Page 223 - Power Quality in Electrical Systems

Power Quality Measurements 205

Power-Quality Meters and Analyzers

A number of manufacturers make “power-quality meters” and “power-
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quality analyzers,” which are instruments similar to oscilloscopes, but that
have a number of functions particularly suited to making power-quality
measurements. In measuring power quality, we must be able to trigger on
events that are not continuous (such as a voltage sag) or transient (like that
caused by lightning strikes or utility faults). For instance, typical power-
quality meters/analyzers and analyzers have the following functionalities:
■ Datalogging—capturing waveforms in real-time for later display
■ The ability to trigger on power-quality events such as sags, swells, or
transients
■ Calculation of power-quality metrics such as total harmonic distortion
in real-time
■ Spectrum analysis
■ Inputs for high-voltage probes and high-current probes

Numerous factors [14.3] should be considered when selecting a power-
quality measurement meter/analyzer, including:

■ The number of channels (for instance, single-phase or three-phase)
■ The input voltage range
■ The current measurement range
■ Isolation
■ Communication capabilities (For example, can the instrument be net-
worked or tied to a stand-alone computer?)

The block diagram of a typical system with a power-quality analyzer
is shown in Figure 14.5. The power-quality analyzer A/D converts system
voltages and currents. Power indices such as THD, harmonic content,
and the like are calculated in real-time. In addition, the analyzer has
on-board memory so data may be saved for future analysis.
The output of a typical power-quality analyzer is shown in Figure 14.6,
where we see the time waveform of the line current in a copy machine (top
trace) and the harmonics (bottom trace).

Current Transformer Analysis in Detail
A typical setup for high-current sensing is shown in Figure 14.7. The pri-
, and is supplied by a
mary side current we are trying to measure is I p
bus bar. The CT steps the current down with the ratio 1:N . We resistively
s
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See, for example, Fluke and Dranetz BMI, among others.