Fluke Power Quality Troubleshooting

Power Quality Troubleshooting

2 Fluke Corporation Power Quality Troubleshooting

While we’ve enjoyed enormousbenefits from the evolution ofsolid state technology, the factis that the microelectronics atthe heart of that technologyrequires clean power. Fasterspeeds and lower voltagesmean that there is less and lesstolerance for anything less thanquality power.

Power Quality (PQ) covers awide range of issues, from volt-age disturbances like sags,swells, outages and transients,to current harmonics, to perfor-mance wiring and grounding.The symptoms of poor PQ in-clude intermittent lock-ups andresets, corrupted data, prema-ture equipment failure, over-heating of components for noapparent cause, etc. The ulti-mate cost is in downtime,decreased productivity andfrustrated personnel.

This application note givesyou information on how totroubleshoot PQ problems. Italso gives you information onhow to start fixing those

Introduction

problems. But before grabbingthat meter, please read thefollowing cautionary notes:1. Suggested measurements

should only be made byqualified personnel who havebeen trained to make thesemeasurements in a safemanner, using properprocedures and test toolsrated for work on electricalpower circuits.

2. To the best of our knowledge,recommended solutions areconsistent with the NationalElectric Code (NEC), but inany case, NEC requirementsmust not be violated.

3. We have tried to make theinformation accurate andcurrent, but it is not intendedto be a substitute for thespecialized knowledge andexperience of professionalpower quality practitioners.

What this application note offersis a “starter kit,” not the finalword on PQ troubleshooting.

IEC 61010 establishes interna-tional safety requirements forlow voltage (1000V or less)electrical equipment for mea-surement, control and labora-tory use. The low voltage powerdistribution system is dividedinto four categories, based onthe proximity to the powersource. Within each categoryare voltage listings—1000V,600V, 300V, etc.

The key concept to under-stand is that you should use ameter rated to the highest cat-egory, as well as the highestvoltage, that you might beworking in. For PQ trouble-shooters, that means a meterrated to CAT III-600V or CAT III-1000V (the specifications forCAT IV have not yet been de-fined by IEC). We recommendthat 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-600/1000V.

IEC 61010 requires increasedprotection against the hazardsof transient overvoltages. Tran-sients can cause an arc-overinside an inadequately pro-tected meter. When that arc-over occurs in a high energyenvironment, such as a three-phase feeder circuit, the resultcan be a dangerous arc blast.The potential exists for seriousharm to personnel as well asdamage to the meter. For moreinformation, see the Fluke appli-cation note “ABCs of MultimeterSafety” (document numberB0317UEN) and the Fluke video“The ABCs of Digital MultimeterSafety” (P/N 609104).

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 re-quirements. Look for a symboland listing number of an inde-pendent testing lab such as UL,CSA, TÜV, VDE, etc. UL 3111, forexample, is based on IEC 61010.

OvervoltageCategory Summary Description

CAT IV* Three-phase at utility connection, anyoutdoors conductors (under 1000V)

CAT III Three-phase distribution (under 1000V),including single-phase commercial lightingand distribution panels

CAT II Single-phase receptacle connected loads

CAT I Electronic

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

International SafetyStandards for Test Tools

Table of Contents Page

Safety ............................................................ 2

Getting Started ...............................................3

First Steps .....................................................3

Part I: Facility Distribution SystemSection 1: Receptacle Branch Circuit ................... 4

Section 2: Service Panels ...................................... 8

Section 3: Transformers ....................................... 10

Section 4: Electrical Noise and Transients ........ 16

Section 5: Lightning Protection .......................... 21

Part II: Three-Phase LoadsSection 6: Polyphase Induction Motors ............. 22

Section 7: AC Adjustable Speed Drives ............. 25

Section 8: Commercial Lighting .......................... 30

A Lineup of Power Quality Culprits ............... 31

Test Tools ....................................... Back cover

Power Quality Troubleshooting Fluke Corporation 3

Recept.L.C.

ASD

Recept.XFMR480/208Y

UtilityXFMRMV/480Y

Switch Gear

Lighting Panel

LightingLoad

MotorM.C.C.

PFCorrectionCapacitor

InductionMotor

ReceptacleLoad

Start at the sceneof the crimeTo 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 malfunction-ing. Poor PQ is suspected, butpart of your job is to isolate PQas a cause from other possiblecauses (hardware, software?).Like any detective, you shouldstart at the scene of the crime.This bottom-up approach cantake you a long way. It relieson making use of a sharp eyeand on taking some basicmeasurements.

An alternative is to start atthe service entrance, using athree-phase monitor, and workback to the “victim load.” This ismost useful if the problemsoriginate with the utility. Yetsurvey after survey has con-cluded that the great majorityof PQ problems originate in thefacility. In fact, as a general rule,PQ is best at the service en-trance (connection to utility)and deteriorates as you movedownstream through the distri-bution system. That’s becausethe facility’s own loads arecausing the problems. Anotherilluminating fact is that 75% ofPQ problems are related to wir-ing and grounding problems!

