变压器差动保护误动原因的复杂电磁暂态分析

炼钢炉等大容量非线性负荷的投入所导致变压器差动保护误动作的案例常常被归入原因不明的一类，因为此类操作使变压器位于一个具有2个非线性元件的网络中，各种电气量之间的相互作用使得电磁暂态分析变得相当复杂。结合解析分析和数值分析，建立了计及变压器铁心饱和以及非线性负荷特性的微分方程组，生成各支路的磁链波形和电流波形。对变压器差动保护差流波形及其2次谐波特征进行分析。得出结论：非线性负荷投入时，由于2个非线性特性元件的相互作用，变压器励磁支路进入并停留在饱和点附近，从而造成差流幅值很大且2次谐波含量低于门槛值，2次谐波制动判据失去了效用，引起变压器差动保护的误动作。通过仿真试验验证了所得结论。

Analysis of Complex Electromagnetic Transient Leading to the Mal-operation of Power Transformer Differential Protection

When a nonlinear load with high capability, such as a furnace, is switched in a transformer-involved power system, the interactions among various electric quantities make the analyses complex because the transformer is located in a network with two nonlinear components. In this case, the mal-operation of transformer differential protection is difficult to analyze and usually attributed to a category of uncertain reasons. In order to illustrate the mechanism of differential protection mal-operation, a novel model including transformer and nonlinear load was proposed and the analytic and numerical analyses were done. Based on this model, a system of non-linear differential equations was formed. The waveform and harmonic characteristic of the differential current during nonlinear load switching in were analyzed. It is pointed out that when the nonlinear load is switched in, the interaction between two nonlinear components makes the transformer core to enter and stay within the area around the saturation point. Therefore, the differential current may exceed the threshold and the percentage of second order harmonic is low, which leads to the mal-operation of the differential protection. The conclusions were verified by simulation tests.