Superconducting Fault Research Articles

Electrical Strength of Layered Insulation Systems in Liquid Nitrogen for Resistive Superconducting Fault Current Limiters

Electrical Strength of Layered Insulation Systems in Liquid Nitrogen for Resistive Superconducting Fault Current Limiters

Alternative analytical models for HTS tapes considering their AC hysteretic and resistive losses

This work proposes two alternative analytical models to evaluate the ac losses of high-temperature superconducting (HTS) tapes during their hysteretic and resistive modes. These models intend to extend the application range of state-of-the-art analytical models for current values higher than the critical one, i.e. for the resistive state, and to correctly predict the ac losses during the transition between the hysteretic and resistive modes. Two analytical models are proposed, one considering an extension of the Norris model for the HTS tape’s resistive mode and the other based on a sigmoid function to characterize the hysteretic losses and their smooth transition to the resistive mode. Analytical models capable of estimating ac losses of superconducting (SC) tapes are an important tool for the design of complex SC systems, such as SC fault current limiters, SC electrical machines and SC cables. The proposed models are validated experimentally, for a 1st generation BSCCO tape and a 2nd generation REBCO tape. Finite element simulation is also carried out to verify the accuracy of the proposed models. Results show that the proposed extended-Norris model presents some deviation at the transition between the hysteretic and resistive modes, while the sigmoid model presents very accurate results for the whole spectrum of applied current. Also, the parameters of the sigmoid models are independent of the tape geometry.

New technique for improving transient stability of standalone microgrid using superconductor fault current limiter

This study proposes a new insertion topology for Superconductor Fault Current Limiter (SFCL) to maintain and enhance the transient stability of a wind generation system in a standalone microgrid. The proposed topology is inserting the SFCL in series with the faulty phase only. The new insertion topology performance is compared with the traditional insertion method. The proposed insertion topology of the SFCL displays excellent performance compared to the traditional method. The overall performance of the Micro-Grid is improved. The performance of the wind generator is improved dramatically after employing the proposed controller. The proposed insertion topology suppresses the fluctuations of the wind generator speed (from ± 6% to less than ± 1%) as well as the post fault wind generator real power and imaginary power. For a single line to ground fault, the wind generator active power fluctuates between + 80 kW, and − 40 kW with traditional SFCL insertion topology. When using the proposed insertion topology, the wind generator active power fluctuates between + 27 kW, and + 12 kW. Also, the proposed SFCL insertion topology mitigates the fluctuations in voltage, real power and imaginary power not only at the generator bus but also at all other Micro-Grid buses (PV bus, and storage batteries bus). The voltage at all buses recover to steady state values fast (0.01 s) compared with the recovering time when using the traditional method (nearly 1 s). The proposed method is very simple and low cost because there is no additional hardware is required. This fact makes the proposed method highly attractive from the economical and implementation point of views.

Study of Resistive SFCLs for Transient Stability Enhancement of Paralleled Synchronous and Virtual Synchronous Generators in Weak Grid

Regarding a paralleled synchronous generator (SG) and virtual synchronous generator (VSG) system connected to a weak grid, the transient stability should be given more concern. This paper proposes applying resistive superconducting fault current limiters (R-SFCLs) to boost the paralleled SG-VSG system's transient stability while meeting the current limitation requirement (CLR). Firstly, the theoretical modeling is carried out, and the influencing mechanism of the grid strength on the transient stability is clarified. Then, the functions of the R-SFCLs to the paralleled system are analyzed, and the parameter design guidelines for the R-SFCLs are stated. Time-domain simulations are implemented in MATLAB, and a detailed comparison of using the R-SFCLs and the additional torque control for the paralleled system is made. From the simulations, the proposed approach offers a more significant stabilization effect. It outperforms the additional torque control to decrease the unbalanced power, eliminate the power angle oscillation, and handle the short-circuit fault with a longer duration for the paralleled system. The viability and appropriateness of the proposed approach are well confirmed.

