Application Note

2 Fluke Corporation Troubleshooting power harmonics
Sources of harmonics
Defining the problem
Harmonics are currents or volt-
ages with frequencies that are
integer multiples of the funda-
mental power frequency. For
example, if the fundamental fre-
quency is 60 Hz, then the second
harmonic is 120 Hz, the third is
180 Hz, etc.
Harmonics are created by non-
linear 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.
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 equip-
ment are typically single-phase,
non-linear loads (Figure 3A).
In industrial plants, the most
common causes of harmonic cur-
rents 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.
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 comput-
ers, printers and medical test
equipment.
Electrically what happens is
the incoming ac voltage is diode
rectified and is then used to
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 insuf-
ficient peak voltage.
In the industrial environment, the induc-
tion 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 reso-
nant 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.