The paper presents the S-transform (ST) technique as a new tool for power quality indices measurement. A key feature of the ST is its accurate time-frequency (amplitude and phase) domain. With this feature, it is possible to redefine the calculation of the power quality indices in the ST domain based on the harmonic content of the signals. In addition, using the time information provided by this technique, the definitions of the power quality indices can be extended to be instantaneous. The accuracy of the instantaneous versions of different indices in the ST domain is verified on a number of simulated and real power quality signals.
Keywords: power quality indices, S-transform, non-sinusoidal signals, stationary signals, non-stationary signals