Page 121 - Uninterruptible Power Supplies
P. 121
Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 119
Conventionally, nonlinear loads are regarded as loads which take
current at the fundamental frequency and which include, in series,
harmonic current generators having zero impedance.
There are a number of formulae likely to be encountered in connection
with nonlinear loads; they appear below and use the following convention:
a rms value of the total current
a 1 rms value of the fundamental current component
a n rms value of the nth harmonic current component
The rms value of total current a
a a a a … (4.1)
2
2
2
2
1 2 3 4
rms value of the fundamental current component
Distortion factor
rms value of the total current
a 1
(4.2)
a a a a
2 …
2
2
2
1 2 3 4
rms value of the harmonic components
Total harmonic distortion
rms value of the fundamental component
a a a a
2 …
2
2
2
1
2
3
4
(4.3)
a
1
Total harmonic distortion is often abbreviated to thd and is usually
expressed as a percentage of the fundamental component (either cur-
rent or voltage).
instantaneous peak value of current
Peak or crest factor
rms value of the total current
instantaneous peak value of current
(4.4)
a a a a …
2
2
2
2
1 2 3 4
For a sinusoidal supply voltage, and where cos is the power factor
of the fundamental current component:
Power v a 1 cos
v a cos (4.5)
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