Flux-coupling Type Superconducting Fault Current Limiter Research Articles

The optimal allocation of H-SFCL in MVDC SPS

Medium voltage direct current shipboard power system (MVDC SPS) is the development trend of navy in the future. At present, one of the bottlenecks restricting the development of MVDC SPS is that the short fault. There is a lack of effective means to limit short fault current to protect MVDC SPS. This paper presents a method of applying a hybrid type superconducting fault current limiter (H-SFCL) to MVDC SPS protection. The architecture and system modeling of MVDC SPS are introduced, and the mechanism of bipolar DC short fault is analyzed. The topology and working principle of H-SFCL are introduced. The small-scale prototype experiment is done, the current limiting effect of H-SFCL is compared with resistive type superconducting fault current limiter (R-SFCL) and flux-coupling type superconducting fault current limiter (FC-SFCL) in the prototype test, so as to verify the superiority of H-SFCL. A dual objective hierarchical optimization configuration method is proposed, and the number and position of H-SFCLs are verified on a four-terminal system, as well as multi-objective optimization for H-SFCLs. A fault isolation and H-SFCL optimal allocation evaluation model is proposed; An improved method suitable for fault isolation and multi-objective optimal allocation of H-SFCLs is proposed to address the issue of breaking timing between internal switches in H-SFCLs and external circuit breakers in the optimization process. The research results indicate that the proposed optimal allocation method can optimize the inductance, resistance, and switch breaking time inside the current limiter, and can match the timing with the external circuit breaker; The proposed multi-objective optimization configuration method has a maximum effective individual ratio of 88% in the first 20 generations of improvement, and the improved iteration speed has significantly increased. In the 5th generation, the effective individual ratio can reach 100%.

Quenched Flux-Coupling Superconducting Fault Current Limiter Scheme and Its Electromagnetic Design Method.

In order to solve the problem of excessive short-circuit current in the present power system, a fault current limiter has become a new type of power device with high demand and is one of the current research hotspots. The flux-coupling type superconducting fault current limiter (FC-SFCL) generates a current-limiting impedance through decoupling superconducting parallel inductance based on the circuit breakers' fractional interruption. The principle is simple, and the impedance is low during normal operation. It can directly use the existing circuit breaker to open a short circuit that is much higher than its own breaking capacity. Thus, it can be used for large-capacity fault current limiting and effective failure breaking. This paper focused on exploring and studying the implementation scheme of practical products of FC-SFCL. Considering that the quenched-type parallel inductance can limit the first peak value of the fault current, a quenched-type improvement scheme was proposed. Then, an electromagnetic design method based on the simplified calculation of the number of parallel tapes was proposed, which simplified the design process and reduced the design difficulty of the quenched FC-SFCL. Taking a 10 kV/500 A/5 kA quenched prototype as an example, its electromagnetic design was completed, and the performances of the non-quenched and quenched schemes were compared. The results showed that, compared to the non-quenched structure, the technical economics of the quenched one were more prominent, and it can be used preferentially for engineering prototypes. This study about the scheme of the quenched FC-SFCL and its electromagnetic design method is useful for promoting the implementation of the current limiter engineering prototype.

Transients Analysis in AC Microgrid System

In the modern power system, small scale versions of the power grid termed as Microgrid are being greatly used because of the practical, economic and environmental advantages over conventional structure. Microgrid as the word itself suggests (micro-small and grid-interconnection of different elements) is the advanced technology that is being currently endorsed for future power systems in electrical engineering world. Transients are produced in microgrids due to the fluctuating nature of loads and the irregular behaviour of renewable power generation, particularly whenever the transition from grid linked to islanded state happens. In this paper, for controlling the transients during the transition in islanded mode in microgrid comprising of various energy resources like Diesel Generator, Solar Photovoltaic System, Energy Storage System, Flux Coupling Type Super Conducting Fault Current Limiter (FSFCL) is used. By incorporating the FSFCL in the microgrid, transient behaviour of the microgrid is greatly improved. The model analysis is done using the MATLAB’s Simulink Software and the output results in presence and absence of Flux coupling type SFCL is compared. The output results which are obtained show the supremacy of the suggested device in reducing the transients in microgrid.

