POWERQUALITY BY RICHARD P. BINGHAM
However, this and the aforementioned sources fail to account for
harmonics, one of the most prevalent
power quality phenomena. Harmonics
have affected electric power systems for
years, since the widespread use of fluorescent lights began in 1938 when GE
introduced fluorescent Mazda lamps
as a regular product line, selling about
200,000 in the first year. At conferences
and in journals, people began to question
the effect of harmonics on the accuracy
of the traditional power measurements
used in revenue meters.
The Institute of Electrical and Electronics Engineers (IEEE) organized a
task force, chaired by Alex Emmanuel of
Worcester Polytechnic Institute. This task
force developed Standard 1459 2000, the
“IEEE Trial Use Standard for Standard
Definitions for the Measurement of Electric Power Quantities Under Sinusoidal,
Nonsinusoidal, Balanced, or Unbalanced
Conditions” (now IEEE 1459 2010).
The problem with using the old power
triangle is in the reactive power leg of the
triangle. Reactive power is the result of
inductance or capacitance in the elec-
trical system, which are both common
in nearly every circuit. Nonlinear loads
cause harmonic currents that result in
harmonic voltage, which propagates to
other system circuits. Voltage distortions
of 5 percent or more are not uncommon
anymore. In many locations, the replace-
ment of incandescent lamps with compact
fluorescent or light-emitting diode tech-
nology increases those numbers. Whereas
a resistive load’s impedance is frequency
insensitive, inductive and capacitive loads
are affected. The higher the harmonic
frequency, the higher the impedance of
an inductor and the lower the impedance
of a capacitor. The reactive power value
is different for the fundamental, second
harmonic, third harmonic, all the way
to the nth harmonic, and must be com-
puted separately.
To simplify the math, we typically
compute the fundamental frequency
reactive power as the other vector at
90 degrees to the real power vector and
combine all of the harmonic power into
a single vector called the “distortion
power.” In Figure 2, the apparent power
is kept at the same value as Figure 1, but
the “nonworking power” now consists
of the fundamental reactive power and
distortion power vectors. The net result
is that there is less “working” or real
power for the same apparent power. In
equine terms, less horsepower is applied
to actually move the railcar down the
tracks. In electrical terms, the power
factor (PF) is lower since there are fewer
watts in the numerator for the same
volt-amperes in the denominator of the
equation in Figure 1.
As I explained in “Imperfect Harmony”
(October 2015, ELECTRICAL CONTRACTOR),
some of the harmonics are effectively
“negative sequence” components, which
try to turn an electric motor in the opposite
rotation from the fundamental frequency
power. Others are zero-sequence components, generating no useful work. These
values of the distortion power vector work
against the “horse” to reduce its efficiency.
In mechanical terms, bad bearings on the
wheels or incorrectly aligned railroad
tracks are horsepower losses.
An electric utility has to generate
and distribute the volt-amperes the customer’s loads require, so they become
less efficient as the PF decreases. Hence,
tariffs or rate schedules for some industrial and commercial customers add a PF
penalty or extra expense for inefficiently
using their power. Some old electromechanical meters used to display the
displacement power factor (DPF), or
the cosine of the angle between the voltage and current, as the PF. It is possible
for the DPF to be 1.0 and the true PF be
significantly less, depending on the harmonic content.
Short of requiring all students
and professionals to read ELECTRICAL
CONTRACTOR, one must continue to exercise caution while surfing the web when it
comes to PF (or just about anything else).
A Wealth of Misinformation
Harmonics and online-information sources
WHILE FEW WOULD ARGUE ABOUT of the wealth of information
on the Internet, there isn’t a mechanism for qualifying its accuracy.
Basically, anyone can post anything. Even such peer-reviewed sites
as Wikipedia can fall short of the facts. A relatively simple concept in
power is a good example. Online textbooks, PowerPoint presentations
and numerous websites from some of the most reputable universities
and companies (including electric utilities) still promote the “power
triangle” as a method to describe the relationship between real power,
reactive power and apparent power. Even the U.S. Department of
Energy (DOE) contributes to this inaccurate information, as shown
in Figure 1.
BINGHAM, a contributing editor for power quality, can be reached at 732.248.4393. DIA
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Figure 1: Using horse
and railcar to explain
power factor
Figure 2: Adding
distortion power
to the mix
