Dc bias system of a 35 kV/90 MVA saturated iron core SFCL

Saturated iron core superconducting fault current limiter (SIC-SFCL) is a promising fault current limiting device for high or extra-high voltage power grids. It has low impedance in normal power transmission and turns high impedance when a short-circuit takes place. The dc bias system of a saturated iron core superconducting fault current limiter plays a key role in realizing these features. Our 35 kV/90 MVA SIC-SFCL has been running live-grid since January 2008 at Puji substation in Yunnan, China. In this paper, the working principle of the dc bias system will be introduced and results of artificially imposed short-circuit tests will be provided, which verifies the validity of this system.     

14 kV single-phase superconducting fault current limiter based on YBCO films

We present the fabrication and short circuit test results of a 14 kV single-phase resistive superconducting fault current limiter (SFCL) based on YBa₂Cu₃O₇ (YBCO) films. Individual components were processed using the 4″ YBCO films and have the rated voltage and current of 600 V and 35 A at 77 K, respectively. Twenty four components, eight components in series and three lines in parallel, make a module having the rated voltage and current of 4.8 kV and 105 A, respectively. Three modules were assembled in series to produce the SFCL working at 77 K, a 14 kV single-phase machine for the 22.9 kV Y-Y grid. short circuit tests were successfully conducted in an accredited test facility with the maximum fault currents up to 14.1 kA_P. All components quenched together upon faults and shared the rated voltage evenly without any supplementary device between the modules. This proves that the SFCL based on YBCO films may not only work reliably at 22.9 kV, but also provide technical feasibility for higher voltage application including the transmission grids.     

Characteristics of critical current of superconducting solenoid wound with the stacked tape

The high-T_c superconducting (HTS) magnet is an important element for developing HTS power equipments such as the dc reactor of the inductive type superconducting fault current limiter (SFCL). In order to use the HTS magnet for the large-scale power system, its critical current needs to be high enough. Generally, the double pancake HTS magnet has the severe decrease in the critical current because of magnetic field perpendicular to the tape surface. To fabricate a high critical current magnet, we wound a solenoid with the stacked tape. In this paper, the characteristics of the critical current of the HTS solenoid wound with the stacked tape were investigated. The results of this research can be used as the background data for the design of the large-scale HTS magnet.     

Comparative Study of Resistive and Inductive Superconducting Fault Current Limiters SFCL for Power System Transient Stability Improvement

This paper presents a comparative study of resistive and inductive superconducting fault current limiter (SFCL) for power systems transient stability improvement. Two applications of transient stability assessment are presented in this paper: The first shows the efficiency of the resistive and inductive SFCL in series with a generator, the second uses SFCL installed in series with a transmission line. SFCL can just be operated during the period from the fault occurrence to the fault clearing; the modeling and the effect of SFCL has been investigated to have higher benefits for the power system. In the present work, modification of the admittance matrix method is used for modeling of SFCL; Critical Clearing Time (CCT) has been used as an index for evaluated transient stability. The transient stability is assessed by the criterion of relative rotor angles, using the Runge–Kutta method. The effectiveness of the proposed method is tested on the WSCC3 nine-bus system applied to the case of three-phase short circuit fault in one transmission line. A simulation and comparison are presented in this document.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

Development of a superconducting fault current limiter using various high-speed circuit breakers

The authors constructed and tested a model superconducting fault current limiter (SFCL) using a high-temperature superconducting film according to a design that includes a vacuum interrupter with an electromagnetic repulsion mechanism. The superconductor and the vacuum interrupter are connected in parallel with a bypass coil. If a fault occurs and current flows through the system, the superconductor is quenched and the current is transferred to the parallel coil because of the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter. On opening the vacuum interrupter, the current in the superconductor is interrupted. This model is expected to exhibit very low-energy consumption by the superconductor. The authors succeeded in interrupting the current flowing in the superconductor within a half-cycle using a prototype SFCL. An improved SFCL with higher voltage and current ranges was used to carry out current-limiting tests and to investigate the possibility of adapting our SFCL in a power system. The authors also carried out a currentlimiting test using a conventional high-speed vacuum circuit breaker (HSVCB) as a new method for realising our concept.     

