一起特高压变压器的差动保护误动分析及防范措施

通过建立MATLAB仿真模型,分析了在特高压交流试验示范工程投运调试过程中出现的一起由于中压侧空投主变压器产生励磁涌流而导致调压变压器差动保护误动作的案例。针对特高压变压器结构上的特殊性,分析了合闸角、剩磁等因素对励磁涌流的影响,仿真复现了现场故障录波波形。针对调压变压器差动电流中二次谐波含量较低可能引起的误动,从2个方面提出了相应的解决方案：指出二次谐波涌流闭锁判据宜采用三取二闭锁方式,但在某些特殊情况下,依然可能失效,进而提出了利用二次谐波变化趋势来识别励磁涌流和故障电流的辅助判据,并对综合判据进行了仿真验证;分析了调压变压器容量对励磁涌流的影响,提出在条件允许的情况下,适当增加调压变压器容量来避免励磁涌流可能引起的误动,为提高调压变压器差动保护可靠性提供了新思路。

Analysis on a Mal-operation of Differential Protection for Ultra-high Voltage Transformer and Its Countermeasure

A simulation model based on MATLAB is developed to analyze the mal-operation of differential protection in voltage regulating transformer(VRT) in an experiment project of ultra-high voltage system,which is caused by inrush current during energizing of the transformer from medium-voltage side.According to particular structure of ultra-high voltage transformers,the influence of closure angle and residual magnetism,etc.on inrush current is analyzed and the waveform of field failure is resembled with simulation.In order to solve mal-operation caused by less second harmonic in differential current of VRT,lock criteria for second harmonic of inrush current should adopt two-out-of-three principle,but it may not valid in some situation.Furthermore,auxiliary criteria with second harmonic tendency is proposed for identifying inrush current or faulty current.Besides,it analyzes the influence of VRT capacity on inrush current and also suggests that suitably increasing the capacity of VRT should be considered in order to avoid mal-operation resulted from inrush current,if possible.The simulation results verify that the improved scheme is able to effectively avoid mal-operation of differential protection from inrush current.