The neutral point potential of the grounding transformer will be changed when HVDC system operates in a dc monopole ground return mode and dc bias may be generated. Dc bias will change exciting current‘s waveform. Then large quantity of even and odd harmonics will be produced. The harmonics will do harm to the transformer and its protection. This paper uses PSCAD/EMTDC to create a simulation model. It emulates exciting current of a practical 750kV single-phase transformer in variant cases under dc bias. It studies the distortion of exciting current, harmonic distribution, the peak value of the exciting current under dc bias. Meanwhile, the performance of transformer protection in this harmonic environment is analyzed. The loss made by exciting current’s distortion and elevated temperature rise of core tie-plate are quantitative calculated.
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