双极MMC-HVDC系统站内接地故障特性及保护策略

针对双极柔性直流输电系统模块化多电平换流器(MMC)交直流出口接地故障,研究了换流器闭锁后的电压电流暂态特性,并推导了故障分量的数学解析式。研究结果表明,交流出口发生单相接地故障时,换流器闭锁后非故障相上、下桥臂分别出现了过电压和过电流现象,并且交流侧电流出现直流偏置导致故障相短路电流不存在过零点。直流出口发生单极接地故障,换流器闭锁后桥臂短路电流主要由交流系统注入的稳态电流和上下桥臂电抗间衰减的环流构成。针对交流出口发生单相接地故障这一特殊的故障特性,提出了一种选相跳闸保护策略,解决了故障电流不存在过零点时交流断路器无法正常断开的难题。最后搭建了张北四端环网结构柔性直流电网仿真模型,仿真结果验证了换流器出口故障特性分析的准确性以及所提选相跳闸保护策略的有效性和可行性。

Converter Grounding Fault Characteristics and Protection Strategy in Bipolar MMC-HVDC System

In view of AC/DC outlet grounding faults of modular multilevel converter (MMC) in bipolar flexible high-voltage direct current (HVDC) transmission systems, the transient characteristics of voltage and current after converter fault blocking are studied. The mathematical analytical formula of the fault component is derived. The research results show that a single-phase grounding fault at the AC outlet will lead to the overvoltage and the overcurrent of the upper arm and the lower arm of blocked converter on the non-fault phase, respectively. And the DC bias on the AC-side current causes the fault phase short-circuit current to have no zero-crossing. When a pole-to-ground fault occurs at the DC outlet, the short-circuit current of the bridge arm of the blocked converter is mainly composed of the steady-state current injected by the AC system and the circulating current between the upper and lower arms. A phase-selective trip protection strategy is proposed for this special fault characteristic of single-phase grounding fault at AC outlet, which can solve the problem that the AC circuit breaker cannot be normally opened when the zero-crossing point of the fault current does not exist. A simulation model of the Zhangbei flexible DC grid of China is built. The simulation results show that the analysis of the fault characteristics at the converter outlet is accurate and the proposed phase-selective trip protection strategy is effective and feasible.