Application Note

12 Fluke Corporation Power Quality Troubleshooting
Voltage distortion, however,
depends on source impedance,
i.e., on system capacity. It was
quite possible for the first (or
second or third) customer to in-
ject significant harmonic cur-
rents into the system and not
cause voltage THD to exceed
5%. The entire responsibility for
harmonic mitigation could fall
on the last customers unlucky
enough to push V—THD over
5%, even if their particular har-
monic load was relatively
small—literally the straw that
broke the camel’s back.
3b. Current THD
To restore some fairness to this
situation, standards for maxi-
mum current harmonics were
added, since current harmonics
were under the control of the
local facility and equipment
manufacturer (remember, har-
monic “loads” act as “genera-
tors” of harmonics). This
emphasis on the mitigation of
current harmonics at the load,
including the not-too-distant
requirement that the load gen-
erate virtually no harmonics,
has become the prevailing
regulatory philosophy. It puts
the burden of responsibility on
the local site and on the equip-
ment manufacturers.
For equipment manufactur-
ers, IEC 1000-3-2, published
in 1995, is the applicable stan-
dard. It specifies maximum
current levels out to the 40th
harmonic. Its expected effective
date is projected to be early
2001. To certify for CE, a re-
quirement 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 Eu-
rope). The limits set in IEEE 519
for harmonic currents depend
on the size of the customer
relative to the system capacity.
(See Table 3.2.)
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 source and the more gen-
erous the harmonic limits. The
larger the customer (smaller
SCR), the more stringent the
limits on harmonic currents.
3c. TDD and THD
TDD (Total Demand Distortion) is
the ratio of the current harmon-
ics to the maximum load (I
L
).
It differs from THD in that THD
is the ratio of harmonics to the
instantaneous load. Why TDD
instead of THD? Suppose you
were running a light load (using
a small fraction of system ca-
pacity), but those loads were
nonlinear. THD would be rela-
tively high, but the harmonic
currents actually being gener-
ated would be low, and the ef-
fect 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 current THD
reading(s) is taken to conditions
of maximum load, the closer it
approximates TDD.
Table 3.3
Inspection of Transformer Ground Explanation
Check for N-G bond. A high impedance N-G bond will cause
voltage fluctuation.
Check for grounding conductor and Fault 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 conduit is not itself grounded, it will
conduit connections. 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 suspicion (there is
no hard and fast rule, but experienced PQ
troubleshooters develop a feel for possible
problems).
Table 3.2 IEEE 519 limits for harmonic currents at the point of common coupling.
(All percentages are % of I
L
, maximum demand load current.)
Odd Harmonics
SCR=Isc/I
L
<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%
50-100 10.0% 4.5% 4.0% 1.5% 0.7% 12.0%
100-1000 12.0% 5.5% 5.0% 2.0% 1.0% 15.0%
>1000 15.0% 7.0% 6.0% 2.5% 1.4% 20.0%
SCR= 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 3.2. For existing
facilities, I
L is calculated by averaging the maximum demand current for 12 consecutive
months (information available in billing records). For new installations, I
L must be esti-
mated. 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.