半桥型MMC直流侧故障限流组合控制策略

基于模块化多电平换流器(modular multilevel converter, MMC)的柔性直流电网在直流短路故障时电流峰值较高且上升速度极快，严重时会造成MMC闭锁从而导致系统大面积停运。为在短时间内限制故障电流对系统的影响，文中提出一种对半桥型MMC适用的故障限流组合控制策略，利用MMC自身的高度可控性，无须外加限流装置，即可达到故障限流效果，并降低对直流断路器的技术需求。首先，文中阐述了限流组合控制策略中2种不同的限流环节及其基本原理。其次，分别分析2种限流环节对直流故障电流、交流电流以及桥臂电流的影响，推导限流组合控制下的直流故障电流计算式。最后，在PSCAD/EMTDC平台搭建半桥型MMC四端直流电网模型进行仿真分析，结果表明所述限流组合控制策略能够有效限制直流故障电流，减小故障点近端换流器的功率和电压波动，降低交流电流和桥臂电流的过流峰值。

Combined control strategy of half-bridge MMC DC side fault current limiting

The flexible direct current (DC) grid based on modular multilevel converter (MMC) has a high current peak value and a very fast rising speed during a DC short-circuit fault. In order to limit the impact of fault current on the system in a short period of time,a fault current-limiting combined control strategy suitable for half-bridge MMC was proposed. By using the high controllability of MMC itself,the fault limit can be achieved without external current limiting device,and it can reduce the technical requirements for DC circuit breakers. Firstly,two different current-limiting links and their basic principles in the current-limiting control strategy are explained. Secondly,the influences of the two current-limiting links on the DC fault current,AC current and bridge arm current are analyzed respectively,and the calculation formula of DC fault current under current-limiting control is deduced. Finally,a half-bridge MMC four-terminal DC grid model is built on the PSCAD/EMTDC platform for simulation analysis. The results show that the current limiting combined control strategy can effectively limit the DC fault current and reduce the power and voltage fluctuations of the near-end converter at the fault point. It can also reduce the peak value of AC current and bridge arm current.