MMC-HVDC直流侧故障特性仿真分析

Simulation and analysis of DC-link fault characteristics for MMC-HVDC

由于拓扑和调制策略的不同,基于模块化多电平换流器的高压直流输电(MMC-HVDC)系统在直流侧发生故障时呈现出与两电平电压源型直流输电(VSC-HVDC)系统不同的故障特性。在PSCAD/EMTDC中搭建的仿真模型基础上,首先分析了MMC-HVDC直流侧线路单极接地、断线和两极短路的故障特性及其对系统运行的影响;然后针对半桥型子模块结构不能够双向阻断故障电流的问题,对子模块拓扑进行了重新设计,通过改变流经子模块电流方向,实现了桥臂电容双向充电,从而提供了续流二极管阻断电压;最后对直流侧两极短路故障进行了仿真分析,仿真结果表明,改进拓扑有效地抑制了直流侧故障电流,避免了交流断路器动作。

The DC-link fault characteristics of MMC-HVDC system are different from those of two-level VSC- HVDC system because of different topologies and modulation methods. Based on the simulation model established with PSCAD/EMTDC,the DC-link fault characteristics of line-to-ground,broken line and line-to- line,as well as their effects on system operation,are analyzed. The topology of half-bridge sub-module is improved to block the fault current in both directions,which,by changing the arm current path,charges the arm capacitor in both directions to set the blocking voltage of free-wheel diode. Simulation is carried out for the line-to-line fault and results show that,the improved topology suppresses the DC-link fault current effectively to prevent the trip of AC circuit breaker.