Actual Measurement and Analysis of DC Magnetic Bias of 750kv Autotransformer

Monopole to ground operation mode of ultra high-voltage DC transmission system will result in DC magnetic biasing phenomenon to transformers in surrounding AC substation. Under monopole to ground operation mode of ±800kV “Qi-Shao” ultra high-voltage DC transmission project, DC magnetic biasing hazard of transformers in AC power network in sending terminal—Jiuquan region was tested. It’s found that after capacitive DC blocking device of 750kV transformer neutral point was put into action, transformer vibration and noise still gradually increased with ground current at ground electrode, so ideal governance effect was not reached. Regional DC current distribution after series capacitance at transformer neutral point in 750kV Dunhuang substation was analyzed in this paper. It’s found that DC current could still flow through autotransformer winding so as to make the transformer be still influenced by DC magnetic bias. Meanwhile, based on analysis of secondary-side waveforms at neutral point of 750kV autotransformer, it’s found that transformer magnetizing current went through serious distortion with current increase at ground electrode after action of DC blocking device. Harmonic analysis was conducted for distorted waveforms and change features of secondary harmonics were used, which provided a convenient and effective means of monitoring DC magnetic biasing hazard of 750kV autotransformer.