Neutral Grounding Mode of MMC-based DC Distribution Grid

With a demonstration project of MMC-based DC distribution grid as the background, this paper studies the neutral grounding mode of DC distribution system with MMC converters and symmetrical single pole wiring structure. When a grounding fault occurs in DC grid, the fault current loop is related to the neutral grounding mode of both AC and DC sides of the converter station. Therefore, the over-current and over-voltage imposed on the converter valve during the fault period can be decided by the grounding mode. This paper selects three optional grounding modes which are the combination of AC side connection transformer neutral point ungrounded or grounded through resistance and DC side neutral point ungrounded or grounded through clamping resistance. Fault characteristics of DC single pole grounding fault for these three neutral grounding modes are analysed using the fault current loop method and simulation results. According to the analysis results, fault current and voltage between the poles for different grounding modes are compared. The results show that the scheme of AC side connection transformer neutral point grounded through appropriate resistance and DC side ungrounded can ensure the safety operation of DC grid during the fault and facilitate fault detection, and is economical.