For this reason, many PQauthorities recommend that alogical troubleshooting flow is

to first diagnose the electricalinfrastructure of the building,then monitor if necessary. Ourbottom-up troubleshooting pro-cedure is designed to help youdo this detective work.

First steps1. Make a map: Obtain orcreate a current one-lineIt’s 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 theac power sources and the loadsthey serve. The “as built” one-line, the one with red-lines, isthe one you want.

If you work on-site, the mapmight already exist in yourhead, but it will be a big help toeveryone, including yourself, ifit’s on paper. If you’re coming toa work site for the first time,getting an up-to-date one-linemeans identifying new loads orother recent changes in the sys-tem. Why go to this effort? Sys-tems are dynamic; they changeover time, often in unplannedand haphazard ways. Further-more, while some problems arelocal in origin and effect, thereare many problems that resultfrom interactions between onepart of the system and another.

Getting Started

Your job is to understand thesesystem interactions. The morecomplete your documentation,the better off you’ll be.

It’s true, however, that thesites that need the most helpare the ones least likely to havea good record of what’s goingon in their system. Many a con-sultant has earned his fee byupgrading the documentationhanded him with what actuallyexists on-site. So the simple ruleis, at this point in the investiga-tion, do the best you can to getgood documentation, but don’tcount on it being available.

2. Do a walk aroundof the siteSometimes a visual inspectionwill offer immediate clues:• A transformer that’s much

too hot• Wiring or connections discol-

ored from heat• Receptacles with extension

strips daisy-chained to exten-sion strips

• Signal wiring running in thesame trays as power cables

• Extra neutral-ground bondsin sub-panels.

• Grounding conductors con-nected to pipes that end inmid-air.

At a minimum, you will get asense of how the facility iswired and what the typicalloads are.

3. Interview affectedpersonnel and keepan incident logInterview the people operatingthe affected equipment. You willget a description of the problemand often turn up unexpectedclues. It’s 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 the“victim load” to a simultaneousevent elsewhere. Logically, thistrouble-logging is the responsi-bility of the operator closest tothe affected equipment.

Simplified electrical distribution system typical of commercial and industrial facilities.


Part I: Facility Distribution System

Many PQ problems show up atthe branch circuit level. There’sa simple reason for this: that’swhere most of the sensitiveloads (and sensitive employees)are located. It’s also the “end ofthe line” of the electrical sys-tem, and the place where short-comings can’t be hidden. Let’sassume you’ve been called into solve the problem. You’vealready talked to the people in-volved, have a rough idea of thesymptoms (equipment lock-ups,intermittent resets or crashes,etc.) and as much sense of thetiming and history of the prob-lems as you can get. So it’s timeto gather hard evidence: it’stime to take measurements.

Our primary focus withtroubleshooting at the recep-tacle level is to determine if theLine-Neutral (L-N) voltage avail-able is of sufficient stability andamplitude to supply the needsof the load(s).

Measurement

1. WaveformThe waveform gives us quicksnapshot information. An idealwaveform would be a sinewave. In this case, (see Fig 1.1)the voltage waveform is flat-topped, which is typical of abuilding with many non-linearloads such as computers andother office equipment (see“Flat-topped voltage,” page 5).

Our other measurements willtell us whether this flat-toppingis excessive.

2. Peak voltageThe peak value is critical toelectronic loads because theelectronic power supply chargesits internal capacitors to thepeak value of the line voltage.If the peak is too low, it affectsthe ability of the caps to chargefully and the ability of thepower supply to ride throughmomentary dips in the line volt-age. For an RMS voltage of115V, the peak value would be1.414 x 115V = 162.6V, if thewaveform were a sine wave.However, as we just saw fromthe flat-topped waveform, whatwe have is far from a sine waveand will have a lower peakvalue.

3. RMS voltageNominal line voltage is measuredin RMS (root-mean-square)

which corresponds to the effec-tive heating value. Equipment israted in RMS, not peak, becausetheir main limitation has to dowith heat dissipation.

RMS voltage can be too highor too low, but it is usually thelow voltage that causes prob-lems. Low RMS voltage com-bined with flat-topping (lowpeak) is a deadly combinationfor sensitive loads.

Voltage drop is a functionof both the loading of the circuitand the source impedance,which in effect means thelength and diameter (gauge)of the wire run. The NEC (210-19.a, FPN No. 4) recommends alimit of a 3% voltage drop fromthe branch circuit breaker tothe farthest outlet, and a totalvoltage drop of less than 5% in-cluding the feeder and branchcircuit.

4. Recording (short-term)The limitation of the abovemeasurement is that it is static.Many loads require more cur-rent, usually referred to as in-rush current, when they are firstturned on. This momentary highcurrent may cause a momentarylow voltage (sag) because of theadditional IR drop through theconductors. Such sags are oftencaused by loads drawing inrushcurrents on the same branchcircuit, or on the samepanelboard.

You can measure a worst-case sag of 100 ms or more(about 6 cycles at 60 Hz) by us-ing the MIN MAX function of theFluke 87 while energizing theload. What if you want to knowif there are recurring sags? TheSags & Swells trending featureof the Fluke 43 Power QualityAnalyzer will continuously cap-ture sags of as little as singlecycle duration (17 ms). A four-minute to a one-hour recordingtime (i.e., anywhere from asingle cup of coffee to a lunchbreak) may be enough to tellyou if there are recurring sagsand swells.

