The Research Of Transformer Differential Protection Based On Wavelet Transforms

As EHV power network is developing rapidly, large capacity power transformers are used widely in the transmission system and distribution system. Therefore, high performance and high reliability of power transformer protection are required urgently. At present, the main protection of internal faults of the power transformer is the differential protection. When the unloaded power transformer is switched on the power system or in the process that a fault, outside the protected zone, is cleared, the magnetizing inrush current will usually produced. The differential protection relays will maloperate. So distinguishing between magnetizing inrush current and fault current correctly is the key to ensure transformer differential relay to operate credibly.In recent years, the wavelet transform analysis method has been developed rapidly. As the wavelet transform possesses the properties of multi-scale analysis and time-frequency domain localization, it is particularly well adapted to transient sharply changing signals and slight changing signals, and to extracted characteristics of these signals. At present used wildly Fourier algorithm is affected by non-integral harmonious that integrates digital filtering and protection algorithm. This paper studies digital filtering and protection algorithm for peek value of base frequency based on Wavelet Transform.In this paper the transient course after an external fault of transformer is cleared is analyzed from the variation of flux .It is proved from the method of simulation that the transformer differential protection and the current protection possibly maloperates due to the recovery surge. Base on the MATLAB simulation, this thesis chooses db wavelet that adapts to the analysis of power system signal, and uses the wavelet transform to identify the magnetizing inrush current and inner fault current, and on the base of intermission angle principle. A new criterion is raised,which pierces through the traditional method with accurate measurement of intermission angle,and analyses the wave by wavelet transform. This thesis proposes a method that can discriminate inrush currents from internal faults, The simulation studies have verified this methods is an effective way.