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.