20
Press the MIN MAX pushbutton for 2 seconds and then
release in order to exit RECORD (or HOLD and RECORD
it selected). This Meter acknowledges with a beep sound
and RECORD is no longer displayed.
6.1 Non-Linear Loads
True-RMS current flow is very important because it directly
relates to the amount of heat dissipated in wiring,
transformers, and system connections. Most ammeters
i
n
the market measure average current flow, not true rms
current flow, even if this average current flow is displayed
on a scale calibrated in rms. These average-sensing
ammeters are accurate only for a pure sign-wave current.
All current waveforms are virtually distorted in some way.
The most common is harmonic distortion cau
s
ed by non-
linear loads such as household electrical app
l
iances,
personal computer
s
or
s
peed controls for motor drives.
Harmonic distortion caused significant current flow at
frequencie
s
that are at odd multiple
s
of the power line
frequency. Harmonic current flow gives a substantial
impact on the neutral wires of star (wye)-connected power
distribution systems.
In most countries a power distribution system uses
commercia
l
3-phase 50Hz/60Hz power applied to
transformer with a delta-
c
onnected primary, and a star
(wye)-connected secondary.
The secondary generally provides 120V AC from phase to
neutral, and 208V AC from phase to phase. To balance the
loads for each phase was a big headache for the electrical
system designer, historically.
21
6. Applications of Measurement
The vector addit
i
on of the currents
i
n the transformer’s
neutral wire was zero or quite
l
ow (because perfect-
balance was rarely a
c
hieved) in a well-balanced system,
devices connected to which were incandescent light
i
ng,
small motors, and other devices that presented l
i
near
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
ma
c
hines, 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
i
n the
transformer of today
c
ontains 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-
l
inear loads may still be
quite low. But, the addition does not cancel al
l
the
harmonic currents. The odd multiples of the 3rd harmonic
(called the “TRIPLENS”) are, particularly, added together in
the neutral. These harmonics can from a total rms current
in the transformer’s neutral wire that is normally 130% of
the total rms current measured in any individual pha
s
e,
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 is
s
ues when he de
s
i
gns a power distribution system
containing harmonic current flow.
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