Transient Performance Augmentation of DFIG Based Wind Farms by Nonlinear Control of Flux-Coupling-Type Superconducting Fault Current Limiter

Any fault related to grid is a matter of great concern for doubly fed induction generator (DFIG) based power system as DFIG's stator windings are connected to the grid directly. To augment the transient performance of the DFIGs, superconducting fault current limiter (SFCL) is a certified device. To boost the performance of a flux-coupling-type SFCL (FC-SFCL) by ensuing the adaptive use of fault current limiting impedance based on fault severity, rather involving the full impedance unnecessarily, a nonlinear controller (NC) for FC-SFCL (NC-FC-SFCL) is presented in this paper. Reason behind choosing a straightforward NC for this work is to have simple implementation capability with the full flavor of a nonlinear controller. Effectiveness of the NC-FC-SFCL is compared with conventionally controlled FC-SFCL for various fault scenarios. Simulation results suggest that, NC-FC-SFCL can improve the overall fault ride through (FRT) capability which is verified both graphically and numerically. Additionally, this effective use of the fault current limiting impedance guarantees better transient sub-synchronous resonance (SSR) performance, and exhibits better total harmonic distortion responses.

Performance Analysis of an Improved Flux-Coupling-Type SFCL in a Medium-Voltage Wind Turbine Generation System

This paper proposes an improved flux-coupling-type superconducting fault current limiter (FC-SFCL) with multiple working modes, and investigates the efficacy of the improved FC-SFCL in guarding a medium-voltage wind turbine generation system (WTGS). The improved FC-SFCL is mainly consisting of a coupling inductance with superconducting and conventional coils, two high-speed switches, a shunt resistance and an additional limiting resistance. Under normal circumstances, the improved FC-SFCL shows little impedance to not disturb the system. In terms of fault severities, the two high-speed switches are flexibly and separately controlled to change the current paths, and the improved FC-SFCL can appear two-level limiting impedances to affect the system's transient response. The performance validation of the improved FC-SFCL in a 35 kV class WTGS is done. From the theoretical derivation and simulation assessment, the results well testify the effectiveness of the improved FC-SFCL. The fault currents in the WTGS are timely suppressed to reasonable levels, and the fluctuations in the power and torque of the WTGS are visibly reduced. A beneficial voltage quality improvement is to maintain the grid connection of the WTGS and make it ride through the serious short circuit fault successfully.

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.

AC Loss Analysis of a Flux-Coupling Type Superconducting Fault Current Limiter

Fault current limiter (FCL) is commonly applied as a current-limiting device to improve the stability of the power system. A flux-coupling type superconducting FCL (FC-SFCL) which mainly consists of a current-limiting unit has been developed. Because of its low steady-state impedance and larger current-limiting impedance at pre-and post-fault, it is regarded as a possible way to restrict fault currents. In this paper, ac loss performances of three structures with different winding methods under three operations were discussed. Then an experimental platform for a small-scale prototype was set up, and several relevant experiments were implemented to test current reduction capacity and loss levels. In theory, the resistive loss obtained by integrating the instantaneous power corresponds to the ac losses of the current-limiting unit. It provides a feasible way to measure ac losses of a magnet, but it needs further verification.

Magnetizing Characteristics of a Flux-Coupling Type Superconducting Fault Current Limiter Connected in Parallel between Two Coils Using an E–I Iron Core

In power systems, there are short-circuit faults due to various causes. One way to reduce this fault current is the superconducting fault current limiter (SFCL). Since the current limiter model using a winding includes an iron core, the magnetizing current generated in the iron core when the fault current is limited should not be as large as possible. That is, when saturation of the iron core occurs, sufficient magnetic flux is not generated and the fault current limiting characteristics are degraded. Therefore, in this paper, a flux-coupling type SFCL using an E–I iron core was fabricated to effectively reduce the saturation of the iron core and limit the fault current to twice. The two major windings $$N_{1}$$ and $$N_{2}$$ were connected in parallel, and the peak current limiting characteristics and voltage waveforms were compared based on the winding directions of the two coils. The two main coils were also connected in parallel to analyze the flux linkage and instantaneous power characteristics. The magnetization power area and operating range of the flux linkage based on the magnetic flux energy accumulated in the E–I iron core were compared and analyzed in relation to the increase or decrease of the magnetizing current.

