Application Note

14 Fluke Corporation Power Quality Troubleshooting
Solutions
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 trans-
former 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
feeders.
•
It lowers source impedance
by decreasing, sometimes
drastically, the distance
between the load and the
source. The potential for volt-
age disturbances, notably
sags, is minimized.
•
It achieves isolation. Since
there is no electrical connec-
tion, only magnetic coupling,
between the primary and
secondary, the SDS isolates
its loads from the rest of the
electrical system. To extend
this isolation to high fre-
quency disturbances, spe-
cially constructed “isolation
transformers” provide a
shield between the primary
and secondary to shunt RF
(radio frequency) noise to
ground. Otherwise, the ca-
pacitive coupling between
primary and secondary
would tend to pass these
high-frequency signals right
through.
•
A new ground reference is
established. Part of the defi-
nition of the SDS is that it
“has no direct electrical con-
nection, including a solidly
connected grounded circuit
conductor, to supply conduc-
tors originating in another
system.” (NEC 100) The op-
portunity exists to segregate
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 fundamen-
tal currents would. This is be-
cause of their higher frequency.
There are three heating effects
in transformers that increase
with frequency:
•
Hysteresis. When steel is
magnetized, magnetic 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 frequency, the
more often the switching
occurs, and, in a process
analogous to the effects of
friction, heat losses increase.
•
Eddy currents. Alternating
magnetic fields create local-
ized whirlpools of current
that create 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 fundamental.
•
Skin effect. As frequency
increases, electrons migrate
to the outer surface of the
conductor. More electrons are
using less space, so the effec-
tive impedance of the con-
ductor has increased; at the
higher frequency, the con-
ductor behaves as if it were a
lower gauge, lower ampacity,
higher impedance wire.
The industry has responded
with two general solutions to
the effects of harmonics on
transformers: 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
Figure 3.5 Typical K-factor in commercial
building.