Case Studies

The system
monitor function
Most power quality problems
can be detected by taking
detailed voltage measurements
and tracking how they vary
over time. A power system that
consistently delivers strong,
stable voltage is a healthy
system. The system moni-
tor function in the Fluke 434
analyzes dozens of voltage
characteristics against preset
limits for up to a week, and
presents this wealth of data in
a single pass/fail display. To
look at how Benoit uses this
function, we’ll examine an
evaluation he did on the power
feeding an MRI system.
System monitor uses limits
from the EN50160 Power
Quality Standard to determine
if voltage characteristics are
within acceptable tolerances.
The characteristics it looks
at are:
•
Average rms voltage
•
Harmonics
•
Flicker
•
Voltage events (dips,
swells, rapid changes,
or interruptions)
•
Unbalance
•
Frequency
System monitor tracks all of
these characteristics for up to
a week. It also records current
to help determine the cause of
voltage changes.
You can use the EN50160
limits which are the defaults,
or you can adjust them to suit
your needs. In this case Benoit
reviewed manufacturer’s spec-
ifications and decided to set
the voltage limits a bit tighter,
to 92 % instead of 90 %.
The details behind pass
or fail
Each of the voltage charac-
teristics on the main System
Monitor display as a pass/fail
bar. But behind this simple
display is a lot of detail.
System monitor summary shows
pass/fail results for average voltage,
harmonics, flicker, dips, unbalance and
frequency. In this case all parameters
were in tolerance.
3 Fluke Corporation Tips for learning power quality analysis
These limits determine when the 434
stores voltage dips in System Monitor.
In Benoit’s voltage data you
can see the weekly genera-
tor test required for hospitals.
The transfer to backup power
shows up as a dip of roughly
two cycles. You can see the
working rhythm of the build-
ing—low currents during the
weekends and nighttime, high
current during working hours.
Stan Benoit has learned a
lot about power quality over
the past year. To some extent
he is becoming a victim of his
own success. He’s worked with
other biomedical engineers
to debug a problem with a
cardiac computer system. He’s
been called in to check power
feeding HVAC chillers. “Since
I’ve gotten the 434 I’ve been
doing favors for people I didn’t
even know!” He’s counting on
two things that will make him
less “popular”: first that he can
teach some of his co-workers
what he’s learned, and second
that other departments will
get their own power quality
analyzer.
Dips and swells trend shows
generator test.
Currrent trend shows lower loading on
weekends and most nights.
The events screen shows the same
event in a different way.
Harmonics monitor shows the
worst-case harmonics and THD
over the week.