engLobos, TRezmer, JSikorski, TSchegner, PTime-frequency analysis of non-stationary signals in power systemshttp://zet10.ipee.pwr.wroc.pl/record/16/files/Representation of signals in time and frequency domain has been of interest in signal processing areas for many years, especially taking in the limelight time-varying non-stationary signals. The standard method for study time-varying signals is short-time Fourier transform (STFT) that is based on the assumption that for a shorttime basis signal can he considered as stationary. The spectrogram utilizes a short-time window whose length is chosen so that over the length of the window signal is stationary. Then, the Fourier transform of this windowed signal is calculated to obtain the energy distribution along the frequency direction at the time corresponding to the canter of the window. The crucial drawback of this method is that the length of the window is related to the frequency resolution. Increasing the window length leads to improving frequency resolution hut it means that the nonstationarities occumng during this interval will he smeared in time and frequency. This inherent relationship between time and frequency resolution becomes more important when one is dealing with signals whose frequency content is changing rapidly, Baraniuk (l),Choi and Williams (2).2008-03-06