Page 219 - Power Quality in Electrical Systems

Chapter
14

Power Quality Measurements

In this chapter, we shall discuss practical issues related to
making power-quality measurements. Any measurement job
requires the proper tools to do that job. The monitoring of
power quality at the supply end, at distribution and
transmission, and at the load end has become of major interest
in recent years. In the following, we will discuss various types
of equipment used in power-quality measurements, and the
proper use and limitations of such equipment.

Multimeters
Multimeters are used to measure voltages, currents, and resistances.
Typical applications of multimeter measurements are line-voltage meas-
urements and imbalance measurements. In making voltage and current
measurements, generally we’re interested in measuring the rms value
of the waveforms. Some subtleties are associated with this measurement.
In one type of multimeter, the input waveform is rectified and aver-
aged, as shown in Figure 14.1a. This method works well for pure sine
waves (where the rms value is the peak value divided by the square root
of two), but for waveforms with harmonics, this method is prone to errors.
Error is especially high for waveforms with a high crest factor.
Several types of multimeters produce “true-rms” readings—that is,
they accurately report the rms value of waveforms with harmonic
distortion. One type of true-rms multimeter employs an rms-to-DC
converter (Figure 14.1b). Several IC manufacturers produce rms-to-
1
DC converter integrated circuits. Another method of producing true-rms
readings involves a thermal measurement method (Figure 14.1c) [14.1].

1
See, for example, Analog Devices and Linear Technology.
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