. In this article, according to the design requirement of a 160 kV/1 kA resistance type DC SFCL in the Nan’ao multi-terminal flexible HVDC transmission demonstration project, the quench recovery time of a SCFL coil wound by two optical fiber encapsulated high temperature superconducting (OFE-HTS) tapes was checked by our optimized Raman-based distributed temperature sensing (Rm-DTS) system. The experimental results show that the used OFE-HTS tapes can withstand a 1500 A/100 ms AC short circuit current impact and are qualified with the SFCL. Furthermore, the quench recovery time of the SFCL coil measured by the Rm-DTS was 2930 ms. During the quench, the highest temperature zone occurred near the center of the SFCL coil.

Experimental Tests of DC SFCL Under Low Impedance and High Impedance Fault Conditions

DC system protection is more challenging than that for AC system due to the rapid rate of rise of the fault current and absence of natural current zero-crossing in DC systems. Superconducting fault current limiter (SFCL) in DC systems is a promising technology to reduce the fault current level and the rate of rise of the fault current, and also SFCLs have no resistance during normal operation. In this paper, the behaviors of an SFCL coil are investigated under both low impedance and high impedance fault conditions in DC systems. In the low impedance fault condition system, the SFCL coil performs effective limitation of the fault current level under different prospective fault current levels. The application of SFCLs with limited inductance in the DC system can be a potential solution to effectively suppress the fault current under low impedance short-circuit faults. The SFCL coil under the high impedance fault condition can only limit the prospective fault current when it is much higher than the critical current of the coil.

Study on Improvement of Recovery Characteristics of GdBCO Non-inductive coil for resistive SFCL

Using the practical application of the resistance SFCLs (Superconducting Fault Current Limiter), it is required to quickly return to the superconducting state after interruption of the fault current. The quick recovery needs to improve the cooling property of a superconducting tape. It is confirmed that the recovery characteristics of a superconducting tape can be improved by the PTFE (PolyeTetraFluoroEthylene) coating or applying pressure. In this paper, we used GdBCO tapes for the resistive SFCL and investigated the effect of the PTFE coating or applying pressure for improvement of its recovery characteristics of non-inductive coil. Besides, we observed the boiling phenomena on the surface of the part of GdBCO tapes between under pressurized conditions and ambient pressure by a high-speed video camera.

Experimental AC loss analysis of braid type non-inductive coil for superconducting fault current limiter

Resistive superconducting fault current limiter (SFCL) plays a key role to realise the stability and safety of power grid and the future electric aircraft. This paper presents the design and test of a braid type non-inductive coil for SFCL. One inner coil and one outer coil were wound around a G10 tube using 4 mm-wide SuperPower wires. Two configurations of the braid type non-inductive coil were achieved by connecting the inner coil and outer coil in series and in parallel, respectively. AC loss results of these two non-inductive coils were measured and compared to investigate the AC loss dependency on coil configuration and current distribution. It has been concluded that braid type non-inductive coil has low AC loss due to magnetic field cancellation. Serial connection of the inner and outer coils leads to a good degree of field cancellation, which results in higher critical current and lower AC loss of the non-inductive coil, compared to parallel connection.

Experimental Testing and Development of Improved Modelling for Multistrand Resistive SFCL

Magnesium diboride (MgB2) in a simple round wire form has been tested and shown to be suitable as a low-cost resistive superconducting fault current limiter (SFCL). The commercial exploitation of MgB2 SFCLs requires a considerable scale-up of the current-carrying capability of the MgB2 wire. A multistrand MgB2 wire was developed for an SFCL coil to increase the current capacity. This paper will briefly report on the experimental results on a three-strand MgB2 coil used as a resistive SFCL. An improved analytical model that predicts the behavior of the three-strand SFCL coil was developed, taking the temperature and critical current variation along the wire into consideration. Variations in the critical current along the wire are to be expected as a consequence of normal manufacturing tolerances. The predicted current using the improved analytical model showed good correlation with the experimental test results at different fault current levels. The improved analytical model is a useful tool for the practical design of commercial SFCLs.

Experimental Tests of a Resistive SFCL Integrated With a Vacuum Interrupter

A resistive superconducting fault-current limiter (SFCL) has been developed using round magnesium diboride (MgB2) wire. The SFCL coil was wound using an interleaved coil arrangement to minimize the total coil inductance. The SFCL coil demonstrated reliable and repeatable current-limiting properties during testing. However, the wire temperature of the SFCL coil increases quickly during quench tests, and several minutes are required for temperature recovery after the fault is cleared. The SFCL coil therefore was fully integrated with a vacuum interrupter to quickly remove the SFCL coil from the circuit once a fault occurred. This allowed the SFCL coil to recover quickly while a bypass resistor acted as the current limiting resistance. A fast-acting actuator and its control circuit were designed and built to provide automatic control for the operation of the vacuum interrupter. The SFCL with the prototype vacuum interrupter was successfully tested. The energy dissipated in the SFCL coil was significantly reduced by integrating the vacuum interrupter. The fault tests with different potential fault currents also proved that the operation of the vacuum interrupter is independent of the fault current level. This prototype demonstrated the potential of a cost-effective and compact integrated SFCL and vacuum interrupter for power system applications.

Experimental testing and modelling of a resistive type superconducting fault current limiter using MgB2 wire

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault.

