Application Note

3 Fluke Corporation Common power quality factors affecting transformers
Voltage distortion, however,
depends on source impedance,
i.e., on system capacity. It was
quite possible for the first (or sec-
ond or third) customer to inject
significant harmonic currents into
the system and not cause voltage
THD to exceed 5 %. The entire
responsibility for harmonic miti-
gation could fall on the last
customers unlucky enough to
push V-THD over 5 %, even if
their particular harmonic load
was relatively small-literally the
straw that broke the camel’s
back.
3b. Current T
H
D
To restore some fairness to this
situation, standards for maximum
current harmonics were added,
sinc
e current harmonics were
under the control of the local
fac
ility and equipment manufac
-
turer (rememb
er, harmonic
“loads” act as “generators” of har-
monics). This emphasis on the
mitigation of current harmonics at
the load, including the not-too-
distant requirement that the load
generate virtually no harmonics,
has become the prevailing regu-
lator
y philosophy
. It puts the
burden of responsibility on the
local site and on the equipment
manufacturers
.
For equipment manufacturers,
IEC 1000-3-2, published in 1995,
is the applicable standard. It
specifies maximum current levels
out to the 40th harmonic. Its
expected effective date is pro-
jected to be early 2001. To certify
for CE, a requirement for the
European market, manufacturers
will have to meet this standard.
This edict will have a major effect
on power supply design.
For the facility, IEEE 519 is the
standard (EN 50160 in Europe).
The limits set in IEEE 519 for
harmonic currents depend on the
size of the customer relative to the
system capac
ity. (See Table 2.)
The SCR (Short Circuit Ratio) is
a measure of the electrical size of
the customer in relation to the
utility source. The smaller the
customer (higher SCR), the less
the potential impact on the utility
sourc
e and the more generous
the harmonic limits. The larger
the customer (smaller SCR), the
more stringent the limits on har-
monic currents.
3c. TDD and THD
TDD (Total Demand Distortion) is
the ratio of the current harmonics
to the
maximum load (I
L
). It dif-
fers from THD in that THD is the
ratio of harmonics to the
instan-
taneous
load. Why TDD instead
of THD? Suppose you were run-
ning a light load (using a small
fraction of system capacity), but
those loads were nonlinear. THD
would be relatively high, but the
harmonic currents actually being
generated would be low, and the
effect on the supply system
would in fact be negligible. So
who cares? TDD acknowledges
this, and allows harmonic load to
be referenced to the maximum
load: if harmonic load is high at
maximum load, then we have to
watch out for the effect on the
supply source. So where does
that leave current THD as a useful
measurement.
The closer the cur-
rent THD reading(s) is taken to
conditions of maximum load, the
closer it appr
oximate
s T
D
D.
Table 2: IEEE 519 limits for harmonic currents at the point of
common coupling
(All percentages are % of IL, maximum demand load current)
Odd Harmonics
S
CR=Isc/IL <11 11-17 17-23 23-35 >35 TDD
<20
4.0 % 2.0 % 1.5 % 0.6 % 0.3 % 5.0 %
20-50
7.0 % 3.5 % 2.5 % 1.0 % 0.5 % 8.0 %
50-1
00 10.0 % 4.5 % 4.0 % 1.5 % 0.7 % 12.0 %
1
00-1000 12.0 % 5.5 % 5.0 % 2.0 % 1.0 % 15.0 %
>1
000 15.0 % 7.0 % 6.0 % 2.5 % 1.4 % 20.0 %
S
CR
=
Short circuit ratio (Isc/I
L
)
Isc =
Available short circuit current at PCC
I
L
=
Maximum demand load current (rms amps)
TDD = Total demand distortion
Note: IEEE allows these limits to be exceeded for up to one hour per day, while IEC allows
them to be exceeded for up to 5 % of the time.
The concept of
I
L
, maximum demand load current, is key to using Table 2. For existing facili-
ties,
I
L
is calculated by a
verag
ing the maximum demand current for 12 consecutive months
(information a
vailable in billing records). For new installations,
I
L
must b
e estimated.
Transformer rating could be used and would be the most conservative estimate (i.e., it would
result in the lowest SCR), since it assumes that the transformer would be used at full capacity.
Table 3
Inspection of Transf
ormer Ground Explanation
Check for N-G bond. A high impedance N-G bond will cause
voltage fluctuation.
Check for g
rounding c
onductor and
F
ault currents will return to the source via
integrity of connection to building steel these connections, so they should be as low
(exothermic weld). impedance as possible.
Check for tightness of all
If the c
onduit is not itself g
rounded, it
conduit connections. will tend to act as a “choke” for higher
frequencies and limit fault current
(remember that fault currents are not just
at 60 Hz but have high-f components).
Measure for
ground currents on the Ideally there should be none, but there will
grounding conductor. always be some ground current due to
normal operation or leakage of protective
components (MOVs, etc.) connected from
phase or neutral to ground. However,
anything above an amp should be cause
for suspic
ion (there is no hard and fast rule,
but experienc
ed PQ troubleshooters develop
a feel for possible problems).