切除外部故障时电流互感器局部暂态饱和对变压器差动保护的影响及对策

THE INFLULENCE AND COUNTERMEASURE TO TRANSFORMER DIFFERENTIAL PROTECTION OF CT PARTIAL TRANSIENT SATURATION CAUSED BY EXTERNAL FAULT REMOVAL

针对近年来相继出现的变压器差动保护在外部故障切除时发生误动的现象,研究了TA(电流互感器)局部暂态饱和的物理现象和TA局部暂态饱和的数学模型。通过模型仿真、实验室小TA的试验、现场误动录波数据的分析,指出TA工作在饱和点附近的小工频电流传变是引起差动保护误动的根本原因,此时差动保护因制动量小而非常灵敏,如果处理不当可能会导致差动保护的误动,同时分析出TA在较长时间的非周期分量作用下(如和应涌流、穿越性涌流等),即使只是带负荷电流,也会出现局部暂态饱和现象。针对此问题提出了基于反时限特性的电流差动保护方案,该方法能有效地防止因外部故障切除TA局部暂态饱和引起的差动保护误动。

The mal-operation of the transformer differential protection will occasionally take place after the removal of external faults. The physical phenomenon of partial transient saturation for current transformer was studied in this paper. Through model emulation, the experiment of small CT and the waveform of field fault record for mal-operation, it is pointed out that the main cause of the malfunction in differential protection is partial transient saturation of current transformer while transferring small load AC current at large remnant flux. The differential protection is very sensitivity to malfunction since the restrained current is small. At the same time, partial transient saturation phenomenon will also occur while CT transferring small AC current combined with long time decaying current (such as sympathetic inrush or through inrush). In order to solve this problem, an inverses time current differential protection scheme based on detecting external faults removal are proposed. This scheme could avoid the maloperation of the different protection due to CT partial transient saturation during the removal of external fault. It has great perspective if the method is realized for transformer protection.