Interharmonic currents from a Type-IV wind energy conversion system

•The generation of interharmonic emission, from Type-IV wind turbines, is for the first time explained and validated experimentally based on the classical model of the frequency conversion between two AC systems.•A novel graphical method for analyzing time- and frequency-varying interharmonics is proposed: a plot showing correlation between different frequency components to visualize interharmonic emission, even in case of limited frequency resolution in the individual measurements.•The observed increase of the magnitude of interharmonic emission with increasing power production has been explained from the model for the interharmonics emission.•The ratio of the magnitude of pairs of interharmonic frequencies, as predicted by the model, has been verified by the measurements.

This paper presents, for the first time, the verification of the classic model for the origin of interharmonic emission from a frequency converter, for a Type-IV wind turbine by using long-term measurements. Interharmonic variations in magnitude and in frequency are due to the difference between the generator-side and grid-side frequency of the full-power converter. The model verification consists of three parts: correlation between frequencies; relation between magnitude of interharmonics and active-power production, and relation between magnitudes of different interharmonics. The measurements are in agreement with the model predictions. The paper also introduces a novel graphical correlation method to extract information on interharmonics from long-term measurement series.