Protection and pole voltage rebalancing for pole-to-ground faults in symmetrical monopolar HVDC grids

Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from pole-to-ground faults. For selective protection employing DC circuit breakers, pole rebalancing considering backup protection operation in case of breaker failure has until now not been studied. This paper proposes fault clearing and post-fault restoration sequences including pole rebalancing to deal with DC-side faults, considering both primary and backup protection operations. The performance of the proposed sequences and the impact of the key breaker parameters on pole rebalancing are investigated in a four-terminal test system using PSCAD/EMTDC.