Switching Overvoltage Characteristics of $\pm$ 1100-kV UHVDC Converter Station

The present study analyzed the characteristics of switching overvoltage, and discussed in detail the main influencing factors and worst-case conditions of switching overvoltage in the ±1100-kV ultra-high-voltage direct-current (UHVDC) converter station. The results showed that ac bus overvoltage was produced by oscillation, and would not be a threat to the equipment in the converter station. The results also showed there were two cases for overvoltage on related valves. In the first case, the valve overvoltage was on converter valves in the upper three-pulse group of the 12-pulse converter, and was caused by the voltage difference between the dc line and ac bus at the valve side under a specific fault. In the second case, the overvoltage was on other valves, and was caused by a phase-to-phase switching surge that penetrated into the valve hall. Other results were that overvoltage on the dc bus was mainly caused by the full reflection of the wave, and overvoltage on the neutral bus was caused by high-voltage sources, such as the dc source creating stress on the neutral bus under specific faults, or the freewheeling current of the metal return line and the smoothing reactors.