Analysis of a Mal-operation of Differential Protection After the DC Resistance Testing

After the DC resistance testing of a transformer, the residual magnetism will affect the judgement of transformer’s differential protection. Upon charging the transformer with no-load, large inrush current will be created with big magnitude and low harmonics, which may lead to the secondary harmonic blocking failure of differential protection. The combined judgement method of secondary harmonics and waveform symmetry will improve the reliability of the differential protection action. An analysis is conducted of the mal-operation of differential protection after the DC resistance testing, and the difference in identifying the inrush current is compared between the blocking criteria of secondary harmonics and waveform symmetry. Based on the analysis, some feasible measures are proposed to avoid the mal- operation of differential protection. At last, with different residual magnetism modeling, the different blocking methods are tested again.