Research on inrush current characteristics of converter transformers

The three-phase transformer will produce great magnetizing inrush current with high secondary harmonic component at the time of energization, while differential protection can effectively block by the second harmonic principle and interval angle principle. For paralleled converter transformers, the differential current of the integral differential protection equals the sum of the differential current of the two partial differential protections. However, the secondary harmonic component in the two partial differential currents is not simple superposition, which may cause the failure of blocking of the integral differential protection, and therefore cause false tripping. Based on a false tripping accident of the integral differential protection, the paper further analyses the occurrence mechanism of magnetizing inrush current and the secondary harmonic component, proves the possibility of the offsetting of secondary harmonic component in the two partial differential current, and proposes solution accordingly.