Features and mechanism of negative-sequence second harmonics of magnetizing inrush current in transformers

The no-load switching of transformers is the main influencing factor of malfunction of current differential protection, which will cause power quality problems such as harmonics and voltage dips as well. In the AC/DC hybrid power grid, because of the harmonic coupling property of the converter, the harmonics on the AC side will cause protection malfunction and blocking on the DC side, thereby threatening grid security. Based on the approximation magnetization curve of the transformer core, the analytical expression of magnetizing inrush current and its second harmonic component is deduced. The influence of switching parameters on the negative-sequence second harmonic component is analyzed. It is pointed out that the three-phase balance remanence caused by breaker switching will not result in the phenomenon of the negative-sequence second harmonic current being the main ingredient. Only the three-phase unbalanced remanence will result in obvious negative-sequence characteristics of magnetizing inrush current. The conclusion is verified by simulation using PSCAD software and provides an important theoretical basis for solving problems in practical engineering.