Table 1.1 Measurements on receptacle branch circuits.

Voltage Measurements Look for Instrument

1. Waveform Snapshot of severity of 43 PQ Analyzervoltage distortion 41B Harmonics Analyzer

2. Peak voltage Excessive flat-topping 43 PQA, 41B87 DMM (Peak MIN MAX)

3. RMS voltage Low rms (steady-state low 43 PQA (Sags/Swells)rms or intermittent/cyclical 41B (MIN MAX)sags) 87 DMM (MIN MAX)

4. Recording (short-term) Sags, swells, interruptions 43 PQAwhile troubleshooter remains (Sags/Swells or Transients)on-site (4 minutes to 1 hourtypical recording time)

5. Recording (long-term) Up to 4,000 sags, swells, VR101Soutages, transients

6. Neutral-ground N-G voltage too high 43 PQA, 87 DMM(or close to zero)

Figure 1.1 Flat-topped voltage at receptacle.

Section 1Receptacle Branch Circuit


5. Recording (long-term)For longer term recording, theVR101S Voltage Event Record-ers will record sags, swells, out-ages, transients and frequencydeviations while plugged intothe outlet (see “Recording at theReceptacle Outlet,” page 7). Thedevice can be left on-site, unat-tended, for days and weeks, allthe time catching intermittentevents (4000 event buffer). Nowyou can see why it’s so impor-tant to ask the user to keep atroubleshooting log: correlationof equipment malfunction withvoltage events is hard evidenceof a PQ problem.

6. Neutral-to-ground voltageLet’s say that you make asimple L-N measurement at theoutlet and get a low reading.You can’t tell if the reading islow because the feeder voltageis low (at the subpanel), or if thebranch circuit is overloaded. Youcould try to measure the voltageat the panel, but it’s not alwayseasy to tell which panel feedsthe outlet you’re measuring andit’s also sometimes inconvenientto access a panel.

N-G voltage is often an easierway of measuring the loadingon a circuit. As the current trav-els through the circuit, there is acertain amount of voltage dropin the hot conductor and in theneutral conductor. The drop onthe hot and neutral conductorswill be the same if they are thesame gauge and length. The to-tal voltage drop on both con-ductors is subtracted from thesource voltage and is that muchless voltage available to theload. The greater the load, thegreater the current, the greaterthe N-G voltage.

Think of N-G voltage as themirror of L-N voltage: if L-Nvoltage is low, that will showup as a higher N-G voltage(see Fig. 1.4).

The flat-topped waveform istypical of the voltage in a com-mercial building with computerloads. What causes flat-topping?

The utility supplies ac power,but electronic equipment runson dc power. The conversionof ac into dc is done by a powersupply. The PS has a diodebridge which turns ac into pul-sating dc, which then charges acapacitor. As the load draws thecap down, the cap recharges.However, the cap only takespower from the peak of thewave to replenish itself, sincethat’s the only time the suppliedvoltage is higher than its ownvoltage. The cap ends up draw-ing current in pulses at eachhalf-cycle peak of the suppliedvoltage. This is happening withvirtually all the electronic loadson the circuit. Now that we seewhat the loads are demandingfrom the source, let’s take a lookat what the source can supply.

If the source were perfectly“stiff,” meaning that it had aninfinite capacity to supply allthe current that was required,then there would be no suchthing as flat-topping (or sagsor any voltage distortion). Thinkof it this way: if you had all themoney in the world, youwouldn’t get distorted eitherwhen the bills came in. But inthe real world there are practi-cal limits to what a source can

supply. This limit is usuallydescribed by a concept calledsource impedance, which is thetotal impedance from the pointyou’re measuring (or the pointwhere the load is located) backto the source. There are twomajor contributors to this sourceimpedance. One is the wiring;the longer the conductor andthe smaller the diameter (highergauge), the higher the imped-ance. The other factor is theinternal impedance of the trans-former (or other source equip-ment). This internal impedanceis simply a way of saying that atransformer of a given size/rat-ing can only supply so muchcurrent.

The source impedance isnaturally greatest at the endof a branch circuit, the farthestpoint from the source. That’s thesame place where all thoseelectronic loads are demandingcurrent at the peak of the wave.The result is that the voltagepeak tends to get draggeddown—in other words, flat-topped. Maybe you’ve felt thesame way when all the billscome in at the same time of themonth. The more loads thereare (the more the bills), thegreater the flat-topping. Also,the greater the source imped-ance (the less the cash), thegreater the flat-topping.

Flat-topped voltage

Figure 1.2 Flat-topped voltage.

Diode-CapacitorInput Circuit

SwitchingPower Supply


N-G voltage exists becauseof the IR drop of the currenttravelling through the neutralback to the N-G bond. If thesystem is correctly wired, thereshould be no N-G bond exceptat the source transformer (atwhat the NEC calls the source ofthe Separately Derived System,or SDS, which is usually atransformer). Under this situa-tion, the ground conductorshould have virtually no currentand therefore no IR drop on it.In effect, the ground wire isavailable as a long test leadback to the N-G bond.

