Investigation of Input Harmonic Distortions of Variable Frequency Drives

Knowledge of harmonic distortions at the input of Variable Frequency Drives (VFD) as a function of certain variables is essential for the harmonic modeling and evaluation. IEEE std. 519-1992 proposes current harmonic spectrums and current harmonic distortions for the drive input harmonics. However, field engineers see large current harmonic distortions at the input of light-loaded drives, which are far away from the IEEE proposed values. In this paper input harmonics of a single VFD measured in a power grid and in a laboratory testing circuit are analyzed. Harmonics measured at the input of 55 VFDs in a large industrial distribution system are also investigated. It is found that the drive loading condition and the system impedance at the drive input are two factors determining the input current harmonic distortions of VFDs. Current magnitudes for higher order harmom´cs above the 17^th are small and do not change much. However, the 5^th harmonic current magnitude varies significantly with the drive loading conditions especially for voltage-source type VFDs. This paper proposes a modification of the 5^th harmonic current spectrum recommended by IEEE std. 519-1992 for 6-pulse drives. A simulation using a fixed drive load factor verifies the influence of the system impedance on the drive input harmonic distortions.