Byoung-Sung Han

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.

Study on Maximum Operating Condition of Resistive Type SFCL Using YBCO Coated Conductor

The most important thing in developing a superconducting fault current limiter (SFCL) is to find the maximum operating condition for each current-limiting element to increase its SFCL capacity. The maximum operating condition can be defined using Vmax, Tr, Imax and Iq, which are detected after the quenching of the SFCL. In this study, YBCO coated conductor whose characteristics were proved as the superconducting fault current limiting element was used to test the operating characteristics of the unit current limiting element and to find the maximum operating condition using Vmax, Tr, Imax and Iq, which are detected after the quenching against the fault current according to the fault angle. YBCO coated conductors used in the test are one wire with stainless-steel stabilizing layer and one wire with no stabilizing layer. Critical current value is 70 Arms and the critical temperature is 90 K. The YBCO coated conductor with non stabilizing layer had a critical current of 80 Arms and a critical temperature of 90 K. Therefore, the two YBCO coated conductors in this study had different critical currents and an identical critical current of 90 K.

Current Limiting Characteristics of a Flux-Lock Type SFCL Dependent on Fault Angles and Core Saturation

The flux-lock type superconducting fault current llmiter (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-TC superconducting (HTSC) element and the limiting current capacity can be simultaneously increased through the application of magnetic field into HTSC element. However, like other type of SFCLs using an iron core, unless the saturation of the iron core in the design is considered, the actual limited fault currents can be higher than expected as the saturation of the iron core occurs, and can lead to damage the electrical equipment of the power system. To analyse the dependence of the fault current in the flux-lock type SFCL on the fault angles, its electrical equivalent circuit with the magnetization branch was derived. From the electrical equivalent circuit, the fault current and the inner magnetic flux considering the fault angles as well as the design parameters such as winding direction and inductance ratio of two coils were calculated. The analysis results were compared with the experimental ones, and both the results agreed.

Quench Characteristics of Current Limiting Elements in a Flux-Lock Type Superconducting Fault Current Limiter

We investigated the quench characteristics of a flux-lock type superconducting fault current limiter (SFCL) according to the number of the serial connection between the superconducting elements at the subtractive polarity winding of a transformer. The flux-lock type SFCL consists of two coils. The primary coil is wound in parallel to the secondary coil through an iron core, and the secondary coil is connected to the superconducting elements in series. The operation of the flux-lock type SFCL can be divided into the subtractive and the additive polarity windings according to the winding directions between the primary and secondary coils. In this paper, the analyses of voltage, current, and resistance in serial connection between superconducting elements were performed to increase the applied voltage of flux-lock type SFCL. The power burden was reduced through the simultaneous quenching between the superconducting elements. This enabled the flux-lock type SFCL to be easy to increase the capacity of power system

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.

Responses of resistive superconducting-fault-current-limiters to unbalanced faults

We analyzed the unsymmetrical fault characteristics of resistive superconducting-fault-current-limiters (SFCL) based on YBCO thin films with the unbalanced faults such as a single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault in a three-phase system. The unsymmetrical rates of fault phases were 6.4, 9.2, 8.8 at the fault onset, but decreased by 1.4, 1.5, 3.7 after 50 ms in the fault types, respectively. The positive sequence current I/sub 1/ was the highest in a double line-to-ground fault, immediately after the fault onset, but that of a line-to-line fault was the highest after 50 ms. This means the current limiting effect was the worst in a line-to-line fault, due to the unbalanced quench between the SFCL units. The negative sequence currents I/sub 2/ of a single and double line-to-ground faults were relatively low, except for the quench instant, because of the rapid interruption of fault currents by the SFCL. The zero sequence current I/sub 0/ was similar to the behavior of the negative sequence current. Finally, the positive sequence resistance Z/sub 1/ was reduced remarkably immediately after the fault but gradually approached the balanced positive sequence resistance prior to the system fault, except during a line-to-line fault. The simultaneous quench between the SFCL units was important for low line-to-line fault currents.

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

Quench characteristics of HTS tapes with alternating currents above their critical currents

In this study, the dependence of quench characteristics of HTS tapes on the amplitude of current and duration time was investigated by applying alternating current (AC) that is several times over the critical currents of HTS tapes. When the applied AC/sub peak/ was up to three times of critical current, the joule heat was not cooled down by LN/sub 2/ completely. However, when the over-current reached four times of critical current, the superconducting region began to decrease gradually and the total resistance of HTS tapes increased rapidly. Finally, we analyzed the quench development using voltage-current characteristics and examined the influence of pitch angles on resistance increase.