Based On Wavelet Transform Electrified Railway Harmonic Analysis

In recent years, there is a rapid development in electrical railway in China. However, the harmonics it generates also requires more attentions. Harmonic analysis can be used to get the harmonic content and then provide data for some further work such as harmonic elimination. Thus it is necessary to implement harmonic analysis in electrical railway. The harmonic content of electrical railway will be different when the electrical locomotive runs in different state. So when the harmonic changes, it would not be able to get accurate result using traditional FFT method.Wavelet Transform (WT), as a hot issue discussed in recent years, has a good performance in both frequency and time domain analysis. Wavelet Packet Transform (WPT) has a better performance in frequency domain analysis because it can divides the frequency domain into small parts, thus it is more suitable for harmonic analysis. So in this thesis, WPT is adopted to implement electrical railway harmonic analysis.Then some improvements are made to WPT to make it perform better in harmonic analysis. Firstly, the order of frequency bands is corrected. Secondly, some changes are made to the method of dividing frequency bands to make sure that the frequency of every order of harmonic is at the centers of the sub-frequency bands. Furthermore, a method to reduce the wavelet aliasing is adopted to improve the accuracy of WPT. Besides, a method to reduce the calculation amount is also presented.In order to prove the performance of WPT in time-varying harmonic analysis, a model of electrical locomotive is built in Matlab with Simulink toolbox. Then the sampled data of the current which contains harmonics are obtained. After that harmonic analysis is implemented and the result shows that the improved WPT algorithm presented in this paper has a satisfying time and frequency domain analysis ability when deals with time-varying harmonic analysis.