19 5.4 Cycle Histograms
will have all zeroes for a Daily Profile.
A Stripchart can also be used for profiling tasks, but is not ideal. The
stripchart interval is usually set to an interval faster than 15 minutes; a fast
interval can show too much information, making it hard to form a good aver-
age Profile. Often the stripchart only has enough memory for a week or two,
limiting the averaging time; the Daily Profiles have no such limit. Most im-
portantly, the stripchart does not divide the data into an averaged day period,
so it can difficult to spot daily trends in the graph.
5.4 Cycle Histograms
The cycle histograms contain valuable power quality information as well as
information for distribution line profiling. Questions such as “what were the
absolute highest and lowest RMS voltage?”, “how many cycles was the voltage
below 80 volts?”, and “what are the most common load currents?” are easily
answered. The histograms also contain the raw data necessary to answer more
complicated statistical questions such as “what is the probability of a voltage
sag below 100 volts?” and “what high and low limits does the line voltage
meet 99.99% of the time?”. Where the Daily Profiles give average current,
power factor, etc. for distribution profiling, the histograms show what values
are the most common– the “mode” in statistical terms.
5.4.1 What’s Recorded
A Histogram divides a measurement range into many bins. For example, in
the Vip, the voltage Histogram divides the 290V voltage range into 290 bins,
each one volt wide, giving a bin for zero volts, a bin for one volt, two volts,
all the way to 600 volts. After each 60Hz cycle is measured, the voltage is
rounded to the nearest volt and “put” in the appropriate bin. The bins are
really counters that count how many cycles were at that voltage. If the 108
volt bin has a count of 45, then there have been 45 cycles with an RMS voltage
of exactly 108 volts, sometime during the recording session. The Histogram
throws away time information: those 45 cycles could have occurred anytime
during the recording session. They may have been 45 cycles in a row, or
three 15-cycle sags, or 45 isolated sags spread out during the entire recording
session. (To recover the time information, use the Stripchart or an event-based
report.)
Every Stripchart max and min value will have a non-zero count in the
corresponding Histogram. For example, if the voltage Stripchart shows six
sags to 108 volts sometime during the recording session, there should be a
