basic power quality

8/13/2019 Basic Power Quality

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Application Note

While weve enjoyed enormousbenefits from the evolution ofsolid state technology, the fact isthat the microelectronics at theheart of that technology requiresclean power. Faster speeds andlower voltages mean that there is

less and less tolerance for any-thing less than quality power.Power Quality (PQ) covers

a wide range of issues, fromvoltage disturbances like sags,swells, outages and transients,to current harmonics, to perfor-mance wiring and grounding.The symptoms of poor PQ includeintermittent lock-ups and resets,corrupted data, premature equip-ment failure, overheating ofcomponents for no apparentcause, etc. The ultimate cost is in

downtime, decreased productivityand frustrated personnel.

Start at the scene of thecrime

To troubleshoot PQ problems,one approach is to start as closeto the victim load as possible.The victim load is the sensi-

tive load, typically electronic,that is somehow malfunctioning.Poor PQ is suspected, but partof your job is to isolatePQ as acause from other possible causes(hardware, software?). Like anydetective, you should start at the

scene of the crime. This bottom-up approach can take you a longway. It relies on making use ofa sharp eye and on taking somebasic measurements.

An alternative is to start at theservice entrance, using a three-phase monitor, and work backto the victim load. This is mostuseful if the problems originatewith the utility. Yet survey aftersurvey has concluded that thegreat majority of PQ problemsoriginate in the facility. In fact,

as a general rule, PQ is best atthe service entrance (connec-tion to utility) and deteriorates asyou move downstream throughthe distribution system. Thatsbecause the facilitys own loadsare causing the problems. Anotherilluminating fact is that 75 % of

PQ problems are related to wiringand grounding problems!

Basic power qualitymethodology and

common culprits

For this reason, many PQauthorities recommend that alogical troubleshooting flow is tofirst diagnose the electrical infra-structure of the building, thenmonitor if necessary. Our bottom-up troubleshooting procedure

is designed to help you do thisdetective work.

First steps

1. Make a map: Obtain orcreate a current one-lineIts tough to diagnose PQ prob-lems without having a workingknowledge of the site beinginvestigated. You can start bylocating or reconstructing aone-line diagram of the site.The one-line will identify the acpower sources and the loads they

serve. The as built one-line, theone with red-lines, is the oneyou want.

If you work on-site, the mapmight already exist in your head,but it will be a big help to every-one, including yourself, if its onpaper. If youre coming to a worksite for the first time, getting anup-to-date one-line means iden-tifying new loads or other recentchanges in the system. Why go tothis effort? Systems are dynamic;they change over time, often in

unplanned and haphazard ways.Furthermore, while some prob-lems are local in origin and effect,there are many problems thatresult from interactions betweenone part of the system andanother. Your job is to understandthese system interactions. Themore complete your documenta-tion, the better off youll be.

Simplified electrical distribution system typical of commercial and industrial facilities.

Recept.L.C.

ASD

Recept.XFMR480/208 Y

UtilityXFMRMV/480 Y

Switch Gear

Lighting Panel

LightingLoad

MotorM.C.C.

PFCorrectionCapacitor

InductionMotor

ReceptacleLoad


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2 Fluke Corporation Basic power quality methodology and common culprits

Its true, however, that thesites that need the most help arethe ones least likely to have agood record of whats going on intheir system. Many a consultant

has earned his fee by upgradingthe documentation handed himwith what actually exists on-site.So the simple rule is, at this pointin the investigation, do the bestyou can to get good documenta-tion, but dont count on it beingavailable.

2. Do a walk around of the siteSometimes a visual inspectionwill offer immediate clues: A transformer thats much too

hot

LightningCan be extremely destructive ifproper surge protection is notinstalled. It also causes sags andundervoltages on the utility lineif far away. If close by, it causesswells and overvoltages. But inthe final analysis, lightning is anact of nature and not in the samecategory as the damage mandoes to himself.

Utility automatic breakerreclosureCauses short duration sags/outages, but better than the alter-native, a longer-term outage.

Utility capacitor switchingCauses a high-energy voltagedisturbance (looks like an oscil-lating transient riding on thewave). If the cap bank is near thefacility, this transient can propa-gate all through the building.

Commercial high riseswithout enough distributiontransformersTrying to cut corners in thewrong places; running 208 Vfeeder up twenty stories is notthe road to PQ.

Gen-sets not sized forharmonic loadsExcessive voltage distortionaffects electronic control cir-cuits. If SCR converter loads arepresent, notching can affect fre-

quency control circuits.

Applying PF correctioncapacitors without consideringthe effects of harmonicsHarmonics and caps dont mix.Those bulging capacitors are cry-ing for help.

Inrush currents from hightorque motor loads startedacross-the-line

Causes voltage sags if the loadis too large or the source imped-ance too great. Staggered motorstarts can help.

