Studies On Extracting Characteristic Values Of Transformer Protection By Wavelet Transform

Power transformers are an important equipment in power system, the safe operation of power transformers has much effect on the continuous running of power system. So it's valuable to research the algorithms of relay protection for transformer. Microcomputer-based protection uses the computer's advanced signal processing function, which gives help in enhancing the protection algorithms' operational speed and precision. This paper mainly focuses on the implement of wavelet to the Microcomputer-based protection for power transformer, especially to extract some useful information for the protection algorithms. A wavelet-based digital filter is designed in this paper, and the applications of wavelet to the protection for transformer are discussed. Aiming at extracting harmonic components of signals, a filter progress is designed. During the progress, the technology of sampling rate alteration worked as an aid in dividing different frequency signals. The wavelet filters adopted in the progress are the conjugate quadrature filter bank(CQF) and the Daubechies (Daub)based filter. Analysis and comparison of the results are made when different wavelet filters work. For example, to the frequency splitting between fundamental wave and second harmonics, the filter Daub6 and CQF12 do well in extract the fundamental wave, whereas the filter Daub5 and Daub6 do well in extract the second harmonics. To test the filter's effect, the half-wave Fourier algorithm is used to calculate the amplitudes of harmonics. Through the filter progress, the errors of results are reduced compared with that are before the filter. The errors are less than 10%.An algorithm that can extract harmonics' amplitudes using Morlet wavelets is put forward. Compared with the full-wave Fourier algorithm, the algorithm proposed in this article is more stable when the frequency of electric network flutters. Meanwhile the algorithm has the rapidity of giving out theresults in one to three periodic times. This algorithm works effectively in extracting harmonics' amplitudes of currents when transformer occurs exciting in-rush. A visual signal analyzing software that serves in transformer substation automation system is developed based on the programming language of Borland Delphi 6.0. The function of the software can be an aid to the field engineer in judging the working condition of transformers.