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

doi:10.3969/j.issn.1000－7229.2018.09.005

Abstract

Abstract: Pole-to-ground faults in the hybrid AC/DC power system may result in DC-bias voltage on the AC side of the MMC converter. If the transformer is Dyn-11 connected with small grounding resistance, the DC current caused by the pole-to-ground fault would pass the transformer and lead to DC bias in magnetic field, which would further produce the magnetizing inrush current and the malfunction of differential current protection of transformers. In this paper, with respect to YNd-11 connected transformers and Dyn-11 connected transformers with different grounding resistance in hybrid AC/DC distribution grids, the impact of pole-to-ground faults on the DC bias of transformers are analyzed. Influencing factors are subsequently concluded. Secondly, as an example of hybrid AC/DC systems, the DC-access PV grid-connection system model is established in PSCAD/EMTDC to verify the correctness of the theoretical analysis. Finally, countermeasures for DC bias, as well as the identification of pole-to-ground fault and single-phase grounding fault are given after analyzing the protection misoperation caused by DC bias.

Key words: pole-to-ground fault, DC bias, the magnetizing inrush current, grounding mode, hybrid AC/DC power systems

CLC Number:

TM 773

DAI Zhihui，SU Huaibo，WANG Xue，JIAO Yanjun. Impact of Pole-to-ground Faults on Transformer DC Bias and Differential Current Protection in Hybrid AC/DC Power Systems[J]. Electric Power Construction, 2018, 39(9): 39-46.

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