Page 1
PART FOUR
of a predictive
maintenance series
Applying power quality
measurements to
predictive maintenance
You may already be using predictive maintenance (PdM)
techniques on your motors and
drives. But how often do you
inspect the power to your equip
ment? By adding basic power
quality measurements to produc
tion equipment maintenanc
procedures you can head off
unexpected failures in both
production equipment and your
power system.
Cost savings
Insurance claims data in the
NFPA 70B maintenance standard
shows that roughly half of the
cost associated with electrical
failures could be prevented by
regular maintenance. A study
published in IEEE 493-1997 says
that a poorly maintained system
can attribute 49 % of its failures
to lack of maintenance.
To determine the cost of a
failure, it helps to consider three
key categories:
ross marg
ome (g
ost inc
L
•
due to dow
Cost of labor to troubleshoot,
•
patch, clean up, repair and
restart
Cost of damaged equipment
•
and materials, including
repairs, replac
scrapped material
To calculate power quality
costs and potential savings
at your facility, reference the
online calculator at
www.fluke.com/pqcalculator.
ntime
ements and
-
-
e
in)
Application Note
e 1.
Figur
limit and then the effect of the corrective action.
This maintenanc
e control chart tracking voltage unbalance shows a Nov-03 reading above the 2.5 %
For more information on Fluke Predictive
Maintenance Products and Services go to
www.fluke.com/pdm
Page 2
Integrating power quality
into PdM
Unlike a comprehensive electrical
system survey, predictive maintenance 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, gearboxes, or chillers on site.
Measurement Guidelines
Voltage Measurements Phase-to-Neutral 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
Voltage stability, harmonic
distortion, and unbalance are
good indicators of load and distribution 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 measurements can be taken without
halting operations and generate
numbers that can easily be
entered into maintenance software and plotted over time.
For each measurement point or
piece of equipment, determine
what limit should trigger c
tive action. Limits should be set
elow the point of failure,
well b
Neutral-to-Ground Voltages
orrec-
and as time goes on limits may be
“tightened” or “loosened” by analyzing historical data. The appropriate 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 discussed in this article. (Storing voltage sag data will add more time,
e it requires picking up the
sinc
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 reliability problems and failures.
Verify that line voltage is
within 10 % of the nameplate
rating.
As connections in your system
•
deteriorate, the rising impedance will cause “insulation
resistance drops” in voltage.
2 Fluke Corporation Applying power quality measurements to predictive maintenance
Added loads, espec
with high inrush, will also
cause voltage decline over
time. The loads farthest from
vice entrance or trans-
the ser
former will show the lowest
voltage.
eutral to ground voltage tells
N
•
you how heavily your system
is loaded and helps you track
harmonic current
ground voltage higher than
3 % should trigger further
investigation.
ially those
. Neutral to
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.
Page 3
oltage sag count
V
Taking a single voltage reading
tells only part of the story. How is
the voltage changing during an
hour? During a day? Sags, swells
and transients are short-term
variations in voltage. The voltage
sag (or dip) is the most common
and troublesome variety.
Sags indicate that a system is
having trouble responding to load
requirements and significant sags
can interrupt production. Voltage
sags can cause spurious resets on
electronic equipment such as
computers or controllers, and a
sag on one phase can cause the
other two to overc
ompensate,
potentially tripping the circuit.
Sags ha
ve several dimensions:
depth, duration, and time of day.
Utilities use a special index to
track the number of sags that
occur over a period of time. To
gauge the depth of the sags, they
count how often voltage drops
below various thresholds.
The longer and larger the
voltage variations, the more likely
equipment is to malfunction.
For example, the Information
Technology Industry Council
(ITIC) curve specifies that 120 V
computer equipment should be
able to run as long as voltage
does not drop below 96 V for
more than 10 seconds or below
84 V for more than 0.5 seconds.
Current
Current measurements that trend
Unbalance is tracked in per-
•
centages (see Figure 3). The
negative sequence voltage
(Vneg) and zero sequence
voltage (Vzero) together identify any voltage asymmetry
between phases.
Using a power quality analyzer
•
to do the math, high percentages indicate high unbalance.
EN 50160 requires Vneg to be
less than two percent.
Voltage harmonic distortion
Harmonic distortion is a normal
consequence of a power system
supplying electronic loads such
as computers, business machines,
electronic lighting ballasts, and
control systems. Adding or
removing loads from the system
changes the amount of distortion,
so it’s a good idea to regularly
check harmonics.
Harmonics cause heating and
reduced life in motor windings
and transformers, excessive neutral current, increased susceptibility to voltage sags, and
reduced transformer efficiency.
As current harmonics interact
with impedance, they’re converted into voltage harmonics.
Total Harmonic Distortion (THD) is
a sum of the contributions of all
harmonics. By tracking Voltage
THD over time you can determine
if distortion is changing. For voltage harmonics, IEEE 519 recom-
.
D
mends less than 5 % T
H
upward are a key indicator of a
problem or deg
radation in your
load. While equipment is running, monitor phase, neutral and
round current over time. Make
g
sure none of the currents are
increasing sig
nificantly, verif
that they’re less than the name
y
plate rating, and keep an eye out
for high neutral current, which
can indicate harmonics and
unbalance.
oltage unbalance
V
-
In a three-phase system, sig
nifi
cant differences in phase voltage
indicate a problem w
ith the sys
tem or a defect in a load
High voltage unbalance causes
•
three-phase loads to draw
essive current and causes
exc
-
e 4.
.
Figur
phase. N
close to 100 %.
This Harmonics table show
ote that it
’s normal for the voltage T
s the voltage T
motors to deliver lower torque.
3 Fluke Corporation Applying power quality measurements to predictive maintenance
Figure 3. This unbalance display shows the voltage unbalance
parameters (Vneg and Vzero) as well as current unbalanc
International safety standards for test tools
voltage Summary Description
Over
Category
CAT IV* Three-phase at utility connection,
CAT III Three-phase distribution (under 1000 V),
CAT II Single-phase receptacle connected loads
CAT I Electronic
*CAT IV product specifications are not yet defined in the standard.
any outdoors conductors (under 1000 V)
including single-phase commercial lighting
and distribution panels
eping your world
Fluke. K
e
up and running.
D for each
H
D on the neutral to run
H
Fluke Corporation
O Box 9
P
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The N
For more information call:
In the U.S.A. (800) 443-5853 or
ax (425) 446-5
F
In Europe/
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ax (9
F
From other countries +1 (425) 446-5500 or
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Web access: http://www.fluke.com
©2005 Fluke Corporation
Printed in U.S.A. 3/2005 2435544 A-US-N Rev A
0
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