Stability Enhancement of Grid-Connected Wind Power Generation System Using PSS, SFCL and STATCOM

The stability related issues may occur in a power system due to disturbances in generating or loading conditions, especially in the presence of distributed generation (DG) based on renewable energy resources (RERs). This paper proposes a novel strategy for the stability enhancement of a wind power generation system (WPGS) by using a combination of three devices, namely, a power system stabilizer (PSS), resistive superconductor fault current limiter (R-SFCL) and static synchronous compensator (STATCOM). The small signal (SS) stability of the test system is enhanced by selecting the best PSS type from the different types of PSS. An R-SFCL is used for improving the rotor angle and the frequency stability of the test system. Two indices, namely, transient stability index and sum of maximum deviations (SMD) index are introduced for determining the optimal locations of different sized R-SFCLs for increasing the rotor angle stability. The sensitivity index (SI) based on the power change between areas is applied for determining the optimal locations of different sized R-SFCLs for enhancing the frequency stability. Along with rotor angle and frequency stability, LVRT capability improvement of the wind farm using STATCOM is also considered. Finally, the combined effect of R-SFCL and STATCOM on the rotor angle and the frequency stability, for different fault locations, is also investigated for determining the optimal location of an R-SFCL in the presence of STATCOM. The results presented in the paper show that STATCOM affects both the number of feasible locations and the optimal locations that can be selected for different sized R-SFCLs for augmenting the rotor angle and the frequency stability of the system during faults. Moreover, it is pointed out that an optimal combination between the different sizes and the locations of R-SFCLs and STATCOM exists to enhance the overall stability of the test system under fault conditions.

Shielding Characteristics of Solenoidal Superconducting Screens Made From HTS Tape, for SFCL Applications

In the design and prototyping of an Inductive-Resistive Superconductor Fault Current Limiter (IR-SFCL), previously presented by the authors, superconducting screens made from BSCO bulk have been used for shielding the inductive stage of the device. This kind of screens has several problems as rigidity, brittleness, commercial dependence on size and high cost. In order to avoid such issues, replacing the bulk-type superconducting screens by screens made from high temperature superconducting (HTS) tape is proposed. Tapes are more flexible and allow to easily make screens with several sizes and configurations, and better behavior against temperature changes and dynamic effects. To select a suitable configuration for this type of screen, a preliminary study of the shielding capacity of screens with different configurations, made from tape, is presented. This study, as well as literature, led us to select the solenoidal configuration as the most suitable for our application. In this work, we study solenoidal screens built with independent concentric layers, each one made from non-insulated tape, and all in contact just by the Ag external cover of the tape. A test method has been developed, and screens with 1 to 10 solenoid layers have been tested to find out their shielding factor. The shielding mechanism (diamagnetism vs. induced currents) has been studied by measuring of the transport current generated in between the non-insulated solenoids. The low incidence of these currents in the magnetic shielding process of the superconducting tape screens is reported.

A Power-Electronics Free Protection Device for Superconducting Electrical Propulsion Aircraft

Research into developing cryogenic protection devices for superconducting electrical systems is lacking. One of the few investigations was of cryogenic switches in the form of a solid-state circuit breaker (CB). It was reported that these breakers would take up nearly 40% of the superconducting electrical system mass and losses. This article designs a cryogenic switch, free of power electronics, whose conduction loss is only 4.1% of a comparable solid-state CB’s total loss. Protection considerations of a 1-kV/40-MW cryogenic system are referred to validate its application in a superconducting electrical powertrain. The designed cryogenic switch’s dc and ac interrupting ability were verified by experiments to interrupt 750-V/1500-A dc and 1-kV/6.3-kA ac, very close to the dc and ac limits of the proposed 40-MW Center for High Efficiency Electric Aircraft (CHEETA) system. For fault conditions, a noninductive pancake resistive superconducting fault current limiter unit was designed to help with the unacceptable fault current. The 750-V/10-kA prospective current was suppressed to 1.4 kA in 588 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> and interrupted by the switch in 12 ms. Cooperation of the cryogenic switch and superconducting fault current limiter shows potential in dealing with fault currents in future electric aircraft.

Shortening Recovery Time with Bypass Breaker for Resistive Superconductor Fault Current Limiters

Shortening Recovery Time with Bypass Breaker for Resistive Superconductor Fault Current Limiters

Improved Recovery Time and Voltage Stability for Resistive Superconductor Fault Current Limiters

Fault current levels occurring in power systems force the breaking capacities of the breakers. In the power system elements, the generator, transformer, cables, etc., are exposed to the dangerous effects of fault current until the breaker clears the fault. Various limiting methods have been developed to eliminate this adverse situation in recent years. One of these methods is Superconducting Fault Current Limiters (SFCL). With the discovery of High-Temperature Superconductors (HTS), SFCL's Resistive type has become more popular. Resistive SFCL (R-SFCL), based on the non-linear resistance property of superconducting materials, limits the fault within the first half period. However, it cannot provide such a fast response after the fault. The time required to regain the superconducting state after the fault is the recovery time. For R-SFCL manufacturers, recovery time is a design parameter that must be kept short. This study examines limiting performance, recovery time, and voltage stability with the dynamic model created using MATLAB/Simulink. With the shunt breaker method proposed in the study, the recovery time was shortened, and the voltage stability improved.

