Application Note
3 Fluke Corporation Troubleshooting power harmonics
Symptoms of harmonics usually
show up in the power distribu-
tion equipment that supports the
non-linear loads. There are two
basic types of non-linear loads:
single-phase and three-phase.
Single-phase, non-linear loads
are prevalent in offices, while
three-phase loads are wide-
spread in industrial plants.
Each component of the power
distribution system manifests
the effects of harmonics a little
differently, yet all are subject to
damage and inefficient perfor-
mance if not designed to handle
electronic loads.
Neutral conductors
In a three-phase, four-wire
system, neutral conductors can
be severely affected by non-
linear loads connected to the 120
V branch circuits. Under normal
conditions for a balanced linear
load, the fundamental 60 Hz
portion of the phase currents will
cancel in the neutral conductor.
In a four-wire system with
single-phase, non-linear loads,
certain odd-numbered harmon-
ics called triplens—odd multiples
of the third harmonic: 3rd, 9th,
15th, etc—do not cancel, but
rather add together in the neutral
conductor. In systems with many
single-phase, non-linear loads,
the neutral current can actually
exceed the phase current. The
danger here is excessive over-
heating because, unlike phase
conductors, there are no circuit
breakers in the neutral conductor
to limit the current.
Excessive current in the neu-
tral conductor can also cause
higher-than-normal voltage
drops between the neutral con-
ductor and ground at the 120 V
outlet.
Circuit breakers
Common thermal-magnetic
circuit breakers use a bi-metallic
trip mechanism that responds to
the heating effect of the circuit
current. They are designed to
respond to the true-rms value of
the current waveform and will
trip when the trip mechanism
gets too hot. This type of breaker
has a good chance of protect-
ing against harmonic current
overloads.
A peak-sensing, electronic trip
circuit breaker responds to the
peak of current waveform. As a
result, it won’t always respond
properly to harmonic currents.
Since the peak of the harmonic
current is usually higher than
normal, this type of circuit
breaker may trip prematurely at a
low current. If the peak is lower
than normal, the breaker may
fail to trip when it should.
Bus bars and
connecting lugs
Neutral bus bars and connecting
lugs are sized to carry the full
value of the rated phase current.
They can become overloaded
when the neutral conductors are
overloaded with the additional
sum of the triplen harmonics.
Electrical panels
Panels that are designed to carry
60 Hz currents can become
mechanically resonant to the
magnetic fields generated by
higher frequency harmonic
currents. When this happens,
the panel vibrates and emits a
buzzing sound at the harmonic
frequencies.
Telecommunications
Telecommunications systems
often give you the first clue to
a harmonics problem because
the cable can be run right next
to power cables. To minimize
the inductive interference from
phase currents, telecommunica-
tions cables are run closer to the
neutral wire.
Triplens in the neutral conduc-
tor commonly cause inductive
interference, which can be heard
on a phone line. This is often the
first indication of a harmonics
problem and gives you a head
start in detecting the problem
before it causes major damage.
Transformer
Commercial buildings commonly
have a 208/120 V transformer
in a delta-wye configuration.
These transformers commonly
feed receptacles in a commer-
cial building. Single-phase,
non-linear loads connected to
the receptacles produce triplen
harmonics, which add up in the
neutral. When this neutral cur-
rent reaches the transformer, it is
reflected into the delta primary
winding where it causes over-
heating and transformer failures.
Another transformer problem
results from core loss and copper
loss. Transformers are normally
rated for a 60 Hz phase current
load only. Higher frequency har-
monic currents cause increased
core loss due to eddy currents
and hysteresis, resulting in more
heating than would occur at the
same 60 Hz current.
These heating effects demand
that transformers be derated for
harmonic loads or replaced with
specially designed transformers.
Effects of harmonic currents
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02
03
120 V Branch Circuits
208/480 Volt Transformer
Secondary
Primary
Neutral
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B
C