Page 119 - Uninterruptible Power Supplies

Source: Uninterruptible Power Supplies and Standby Power Systems

Chapter
4

Harmonic Distortion of the

Supply

Acknowledgment
This text originates from Chap. 10 of a book titled Uninterruptible
Power Supplies published in 1992 by Peter Peregrinus Ltd. on behalf of
the Institution of Electrical Engineers of London. It has been modified
in parts but much of the material is unchanged and is published here
with the permission of the Institution of Electrical Engineers.

Nonlinear Loads and Current Distortion
Introduction
Anonlinear load may be defined as a load which, having a sinusoidal volt-
age applied to it, passes a nonsinusoidal current. Many everyday loads
are nonlinear but the nonlinearity is often unimportant; an unloaded
transformer is nonlinear owing to magnetic saturation. Significant
nonlinear loads associated with UPS equipment include rectifiers and
switched mode power supplies.
Any regular nonsinusoidal waveform may be regarded as a compound
wave made up from a fundamental component and harmonic compo-
nents; the composition of such waveforms may be established mathe-
matically by applying Fourier’s analysis. Even harmonic currents and
voltages indicate a lack of symmetry between positive and negative half
cycles.
The phase sequence of three-phase harmonics is in accordance with
the following pattern:

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