Guide to Harmonics
with AC Variable Frequency Drives
Contents
Introduction
General
This guide continues ABB's technical guide series, describing harmonic distortion, its
sources and effects, and also distortion calculation and evaluation. Special attention has
been given to the methods for reducing harmonics with AC drives.
Basics of the harmonics phenomena
Harmonic currents and voltages are created by nonlinear loads connected on the
power distribution system. Harmonic distortion is a form of pollution in the electric
plant that can cause problems if the sum of the harmonic currents increases above certain
limits.
All power electronic converters used in different types of electronic systems can
increase harmonic disturbances by injecting harmonic currents directly into the grid.
Figure 2.1 shows how the current harmonics (ih) in the input current (is) of a power
electronic converter affect the supply voltage (ut).
Figure 2.1 Plant with converter load, mains transformer and other loads.
The line current of a 3phase, 6pulse rectifier can be calculated from the direct output
current by using the following formula.

, where 

the total RMS current and 

direct current output from the rectifier.
(valid for ideal filtered DC current) 
The fundamental current is then
In a theoretical case where output current can be estimated as clean DC current, the
harmonic current frequencies of a 6pulse three phase rectifier are n times the fundamental
frequency (50 or 60 Hz). The information given below is valid in the case when the line inductance is insignificant
compared to the DC reactor inductance. The line current is then rectangular with 120° blocks. The order numbers n
are calculated from the formula below:

where 

The rms values of the harmonic components are:
and the harmonic components are as shown in Figure 2.2.
Figure 2.2 The harmonic content in a theoretical rectangular current of a 6pulse
rectifier.
The principle of how the harmonic components are added to the fundamental current is
shown in Figure 2.3, where only the 5th harmonic is shown.
Figure 2.3 The total current as the sum of the fundamental and 5th harmonic.
