Pole Rebalancing Methods for Pole-to-Ground Faults in Symmetrical Monopolar HVDC Grids

Pole rebalancing in symmetrical monopolar high-voltage direct current (HVDC) grids is necessary to remove pole imbalances resulting from pole-to-ground faults. For selective protection employing DC circuit breakers, the interaction between DC circuit breakers and pole rebalancing methods have not been studied. This paper proposes new strategies for pole rebalancing methods to deal with DC circuit breaker operation in HVDC grids. A complete analysis of pole rebalancing using equipment at DC or AC side is performed for all stages of the fault clearing process. Based on the analysis, new control strategies are proposed to optimize the use of the pole rebalancing equipment. The proposed control methods are shown to enable the pole rebalancing equipment to meet the required high-protection speed and low losses. Both DC and AC side equipment such as dynamic braking systems and AC groundings are investigated and proven to be applicable for pole rebalancing in selective protection strategies. The impact of the breaker technology on the interaction between DC circuit breaker requirements and pole rebalancing needs is investigated in detail. The conclusions are validated using electromagnetic transients program (EMTP) simulation on a four terminal test grid.