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Techinal Guide to Harmonics with AC Drives

Guide to Harmonics 
with AC Variable Frequency Drives

Guide to Harmonics with AC Drives

ABB Guide to Harmonics with AC DrivesContents

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 non-linear 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). 

Plant with converter load, mains transformer and other loads.
Figure 2.1 Plant with converter load, mains transformer and other loads.

The line current of a 3-phase, 6-pulse 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 6-pulse 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.

The harmonic content in a theoretical rectangular current of a 6-pulse rectifier.
Figure 2.2 The harmonic content in a theoretical rectangular current of a 6-pulse 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. 

The total current as the sum of the fundamental and 5th harmonic.
Figure 2.3 The total current as the sum of the fundamental and 5th harmonic.

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