Pole-to-ground fault protection strategy for HVDC grids under symmetrical monopolar configuration

Symmetrical monopolar is nowadays one of the most common architecture employed in HVDC point-to-point interconnectors using Modular Multilevel Converter (MMC) technology. Future HVDC grids built in a stepwise approach, starting from the connection of the existing and future symmetric monopolar HVDC links, are therefore a possibility that can be taken into account. In such symmetrical monopolar configuration, a pole-to-ground fault in a DC link results in a low steady state DC fault current but it entails large overvoltage of the healthy pole. This paper proposes a novel pole-to-ground fault protection strategy for HVDC grids under symmetrical monopolar configuration employing low-speed DC breakers and pole rebalancing reactor (PRR) located at AC side to manage the rebalancing of the pole voltages. The first part of the paper is dedicated to the detailed description of the primary and backup protection sequences. In the second part, the proposed concept is validated through EMT simulations performed on a 4-terminal HVDC grid.