High Temperature Superconducting Fault Current Limiter Research Articles

Degradation Characteristics of REBCO Tapes According to Electrical Breakdown

Second-generation (2G) high-temperature supercon-ducting (HTS) coated conductors are widely used to develop high-voltage superconducting apparatuses, such as HTS fault current limiters and HTS cables. In this study, experiments on electrical breakdown were performed in saturated liquid nitrogen (LN2) at a temperature of 77K to verify the relation between the applied voltage and electrically degraded characteristics of REBa2Cu3Ox (REBCO, RE: rear earth) tapes, such as the critical current (Ic) and index n. We used three types of REBCO tapes to determine the effect of the stabilizer material on the electrical degradation characteristics of silver, copper, and brass. A sphere electrode sys-tem was used to conduct electrical breakdown experiments under AC and lightning impulse voltages. Accordingly, we determined that the electrical degradation characteristics of REBCO tapes de-pend on the stabilizer material according to the applied voltage.

Practical Calculation Method for the Short-Circuit Current of Power Grids with High Temperature Superconducting Fault Current Limiters

With the expansion of the power grid scale and the enhancement of network structure interconnection, the short-circuit current of power grids is increasingly close to the upper capacity limit of circuit breakers. The fault current limiter is effective in suppressing the short-circuit current of power grids, and with the mature development of superconducting technology, high-temperature superconducting fault current limiters (HTS-FCLs) have been widely used in power grids. Due to the nonlinear impedance–current characteristic of HTS-FCLs, the traditional short-circuit current calculation method is inapplicable to power grids with HTS-FCLs. First, based on the establishment of the electromechanical transient model and the short-circuit calculation model, the short-circuit current calculation procedure for power grids with HTS-FCLs is proposed. Second, the calculation methods for three-phase and single-phase short-circuit currents at the substation bus in power grids with HTS-FCLs are analyzed. Finally, three-phase and single-phase short-circuit currents at the 500 kV Xijiang substation bus in the Guangdong power grid are calculated as the study case. Simulation results show that compared with the situation where there is no HTS-FCL installed, the short-circuit current on the 500 kV bus decreases from 65.34 to 58.31 kA in terms of the three-phase short-circuit fault, and decreases from 54.56 to 46.62 kA in terms of the single-phase short-circuit fault, thus verifying the effectiveness and practicability of HTS-FCLs in suppressing the short-circuit current, which is significant for the safe, stable and reliable operation of the power grid.

Numerical Study on Transient State of Inductive Fault Current Limiter Based on Field-Circuit Coupling Method.

As the capacity of the power grid continues to expand, high-level fault currents might be caused during a contingency, and the problem of short-circuit current over-limitation is imminent. The high-temperature superconducting (HTS) fault current limiter (FCL) is an effective method to solve this problem. In this paper, a transient numerical model for the process of limiting current in the inductive FCL is proposed. The model is based on the coupling of multiphysics finite element simulation and a circuit model. The voltage source is used as input, which can simulate the macroscopic characteristics in the process of limiting current, such as the voltage and current waveforms, and can also simulate microscopic characteristics, such as temperature, magnetic field, and electrodynamic force distribution. The short-circuit experimental data of an air core inductive superconducting fault current limiter (SFCL) prototype was compared with the simulation results to verify the reliability of the simulation.

Enhancement of DUBAL Network Operational Performance Using Resistive High Temperature Superconducting Fault Current Limiter

Power systems under expansion suffer from escalating fault levels that impact equipment integrity, operational flexibility, and the overall security of the system. The fault current limiter (FCL) is one of approaches used by utilities to limit fault current levels and in Dubai Aluminum (DUBAL) series current limiting reactors are currently used. However, more effective (FCL) topologies are sought and, in this paper, a case study is proposed using resistive high temperature superconducting fault current limiters (HT-SFCLs). The application of HT-SCFLs is aimed here at reducing short-circuit currents, while at the same time enhancing the stability and security of the network. The study involves analysis of three-phase and single-line-to-ground faults, evaluation of the voltage levels and total harmonic distortion (THD) levels at busbars considering different fault scenarios, and demonstrates how the use of HT-SFCLs at various locations improves the plant performance. The ideal HT-SFCL model is adopted for this analysis since the aim is to look at the steady-state performance rather than the transient performance. Comparison with series reactor FCLs which are currently installed in the plant show better performance with the proposed HT-SFCL. Voltage profile values and total harmonic content were also compared with measurement data available at the plant.

