| 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.Owing to relay protection acts promptly, extracted signals are including abundant harmonious, high-frequency and damping non-periodic components. Although some digital filtering measures are also adopted, it is an inevitable error because the damping non-periodic components is considered as steady direct current or line component to deal with on most occasions. And that, 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. Compared with the traditional Fourier algorithm, it is more reliable and sensitive due to it can consumedly filter damping non-periodic component, noises, and non-integral harmonious.It is very important and difficult for investigator to identify magnetized inrush currents and internal faults for large transformers relay protection. Firstly, this thesis analysis the mechanism of magnetizing inrush currents in detail, and get the difference in physical characteristic between inrush currents and internal faults. Subsequently, by studying the waveform features extracted by wavelet transform, this thesis proposes a method that can discriminate inrush currents from internal faults. The extensive EMTP simulation results have verified the method is a reliable and feasible way, and can improve sensitivity of computerized protection devices when slight faults happen, also have drastically solved a problem that protection devices act in delay even in error as an internal fault transformer is closed emptily.
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