Abstract:This paper presented a new detection method of voltage sag feature value, which was extracted from the module matrix of modified Stransform. In order to get the optimal performance, the parameter related to timefrequency resolution was determined on the basis of the flatness and minimal transition time. By means of the optimal timefrequency resolution, the amplitude of voltage sag duration less than 2 cycles could be detected accurately. The difference vector of base frequency was employed to detect sag duration so as to overcome the flaws of exiting Stransformbased methods, whose effects were unsatisfied when there were strong noise and no amplitude abrupt. A comparison between Stransformbased method and modifiedStransformbased method was made, and the results showed that the later was superior to the former in magnitude detection and duration localization.