Design and Evaluation of a Novel Non-Inductive Unit for a High Temperature Superconducting Fault Current Limiter (SFCL) with Bias Magnetic Field

The level of the fault current is enlarged against the capacity of conventional circuit breaks in some urban power grids with the rapid increase of power load. Thus, a high-temperature superconducting fault current limiter (HT SFCL) has been suggested to provide effective and reliable protection of power devices against fault currents. This paper deals with the design of a prototype hybrid SFCL, which is integrated by a double-split reactor with asymmetric noninductively wound copper windings and a noninductive HTS magnet in series to one branch of the reactor. There is a fast switch in series to the magnet as well. The role of non-inductive magnet is for reducing the fault current in the first swing and the double-split reactor further decreases the fault current. The noninductive HTS unit is designed based on the magneto-thermal coupled model. A noninductive superconducting unit prototype is constructed using a YBCO-coated conductor and is immersed in the liquid nitrogen. A high-speed Data Acquisition System of National Instruments based on LABVIEW is built. The critical current and current impact performance for this prototype under the operating temperature of 77 K are investigated experimentally. The experimental results validate the current limiting effectiveness of the noninductive unit for the SFCL with bias magnetic field.

Study on the Working Current of the Superconducting Coil and the Equivalent Circuit of HTSFCL

This paper mainly describes the designing progress of iron-core saturated high-temperature superconducting fault current limiter (HTSFCL). First, the relationship between magnetic field and current of high temperature superconducting coil is simulated in the finite element software, which can get the working current of superconducting coil of 70 A. Then, using the simulation results of coil, the design formula is deduced, and the size of HTS current limiter and the number of coil turns are calculated. Third, the HTSFCL model is established in the finite element analysis software to obtain the field distribution and short-circuit characteristics from the simulation results, and calculate the equivalent parameters. The equivalent circuit of HTSFCL is established in the circuit analysis software, and the correctness of the equivalent circuit is verified by comparing different simulation results. Finally, through the test of HTS coil, the working current of the superconducting coil is verified to be 70 A.

Trap Distribution and Breakdown Characteristics of Direct-Fluorinated PI Film for DC-HTSFCL

High-temperature superconducting fault current limiter (HTSFCL) is an important application in the power industry. In high-voltage dc system, a polyimide (PI) is used as superconducting winding insulation in HTSFCL also withstands pulse voltage of lightning and operating voltage caused by line fault and switching. The combined dc-pulse voltage makes the electric field more deformed, which induces partial discharge and accelerates insulation failure. Direct fluorination is a good means of regulating the molecular structure and electrical properties of polymers. This paper studies the trap distribution and breakdown characteristics of direct-fluorinated PI film. The corona charging was implemented in an environment with a relative humidity of 40%, using a combined dc-pulse voltage. To calculate the trap distribution, the surface potential was obtained. The breakdown voltage was also measured in LN 2 under different voltages. The experimental results indicate that moderate fluorination can effectively reduce the depth of trap level, accelerate charge detrapping, and improve the breakdown voltage of the PI film.

Surface Potential and Breakdown Characteristics of Epoxy/AlN Nanocomposites Under DC and Pulse Voltages

In dc high-temperature superconducting fault-current-limiter (HTSFCL) operation, epoxy resin used as the current lead insulation withstands dc voltage. In the case of lightning stroke or line fault, HTSFCL needs to withstand the additional pulse voltage, which is several times greater than that of the dc voltage, and the polarity may be different from that of the dc voltage. It intensifies the electric field distortion, induces partial discharge, and causes insulation failure. To improve the insulation properties of the current lead, the epoxy/aluminum nitride (AlN) nanocomposites with the filler content of AIN nanoparticles of 0, 1, 3, and 5 wt% were prepared. The trap distribution of the sample was acquired to clarify the effects of AlN content on the surface potential behaviors. The results show that the amplitude and the polarity of the pulse voltage can affect the surface charge accumulation of the sample under dc voltage. Moreover, the addition of AlN nanoparticles improves the breakdown voltage of the sample in LN 2 under dc and pulse voltages.