Figure 1.3. Neutral-to-ground voltage increases with shared neutrals.

Ø1

Ø1

Ø2

Ø2

Ø3

Ø3

To panelground buss

N-G voltage

To transformerX0

Figure 1.4 Neutral-ground voltage increases as load current goes up.

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1. A rule-of-thumb used by many in the industryis that N-G voltage of 2V or less at the recep-tacle is okay, while a few volts or moreindicates overloading; 5V is seen as the upperlimit. There’s obviously some room for judgmentin this measurement.

2. A high reading could indicate a shared branchneutral, i.e., a neutral shared between morethan one branch circuit. This shared neutralsimply increases the opportunities for overload-ing as well as for one circuit to affect another.

3. A certain amount of N-G voltage is normal in aloaded circuit. If the reading is stable at closeto 0V, suspect an illegal N-G bond in thereceptacle (often due to loose strands of theneutral touching some ground point) or at thesubpanel. Any N-G bonds other than those atthe transformer source (and/or main panel)should be removed to prevent return currentsflowing through the ground conductors.

4. If N-G voltage is low at the receptacle, you’re ingood shape (see Measurement Note #3 for theexception to the rule). If it’s high, then you stillhave to determine if the problem is mainly atthe branch circuit level, or mainly at the panellevel. Remember, assuming there’s no illegalN-G bond in intervening panels or receptacles,your ground “test lead” goes all the way back tothe source, so you’re reading voltage drops allthe way to the source.

Receptacle N-G VoltageMeasurement Notes

Shared neutralsSome buildings are wired sothat two or three phases share asingle neutral. The original ideawas to duplicate on the branchcircuit level the four wire (threephases and a neutral) wiring ofpanelboards. Theoretically, onlythe unbalanced current will re-turn on the neutral. This allowsone neutral to do the work forthree phases. This wiring short-cut quickly became a dead-endwith the growth of single-phasenon-linear loads. The problemis that zero sequence current

from nonlinear loads, primarilythird harmonic, will add uparithmetically and return on theneutral. In addition to beinga potential safety problembecause of overheating of anundersized neutral, the extraneutral current creates a higherN-G voltage. Remember thatthis N-G voltage subtracts fromthe L-N voltage available to theload. If you’re starting to feelthat shared neutrals are one ofthe worst ideas that ever gottranslated to copper, you’re notalone.


Solutions

Performance Wiring vs.Code MinimumAny experienced PQ trouble-shooter will tell you that thefirst place to look for most prob-lems is in the building wiringsystem (including its groundingsystem). Quality power dependson quality wiring; the term theindustry uses is performancewiring (See Table 1.2). The ba-sic intent of performance wiringis to maintain or restore L-Nvoltage to the load. There is adistinction between “perfor-mance wiring” and “code mini-mum” wiring. The NEC sets theabsolute minimum requirementsfor a wiring job and is primarilyconcerned with fire preventionand personnel safety. The NEC

should, of course, never be vio-lated, but it is also important tounderstand that the Code’s ob-jective is not to establish stan-dards to achieve power quality.However, many facilities arefinding that it pays to take theextra step and install or evenretrofit a performance wiringjob. As one veteran said, “Ifevery building were perfor-mance wired, I’d be out ofbusiness. . . But there’s no fearof that happening.”

Power conditioningThere are also situations wherereceptacle-installed power con-ditioning devices are a good so-lution, either as a complementto the wiring changes or as aneconomically viable alternativeto some wiring changes.

Recording at thereceptacle outletBy monitoring voltage events atthe receptacle, you can see ex-actly the same voltage that thesensitive load sees.

The VR101 is plugged intoan outlet, and can record upto 4000 events, including:• Voltage sags and swells (rms)• Outages• Transients (L-N and N-G)

with peak values• Frequency deviationEvents are identified by type,real-time stamp, and duration.

VR101S operationSet upUse EventView software toconfigure the device. The unitcomes with default thresholds,but users can enter new thresh-olds. An optical wand, suppliedwith the software, transfersnew configurations to the VR101.

Plug inThe VR101 is left on-site for aslong as needed. No computerconnection is necessary. Itdraws power from the line andin the event of outages, a built-in battery saves data.

DownloadThe VR101 is taken to thecomputer. The optical wandretrieves its data.

AnalyzeEvents are displayed in spread-sheet format in EventViewsoftware. Charts, graphs andwaveform graphics are alsoprovided for report generation.Table 1.2 Suggestions for performance wiring of branch circuits.

Recommendation Reason

Check for loose connections. It’s easy to overlook the obvious.

Eliminate shared neutrals. In new Minimize load interaction and sourceinstallations, pull individual neutrals impedance.for each branch circuit.

Limit the number of receptacles per Minimize loading and load interaction.branch circuit to three.

Limit length of 120V branch circuits Minimize source impedance.to 50 ft. (15m).

