柔性直流配电系统换流器交流侧接地故障的分析

柔性直流配电网中的换流器若经联结变压器接入交流电网，联结变与换流阀之间发生绝缘损坏会产生换流器交流侧接地故障。接地故障对系统运行的影响及其保护方案值得研究。首先分析了柔性直流配电网中各部分的共模等效模型。考虑联结变不同的接地方式，建立了一个典型的五端柔性直流配电系统共模等效电路。推导了各端基频共模电流、电压的计算公式。其次考虑系统可能存在的各种谐振，提出了一种基于换流器直流侧基频共模电流、电压以及功率方向的多端柔性直流配电系统交流侧接地故障的保护方案。最后在PSCAD仿真平台上进行了验证。

Analysis of grounding fault on the AC side of converter station in a flexible DC distribution system

In the flexible DC distribution power system, if the converter station is connected to the AC grid via a connecting transformer, the insulation damage between the connecting transformer and the converter valves will cause a grounded fault on the AC side of the converter station. The influence of this grounded fault and the protection strategy is worth researching. First, the equivalent common-mode circuit model of the components in the flexible DC distribution power system is analyzed. Subsequently, the equivalent common-mode circuit model of a flexible five-terminal DC distribution power system with different grounding methods is built. The calculation formulas of fault fundamental frequency common-mode current and voltage after a converter AC side single-phase grounded fault occurs are obtained. Then, considering various resonances that may exist in a DC system, a protection strategy based on a converter DC side fundamental frequency common-mode voltage, current and power direction to identify the single-phase grounded fault on the converter AC side is proposed and verified on the PSCAD platform. This work is supported by National Key Research and Development Program of China (No. 2017YFB0903100).