Case Studies

Application Note
Performing power
quality studies and
troubleshooting loads
Working from his offices in
Puyallup, Washington, just
east of Tacoma, Mac McArthur
performs power quality stud-
ies for commercial, industrial,
and residential clients. He
also serves as an agent for
the commissioning of electri-
cal systems in new buildings
and the retro-commissioning of
older buildings.
Prelude to a power-logger
purchase
In late 2006, McArthur found
himself in the middle of a
challenging assignment. The
subject of his investigation was
a facility that makes corrugated
paperboard and whose elec-
trical load was handled by a
1,500 kVA transformer owned
by the local utility. The trans-
former, a 277/480-volt, 4-wire,
Wye, had failed because of
thermal stress with only a 65
percent load on it. The util-
ity hired McArthur to find the
cause of the failure.
“I went in with my basic
instruments, including a single-
phase power quality analyzer
and my Fluke 189 (true-rms)
Digital Multimeter, but I ended
up renting an expensive
power logger—a black box
that allowed me to see what
was going on, but would not
allow me to zero in on just the
parameters that interested me,”
McArthur explains. That meant
a lot of data he didn’t need, fill-
ing up the memory too fast. And
without a screen on the unit, he
couldn’t verify the setup.
Field Applications
Case Study
Profile: Mac McArthur, power quality
consultant
Tool: Fluke 1735 Three-Phase
Power Logger
Tests performed:
Tracing and
isolating harmonics; verifying
voltage drops.
From the Fluke Digital Library @ www.fluke.com/library
Using his instruments and the
rented power logger, McArthur
determined that the problem
was on the customer side of
the transformer. Of course,
the utility required that the
company correct whatever had
caused the transformer failure
in order to avoid burning up
another one. So, the company
retained McArthur to diagnose
the problem.
“To put it in numbers,”
McArthur says, “the transformer
is good for approximately 1,800
amps. The company was draw-
ing between 1,100 and 1,200
amps at full load, but the envi-
ronment was full of dc drives.
We noticed that dc current was
saturating the transformer’s
core with all of the equipment
on. This condition was evident
because the even harmon-
ics were higher in amplitude
especially the 2
nd
, 4
th
and 6
th
harmonics. Further testing
showed that the transformer’s
output voltage was approxi-
mately six to eight volts lower
because of the core saturation.
That was a problem.”
McArthur started taking read-
ings at the service entrances
and went from there down to
the equipment—the individual
loads. He found a relatively
high dc component being
reflected back to the trans-
former and set about to isolate
the source.