交流系统电压对MMC-HVDC接地故障时换流器闭锁前桥臂电流的影响机理

直流线路接地故障是模块化多电平换流器(Modular Multilevel Converter, MMC)的主要故障类型。发生故障时,为满足直流断路器切断电流要求,应在MMC闭锁前切除故障,而MMC的闭锁时刻取决于子模块中绝缘栅双极晶体管(Insulated Gate Bipolar Transistor, IGBT)承受故障时桥臂电流的能力,因此对故障时单桥臂电流的特性分析提出了更细致具体的要求。首先介绍了真双极MMC的拓扑结构和工作原理,分析了在发生直流线路单极接地故障时闭锁前故障电流不同成分在MMC桥臂上的流通路径。然后采用复频域计算法,构建了故障时桥臂电流的数学模型,推导了交流系统电压对MMC闭锁前桥臂短路电流的影响机理。研究表明交流系统电压相角对故障时桥臂电流的幅值影响显著。最后,基于PSCAD仿真实验平台,搭建31电平单端及51电平双端MMC-HVDC,在不同电压幅值与相角取值下的仿真结果验证了该机理的正确性。

Influence of AC system voltage on the current of the arm of the converter during an MMC-HVDC grounding fault before converter latching

DC line ground faults are the primary fault type of Modular Multilevel Converters (MMC). In the event of a fault, in order to meet the cut-off current requirement of DC circuit breaker, the fault should be cut off before the MMC latching, and the time latching of the MMC depends on the ability of the Insulated Gate Bipolar Transistor (IGBT) to withstand the bridge arm fault current. Therefore, more detailed and specific requirements are put forward for the analysis of the characteristics of the single-bridge arm current in the event of a fault. First, the topology and working principle of the true bipolar MMC are introduced, and the flow path of the different components of the fault current on the MMC bridge arm before the converter latching during the DC line grounding fault is analyzed. Then, using the calculation method of the complex frequency domain, a mathematical model of the bridge arm current at fault is constructed, and the influence mechanism of the AC system voltage on the short-circuit current of the bridge arm before MMC latching is derived. Studies have shown that the voltage phase angle of the AC system has a significant effect on the amplitude of the bridge arm current at fault. Finally, based on the PSCAD simulation experiment platform, 31-level single-ended and 51-level double-ended true bipolar MMC-HVDC models are built. The simulation results under different voltage amplitudes and phase angles verify the correctness of the mechanism. This work is supported by National Key Research and Development Program of China (No. 2018YFB0904600).