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

6.5.2.4 Graphical display monitors
The fourth type of power-line disturbance monitor is a graphical display monitor. These instruments collect
and record power system variations in a graphical format that is enhanced with alphanumeric descriptions
that are similar to the text monitors previously discussed. Power system variations are recorded by the time
of the occurrence with a graphical representation of the variation. These variations are further enhanced with
alphanumeric descriptions. The data collected may be displayed on a paper tape, a CRT type of display, or
stored on some type of electronic media.
Data collection used in the graphical display monitors is based on Þxed or variable sampling techniques that
break down the ac voltage waveform into a series of discrete steps that can be stored. This stored data is then
recombined to present a representation of the original ac waveforms. The speed of the sampling rate dictates
the degree of detail available to reconstruct the ac waveform.

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
34
Steady-state values are recorded either at some manufacturer-selected interval, or can be recorded when a
user-selected sensitivity threshold is surpassed. In addition, out-of-limits data is recorded when a power sys-
tem variation exceeds the threshold parameters preset in the machine or adjusted by the user. These variables
include, but should not be limited to, sags and swells, waveform distortion, and impulsive transients. As in
the previous instrument, comparison circuits may be analog and/or digital.
Thresholds for triggering in the event of a power system variation are based on software controls. Compari-
son algorithms are installed in the system software that allow a wide range of thresholds to be established for
the collection of data.
When a power system variation that exceeds the established thresholds is detected, the digitized data of the
variation is stored in memory. This data is then measured in numerous ways to establish the parameters of
the power system variation. Once these measurements are complete, the digitized data is used to provide a
detailed graphical representation of the power system variation.
As mentioned earlier, reporting of the data collected may be provided through a paper tape record or trans-
ferred to some form of electronic media for storage and future reference. Graphical representation may also
be displayed through the use of a CRT type of display.
These monitors may be set to collect large amounts of data and many events over a period of time. The sub-
sequent analysis is time consuming and difÞcult. Prior planning and thoughtful application of thresholds
may be required to control data collection and analysis.