Study On Adaptive Transformer Longitudinal Differential Protection

The longitudinal differential protection is one of the primary protections fortransformer. But, there are still some problems in practical applications. Magnetizinginrush causes mal-operation of transformer differential protection occasionally, andmoreover, in the case of internal fault, the relay is not unblocked until the level of therestraint factor drops below the threshold setting, so the speed of protection operationis constrained. Besides, in the case of severe CT saturation, the mal-operation ofpercentage differential protection during external fault conditions may be caused.Adaptive protection can adjust the operating characteristics and threshold settings inresponse to the changing system operating conditions and fault states, so itsperformance is superior to the traditional protection. Therefore, studying on theadaptive transformer longitudinal differential protection is of great importance toincrease the level of power transformer relay protection.First, in order to propose a more reliable method of magnetizing current restraint,in the paper, positive sequence component of the second and third harmonic indifferential current is extracted and analyzed, respectively in transformer internalfault cases and magnetizing inrush cases, and variable situations of CT saturation areconsidered in the process. Then a new criterion based on the positive sequencecomponent to identify magnetizing inrush current is proposed in the paper. Thesimulation results of EMTP show that the new criterion can restrain differential relayduring magnetizing current conditions more reliably, in addition, it can initiate a tripsignal during internal fault conditions slightly faster than the method of secondharmonic restraint.Then, by extracting, analyzing and using the appropriate system information offault states and operating conditions, in the paper, adaptive control factors for theparts of magnetizing current restraint and percentage restraint are determined respectively. These factors are added to the magnetizing current criterion of positivesequence and the percentage restraint criterion, so the adaptive scheme ofmagnetizing current restraint and percentage restraint are proposed. The two adaptiveschemes are combined with the parts of the instantaneous differential protection andover-excitation restraint, so as to form a new and whole adaptive scheme oftransformer differential protection.Finally, by EMTP simulation and dynamic experiment data, the correctness ofthe adaptive transformer differential protection scheme is verified. The results showthat by comparison with the traditional protection, the new protection scheme canrestrain the differential relay during magnetizing current conditions more reliably andinitiate a trip signal during internal fault conditions quickly. In addition, the stabilityof new protection technique during external fault conditions is higher than thetraditional protection, and the differential protection can avoid mal-operationeffectively.