(^34) DESIGN WORLD — EE NETWORK 6 • 2019 eeworldonline.com | designworldonline.com
TEST & MEASUREMENT HANDBOOK
Amplitude, in units of power (dB)
rather than volts, displays on the Y-axis
and frequency, rather than time, displays
along the Y-axis. These are the harmonics
that, added together and divided by the
fundamental, make up THD.
A high THD level in power systems
is harmful for the system as well as for
connected equipment. Lower THD equates
to lower peak currents, higher efficiency and
higher power factor.
Power factor is generally thought of
as determined by the phase relationship
between voltage and current, in accordance
with: Power Factor PF = cos θv - cos θi, where
θv is the phase angle of the voltage and θi is
the phase angle of the current.
While this equation, known as the
displacement factor, is valid when voltage
and current are sinusoidal, it does not
account for THD in non-sinusoidal circuits,
which are prevalent today thanks to the rise
of nonlinear loads with abundant harmonics.
Loads that include power conversion
equipment -- such as ac-dc, dc-ac and dc-
dc, or nonlinear loads such as fluorescent
ballasts -- create a heavy nonlinear
environment in which harmonics and THD
abound. Switching power supplies, now
common in office and home, contribute
to this mix. This loading modifies the
higher-quality sinusoidal power at the
utility generator terminals. (Generators
do contribute some fifth-order harmonics
because of magnetic flux that takes place at
the stator slots in addition to non-sinusoidal
flux across the air gap.)
Scopes and THD meters aren’t the only
instruments capable of gauging harmonic
content. Power analyzers, such as the
PA3000 from Tektronix, are optimized for
characterizing power sources, including their
harmonic content.[/caption]
VFDs, welders and arc furnaces also
generate prodigious amounts of THD.
Because harmonic currents are at
higher frequencies that the power system
fundamental, they see greater impedances.
The cause of this strange phenomenon is
that greater amounts of higher-frequency
current flow near the surface of a conductor.
With less usable cross-sectional area, the
effective resistance of the conductor rises,
resulting in more heat. This is seen in three-
phase neutral conductors and transformer
windings.
When an ac motor is powered by
a VFD, it gets a powerful direct dose of
harmonics. This is a consequence of the
high-speed switching in the VFD inverter
section. Most of the ambient harmonics
caused by other nonlinear loads in the same
building or neighborhood are not much
of a problem because they generally get
suppressed when the power goes through
the dc bus midway through the VFD. These
outside harmonics do, however, assault the
many autonomous motors that are found in
the workplace.
For one thing, harmonics create flux
distribution in motor air gaps, causing
poor start-ups and abnormally high slip
in induction motors. A serious problem in
motors and generators is pulsating torque,
causing losses and mechanical oscillations
with harmful heat.
Here’s the greatest problem in motors when
there is high THD riding on the good power
at the input:
Input Impedance matching
attenuator
Input
Sensitivity
control
Frequency range
select
Bridge amp Wien bridge Pre-amp
Post
attenuator Meter amp Output
Typical block diagram, THD meter
Before scopes began to double as spectrum
analyzers, the typical means of gauging THD
was with a fundamental suppression THD
analyzer. The instrument input is typically
impedance-matched with the rejection circuit
via an attenuator and an impedance matcher.
This signal is then pre-amplified and sent to
a Wien bridge notch filter tuned to reject
the fundamental frequency and balanced
for minimum output by adjusting the bridge
controls. The output is the remaining signal
after the fundamental has been suppressed.
A feedback loop from the bridge amp output
to the pre-amp input helps eliminate any
remaining contribution from the fundamental
frequency. The output from these blocks is
measured, typically using an instrumentation
amp driving an analog or digital meter.
The voltage at the meter is caused by the
harmonic distortion products plus noise.
Herres — Test and Measurement HB 06-19.indd 34 6/10/19 11:09 AM
amelia
(Amelia)
#1