Effects of Neutral- Point of Rectifier on Insulation Monitoring and Its Solution

In IT system,the changes of rectifier diode conduction voltage and the asymmetric decline of the DC side line- to- ground insulation resistance will cause the neutral- point voltage of rectifier unbalance. DC current flowing into the neutral point of the AC system through the earth will makecause false alarm or leakage alarm for the DC injection method based IMD. Analysis indicate that the neutral- point voltage to ground of rectifier can be equivalent to a Thevenin model,which can act on the DC impedance network of the original system,so as to derive the mechanism of neutral- point voltage on IMD. Meanwhile,the idea of changing injection signal direction has been presented,which will record the insulation resistance values in forward and reverse way,and the real insulation value can be calculated after record data processing. Finally,experiment results validate the correctness of the proposed scheme.