Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6. Measurement instruments
6.1 Introduction
Instruments used to monitor electromagnetic phenomena can be as simple as an analog voltmeter to an
instrument as sophisticated as a spectrum analyzer. Selecting and using the correct type of monitor requires
the user to understand the capabilities and limitations of the instrument, its responses to power system varia-
tions, and the speciÞc objectives of the analysis. This clause will focus on the capabilities and limitations of
various monitoring equipment.
Instrument features required are dependent on the monitoring location and objectives. If assessing power
quality at the service entrance, for example, the emphasis may be only on long-term steady-state conditions
and utility-transmitted anomalies. The level of detail requiredÑrms voltage stripcharts or high-speed wave-
form capturesÑis indicated by the type of phenomena likely to be causing problems.
6.2 AC voltage measurements
The analog electromechanical voltmeter is the oldest type of voltmeter and is the one that is most familiar.
Once the correct scale is selected, voltage is read directly from an analog scale. AC measurements made with
this type of instrument require an understanding of the waveform to be measured. The ac scales are cali-
brated on the basis of a sinusoidal wave form. If the voltage being measured does not have a sinusoidal
waveform, the voltage reading is not correct. Internally, the voltage being measured is rectiÞed with either a
half-wave or a full-wave bridge, and the resulting average dc voltage is measured. The meter scale is then
calibrated with a conversion factor to obtain the correct ac voltage readings.
Another type of ac voltmeter is the digital voltmeter (DVM). These meters are more accurate and are easier
to use than their electromechanical counterparts. Two common measurement techniques used in these
meters are average and peak-sense. As with the analog meter discussed above, the averaging meter takes the
average of the absolute value of the instantaneous voltages over a cycle, and the peak sense meter detects the

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
30
highest instantaneous voltage during a cycle. Most digital voltmeters use some form of average conversion
for ac measurements as this is the easiest and least expensive measurement technique to implement. Both of
these techniques, average and peak-sense, are calibrated for a sinusoidal waveform. An average sense meter
is calibrated to display 1.11 times the average voltage, and the peak sense meter is calibrated to display
0.707 times the peak voltage. If the waveform being measured is sinusoidal, these calibration factors will
yield ac voltage measurements that will agree with measurements being made with a true rms voltmeter.
True rms meters accurately measure the effective heating value of current, distorted or sinusoidal.