VFD harmonics injected into grid: FAQ
Working in drives business for over 15 years I have heard many questions related to VFD harmonics. Some of them come repeatedly signalling that they may not be well understood. In this post we list some of them including brief answers.
What is the drive’s behavior/character with respect to harmonic injection?
The short answer is:
- DFE VSI drives: Harmonic current source
- AFE VSI drives: Harmonic voltage source
- CSI PWM / LCI drives: Harmonic current source
For more detailed answer purchase our publication on harmonics.
What is the phase angle of harmonic currents?
Drive manufacturers often provide just a table with harmonic amplitudes. The phase angles may not be provided. But your network software tool (e.g. DIgSilent PowerFactory, ETAP, Neplan) needs both amplitude and angle. Where do you get that information?
How to model VFD with regards to harmonic injection?
Is the input transformer included in the VFD harmonic modelling?
Next frequent question is addressing the point (node) where the harmonic currents or voltages are declared. Is it the input of the VFD or the primary (grid) side of the converter transformer?
Normally, harmonics are provided on the primary side of the transformer, i.e. the entire drive system (input transformer + VFD + motor) is a black box. The user does not need to care about modelling of the input (phase shifting) transformer and corresponding harmonic cancellation inside the transformer. Instead, he receives the harmonics injected into the grid. A confusion may occur in case of a detailed network harmonic study. The party responsible for the modelling often introduces the input multi-winding transformer although the harmonics from the VFD manufacturer apply “upstream” of the transformer.
Where do non-characteristic harmonics come from?
What system parameters affect the harmonic distortion?
There are several parameters influencing the harmonic distortion. Some of them have major impact while others affect the harmonic distortion just marginally.
The full article helps you to understand what these factors are and how important role they play.
See section “Our publications”:
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