Hyo-Sang Choi

Operating Performance of the Flux-Lock and the Transformer Type Superconducting Fault Current Limiter Using the YBCO Thin Films

The operating characteristics of the flux-lock and the transformer type superconducting fault current limiters (SFCLs) have been tested and compared each other. The SFCLs are composed of the primary and secondary coils and the YBCO thin film as a current limiting unit. The turn ratios between the primary and secondary windings were 63:21 and 63:42, respectively. The winding direction between the primary and secondary windings was subtractive. When a fault occurred under the same conditions, the line current was limited more effectively in the transformer type SFCL. That is, the current value of the flux-lock type SFCL was approximately 2 times as high as that of the transformer type SFCL at the turn ratio of 63:42. However, the flux flowing into the iron core of the flux-lock type was less saturated than in the transformer type due to its flux distribution. The resistance generated in the YBCO thin film was higher in the flux-lock type where its voltage was also higher because of the winding direction between the primary and secondary windings. The initial limiting current of the flux-lock type was 2 times as high as that of the transformer type at the turn ratio of 63:42. The power consumed in the YBCO thin film of the transformer type was also remarkably low because the resistance, the current, and the voltage of the YBCO thin film are all lower. Consequently, we found that the transformer type had advantages for the current limiting effect and power burden of the YBCO thin film.

Transient Characteristics of a Flux-Coupling Type Superconducting Fault Current Limiter According to Winding Direction

The flux-coupling type superconducting fault current limiter (SFCL) is made by using the transformer. The flux-coupling type SFCL consists of the primary and the secondary coils connected in series, and the secondary coil has a superconducting unit connected in parallel. Before the fault occurrence in power system, the SFCL is operated without power loss (I2R) because of the zero impedance behavior of the superconducting unit. When the fault occurs and the short-circuit current exceeds the critical current in the superconducting unit, the superconducting unit is quenched, and the short-circuit current is limited. The flux-coupling type SFCL could be divided into the additive and the subtractive polarity windings according to winding direction. The short-circuit current of the flux-coupling type SFCL with the additive polarity winding was limited more effectively than that of the subtractive polarity winding. It was because the direction of current according to the winding direction of a secondary coil was reversed. In the case of the voltage generated in the superconducting unit, the voltage in additive polarity winding was generated more than in the subtractive polarity winding. Consequently, we found that the additive polarity winding could reduce the power burden of the superconducting unit in the comparison with the subtractive polarity winding.

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

Operational Characteristics of Hybrid-Type SFCL by the Number of Secondary Windings With YBCO Films

We investigated the operational characteristics of the hybrid-type superconducting fault current limiter (SFCL) according to the number of secondary windings. The SFCL consists of a transformer, which has a primary winding and several secondary windings with serially connected YBa2Cu3O7 films. In order to increase the capacity of the SFCL, the serial connection between each current limiting unit is necessary. Resistive-type SFCL has a difficulty in quenching simultaneously between the units due to slight differences of their critical current densities. The hybrid-type SFCL could achieve the simultaneous quenching through the electrical isolation and the mutual flux linkage among the units. We confirmed that the capacity of the SFCL could be increased effectively through the simultaneous quenching among the units. In addition, the power burden of the system could be reduced by adjusting the number of secondary windings. We will investigate the method to increase the capacity through serial and parallel connections among current limiting units

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.

Critical Current Equalization via Neutral Lines in a Transformer-Type SFCL

In order to increase the capacity of the superconducting fault current limiter (SFCL), serial and parallel connections of the superconducting units are necessary. The resistive-type SFCL with serial and parallel connections has the unbalance of power burden because of the difference of critical current between the superconducting units. The transformer-type SFCL using a neutral line consists of a primary coil and several secondary coils with the superconducting units. Because the neutral line is connected with the secondary coils in the SFCL, it induces the simultaneous quench from compulsive current distribution in the superconducting units. The simultaneous quench enables the power burden between the superconducting units to be distributed equally. The neutral line also induces the equal voltage distribution between the superconducting units due to the equal turns ratio of the secondary coil. In conclusion, the increase of the capacity in the transformer-type SFCL could be achieved easily through the application of the neutral line between the secondary coils and the superconducting units.

Current Limitation and Power Burden of a Flux-Coupling Type SFCL in the Three-Phase Power System According to Turn's Ratio and Fault Type

Most of the transmission system has a network structure to improve the reliability and stability of a power system. Fault current is continuously expected to increase by the increase of the power demand. If fault current exceeds the cutoff capacity of a circuit breaker, the circuit breaker is broken and the damage by fault current is expanded throughout the power system. Superconducting fault current limiter (SFCL) was designed to solve this problem in a power system. In this paper, we investigated the current limiting characteristics and power burden of superconducting elements of a flux-coupling type SFCL in three-phase power system. A Flux-coupling type SFCL is one of the resistive type SFCLs. The flux-coupling type SFCL was made by using a transformer. Reactors connected in each phase shared an iron core. When the superconducting elements were quenched in fault phase, the fault current flowed into the primary and secondary coils simultaneously. Thus, the current flowed into primary and secondary coils of sound phase by the magnetic coupling flux. Meanwhile, when the current of sound phase exceeded the critical current of the SFCL, superconducting elements connected in the sound phase were quenched. The value of the fault current tended to decrease as the first reactors ratio increased. Furthermore, the power burden of the superconducting element was reduced. The reduced power burden of the superconducting elements shortens the recovery time of the superconducting element, which is advantageous for cooperation with a reclosing system when the SFCL is applied to the system. As a result, we confirmed that the flux-coupling type SFCL operated effectively in the three-phase power system.

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

Recovery Behaviors of the Transformer-Type SFCL With or Without Neutral Lines

This study was to analyze the recovery characteristics for the superconducting element in the transformer-type superconducting fault current limiter (SFCL) consisted of iron core, elements in primary and secondary windings. The superconducting element was connected to the secondary windings of a transformer-type SFCL in series. In order to increase the power capacity of the SFCL, the number of the superconducting elements should be increased and the recovery time of each superconducting element should be also shortened. When the number of superconducting elements in secondary winding was increased, the recovery time of the superconducting elements according to the voltage increase in the transformer-type SFCL grew longer because of unbalanced quenching of the superconducting elements. However, as for the transformer-type SFCL that has neutral lines connected between the secondary winding and the superconducting elements, the recovery time grew shorter due to the enhancement of quenching behavior. The transformer-type SFCL with the neutral lines was more profitable for the capacity increase of the SFCL.

Limitation of Fault Current and Burden of Superconducting Element Applied to Neutral Line of the Transformer

The consumption of electrical energy has been increased since industrial revolution and still increasing so far. As the consumption of electricity increases, the problem of the failure of electric device and degrading of electric reliability by over current from the related fault happened. Therefore, the research of electrical device using high-temperature superconducting element is going on now and superconducting fault current limiter (SFCL) was applied on the neutral line of common transformer. As the applied voltage increases, the characteristics of the electrical power burden of SFCLs were studied. In case of fault in the power system, superconducting element restricted the fault current by normal conductivity element connected in third winding of a transformer. As a result of the test, it was confirmed that the conduction element limited initial fault current promptly as the voltage increases. After this, the conduction element took the power burden of fault current by changing the way of current and the limit rate became high as well. Also, it was confirmed that the power burden of conduction element was reduced due to no change of limit rate of fault current. By adapting of the SFCL to neutral line of a transformer and of normal conduction element to power system in the third winding, the use of current protection facility became available without increase and change of the capacity. Accordingly, the current limiter which was applied on the neutral line of a transformer was considered to have effects of cost saving, preventing mechanical failure and expansion of fault.