Case Studies

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©2004 Fluke Corporation. All rights reserved.
Printed in U.S.A. 12/2004 2398543 A-ENG-N Rev A
K-factor measures the heat-
ing effects of harmonics on
transformer loading and losses.
K-1 is the baseline for a stan-
dard transformer. K-4 would be
for times the “normal” heating.
K-rated transformer are specifi-
cally designed to handle
harmonics.
Derating is a way of calcu-
lating the realistic load a
transformer can carry when
harmonics are present. For
example, while a transformer
may be rated at 150 kVA,
harmonics would derate it to a
75 kVA load availability.
The higher the harmonics
the less efficient the trans-
former is and the less current it
can carry — the harmonics are
competing for the total rating.
In this case, the combination of
customer load and harmonics
had resulted in an overloaded
transformer.
So, were the harmonics
too high or the transformer
insufficient? The technician
began tracing the load. THD
and K-factor are worst-case at
the load vs. elsewhere in the
distribution system. This is
because source impedance is
highest at the load, and source
impedance is what drives THD
and K-factor higher.
He realized that while the
plant floor had b
een updated
to include a full assortment of
harmonic-laden electrical and
electronic technology, the entire
electrical system was still
designed to support the origi-
nal, 75 year-old electrical load.
Conclusion
Based on the harmonic levels
and loading figures gathered by
power quality analyzer, the
technician went to the floor
manager and recommended a
new K-rated transformer
designed to support the real
load for that production area.
In the meantime, he
suggested either transferring
some of the current load to
other transformers or staggering
the timing of the loads. He
also recommended regular
harmonics and system imped-
ance measurements. The new
transformer wouldn’t overheat,
but given the amount of
harmonics on this system they
would have to consistently
track THD throughout the floor.
The technician selected the
Harmonics function from the
power quality analyzer menu
and reviewed the Total
Harmonic Distortion (THD)
bar-graph. THD measures the
combined action of all the
harmonics (on multiple frequen-
cies) present, compared to the
overall system. THD should not
exceed 5 %.
2 Fluke Corporation Power Quality Case Study: Overheated transformer
At an average of 13.8 %
across all phases, this THD
reading was way too high. He
selected the Watts reading (W)
to get an idea of what the K
factor was for the transformer’s
derating.