Analysis And Research Of AC Transformer DC Bias Magnetization Suppression

High-voltage direct current (HVDC) transmission system has advantages over traditionalalternating current (AC) power transmission system, such as long conveying distance, largetransmission capacity and little line loss. However, the produced DC current will get into theAC transformer because of the unbalance of double polar or single polar back to the earth inDC transmission system, thus causes a DC bias problem which brings serious threat to theoperation of the transformer. DC bias will make the noise of neutral point grounded ACtransformer much louder, as well as strengthen vibration and the iron core overheatingsomewhere, further lead to increasing iron loss, besides, a series of problems such as voltagewaveform distortion will occur.This paper mainly introduces the history of HVDC transmission, the generatingmechanism of DC bias, and several typical theories and methods, such as AC transformerneutral point series with a certain value of resistance, connection in series with capacitance onthe AC line, furnish with resistance capacity DC-blocking device or reverse suppressiondevice on transformer neutral point, the comparative analysis of the advantages anddisadvantages of these methods are also provided. An example of emergency replacement oftransformer substation500kV Luo Dong caused by DC bias is exhibited, two kinds ofmeasures applied in Luo Dong are analyzed, thus the most feasible scheme is put forward. Inthis paper, through the comparison between theoretical research and practical applicationsituation, we can draw the following conclusions:The connection in series with a small resistance on AC transformer neutral point willchange the system operation mode, which also makes the corresponding protectionconfiguration more complicated, thus rarely used in practice.Method of connection in series with capacitors on AC line, due to the enormous projectand high cost, is not suitable for practical operation and there is also no specificimplementation.Methods of resistance capacity DC-blocking and reverse suppression, no matter in theory and practical application, have a high feasibility and decrease the impact of the DC biasphenomenon, but its efficiency needs to be improved. This paper mainly introduces theprinciple of the two methods, the usage of device,the configuration of device and the actualoperation effects, it will provide reference for further research to reduce transformer vibrationand noise, extend the life of the transformer.