Application Note

4 Fluke Corporation Common power quality factors affecting transformers
A final word on measuring
THD: the one place not to apply
the specs is at the individual har-
monic-generating load. This will
always be a worst-case distortion
and a misleading reading. This is
b
ecause as harmonics travel
upstream, a certain amount of
cancellation takes place (due to
phase relationships which, for
practical purposes, are unpre-
dictable). Measure at a PCC, or at
the source transformer.
4. K-factor
K-factor is a specific measure of
the heating effect of harmonics in
general and on transformers in
particular. It differs from the THD
calculation in that it emphasizes
the frequency as well as the
amplitude of the harmonic order.
This is because heating effects
increase as the square of the fre-
quency.
A K-4 reading would mean
that the stray loss heating effects
are four times normal. A standard
transformer is, in effect, a K-1
transformer. As with THD, it is
misleading to make a K-factor
reading at the load or receptacle
because there will be a certain
amount of upstream cancellation;
transformer K-factor is what
counts. Once the K-factor is
determined, choose the next
higher trade size. K-factor rated
transformers are available in
standard trade sizes of K-4, K-13,
K-20, K-30, etc. K-13 is a com-
mon rating for a transformer
supplying office loads. The higher
ratings tend to be packaged into
PDUs (Power Distribution Units)
which are spec
ially designed to
supply computer and other PQ-
sensitive installations.
5. Ground currents
Two prime suspects for excessive
ground current are illegal N-G
bonds (in subpanels, receptacles
or even in equipment) and so-
called isolated ground rods:
•
Subpanel N-G bonds create a
parallel path for normal return
current to return via the
grounding conductor. If the
neutral ever becomes open, the
equipment safety ground
becomes the only return path;
if this return path is high
impedance, dangerous voltages
could develop.
•
Separate isolated ground rods
almost always create two
ground references at different
potentials, which in turn
causes a “ground loop” current
to circulate in an attempt to
equalize those potentials. A
safety and equipment hazard is
also created: in the case of
lightning strikes, surge currents
travelling to ground at different
earth potentials will create
hazardous potential differences.
Transformer grounding
The proper grounding of the
transformer is critical. (Table 3.3.)
NEC Article 250 in general and
250-26 in particular address the
grounding requirements of the
SDS.
•
A ground reference is estab-
lished by a grounding
connection, typically to build-
ing steel (which, in turn, is
required to be bonded to all
cold water pipe, as well as
any and all earth grounding
electrodes). Bonding should be
by exothermic weld, not
clamps that can loosen over
time. The “grounding electrode
conductor” itself should have
as low a high-frequency
impedance as possible (not
least because fault current has
high frequency components).
Wide, flat conductors are pre-
ferred to round ones because
they have less inductive reac-
tance at higher frequencies.
For the same reason, the dis-
tance between the “grounding
electrode conductor connection
to the system” (i.e., N-G bond
at the transformer) and the
grounding electrode (building
steel) should be as short as
possible: in the words of the
Code, “as near as practicable
to and preferably in the same
area
...”
•
The neutral and ground should
be connected at a point on the
transformer neutral bus
.
Although permitted, it is not
advisable to make the N-G
b
ond at the main panel, in
order to maintain the segrega-
tion of normal return currents
and any g
round currents
. This
point at the transformer is the
only point on the system
where N
-
G should be bonded.
480 V
208 Y/120 V
Neutral
Grounding electrode nearby,
preferably structural metal
Figure 3. Transformer grounding.