Install dedicated branch circuits for all Keep victim loads and culprit loads separated.laser printers and copy machines. Dedicated Conduit prevents coupling between circuits.circuits should be run in their own conduit.

Install a green wire ground (don’t just Maintain a continuous, low impedancedepend on the conduit connection). ground.

Label all panels, circuit breakers and Strictly speaking, this won’t improve powerreceptacles. quality, but it will sure make life easier for

the troubleshooter and the installer.

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Figure 1.5 Performance wiring.


Section 2Service Panels

Check-out the service panelas follows:• Visual inspection• Feeder conductor current test• Neutral conductor current test

(feeder and branch)• Phase-to-neutral voltage test

(feeder and branch)• Neutral-to-ground voltage

test (feeder)• Circuit breaker voltage drop

and current on branch phaseconductors

The service panel is where theeffects of single-phase harmonicloads are easy to measure. Atrue-rms meter ensures accuratereadings of non-linear voltagesand currents (see “Why True-rms, page 27).

Visual inspection• Look for an illegal Neutral-

Ground bond in subpanels.This is a violation of the NECas well as of PQ wiring. It isalso extremely common. If anillegal N-G bond is found inone panel at a site, it is likelyto be in any number of them.Who knows why they’rethere: perhaps the installerwas thinking that all panelsare wired like residential ser-

vice panels; or that thequickest way to reduce N-Gvoltage was to install ajumper, or that the moregrounds the better. In anycase, remove all illegalN-G bonds—no exceptions.

• Look for signs of overheat-ing, such as discolored con-necting lugs. Loose con-nections and excessive load-ing show up as heat. Highlevels of harmonic currentthat were not accounted forin the original wire sizingcan also cause overheating.Infrared sensors are thepreferred method fornon- contact temperaturemeasurement.

• Of particular concern is thesize of the feeder neutralconductor. It has long been

understood that any funda-mental current resulting fromthe unbalance of single phaseloads among the three phaseswill return on the neutral, buta relatively recent phenom-enon is the third harmonic(triplen) currents generated bynonlinear single-phase loadsthat all return on the neutral.

The 1996 NEC for the firsttime stated that “On a 4-wire,3-phase wye circuit where themajor portion of the load con-sists of nonlinear loads, thereare harmonic currents presentin the neutral conductor, andthe neutral shall be consideredto be a current-carrying con-ductor.” (Article 310, “Notes toAmpacity Tables of 0 to 2000Volts,” Note 10.c). In effect, thisrequires that the neutral con-ductor at least equal the size ofthe phase conductor. This re-quirement is based on solid re-search: a 1990 survey of 146sites nation-wide found that22.6% of them had neutralcurrent in excess of 100% ofphase current!

Many experts would recom-mend that the neutral bedouble the size of the phaseconductor.

• Check for shared branchneutrals. Count neutral con-ductors for branch circuits: ifthere are fewer than the phaseconductors, there are sharedneutrals.

• Check tightness of conduitconnections, especially if theconduit is being used exclu-sively as the grounding con-ductor (not recommended).

Figure 2.1 Sub-panel N-G bonds cause load return currents to flow on ground conductors.This causes corrosion of pipes in grounding system as well as noisy grounds.

Panel Sub-Panel

Line

Neutral

GroundN-GBond

Table 2.1 Service panel measurements.

Measurement Look for Instrument

1. Feeder phase current Overloading and balance. 43, 41B, 87 w/80i-400,True-rms ClampMeter

2. Feeder neutral current High currents from unbal- 43, 41B for spectrum.anced fundamental and 87 to find dominant3rd harmonics. frequency.

3. Feeder N-G voltage High voltage indicates Sameexcessive current, near-zeroindicates possible subpanelN-G bond.

4. Branch L-N voltage Low voltage. Same

5. Branch neutral current Shared neutrals. Same

6. Voltage drop across Worn contacts. Breakers in 43, 87breaker contacts. need of replacement.Hot breakers.


SolutionsTable 2.2 Service panel recommendations.

Recommendation Reason

Limit length of 208V feeder runs to 120V Minimize source impedance and chancesubpanels to 200 ft. (65m). of voltage sags.

Don’t cascade (daisy chain) subpanels off Upstream loads can cause voltage sags thatof other subpanels if possible, and will affect all downstream loads.especially if the upstream panel is heavilyloaded or has loads with high inrushcurrents.

Install a green wire ground conductor Maintain a continuous, low impedance(don’t rely on conduit connections). ground.

Reduce the load on the panel if necessary. Minimize heat, voltage sags.

Redistribute branch circuit loads to improve Reduce neutral return current (of thebalance of the three phases. fundamental current).

Upsize the feeder neutral if necessary, to Prevent overloading and heating of feederaccommodate the third harmonic. This can neutral. Will reduce N-G voltage.be done by running another neutralin parallel.

Install 3rd harmonic filter. Reduce neutral current.

Nonlinear load panel. Manufacturer-designed for nonlinear loads.

Measurements

1. Feeder phase currentCheck each phase to make sureit is not overloaded. Also checkfor excessive unbalance.

2. Feeder neutral currentMeasure the feeder neutralconductor for cumulative neu-tral current. Third harmoniccurrents from all three phaseswill add arithmetically in theneutral.

