Research On Differential Protection Of Power Transformer Based On Fuzzy Multi-Criteria

With continuous development of the power system, large-capacity, the EHV power transformer is more and more used in system operation, which poses a higher demand for transformer protection. Differential protection, which is a simple and reliable protection mechanism, has long been the core of transformer protection.So far, the key problem of transformer differential protection is still how to precisely identify the magnetizing inrush currents of the transformer. Among those algorithms currently being used, the second harmonic current restraint, waveform symmetry and dead angle method and others have both advantages and shortcomings. To meet the demand of transformer relaying protection, it is important to search for a new method which can discriminate the magnetizing inrush current and short current faster and more reliably.Based on the brief analysis of the power transformer magnetizing inrush currents and those known methods of discrimination between magnetizing inrush current and internal fault current of transformers, the paper has reached a best algorithm of second harmonic, discussed the relationship between symmetry degree and the range of symmetry and proposes the approaches of digital filter to measurement the first derivatives. On the basis of in-depth analysis of above three criteria, a new method based on fuzzy logic theory is proposed. The new method integrated a number of criteria and gave full play to the various criteria for their own advantage. It discriminates the magnetizing inrush and fault current not only quickly, but also dependably.This paper discussed in detail the features of ratio-restrained differential protection, analyzed the braking current selection method, and draw some useful conclusions. The paper made a way of coordinating the fault component ratio differential protection with the steady ratio differential protection to improve the overall performance of the transformer protection. In this way, it can overcome the disadvantage that the steady ratio differential protection lacks sensitivity when minor faults occur in the internal region. For the features of fault component ratio differential protection, the paper discussed the feasibility of applying it to transitional fault and presented the method to prevent incorrect operation.Finally, the paper used MATLAB language to compile program to simulate algorithm of protection and achieved satisfactory results.