Application Note

6 Fluke Corporation Common power quality factors affecting transformers
3. Derating standard
transformers
Some facilities managers use a
50 % derating as a rule-of-thumb
for their transformers serving
single-phase, predominantly
nonlinear loads. This means that
a 150 kVA transformer would
only supply 75 kVA of load. The
derating curve, taken from IEEE
1100-1992 (Emerald Book),
shows that a transformer with
60 % of its loads consisting of
SMPS (switched-mode power
supplies), which is certainly
possible in a commercial office
building, should in fact be
derated by 50 %.
The following is an accepted
method for calculating trans-
former derating for single-phase
loads only. It is based on the very
reasonable assumption that in
single-phase circuits, the third
harmonic will predominate and
cause the distorted current wave-
form to look predictably peaked.
Use a
true-rms meter to make
these current measurements:
1. Measure rms and peak current
of each secondary phase.
(Peak refers to the instanta-
neous peak, not to the inrush
or “peak load” rms current).
2. Find the arithmetic average of
the three rms readings and the
three peak currents and use
this average in step 3 (if the
load is essentially balanc
ed,
this step is not nec
essary).
3. Calculate Xformer Harmonic
Derating Factor:
xHDF = (1.414 * IRMS) / IPEAK
4. Or, since the ratio of
Peak/RMS is defined as Crest
Factor, this equation can be
rew
ritten as:
xH
DF = 1.414 / CF
If your test instrument has the
capability, measure the CF of
each phase directly. If the load
is unbalanced, find the aver-
age of the three phases and
use the average in the above
formula.
Since a sine wave current
waveform has a CF=1.414, it will
ha
ve an xH
DF=1; there will be
no derating. The more the 3rd
harmonic, the higher the peak,
the higher the CF. If the CF were
2.0, then the xH
DF=1.414 / 2
=.71. A CF=3 gives us an xHDF
=.47. A wave with CF=3 is about
as badly distorted a current
waveform as you can expect to
see on a single-phase distribution
transformer.
Caution: This method does not apply to
transformers feeding three-phase loads,
where harmonics other than the third tend to
predominate and CF is not useful as a simple
predictor of the amount of distortion. A calcu-
lation for three-phase loads is available in
ANSI/IEEE C57.110. However, there is some
controversy about this calculation since it
may underestimate the mechanical resonant
vibrations that harmonics can cause, and that
accelerate transformer wear above and
beyond the effects of heat alone.
4. Forced air cooling
If heat is the problem, cooling is
the solution. Break out the fan,
turn it on the transformer and use
forced air cooling. Some experi-
enc
ed hands fig
ure that’s worth
20-30 % on the up side. In any
case, it can only help.
order is squared in the equation
and that is precisely where the
high- frequency heating effects,
like eddy current losses, are
taken into account.
K-rated transformers are
desig
ned to minimize and accom-
modate the heating effects of
harmonics. K-rated transformers
do not eliminate harmonics
(unless additional elements like
filters are added). They accom-
modate harmonics with
techniques such as the use of a
number of smaller, parallel wind-
ings instead of a single large
w
inding: this gives more skin for
the electrons to travel on. The
primary delta winding is up-sized
to tolerate the c
irculating third
harmonic currents without over-
heating. The neutral on the
secondary is also up-sized for
third harmonics (typically sized at
twice the phase ampacity).
Application issues with
K-factor transformers
K-rated transformers have been
widely applied, but there are cer-
tain issues with them. Many
consultants do not see the need
for using transformers with a rat-
ing higher than K-13 although
K-20 and higher might be sup-
plied as part of an integrated
Power Distribution Unit (PDU).
Also, early applications some-
times overlooked the fact that
K-rated transformers necessarily
have a lower internal impedance.
Whereas a standard transformer
has an impedanc
e typically in the
5-6 % range, K-rated transform-
ers can go as low as 2-3 %
(lower as the K
-rating increases).
In retrofit situations, where a
standard transformer is being
replac
ed by a K
-rated transformer
of equivalent kVA, this may
require new short circuit calcula-
tions and re-sizing of the
secondary overcurrent protective
devices.
Fluke Corporation
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etherlands
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ax (9
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©2004 Fluke Corporation
. All rights reserved.
Printed in U.S.A. 10/2004 2403202 A-US-N Rev A
Fluke. K
e
eping your world
up and running.
0
0
20
40
60
80
100
20 40 60 80 100
Transformer Capacity (%)
After Derating for
Electronic Load
Switched-Mode Power Supply Load (% of Overall Load)
Figure 5. Transformer derating curve (IEEE 1100-1992)