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AMPROBE
and 400V AC from phase to phase. Balancing the loads on each phase has always
represented a headache for electrical systems designers.
Until some ten years ago, in a well-balanced 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
50Hz (or 300Hz) component 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.
16.6.4. Presence of harmonics: consequences
In general, even harmonics, i.e. the 2
odd multiples of three, are added on the neutral (instead of cancelling each other) thus
creating a condition of overheating of the wire, which is extremely dangerous.
Designers should take into consideration the three issues given below when designing a
power distribution system that will contain harmonic current:
• The neutral wire must be of sufficient gauge.
• The distribution transformer must have an additional cooling system to continue
operating at its rated capacity when not suited to the harmonics. This is necessary
because the harmonic current in the neutral wire of the secondary circuit circulates in
the delta-connected primary circuit. This circulating harmonic current heats up the
transformer.
nd
th
, 4
etc., do not cause problems. Triple harmonics,
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