Superconducting Fault Current Limiter Research Articles

Superconductors for power applications: an executable and web application to learn about resistive fault current limiters

High-temperature superconductors (HTS) can be superconducting in liquid nitrogen (77 K) at atmospheric pressure, which holds immense promises for our future such as nuclear fusion, compact medical devices and efficient power applications. In a power system, high short-circuit currents can exceed the operational current by more than ten times, putting many parts of the system at risk. Superconducting fault current limiters (SFCLs) can limit the prospective fault current without disconnecting the power system, and are thus becoming increasingly attractive for future grids. With a growing interest in modeling and commercializing SFCL, the question of how to teach and to explain their operation to students has arisen. In order to help students visualize the potential use and benefits of an SFCL, we created an executable and a web application using COMSOL Multiphysics. This executable allows students to investigate the electro-thermal response of a resistive SFCL. The executable solves a 1D electro-thermal model of the SFCL under AC fault conditions, evaluating important figures of merit such as the limited current, the prospective current and the maximum temperature reached within the tape. Finally, the geometrical parameters as well as the superconducting properties of the device can be modified. The importance of the amount of silver stabilizer necessary to protect the device from over-heating occurring during a fault current can be investigated. In addition, the effects of having a sharp nonlinear transition from the superconducting to the normal state (intrinsic property of the superconductor) to obtain a current limitation can be explored. The executable allows the users to learn about the benefits of superconductors in real-life applications, without the prerequisite of extensive modeling or experimental setup. The executable can be downloaded from the HTS modeling website and run on the most commonly used operating systems.

Design and Tests of a 160-kV/1-kA DC Superconducting Fault Current Limiter

DC Superconducting fault current limiter (SFCL) can be used to reduce the fault current of voltage source converter-based high-voltage direct current (VSC-HVdc) transmission systems, thereby significantly reducing the manufacturing difficulty and cost of dc circuit breakers. The authors aim to develop a 160-kV/1-kA resistive type dc SFCL, and apply it in the VSC-HVdc transmission system in Nan'ao of China. In order to verify the feasibility of the transmission voltage dc SFCL, the research is divided into two stages, and the results of the first stage are presented. The rated voltage and current of the developed dc SFCL is 160 kV and 1 kA, respectively, and the current limiting resistance is no less than 2 Ω within 10 ms, which is 1/4 of the second stage prototype. The used superconducting material is stainless steel reinforced yttrium barium copper oxide (YBCO) conductor produced by Shanghai Superconductor. A three terminal structure is presented to meet the different operation parameters. A 160-kV dc SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the dc withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 μs/50 μs, and the operating impulse test of 450 kV/250 μs/2500 μs. The average operating thermal loss of the device at rated current is 652 W. Power switching test results show that the developed SFCL could generate the desired limiting resistance of about 2.2 Ω in less than 10 ms, and the maximum fault current duration time could be as high as 60 ms.

Effect of the Conductor Length on the Hot-Spot Regime for Resistive-Type Superconducting Fault Current Limiter Applications

The design of the conductor of a resistive-type superconducting fault current limiter (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) tapes is driven by two operation regimes. On one hand, when the quench occurs on the overall conductor (i.e., the limitation regime), it should withstand the highest possible electric field to reduce its length and make it cost-effective. On the other hand, it has also to cope with the hot-spot regime. Fault currents in the range of the critical current <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> can lead to localized dissipation along the length of the conductor over the parts showing the lowest <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> values. The almost nonlimitation of the current coming from the low normal zone propagation velocity of 2G HTS tapes causes temperature elevations in these zones, which highly threaten their integrity. To summarize, the conductor architecture is adapted to withstand a high electric field and to obtain a nondestructive value of the maximum temperature in a hot-spot regime. However, the <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> variations, causing this last-mentioned regime, depend on the position along with the tape. This article aims to qualify the effect of a variable conductor length on the <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> variations and, as a consequence, on the hot-spot regime. We first study the influence of the length on the <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> variations. The minimum critical current tends to decrease when the conductor length increases. This behavior can be modeled by a Weibull distribution assuming a minimum critical current different from zero with an infinite length of the conductor. To assess this impact on the hot-spot regime, we develop a probabilistic approach using the deterministic one-dimensional modeling of 2G HTS conductor considering the <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> inhomogeneity along its length to simulate a R-SFCL behavior. It appears that the more the conductor is long, the more the maximum temperature in the hot-spot regime is high. Moreover, the fact that two <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> measurements corresponding to the same length of conductor present different maximum temperatures in hot-spot regime leads to present a method to design large-scale manufacturing conductors of the desired length, robust to survive hot-spot regime due to any <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> variations.

