Sung-Hun Lim

Fault current limiting Characteristics of resistive type SFCL using a transformer

A transformer is expected to be an useful component of resistive type superconducting fault current limiter (SFCL) using high-T/sub C/ superconducting (HTSC) element with constant critical current for easier adjustment of both the limiting impedance and the initial limiting current level. Therefore, the analysis for the fault current limiting characteristics of the resistive type SFCL using a transformer considering the resistance variance of HTSC element is needed because the resistance of HTSC element during a fault period is affected by the design condition of the transformer. In this paper, we investigated the limiting impedance and the initial limiting current level of the resistive type SFCL using a transformer as a function of the turn numbers ratio between the primary winding and the secondary winding. The limiting impedance of SFCL was extracted by applying discrete Fourier transform (DFT) equation for the measured voltage and current data of SFCL. It was confirmed from the analysis that the transformer design with the lower turn numbers ratio of the primary winding for the secondary winding was advantageous for the larger load current capacity of SFCL and that the transformer with the higher ratio design condition, on the other hand, was favorable for the larger fault current limiting capacity of SFCL.

Impedance variation of a flux-lock type SFCL dependent on winding direction between coil 1 and coil 2

The flux-lock type superconducting fault current limiter (SFCL) uses the magnetic coupling between two coils connected in parallel with each other. This SFCL has the merit that both the resistance of high-T/sub C/ superconducting (HTSC) element and the limiting current capacity can be simultaneously increased through the application of magnetic field into HTSC element. However, unlike other types SFCLs, the different fault current limiting characteristics can appear according to the winding direction between the two coils. In this paper, the relation of the impedance of the flux-lock type SFCL with the resistance of HTSC element was derived from its equivalent circuit using the current and the voltage relationship between the two coils. The limiting impedances of the flux-lock type SFCL due to the winding direction through the discrete fourier transform (DFT) analysis for the voltage and the current data obtained from the fault current limiting experiments were extracted and analyzed from the point of view of the limited line current and the resistance of HTSC element.

Operational characteristics of a flux-lock-type high-T/sub c/ superconducting fault current limiter with a tap changer

The authors investigate the operational characteristics of a flux-lock-type high-T/sub c/ superconducting (HTSC) fault current limiter (SFCL) with a tap changer which could adjust the number of turns of the third winding. In the case of conventional flux-lock-type SFCL, the phase adjusting capacitor is connected in series with magnetic field coil to adjust the magnetic field applied to the HTSC element in phase with the current flowing through the HTSC element during a fault time. However, the current flowing at the third winding, which is connected with magnetic field coil, affects the fault current limiting characteristics. To analyze the influence of current flowing at the third winding on the fault current limiting characteristics, the fault current limiting characteristics of the flux-lock type SFCL, whose inductance of coil 3 could be adjustable through a tap changer, are investigated through the experiments and the computer-aided simulations. The relation of line currents flowing into the flux-lock-type SFCL during a fault time and numbers of turns of a tap changer is drawn.

Current limiting characteristics of flux-lock type high-T/sub C/ superconducting fault current limiter with control circuit for magnetic field

A flux-lock type superconducting fault current limiter (SFCL) can change the amplitude of the magnetic field by adjusting either the inserting resistance or the phase adjusting capacitor. However, the magnetic field coil cannot generate enough magnetic field for some time after a fault happens due to resonance between the phase adjusting capacitor and the magnetic field coil. It is also required for the magnetic field generated to be controlled for the application to high-T/sub C/ superconducting (HTSC) elements which have different critical characteristics. This paper proposes a flux-lock type SFCL with a control circuit for the magnetic field, which is composed of solid state switches connected with the magnetic field coil. A current limiting experiment of this model was carried out. We showed that the amplitude of the fault current as well as the magnetic field could be controlled by the sinusoidal pulse width modulation (SPWM) operation, one of the switching techniques for controlling the magnetic field.

Analysis of operational characteristics of flux-lock type SFCL combined with power compensator

The flux-lock type superconducting fault current limiter (SFCL) combined with a power compensator was proposed in this paper. This SFCL consists of a flux-lock reactor and a power compensator. The former, which has coil 1 and coil 2 wound in parallel through high-T/sub C/ superconducting (HTSC) element, can perform the fault current limiting operation during a fault period. The latter is composed of a current-controlled inverter and an AC/DC converter between coil 3 and grid, which can be operated as the power compensator for nonlinear load during a normal operation. The specification for a test model was determined and its operational characteristics were analyzed through computer simulation using PSIM program. It was confirmed that the suggested flux-lock type SFCL, combined with the power compensator, could protect the system from over-current by a short circuit accident and compensate the reactive power due to a nonlinear load during a normal operation as well.

