Application Note

Power Quality Troubleshooting Fluke Corporation 13
A final word on measuring
THD: the one place not to apply
the specs is at the individual
harmonic-generating load. This
will always be a worst-case
distortion and a misleading
reading. This is because as har-
monics travel upstream, a cer-
tain amount of cancellation
takes place (due to phase rela-
tionships which, for practical
purposes, are unpredictable).
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 em-
phasizes the frequency as well
as the amplitude of the har-
monic order. This is because
heating effects increase as the
square of the frequency.
A K-4 reading would mean
that the stray loss heating
effects are four times normal. A
standard transformer is, in ef-
fect, 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-fac-
tor 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 common rating for a
transformer supplying office
loads. The higher ratings tend
to be packaged into PDUs
(Power Distribution Units)
which are specially designed
to supply computer and other
PQ-sensitive installations.
5. Ground currents
Two prime suspects for exces-
sive ground current are illegal
N-G bonds (in subpanels, re-
ceptacles or even in equip-
ment) and so-called isolated
ground rods:
•
Subpanel N-G bonds create
a parallel path for normal re-
turn 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.
(Figure 2.1, page 8.)
•
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 at-
tempt to equalize those po-
tentials. A safety and
equipment hazard is also
created: in the case of light-
ning strikes, surge currents
travelling to ground at differ-
ent earth potentials will
create hazardous potential
differences. (See page 31.)
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 require-
ments of the SDS.
•
A ground reference is estab-
lished by a grounding con-
nection, typically to building
steel (which, in turn, is re-
quired 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 elec-
trode conductor” itself should
have as low a high-frequency
impedance as possible (not
least because fault current
has high frequency compo-
nents). Wide, flat conductors
are preferred to round ones
because they have less in-
ductive reactance at higher
frequencies. For the same
reason, the distance between
the “grounding electrode con-
ductor connection to the sys-
tem” (i.e., N-G bond at the
transformer) and the ground-
ing electrode (building steel)
should be as short as pos-
sible: 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 neu-
tral bus. Although permitted,
it is not advisable to make
the N-G bond at the main
panel, in order to maintain
the segregation of normal re-
turn currents and any ground
currents. This point at the
transformer is the only point
on the system where N-G
should be bonded.
480V
208Y/120V
Neutral
Grounding electrode nearby,
preferably structural metal
Figure 3.4 Transformer grounding.