Byeong-Hwa Lee

Design and Experiments of Novel Hybrid Type Superconducting Fault Current Limiters

In order to apply resistive superconducting fault current limiters into electric power systems, the urgent issues to be settled are as follows, such as initial installation price of SFCL, operation and maintenance cost due to ac loss of superconductor and the life of cryostat, and high voltage and high current problems. The ac loss and high cost of superconductor and cryostat system are main bottlenecks for real application. Furthermore in order to increase voltage and current ratings of SFCL, a lot of superconductor components should be connected in series and parallel which resulted in extreme high cost. In addition, the method to quench all components at the same instant needs very sophisticated skill and careful operation. Due to these problems, the practical applications of SFCL were pending. Therefore, in order to make practical SFCL, the price of SFCL should be lowered and should meet the demand of utilities. LSIS and KEPRI designed novel hybrid SFCL which combines superconductor and conventional electric equipment including vacuum interrupter, power fuse and current limiting reactor. The main purpose of hybrid SFCL is to drastically reduce total usage of superconductor by adopting current commutation method by use of superconductor and ultra fast switch. Consequently, it was possible to get the satisfactory test results using this method, and further works for field tests are in the process.

Practical Application Issues of Superconducting Fault Current Limiters for Electric Power Systems

The discovery of high-temperature superconductors were the great opportunity for developing fault current limiters using quenching phenomena. Superconducting fault current limiters is highly effective solutions to reduce and control excessive fault current which is inevitable to electric power systems. So, many companies and national research institutes were trying to develop this novel electric equipment under the assistance of national government. Due to their efforts, various types of superconducting fault current limiters were designed and tested. But, unfortunately, the practical applications and commercialization of superconducting fault current limiters are pending due to many unsolved problems. This paper discusses the current issues and commercialization problems of superconducting fault current limiters considering various aspects such as coordination with conventional relay, high voltage and high current issues, performance, cost, size, and life and maintenance issues. Then, emerging solutions for fault current limiters using hybrid method were introduced and analyzed. Finally, the viable method and our newly developed hybrid superconducting fault current limiters in order to solve the practical problems of conventional superconducting fault current limiters was briefly introduced.

Feasibility Analysis of the Positioning of Superconducting Fault Current Limiters for the Smart Grid Application Using Simulink and SimPowerSystem

One of the most important topics regarding the application of superconducting fault current limiters (SFCL) for upcoming smart grid is related to its possible effect on the reduction of abnormal fault current and the suitable location in the micro grids. Due to the grid connection of the micro grids with the current power grids, excessive fault current is a serious problem to be solved for successful implementation of micro grids. However, a shortage of research concerning the location of SFCL in micro grid is felt. In this work, a resistive type SFCL model was implemented by integrating Simulink and SimPowerSystem blocks in Matlab. The designed SFCL model could be easily utilized for determining an impedance level of SFCL according to the fault-current-limitation requirements of various kinds of the smart grid system. In addition, typical smart grid model including generation, transmission and distribution network with dispersed energy resource was modeled to determine the location and the performance of the SFCL. As for a dispersed energy resource, 10 MVA wind farm was considered for the simulation. Three phase faults have been simulated at different locations in smart grid and the effect of the SFCL and its location on the wind farm fault current was evaluated. Consequently, the optimum arrangement of the SFCL location in Smart Grid with renewable resources has been proposed and its remarkable performance has been suggested.

Mechanical Stress Reduction of Rotor Core of Interior Permanent Magnet Synchronous Motor

In this paper, the bridge shape of interior permanent magnet synchronous motor (IPMSM) is designed for integrated starter and generator (ISG) which is applied in hybrid electric vehicle (HEV). Mechanical stress of rotor core which is caused by centrifugal force is the main issue when IPMSM is operated at high speed. The bridge is thin area in rotor core where is mechanically weak point and the shape of bridge significantly affects leakage flux and electromagnetic performance. Therefore, bridge should be designed considering both mechanic and electromagnetic characteristics. In the design process, we firstly find a shape of bridge has low leakage flux and mechanical stress. Next, the calculation of mechanical stress and the electromagnetic characteristics are performed by finite element analysis (FEA). The mechanical stress in rotor core is not maximized in steady high speed but dynamical high momentum. Therefore, transient FEA is necessary to consider the dynamic speed changing in real speed profile for durability experiment. Before the verification test, fatigue characteristic is investigated by using S-N curve of rotor core material. Lastly, the burst test of rotor is performed and the deformation of rotor core is compared between prototype and designed model to verify the design method.

Temperature Estimation of IPMSM Using Thermal Equivalent Circuit

This paper deals with the temperature estimation of interior permanent magnet synchronous motor (IPMSM). A thermal equivalent circuit of IPMSM is proposed with considering eddy current loss of PM as well as core losses of rotor. Heat sources, which are core loss of stator/rotor core and eddy current loss of PM, are calculated by numerical method to enhance the accuracy of thermal estimation. The thermal equivalent model is represented by the thermal resistances and thermal capacitances, which are determined by motor configuration. Meanwhile, a temperature test is performed using prototype to determine the heat transfer. Finally, this thermal equivalent model is verified by a temperature test in a 25 kW 12-pole/18-slot IPMSM.

