Research On Digital Differential Protection For Transformer Based On Fuzzy Logic Theory

With the development of electrical industry, more and more large power transformers will be widely used in power system. Large power transformers belong to a class of vital and very expensive components in electrical power system. Accordingly, high demands are imposed on power transformer protective relays. Differential relay is the key protection for power transformer. So far, the key problem of transformer differential protection is still how to precisely identify the inrush of the transformer. Among those algorithms currently being used, the second harmonic current restraint method and dead angle method and others have both advantages and shortcomings. To meet the demand of speediness of transformer relaying protection, it is important to search a new method which can discriminate the inrush current and short current faster and more reliably.Based on the brief analysis of the power transformer inrush currents and those known methods of discrimination between inrush current and internal fault current of transformers, a new method based on fuzzy logic theory is proposed, which not only can discriminate inrush current from internal fault current, but also can identify the other abnormal states through information searching. The theory analysis and the simulation results show that the method is characterized with quick identification, reliable operation and less affected by noise interfere.In this thesis, the influence of decaying DC component on the new criterion and Fourier algorithm is analyzed. Then a novel algorithm for decaying DC component filtration is proposed. A complete microprocessor-based transformer differential protection scheme based on the new method for fault identification is presented. Transformer differential protection program is designed by using C language. The new method for fault identification is validated by the simulation of switching on the unloaded transformer and the fault transformer.The paper also points out the direction of differential relay development.