When the single earth-return line of HVDC (High voltage direct current) system is functioning, the AC system in the same area will probably be influenced. Because two converting station at different DC potential constitute a loop via transmission line, causing certain direct current moving into the transformer neutral point and winding. The transformer DC magnetic biasing phenomenon will appear, leading increased noise and vibration of transformer. The influence of transformer DC magnetic biasing already exists and will do harm to transformers in a long term, while there's no set of effective quality inspection method. Therefore, it is necessary to establish a set of real-time current monitoring system for transformer DC magnetic biasing. In this paper, a set of automatic, high-accuracy and real-time current monitoring system is proposed. The monitoring system can display the amplitude and polarity of main transformer's neutral point direct current.
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