Research on Endurable Electrodynamic Force of YBCO Coated Conductors During Quenching Process

In a resistive superconducting fault current limiter (SFCL), the electrodynamic force among the YBCO tapes is enormous when experiencing a high short circuit current, which may cause deformation of tapes and damage of their electrical characteristics. The purpose of this paper is to investigate how much electrodynamic force the YBCO tapes can bear. In this paper, an electrodynamic force simulation model is built to simulate the force, which the two parallel tapes experience and their deformation when short-circuit currents of different value flow through. Then we conducted experiments to observe the deformation of the tapes when they are experiencing a short-circuit current and see if the volt-ampere characteristic of the tapes has a noticeable change after that. Combining the results of the simulation and the experiments, we can obtain the electrodynamic force that tapes can bear. The result can guide us to design the layout of the tapes in the SFCL in accordance with the required fault current level.     

Analysis on the Fusion Technology of a Commercial Transformer and an SFCL

In this study, a new concept is proposed on a combination device with functions of a commercial transformer and a superconducting fault current limiter (SFCL). This device serves as a transformer by stepping the voltage up or down in normal condition. When a transient phenomenon occurs in the power system, it serves as an SFCL to limit the fault current. The device quickly detects the line current using a current transformer (CT), and is based on the high-speed, silicon-controlled rectifier (SCR) interrupter operation. This is done by identifying the fault using an SCR switching control system. The test results showed that the fault current was limited by the impedance of the superconducting element a half cycle after a fault occurred. An SCR that initially had a normally open contact was turned on within a half cycle. However, an SCR with a normally close contact was turned off after a half cycle because the current dropped below the holding current after a half cycle. The voltage of the superconducting element was low in the step-down turn ratio condition of the transformer. This was because the secondary and tertiary windings were connected in series due to the SCR-1 turn-off condition, and the sum of voltages on each winding appeared on the superconducting element. By combining the existing power device technology and an SFCL technology, it is expected that the existing problems of an SFCL can be addressed to construct a smart power system.     

Experimental and Numerical Study of Co-ordination of Resistive-Type Superconductor Fault Current Limiter and Relay Protection

Superconducting fault current limiter (SFCL) has become one of the most ideal current limiting devices to solve the problem of increasing short-circuit current in high-voltage power grid. This paper presents a resistive-type SFCL model developed using simulation software PSCAD/EMTDC. After being verified by finite-element model and experimental results, the model is used to study the impact of SFCLs on the power grid and the co-ordination between SFCL and relay protections in 10 kV distribution network. A series of simulations are carried out to find appropriate parameters of SFCL model to cooperate with relay protection devices. The final result in this paper could provide important quantitative basis of parameters for SFCL to be applied in a real power system.     

Recovery estimation for over-current test of non-inductive fault current limiters using numerical analysis

When an HTS coated conductor (CC) is used as a conductor of a superconducting fault current limiter (SFCL), the CC is expected to be exposed to the over-current and temperature of the CC is expected to be increased rapidly by electrical joule heating. Because the CC is a composite tape, thermal and electrical properties of composite materials could affects over-current limiting capacity and recovery time of SFCL. This paper presents experimental and numerical results of over-current test and recovery time measurement test on four bifilar wound SFCL modules. The temperature transitions of the samples were estimated from total electrical resistance of the coils. We fabricated one bifilar solenoid coil and three bifilar pancake coils whose cryogenic conditions were different from the other coils. An numerical model was also fabricated to simulate the temperature transition and the numerical results were compared with experimental results.     

Sub-cooled nitrogen cryogenic cooling system for superconducting fault current limiter by using GM-cryocooler

The 21st Century Frontier R&D Program was planned to develop and commercialize the inductive Superconducting Fault Current Limiter (SFCL) in Korea until 2011. The 1.2 kV/80 A inductive SFCL was planned to develop at the first year in the first phase (2001-2002) and the 6.6 kV/200 A inductive SFCL for short run operation test was planned to develop at the second and third year in the first phase (2002-2004). The experimental characteristics of conduction-cooled cooling system developed in the first year was very weak from the sudden large thermal disturbance. Therefore, the conduction-cooled cooling system was concluded not appropriate for the cryogenic technology of the application of superconducting fault current limiter. In the third year research, the improved sub-cooled nitrogen cooling system was adopted and investigated.In this paper, the characteristics of each cooling type was compared and the basic deign of ameliorated cooling system was introduced and the total heat load of the cooling system was calculated and compared with the heat load of the cooling system developed at 2nd year research.     