Magneto-Thermal Coupling Design and Performance Investigation of a Novel Hybrid Superconducting Fault Current Limiter (SFCL) With Bias Magnetic Field Based on MATLAB/SIMULINK

Energy demand is growing rapidly worldwide, and sustainable solutions are required to improve the energy supply. The new devices like high-temperature superconducting fault current limiters (SFCLs) are popular in recent years. They use the electrical properties of high-temperature superconductivity to instantaneously protect power grids against short circuits and thereby prevent costly outages. They can not only reduce the capacity of the circuit breaker but also lower the cost of line construction. They have become much promising for application in a high-voltage transmission line. A novel hybrid SFCL with a bias magnetic field is proposed by combining a reactor with double-split symmetrical windings and a non-inductive high-temperature superconducting (HTS) magnet, which is in series to one branch of the reactor. An electro-magnetic model of the reactor and a magneto-thermal coupling model of the non-inductive HTS element have been built, and the simulation model of this SFCL is developed in MATLAB/SIMULINK. Its performance is investigated through the simulation considering a serious short-circuit fault and is compared to the performance without SFCL. The results validated the effectiveness of this kind of hybrid SFCL.

An effective way to reduce AC loss of second-generation high temperature superconductors

Second-generation (2G) high temperature superconducting (HTS) devices have high power density and efficiency for energy applications. However, in alternative current conditions, the vortex movement in 2G HTS leads to energy dissipation at low temperatures, which significantly affects efficiency and adds an extra burden to cooling systems. This energy dissipation is identified as AC loss. Reducing AC loss to improve efficiencies of 2G HTS devices, e.g. HTS machines, cables and fault current limiters, has become a key research focus. We report here on an effective way to reduce AC losses by using a new 2G HTS wire. The principle is to stack narrow 2G HTS tapes into a wire structure, which is called a soldered-stacked-square (3S) wire. Our experiments on solenoid coils have proved that the 3S wire helps to reduce the AC loss by 80%. Moreover, the critical current of the 3S wire can be easily adjusted for various applications. It is envisioned that the 3S wire will become a powerful technology enabling the development of 2G HTS power devices with high efficiency.

Transport AC Loss Measurements in Bifilar Stacks Composed of YBCO-Coated Conductors

Increasing the current-carrying capacity of bifilar coils is essential for the high-temperature superconducting (HTS) fault current limiter (FCL) application. We built three bifilar stacks with two, four, and six conductors using 4 mm wide stabilized SuperPower YBCO wires to investigate ac loss dependence of the bifilar stacks on the number of conductors. In the three basic configurations, the upper half conductors carry current in one direction, and the lower half conductors carry current in the opposite direction. We report transport ac loss results on these stacks measured at three different frequencies. The measured loss values were compared with those in stacks with the same conductor number and geometry but producing different degrees of field cancellation. We also explored the dependence of ac loss on a central gap in bifilar stacks composed of four and six conductors and the results were compared with the numerical results obtained from a two-dimensional FEM model. Results show that the bifilar stacks where each turn is stacked with multiple wires is a promising option which compromise both high current-carrying capacity and lower ac loss for HTS FCL application.

A Novel DC Fault Ride-Through Scheme for MTDC Networks Connecting Large-Scale Wind Parks

This paper proposes a novel fault ride-through protection scheme for multiterminal dc (MTDC) networks connecting large-scale offshore wind parks. The proposed scheme introduces an MTDC network with a hybrid fault current limiter (FCL), which consists of a high-temperature superconducting FCL (HTS-FCL) and series braking resistor (SBR) integrated with low-loss mechanical dc circuit breaker (DCCB). The aim of HTS-FCL is to provide fast quenching, limit the dc fault current within the interrupting capacity of the DCCB, and reduce the dc fault current to certain levels such that converters can sustain prolonged periods of operation without blocking. Also, SBR allows automatic reclosing of DCCBs following a temporary fault, thereby enhancing the network security and reliability. Mechanical DCCBs with the proposed hybrid limiter are installed at the terminating points of each dc cable. The proposed protection scheme is based on differential current approaches with fault clearing time of few milliseconds, which gives enhanced network selectivity and reliability. A four-terminal meshed MTDC network with the proposed hybrid FCL is modeled in PSCAD/EMTDC. The obtained simulation results confirm successful dc fault isolation and network recovery for different dc fault scenarios.

