Application Note

4 Fluke Corporation Troubleshooting power harmonics
Finding harmonics
Generators
Standby generators are subject
to the same kind of overheat-
ing problems as transformers.
Because they provide emergency
backup for harmonic producing
loads such as data processing
equipment, they are often even
more vulnerable. In addition to
overheating, certain types of
harmonics produce distortion at
the zero crossing of the current
waveform, which causes inter-
ference and instability for the
generator’s control circuits.
Classification of harmonics
Each harmonic has a name, frequency and sequence. The sequence refers
to phasor rotation with respect to the fundamental (F), i.e., in an induction
motor, a positive sequence harmonic would generate a magnetic field that
rotated in the same direction as the fundamental. A negative sequence har-
monic would rotate in the reverse direction. The first nine harmonics along
with their effects are listed below:
Name F 2nd* 3rd 4th* 5th 6th* 7th 8th* 9th
Frequency 60 120 180 240 300 360 420 480 540
Sequence + — 0 + — 0 + — 0
*Even harmonics disappear when waves are symmetrical (typical for electrical circuits)
Sequence Rotation Effects (from skin effect, eddy currents, etc.)
Positive Forward Heating of conductors, circuit breakers, etc.
Negative Reverse Heating as above plus motor problems
Zero** None Heating, plus add in neutral of 3-phase, 4-wire system
**Zero sequence harmonics (odd multiples of the 3rd) are called “Triplens” (3rd, 9th, 15th, 21st, etc.)
A harmonic survey will give you
a good idea if you have a prob-
lem and where it is located. Here
are a few guidelines to follow.
1. Load inventory. Make a
walking tour of the facility
and take a look at the types of
equipment in use. If you have
a lot of personal computers
and printers, adjustable speed
motors, solid-state heater
controls, and certain types of
fluorescent lighting, there’s a
good chance that harmonics
are present.
2. Transformer heat check.
Locate the transformers feed-
ing those non-linear loads and
check for excessive heating.
Also make sure the cooling
vents are unobstructed.
3. Transformer secondary
current. Use a three-phase
true-rms power quality
analyzer to check transformer
currents.
•
Verify that the voltage rat-
ings for the clamp meter are
adequate for the transformer
being tested.
•
Measure and record the
transformer secondary cur-
rents in each phase and in
the neutral (if used).
•
Compare the kVA deliv-
ered to the load against the
nameplate rating. (If har-
monic currents are present,
the transformer can overheat
even if the kVA delivered
is less than the nameplate
rating.)
•
Use the k-factor measure-
ment from a three-phase
analyzer to determine
de-rating or transformer
replacement.
•
Measure the frequency of
the neutral current. 180 Hz
would be a typical reading
for a neutral current consist-
ing of mostly third harmonic.
4. Sub-Panel neutral current
check. Survey the sub-
panels that feed harmonic
loads. Measure the current
in each branch neutral and
compare the measured value
to the rated capacity for the
wire size used. Check the
neutral bus bar and feeder
connections for heating or
discoloration. A non-contact
infrared thermometer, like the
Fluke 61, is useful for detect-
ing excessive overheating on
bus bars and connections.
5. Receptacle neutral-to-
ground voltage check.
Neutral overloading in
receptacle branch circuits
can sometimes be detected
by measuring the neutral-
to-ground voltage at the
receptacle.
•
Measure the voltage when
the loads are on. Two volts
or less is about normal.
Higher voltages can indicate
trouble depending on the
length of the run, quality of
connections, etc. Measure
the frequency. A frequency
of 180 Hz would suggest
a strong presence of har-
monics, while 60 Hz would
suggest that the phases are
out of balance. Pay special
attention to under carpet
wiring and mod-ular office
panels with integrated
wiring that uses a neutral
shared by three-phase
conductors.
•
Because the typical loads in
these two areas are com-
puter and office machines,
they are often trouble spots
for overloaded neutrals.