Fluke 1750, 1577, 1743, 1744, 1735 Service Guide

Power quality recording

and analysis: Techniques

and applications

Application Note

Hooking up a power quality recorder and taking
days’ worth of data can give you a rich picture
of your power. In this article we’ll talk about the
various recording techniques available in power
loggers and recorders—understanding the tools
and techniques you have available will be key to
your strategy. What should you be looking for?
And when does recording make sense?

Recording techniques

To really know your power, ideally,
you’d want to look at every line
cycle to see even the smallest
changes. However, since our power
swings through more than four
million cycles per day, it’s impracti­cal to look at tiny changes—and
often unnecessary. Few loads even
notice occasional voltage dips of
one or two cycles.

How long you record depends
somewhat on the rhythm of your
building. If you are working on a
typical commercial or light indus­trial building, then a week is long
enough for a building to go through
its normal cycle. If you are work­ing in a plant that runs special
equipment only periodically (say
a furnace that runs only monthly),
you’ll need to be sensitive to the
timing of these loads.

Different recording techniques
have been developed to look at
small changes over relatively long
time periods. Many instruments
combine several techniques to
improve their coverage. We’ll
describe the common techniques
and some of their pros and cons.

By understanding the different
techniques. you’ll be better able to
choose an appropriate tool for the
job at hand.

Setting up a Fluke 1735 Power Logger at the service entrance for a
30-day load study.

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Techniques for tracking

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2

4

10 min.

10 min.

20 min.

20 min.

30 min.

30 min.

40 min.

40 min.

3

2

4

X

X

X

X

Actual variation over time

Plot using fixed 10 minute interval, extremes are lost

trends

Trending tracks power quality
parameters over hours or days.
Power loggers measure param­eters like voltage, current, or
power and log them over time.
Trend recording is good for
tracking normal power, subtle
changes, and exceptional condi­tions but may have a limited
ability to catch fast events.
However, instrument makers
have come up with some creative
ways of showing faster events
while allowing recording lengths
of weeks or even months.

Fixed interval logging

This is the simplest form of
digital recording. To set it up, you
choose a time period, or interval,
between readings—usually in
seconds or minutes. The instru­ment calculates an average of the
rms values during each interval
and stores it in memory. This
technique is useful for tracking
changes longer than the logging
interval. Unfortunately, a very
short measurement interval will
catch fast events, but will also
use memory quickly. So even
though fixed interval logging is
easy to set up, it can’t capture
fast events over hours or days.

Figure 2. Min/Max/Avg trendplot.

Min/Max/Avg logging

This technique is similar to fixed
interval logging since it uses
a preset interval. But instead
of taking just one reading per
interval, the instrument takes
many high-speed measurements
over each interval. Processors
within the instrument crunch
through the measurements and
log three numbers for each
interval: a minimum, a maximum
and an average. The min and
max indicate the worst-case,

short-duration events, in some
instruments as short as a single
power cycle. The average tracks
the overall trend. Graphs from
these instruments will often plot
min, average and max on the
same graph.

Automatic time compression,
TrendPlot

TrendPlot is a logging technique
in some Fluke instruments. It is
a form of min/max/avg recording
in which the timescale auto­matically compresses whenever
the trend approaches the end
of memory. When the recorder
starts to run out of memory,
signal processors quickly go to
work. They combine adjacent
intervals into a new min, max,
and average. You still get to
see the worst-case measure­ments and the overall trend. And
because you choose when to stop
the measurement, you automati­cally get the best time resolution
with the available memory.

Figure 1. Fixed interval voltage trend.

2 Fluke Corporation Power quality recording and analysis: techniques and applications