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.     

Modeling of Shield-Type Superconducting Fault-Current-Limiter Operation Considering Flux Pinning Effect on Flux and Supercurrent Density in High-Temperature Superconductor Cylinders

Superconducting fault current limiter, SFCL, forms an important category of fault-current-limiting devices which limit the short-circuit current levels in electrical networks. Therefore, modeling its operation and anticipating its characteristic parameters are too important in its design and optimization process. In this paper a novel integrative method has been proposed which predicts, with a good accuracy, the behavior of inductive shield-type SFCL in different circumstances and approximates its main operational characteristics, as the through current, the inductance and the voltage-current characteristics. An algorithm is presented to calculate the exact distribution of magnetic flux and supercurrent density inside the superconductor bulk in different operational conditions using the well-known Bean model and for the first time the flux pinning effect has been taken into account in SFCL operation modeling. For estimation of flux density distribution outside the superconductor bulk, the FEM analysis has been utilized. An iterative method has been used, based on the numerical solution of differential equations, to calculate the instant value of the SFCL through-current and inductance. The proposed method of modeling has been studied on a specific design of shield-type SFCL and its through current in normal and fault conditions of a test circuit, variation of its inductance with time and its voltage-current characteristic are calculated theoretically. A prototype has been fabricated based on the studied SFCL design and has been tested experimentally. The comparison of the experimental and theoretical results shows that this modeling predicts the SFCL operation with a good accuracy.     

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.     

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.     

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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Influence of the Substrate Layer on the Lightning Current Performance of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> Tapes

In a power grid, the superconducting power devices might also experience lightning impulse current except for the common over-currents. However, the study of the performance of YBCO tapes suffering a lightning current is scarcely reported. This paper mainly focuses on the influence of the substrate layer on the thermal stability of YBCO tapes suffering a lightning current. A numerical model which took into account both the thermal and the electromagnetic aspects was proposed. The validity of this model was verified by experiment. Based on this model, the influence of the dimension and material type of thesubstrate layer on the thermal stability were investigated in detail. The simulated results showed that the substrate layer could affect the temperature distribution on different layers, and stainless steel substrate layer is a more desired choice for decreasing the maximum temperature. Moreover, a theoretical explanation based on a simplified equivalent circuit was also used to study the influence of the substrate layer.     

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.     

Spatial generalized linear mixed models of electric power outages due to hurricanes and ice storms

This paper presents new statistical models that predict the number of hurricane- and ice storm-related electric power outages likely to occur in each 3 km×3 km grid cell in a region. The models are based on a large database of recent outages experienced by three major East Coast power companies in six hurricanes and eight ice storms. A spatial generalized linear mixed modeling (GLMM) approach was used in which spatial correlation is incorporated through random effects. Models were fitted using a composite likelihood approach and the covariance matrix was estimated empirically. A simulation study was conducted to test the model estimation procedure, and model training, validation, and testing were done to select the best models and assess their predictive power. The final hurricane model includes number of protective devices, maximum gust wind speed, hurricane indicator, and company indicator covariates. The final ice storm model includes number of protective devices, ice thickness, and ice storm indicator covariates. The models should be useful for power companies as they plan for future storms. The statistical modeling approach offers a new way to assess the reliability of electric power and other infrastructure systems in extreme events.     

Deriving operating characteristic equations for overcurrent protection devices based on eigensystem realization algorithm

Accurate representation of the operating characteristics of conventional inverse-time overcurrent (OC) protection devices plays an important role in protection coordination schemes. Based on the singular value decomposition of the Hankel matrix, this study uses the eigensystem realization algorithm to curve-fit the characteristic curves of OC protection devices under the digital state-space model and obtain the equations of their operating characteristics. This study applies the proposed method to four types of OC protection devices, including two electromechanical OC relays, one digital OC relay, and one power fuse. One characteristic curve from each protection device is selected for curve-fitting. For all four OC protection devices, the absolute error values for the hundreds of sample points between the actual characteristic curves and the corresponding curve-fitting equations are all less than 10 ms. The numbers of fitting components required are determined by the desired maximum absolute values of errors for the fitting equation. Finally, this study uses the derived equation to construct the characteristic curve of customized OC relay to solve the coordination problem of power system protection.     

Software reliability allocation of digital relay for transmission line protection using a combined system hierarchy and fault tree approach

Digital relay is a special purpose signal processing unit in which the samples of physical parameters such as current, voltage and other quantities are taken. With the proliferation of computer technology in terms of computational ability as well as reliability, computers are being used for such digital signal processing purposes. As far as computer hardware is concerned, it has been growing steadily in terms of power and reliability. Since power plant technology is now globally switching over to such computer-based relaying, software reliability naturally emerges as an area of prime importance. Recently, some computer-based digital relay algorithms have been proposed based on frequency-domain analysis using wavelet-neuro-fuzzy techniques for transmission line faults. A software reliability allocation scheme is devised for the performance evaluation of a multi-functional, multi-user digital relay that does detection, classification and location of transmission line faults.     

异步电动机电磁参数引起的失稳振荡的分析

交流异步电动机的失稳自激振荡,会引起较大的有害机械振动,并使电机电流、功率的波动增大,超过许可值,因而烧毁电机和电气设备。本文建立了电动机的机电耦联振动方程组,经过变换得到了无周期系数的标幺化非线性方程组,采用Hopf分叉理论研究了系统电磁参数变化引起的自激振荡,指出了系统失稳的数学机理及其变化规律;给出了相应的数值计算结果,并进行了实验研究,实验结果与结论计算结果相吻合。