Application Note

4 Fluke Corporation Power quality recording and analysis: techniques and applications
Power quality is recorded in
several general situations. This
section describes the most
common applications.
Troubleshooting with
long-term analysis
Troubleshooting intermittent
failures is challenging. When a
piece of equipment fails or resets
itself mysteriously, it’s tempting
to just swap gear out or reset a
circuit breaker and hope for the
best. For equipment with a high
cost of downtime, the risk of a
repeat failure is too great to rely
on a quick fix. Monitoring the
power after getting your equip-
ment running again will reduce
the number of repeat failures
and rule out power problems if a
failure does recur.
The first decision you’ll have
to make is where to connect
the monitor or analyzer. In
general you should start with the
recorder close to the “victim load”
(the equipment that’s having
problems). This way the monitor
will “see” what the load “sees”. If
you have multiple tools available
it can help to record at different
points in the power system.
Before recording, start by
taking some spot measurements
to answer some basic questions.
Is the voltage level right? Is the
voltage waveform a clean sine
wave or is it noisy or distorted?
If the victim is a 3-phase load,
are the phases balanced? Is the
current being drawn by the load
too high?
The next question is: what
should I record? Unless you are
lucky enough to have a sophis-
ticated power analyzer, you’re
probably going to have to decide
whether to start by tracking
trends or hunting for transients.
Trends will uncover more prob-
lems so try trend recording first
(use fixed interval logging or min/
max/avg). Recording rms voltage
trends on all relevant phases is
the most basic approach. This
will determine if the supply is
subject to voltage dips or outages
which can cause load dropouts
or resets. Recording voltage will
also uncover swells or unbalance
which can cause overheating.
Current trends can also help
in troubleshooting. Excessive
current draw will cause over-
heating. If the voltage is stable,
clean and balanced, high current
indicates a problem with the load
itself. By comparing the voltage
and current recordings you can
tell whether the voltage drop is
being caused by high current
being drawn by the victim load
or is being caused by some other
load upstream.
Voltage distortion can also
cause overheating and should
be recorded if possible. The
most basic way to track voltage
distortion is by recording total
harmonic distortion (THD). Some
analyzers can also track indi-
vidual harmonics which can help
point to the source of high THD.
If the victim load shows
evidence of arcing or blown
input circuitry transients may
be to blame, but don’t jump to
this conclusion too quickly. After
you’ve performed some trend-
ing, or if the evidence strongly
suggests transient damage, then
it’s time to try transient capture
and look at waveforms.
Recording applications
Checking data during recording with the wireless PDA feature of the
FLuke 1750 Power Recorder.