MMC-MTDC系统单极接地故障电流计算方法

Pole-to-ground Fault Current Calculation Method for MMC-MTDC Systems

直流线路单极接地故障是基于模块化多电平换流器(modularmultilevelconverter,MMC)的多端直流(multiterminal high voltage direct current,MTDC)输电系统最常见的故障类型,分析其故障电流暂态特性对于故障类型的判断、保护配置的设计、系统参数的优化具有较大的工程意义。提出了一种适用于MMC-MTDC输电系统处于单极接地故障下,故障电流的计算方法。首先完整分析了多端直流系统单极接地故障演化机理;然后对单个换流站进行简化等效,进而将多端直流系统简化为RLC等效电路;根据基尔霍夫定律推导出故障前系统的状态方程;故障发生后,通过修改故障前状态方程中的状态变量及系数矩阵,用微分方程数值解的方法求解状态方程,进而求得故障电流。最后在PSCAD/EMTDC仿真软件上,搭建了三端MMC直流输电模型,仿真结果验证了故障电流计算方法具有较高的精确度。

DC pole-to-ground fault is a typical fault of modular multilevel converter(MMC) based multi-terminal HVDC(MTDC) transmission systems, and detailed analysis of the transient characteristics of fault current is of great engineering significance for judgment of fault type, configuration of protection, and optimization of system parameters. This paper proposed a pole-to-ground fault current calculation method for MMC-MTDC. The mechanism of pole-to-ground fault was completely analyzed. And a simplified RLC equivalent model of a modular multilevel converter was established, and then the MMC was expanded to MMC-MTDC system. According to Kirchhoff law, prefault state space description was established, and post-fault state equations were built by modifying the state variable and coefficient matrix of prefault state equations. These state equations were solved with numerical solution method. Finally, a 3-terminal MMC-MTDC system was established in PSCAD/EMTDC, and simulation results validated accuracy of the fault current calculation method.