전력계통용 자속결합형 한류기의 특성 연구

Abstract

The power demand is rapidly increasing in Korea, especially in the urban areas that include the metropolitan area. The increase in the power demand requires the extension of power facilities, but it is difficult to secure spaces for equipment installation in the limited space of urban areas. In addition, the transmission system in Korea has a short transmission distance, and is a network structure that ensures the high reliability and stability of power supply. Accordingly, the fault current rapidly increases in the case of a system fault, due to the decrease in the line impedance. With the continuous increase in the power demand and large capacity of power facilities, the fault current increases and eventually exceeds the breaking capacity of the circuit breaker. Where the breaking capacity of the circuit breaker is exceeded, breakers must be maintained or replaced. There are many economic and technical problems, however, to prepare for the increasing power demand. For example, if high capacity of circuit breaker is replaced, the low capacity of circuit breakers must also be replaced. This leads to an enormous economic burden. In addition, a large-capacity of circuit breaker must also be developed, which requires additional efficient measures. Air-core reactors are being applied to some systems as a fault current limiting measure, but it involves continuous loss due to the increase in the line impedance during normal operation. It also requires larger installation space and the consideration of its effect on the surrounding devices. The bus separation is also being used, but it is not effective because it reduces the power quality and reliability due to the overload on the adjacent system and excessive voltage fluctuation. The superconducting fault-current limiter (SFCL) was devised to solve these existing problems. The fault-current limiting technology using the superconductor is being actively studied around the world. The SFCL in the system does not affect the surrounding equipment without loss during normal operation, but if the fault current exceeds the critical current of the superconductor in the case of a fault in the system, it is quenched and it provides high impedance to quickly limit the fault current. Because the SFCL limits the fault current below the breaking capacity, the existing breaker can stably operate and the system is effectively protected against the fault, without the need for replacing or increasing the capacity of the existing breaker. In particular, the smart-grid-based future power system, which is recently drawing attention as a next-generation power grid, requires high-quality stable power supply. Therefore, it is expected that the introduction of the SFCL for the system protection will be an essential technology. Based on the simple resistive-type SFCL, this study addressed the SFCL with a three-phase transformer. To apply the existing resistive-type SFCL to the actual system, the serial or parallel connection of SFCL elements was essential, and individual SFCL elements showed the ununiform quench characteristic due to the manufacturing problems. Accordingly, the burden from the fault was not uniformly distributed, but concentrated on a specific superconducting element, which may lead to the breakage of the superconducting element. The SFCL in this study could reduce the use of expensive superconducting elements by reducing the burden on the superconducting elements in the case of a system fault, and control the fault current magnitude by changing the primary-secondary turn ratio of the transformer. This study examined the performance and operating characteristics of the current limiter that were required by the system, considering the expected coordination with the protective devices in the actual power system.