How do system parameters impact the grid harmonic distortion?
VFD as a non-linear load produces harmonics that are injected into the grid and cause harmonic distortion. The level of harmonic distortion depends on the type of VFD, specifically the rectifier topology. However, it is also strongly affected by the parameters of the system and particularly the grid. Subject of this post is to briefly explain how the system parameters impact the grid harmonic distortion.
System parameters
Grid harmonics can be assessed either by a simplified harmonic calculation or by performing a network study [1]. The harmonic calculation usually assumes a linear inductive network impedance [2]. For the sake of easy understanding we will assume such inductive grid in our explanation. In this system the key parameters are:
- Grid impedance (reactance)
- Transformer short circuit impedance
How do these system parameters affect the current and voltage harmonics?
Impact of system parameters on grid harmonic distortion
Before jumping into the effect of system parameters on harmonics we just recap our base. We assume a (purely) inductive grid, i.e. grid impedance linearly increasing with frequency. The VFD is connected to the grid through an input transformer (usually step down). The VFD is a voltage source inverter and the rectifier consists of a diode bridge with certain pulse number (typically 12-pulse, 18-pulse, 24-pulse etc). Of course, other VFD topologies exist as well, but for the explanation we focus on the “mainstream” technology.
From the grid perspective a diode rectifier feeding a dc bus has a character of harmonic current source. This is very important fact in the understanding of the phenomena. It means that current drawn from the grid is not purely sinusoidal but contains certain distortion. The distorted signal can be decomposed into the dominant component of fundamental frequency and other (smaller) components of frequencies being integer multiples of the fundamental frequency (⇒ harmonics). The dominant harmonic orders are determined by the rectifier pulse number and transformer winding connection (e.g. phase shifting). The amplitudes of the harmonic frequencies do not depend on VFD design only but are impacted by system parameters.
At this stage it might be useful to depict a simplified equivalent circuit diagram for grid harmonics produced by a VFD (focus on harmonic current source).
Impact of grid short circuit power
Short circuit power indicates the strength of the grid. It is inverse proportional to the grid impedance. High short circuit power means low short circuit impedance and vice versa. Strong grid can better cope with harmonics. Same magnitude of current harmonic causes less voltage distortion (product of injected harmonic current and corresponding grid impedance). This is widely understood and somehow intuitive. Less obvious might be the fact that current harmonics slightly increase when the grid short circuit power goes up. However, this increase is rather marginal. Relevant for the current harmonics is the sum of transformer impedance and grid impedance. Since the transformer impedance is normally much larger (more dominant) than the grid impedance, the effect of grid impedance on the magnitude of current harmonics is limited.
Impact of transformer short circuit impedance
Input transformer acts with its short circuit impedance as a smoothing reactance for harmonics (equivalent effect like an input line reactor). The higher the short circuit impedance the more smoothing and the less harmonics. Transformer impedance has positive effect on both voltage and current harmonics. The higher the transformer impedance the lower the harmonic distortion.
Summary
Transformer short circuit impedance helps to reduce both voltage and current harmonic distortion at the point of connection/point of common coupling. Increasing grid short circuit power reduces the voltage harmonics while current harmonics marginally increase. Nevertheless, high grid short circuit power (strong grid) is positive from harmonic point of view. Several standards even scale the limits of harmonic current with the ratio of grid short circuit power and VFD installed power (meaning that for a stronger grid the standard allows higher current harmonics).
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References
[1] Harmonic calculation versus network harmonic study, https://mb-drive-services.com/harmonic-calculation-versus-network-harmonic-study/
[2] What does purely inductive network actually mean? https://mb-drive-services.com/what-does-purely-inductive-network-actually-mean/