HTS Applied to Power System: Benefits and Potential Analysis for Energy Conservation and Emission Reduction

Energy conservation and emission reduction is a critical task for power systems. Innovative technology such as applied superconductivity has great potential. With the rapid advances in high temperature superconducting (HTS) technology, various HTS power applications, such as HTS cables, HTS transformers, HTS generators, HTS fault current limiters, and superconducting magnetic energy storage can be of great help for energy conservation and emissions reduction in modern power systems. A brief review of energy losses and CO 2 emissions in power systems is first presented with various currently existing solutions and techniques analyzed and compared. After analysis of the inherent characteristics of HTS applications, the potential and possible benefits of HTS power applications for energy conservation and emission reduction are studied in detail. In addition, the technical and economic challenges for HTS power applications are also discussed. It is obvious that HTS power applications have great potential and extensive direct and indirect benefits for energy conservation and emissions reduction in the future power systems, after some challenges for HTS power applications have been solved.

A multi-objective heuristic method for optimal design of HTS fault current limiters

The optimization algorithm for High-Temperature Superconducting (HTS) Fault Current Limiters (FCL) normally includes some non-commensurable criteria or objectives such as cost, limiting factor, thermal stresses, and mechanical stresses that should be minimized or maximized concurrently in a protracted optimization process. It can be performed only by tools which rstly describe the HTSFCL precisely and predict its limitation behavior and then select the optimum design using optimization algorithm. Multi-Objective Decision Making (MODM) is one of the most widely used decision techniques in the business and engineering worlds. In the MODM problems, there are several objectives of the system concurrently optimized, and a solution set, i.e. the Pareto front, is usually obtained instead of a real optimal solution. This paper explains in details the combination models of HTSFCL as a component in PSCAD/EMTDC and presents the optimization algorithm based on a new approach of normalized multi-objective simulated annealing.

Conceptual Design of a Resistive 154-kV/2-kA Superconducting Fault Current Limiter

The expansion of electric capacity is inevitable to satisfy the demands of rapid economic growth. An increase in electricity load causes a decrease in load impedance. As a result, the fault current would exceed the capacity limit for the short-circuit breaker installed in a power system. Fault current that exceeds the rating of a conventional circuit breaker will degrade the stability of the power grid system. Many types of high-temperature superconducting (HTS) fault current limiters (FCLs) have been developed to enhance the stability and reliability of a power grid system. In this paper, a resistive HTS FCL using 2G HTS wires is designed, taking into consideration the demand quantity of 2G HTS wires, the winding type of the coils, the number of series and parallel connections, electrical insulation, and mechanical stability in the fault state. The length of the superconducting coils and the number of series and parallel connections are calculated. A finite-element method is used to perform dielectric and mechanical design analysis.

Investigation of the flashover and breakdown voltage of thin polymer films in liquid nitrogen

Resistive high-temperature superconducting fault current limiters (R-SFCL's) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium voltage level. To develop R-SFCL's for high-voltage applications exceeding 100 kV, the dielectric properties of liquid nitrogen as insulation material in different arrangements have to be investigated more in detail. Considering R-SFCL's, the 2G tapes are typically arranged in bifilar coils. Adjacent tapes are insulated by a combination of thin polymer foils and liquid nitrogen resulting in a surface discharge arrangement, which dielectric properties were investigated. Specimens of foil material were inserted between the two electrodes and immersed in liquid nitrogen within an open isolated vessel. The applied ac voltage was raised until flashover occurred, and the shortest overhang of the foil material was recognized as flashover length. The investigations were performed for the polymer materials polyimide, PTFE, and PET and thicknesses of the foils between 75 and 250 $\mu\textrm{m}$ in dependence on the specific properties of the electrode arrangement. The results reveal that the flashover lengths are rather small, so that in most cases, the breakdown of the polymer foil occurs before the flashover.