Current Limiting Characteristics of Integrated Three-Phase Flux-Lock Type SFCL

The short circuit analysis of the integrated three phase flux-lock type superconducting fault current limiter (SFCL), which consisted of three-phase flux-lock reactor wound on an iron core with the same turns ratio between coil 1 and coil 2 for each single phase, was performed. In a normal time, the flux generated in the iron core is zero because the flux generated between two coils of each single-phase is canceled out. In a fault time, the integrated three-phase flux-lock type SFCL showed the different fault current limiting characteristics from other three-phase SFCLs with the separated three iron cores. Through the computer simulation for the three-phase circuit with its equivalent circuit, the current limiting characteristics of the integrated three-phase flux-lock type SFCL were analyzed

Fault Current Limiting Characteristics of DC Dual Reactor Type SFCL Using Switching Operation of HTSC Elements

The fault current limiting characteristics of DC dual reactor type superconducting fault current limiter (SFCL) using switching operation of high-TC superconducting (HTSC) elements were analyzed. The suggested SFCL consists of a diode bridge, DC dual reactor with magnetic coupling and HTSC elements. Unlike the conventional bridge type SFCL, which requires the controller for the operation of the interrupter to prevent the continuous increase of fault current after a fault happens, this SFCL can be operated without the interrupter and the controller for its operation. In addition, despite different critical currents after a fault accident, the balanced power burden between HTSC elements can be achieved by the magnetic coupling between two coils of DC dual reactor. It was confirmed through the experiments for the fault current limiting characteristics that the suggested SFCL performed the advantageous current limiting operations compared to the conventional bridge type SFCL using HTSC coil

Alternating Over-Current Characteristics of HTSC Tape Using DFT

The quench characteristics of high-TC superconducting (HTSC) tape for alternating over-current applications have been investigated at various amplitudes of the over-current and its quench developments have been analysed by applying discrete Fourier transform (DFT) for the voltage and the current waveforms of HTSC tape. Generally, the HTSC tape shows a broad superconducting and normal transition region above its critical current due to the metal sheath unlike HTSC thin film or HTSC bulk. Moreover, the voltage-current characteristics of HTSC tape are complicated because the quench and the recovery between the superconducting state and the normal state are repeated. In this paper, the alternating over-current experiments for the HTSC tape were performed to analyse its quench developments. The fundamental frequency components for the current and the voltage waveforms of the HTSC tape were abstracted by applying DFT. From those fundamental frequency components, the numerical formula for HTSC tapes resistance has been derived. By applying its numerical formula into circuit equation, the quench developments of HTSC tape dependent on alternating over-current have been estimated and the agreement with the experimental results has been found

Analysis of new bridge-type superconducting fault current limiter applying modified magnetic shielding model

Abstract To reduce the difficulty and cost for the fabrication of high- T c superconducting wire consisting of bridge-type superconducting fault current limiter (SFCL), we suggested the new bridge-type SFCL applying modified magnetic shielding model using high- T c superconducting tube. In this work, the modified magnetic shielding model has been designed that replaced the DC reactor of bridge-type SFCL. Through the circuit analysis for operational characteristics, we analyzed the fault current-limiting characteristics for the proposed SFCL, and confirmed that it can perform the same fault current-limiting operation as existing bridge-type SFCL required for high- T c superconducting wire.

Conditions of ICP for a superconducting flux flow transistor and its etching characteristics

Abstract Superconducting flux flow transistors with a micron channel (3 μm) have been fabricated based on the flux flow by the inductively coupled plasma (ICP) etching technique. For a channel of superconducting vortex flow transistor (SFFT) with the high- T c superconducting characteristic in YBaCuO, the proper ICP etching conditions were an ICP power of 700 W, r.f. power of 150 W, pressure in chamber of 5 mTorr, and mixing rate of etching gas for Ar:Cl 2 , 1:1. The sample etched by ICP produced smoother morphology than by H 3 PO 4 etchant, while it made little difference with a sample by non-aqueous etchant, Br. The transresistance of an SFFT etched by the ICP technique was below 0.1 Ω at an I bdy of 40 mA. Output resistance was below 0.2 Ω. It is expected that the transresistance can be improved by decreasing the thickness of the link and introducing the multi-link structure.