Methods for detection of sub-harmonics in power systems

With a wide range of power electronics-related applications in power systems, harmonic currents are increasing at an alarming rate which has greatly deteriorated the power quality in electrical power networks. Moreover, some of electronic controlled equipments used in power systems, such as cycloconverters, produce sub-harmonics, a type of waveform distortion, which can severely degrade the power system performance. Therefore, they must be closely monitored. Moreover, Fast Fourier Transform cannot accurately analyze waveforms containing sub-harmonics because the synchronization of the sampling procedure to subharmonics is practically infeasible. The detection of subharmonics requires a different approach from that used for harmonics analysis. In most analysis methods the voltage waveform is expected to be a pure sinusoid with a given frequency and amplitude. Standard tools of harmonic analysis based on the Fourier transform assume that only harmonics are present in the investigated signal and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics and sub-harmonics can be variable and very long. Two novel approaches to analyze non-stationary signals are shown in this paper. The first is the “Root-Music” harmonic retrieval method that is an example of high-resolution eigenstructure-based method, the second is a numerical method based on moving average.