Application Note

7 Fluke Corporation Troubleshooting power harmonics
The high voltage and current
present in electrical power
systems can cause serious
injury or death by electrocu-
tion. Consequently, only trained,
experienced electricians who
have knowledge of electri-
cal systems in general and the
equipment under test should
perform testing and modification
of electrical systems.
Fluke cannot anticipate all
possible precautions that you
must take when performing the
measurements described here.
At a minimum, however, you
should:
•
Use appropriate safety equip-
ment such as safety glasses,
insulating gloves, insulating
mats, etc.
•
Be sure that all power has
been turned off, locked out,
and tagged in any situation
where you will be in direct
contact with circuit compo-
nents. Be certain that the
power can’t be turned on by
anyone but you.
•
Read and understand all of
the applicable manuals before
using the application informa-
tion in this application note.
Take special note of all safety
precautions and warnings in
the instruction manuals.
This article is a general guide to
understanding harmonics. It is
not intended to substitute for the
services of a professional electri-
cal systems consultant. Before
you take any measures to diag-
nose or address your potential
harmonics problems, you should
have your operation thoroughly
analyzed by a professional elec-
trical engineer.
Situation
A modern office building dedi-
cated primarily to computer
software development contained
a large number of personal com-
puters and other electronic office
equipment. These electronic
loads were fed by a 120/208 V
transformer configured with a
delta primary and a wye second-
ary. The PCs were fairly well
distributed throughout the build-
ing, except for one large room
that contained several machines.
The PCs in this room, used exclu-
sively for testing, were served by
several branch circuits.
The transformer and main
switch gear were located in a
ground floor electrical room.
Inspection of this room immedi-
ately revealed two symptoms of
high harmonic currents:
•
The transformer was generat-
ing a substantial amount of
heat.
•
The main panel emitted an
audible buzzing sound. The
sound was not the chatter
commonly associated with
a faulty circuit breaker, but
rather a deep resonant buzz
that indicated the mechanical
parts of the panel itself were
vibrating.
Ductwork installed directly over
the transformer to carry off some
of the excess heat kept the room
temperature within reasonable
limits.
Defining the problem
Transformer – Current mea-
surements (see Table 1) were
taken on the neutral and on
each phase of the transformer
secondary using both a true-rms
multimeter and an average-
responding unit. A 600 A
clamp-on current transformer
accessory was connected to each
meter to allow them to make
high current readings. The cur-
rent waveshapes are shown in
Figures 4 and 5.
Conductor name
True-rms multimeter
(amps)
Average responding
multimeter (amps)
Instantaneous peak
current (amps)
Phase 1 410 328 804
Phase 2 445 346 892
Phase 3 435 355 828
Neutral 548 537 762
Work safely
Case study