Research On Relay Protection Of The Power Transformer

With the rapid progress of social science and technology, the continuous development of human civilization, the power industry has entered a new stage which takes ultra high pressure and high intelligence as characteristics. In the new stage the reliability, stability and security of power supply system becomes more and more important. With the rapid development of the electric power industry, the large power transformers have been widely used, which have become an indispensable and irreplaceable electrical equipment in the power system. If the fault of transformer can not be processed quickly and correctly, it will bring serious influence to the whole system. Therefore we have put forward high requirements of relay protection for power transformer.In this paper, there is a in-depth research on the differential and backup protection of power transformer. First of all, it does a study on principles and related fields of the power transformer differential and backup protections, and builds simulation models on SIMULINK platform according to the principles of protection and the need of study. One is simulation model of double winding transformer system with dual power supply, the other is simulation model of three windings transformer system with multiple outlets. By analyzing the simulation results, it is proved that the two simulation models can simulate the process of inrush current occurs due to the transformer no-load closing and fault current occurs due to the internal and external faults of transformer, which provide a simulation platform for the study.Subsequently, aimed at the limitation of traditional inrush current braking method in differential protection, this paper makes inrush current and fault current to be decomposed into 5 levels selecting wavelet ’ db4’ on the basis of deep study of wavelet transform. Then observe and analysis the characteristics of the performance of magnetizing inrush current in some kinds of situations and fault current under various faults in transformer after having wavelet transform decomposed. Through analyzing the changes of wavelet coefficients in different levels, a new method to identify the inrush current and internal fault current based on the distribution of wavelet coefficients is proposed. The method is based on the wavelet coefficients obtained from the second level of the wavelet transform, then calculates the variance of the variance and set a reasonable criterion which can distinguish between magnetizing inrush current and fault current. The simulation results show that the method can quickly and accurately distinguish the magnetizing inrush current and fault current, overcoming the shortcomings of the traditional method, and preventing the differential protection malfunction effectively, which has a application prospect.On the other hand, a backup protection criterion scheme is presented which using by phase difference of fault sequence component mainly on solving the insufficient sensitivity of transformer over-current protection in backup protection and problems of complicated coordination of each side protection. This scheme extracts fault component by using redundancy of line information collection volume in digital protection, and makes analysis and compare on phase of fault component in each side line and each outgoing line of this side, according to the characteristic that the fault component will appear in the fault state. It verified by simulation that the method can correctly judge the position of fault and solve the problem of low sensitivity and complex coordination among transformer backup protection, which ensure the relay protection device to make a correct action, and the transformer to have a safe and reliable operation.