Application Note

5 Fluke Corporation Common power quality factors affecting transformers
There are a number of solutions
for transformer-related PQ
problems:
•
Install additional distribution
transformers (Separately
Derived Systems)
•
Derate transformers
•
Install K-rated transformers
•
Used forced air cooling
1. Separately Derived System
(SDS)
The distribution transformer is
the supply for a Separately
Derived System (SDS), a term
which is defined in the NEC
(Article 100). The key idea is that
the secondary of this transformer
is the new source of power for all
its downstream loads: this is a
powerful concept in developing a
PQ distribution system. The SDS
accomplishes several important
objectives, all beneficial for PQ:
•
It establishes a new voltage
reference
. Transformers have
taps which allow the second-
ary voltage to be stepped up
or down to compensate for
any voltage drop on the feed-
ers.
•
It lowers source impedance by
decreasing, sometimes drasti-
cally, the distance between
the load and the source. The
potential for voltage distur-
bances, notably sags, is
minimized.
•
It achieves isolation. Since
there is no electrical connec-
tion, only magnetic coupling,
between the primary and sec-
ondary, the SDS isolates its
loads from the rest of the elec-
trical system. To extend this
isolation to high frequency dis-
turbances, specially
constructed “isolation trans-
formers” provide a shield
between the primary and sec-
ondary to shunt RF (radio
frequency) noise to ground.
Otherwise, the capacitive cou-
pling between primary and
secondary would tend to pass
these high-frequency signals
right through.
•
A new ground reference is
established
. Part of the defini-
tion of the SDS is that it “has
no direct electrical connection,
including a solidly connected
grounded circuit conductor, to
supply conductors originating
in another system.” (NEC 100)
The opportunity exists to seg-
regate the subsystem served
by the SDS from ground loops
and ground noise upstream
from the SDS, and vice versa.
2. K-
rated transformers
Harmonics cause heating in
transformers, at a greater rate
than the equivalent fundamental
currents would
. This is because
of their higher frequency. There
are three heating effects in trans-
formers that increase w
ith
frequency:
•
Hysteresis. When steel is
mag
netized, mag
netic dipoles
all line up, so that the North
poles all point one way, the
South poles the other
. These
poles switch with the polarity
of the applied current. The
higher the frequenc
y, the more
often the switching occurs,
and, in a process analogous to
the effects of friction, heat
losses increase.
•
Eddy currents. Alternating
mag
netic fields create localized
whirlpools of current that cre-
ate heat loss. This effect
increases as a square of the
frequency. For example, a 3rd
harmonic current will have
nine times the heating effect
as the same current at the fun-
damental.
•
Skin effect. As frequency
increases, electrons migrate to
the outer surface of the con-
ductor. More electrons are
using less space, so the effec-
tive impedance of the
c
onductor has increased; at
the higher frequency, the con-
ductor behaves as if it were a
lower gauge, lower ampac
ity,
higher impedanc
e w
ire.
The industry has responded
with two general solutions to the
effects of harmonics on trans-
formers: install a K-factor rated
transformer or derate a standard
transformer. Let’s look at pros
and cons of the K-factor approach
first. K-factor is a calculation
based on the rms value, %HD
(harmonic distortion) of the har-
monic currents, and the square of
the harmonic order (number). It is
not necessary to actually perform
the calculation because a har-
monic analyzer will do that for
you. The important thing to
understand is that the harmonic
Figure 4. Typical K-factor in commercial
building.
Solutions