Interharmonics analysis using Fourier Transform and virtual instrumentation

Interharmonics represent those electrical signals whose frequencies are not integer multiples of the supply fundamental frequency. They correspond to electrical components that are not synchronized with the systems fundamental frequency; due to this fact, they have a more destructive impact than harmonics on the elements of the power system. International organizations like IEC and IEEE International Force Task Committee have defined the terminology, limits and measurement guidelines for interharmonics; even though, there still are difficulties in their detection and measurement with an acceptable accuracy. The paper presents the mathematical basis of Fourier transformation and some general aspects regarding a new algorithm that authors propose for interharmonics analysis. The classic Discrete Fourier Transform is adjusted using windows with different widths accordingly to the interharmonics spectrum. The paper also contains a numerical example and the description of the virtual instrument developed specifically for interharmonics detection and evaluation.