HVDC 계통 신뢰도 향상을 위한 한류형 초전도 DC 차단기에 관한 연구

Abstract

Distributed power systems using renewable energy and power grids based on DC systems such as High Voltage Direct Current(HVDC) systems, which can be connected to different frequencies, have been expanding of late. HVDC is economical due to the low insulation level of the line, and has high transmission efficiency because there is no loss through the skin effect and reactance component. Because of these and its other advantages, HVDC is actively being researched on both in and outside the country. In the past, grids were constructed with CSC-HVDC systems, but VSC-HVDC systems are now in use. Thus, a DC circuit breaker technology with excellent interruption performance and stable operation reliability is essential. It is difficult to develop and commercialize DC circuit breakers, however, due to the problems of the existing circuit breakers, such as limited capacity, high prices, and poor protection coordination with the system. Therefore, this paper proposes a new Current-limiting Superconducting DC Circuit Breaker(CLS-DCCB). CLS-DCCB has a simple structure, in which a superconductor and a DC circuit breaker are connected in series. It is a protective device that combines the fault-current-limiting technology and the line interruption technology of the DC circuit breaker. The DC circuit breaker generates an operation delay due to open contact and relay signaling. If only the DC circuit breaker is applied, the system must fully bear the burden of faults during the operation delay. The superconductor maintains the superconducting state with zero resistance at below the threshold current. If a current above the threshold current flows, however, it changes into its normal conducting state within several milliseconds, and generates current-limiting impedance, which limits the maximum magnitude of the fault current. Furthermore, the superconductor shares the burden of faults during the operation delay of the DC circuit breaker. As a result, an initially limited fault current flows, and the interruption time is shortened due to the reduced arc magnitude. The single-operation characteristics of CLS-DCCB were verified using the grid analysis application PSCAD/EMTDC. Next, a grid-connected PV system and a VSC-HVDC system, which are representative DC power systems, were designed. Then the transient state was analyzed by applying CLS-DCCB to a DC line. For the transient state, two fault points were set in the DC and AC lines. The simulation results showed similar current limiting and interruption trends when CLS-DCCB was applied to the grid. Based on this, the capacity indicator of the circuit breaker to be produced in the future was additionally proposed. To improve the operation reliability of CLS-DCCB, a simulated DC power test system was constructed, and the CLS-DCCB was fabricated. The simulated DC power test system was constructed in a way to enable experimentation on a DC device by considering the essential elements of the experiment. Considering the actual application to the grid, a CLS-DCCB was fabricated by combining a DC circuit breaker with a superconducting wire. The operation characteristics with increasing voltage by capacity were analyzed, and the results confirmed that the simulation data of the HVDC system and the experimental data of the LVDC system had the same trends of limiting the maximum fault current and shortening the interruption time. The results of this study about CLS-DCCB can be used to establish the basic concepts and operation characteristics of such device. Furthermore, it was verified that when the capacity is increased with the structure proposed in this study, it can be applied to the HVDC system. Therefore, the results of this study can be used as reference data and will greatly contribute to the production of a 100[kV]-scale HVDC circuit breaker in the future. Moreover, when the CLS-DCCB proposed in this study is applied to the HVDC grid, it can improve the stability and reliability of the grid by reducing the possibility of interruption failure and shortening the fault interruption time.