Impact of Pole-to-ground Faults on Transformer DC Magnetic Bias and Differential Current Protection in Hybrid AC/DC Power Grids

Pole-to-ground faults in hybrid AC/DC power grids may result in DC-bias voltage on AC side of the MMC. If the transformer is YNd11 connected, or Dyn11 connected with small grounding resistance, the DC current caused by the fault will pass the transformer and lead to DC magnetic bias, which would further produce the magnetizing inrush current and the malfunction of differential current protection of transformers. In this paper, with respect to YNd11 connected transformers and Dyn11 connected transformers with different grounding resistance, the impact of pole-to-ground faults on the DC magnetic bias of transformers was analyzed. Influencing factors were subsequently concluded. Secondly, as an example of hybrid AC/DC systems, the DC-type PV grid-connection system model was established in PSCAD/EMTDC to verify the theoretical analysis. Finally, countermeasures, as well as the identification of single-pole fault and single-phase fault were given after analyzing the protection failures caused by DC magnetic bias.