变压器励磁涌流负序二次谐波特征及机理

空载合闸产生的励磁涌流是导致变压器差动保护不正确动作的主要因素,同时也会造成电网中谐波、电压暂降等电能质量问题。交直流混联电网中,换流器的谐波耦合特性使得交流电网中的谐波会引起直流保护的误动作,导致直流闭锁,威胁电网安全运行。文中基于变压器铁芯近似磁化特性曲线,推导了励磁涌流及其中二次谐波分量的解析表达式,分析了合闸参数对负序二次谐波分量的影响,指出由于断路器开断引起的三相平衡性剩磁不会造成励磁涌流中二次谐波以负序为主的现象,只有三相非平衡性剩磁才可能引起励磁涌流中的二次谐波呈现明显的负序特征。PSCAD仿真验证了分析及结论的正确性,为解决工程实际问题提供了理论依据。

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. This work is supported by National Natural Science Foundation of China (No. 51377129).