内桥接线方式下变压器差动保护误动原因及防范措施

Mal-operation of Transformer Differential Protection with Inner Bridge Connection and Countermeasure

为了研究在内桥接线变电站中,2台主变压器并列运行,其中1台主变通过内桥开关空载合闸,另1台正常运行时主变差动保护误动的原因,结合一起现场操作过程中内桥接线方式下变压器差动保护的误动事故,详细分析了励磁涌流、和应涌流以及电流互感器(TA)饱和对差动保护的影响,最终认为励磁涌流中非周期分量引起的进线侧TA局部暂态饱和是差动保护误动的主要原因。针对误动实例中变压器采用西门子7UT512型微机差动保护的情况,通过对保护误动作点的分析,提出利用抬高比率制动门槛值的方法防止内桥接线方式下变压器差动保护误动。仿真结果表明了该文分析的正确性,并验证了所提改进比率制动特性方法的有效性。

In the substation of inner bridge connection,two main transformers operate in parallel connection.The differential protection of normal operating transformer may mal-operate when the other transformer switching-on unloads through bridge breaker.Considering a case of mal-operation,we analyzed several influential factors on transformer differential protection,such as magnetizing inrush,sympathetic inrush,and current transformer（TA） saturation.It is concluded that TA partial transient saturation caused by the decaying DC component in magnetizing inrush is the main cause that results in the mal-operation of differential protection.By analyzing the mal-operation point,a new method of improving the threshold of ratio restraint characteristic is proposed to prevent differential protection from mal-operation in condition of transformers equipped with Siemens 7UT512 Numerical Current Differential Protection.Simulation results show the validity of analysis and verify the feasibility and practicability of the new modified ratio restraint scheme.