Electromagnetic Design and Performance Analysis of a Flux-Coupling-Type SFCL

Superconducting fault current limiter (SFCL) is an efficient way to solve the short-circuit problems in modern power systems. A flux-coupling-type SFCL (FC-SFCL) based on disconnecting coupling windings for current reduction has been developed in this study. FC-SFCL is considered as a potential scheme to limit fault currents due to its low steady impedance before fault and higher current-limiting impedance after fault. It contains a pair of high-temperature superconducting (HTS) coupling coils, wound on an iron core with air gaps. Raising the coupling coefficient of the HTS coils and reducing the loss are essential issues to improve its performance, which is mainly affected by the iron core structure and winding processes of HTS coils. In this study, the electromagnetic design of a 10 kV/500 A FC-SFCL one-phase prototype is completed based on a genetic algorithm, along with the performances of current-limiting capacity, response time, and loss of the prototype. The simulation results show the FC-SFCL prototype with electromagnetic parameters meets the design requirements, and has good performances in terms of current reduction and loss.

Design and Test of 10 kV/400 A Flux-Coupling-Type Superconducting Fault Current Limiting Module

The flux-coupling-type superconducting fault current limiter (FC-SFCL) is suggested to suppress the surge current in voltage sourced converter based HVdc (VSC-HVdc) power systems. One 10 kV/400 A FC-SFCL module with two branches is designed and tested. For the FC-SFCL module, the two branches could use the same or different YBCO tapes. Furthermore, the electromagnetic analysis and parameter calculation of the module are performed. In order to study the current-limiting performance of the FC-SFCL under the rapid change of fault current, a dynamic simulation model is established. Finally, the transient properties of FC-SFCL module are simulated and tested. According to the measurement results, the maximum impact current could be limited from 12 kA to 6264 A, and the largest resistance of the SFCL module is about 1.5 Ω when the dc impact voltage is 10 kV, which is close to the simulation results.

Investigation of Flux-Coupling Type Superconducting Fault Current Limiter With Multiple Parallel Branches

The flux-coupling type superconducting fault-current limiter (FC-SFCL) is discussed in this paper. Considering the shortcomings of conventional inductive and resistive SFCLs, two novel FC-SFCLs with multiple parallel branches are carried out. Considering the characteristics of YBCO tapes in 77-K LN2 environment, the design scheme of the HTS module is studied according to the impedance parameters, the number of parallel branches, and insulation distance. Furthermore, the electromagnetic analysis and parameter calculation of the module are performed. In order to identify the status of the HTS module in normal operating, current limiting, and reclosing processes, the status identification method for HTS module is analyzed. Finally, the possible application of FC-SFCL in power grid is discussed.

Simulation Analysis and Experimental Tests of a Small-Scale Flux-Coupling Type Superconducting Fault Current Limiter

Superconducting fault current limiter (SFCL) is able to efficiently solve the short-circuit problems occurred in the modern power system. In this paper, the simulation analysis and experimental tests of a small-scale flux-coupling type SFCL (FC-SFCL) are carried out. In regard to the developed 400 V/20 A small-scale prototype of the FC-SFCL, it is composed of two coaxial HTS windings carrying current in reverse directions, two metal oxide varistors, and a controlled switch. The parameter design of the FC-SFCL prototype is presented. Based on the simulations, the design scheme's effectiveness can be confirmed from the higher current-limiting ratio, and the SFCL has lower average ac losses under normal condition. The experimental results are in accord with the simulation ones, and the current-limiting characteristics of the FC-SFCL can be well verified.

Operational Behaviors of Flux-Coupling Type SFCL Using Integrated Three Phase Transformer Under Transient State

A superconducting fault current limiter (SFCL) has been proposed as an efficient method to protect a power grid from fault current. For applying a SFCL to the power grid, the various ways to increase the capacity of a SFCL should be researched in advance. Thus, the study on SFCL using a transformer has been performed. In this paper, we analyzed operational behaviors of integrated three-phase flux-coupling type SFCL under transient states in the power grid. When single and double line-to-ground fault occurred, all superconducting elements of sound and fault phase were quenched unlike segregated type SFCL. The fault current was limited by mutual supplementing in the all phases. As a result, the limited rate of fault current was small, even though the applied voltage was increased. It means this type SFCL limited the fault current efficiently by sharing of magnetic flux.

