Determination of frequency variation effect on DFT-based algorithms for harmonic and flicker measurements

Harmonic distortion and voltage flicker are among the major causes resulting in growing concerns about electric power quality, and DFT-based algorithms have been widely used in their instrumentation and measurement. Though the pit-falls of DFT had been noticed, the effects of system frequency variation on the accuracy of the measurements have never been clearly determined. In this paper, to facilitate the accurate application of DFT under the situation of ever-changing system frequency, the effects are theoretically derived from the harmonic and flicker computations for the case of constant sampling time interval and truncating duration. The results show that the discrepancies in amplitude of individual harmonic and in total harmonic distortion factor over the normal system frequency variation range of 60±0.1 Hz are trivial, but the errors in harmonic phase and flicker are much more serious. In addition, different harmonic-distorted and flicker-modulated waveforms are simulated, and the effects of the system frequency variation on their measurements are evaluated. The results accord with those in theory.