Analysis Of The Restraint Methods And Misoperaton Of Transformer Differential Protection

Power Transformer is one of the most important equipment in power system. With the presence of nonlinear magnetic branch the differential protection of transformer has a theoretic defect. As a result, protection misoperation occurs frequently and becomes one hidden trouble for the power system safety.In fact, the primary and secondary sides of transformer are connected through magnetic circuit other than electrical circuit. This situation results in a theoretic defect in any protection scheme basing on the kirchhoff current law. To make up this defect some methods of restraint are involved in the protection. However, due to the variety of system parameters, magnetic characters of core and the effect of current transformer on the secondary current, those protection scheme can not guarantee a complete precise restraint, including the discrimination method basing on the characteristic of wave form, harmonic discrimination involving the second-harmonic restraint, blocking method basing on the theory of discontinuous angle, etc. At present some new theories and techniques adopted in differential protection, such as artificial neural network and discrimination method basing on the magnetic flux characteristic, still wait for further consummation.For the seasons above, it is necessary for us to investigate to a deeper extent on the theory and practice of transformer differential protection, to study the impact of nonlinear magnetic characteristics on protection performance, so that we can get a better application.Basing on a deep acquaintance of the theory of transformer differential protection, the dissertation emphasizes the study of the discrimination method basing on the characteristic of waveform, harmonic discrimination involving the second-harmonic restraint, blocking method basing on the theory of discontinuous angle, establishing models for analysis and searching for the characteristics of inrush current through power system simulation. This simulation cases include energization of no-load transformer and full-load transformer, different types of external short circuits. The analytic aspects are the waveform, content of secondary harmonic and the value of discontinuous angle, which are obtained through the current transformers under different switching phase angles. These results about the influence on the performance of differential protection are useful to the design of protection scheme and setting of the threshold, thus have certain significance for the performance of the differential protection of transformer in power system.