Undersized neutrals atpanelboardIn the era of the 3rd harmonic,neutrals can easily carry as muchcurrent or more current than the

A Lineup of Power Quality CulpritsFrom utility source to receptacle

Isolated ground rod can cause ground loops. Common problem with CNC machine tool installations.

Load Current

Neutral ReturnCurrent Error

Panel

Line Line

Neutral Neutral

Ground Ground

SeparatelyDerivedSystem

N - GBond

Earth Ground Isolated Ground,Ground Rod, Cold Water Pipe, Etc.

3. Interview affected personneland keep an incident logInterview the people operatingthe affected equipment. You willget a description of the problem

and often turn up unexpectedclues. Its also good practice tokeep a record of when problemshappen and what the symptomsare. This is most important forproblems that are intermittent.The goal is to find some patternthat helps correlate the occur-rence of the problem in thevictim load to a simultaneousevent elsewhere. Logically, thistrouble-logging is the responsi-bility of the operator closest tothe affected equipment.

Wiring or connections discol-ored from heat

Receptacles with extensionstrips daisy-chained to exten-sion strips

Signal wiring running in thesame trays as power cables Extra neutral-ground bonds in

sub-panels. Grounding conductors con-

nected to pipes that end inmid-air.

At a minimum, you will get asense of how the facility is wiredand what the typical loads are.

phase conductor. Keeping themundersized leads to overheatedlugs, potential fire hazards andhigh N-G voltage.

Running power and signalcables togetherThink of the signal cable as asingle-wire transformer second-ary and the power cable as the

primary. The opportunities forcoupling are endless.

Loose conduit connections andlack of green wire groundingconductorCauses open or high impedanceground circuit. Not good for PQ orsafety.


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3 Fluke Corporation Basic power quality methodology and common culprits

Shared neutrals on branch circuitsCauses load interaction and over-loaded neutrals.

Laser printers and copierssharing branch circuits with sensi-tive loadsGuaranteed periodic voltage sags andswitching transients.

Miswired receptacles(N-G swapped)Hard to believe, but they are outthere in quantity. Guaranteed to putreturn currents on the ground con-ductor and create a noisy ground.

Data cables connected to differentground references at each endOuch! Shows up as voltage betweenequipment case and the data cable

connector.Hi-frequency noiseThe most effective high frequencygrounding technique is the installa-tion of a Signal Reference Grid (SRG).

And in a class bythemselves

Isolated ground rods (below)Theyre a safety hazard because theearth is a high impedance path andwill prevent enough current fromflowing to trip the breaker. They also

cause ground loops; after all, everyelectron still has to go back where itcame from. One of the great mysteriesof PQ is how some manufacturers caninsist that their equipment warrantyis void unless an isolated ground rodis installed.

Illegal N-G bondsGuaranteed to put return currents onground. Not only is it a PQ problem,its a plumbing problem. Circulatingground currents cause corrosion ofwater pipes.

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

International Safety Standards for Test Tools

IEC 61010 establishes interna-tional safety requirements for lowvoltage (1000V or less) electricalequipment for measurement, con-trol and laboratory use. The lowvoltage power distribution systemis divided into four categories,

based on the proximity to thepower source. Within each cat-egory are voltage listings1000V,600V, 300V, etc.

The key concept to understandis that you should use a meterrated to the highest category, aswell as the highest voltage, thatyou might be working in. For PQtroubleshooters, that means ameter rated to CAT III 600 V orCAT III 1000 V (the specificationsfor CAT IV have not yet beendefined by IEC). We recommend

that you do not use CAT II ratedmeters, scopes or test leads andprobes on CAT III circuits. TheCAT ratings should be markednear the voltage inputs of theinstrument. Meters designed toIEC 348, the previous standard,will typically not meet the morestringent safety specs of IEC61010 CAT III 600/1000 V.

IEC 61010 requires increasedprotection against the hazards oftransient overvoltages. Transientscan cause an arc-over inside an

inadequately protected meter.When that arc-over occurs in ahigh energy environment, suchas a three-phase feeder circuit,the result can be a dangerousarc blast. The potential exists forserious harm to personnel as wellas damage to the meter.

Independent Testing andCertificationManufacturers can self-certifythat they meet IEC 61010 specs,

but there are obvious pitfalls forthe end-user in self-certification.Certification by an independenttesting lab provides assurancethat the meter meets IEC require-ments. Look for a symbol andlisting number of an independenttesting lab such as UL, CSA, TV,VDE, etc. UL 3111, for example, isbased on IEC 61010.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For 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

2004 Fluke Corporation. All rights reserved.Printed in U.S.A. 10/2004 2403240 A-US-N Rev A

Pub-ID: 11600-eng

Fluke.Keeping your world upand running.

OvervoltageCategory Summary Description

CAT IV* Three-phase at utility connection, any outdoors conductors (under 1000 V)CAT III Three-phase distribution (under 1000 V), including single-phase commer-

cial lighting and distribution panels

CAT II Single-phase receptacle connected loads

CAT I Electronic

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