Electrothermal Modeling of Coated Conductor for a Resistive Superconducting Fault-Current Limiter

Coated conductors are very promising for the design of a novel and efficient superconducting fault-current limiter (SFCL). The thermal and electrical behaviors of this type of SFCL in the presence of over-critical currents need to be investigated in detail to master its performance in a power grid. In this paper, an Electrothermal Model of a Coated Conductor (ETMCC), not simulated in the literature, is implemented and introduced in the library of MATLAB software. An algorithm to solve the differential equations characterizing the superconducting material is developed using the Runge-Kutta method. In this context the ETMCC under over-critical current is performed. Different dimensions and substrate configurations of the sandwich layers are considered. In order to improve the high-temperature superconductor (HTS)-FCL design, the influence of the substrate and shunted layers (using different materials) on the thermal stability is investigated. The simulation results are generalized, thus allowing us to determine the current threshold to achieve thermal stability of the HTS-FCL at any point of the coated conductor.     

Analysis on the thermal and electrical characteristics of impregnating materials for the bifilar winding-type superconducting fault current limiter

The resistive type high temperature superconducting fault current limiter (HTSFCL) limits the fault current with the resistance that generated by fault current. The generated resistance by fault current makes large pulse power which makes the operation of HTSFCL unstable. So, the cryogenic cooling system of the resistive type HTSFCL must diffuse and eliminate the pulse energy very quickly. Although the best way is to make wide direct contact area between HTS winding and coolant as much as possible, HTS winding also need the impregnation layer which fixes and protects it from electromagnetic force. This paper deals with thermal conductivity and dielectric strength of some epoxy compounds for the impregnation of high temperature superconducting (HTS) winding at 77 K. The measured data can be used in the optimal design of impregnation for HTS winding. Aluminar filling increased the thermal conductivity of epoxy compounds. Hardener also affected the thermal conductivity and the dielectric strength of epoxy compounds.     

Empirical Modeling of Cryogenic System for Hybrid SFCL Using Support Vector Regression

The hybrid superconducting fault current limiter (SFCL) is now at the stage of practical use in a power grid in Korea. A cryogenic cooling system was designed, fabricated, and successfully tested for a prototype of 22.9 kV/630 A SFCL. The operation scheme of cryogenic system has been investigated in preparation for temporary loss of cryocooler power in hybrid SFCL (in Kim et al., IEEE Trans. Appl. Supercond. 21(3):1284–1287, 2011). In this paper, we investigated the empirical modeling of cryogenic cooling system for SFCL using principal components and auto-associative support vector regression (PCSVR) for the prediction and fault detection of the cryogenic cooling system. For empirical model, data were acquired during a blackout test of cryogenic cooling system. Blackout times of the test were 1 hour and 4 hours at two operation current levels. Three set of data were used for training and optimization of the model and the rest set of data was used for verification. Signals for the model are temperatures measured at copper band and cold head of cryocooler, system pressure and liquid temperatures measured at two locations in liquid-nitrogen pool. For optimization of the SVR parameters, the response surface method (RSM) and particle swarm optimization (PSO) were adopted in this paper. After developing the empirical model we analyzed the accuracy of the model. Also, these results were compared with that of auto-associative neural networks (AANN). RSM and PSO gave almost the same optimum point. PCSVR showed much better performance than AANN in accuracy aspects. Moreover, this model can be used for the prognosis of cryogenic cooling system for SFCL.     

Design,fabrication techniques and test results of 1.2 kV/80 A inductive fault current limiter by using conduction-cooled system

An inductive superconducting fault current limiter protects power system by limiting the amplitude of fault current by the inductance of its dc reactor. Therefore, it is very important to design the dc reactor of high critical current prior to fabrication. At first, the optimal design parameters were calculated by using finite element method and then the superconducting dc reactor for 1.2 kV/80 A_rms inductive superconducting fault current limiter was designed by considering the conduction-cooling characteristics. Moreover, the design, fabrication and conduction-cooling method of the superconducting dc reactor were introduced. Actually, the superconducting dc reactor was fabricated and cooled down to 20 K by using GM cryocooler. Finally, the short-circuit test was performed and the experimental results were discussed.     

