Instantaneous-frequency measurement for noisy nonstationary distorted signals

A correct application of the interpolated short-time Fourier transform (STFT) to time-varying electrical quantities is made. The authors show also how the suggested technique can be implemented in a dedicated digital instrument to perform accurate time-frequency analysis of the instantaneous harmonic contents of network load currents, typically nonstationary distorted signals. Original interpolation formulae, derived from the windowing theory, have been developed with the aim of reducing the incorrect spectral estimates, due to leakage effect during instantaneous frequency changes. High levels of accuracy, flexibility and efficiency have been attained by applying the proposed algorithms, also when only noisy input data are available. Several simulations confirm the effectiveness of suggested technique.