AEMC 721 User Manual page 19

Harmonics can cause a variety of problems to any electrical power user.
For large users, the problems can be intense. For electronic equipment
that relies on the zero crossing of the sinusoidal waveform, such as clock
timing devices, heavy harmonic content can cause a zero crossing point
offset.
Odd number harmonics (3rd, 5th, & 7th) cause the greatest concern in
the electrical distribution system. Because the harmonic waveform usu-
ally swings equally in both the positive and negative direction, the even
number harmonics are mitigated.
Heating effect causes the greatest problem in electrical equipment. Many
times, electrical distribution equipment has overheated and failed even
when operating well below the suggested rating requirements. Tempera-
ture increase is directly related to the increase in RMS current.
Because harmonic frequencies are always higher than the 60 Hz funda-
mental frequency, "skin effect" also becomes a factor. Skin effect is a phe-
nomenon where the higher frequency causes the electrons to flow toward
the outer sides of the conductor, effectively reducing the cross-sectional
diameter of the conductor and thereby reducing the ampacity rating of the
cable. This effect increases as the frequency and the amplitude increase.
As a result, higher harmonic frequencies cause a greater degree of heat-
ing in conductors.
On balanced three-phase systems with no harmonic content, the line cur-
rents are 120 degrees out-of-phase, canceling each other and resulting in
very little neutral current. However, when there is distortion in any one of
the phase currents, the harmonic currents increase and the cancellation
effect is lessened. The result is typically a neutral current that is signifi-
cantly higher than planned. The triplen harmonics (odd multiples of three)
are additive in the neutral and can quickly cause dangerous overheating.
In theory, the maximum current that the neutral will carry is 1.73 times
the phase current. If not sized correctly, overheating will result. Higher
than normal neutral current will cause voltage drops between neutral and
ground which are well above normal. Readings above 4 volts indicate high
neutral current.
False tripping of circuit breakers is also a problem encountered with the
higher frequencies that harmonics produce. Peak sensing circuit breakers
often will trip even though the amperage value has not been exceeded.
Harmonic current Peak values can be many times higher than sinusoidal
waveforms.
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Harmonic Meter Model 721