Coordination of Mechanical DCCBs and Temporary Blocking of Half Bridge MMC

This paper proposes a dc grid protection strategy based on temporary MMC blocking in combination with mechanical DCCBs on dc lines. MMCs are blocked for only a short period of time while DCCBs operate and resume operation afterwards. A comparison is made with a protection strategy in which MMC blocking is avoided. The study analyses the impact of dc faults on dc power flow, ac system, DCCB dimensioning and MMC’s antiparallel diodes. Operation is demonstrated on a point-to-point HVDC and a three-terminal dc grid, also using thermal model of MMC’s IGBT module. Main benefits of the proposed strategy are simplicity and low protection system cost. On the downside, antiparallel diodes are exposed to greater current and thermal stress.