Modeling of Core Loss Resistance for

This paper presents modeling of core loss resistance for d-q equivalent circuit analysis of IPMSM considering harmonic linkage flux. High efficiency is one of the major advantages of IPMSM, therefore precise loss analysis is required for high efficiency design. However, there is practical limitation of d-q equivalent circuit analysis in the calculation of core loss due to harmonic components of flux especially in high speed region where field weakening is generally applied. Harmonic components of linkage flux leads to the underestimation of core loss. Especially operation with high constant speed power range results in severe underestimate of core loss. Harmonic components of linkage flux are added to the general d-q axis equivalent circuits in this paper, therefore core losses due to harmonic components are considered by increased voltage drop across core loss resistance. From the presented modeling of core loss resistance, precise core loss can be obtained. Verification with FEA and experiments are presented.

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This paper deals with optimum design criteria for maximum torque density and efficiency of a single phase line-start permanent-magnet motor (LSPMM) using response surface methodology (RSM) and finite element method (FEM). The focus of this paper is to find a design solution through the comparison of maximum torque density and efficiency resulting from rotor shape variations. Then, a central composite design (CCD) mixed resolution is introduced, and the motor parameters meeting the required efficiency and maximum torque are determined by the characteristic map obtained by the d-q axis equivalent circuit analysis. Then the geometric shape design satisfying the motor parameters are performed. In the end, the improved LSPMM is fabricated and tested in order to measure the maximum torque and efficiency.

Equivalent Circuit Analysis of IPMSM considering Harmonic Linkage Flux

The resistive superconducting fault current limiters (SFCLs) are very attractive devices for the electric power network. But they have some serious problems when the YBCO thin films were used for the current limiting materials due to the inhomogeneities caused by manufacturing process. When the YBCO films have some inhomogeneities, simultaneous quenches are difficult to achieve when the fault current limiting units are connected in series for increasing operating voltage ratings. In order to solve these problems, vertical magnetic fields varied from 0 to 130 mT were applied to the YBCO elements. Then, extensive electric field-current (E-I) and quench characteristics were investigated for all elements by using both electrical measuring method and observations of bubble behaviors. The experimental results were compared with the quench properties of YBCO elements, which were connected in series. From the experiment works, it was revealed that applied magnetic fields generated by surrounding coils could induce uniform quench distribution for all strips and simultaneous quenches were realized in all YBCO elements. Finally, by applying vertical magnetic fields perpendicular to the limiting devices, 1.2 kV/sub rms/ rated resistive fault current limiter were realized using five YBCO films in series.

Optimum Design Criteria for Maximum Torque and Efficiency of a Line-Start Permanent-Magnet Motor Using Response Surface Methodology and Finite Element Method

Due to the existence of AC loss in superconducting materials when an alternating voltage is applied, high cryogenic costs are inevitable to operate superconducting devices in AC networks. Therefore applications of superconducting devices in DC electric power networks could be regarded as the optimum choice for superconducting devices, because superconductors show exactly zero resistance to a DC source. Recently, DC superconducting devices such as DC cables have received noticeable attention as DC power transmission lines. In order to develop DC superconducting devices, the DC insulation characteristics in cryogenic liquids should be clarified. However, up to now, limited research has been reported in this field. In this paper, to clarify the different breakdown characteristics of DC and AC applications, various kinds of cryogenic dielectric sheets including Kraft, Kapton (polyimide) and Nomex (polyamide) papers have been prepared. Furthermore, a penetrating breakdown test for three kinds of sheets and turn-to-turn breakdown tests have been performed in liquid nitrogen (LN2). Consequently, it was found that the prepared sheets have shown 1.7-2.7 times higher dielectric strength than those of AC. Moreover, the Nomex and Kraft sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared to air. However, the dielectric strength of the Kapton sheet did not show a remarkable increase in liquid nitrogen. From the turn-to-turn breakdown test, it was proved that the dielectric strength has been linearly increased according to the wrapping number of the sheets and that the DC breakdown voltage was 1.1-2.5 times higher than AC.

Quench behavior of YBaCuO films for fault current limiters under magnetic field

This paper deals with finding the optimal ratio of height and length of Single-Sided Linear Induction Motors (SLIM) using Finite Element Method (FEM) for magnetic field analysis coupled with optimal design methodology. For effective analysis, FEM is conducted in time harmonic field which provides steady state performance with the fundamental components of voltage and current. The ratio of height to length providing the required output power is obtained by Response Surface Methodology (RSM) and optimal values are presented by the variation in output power. When output power is small, the ratio is high and as the power increases, the ratio shows a converged value. Considering the general application of linear motors, using a small ratio can be limiting, however, the shape ratio for maximum thrust can be identified.