DC Current Limiting Operation and Power Burden Characteristics of a Flux-Coupling Type SFCL Connected in Series between Two Windings

In this paper, a DC fault short circuit was conducted to analyze the DC fault current limiting characteristics of a flux-coupling type superconducting fault current limiter (SFCL) that has two coils connected in series via one iron core. Similar to the AC power system, the flux-coupling type SFCL in a DC system, which has the two coils connected with each other in series and the secondary coil connected with the superconducting element in parallel, remains in the superconducting state before a short-circuit accident occurs. This results in magnetic flux getting generated by the two windings connected in series offsetting each other and the induced voltage at the two windings remaining at zero. However, in the event of a short-circuit accident on the DC line, a resistance is generated on the superconducting element, so that the magnetic flux generated at the two windings no longer offsets each other. Therefore, a voltage is induced on the two windings, and the fault current is limited accordingly. As a result of configuring a DC short-circuit device and experimenting with this SFCL, we could confirm the DC fault current limiting effect of a flux-coupling type SFCL with two windings connected in series. In addition, we could establish performance conditions of the flux-coupling type SFCL in a DC system by inferring the fault current, operating current, and limited impedance equations according to the connection direction of the flux-coupling type SFCL with two windings connected in series and by analyzing fault current limiting degree, power burden, magnetic flux, and energy consumption for each element composing the SFCL.

Design and performance tests of a 160 kV/1.0 kA DC superconducting fault current limiter

Superconducting fault current limiter (SFCL), which is used in voltage source converter based high voltage direct current (VSC-HVDC) transmission systems to limit fault currents and protect the grid together with DC circuit breakers, could improve system safety and stability. The first transmission voltage DC SFCL is developed in China, and it has been applied in the 160 kV VSC-HVDC transmission system in Nan’ao. The electromagnetic design as well as insulation analysis of the current limiting unit is presented, where the current limiting unit is composed of 96 bifilar coils made of stainless steel reinforced YBCO tapes. The liquid nitrogen at the temperature of 77 K is used as the cooling and insulation medium, and the liquid nitrogen is cooled by a closed-loop cryogenic system consists of 3 G-M refrigerators. The 160 kV DC SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the DC withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 µs/50 µs and the operating impulse test of 450 kV/250 µs/2500 µs. Power switching test results show that the developed SFCL could generate the desired limiting resistance, and the maximum fault current duration time could be as high as 60 ms. The developed prototype has been installed in the Nan’ao VSC-HVDC transmission system, and put into operation on August 17, 2020.

Performance of Ratios‐Based Transformer Differential Protection Scheme in the Presence of Resistive Superconductor Fault Current Limiter

With the increased demand for electrical power, it is required to increase the number of power transformers in power systems. One of the main challenges facing protection engineers is the protection and isolation of power transformers in the shortest time possible, during internal faults. One of the alternative solutions to reduce the fault current in the power system is the application of a superconductor fault current limiter (SFCL). In this article, the ratios-based transformer differential protection algorithm is evaluated in the presence of resistive SFCL during internal fault conditions. A model of the system under study, including SFCL, is simulated and studies are performed in MATLAB/Simulink environment. The results demonstrate the capability of the ratios-based differential protection scheme to detect internal faults in the presence of SFCL.

Experimental Study on the Current Limiting Characteristics of a Saturated Iron-Core Superconducting Fault Current Limiter in DC Power Systems

Several recent studies have proposed a conceptual design of a saturated iron-core superconducting fault current limiter (SI-SFCL) for DC power systems, but experimental studies to verify its operating characteristics and performance have not yet been conducted. In this paper, a laboratory-scale SI-SFCL was designed and tested to operate in a 500 V, 50 A DC power system. First, the detailed configuration, specifications, and hardware manufacturing contents of the SI-SFCL were presented, and then an experimental circuit was developed and installed for the performance test of the SI-SFCL in normal operation and failure conditions. A 3D finite element method model was constructed to compare the simulation results and experimental results for the operating function of the SI-SFCL. Without the SI-SFCL, the maximum fault current level was 500 A, as a result, the SI-SFCL has a large current limit function, which can reduce the magnitude of the fault current up to the design target value of 73.6%. The results obtained in this study can be effectively applied to large-scale SI-SFCL development studies for HVDC power systems.

