A contribution for the measuring process of harmonics and interharmonics in electrical power systems with photovoltaic sources

The main standards for quantifying waveform distortions in power systems are based on the Discrete Fourier Transform (DFT) through the use of a rectangular time-window of fixed length. However, limitations and imprecisions of the DFT related to the analysis of time-varying signals, typical of environments with renewable sources, have been reported in the literature. This study presents a methodology for identifying the most appropriate time-window length to quantify the emissions of harmonics and interharmonics, considering time-varying signals. The methodology in question is substantiated on the calculation of the harmonic and interharmonic distortions using the DFT according to IEC 61000-4-7 and employing the spectral components acquired from the application of the Prony method and Kalman Filter. It is tested in view of the current signals measured in a real grid-connected photovoltaic installation. The methodology proposed in this work improves the accuracy of the harmonic and interharmonic quantification process using DFT by selecting a proper time-window length, and in addition, identifies the error attributed to the use of a 200 ms time-window length, as premised by the IEC 61000-4-7.