Study On Technology Of Inrush Current Identifying And Restraining For Transformer

Power transformer is one of the key power equipments, being a transform hub in the power system, whether it operates well or not, and whether it has perfectly protective measures when under the fault conditions, and whether it can operate reliably when it is disturbed, is directly affect the safety and stability of the whole power grid. Therefore, it must be outfitted the high-performance and high-reliability protective relaying devices. With the increasing of system voltage level and the augment of transformer capacity, the dependent upon the transformer relaying protection become up and up. With the development of technology of relay protection in our country, the research and actual operation level of the transformer relaying protection is substantial, but if compare with line protection and bus protection, the correct action rate of transformer relaying protection is still low. The fact reflects that there are still some problems crying out for solutions in the transformer relaying protection. For this reason, this paper finished some relevant study of how to identify and how to restrain the inrush current, which are two key problems of the transformer relaying protection.The mechanisms of production of the inrush current when energizing no-load transformer, over excitation and recovery inrush current are derived by theoretical analysis. Simulation models for these three circumstances are established by PSCAD/EMTDC which is electromagnetic transient simulation software for the power system. Characteristics of these three kinds of inrush currents are studied by the simulation results, and the real reason, which causes the differential protection false tripping when the external fault is cleared, is found out.Based on widely collected and referred to relevant documents and materials, studied the existing methods of identifying and restraining inrush current, advantages and disadvantages of these methods are pointed out. Furthermore, finding out that for modern transformer protection, only use one kind of identifying method can not absolutely right to identify the inrush current.Base on the detailed analysis of reasons and characteristics of overcurrent in the differential circuit, a new criterion of the transformer differential protection based on fuzzy information retrieval is proposed which the fuzzy theory and information retrieval is adopted. The difficulty of this method is how to estimate the excitation inductance when transformer core is saturated. Therefore, a method, using the equivalent instantaneous inductance to estimate the excitation inductance when transformer core is saturated, is proposed. Using the PSCAD/EMTDC, electromagnetic transient simulation software, various kinds of working conditions are simulated, and the simulation results prove this algorithm is right and this algorithm can overcome problems like misjudgment and delaying action under some working conditions.In view of these facts that inrush current is harmful to the transformer relaying protection and power system, a simple and effective restraining method, implementing capacitors mounted on the low voltage side of the transformer, is proposed. Simulation results show that this method can effectively restrain inrush current whenever the transformer is put into operation with different remanence.