R-SFCL and RRCOCS-TVC Based Cooperative Control for Enhancing LVRT Capability of DFIG System

For the extraction of wind energy through a doubly-fed induction generator (DFIG), low voltage ride through (LVRT) is an essential technical requirement specified by the transmission system operator (TSO). Under a grid fault condition, DFIG should remain in connection with the grid for a certain minimum period and offer reactive power support as required by the TSO. A cooperative control scheme consisting of hardware solution through a superconducting resistance type fault current limiter (R-SFCL) and software solution constructed on the rotor reference current orientation control strategy (RRCOCS) with transient voltage control (TVC), is proposed in this paper to address the LVRT requirement. In the proposed control strategy, RRCOCS will limit the rotor current directly during a fault condition. The reactive power needs to be generated during fault to maintain grid code which is achieved through TVC. At the same time, improvement of stator terminal voltage, as well as suppression of stator current, is achieved by R-SFCL. The suppression of stator current by R-SFCL is also transformed to the rotor side aiding the rotor current limiting. The proposed cooperative scheme’s performance is simulated and tested on a 9 MW grid-connected DFIG wind system. The results obtained by the proposed strategy are compared with RRCOCS and RRCOCS-TVC.&#x0D; HIGHLIGHTS&#x0D; &#x0D; Independent software and hardware LVRT methods fail under severe faults and strict grid codes&#x0D; Software methods are challenged by current suppression and voltage support limits while the hardware methods involve high costs and undergo high thermal stress&#x0D; A cooperative LVRT can overcome the challenges and limitations associated with these independently deployed methods. The proposed R-SFCL device, in cooperation with RRCOCS-TVC, successfully meets the grid codes, and the transient behavior of the system's electrical and mechanical variables gets improved&#x0D; The proposed scheme helps reduce R-SFCL resistance, reduces its thermal stress, brings cost reduction in R-SFCL design, and improves its recovery characteristics. The listed improvement in R-SFCL can prevent the system from overcompensation and the adverse effects of recurring faults&#x0D; &#x0D; GRAPHICAL ABSTRACT

Limitation Analysis of 1st and 2nd Generation Superconductors used in Resistive Superconductor Fault Current Limiters

Fault currents in power systems force valuable power system elements thermally, dynamically, and electromagnetically until the arc disappears. Installation elements that can withstand fault currents or damage existing components require a high cost. Installing components that withstand fault currents and the damage of fault currents to existing components are costly. Resistive Superconducting Fault Current Limiter (R-SFCL), one of the modern limiting methods, increases the safety and sustainability of the system by eliminating these risks. In this study, a dynamic model was created in MATLAB/Simulink for 1G and 2G HTS used in R-SFCL, and their response to single phase-to-ground fault was observed. According to the simulation results, the most advantageous HTS type for R-SFCL was determined. The fault current level, limitation rate, resistance and temperature values, and response times were compared in terms of limiting performance.

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.

Demonstration of a Three-Dimensional Current Mapping Technique Around a Superconductor in a Prototype of a Conventional Superconducting Fault Current Limiter

Here, we describe a three-dimensional (3-D) current mapping technology developed for a superconductor using an array of Hall sensors distributed around it. We demonstrate this in a prototype similar to a conventional resistive superconducting fault current limiter (SCFCL). By calibrating the Hall sensor voltage, we can directly measure the distribution of the currents in the superconductor and the shunt. Using pulsed measurements, we measure the fractions of current distributed between the superconductor and shunt resistor parallel combination when a fault-like condition is mimicked in the system. Using the Hall array measurements, we generate a real-time 3-D map of local average current distribution around the superconductor used in our prototype of SCFCL. Our measurements show that even for currents less than the critical current a nonuniform current flow pattern exists around the superconductor, which we have used in the prototype. The capability of real-time, 3-D monitoring of the average local current distribution offers a way for the early detection of instabilities like hotspots developing in a superconductor. We discuss the use of this technique to not only show how it offers early detection and protection against instabilities developing in the superconductor, but also how it offers an added flexibility, namely, a user-settable fault current threshold.

