616 / 2019-03-18 23:06:43

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

DC bias; grounding mode; magnetizing inrush current; pole-to-ground fault

Pole-to-ground faults in hybrid AC/DC power grids may result in DC-bias voltage at the AC side of the converter. If the transformer is Dyn11 connected with small grounding resistance, the DC current caused by the pole-to-ground fault will pass the transformer and lead to DC bias, which would further produce the magnetizing inrush current and the malfunction of differential current protection of transformers. In this paper, considering different transformer connection groups with different grounding resistance, the impact of pole-to-ground faults on the DC bias of transformers is analyzed. Influencing factors are concluded as well. Secondly, a hybrid AC/DC system is modeled to verify theoretical analysis. Finally, counter-measures to DC magnetic bias, and the identification method of pole-to-ground fault and single-phase grounded fault are given.