Power quality measurement
It used to be with large AC power systems
that "power quality" was an unheard-of concept, aside from power factor.
Almost all loads were of the "linear" variety, meaning that they did not
distort the shape of the voltage sine wave, or cause non-sinusoidal
currents to flow in the circuit. This is not true anymore. Loads
controlled by "nonlinear" electronic components are becoming more
prevalent in both home and industry, meaning that the voltages and
currents in the power system(s) feeding these loads are rich in
harmonics: what should be nice, clean sine-wave voltages and currents
are becoming highly distorted, which is equivalent to the presence of an
infinite series of high-frequency sine waves at multiples of the
fundamental power line frequency.
Excessive harmonics in an AC power system
can overheat transformers, cause exceedingly high neutral conductor
currents in three-phase systems, create electromagnetic "noise" in the
form of radio emissions that can interfere with sensitive electronic
equipment, reduce electric motor horsepower output, and can be difficult
to pinpoint. With problems like these plaguing power systems, engineers
and technicians require ways to precisely detect and measure these
conditions.
Power Quality
is the general term given to represent an AC power system's freedom from
harmonic content. A "power quality" meter is one that gives some form of
harmonic content indication.
A simple way for a technician to
determine power quality in their system without sophisticated equipment
is to compare voltage readings between two accurate voltmeters measuring
the same system voltage: one meter being an "averaging" type of unit
(such as an electromechanical movement meter) and the other being a
"true-RMS" type of unit (such as a high-quality digital meter). Remember
that "averaging" type meters are calibrated so that their scales
indicate volts RMS, based on the assumption that the AC voltage being
measured is sinusoidal. If the voltage is anything but sinewave-shaped,
the averaging meter will not register the proper value, whereas
the true-RMS meter always will, regardless of waveshape. The rule of
thumb here is this: the greater the disparity between the two meters,
the worse the power quality is, and the greater its harmonic content. A
power system with good quality power should generate equal voltage
readings between the two meters, to within the rated error tolerance of
the two instruments.
Another qualitative measurement of power
quality is the oscilloscope test: connect an oscilloscope (CRT) to the
AC voltage and observe the shape of the wave. Anything other than a
clean sine wave could be an indication of trouble:
Still, if quantitative analysis
(definite, numerical figures) is necessary, there is no substitute for
an instrument specifically designed for that purpose. Such an instrument
is called a power quality meter and is sometimes better known in
electronic circles as a low-frequency spectrum analyzer. What
this instrument does is provide a graphical representation on a CRT or
digital display screen of the AC voltage's frequency "spectrum." Just as
a prism splits a beam of white light into its constituent color
components (how much red, orange, yellow, green, and blue is in that
light), the spectrum analyzer splits a mixed-frequency signal into its
constituent frequencies, and displays the result in the form of a
histogram:
Each number on the horizontal scale of
this meter represents a harmonic of the fundamental frequency. For
American power systems, the "1" represents 60 Hz (the 1st harmonic, or
fundamental), the "3" for 180 Hz (the 3rd harmonic), the "5" for
300 Hz (the 5th harmonic), and so on. The black rectangles represent the
relative magnitudes of each of these harmonic components in the measured
AC voltage. A pure, 60 Hz sine wave would show only a tall black bar
over the "1" with no black bars showing at all over the other frequency
markers on the scale, because a pure sine wave has no harmonic content.
Power quality meters such as this might
be better referred to as overtone meters, because they are
designed to display only those frequencies known to be generated by the
power system. In three-phase AC power systems (predominant for large
power applications), even-numbered harmonics tend to be canceled out,
and so only harmonics existing in significant measure are the
odd-numbered.
Meters like these are very useful in the
hands of a skilled technician, because different types of nonlinear
loads tend to generate different spectrum "signatures" which can clue
the troubleshooter to the source of the problem. These meters work by
very quickly sampling the AC voltage at many different points along the
waveform shape, digitizing those points of information, and using a
microprocessor (small computer) to perform numerical Fourier analysis
(the Fast Fourier Transform or "FFT" algorithm) on those
data points to arrive at harmonic frequency magnitudes. The process is
not much unlike what the SPICE program tells a computer to do when
performing a Fourier analysis on a simulated circuit voltage or current
waveform.
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