인가전압의 크기에 따른 2세대 고온 초전도선재의 열화특성에 관한 연구

2G High temperature superconducting (HTS) GdBCO conductors are usually considered as proper superconductors to develop a superconducting fault current limiter and a superconducting cable which are operated in a high voltage condition. 2G HTS GdBCO conductors manufactured by SuNAM Co., Ltd. can be classified by a metal stabilizer: silver, copper, brass, and stainless steel. In previous study, the electrical performance of 2G HTS GdBCO conductors with copper stabilizer according to electrical breakdown voltage is reported. In this study, dielectric experiments are performed in saturated liquid nitrogen (LN₂) at 77K temperature to analyze the characteristics of electrical degradation such as critical current and index number of 2G HTS GdBCO conductors with copper and brass stabilizer according to amplitude of applied voltage. All dielectric experiments are performed with AC and lightning impulse voltages by using a sphere-to-plane electrode system. It is found that the electrical performance of 2G HTS GdBCO conductors is degraded according to the amplitude of applied voltage.

Modeling the limiting performance of resistive superconductor fault current limiters for 2G HTS tape

Modeling the limiting performance of resistive superconductor fault current limiters for 2G HTS tape

High-Temperature Superconducting Cable Fault Location Method Based on Improved Time-Frequency Domain Reflection Method and EEMD Noise Reduction

Aiming at the operation and maintenance requirements of the fault location of high-temperature superconducting cables, a fault location method of high-temperature superconducting cables based on the improved time-frequency domain reflection method and EEMD noise reduction is proposed. Considering the cross-term interference problem in the traditional time-frequency domain reflection method, this paper introduces the affine transformation to project the time-frequency distribution of the self-term and the cross term and further highlights the characteristic differences between the two through coordinate transformation, and the particle swarm algorithm is employed to solve the optimal stagger angle of the affine transformation. The unscented particle filter is adopted to separate the cross term, and EEMD noise reduction is introduced to solve the signal noise problem. Finally, two software programs, PSCAD and MATLAB, are employed for joint simulation to build a model of high-temperature superconducting cable. The simulation example shows that the proposed method in this paper can eliminate the cross-term interference of the traditional time-frequency domain reflection method, effectively locate the fault of the high-temperature superconducting cable, and improve the positioning accuracy.

Characteristics of a Superconducting DC Circuit Breaker According to L and C Elements of LC Divergent Oscillation Circuit

Currently, we are developing an AC/DC conversion technology with the development of power semiconductors. Accordingly, DC is attracting attention because it is advantageous in the current transmission system. However, it has a problem that the cut-off technology required for DC system operation has not been secured yet. We are researching and developing a DC cut-off technology in several ways. We proposed a model that combines a superconducting fault current limiter and a DC circuit breaker. This is a configuration which a superconducting fault current limiter and a mechanical DC circuit breaker are connected in series. The superconducting fault current limiter limits the initial fault current and the mechanical DC circuit breaker cuts off the fault current. The mechanical DC circuit breaker consists of a high-speed DC switch, a LC divergent oscillation circuit and a surge arrester. This creates an artificial zero-point by affecting the blocking of the DC fault current according to the LC diverging oscillation circuit. In this paper, we analyzed the effect of the LC divergent oscillation circuit on the superconducting DC circuit breaker according to Reactor (L) and Capacitor (C). As a result, we analyzed the operating characteristics of the superconducting DC circuit breaker according to the application of the LC divergent oscillation circuit using the PSCAD/EMTDC program. Also, we confirmed the characteristics of the initial fault current limit rate of the superconducting fault current limiter and the possibility of controlling how the LC divergent oscillation circuit affects the DC arc generated at the opening of the mechanical DC contacts.

한류기 보호 MTDC시스템에 과전류 계전기적용을 위한 전압기울기 방향 보정 연구

In the DC system, the fault current is quite fast and rises significantly due to capacitor discharge. Due to this, DC fault conditions must be cut off more quickly and accurately than AC. However, DCCB is more complicated to operate and more expensive than AC system circuit breaker. Therefore, it is necessary to introduce a superconducting fault current limiter, and the trip delay may occur in the overcurrent relay due to the introduction of the superconducting fault current limiter. To prevent this, a correction algorithm is needed. In this paper, an algorithm that considers the voltage slope and the direction of the current is applied to effectively limit the fault current through a superconducting fault current limiter, and a method to prevent the malfunction of the circuit breaker.

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.

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.