Comparative Study on Temporary Blocking Strategies of Converter Valve in MMC-based DC Grid with Overhead Transmission Line

Compared with the AC grid, DC grid based on Modular Multilevel Converter (MMC) with overhead transmission line has problems such as faster development of fault current and wider range of fault influence. To ensure the safety of the converter valve and the continuous operation of the system, blocking the converter valve temporarily is usually necessary. Three blocking schemes can be used according to the range of blocking, i.e. global temporary blocking, phase-based temporary blocking and arm-based blocking. Based on a four-terminal DC grid model, the strategies are compared and analyzed from the perspectives of the safety of the converter valve, the impact on the system under fault ride through, and on the fluctuation of internal circulating current. The results show that global blocking is easy to implement but the system may become uncontrollable, arm-based blocking will cause less fluctuations in system under fault ride through, and it has a better fault recovery performance. The research results can provide reference to the strategy design for the projects with similar operation requirements.