Page 127 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 125

A calculation of the effect of harmonic currents on a supply system is
complex and requires a knowledge of the resistance and reactance of
the various components such as overhead lines, cables, and transform-
ers. If it is assumed that the supply impedance is purely inductive the
voltage distortion is related to the formula:

Harmonic voltage n I N
(4.8)
System voltage I SC

where n harmonic order
I N harmonic current of order n
I SC symmetrical prospective fault current

This formula does not provide a reliable prediction of the voltage dis-
tortion and Engineering Recommendation G.5/4 (see below) includes
factors to modify the harmonic impedance to allow for cable capaci-
tance, overhead line inductance, and other random effects.
Any resistive loads connected to the system provide additional
shunt paths for harmonic currents and reduce the voltage distortion.
Harmonic currents and the resulting voltage distortion are undesir-
able for several reasons, the circulating currents cause additional
system losses, the distortion can affect the operation of equipment
connected to the system, the losses and temperature rises of motors
and other machines are increased, and power factor capacitors can be
damaged.

Some Effects of Distortion
Data and control circuits running parallel to circuits carrying distorted
currents may be affected by inductive coupling particularly if the
severe notching associated with thyristors is present. Telephone cir-
cuits may similarly be affected; the human ear is particularly sensitive
to frequencies of about 800 Hz, the sixteenth harmonic of 50 Hz. A
three-phase bridge rectifier produces the seventeenth harmonic.
Within asynchronous induction motors harmonic stator currents
cause flux systems which rotate at harmonic speeds and cause addi-
tional iron losses, particularly in the rotor. The effects of harmonic cur-
rents within synchronous machines is discussed in the section titled
“The Effect of Bridge Rectifier Loads on Local Generators.”
Analogue or digital measuring instruments may display a true rms
value or a rectified average multiplied by the sine wave form factor
(1.11). If the wave form is not sinusoidal the form factor will differ
from 1.11 and the reading will be incorrect. Even more confusion is
caused if the instrument does not inform the user what system of

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