Influence and countermeasure to transformer differential protection of CT partial transient saturation caused by external fault removal

The mal-operation of the transformer differential protection will occasionally take place after the removal of external faults. The physical phenomenon of partial transient saturation for current transformer was studied in this paper. Through model emulation, the experiment of small CT and the waveform of field fault record for mal-operation, it is pointed out that the main cause of the malfunction in differential protection is partial transient saturation of current transformer while transferring small load AC current at large remnant flux. The differential protection is very sensitivity to malfunction since the restrained current is small. At the same time, partial transient saturation phenomenon will also occur while CT transferring small AC current combined with long time decaying current (such as sympathetic inrush or through inrush). In order to solve this problem, an inverses time current differential protection scheme based on detecting external faults removal are proposed. This scheme could avoid the maloperation of the different protection due to CT partial transient saturation during the removal of external fault. It has great perspective if the method is realized for transformer protection.