3. Feeder neutral-to-groundvoltage testAs at the receptacle, excessiveN-G voltage indicates overload-ing. A N-G voltage at or verynear zero indicates the exist-ence of an illegal N-G bond in asubpanel.

4. Phase-to-neutralvoltage testPhase-to-neutral voltages aremeasured and recorded. Theycan be compared with recep-tacle L-N voltages to measurevoltage drop.

5. Branch neutral currentMeasure each branch neutralfor overloading. The neutralsare measured instead of thephase conductors because theymight share the return currentof several phase conductors,yet they are not protectedby breakers.

6. Circuit breakervoltage dropThe voltage drop across a setof breaker contacts will giveyou a quick measure of thewear of those contacts. Ideally,the voltage drop should bezero. In practice, there will besome voltage drop in the mVrange, with the exact value be-ing dependent on the load cur-rent. As a general rule, thevoltage drop should not exceed20-100 mV, depending on load.Replace worn breakers.

Circuit breakervoltage drop

BranchL-Nvoltage

N-G voltage

Branch neutral current

Feederneutralcurrent

Feederphasecurrent(s)

Figure 2.2 Panel with clamps, probes to show measurements.


Unbalanced load: kVATOTAL = kVA1 + kVA2 + kVA3

kVA1

Red

Red

Red

Black Black

Black

kVA2

kVA3

ø1

ø2

ø3

N

For a balanced load: kVATOTAL = kVA1 • 3= (kVA1)(1.732)

Red

Black

Transformers are subject tooverheating from harmonic cur-rents. Transformers supplyingnon-linear loads should bechecked periodically to verifyoperation within acceptablelimits. Transformers are alsocritical to the integrity of thegrounding system.

Measurements1. Transformer loading (kVA)If the transformer has a four-wire wye secondary, which isthe standard configuration forcommercial single-phase loads,actual kVA can be easilydetermined. (See Figure 3.2)• Connect voltage probes on

Phase 1 and Neutral andclamp current probe on samephase. Repeat for Phase 2and 3.

• Read kVA of each phase andsum all three for total trans-former kVA.

• Compare actual load kVAto nameplate kVA rating todetermine % loading.

If the load is balanced, a singlemeasurement is sufficient. (seeFigure 3.1) Transformers loadedat less than 50% are generallysafe from overheating. However,as loads increase, measure-ments should be made periodi-cally. At some point thetransformer may require derat-ing (see page 15).

2. Harmonic spectrumThe harmonic spectrum of thesecondary (load) current willgive us an idea of the harmonicorders and amplitudes present:• In a transformer feeding

single-phase loads, the prin-cipal harmonic of concern isthe 3rd. The 3rd will addarithmetically in the neutraland circulate in the delta pri-mary of a delta-wye trans-former. The good news isthat the delta-wye tends toisolate the rest of the systemfrom the 3rd (though not the5th, 7th or other non-triplenharmonics). The bad news isthat the transformer pays theprice with additional heat.

• In a transformer feedingthree-phase loads which in-clude drives or UPS systemswith 6-pulse converters, the5th and 7th harmonic willtend to predominate. Exces-sive 5th is of particular con-cern because it is negativesequence. It will tend toproduce counter-torque andoverheating in polyphasemotors.

Section 3Transformers

Figure 3.1 Measuring transformer load (balanced).

Figure 3.2 Measuring transformer load (unbalanced).

Figure 3.3 Harmonic spectrum.


• Harmonic amplitudesnormally decrease as thefrequency goes up. If onefrequency is significantlyhigher in amplitude thanlower frequencies, we cansuspect a resonant conditionat that frequency. If such acondition is detected, be sureto take readings at capacitorbanks to see if the caps areexperiencing overcurrent/overvoltage conditions.

• Before-and-after harmonicspectrum measurement is ex-tremely valuable to deter-mine if harmonic mitigationtechniques, like trap filters,which are tuned to specificfrequencies, are sized prop-erly and are working asexpected.

• Different harmonic frequen-cies affect equipment in dif-ferent ways (see below).

3. Total Harmonic DistortionCheck for THD of both voltageand current:• For voltage, THD should

not exceed 5%• For current, THD should not

exceed 5-20% (Table 3.2)IEEE 519 sets limits for harmon-ics at the PCC (Point of CommonCoupling) between the utilityand customer (EN50160 is theEuropean standard). IEEE 519is based on THD measurementstaken at the PCC. Technically,the PCC is the primary of theutility supply transformer (al-though there are cases wherethe PCC is at the secondary ifthe secondary feeds a numberof customers). In practice, thesemeasurements are often madeat the secondary of thecustomer’s main transformer,since that is the point most eas-ily accessible to all parties (andalso since that is generally aLow Voltage measurement).

Some PQ practitioners havebroadened the concept of PCCto include points inside the fa-cility, such as on the feeder sys-tem, where harmonic currentsbeing generated from one setof loads could affect another setof loads by causing significantvoltage distortion. The emphasisis on improving in-plant PQ,rather than on simply not af-fecting utility PQ.

