DC Fault Protection of a Nine-Terminal MMC HVDC Grid

An earthed bi-pole nine converter HVDC grid was modelled in PSCAD. A protection system involving hybrid DC breakers at the ends of each line and cable segment is proposed. Inductors are placed in series with each breaker to limit the rate of rise of the fault currents through the breakers. A fault detection system and localization scheme is proposed using ROCOV measured locally at the line side of these inductors at each breaker location. A systematic process for setting the ROCOV protection thresholds is discussed. Resistive faults are detected using local over-current methods. The protection system was modelled and tested under different fault types and locations. The results show that the proposed method of HVDC grid protection is feasible using hybrid DC breaker technology.