AEMC 721 User Manual page 17

Harmonics in the electric power system combine with the fundamental
frequency to create distortion. The level of distortion is directly related to
the frequencies and amplitudes of the harmonic current. The contribution
of all harmonic frequency currents to the fundamental current is known as
"Total Harmonic Distortion"
or THD. This THD value is
expressed as a percentage
of the fundamental current.
THD values of over 10%
are reason for concern.
THD is calculated as the
square root of the sum of
the squares of all the har-
monics divided by the fun-
damental signal (50 or 60
Hz). This calculation arrives
at the value of distortion as a percentage of the fundamental.
Mathematically, %THD is the ratio of the root-mean-square (RMS) of the
harmonic content to the root-mean-square (RMS) value of the fundamen-
tal 50 or 60 Hz signal. THD is expressed as a percentage, and may be
greater than 100%.
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Wherever there are large numbers of nonlinear loads, there are sure to
be harmonics in the distribution system. Harmonic-producing equipment
is found in varied locations from administrative offices to manufacturing
facilities. In the factory environment, electronic power converters such as
variable speed drives, SCR drives, etc., are the largest contributors to har-
monic distortion. It is not uncommon to have THD levels as high as 25%
within some industrial settings.
16
Total Harmonic Distortion
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Harmonic Meter Model 721
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