Directional Correction of Over-Current Relay Using Voltage Slope in Multi Terminal DC System With SFCL

In the DC system, a larger fault current flows due to capacitor discharging than in an AC system. In the event of a fault in the MTDC (multi-terminal DC), although the magnitude of the current flowing through each DCCB (DC circuit breaker) varies depending on the fault location, all capacitors in MTDC may discharge and all DCCB may interrupt. In this paper, the trigger type SFCL (superconducting fault current limiter) were constructed to limit large fault current, and DC OCR (overcurrent relay) was designed to prevent interrupting malfunction according to the impact of SFCL. When the fault current was small by operation of SFCL, the DCCB did not interrupt or trip time is delayed. These problems cause malfunction even when DC OCR is applied. Therefore, DC OCR requires an additional correction method. In this paper, a new correction method was proposed in consideration of the current direction and voltage slope. The simulation was conducted according to four cases, and the proposed OCR correction was verified to eliminate malfunctions through case studies.

Influence of Current Change Rate During DC Current Limitation on the Coated Conductor Degradation

The paper explores the idea of reducing the amount of REBCO conductor in Resistive Superconducting Fault Current Limiters (R-SCFCL) by reducing the temperature of liquid Nitrogen from 77 K to 66 K. Because of lower temperature, the critical current increase approximately two times, and instead of two parallel tapes, one can use only one tape in a 1000 A class R-SCFCL. However, recent experimental results on short samples show a frequent degradation of superconductors by DC current limitation at 66 K. The paper reports our investigation of this phenomenon by experiments complemented by extensive numerical modeling. We suggest that the primary cause of conductor degradation initially appearing at 66 K should be sought in the first sub-millisecond period of the limitation pulse. We observed that a reduced initial rate of the current rise during the limitation event can prevent the conductor damage.

Fault Current Limiting Operations of Three-Phase Transformer Type SFCL Using Two Common Connection Points Between Secondary Windings

Three-phase transformer type superconducting fault current limiter (SFCL) using two common connection points between secondary windings, which consisted of three-phase transformer windings wound on three legs of E-I iron core and two/three superconducting modules (SCMs), were suggested and its fault current limiting operations according to ground-fault types were analyzed. To verify the effective operation of the three-phase transformer type SFCL using two common connection points between secondary windings, the unsymmetrical ground and the symmetrical ground faults were applied into three-phase power simulated system with the suggested SFCL. For the comparison, the ground faults were generated into three-phase transformer type SFCL with two/three SCMs. Through analysis on the test results, the SFCL with two SCMs was confirmed to have no different fault current limiting operation from the SFCL with three SCMs. The structure of E-I iron core with three magnetically coupled legs and the constitution of three secondary windings with two common connection points were analyzed to be contributed to the same fault current limiting operation. Furthermore, the power consumption and the joule energy of the SCMs comprising both the SFCL with two SCMs and the SFCL with three SCMs during the fault period had shown to be almost the same except for the transient period due to the quench time difference in case of the single-line ground fault.

Recovery Characteristics of GdBCO Series-Connected Non-Inductive Coil in Pressurized Liquid Nitrogen for a Resistive SFCL

A SFCL (Superconducting Fault Current Limiter) is expected to limit a fault current and to improve system stability and operation flexibility in electric power system which is becoming more and more complicated and larger-scale. In order to put SFCL into practical use, it is important for quick recovery to improve the cooling property of a superconducting wire. Especially in resistive SFCL, the superconductor turns to the resistive state for current limiting performance, then the temperature of the superconductor rises up to the film boiling regime of LN2 heat transfer characteristics. After the fault, the SFCL should recover to the superconducting state (stand-by for repetitive fault) within a short period according to the circuit breaker operation. In this study, to improve current-limiting capability, we used GdBCO series-connected Non-inductive coil for the resistive SFCL and investigated the effect of different applying pressure for improvement of its recovery characteristics. In addition, we observed the difference of the boiling phenomena on the surface between under pressurized condition and under ambient pressure by a high-speed video camera.

