Application Note

2 Fluke Corporation Applying power quality measurements to predictive maintenance
Integrating power quality
into PdM
Unlike a comprehensive electrical
system survey, predictive mainte-
nance power quality focuses on a
small set of measurements that
can predict power distribution or
critical load failures. By checking
the power quality at critical
loads, you see the effect of the
electrical system up to the load.
Your predictive maintenance
inspection route probably already
includes any motors, generators,
pumps, A/C units, fans, gear-
boxes, or chillers on site.
Voltage stability, harmonic
distortion, and unbalance are
good indicators of load and distri-
bution system health and can be
taken and recorded quickly with
little incremental labor. Current
measurements can identify
changes in the way the load is
drawing. All of these measure-
ments can be taken without
halting operations and generate
numbers that can easily be
entered into maintenance soft-
ware and plotted over time.
For each measurement point or
piece of equipment, determine
what limit should trigger c
orrec-
tive action. Limits should be set
well b
elow the point of failure,
and as time goes on limits may be
“tightened” or “loosened” by ana-
lyzing historical data. The appro-
priate limits depend somewhat on
the ability of your loads to deal
with power variation. But for most
equipment, your maintenance
team can devise a set of default,
“house limits” based on industry
standards and experience.
The cost of three-phase power
analyzers is lower now than ever
and it should only take roughly 15
minutes to take the readings dis-
cussed in this article. (Storing volt-
age sag data will add more time,
sinc
e it requires picking up the
data after a day or so.)
Voltage
Good voltage level and stability
are fundamental requirements for
reliable equipment operation.
•
Running loads at overly high
or low voltages causes reliabil-
ity problems and failures.
Verify that line voltage is
within 10 % of the nameplate
rating.
•
As connections in your system
deteriorate, the rising imped-
ance will cause “insulation
resistance drops” in voltage.
Measurement Guidelines
Added loads, espec
ially those
with high inrush, will also
cause voltage decline over
time. The loads farthest from
the ser
vice entrance or trans-
former will show the lowest
voltage.
•
N
eutral to ground voltage tells
you how heavily your system
is loaded and helps you track
harmonic current
. Neutral to
ground voltage higher than
3 % should trigger further
investigation.
Voltage Measurements Phase-to-Neutral Voltages
Neutral-to-Ground Voltages
Voltage Sags Phase to Neutral Sag Count
Voltage Harmonics Phase Voltage THD
Current Measurements Phase Currents
Voltage Unbalance Negative Sequence, Zero Sequence
Table 1. Basic power measurements for 3-phase Wye equipment
Figure 2. Recording all phase voltages and neutral to ground voltage
is a good start for a PQ analysis of critical equipment and overall
distribution system health.