E N G L I S H
34
The vector addition of the currents
in the transformer’s neutral wire was
zero or quite low (because perfect-
balance was rarely achieved) in a well-
balanced system, devices connected
to which were incandescent lighting,
small motors, and other devices that
presented linear loading.
The result was an essentially sine-
wave current flow in each phase
and a low neutral current flow at a
frequency of 50Hz/ 60Hz.
But, devices such as TV sets,
fluorescent lighting, video machines,
and microwave ovens are commonly
drawing power line current for
only a fraction of each cycle so that
they cause non-linear loading and
subsequent non-linear current flow.
This generates odd harmonic of the
50Hz/60Hz line frequency.
Therefore, the current in the
transformer of today contains not
only a 50Hz (or 60Hz) component,
but a 150Hz (or 180Hz) component,
a 250Hz (or 300Hz) component,
and the other significant harmonic
components up to a 750Hz (or 900Hz)
component and beyond.
The vector addition in a properly-
balanced power distribution system
feeding non-linear loads may still
be quite low. But, the addition
does not cancel all the harmonic
currents. The odd multiples of the
3rd harmonic (called the “TRIPLENS”)
are, particularly, added together
in the neutral. These harmonics
can go from a total rms current in
the transformer’s neutral wire that
is normally 130% of the total rms
current measured in any individual
phase, whose theoretical maximum
is 173%.
For example, phase currents of 80
amperes may cause harmonic current
flow in the neutral of 104 amperes.
The dominant current flow in the
neutral is most commonly the 3rd
harmonic.
The electrical system designer must
consider the following 3 issues
when designing a power distribution
system containing harmonic current
flow.
1. The AC neutral wires must be
of sufficient gauge to allow for
harmonic current flow.
2. The distribution transformer
must have additional cooling to
continue operation at its rated
capacity, if it is not harmoni crated.
This is because the harmonic
current flow in the secondary
neutral wire is circulating in the
delta-connected primary winding,
after it is reflected to the primary
winding. The circulating harmonic
current heats up the transformer.
3.
Phase current
harmonics are reflected to the
primary winding and they continue
back towards the power source.
This may cause distortion of the
voltage wave so that any power
factor correction capacitors on the
line can be easily overloaded.
We can use this Meter to analyze
components such as power
distribution transformers and power
factor correction capacitors.
6.2 General Applications
