Byung-Ik Jung

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.

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.

Design and Characterization of the Integrated Matrix-Type SFCL

In this paper, we report the improved and integrated matrix-type superconducting fault current limiter (MFCL). The new MFCL proposed in this work has a structure that easily improves the quench characteristics of superconducting elements. This integrated MFCL is simply constructed by mounting a trigger element and superconducting elements in a single reactor, whereas older MFCL designs require several complex reactors for proper operation. Also this integrated type of structure provides a uniform and strong magnetic field with superconducting elements, compared with an old type MFCL. We designed and characterized the integrated MFCL with 1 times 3 superconducting modules and 2 times 3 superconducting modules. From experimental results, we confirmed that our integrated MFCL had advantages including a compact design, better quench characteristics and an easily adjustable increment of the capacity for fault current limiting.

Combined Effect of the SFCL and Solenoid Coils

As the capacity of power facilities increases with the power demand, the fault current also increases in the case of a power system fault. Many studies are underway to limit the increasing fault current. They especially include superconducting fault current limiters (SFCLs). The existing SFCLs operate almost without loss because of their zero resistance characteristic in the normal condition, but limit the fault current by generating impedance in the event of a fault. However, this mechanism gives a significant burden on the SFCL. In this study, a high-speed interrupter and a normal conduction current-limiting unit were applied to the SFCL. In this structure, the current flows through the b-switch of a high-speed interrupter and the current-limiting element in the normal condition, but in the event of a fault, the current was bypassed to the current-limiting unit by the quench of superconducting units and interrupter switching operations. In this case, the magnitude of the fault current was covered by the SFCL, and after the switching operation, the current was limited by the normal conduction current-limiting unit. The results of this study showed that this structure reduced the power burden on the expensive superconducting element, and limited a high fault current that was initially generated from the fault.

Characteristics of the Fault Current and the Protection for Superconducting and Normal Conducting Limiter combined with a Transformer

With increasing demand of power, the equipment of power system is enlarging and the absolute capacity is going up. As a result, when a fault occurs, the fault current is consistently increasing. Therefore, I suggested some solution for limiting the fault current more efficiently. This study shows the characteristics of superconducting limiting elements and normal conducting elements combined with a transformer. We performed a short-circuit test about the fault current by using SCR switching control system operated from a CT. When short circuit accidents happened in the secondary side of a transformer, fault currents flowed and a SCR switching control system was operated. It resulted in a decrease of the fault current in the limited elements of third winding connected in parallel. For this test, we used YBCO thin films and normal conducting elements as the limited elements. Within a cycle, a superconducting fault current limiter with YBCO thin films reduced more than 90% of fault current because the resistance of superconducting elements sustainedly grew. On the other hand, the limiter with normal conductors limited as much as a set value because its resistance characteristic was linear. Consequently, in case of the limiter with superconductor, limiting range of the circuit was wide but the range of protective detection was undefined. In contrast, as for the limiter with normal conductors, limiting range and protection duty were appropriate.

Performance assessment of 3kW grid-connected PV Systems in Korea

In this paper, for the field test of the first photovoltaic (PV) systems in Korea, the performance of a 3kW grid-connected PV systems for house, separately installed in the solar energy demonstration research complex, had been analyzed for the four years from January 2003 to December 2006.

Comparison of the Unbalanced Faults in Three-Phase Resistive and Matrix-Type SFCLs

The symmetrical components of the three-phase resistive-type and three-phase matrix-type superconducting fault current limiters (SFCL) were analyzed in the single line-to-ground fault and the double line-to-ground fault using symmetrical component calculus. Positive sequence current was highest in the resistive-type SFCL with a single line-to-ground fault and in the 380 turns matrix-type SFCL with a double line-to-ground fault, and negative sequence current was lowest in the resistive-type SFCL with a single line-to-ground fault. These indicate that and are determined from the total impedance of the SFCL and the uniform quenching characteristics of the superconducting element. Zero sequence current rapidly appeared from the three-phase resistive-type SFCL, but approached zero after three cycles. The application of magnetic field in the three-phase matrix-type induced the simultaneous quench of the superconducting elements, but a large amount of the symmetrical component appeared after three cycles from the fault occurrence due to the shunt resistor and the shunt reactor.

Characteristics of a FCL Applying Fast Interrupter According to the Current Limitation Elements

With the development in industry, power demand has increased rapidly. As consumption of power has increased, Demand for new power line and electric capacity has risen. However, in the event of fault, problems occur in extending the range of fault coverage and increasing fault current. In these reasons, protection devise is recognized as the prevention of an accident and fault current. This paper dealt with minimizing fault propagation and limiting fault current by adjusting fault current limiter (FCL) with fast interrupter. At this point, we compared and analyzed characteristics between non-inductive resistance and fault current which is limited by superconducting units. In normal state of the power system, power was supplied to the load, but when fault occurred, the interrupter was operated as CT which detected the over-current. Its operation made the limitation of fault current through a FCL. We concluded that the limiter using superconducting units was more efficient with the increase of power voltage. Superconducting fault current limiter with the fast interrupter prevented the spread of a fault, and improved reliability of power system.

Cooperation Between Reclosing System and a Flux Coupling Type SFCL by a Neutral Line

We analysed the protective cooperation between a reclosing system and a flux-coupling type superconducting fault current limiter (SFCL). In order to compare the recovery behavior, the SFCLs without or with a neutral line between secondary coil and superconducting elements were prepared. Most of power grid has a reclosing system, which controls the circuit breaker by the sequential procedure, to cope with various faults. Generally, fast recovery of the SFCL was very important for stable operation between the reclosing system and the SFCL. The reclosing cycles (CO-t-CO-t-CO) for the experiments was set to 5-10-7-20-5. Recovery time in a flux coupling type SFCL without a neutral line was lengthened due to unequal quench between superconducting elements and it caused decrease of the maximum applied voltage. In the meantime, recovery time in the SFCL with a neutral line was shortened due to simultaneous quench between the elements. So, applied voltage of the power system could be higher within same reclosing system. As turns ratio between secondary coil and superconducting elements was increased, recovery time was longer due to increase of their power. We confirmed that a flux-coupling type SFCL with a neutral line was more profitable for good cooperation between superconducting elements and a reclosing system.

Reduction of the Power Burden of a Transformer-Type SFCL Using a Vacuum Interrupter

Various protective equipment have been used to enhance the supply reliability and transient stability of a power system. A superconducting fault current limiter (SFCL) is electric power equipment that reduces the fault current in a power system. In the power system in Korea, an SFCL that can consistently restrict fault currents for 1.5 s or longer without support from a stable current limiting operation, a reclosing operation, and a circuit breaker was required. In this study, we suggested methods of improving the performance of a transformer-type SFCL. The superconductor in the secondary winding of a transformer-type SFCL was connected to a vacuum interrupter in series. As a result, the current limiting performance was enhanced, and the burden of the superconductor was decreased significantly. The superconductor restricted only the initial fault currents, and thereafter, the limiting operation was conducted by reactor of the transformer, which significantly improved the duration time of the current limiting operation. These outcomes satisfy the SFCL conditions required by the power system, so practical applications may be possible.