Comparison between saturated-iron core and shielded-iron core superconducting fault current limiters on recloser-fuse coordination of radial distribution systems connected with distributed generation

Abstract Recently, distributed generators (DGs) are being largely amalgamated with radial distribution systems because of their positive impacts on these systems such as voltage profile improvement and power loss minimization. However, if this integration is not well-planned, it can lead to serious problems in the protection devices. One of these problems is the recloser-fuse miscoordination. This paper presents an effective solution to the recloser-fuse miscoordination due to DGs integration with RDS. The proposed approach is based on suppressing the DG current during fault period so that the contribution of DG to the fault current becomes minimal. Two types of superconducting fault current limiters (SFCLs) namely, saturated iron-core and shielded iron-core SFCLs are studied and a comparison between their performance is presented. The proposed solution was implemented on IEEE 33-bus RDS. All simulation studies are performed on MATLAB script. The simulation results illustrated that saturated iron-core SFCL could not recover recloser-fuse coordination in some of the studied cases. However, shielded iron-core SFCL could successfully restore the recloser-fuse coordination in all the studied cases. This shows that shielded iron-core SFCL is preferred in solving recloser-fuse miscoordination.

ASSiST - A Superconducting Fault Current Limiter in a Public Electric Power Grid

For installation in the public electric power grid of the city of Augsburg, Germany, a resistive superconducting fault current limiter (SFCL) was designed, constructed, tested and operated in the framework of a partially funded project &#x2018;ASSiST&#x2019;. Right from the beginning, it was agreed to aim for a prolonged operation even after closure of the project. This endeavor has been started due to a fundamental need to limit the prospective fault currents at a feeder point of the grid and in order to create long-term operational experience on a device seamlessly integrated into commercial grid components and switchgear, thus demonstrating a high level of readiness for widespread application. It revealed that the SFCL was of robust design &#x2013; withstanding all electrical tests including a special two-phase fault and sustaining stable cryogenic conditions even during partial outages of the cooling system &#x2013; and that the technology is reducing loss by shunt reactors. Furthermore, due to the control equipment on medium voltage level, valuable insights on grid behavior could be gained. The extended operation for 4 years has clearly shown the performance and robustness of SFCL technology in feeder points on MV level and provides a reference point for further commercialization.

Impedance Compensation Method of Protective Relay for SFCL's Application in a Power Distribution System

Recently, the development of the electric power industry has led to an increase in the fault current when a fault occurs. It is important to reduce the fault current, since fault currents exceeding the capacity of protective devices can result in large blackouts. In order to reduce the fault current, there is a method to separate the bus line of main substation. However, this method has the disadvantage of complicating the power system. Another method is to install a superconducting fault current limiter (SFCL). The SFCL has the advantage that the fault current can be quickly limited within 1/4 cycle immediately after the fault occurrence without affecting the power system since its impedance is zero in a normal time. However, the application of SFCL affects the operation of the over-current relay (OCR) due to the reduced fault current such as trip time delay. The trip time delay of OCR was reported to be suppressed by using SFCL's voltage component in previous studies. In this paper, the impedance compensation method was suggested to exclude the effect of SFCL on OCR. To prevent OCR from the influence of SFCL, the component that can exclude the impedance of SFCL was considered to be added to the characteristic formula of OCR. In addition, the effect of correcting this characteristic equation was analyzed by constructing a power distribution system using PSCAD / EMTDC.

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.

Impact of SFCL According to Voltage Sags Based Reliability

Most industrial technologies are automated by sensitive electric parts such as programmable logic controllers(PLCs) and magnetic contactors which are required high power quality. The voltage sags occur in the non-faulted power distribution feeders during the fault duration. Therefore, voltage causes economic losses in the production process of various companies. To solve the fault current and the voltage sags, the superconducting fault current limiter (SFCL) is in the spotlight. The SFCL where was located on starting point of the feeder improves the reliability of the facility by reducing the fault current. In addition, it compensates the voltage sags on the bus-bar. In this paper, evaluating the voltage sags based on fault location in the power distribution system with the SFCL through PSCAD/EMTDC simulation. Next, the expected the voltage sags frequency is predicted by the reliability analysis technique of the power distribution system. The expected voltage sags frequency of the power distribution feeder is compared with before and after the application of the SFCL.

