Research On Differential Protection For Transformer Based On Fuzzy Set Treory

Power transformers are key components in power system, its safety and stability are both important and necessary to power system operation, especially large transformer among modern power grids. Accordingly, high demands are imposed on power transformer protective relays.Differential protection has long been the key protection for power transformer. How to improve its rapidity,sensitivity and reliability is a popular research field.So far, the core problem of transformer differential protection is still how to precisely identify the magnetizing inrush current. Inrush current identification methods currently being used, such as the second harmonic current restraint principle and dead angle method, can't very well meet the requirements of modern large power transformer protection. In order to adapt to the requirements of large transformer protection, it is important to search a new method which can discriminate the inrush current and short current faster and more reliably.The paper firstly introduces the present and future development of the transformer protection. For the main problem, the discrimination of the inrush current, the methods current used is summarized roundly. Secondly based on the depth analysis and research on the formation mechanism of magnetizing inrush current, establishing simulation model of transformer inrush current and internal fault used EMTP/ATP program.Based on the analysis of existing methods of discrimination between inrush current and internal fault current, a new method based on fuzzy set theory is proposed,which combined with the Proximity principle of Fuzzy Pattern Recognition and the principle of Information Retrieval. The simulation results show that it not only can discriminate inrush current from internal fault current, but also can identify the other abnormal states through information searching.Finally, a complete differential key peotection scheme based on the new method for fault identification is presented, and the fault treatment's process of the main differential protection scheme is designed.