Simulation Calculation and Analysis Model of UHV Autoransformer under DC Bias

The DC bias current invading into the transformer neutral point is generated by the high voltage direct current transmission system and the geomagnetic storm, which will cause a half wave saturation of transformer core flux and threaten the safe and stable operation of power system. The experiment made by our research group is based on the 1000kV UHV autotransformer in dependently developed by China, of which the adverse effects of UHV autotransformers under DC bias event such as the distortion rate, harmonic component of excitation current and the reactive power fluctuation are simulated and analyzed. Simulation results show that the half-wave saturation of the UHV autotransformer core is gradually deepened along with the increase of DC bias current, making the excitation current to be more severely distorted with a large number of harmonics generated. Moreover, results of this paper also indicate that the lower is the harmonics number, the sooner is the harmonic growth speed, and the transformer reactive disturbance caused by DC bias is linear with the harmonics number.