Amprobe DM-4 Power-Quality-Recorder - Manual (Page 51)

15.2.3 Presence of Harmonics: Causes
Any apparatus that alters the sine wave or uses only a part of such a wave caus-
es distortions to the sine wave and therefore harmonics. All current signals are in
some way distorted. The most common situation is the harmonic distortion caused
by non-linear loads such as household appliances, personal computers or speed
control units for motors. Harmonic distortion causes significant currents at frequen-
cies that are odd multiples of the fundamental frequency. Harmonic currents affect
considerably the neutral wire of electric installations. In most countries, the main
power is three-phase 50/60Hz with delta primary and star secondary transformers.
The secondary generally provides 230V AC from phase to neutral and 400V AC
from phase to phase. Balancing the loads on each phase has always represented
a headache for electric systems designers. Until some ten years ago, in a well-bal-
anced system, the vectorial sum of the currents in the neutral was zero or quite low
(given the difficulty of obtaining a perfect balance). The devices were incandescent
lights, small motors and other devices that presented linear loads. The result was
an essentially sinusoidal current in each phase and a low current on the neutral at
a frequency of 50/60Hz. “Modern” devices such as TV sets, fluorescent lights,
video machines and microwave ovens normally draw current for only a fraction of
each cycle thus causing non-linear loads and subsequent non-linear currents. All
this generates odd harmonics of the 50/60Hz line frequency. For this reason, the
current in the transformers of the distribution boxes contains only a 50Hz (or 60Hz)
component but also a 150Hz (or 180Hz) component, a 300Hz (or 360Hz) compo-
nent and other significant components of harmonic up to 750Hz (or 900Hz) and
higher. The vectorial sum of the currents in a well-balanced system that feeds non-
linear loads may still be quite low. However, the sum does not eliminate all current
harmonics. The odd multiples of the third harmonic (called “TRIPLENS”) are added
together in the neutral and can cause overheating even with balanced loads.
15.2.4 Presence of Harmonics: Consequences
In general, even harmonics, i.e. the 2
nd
, 4
th
etc., do not cause problems. Triple
harmonics, odd multiples of three, are added on the neutral (instead of canceling each
other) thus creating a condition of overheating of the wire which is extremely
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DM-4 Power Quality Recorder