부산대학교 도서관

In order to solve the problem of abundant renewable energy in the areas where wind farms and photovoltaic power stations with limited local consumption capacity are located in China. The state has planned and constructed several VSC-HVDC grid projects to realize the access of renewable energy to the grid. Due to the complex structure of the flexible DC network, the research of operating characteristics of its devices is very important. Among them, the converter transformer, as an important equipment connecting AC and DC systems. It is necessary to research its operating characteristics under different working conditions. AC harmonics in converter station can be generated by DC converter, which make the impact of harmonics on the converter transformer cannot be ignored. A project is a ±500kV symmetrical true bipolar four-terminal ring grid structure,which using half-bridge type sub-module MMC, DC circuit breaker and overhead lines for power transmission. The overhead lines occur monopolar grounding and bipolar short circuit faults easily, and multiple faults also occur in the commutation valve. Through simulation software, a ±500kV flexible and straight grid engineering simulation model is built to simulate and analyze the voltage and current on the valve side of the converter transformer under the above-mentioned fault conditions, and also calculate the amplitude of the harmonic components of the voltage and current. The effect of bridge arm reactors and the number of submodule MMC on the overvoltage and harmonic components are simulated and analyzed.