Research On Transformer's Differential Protection And Recognition Of Inrush Current

The power transformer is one of the most important equipment in power system, it plays the role of energy and voltage conversion function in the whole power system, and its safe operation directly related to the stability of the entire power system working continuously. However, the average correct action rate of transformer is much lower than the large-capacity generators and high voltage line, one of the important reasons for this situation is the transformer inrush identification.At present, the second harmonic restraint principle and the intermission angle principle are mainly used to distinguish excitation current. However, with the transformer manufacturing technology and the material improved, modern transformer is saturated easier, the second harmonic component reduced significantly, and even less than 10%.Under these conditions, differential protection may maloperation when transformer is charging, and second harmonic restraint ratio will be difficult to select. The intermission angle principle, the saturation of current transformer (TA) will led to disappearance of the intermission angle. In order to restore intermittent angle, complexity of the protection hardware will be increase .This paper from the waveform characteristics of inrush current and features of the magnetic flux in the frequency domain to identifying inrush current and fault current. And verifying identification methods through the data of the Dynamic Simulation Laboratory in North China Electric Power University, Results of dynamic tests show that the proposed method is feasible and correct.