Time frequency distributions applied to the analysis of fluctuating harmonics

This paper looks at the use of time-frequency distributions to determine the nature of current harmonics drawn by electrical equipment connected to the mains supply. The growth in the use of distorting loads, in particular switched mode power supplies, has led to regulation in an attempt to limit the current harmonics drawn from the mains. When the load is steady state, the harmonic content can be readily determined by use of a Fourier transform (FT). Loads, which change with time, give rise to harmonics which fluctuate in amplitude and phase. It is the analysis of this type of non-stationary waveform that is the subject of this paper. In the example given, it is shown that different kernels can be used to give high resolution results with reduced cross-terms. Signals with more complex fluctuations would generally give rise to cross-terms when using the distributions presented. In such cases alternative kernels are necessary for cross-term suppression. It may also be possible to use adaptive kernels