3a. Voltage THDTHD has a long history in theindustry. The underlying con–cept is that harmonic currentsgenerated by loads will causevoltage distortion (E=IZ) as theytravel through the systemimpedance. This voltagedistortion then becomes thecarrier of harmonics system-wide: if, for example, thedistorted voltage serves a linearload like a motor, it will thencreate harmonic currents in thatlinear load. By setting maximumlimits for voltage distortion, weset limits for the system-wideimpact of harmonics.

Table 3.1 Measurements at the distribution transformer.


1. kVA Transformer loading. If loading 43, 41Bexceeds 50%, check for harmonicsand possible need for derating.

2. Harmonic spectrum • Harmonic orders/amplitudes present: Same3rd harmonic (single-phase loads)5th, 7th (primarily three-phase loads)

• Resonance of higher order harmonics• Effectiveness of harmonic trap filters

3. THD Harmonic loading within limits: SameVoltage %THD <5%Current %THD <5-20% (Table 3.2)

4. K-factor Heating effect on transformer from Sameharmonic loads

5. Ground currents • Objectionable ground currents are Samenot quantified but are prohibited by True-rms Clampthe NEC.

• Neutral-ground bond in place• ESG (Electrical Safety Ground)

connector to ground electrode(typically building steel) in place

Harmonic Sequences

Name F 2nd 3rd 4th 5th 6th 7th 8th 9th

Frequency 60 120 180 240 300 360 420 480 540

Sequence + — 0 + — 0 + — 0

Rule: If waveforms are symmetrical, even harmonics disappear.

Effects of Harmonic Sequences

Sequence Rotation Effects (from skin effect, eddy currents, etc.)

Positive Forward Heating of conductors, circuit breakers, etc.

Negative Reverse Heating as above + motor problems

Zero None Heating, + add in neutral of 3-phase, 4-wire system

Harmonics are classified as follows:

1. Order or number: Multiple of fundamental, hence, 3rd is three times thefundamental, or 180 Hz.

2. Odd or even order: Odd harmonics are generated during normal operation ofnonlinear loads. Even harmonics only appear when there is dc in the system.In power circuits, this only tends to occur when a solid state component(s),such as a diode or SCR, fails in a converter circuit.

3. Sequence:

• Positive sequence. Main effect is overheating.

• Negative sequence. Create counter-torque in motors, i.e., will tendto make motors go backwards, thus causing motor overheating.Mainly 5th harmonic.

• Zero sequence. Add in neutral of 3-phase, 4-wire system.Mainly 3rd harmonic.


Voltage distortion, however,depends on source impedance,i.e., on system capacity. It wasquite possible for the first (orsecond or third) customer to in-ject significant harmonic cur-rents into the system and notcause voltage THD to exceed5%. The entire responsibility forharmonic mitigation could fallon the last customers unluckyenough to push V—THD over5%, even if their particular har-monic load was relativelysmall—literally the straw thatbroke the camel’s back.

3b. Current THDTo restore some fairness to thissituation, standards for maxi-mum current harmonics wereadded, since current harmonicswere under the control of thelocal facility and equipmentmanufacturer (remember, har-monic “loads” act as “genera-tors” of harmonics). Thisemphasis on the mitigation ofcurrent harmonics at the load,including the not-too-distantrequirement that the load gen-erate virtually no harmonics,has become the prevailingregulatory philosophy. It putsthe burden of responsibility onthe local site and on the equip-ment manufacturers.

For equipment manufactur-ers, IEC 1000-3-2, publishedin 1995, is the applicable stan-dard. It specifies maximumcurrent levels out to the 40thharmonic. Its expected effectivedate is projected to be early2001. To certify for CE, a re-quirement for the Europeanmarket, manufacturers willhave to meet this standard. Thisedict will have a major effect onpower supply design.

For the facility, IEEE 519 isthe standard (EN 50160 in Eu-rope). The limits set in IEEE 519for harmonic currents dependon the size of the customerrelative to the system capacity.(See Table 3.2.)

The SCR (Short Circuit Ratio) isa measure of the electrical sizeof the customer in relation tothe utility source. The smallerthe customer (higher SCR), theless the potential impact on theutility source and the more gen-erous the harmonic limits. Thelarger the customer (smallerSCR), the more stringent thelimits on harmonic currents.

3c. TDD and THDTDD (Total Demand Distortion) isthe ratio of the current harmon-ics to the maximum load (IL).It differs from THD in that THDis the ratio of harmonics to theinstantaneous load. Why TDDinstead of THD? Suppose youwere running a light load (usinga small fraction of system ca-pacity), but those loads werenonlinear. THD would be rela-tively high, but the harmoniccurrents actually being gener-ated would be low, and the ef-fect on the supply system wouldin fact be negligible. So whocares? TDD acknowledges this,and allows harmonic load to bereferenced to the maximumload: if harmonic load is highat maximum load, then we haveto watch out for the effect onthe supply source. So wheredoes that leave current THDas a useful measurement. Thecloser the current THDreading(s) is taken to conditionsof maximum load, the closer itapproximates TDD.

