The main purpose of this paper is to present some metrological aspects of identification and grouping of harmonics and interharmonics and calculation of total harmonic. The Virtual Instrument for identification of distorted signal spectrum is described and the demonstration may take place at the presentation of the paper at the oral or poster session. We define the total harmonic and interharmonic distortion coefficient and analyse its sensitivity to variation in fundamental and interharmonic frequency and sensitivity to incompleteness of samples for five different cases: rectangular window, triangular, Hanning, Hamming and Blackman windows. The virtual test signal was selected arbitrarily. This signal was processed first by the virtual harmonic analyser, in such a way that all components were identified without any errors, and then the signal was analysed according to the requirements specified in IEC 61000-30-4 draft standard, in which also Harmonic Groups (HG) and Interharmonic Groups (IHG) are defined.
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