Wavelet analysis approach is presented in this paper for the automatic detection and analysis of voltage sag in power systems with noise and without noise. Wavelet analysis is used to process the voltage waveform for detection and better estimation of the time-related parameters of a voltage Sag in voltage supply, The method proposed, which can be processed in real-time, has been simulated for a low voltage distribution system, showing the potential in data reduction and accurate characterization of voltage Sag in power systems .