Analysis of the Unbalanced Fault in Three-Phase Flux-Coupling Type SFCL Using the Symmetrical Coordinate Method

The power demand is rapidly increasing in Korea, especially in the urban areas that include the metropolitan area. In addition, the transmission system in Korea is a network structure that ensures the high reliability and stability of power supply. Accordingly, the fault current rapidly increases in the case of a power system fault, due to the decrease in the line impedance. The superconducting fault current limiter (SFCL) was devised to address these existing problems [1]-[4]. This paper analyses the sizes of fault currents limited by flux- coupling type SFCL using the symmetrical coordinate method in the case of unbalanced faults such as a single and double line-to- ground faults in the power system. An unbalanced current of three- phase was indicated by three balanced symmetrical components using the symmetrical coordinate method comparing the component sizes. This study examines the operating characteristics of the SFCL that are required by the system, considering the expected coordination with the protective devices in the actual system.

Comparison of the quench and fault current limiting characteristics of the flux-coupling type SFCL with single and three-phase transformer

The South Korean power grid has a network structure for the flexible operation of the system. The continuously increasing power demand necessitated the increase of power facilities, which decreased the impedance in the power system. As a result, the size of the fault current in the event of a system fault increased. As this increased fault current size is threatening the breaking capacity of the circuit breaker, the main protective device, a solution to this problem is needed. The superconducting fault current limiter (SFCL) has been designed to address this problem. SFCL supports the stable operation of the circuit breaker through its excellent fault-current-limiting operation [1–5].In this paper, the quench and fault current limiting characteristics of the flux-coupling-type SFCL with one three-phase transformer were compared with those of the same SFCL type but with three single-phase transformers. In the case of the three-phase transformers, both the superconducting elements of the fault and sound phases were quenched, whereas in the case of the single-phase transformer, only that of the fault phase was quenched. For the fault current limiting rate, both cases showed similar rates for the single line-to-ground fault, but for the three-wire earth fault, the fault current limiting rate of the single-phase transformer was over 90% whereas that of the three-phase transformer was about 60%. It appears that when the three-phase transformer was used, the limiting rate decreased because the fluxes by the fault current of each phase were linked in one core. When the power loads of the superconducting elements were compared by fault type, the initial (half-cycle) load was great when the single-phase transformer was applied, whereas for the three-phase transformer, its power load was slightly lower at the initial stage but became greater after the half fault cycle.

Current Limitation and Power Burden of a Flux-Coupling Type SFCL in the Three-Phase Power System According to Turn's Ratio and Fault Type

Most of the transmission system has a network structure to improve the reliability and stability of a power system. Fault current is continuously expected to increase by the increase of the power demand. If fault current exceeds the cutoff capacity of a circuit breaker, the circuit breaker is broken and the damage by fault current is expanded throughout the power system. Superconducting fault current limiter (SFCL) was designed to solve this problem in a power system. In this paper, we investigated the current limiting characteristics and power burden of superconducting elements of a flux-coupling type SFCL in three-phase power system. A Flux-coupling type SFCL is one of the resistive type SFCLs. The flux-coupling type SFCL was made by using a transformer. Reactors connected in each phase shared an iron core. When the superconducting elements were quenched in fault phase, the fault current flowed into the primary and secondary coils simultaneously. Thus, the current flowed into primary and secondary coils of sound phase by the magnetic coupling flux. Meanwhile, when the current of sound phase exceeded the critical current of the SFCL, superconducting elements connected in the sound phase were quenched. The value of the fault current tended to decrease as the first reactor's ratio increased. Furthermore, the power burden of the superconducting element was reduced. The reduced power burden of the superconducting elements shortens the recovery time of the superconducting element, which is advantageous for cooperation with a reclosing system when the SFCL is applied to the system. As a result, we confirmed that the flux-coupling type SFCL operated effectively in the three-phase power system.

Cooperation Between Reclosing System and a Flux Coupling Type SFCL by a Neutral Line

We analysed the protective cooperation between a reclosing system and a flux-coupling type superconducting fault current limiter (SFCL). In order to compare the recovery behavior, the SFCLs without or with a neutral line between secondary coil and superconducting elements were prepared. Most of power grid has a reclosing system, which controls the circuit breaker by the sequential procedure, to cope with various faults. Generally, fast recovery of the SFCL was very important for stable operation between the reclosing system and the SFCL. The reclosing cycles (CO-t-CO-t-CO) for the experiments was set to 5-10-7-20-5. Recovery time in a flux coupling type SFCL without a neutral line was lengthened due to unequal quench between superconducting elements and it caused decrease of the maximum applied voltage. In the meantime, recovery time in the SFCL with a neutral line was shortened due to simultaneous quench between the elements. So, applied voltage of the power system could be higher within same reclosing system. As turn's ratio between secondary coil and superconducting elements was increased, recovery time was longer due to increase of their power. We confirmed that a flux-coupling type SFCL with a neutral line was more profitable for good cooperation between superconducting elements and a reclosing system.