Experimental Tests on a Superconducting Fault Current Limiter Using Three-Strand $\hbox{MgB}_{2}$ Wire

Magnesium diboride (MgB2) in simple round wire form has shown significant potential to be a low-cost resistive superconducting fault current limiter (SFCL). The commercial exploitation of MgB2 SFCLs for typical distribution network voltages requires a considerable scale-up of the current-carrying capability of MgB2 wire. The two obvious options to achieve this are to increase the wire size and use multifilament wire and/or use multiple wire strands. This paper assesses the current capacity for multi-strand wire and presents results of initial tests on a three-strand MgB2 demonstrator SFCL coil. Three strands of monofilament MgB2 wire were braided together to form a single multi- strand wire. The wire was fabricated, wound on a ceramic former and heat treated by Hyper Tech Research, Inc. One of the key performance aspects of this type of wire arrangement is ensuring each strand of wire carries the same current. This paper demonstrates the potential for increasing the current capacity of SFCL using multi-strand MgB2 wire to that required for distribution network levels. The paper includes a detailed analysis of the results and the implications for the practical design of commercial SFCLs.

Experimental Analysis of Bifilar Pancake Type Fault Current Limiting Coil Using Stabilizer-Free Coated Conductor

YBCO coated conductors (CC) have actively been researched and manufactured these days. Recently, stabilizer-free (SF) CC with Hastelloy substrate has been developed for superconducting fault current limiter (SFCL). The SF CC has higher resistivity than other existing CCs with thick metallic stabilizer. Higher voltage per unit length applicable can be increased. In this research, the bifilar pancake coil using SF CC was manufactured and characteristic tests of the coil for SFCL were performed. Though the bifilar coil is supposed to have no inductance during normal operation, the transport current loss is be produced due to AC current flowing through the SF CC. AC loss was measured and short-circuit test was performed in liquid nitrogen. In order to develop large scale SFCL, it is indispensable to consider the dielectric characteristics of pancake module. Dielectric breakdown tests were carried out to find applicable maximum voltage in one pancake module. These results provided to suitable design parameters and showed technical feasibility of pancake type module as an SFCL.

Short-circuit test of a novel solenoid type high- Tc superconducting fault current limiter

In the present study, a novel solenoid type bifilar coil is proposed for high- T c superconducting fault current limiter (SFCL) which is composed of two windings in a bobbin and their winding directions are opposite with each other to have non-inductive characteristics. The two windings are connected in parallel at the current terminals for large current capacity. Besides, the current terminals are at the ends of the coil so that the electrical insulation design may be easier than that of other non-inductive coils. The novel solenoid type bifilar coil for SFCL is fabricated with BSCCO 2223 and tested in LN 2 bath at 77 K to investigate the current and voltage sharing as well as the current limiting effects. According to the test results, the current is limited effectively at first peak after the fault and the current sharing performance in each winding is verified under the normal and fault conditions. In order to compare with other bifilar coils, the AC losses are measured.

A Numerical Study on Temperature Increase in the Resistive SFCL Element Due to the Quench Condition

A transient numerical simulation was conducted to have variation of temperature on an element of resistive SFCL (Superconducting Fault Current Limiter) under quench condition. It is very important engineering information for an optimum design of cryogenic system for cooling of a resistive SFCL element. A bifilar coil for resistive SFCL for 10 MVA system was incorporated as a model in this numerical study. From the result, it was found that the averaged temperature on the shunt and Bi-2212 element at 500 kW, 100 ms was 711.1 K and 198.4 K respectively. The temperature variation with the change of the hot-spot size and time is also obtained. The maximum temperature was continuously increased in all cases until the hot-spot stops at 100 ms and it was going down after then. Such as, the details of temperature distribution on the SFCL element obtained from this numerical study and it should be very valuable information on the decision of the cooling capacity of cryogenic system

AC loss reduction of a 6.6 kV superconducting fault current limiter

The authors have been investigating a 6.6 kV superconducting fault current limiter (SCFCL) for use in power distribution systems. For practical application, heat load at 4.2 K must be reduced to a level which can be compensated by a small refrigerator. This paper describes AC loss reduction of a SCFCL coil, which is comprised of a conductor and junctions. The conductor is a cable of double-twisted multi-filament NbTi strands in a high-resistivity CuNi matrix. The filament diameter in strands and the filament twist pitch were reduced for AC loss reduction. Current junctions made of copper generate eddy current loss because an alternating magnetic field is applied on them by the conductor. It was confirmed experimentally that cylindrical-shaped junctions effectively reduced the loss. A total coil loss of 0.8 W at 1 kA has been achieved, which value is sufficiently low for the development of a closed cryostat of superconducting fault current limiter compensated by a small refrigerator.

Detergents used in oil fields

In the present study, a novel solenoid type bifilar coil is proposed for high-Tc superconducting fault current limiter (SFCL) which is composed of two windings in a bobbin and their winding directions are opposite with each other to have non-inductive characteristics. The two windings are connected in parallel at the current terminals for large current capacity. Besides, the current terminals are at the ends of the coil so that the electrical insulation design may be easier than that of other non-inductive coils. The novel solenoid type bifilar coil for SFCL is fabricated with BSCCO 2223 and tested in LN2 bath at 77 K to investigate the current and voltage sharing as well as the current limiting effects. According to the test results, the current is limited effectively at first peak after the fault and the current sharing performance in each winding is verified under the normal and fault conditions. In order to compare with other bifilar coils, the AC losses are measured.