DC 전력망 공급 신뢰도 확보를 위한 초전도 DC 차단기 특성 연구

Abstract

The topics currently studied on HVDC are control, protection, and security. A new theme has been explored, aiming at the DC grid system of MTDC (Multi-Terminal Direct Current), which is a meshed HVDC type of the future. This study modeled VSC-HVDC based on the ordinary VSC-HVDC actual system model and selected severe contingencies through the P-V characteristics. In addition, this study proposed a superconducting DC circuit-breaking technology to protect power devices from power system transients. A feasibility analysis was performed on current-limiting and circuit-breaking technologies by applying them to the types of contingency accidents based on the simulation results. The feasibility was examined through experiments based on the simulation results. The results of this study are summarized as follows; First, by analyzing the configuration and method of the HVDC system and the considerations for each method (circuit control method, modulation method, and capacitor algorithm), the circuit breaking characteristics required for the HVDC system were established. The operation principle and mechanism of a superconducting current-limiting module (resistance, induction, and saturated iron-core) and mechanical DC circuit breaker (active, passive, LC divergence oscillation, and forced types) of the superconducting DC circuit breaking technology were comparatively analyzed. The combining structure of a superconducting element and mechanical DC circuit breaking units was selected. Second, the TBC VSC-HVDC system was modeled using the PSCAD/EMTDC program. The operating characteristics (steady and transient states) were compared and analyzed. As for the contingency, the study was divided into AC contingency and DC contingency. Severe contingency types were selected through the analysis of voltage, current operation flow, and P-V characteristics generated for each element and used as an HVDC system infrastructure model for the feasibility study of superconducting DC circuit breakers. This study designed a model for each component (superconducting current-limiting module, mechanical DC circuit breaker) of the superconducting DC circuit breaker proposed herein. It analyzed the characteristics of the circuit breaking operation. Also, this study suggested the bifilar-meander winding method to improve the current-limiting rate of the superconducting current-limiting module. Studies on possible improvements and feasibility of the model were performed. A mechanical DC circuit breaker study was conducted on the characteristic of the artificial current zero position of the stable LC divergence oscillation circuit. In addition, the superconducting current-limiting module and the final current-limiting and breaking operation characteristics were reviewed based on frequently occurring and serious accident types. By applying the superconducting DC circuit breaker model to the HVDC system infrastructure model, studies on possible improvements and feasibility were performed according to the presence or absence of a circuit breaker. This stage was to review the DC circuit breaker's reliability through the PSCAD/EMTDC program. The result reduced the initial fault current and a DC fault current cut-off time. Third, this study derived improved simulation results according to the application of a superconducting DC circuit breaker. The design criteria required for modeling PSCAD/EMTDC of the superconducting current-limiting module of the superconducting DC circuit breaker and the modeling (helical, spiral, meander type) characteristics according to the superconducting wire were comparatively analyzed. Each superconducting wire's volume and magnetic field force were comparatively analyzed through Maxwell, the electromagnetic field analysis program, and an appropriate type was selected. The operation characteristics through PSCAD/EMTDC were analyzed by applying the superconducting wire winding method proposed in this study to a superconducting DC circuit breaker. Fourth, the operation characteristics of each current-limiting unit and circuit breaking unit were reviewed through experiments based on the research results above, and their applicabilities were examined. A scaled-down model for the improved superconducting current-limiting module proposed in this study was produced. A DC short-circuit simulation was performed to analyze fault current-limiting and quench operation characteristics in-depth. In addition, a DC short-circuit simulation test was performed by designing and producing a mechanical DC circuit breaker. Finally, the feasibility was examined by conditions through the experiments on the LC divergence oscillation circuit, which creates an artificial zero position for circuit breaking. Based on the data resulting from the experiments, it was confirmed that they could be applied to the VSC-HVDC system of superconducting DC circuit breakers. Therefore, it is deemed that they can be studied as circuit breakers suitable for MTDC in the future. This study could secure the data derived from various design variables and environments through simulations and tests on the superconducting DC circuit breaking technology of VSC-HVDC, which has been the hottest issue worldwide, and confirm the research and development potential of superconducting DC circuit breaking technology. The DC circuit breaking technology based on power semiconductors has been described as a solution to DC system protection. However, it is now deemed that a new hybrid type, DC circuit breaking technology combining a superconducting current-limiting module and a mechanical DC circuit breaker, will lead the way based on the research results above. In addition, it is expected that the research results of this study will be used as primary essential data for the continued analysis of current-limiting and circuit-breaking characteristics in the future.