Harmonic Distortion Feature of AC Transformers Caused by DC Bias

With regard to the first ± 660kV HVDC transmission line from Yinchuan to Qingdao, quantitative harmonic analysis of the main transformer in Yi Ming substation, about 65km away from the Qingdao converter station, is carried out based on an established nonlinear magnetic model by PSCAD/EMTDC. The results indicate that, both the odd and even harmonic components are getting much higher under DC bias than that without DC bias, which thereby makes the power transformers to severe harmonic sources. The relationship between the injected neural DC currents and the even harmonic ratio is established which presents further potential for developing inverse-current-injection based DC bias suppression scheme.