Fluke 434 Series Ii Application Note 2435544

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Figure 1. This maintenance control chart tracking voltage unbalance shows a Nov-03 reading above the 2.5 %

limit and then the effect of the corrective action.

Application Note

Applying power quality

measurements to

predictive maintenance

You may already be using pre-

dictive 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 maintenance

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:

•Lost income (gross margin)

due to downtime

•Cost of labor to troubleshoot,

patch, clean up, repair and

restart

•Cost of damaged equipment

and materials, including

repairs, replacements and

scrapped material

To calculate power quality

costs and potential savings

at your facility, reference the

online calculator at

www.fluke.com/pqcalculator.

PART FOUR

of a predictive

maintenance series

For more information on Fluke Predictive

Maintenance Products and Services go to

www.fluke.com/pdm

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 correc-

tive action. Limits should be set

well below 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,

since 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, especially those

with high inrush, will also

cause voltage decline over

time. The loads farthest from

the service entrance or trans-

former will show the lowest

voltage.

•Neutral 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.

3 Fluke Corporation Applying power quality measurements to predictive maintenance

Voltage sag count

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 overcompensate,

potentially tripping the circuit.

Sags have 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

upward are a key indicator of a

problem or degradation in your

load. While equipment is run-

ning, monitor phase, neutral and

ground current over time. Make

sure none of the currents are

increasing significantly, verify

that they’re less than the name-

plate rating, and keep an eye out

for high neutral current, which

can indicate harmonics and

unbalance.

Voltage unbalance

In a three-phase system, signifi-

cant differences in phase voltage

indicate a problem with the sys-

tem or a defect in a load.

•High voltage unbalance causes

three-phase loads to draw

excessive current and causes

motors to deliver lower torque.

•Unbalance is tracked in per-

centages (see Figure 3). The

negative sequence voltage

(Vneg) and zero sequence

voltage (Vzero) together iden-

tify any voltage asymmetry

between phases.

•Using a power quality analyzer

to do the math, high percent-

ages 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 neu-

tral current, increased suscepti-

bility to voltage sags, and

reduced transformer efficiency.

As current harmonics interact

with impedance, they’re con-

verted 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 volt-

age harmonics, IEEE 519 recom-

mends less than 5 % THD.

Figure 3. This unbalance display shows the voltage unbalance

parameters (Vneg and Vzero) as well as current unbalance.

Figure 4. This Harmonics table shows the voltage THD for each

phase. Note that it’s normal for the voltage THD on the neutral to run

close to 100 %.

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-51

16

In Europe/M-East/Africa (31 40) 2 675 200 or

Fax (31 40) 2 675 222

In Canada (800) 36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2005 Fluke Corporation. All rights reserved.

Printed in U.S.A. 3/2005 2435544 A-US-N Rev A

Fluke. Keeping your world

up and running.

Overvoltage Summary Description

Category

CAT IV* Three-phase at utility connection,

any outdoors conductors (under 1000 V)

CAT III Three-phase distribution (under 1000 V),

including single-phase commercial lighting

and distribution panels

CAT II Single-phase receptacle connected loads

CAT I Electronic

*CAT IV product specifications are not yet defined in the standard.

International safety standards for test tools