Reclosing operation characteristics of the flux-coupling type SFCL in a single-line-to ground fault

The recloser that is used in distribution systems is a relay system that behaves sequentially to protect power systems from transient and continuous faults. This reclosing operation of the recloser can improve the reliability and stability of the power supply. For cooperation with this recloser, the superconducting fault current limiter (SFCL) must properly perform the reclosing operation. This paper analyzed the reclosing operation characteristics of the three-phase flux-coupling type SFCL in the event of a ground fault. The fault current limiting characteristics according to the changing number of turns of the primary and secondary coils were examined. As the number of turns of the first coil increased, the first maximum fault current decreased. Furthermore, the voltage of the quenched superconducting element also decreased. This means that the power burden of the superconducting element decreases based on the increasing number of turns of the primary coil. The fault current limiting characteristic of the SFCL according to the reclosing time limited the fault current within a 0.5cycles (8ms), which is shorter than the closing time of the recloser. In other words, the superconducting element returned to the superconducting state before the second fault and normally performed the fault current limiting operation. If the SFCL did not recover before the recloser reclosing time, the normal current that was flowing in the transmission line after the recovery of the SFCL from the fault would have been limited and would have caused losses. Therefore, the fast recovery time of a SFCL is critical to its cooperation with the protection system.

Recovery behavior based on reclosing operation and a neutral line of three-phase flux-coupling type SFCL

The breaking capacity of circuit breakers currently reaches the saturation point in electric power systems when a fault occurs. This is because the fault current increases due to continuous increases in electric power demand and facilities. It is not efficient to change existing circuit breakers, however, because of some problems that will increase the replacement cost and cause technical difficulties. To solve such problems, a study on the superconducting fault current limiter (SFCL) is being actively conducted.In this paper, a three-phase flux-coupling-type SFCL was composed, and its recovery characteristic according to the change in the number of turns of its secondary coils and its reclosing operation were analyzed. The increase in the number of turns of the SFCL’s secondary coils enhances its limiting efficiency by increasing its total impedance. The recovery time was delayed, however, when the number of turns of the secondary coils increased, because the superconducting element was connected in parallel to the secondary coils, which increased the voltage and current of the superconducting element. In other words, the delay in the recovery time was due to the increase in the power burden between the superconducting elements. Thus, two superconducting elements that were connected in series were used to distribute the power burden to each other, and the recovery characteristics of the SFCL with and without a neutral line were experimented on.

재폐로 동작에 따른 초전도 한류기의 회복성능 분석

The breaking capacity of circuit breakers could be no more increased in the electric power system. This is because the fault current increases due to continuous increases in electric power demand and facilities. To solve the problem, it is necessary to come up with an alternative. The superconducting fault current limiter (SFCL) has received an attention among various alternatives. The SFCL effectively reduce a fault current in cooperation with a power circuit breaker. A various types of the SFCL are suggested and a study on them have been progressed. As a result of it, the SFCL can be applyed to the electric power system in the near future. But, a study on recovery behaviors of the SFCL is not enough for applying to the electric power system. If the superconducting elements do not completely recover to the superconducting state after fault operation, it might be a breakdown of the superconducting elements due to heavy power burden and it gives an bad influence on the working of other electric devices. Additionally, the distribution power system has reclosing operation such as open-0.3sec-closed/open-3min-closed/open procedure. So we need to study more about improvement of the recovery behaviors of the SFCL. In this paper, we analyzed the recovery behaviors of a flux-coupling type SFCL according to reclosing operation when a single line-to-ground fault occurred and we compared recovery behaviors of the SFCL with and without a neutral line between secondary reactors and superconducting elements. Also, the flux-coupling type SFCL has advantageous for increases of capacity by controlling the variation in turn ratios between two reactors. Consequently, when the number of turns of the secondary reactors increased, the power burden of the superconducting elements was bigger due to the increase of impedances of the secondary reactors. To distribute the power burden, two superconducting elements connected in series and the balanced quenching of the superconducting elements was induced by connecting a neutral line.