Modeling and Simulation of Resistive Superconducting Fault-Current Limiters

Superconducting fault-current limiters (SFCL) offer ideal performance in electrical power system. The design of SFCL has to be both flexible, to allow an easy adaptation to the specific requirements of each particular application, and a high quality standard with reproducible properties. Up to now no simulation model of SFCL has been validated or introduced in the Library of MATLAB software. In this paper a simulation model for a novel resistive type superconducting fault-current limiter is proposed. This model includes the electric field-current density (E?CJ) characteristics of High-Temperature Superconductors (HTS). A?graphical interface using Graphical User Interface (GUI) of MATLAB is developed in order to ease the operation of the proposed model. This one facilitates the introduction or the parameter modification of materials candidate to a SFCL model. Thus, the operation characteristics and limitation behavior of SFCL have been investigated. The developed model accurately predicted the current-time waveforms achievable with typical limiters, and improved standard of understanding concerning the fault-current limitation mechanisms.     

Research on resistance characteristics of YBCO tape under short-time DC large current impact

Research of the resistance characteristics of YBCO tape under short-time DC large current impact is the foundation of the developing DC superconducting fault current limiter (SFCL) for voltage source converter-based high voltage direct current system (VSC-HVDC), which is one of the valid approaches to solve the problems of renewable energy integration. SFCL can limit DC short-circuit and enhance the interrupting capabilities of DC circuit breakers. In this paper, under short-time DC large current impacts, the resistance features of naked tape of YBCO tape are studied to find the resistance – temperature change rule and the maximum impact current. The influence of insulation for the resistance – temperature characteristics of YBCO tape is studied by comparison tests with naked tape and insulating tape in 77 K. The influence of operating temperature on the tape is also studied under subcooled liquid nitrogen condition. For the current impact security of YBCO tape, the critical current degradation and top temperature are analyzed and worked as judgment standards. The testing results is helpful for in developing SFCL in VSC-HVDC.     

Experimental analysis of AC loss and recovery characteristics for proposal of effective thickness of spacers in pancake type

For the design of superconducting fault current limiter, the research about recovery time and AC loss is essential issue because this characteristic is closely related to stability and efficiency. In general, superconducting fault current limiter (SFCL) modules can be made into pancake or solenoid type coil. While the pancake type coil has smaller AC loss, it has longer recovery time compared with the solenoid type coil. In this paper, a new pancake type coil was proposed to decrease recovery time by improving their cryogenic condition. A new pancake type coil is made with spacers between adjacent superconducting tapes. However, as the thickness of the spacer was increased, the AC loss was increased unlike the recovery characteristic. From the experimental results, our group confirmed that the pancake type coil with the spacer has trade-off relationship between AC loss and recovery characteristics with the spacer thickness.The proposed results in this paper would be utilized to design superconducting fault current limiters.     

Research and development of superconducting cable in Super-ACE project

Research and development of fundamental technologies for superconducting AC power equipments (called Super-ACE) project started as a national project in 2000 fiscal year by Ministry of Economy, Trade and Industry and New Energy and Industrial Technology Development Organization. This project is to research and develop the basic technology of high temperature superconducting cable (HTS cable), fault current limiter, and transformer. Main subjects of the cable in this project are to study the 3 kA superconducting cable conductor, cooling technology of 500 m HTS cable, and power system analysis of cable and AC equipments. The present paper describes the background of this project, target of each research subject, schedule of research and development, and major results of research activity in 2001.     

Analysis of a flux-coupling type superconductor fault current limiter with pancake coils

The characteristics of a flux-coupling type superconductor fault current limiter (SFCL) with pancake coils are investigated in this paper. The conventional double-wound non-inductive pancake coil used in AC power systems has an inevitable defect in Voltage Sourced Converter Based High Voltage DC (VSC-HVDC) power systems. Due to its special structure, flashover would occur easily during the fault in high voltage environment. Considering the shortcomings of conventional resistive SFCLs with non-inductive coils, a novel flux-coupling type SFCL with pancake coils is carried out. The module connections of pancake coils are performed. The electromagnetic field and force analysis of the module are contrasted under different parameters. To ensure proper operation of the module, the impedance of the module under representative operating conditions is calculated. Finally, the feasibility of the flux-coupling type SFCL in VSC-HVDC power systems is discussed.