Study On The Effect Of TA Neutral Line Anomaly On Line Differential Protection And Its Countermeasures

Split-phase current differential protections,which based on kirchhoffs current law,have been widely used as the main protection of high-voltage transmission lines because of its excellent characteristics.but the selectivity and sensitivity of differential protection is still affected by the TA secondary circuit,there has been many misoperation incidents in the domestic because of TA neutral line abnormality.In view of the misoperation of line differential protection caused by the abnormal of TA second circuit neutral line,this article analyzed its cause and mechanism and Put forward the corresponding countermeasures.In this article,the abnormal condition of TA neutral line is divided into two types,one is the increase of the resistance of the neutral line of TA(Generally caused by bad contact of terminal and aging of second cables),the other type is the complete disconnection of the neutral line.For TA neutral line resistance increasing,this paper elaborates the misoperation mechanism is the the voltage drop generated in the neutral line caused by fault phase second circuit current will generates the effect similar to the inrush current in the non-fault phase TA second circuit,and resulting in the asymmetrical waveform of abnormal-current.The essence of abnormal-current is the non fault TA flux accumulation.By changing the parameters of secondary circuit equation,this article analyzing the influences of different neutral line resistance,non-periodic current,remanence level,and volume of TA to the abnormal-current waveform.The correctness of the conclusion is verified by experiments.In view of the complete disconnection of TA neutral line,the article starts from the case study,and establishes the equivalent model of the second circuit under the condition of TA neutral line disconnection,analyzes the probably three-phase second circuit current distribution that caused by the TA neutral line disconnection.After studying and summarizing the characteristics of such kind of misoperation,the action logic is proposed.The article introduces the three-phase maximum current as the braking criterion,combined with the second harmonic ratio or waveform asymmetry coefficient,to identify the TA neutral line abnormity and then blocking phase separation differential protection for a short time.Lastly,the simulation and waveform playback verifies the correctness and validity of the proposed protection scheme.