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2 Fluke Corporation Applying power quality measurements to predictive maintenance

Integrating power qualityinto PdMUnlike a comprehensive electricalsystem survey, predictive mainte-nance power quality focuses on asmall set of measurements thatcan predict power distribution orcritical load failures. By checkingthe power quality at criticalloads, you see the effect of theelectrical system up to the load.Your predictive maintenanceinspection route probably alreadyincludes any motors, generators,pumps, A/C units, fans, gear-boxes, or chillers on site.

Voltage stability, harmonicdistortion, and unbalance aregood indicators of load and distri-bution system health and can betaken and recorded quickly with

little incremental labor. Currentmeasurements can identifychanges in the way the load isdrawing. All of these measure-ments can be taken withouthalting operations and generatenumbers that can easily beentered into maintenance soft-ware and plotted over time.

For each measurement point orpiece of equipment, determinewhat limit should trigger correc-tive action. Limits should be setwell below the point of failure,

and as time goes on limits may betightened or loosened by ana-lyzing historical data. The appro-priate limits depend somewhat onthe ability of your loads to deal

with power variation. But for mostequipment, your maintenanceteam can devise a set of default,house limits based on industrystandards and experience.

The cost of three-phase poweranalyzers is lower now than everand it should only take roughly 15minutes to take the readings dis-cussed in this article. (Storing volt-age sag data will add more time,since it requires picking up thedata after a day or so.)

VoltageGood voltage level and stabilityare fundamental requirements forreliable equipment operation. Running loads at overly high

or low voltages causes reliabil-ity problems and failures.Verify that line voltage iswithin 10 % of the nameplaterating.

As connections in your systemdeteriorate, the rising imped-ance will cause insulationresistance drops in voltage.

Measurement Guidelines

Added loads, especially thosewith high inrush, will alsocause voltage decline overtime. The loads farthest fromthe service entrance or trans-former will show the lowestvoltage.

Neutral to ground voltage tellsyou how heavily your systemis loaded and helps you trackharmonic current. Neutral toground voltage higher than3 % should trigger furtherinvestigation.

Voltage Measurements Phase-to-Neutral VoltagesNeutral-to-Ground Voltages

Voltage Sags Phase to Neutral Sag CountVoltage Harmonics Phase Voltage THDCurrent Measurements Phase Currents

Voltage Unbalance Negative Sequence, Zero SequenceTable 1. Basic power measurements for 3-phase Wye equipment

Figure 2. Recording all phase voltages and neutral to ground voltageis a good start for a PQ analysis of critical equipment and overalldistribution system health.

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3 Fluke Corporation Applying power quality measurements to predictive maintenance

Voltage sag countTaking a single voltage readingtells only part of the story. How isthe voltage changing during anhour? During a day? Sags, swells

and transients are short-termvariations in voltage. The voltagesag (or dip) is the most commonand troublesome variety.

Sags indicate that a system ishaving trouble responding to loadrequirements and significant sagscan interrupt production. Voltagesags can cause spurious resets onelectronic equipment such ascomputers or controllers, and asag on one phase can cause theother two to overcompensate,potentially tripping the circuit.

Sags have several dimensions:depth, duration, and time of day.Utilities use a special index totrack the number of sags thatoccur over a period of time. Togauge the depth of the sags, theycount how often voltage dropsbelow various thresholds.

The longer and larger thevoltage variations, the more likelyequipment is to malfunction.For example, the InformationTechnology Industry Council(ITIC) curve specifies that 120 V

computer equipment should beable to run as long as voltagedoes not drop below 96 V formore than 10 seconds or below84 V for more than 0.5 seconds.

CurrentCurrent measurements that trendupward are a key indicator of aproblem or degradation in yourload. While equipment is run-ning, monitor phase, neutral andground current over time. Makesure none of the currents areincreasing significantly, verifythat theyre less than the name-plate rating, and keep an eye outfor high neutral current, whichcan indicate harmonics andunbalance.

Voltage unbalanceIn a three-phase system, signifi-cant differences in phase voltageindicate a problem with the sys-tem or a defect in a load. High voltage unbalance causes

three-phase loads to drawexcessive current and causesmotors to deliver lower torque.

Unbalance is tracked in per-centages (see Figure 3). Thenegative sequence voltage(Vneg) and zero sequencevoltage (Vzero) together iden-tify any voltage asymmetrybetween phases.

Using a power quality analyzerto do the math, high percent-ages indicate high unbalance.EN 50160 requires Vneg to beless than two percent.

Voltage harmonic distortionHarmonic distortion is a normalconsequence of a power systemsupplying electronic loads suchas computers, business machines,electronic lighting ballasts, andcontrol systems. Adding orremoving loads from the systemchanges the amount of distortion,so its a good idea to regularlycheck harmonics.

Harmonics cause heating andreduced life in motor windingsand transformers, excessive neu-tral current, increased suscepti-bility to voltage sags, andreduced transformer efficiency.

As current harmonics interactwith impedance, theyre con-verted into voltage harmonics.Total Harmonic Distortion (THD) isa sum of the contributions of allharmonics. By tracking VoltageTHD over time you can determineif distortion is changing. For volt-age harmonics, IEEE 519 recom-mends less than 5 % THD.

Figure 3. This unbalance display shows the voltage unbalanceparameters (Vneg and Vzero) as well as current unbalance.

Figure 4. This Harmonics table shows the voltage THD for eachphase. Note that its normal for the voltage THD on the neutral to runclose to 100 %.

Fluke CorporationPO Box 9090, Everett, WA USA 98206Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The NetherlandsFor more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800) 36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web 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 DescriptionCategoryCAT IV* Three-phase at utility connection,

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

including single-phase commercial lightingand distribution panels

CAT II Single-phase receptacle connected loadsCAT I Electronic

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

International safety standards for test tools