变压器外部故障切除后差动保护误动的机理分析

由于变压器铁心在暂态过程中存在非线性，在实际运行中，变压器保护有时会出现难以解释的误动作。正确分析这些误动原因，对提高变压器保护的运行水平，促进保护生产厂家研制更高性能的产品，都有积极作用。文中通过建立一个基于二阶等效电路的变压器模型，分析了变压器带负荷合闸，尤其在外部故障切除后电压恢复的过程中，变压器差动保护误动的原因。指出：由于负荷支路的存在，导致在暂态过程中通过铁心的磁通可能存在2个具有不同符号和时间常数的衰减直流分量，衰减较慢的磁通在另一磁通充分衰减后仍可能保持较高的数值，导致变压器铁心严重饱和，相应地涌流特征消失，任何利用涌流波形特征进行判别的差动保护均存在误动的可能性。这项工作对变压器保护理论是一种有益的补充。

MECHANISM ANALYSIS OF THE MALOPERATION OF TRANSFORMER DIFFERENTIAL PROTECTION AFTER EXTERNAL FAULT CLEARANCE

The transformer differential protection possibly maloperates due to the non linearization of the core of transformer during transient. It will be of great benefit to the improvement of the operation and design level of transformer protection if these maloperations can be correctly analyzed and described. A two order equivalent circuit based model of transformer switching in with load is proposed to investigate the maloperation of transformer protection during the transient induced by loaded transformer switching in or the isolation of the external faults. It is pointed out the main flux of the transformer may consist of two exponentially decaying DC components with different time constants and signs. The flux with higher time constant possibly keeps quite high residual magnitude after the other one decay sufficiently. Therefore, the core of the transformer will saturate seriously and the characteristics of the inrushes will vanish accordingly. Under this circumstance, any differential protection using characteristics of inrush as restraint criterion is possible to maloperate. The presented work will benefit to the reinforcement of the transformer protection theory.