Experimental Study on Vaporization of Subcooled Liquid Nitrogen by Instantaneous Heat Generation in LN2 Chamber for HTS-FCL

It is one of important design issues in the high-temperature superconducting fault current limiter (HTS-FCL) to estimate how much liquid nitrogen vaporizes during its quench process. This paper describes the small scale experiment about the vaporization of subcooled liquid nitrogen by instantaneous heat generation corresponding to the quench in HTS-FCL. In the experiment, the heat is given liquid nitrogen by Joule heating of the stainless steel strip for the short time of about 100 ms. The time variations of temperature and pressure in the liquid nitrogen test chamber are measured for different subcooling conditions of liquid nitrogen and various heat flux conditions. In addition, the amount of vaporization of liquid nitrogen is calculated from experimental results according to the subcooling of liquid nitrogen. The experimental results show that the vaporization of liquid nitrogen strongly depends on heat flux and subcooling conditions. The amount of vaporization linearly decreases with the increase in the subcooling of liquid nitrogen. The calculation results about the amount of vaporization suggest that the subcooling of more than 20 K is necessary to suppress bubble generation for the heat flux condition of 46 W/cm 2 or more.

Development, Updating and Long-Term Operations of a 10.5 kV HTS Fault Current Limiter

There are many irreplaceable advantages of high temperature superconducting (HTS) fault current limiter, applying in electric utilities. It is expected to be able to solve excessive fault current problems and to enhance safety and stability of power systems. In this paper, the R and D of an improved bridge-type 10.5 kV three-phase superconducting fault current limiter (SFCL) was present. Each phase adopted a HTS coil with inductance of 6.2 mH. The three coils wound with 8,571 m Bi2223/Ag tapes totally. After installation at a 110 kV/10.5 kV substation in Hunan Province of China, the performances of the whole SFCL were tested, including a three-phase-to-ground short circuit experiment at the pre-setup short circuit point. And then, the SFCL was put into operations in the 10.5 kV power grids for more than 11000 h. About 3 years later, the SFCL was moved, reinstalled, and put into operation again since February 16 of 2011 at a 10.5-kV superconducting power substation located in Baiyin, Gansu Province of China. In this paper, the redesigned and manufactured cryostats with nonmagnetic stainless steel, tests, and long-term operations of the SFCL in 10.5 kV power grids were also described in detail.

Optimal Allocation of HTS-FCL for Power System Security and Stability Enhancement

This paper presents a new problem formulation for allocation of high-temperature superconducting fault current limiter (HTS-FCL) in power systems for security and stability enhancement. A dynamic current-dependent model for HTS-FCL is developed using PSCAD/ETMDC. The proposed formulation includes three functions related to security, voltage stability, and rotor angle stability which represent key features of deploying the HTS-FCL. The three functions were combined into a multi-objective function, and the problem is formulated as a mixed-integer nonlinear programming (MINLP) problem and optimal FCL locations are determined using particle swarm optimization. The optimal FCL locations for the New-England 39-bus system are determined, and the system transient performance is validated using MATLAB and PSCAD. The simulation results demonstrate the effectiveness of proposed approach for achieving optimal performance of HTS-FCL in power system in terms of security and system stability.

Voltage Booster Schemes for Fault Ride-Through Enhancement of Variable Speed Wind Turbines

This paper presents series grid interface topologies for enhancing the fault ride-through (FRT) performance of doubly-fed induction generator (DFIG)-based wind turbines (WTs). Two voltage booster schemes, 1) dynamic voltage restorer (DVR) and 2) resistive type high temperature superconducting fault current limiter (HTS-FCL), are designed and implemented. The test system represents a WT connected to an electric grid with alternatively employing DVR and HTS-FCL. Both schemes provide fast mitigation of voltage dip that maintains the nominal operating conditions for DFIG-WT. To achieve a flexible control solution for balanced and unbalanced fault conditions, the DVR employs positive and negative sequence controllers while the HTS-FCL is designed to perform fast quenching for each phase individually. The potential of the two booster schemes is evaluated and analyzed in positive and negative sequence reference frames. Comprehensive simulation studies are presented to verify the capability of the series grid interface schemes for ensuring the normal operation and smooth wind power evacuation with effective isolation from grid faults. Furthermore, the grid code requirements of reactive current support are evaluated for both schemes at various fault scenarios.