16 5 WHAT THE 2SX-SERIES SCANNER RECORDS
one minute setting. Often, the exact time of a voltage dip is not as important
as the size– for this case, any reasonable Interval setting is fine.
The most common setting is one minute. This is a good balance between
frequent data collection and long recording time. Since most loads that start
and stop usually run for longer than a minute, the start and stop effects (such
as inrush current) are easily spotted in the Stripchart. An example is an air
conditioner load: a forty minute period of cycling on and off is obvious in the
Stripchart graph as twenty data points at one load current, then twenty data
points at low current, all connected by straight lines on the graph. The first
interval of the high current period will probably have a much larger current
maximum than the rest due to the starting current of the air conditioner. The
voltage interval will probably have a dip at the same time.
The most frequent reason to use an Interval smaller than one minute is
for large loads that cycle on and off more frequently than one minute. For
example, if an elevator is causing power quality problems, and it only takes 10
or 20 seconds to start at one floor and stop at another, a one second Interval
is probably necessary, otherwise the entire elevator travel will occur during a
single Stripchart Interval. In this case, the Scanner should not be left to record
for days, since it would only hold the last few hours of Stripchart data. The
best use in this case is to set the Stripchart to one second, then cycle the load
(such as the elevator) for a while, in an attempt to reproduce the problem,
then download the Scanner. In general, the Interval should be smaller than
the quickest cycling time of a problem load.
The most frequent reason to use an Interval larger than one minute is to
increase the recording time. Setting the Interval to two minutes doubles the
recording time, without a serious loss of time resolution. Other common set-
tings are five and fifteen minutes, used to match metering or billing increments
or regulatory time periods.
The second Stripchart setting is the “Stripchart Wrap-Around” mode. The
best setting for this depends on how the Scanner will be used. Some users
leave a Scanner at a problem site until the customer calls with a power quality
complaint. The Scanner is set to a small Interval such as one minute or thirty
seconds, and Wrap-Around is enabled. Because Wrap-Around is enabled, the
Stripcharts always have the latest few days of data in memory, by discarding
the old data. The Scanner is downloaded, and has the most recent days
of Stripchart data in memory, no matter how long it was recording. This
recent data will have the voltage disturbance in it. Other users will disable
Wrap-Around, and leave a Scanner at a problem site where the power quality
problem will definitely occur soon. The Scanner will record the first week or
so of Stripchart data, then stop Stripchart recording. The Scanner can be
