Fluke 789, 1587, 1577, 113, 175 Service Guide

Troubleshooting

power harmonics

Basic troubleshooting using

multimeters and current clamps

A mystery is occurring in today’s office buildings and
manufacturing plants. Transformers supplying seemingly
average loads are overheating. Neutral conductors in
balanced circuits are overheating from excessive loads.
Circuit breakers are tripping for no apparent reason.
Yet the standard troubleshooting procedures show
everything to be normal. So what’s the problem?
In one word—harmonics.

New technology,
new challenges

Harmonics are the byproducts
of modern electronics. They
are especially prevalent wher­ever there are large numbers of
personal computers, adjustable
speed drives, and other types of
equipment that draw current in
short pulses.

This equipment is designed
to draw current only during a
controlled portion of the incom­ing voltage waveform. While
this dramatically improves
efficiency, it causes harmon­ics in the load current. And that
causes overheated transformers
and neutrals, as well as tripped
circuit breakers.

If you were to listen to an ordi­nary 60-cycle power line, you’d
hear a monotone hum. When
harmonics are present, you hear
a different tune, rich with high

notes. The problem is even more
evident when you look at the
waveform. A normal 60-cycle
power line voltage appears on
the oscilloscope as a near sine
wave (Figure 1). When harmon­ics are present, the waveform
is distorted (Figure 2A and 2B).
These waves are described as
non-sinusoidal. The voltage and
current waveforms are no longer
simply related-hence the term
“non-linear.”

Getting to the root of the
problem

Finding the problem is relatively
easy once you know what to look
for and where to look. Harmonics
symptoms are usually anything
but subtle. This application note
provides some basic pointers on
how to find harmonics and some
suggestions of ways to address
the problems they create.

Application Note

Figure 1. Near sine wave.

Figure 2A. Distorted current waveform.

Figure 2B. Distorted voltage waveform.

From the Fluke Digital Library @ www.fluke.com/library

Sources of harmonics

Defining the problem

Harmonics are currents or volt­ages with frequencies that are
integer multiples of the funda­mental power frequency. For
example, if the fundamental fre­quency is 60 Hz, then the second
harmonic is 120 Hz, the third is
180 Hz, etc.

Harmonics are created by non­linear loads that draw current
in abrupt pulses rather than in
a smooth sinusoidal manner.
These pulses cause distorted
current wave shapes which in
turn cause harmonic currents to
flow back into other parts of the
power system.

Figure 3A. Single-phase, non-linear load
current waveform.

The inside story

This phenomenon is especially
prevalent with equipment that
has diode-capacitor input power
supplies; i.e., personal comput­ers, printers and medical test
equipment.

Electrically what happens is
the incoming ac voltage is diode
rectified and is then used to

charge a large capacitor. After
a few cycles, the capacitor is
charged to the peak voltage of
the sine wave (e.g., 170 V for a
120 V ac line). The electronic
equipment then draws current
from this high dc voltage to
power the rest of the circuit.

The equipment can draw the
current down to a regulated
lower limit. Typically, before
reaching that limit, the capacitor
is recharged to the peak in the
next half cycle of the sine wave.
This process is repeated over
and over. The capacitor basically
draws a pulse of current only
during the peak of the wave.
During the rest of the wave,
when the voltage is below the
capacitor residual, the capacitor
draws no current.

The diode/capacitor power
supplies found in office equip­ment are typically single-phase,
non-linear loads (Figure 3A).
In industrial plants, the most
common causes of harmonic cur­rents are three-phase, non-linear
loads which include electronic
motor drives, and uninterruptible
power supplies (UPS) (Figure 3B).

Figure 3B. Three-phase, non-linear load
current waveform.

Voltage harmonics

The power line itself can be an indirect
source of voltage harmonics.

The harmonic current drawn by non-linear
loads acts in an Ohm’s law relationship
with the source impedance of the supplying
transformer to produce voltage harmonics.
Source impedance includes the supplying
transformer and branch circuit components.
For example, a 10 A harmonic current being
drawn from a source impedance of 0.1 W will
generate a harmonic voltage of 1.0 V.

Any loads sharing a transformer or a
branch circuit with a heavy harmonic load
can be affected by the voltage harmonics
generated.

The personal computer can be affected by
voltage harmonics. The performance of the
diode/capacitor power supply is critically
dependent on the magnitude of the peak
voltage. Voltage harmonics can cause “flat
topping” of the voltage waveform lowering
the peak voltage (see Figure 2B). In severe
cases, the computer may reset due to insuf­ficient peak voltage.

In the industrial environment, the induc­tion motor and power factor correction
capacitors can also be seriously affected by
voltage harmonics.

Power correction capacitors can form a
resonant circuit with the inductive parts
of a power distribution system. If the reso­nant frequency is near that of the harmonic
voltage, the resultant harmonic current
can increase substantially, overloading the
capacitors and blowing the capacitor fuses.
Fortunately, the capacitor failure detunes the
circuit and the resonance disappears.

2 Fluke Corporation Troubleshooting power harmonics

Effects of harmonic currents

01

02

03

120 V Branch Circuits

208/480 Volt Transformer

Secondary

Primary

Neutral

A

B

C

Symptoms of harmonics usually
show up in the power distribu­tion 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 wide­spread 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 perfor­mance if not designed to handle
electronic loads.

Neutral conductors

In a three-phase, four-wire
system, neutral conductors can
be severely affected by non­linear 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 harmon­ics 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 over­heating because, unlike phase
conductors, there are no circuit
breakers in the neutral conductor
to limit the current.

Excessive current in the neu­tral conductor can also cause
higher-than-normal voltage
drops between the neutral con­ductor 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 protect­ing 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, telecommunica­tions cables are run closer to the
neutral wire.

Triplens in the neutral conduc­tor 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 commer­cial building. Single-phase,
non-linear loads connected to
the receptacles produce triplen
harmonics, which add up in the
neutral. When this neutral cur­rent reaches the transformer, it is
reflected into the delta primary
winding where it causes over­heating 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 har­monic 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.

3 Fluke Corporation Troubleshooting power harmonics