Coordination of MMC Converter Protection and DC Line Protection in DC grids

This paper studies the coordination of MMC converter protection and DC line protection in DC transmission networks. To quickly isolate a DC line fault, hybrid DC circuit breaker is connected at each end of every DC transmission line. DC line protection based on measuring derivative of DC voltage is employed to timely trip faulted DC line only. The goal of protection coordination is to prevent blocking of MMC converter which may happen at high DC current just before DCCB tripping. The current limiting inductors at each DCCB are designed to ensure that DC current stays below the MMC blocking threshold, and their values are 0.5H-0.7H. 3-level cascaded control is employed at MMC converters to improve DC voltage stability of the grid. EMTP simulations are provided to verify theoretical analysis.