Analysis And Countermeasure Of Abnormal Operation Behaviors Of The Transformer Differential Protection During Complex Electromagnetic Transient Courses

The reliable operation of power transformer is very crucial in terms of security andstability of power systems. Among which, the reliable operation of the relay will preventtransformer from being damaged. Meanwhile, unnecessary outage of the transformer can beavoided as well. However, the statistical data related to the operation performance of thetransformer protection show that the correct rate of operation of the differential protection,which serves as the main protection for the transformer, is relatively low. This scenariocannot meet the requirement of latest secure operation of the power system. Recently, withthe increase of the complexity of the structure and components of the power system, theprobability of the abnormal operation of the transformer differential protection is obviouslyrising, and the sequent accident analyses are frequently unclear. It is adverse for the purposeof the security of the power system and will have negative impact on industrial productions.The transformer contains not only electric circuits but the core magnetic circuit couplingwith electric circuits, the complex electromagnetic transient courses may occur due to theswitching operations and fault removals of the transformer. The combination of knowledgeof many respects is required to interpret these phenomena reasonably. So far, the analyticaltheory of the transformer experiencing complex electromagnetic transient courses isrelatively weak. For the purpose of improving the operation correct rate of the differentialprotection, it is greatly important to thoroughly investigate the mechanisms andcharacteristics of the fault or disturbance and find out the genuine reasons leading to themal-operations of the transformer differential protection during various complex transientdisturbances. Therefore, the mechanisms of the abnormal operations of the differentialprotection during the transformer experiencing several kinds of complex electromagnetictransient courses are deeply analyzed in this dissertation. By virtue of the analyses, anintegrated solution of the transformer differential protection criterion is specificallyproposed. The details of this dissertation are as below:A reasonable model for analyzing the transient course of external fault inception andremoval, together with the mathematical model of current transformer(CT), is established.Based on the simulation model, the transforming characteristics of CTs on both sides of thetransformer during the existence of external fault and after the fault being removed areanalyzed. In this case, the reason of a category of mal-operation of the 2^nd harmonicrestraint criterion based transformer differential protection, which results from the lowdifferential current after the fault being removed, is disclosed. A novel model for analyzing the transient course of the loaded transformer energization,together with the existing CT model, is proposed. The waveform characteristics of theprimary and secondary inrushes are analyzed. Together with the transformingcharacteristics of CTs, the harmonic characteristic of the differential current is analyzed.Furthermore, the mechanism of the mal-operation of the differential protection caused bythe low through inrush during the loaded transformer energization is explained.A mathematical model for analyzing the transient course of the nonlinear loadswitching-in to a system with a power transformer is proposed. The characteristics of thesaturation of transformer core and the nonlinear load are taken into account in this model.Together with the analytic and numerical analyses, the waveform characteristics of theinrushes and magnetic linkages are analyzed. On the basis of the analyses of the transientcharacteristics of the differential currents, the reason, that 2^nd harmonic restraint criterionfailing to block the transformer differential protection during the nonlinear load switchingin, is explained.The models of the energization and internal faults of the converter transformer areestablished based on HVDC standard test system I of CIGRE. Furthermore, thecharacteristics of the differential currents during the unloaded transformer energization andinternal faults on the secondary side of the transformer are analyzed. Based on the analyses,the operation performance of the 2^nd harmonic restraint based differential protection for theconverter transformer is evaluated.By virtue of the analysis of the transforming characteristic of the saturated CT duringexternal faults, the variation trends of the primary and secondary currents of the saturatedCT, as well as the differential current, are analyzed. Furthermore, a novel method ofunblocking the differential protection, based on the variation rate and effective point criteriausing the sampling points of the secondary current of the saturated CT and the differentialcurrent, is proposed. The proposed method can be applied to identify the cross-countryfaults of the transformer.According to the investigation of the saturation characteristics of the transformer coreand CT core caused by external faults, a time difference exists between the sudden changeof phase voltage and the emergence of differential current. Based on this characteristic, anovel criterion utilizing the time difference between the superimposed phase voltage anddifferential current to identify the transformer faults is proposed. Furthermore, theintegrated criterion for the transformer differential protection based on the time-differencemethod to effectively discriminate between faults and other disturbances is put forward. The feasibility and effectiveness of the proposed criterion are validated with numeroussimulation and dynamic analog tests.Finally, a systematical summary is presented, and the next stage of research topic isexpected.