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ABSTRACT
A Study on the HVDC System and Fault Analysis of the Northeast Asian Supergrid MOON BYUNG-SOO Advisor: Prof. Choi Hyo-Sang Ph.D. Department of Electrical Engineering Graduate School of Chosun University
The thesis's topic is “A study on the HVDC system and fault analysis of the Northeast Asian Supergrid.”In the 21st century, as energy conversion accelerates due to global warming and fossil fuel depletion, the establishment of a supergrid linking power grids among countries is emerging as a major concern. Northeast Asia is rich in renewable energy resources such as wind and solar power in Mongolia and the interior of China, and is geographically adjacent to large-scale power demands in Korea, Japan, and China, providing favorable conditions for cross-border power supply and demand balancing. As a result, strategies for securing energy security and realizing carbon neutrality by 2050 are being developed, and to achieve these goals, it is necessary to establish measures to address the issues of frequency differences among AC systems when linking power grids among countries and the increase in fault current that occurs when connecting to high-voltage direct-current (HVDC) systems. In this paper, the characteristics of fault currents generated in the HVDC transmission system were analyzed, and the applicability of reduction technology was reviewed. In chapter 2, sections 1 and 2, the technical trends of the HVDC transmission system, the transmission methods and interconnection schemes, and the basic principles and modulation techniques of the VSC-HVDC system were examined to present the system configuration direction suitable for HVDC connection. Section 3 systematically summarized the principle of superconducting fault current limiters (SFCLs), DC interruption theory, types of DC interruption technologies, and the operating principles of DC circuit breakers (DCCBs). Section 4 describes the fault analysis theory for the high voltage alternating current (HVAC) system and the specification of fault current calculation based on the IEC 60909. In addition, the basic theories of unbalanced faults such as single-line-to-ground, double-line-to-ground, line-to-line, and three-phase faults, and protection relay were described. In chapter 3, PSCAD/EMTDC simulation design and interpretation were conducted. In section 1, simulation parameters for the application of the symmetric coordinate method were set up. Using this parameter, a symmetric component (P–N–Z) analysis based on the x-axis was performed on the three-phase asymmetric failure types of one-line ground fault, two-line ground fault, and line-to-line short circuit. Using this, electrical characteristics such as instantaneous values of voltage and current for each failure type, unbalance, and change in symmetric components were analyzed. In section 2, an VSC-HVDC grid based on the modular multi level converter (MMC) was constructed between Weihai, China and Taean, Korea, and its parameters were defined. In section 3, the DC voltage, current, and power graphs of MMC were analyzed by steady-state simulation, and the voltage, current, and power characteristics of AC1 (Weihai, China) and AC2 (Taean, Korea) were analyzed. In section 4, a transient state simulation was executed in a PSCAD/EMTDC environment, and the characteristics of the VSC-HVDC system based on a MMC. Also the DC transmission system, and the AC1 and AC2 systems were analyzed. By setting four fault points (F1-F4) in the HVDC ±500 kV, 350 km transmission section, the DC ground current generated during the one-line ground failure of the P (+) and N (-) poles, and the AC voltage, current, and power characteristics of the sending point and the receiving point were analyzed. In addition, the voltage, current, and power characteristics of the AC1, AC2, and DC systems were analyzed in the event of a one-line ground fault, two-line ground fault, between lines, and three-phase short circuits of the AC1 and AC2 systems.
