MMC换流站阀侧交流接地故障电流解析

对模块化多电平换流器(modular multilevel converter, MMC)换流站阀侧交流接地故障特性进行研究发现：阀侧交流系统发生金属性接地故障时，故障点入地电流由下桥臂电容放电电流和上桥臂以及对端桥臂电容放电电流构成，网侧交流系统不会馈入入地电流；带过渡电阻接地故障时，网侧交流系统将通过过渡电阻作用于故障点入地电流。详细推导了三相接地故障和单相接地故障下的入地故障电流和桥臂电流的数学解析式。采用RTDS仿真验证了该表达式的正确性。由于阀侧交流出口处电位被钳制为0，因此上桥臂的子模块电容将产生过电压。下桥臂与故障点构成放电电容回路，下桥臂流过的故障电流迅速增大，且无法通过断路器切断故障电流。因此，建议采取可靠的限流措施避免阀侧故障对换流站等一次设备造成严重伤害。

Analysis of grounding fault current at the AC valve side of an MMC converter station

In this paper, the AC grounding fault characteristics on the valve side of an MMC converter station are studied. It is found that when the metal grounding fault occurs in the AC system on the valve side, the grounding current at the fault point is composed of the capacitor discharge current of the lower bridge arm and the capacitor discharge current of the upper bridge arm and the opposite bridge arm, and the grid side AC system will not feed the grounding current. For a grounding fault with transition resistance, the AC system at the grid side will act on the grounding current at the fault point through the transition resistance. The mathematical expressions of grounding fault current and bridge arm current under a three-phase grounding fault and single-phase grounding fault are derived in detail. The correctness of the expression is verified by RTDS simulation. Since the potential at the valve side AC outlet is clamped to 0, the sub module capacitance of the upper bridge arm will produce an overvoltage. For the discharge capacitor circuit composed of the lower bridge arm and the fault point, the fault current flowing through the lower bridge arm increases rapidly and cannot be cut off through the circuit breaker. Therefore, it is recommended to take reliable current limiting measures to avoid serious damage to primary equipment caused by a valve side fault, such as to the converter station. This work is supported by the National Natural Science Foundation of China (No. 52167011).