New Power Quality indices based on S-transform for non-stationary signals

The changing power system regulations and increased use of power electronics based equipment, has made power quality (PQ) a highly important issue. The conventional Fourier transform based PQ indices work well with stationary disturbances, but fail to indicate or quantify the PQ issues related to non-stationary disturbances. In order to consider the effects of PQ disturbances, that occur for a short duration and contain frequency components other than fundamental, a method based on time-frequency distribution, called S-transform, is used in this paper. S-transform is used to define two PQ indices, instantaneous form factor, IFF(τ) and instantaneous frequency variation index, IFI(τ), to indicate and quantify PQ. Simulated PQ signals for different short duration PQ disturbances like; voltage sag, voltage swell, momentary interruption, harmonics, impulsive transient and oscillatory transients with different possible variations are used to test proposed PQ indices. Averages and peak values of proposed indices are also computed to justify their indicating capabilities.