Experimental Observations of the Hot-Spot Regime On 2G HTS Coated Conductors

The use of Rare-Earth BaCuO (REBCO) coated conductors is well adapted for Resistive type SFCL (Superconducting Fault Current Limiter) thanks to their high current carrying capability and high resistance per unit length. However, in case of fault currents in the range of the critical current <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> , the dissipation can be highly inhomogeneous along the length, leading to localized temperature elevations in the normal zones. This so-called hot-spot regime is well predicted by simulation tools but was seldom studied experimentally in a non-destructive way. This paper presents two experimental results highlighting the existence of the hot-spot regime. First, localized dissipation on a REBCO tape is observed through high speed recording of nitrogen bubbles synchronized with electric measurements. Second, the maximum temperature at the end of the limitation is studied as a function of the prospective current through measurements carried out on the conductor developed by the European Project FASTGRID. The maximum temperature is found experimentally to be highest for prospective current close to the conductor <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>c</b></sub> .

A coupling method of the superconducting devices modeled by finite element method with the lumped parameters electrical circuit

Superconducting devices have been widely studied over these years. Their application can be found in cables for electric power transmission, energy storage systems, magnetic levitation, electric machines, and fault current limiters. The literature presents some formulations to model the superconductors’ behavior using the finite element method (FEM), such as the H, the AV, and the T formulations, among others. Many superconducting devices have been simulated and designed using some of these formulations. However, none method available offers a coupling between an electric power system, simulated using electrical lumped parameters, to the superconducting FEM model. In this context, this work introduces a methodology for coupling superconducting devices in FEM to lumped parameters composing the power system. Here, a case study with a Saturated Iron Core Superconducting Fault Current Limiter was presented to apply the proposed methodology. This research analyzes the influence of the self and external fields in the superconducting coil on its critical current density. Moreover, it investigates the DC-biased coil voltage drop and the superconducting resistance. Besides, the paper presents the simulations of short circuits for various DC currents applied to the superconducting coil. Short-circuit tests were performed for validating the simulation results, and it showed a maximum error of 15% for the compared points.

Protection of Sensitive Loads in Distribution Systems Using a BSFCL-DVR System.

In this paper, an incorporated bridge-type superconducting fault current limiter (BSFCL) and Dynamic Voltage Restorer (DVR) is presented to improve the voltage quality and limiting fault current problems in distribution systems. In order to achieve these capabilities, the BSFCL and DVR are integrated through a common DC link as a BSFCL-DVR system. The FCL and DVR ports of the BSFCL-DVR system are located in the beginning and end of the sensitive loads’ feeder integrated to the point of common coupling (PCC) in the distribution system. At first, the principle operation of the BSFCL-DVR is discussed. Then, a control system for the BSFCL-DVR system is designed to enhance the voltage quality and limit the fault current. Eventually, the efficiency of the BSFCL-DVR system is verified through the PSCAD/EMTDC simulation.

Fast Hotspot Detection in SFCLs by Exploiting Strain Response in Optical Fiber Sensing

Superconducting fault current limiters (SFCLs) can be used to limit fault currents in both meshed DC and AC grids by transitioning from superconducting to resistive state, in the presence of high currents. While the device is theoretically a great way to protect grids, the inherent inhomogeneity of critical current along the superconductor length can lead to localized heating, called hotspots, and ultimately destruction of the SFCL device. At EPFL under the European Union project Fastgrid, an extremely efficient Mach-Zehnder interferometer (MZI) based optical fiber sensing technique has been developed and patented that can detect even singular hotspots within 15 ms to protect SFCLs. The MZI response is characterized by a strain sensitive as well as a temperature sensitive contribution. This paper outlines an investigation by means of FEM modeling into the response sensitivity. A 2-D thermal model of the superconducting tape and optical fiber was made to study thermal transfer to the optical fiber from the REBCO tape. The simulation results showed that temperature rise observed in the optical fiber is slower than the MZI response time, proving a strain sensitive response in the experiment measurements. Sound mechanical coupling between the optical fiber and the superconductor tape can enhance strain transfer to the optical fiber and hence reduce hotspot detection time. With this improved performance, the health monitoring for SFCLs can be much more efficient and reliable.