Research On Differential Protection Of Power Transformer Based On Wavelet Theory

The major drawback of the differential protection of power transformer is the possibility for false tripping caused by the magnetizing inrush current during transformer energization, so how to discriminate inrush current from internal fault current is always a key problem in protecting power transformer.Since a magnetizing inrush current generally contains a larger second harmonic component than that of internal fault, conventional transformer protection systems are designed to restrain during inrush transient phenomenon by sensing this large second harmonic. However, second harmonic components in the magnetizing inrush currents tend to be relatively small in modern large power transformers because of improvements in the power transformer core material. Moreover, the second harmonic component may also be generated during internal faults due to current transformer saturation or the distributive capacitance in a long extra high-voltage transmission line to which the transformer may be connected. Relay with second harmonic restraining technique may cause mal-operation and delay the operate time. To enhance the reliability of differential protection, it is very essential to research some new protective theory and new protective technology.This paper summarizes some typical methods of discrimination between transformer inrush current and fault current, and introduces their advantages and disadvantages. As a new signal processing technology, wavelet transform has more broad application prospects in transformer protection. this paper analyzes the complexity of transformer inrush’s transient process and the transient characters of transformer internal fault, deeply studies generation mechanism and influence facts of energizing inrush and studies the simulation model of transformer turn to turn fault, which provides theory basis for constructing the transformer model and researching new protection method.A wavelet-based signal processing technique is an effective tool for power system transient analysis and feature extraction. This paper presents a new scheme for power transformer differential relay using wavelet transform energies. The electric power system model is constructed by MATLAB and db4 is chosen as a mother wavelet to analyze kinds of magnetizing inrush currents and internal fault currents. Inrush currents and internal fault currents have different wavelet energy distribution because of the different generation mechanism between them. By extracting d4 (the detail of level 4) and calculating the change rate in wavelet energies of d4, a reasonable criterion is concluded. The simulation results show that the algorithm can distinguish between inrush and internal fault currents accurately in one cycle.