Table 3.3

Inspection of Transformer Ground Explanation

Check for N-G bond. A high impedance N-G bond will causevoltage fluctuation.

Check for grounding conductor and Fault currents will return to the source viaintegrity of connection to building steel these connections, so they should be as low(exothermic weld). impedance as possible.

Check for tightness of all If the conduit is not itself grounded, it willconduit connections. tend to act as a “choke” for higher frequencies

and limit fault current (remember that faultcurrents are not just at 60 Hz but havehigh-f components).

Measure for ground currents on the Ideally there should be none, but there willgrounding conductor. always be some ground current due to

normal operation or leakage of protectivecomponents (MOVs, etc.) connected from phaseor neutral to ground. However, anything abovean amp should be cause for suspicion (there isno hard and fast rule, but experienced PQtroubleshooters develop a feel for possibleproblems).

Table 3.2 IEEE 519 limits for harmonic currents at the point of common coupling.(All percentages are % of IL, maximum demand load current.)

Odd Harmonics

SCR=Isc/IL <11 11-17 17-23 23-35 >35 TDD

<20 4.0% 2.0% 1.5% 0.6% 0.3% 5.0%

20-50 7.0% 3.5% 2.5% 1.0% 0.5% 8.0%

50-100 10.0% 4.5% 4.0% 1.5% 0.7% 12.0%

100-1000 12.0% 5.5% 5.0% 2.0% 1.0% 15.0%

>1000 15.0% 7.0% 6.0% 2.5% 1.4% 20.0%

SCR= Short circuit ratio (Isc/IL)Isc = Available short circuit current at PCC

IL = Maximum demand load current (rms amps)TDD= Total demand distortionNote: IEEE allows these limits to be exceeded for up to one hour per day, while IECallows them to be exceeded for up to 5% of the time.The concept of IL, maximum demand load current, is key to using Table 3.2. For existingfacilities, IL is calculated by averaging the maximum demand current for 12 consecutivemonths (information available in billing records). For new installations, IL must be esti-mated. Transformer rating could be used and would be the most conservative estimate(i.e., it would result in the lowest SCR), since it assumes that the transformer would beused at full capacity.


A final word on measuringTHD: the one place not to applythe specs is at the individualharmonic-generating load. Thiswill always be a worst-casedistortion and a misleadingreading. This is because as har-monics travel upstream, a cer-tain amount of cancellationtakes place (due to phase rela-tionships which, for practicalpurposes, are unpredictable).Measure at a PCC, or at thesource transformer.

4. K-factorK-factor is a specific measure ofthe heating effect of harmonicsin general and on transformersin particular. It differs from theTHD calculation in that it em-phasizes the frequency as wellas the amplitude of the har-monic order. This is becauseheating effects increase as thesquare of the frequency.

A K-4 reading would meanthat the stray loss heatingeffects are four times normal. Astandard transformer is, in ef-fect, a K-1 transformer. As withTHD, it is misleading to make aK-factor reading at the load orreceptacle because there will bea certain amount of upstreamcancellation; transformer K-fac-tor is what counts. Once theK-factor is determined, choosethe next higher trade size. K-factor rated transformers areavailable in standard trade sizesof K-4, K-13, K-20, K-30, etc.K-13 is a common rating for a

transformer supplying officeloads. The higher ratings tendto be packaged into PDUs(Power Distribution Units)which are specially designedto supply computer and otherPQ-sensitive installations.

5. Ground currentsTwo prime suspects for exces-sive ground current are illegalN-G bonds (in subpanels, re-ceptacles or even in equip-ment) and so-called isolatedground rods:• Subpanel N-G bonds create

a parallel path for normal re-turn current to return via thegrounding conductor. If theneutral ever becomes open,the equipment safety groundbecomes the only returnpath; if this return path ishigh impedance, dangerousvoltages could develop.(Figure 2.1, page 8.)

• Separate isolated groundrods almost always createtwo ground references atdifferent potentials, which inturn causes a “ground loop”current to circulate in an at-tempt to equalize those po-tentials. A safety andequipment hazard is alsocreated: in the case of light-ning strikes, surge currentstravelling to ground at differ-ent earth potentials willcreate hazardous potentialdifferences. (See page 31.)

Transformer groundingThe proper grounding of thetransformer is critical. (Table3.3.) NEC Article 250 in generaland 250-26 in particularaddress the grounding require-ments of the SDS.• A ground reference is estab-

lished by a grounding con-nection, typically to buildingsteel (which, in turn, is re-quired to be bonded to allcold water pipe, as well asany and all earth groundingelectrodes). Bonding shouldbe by exothermic weld, notclamps that can loosen overtime. The “grounding elec-trode conductor” itself shouldhave as low a high-frequencyimpedance as possible (notleast because fault currenthas high frequency compo-nents). Wide, flat conductorsare preferred to round onesbecause they have less in-ductive reactance at higherfrequencies. For the samereason, the distance betweenthe “grounding electrode con-ductor connection to the sys-tem” (i.e., N-G bond at thetransformer) and the ground-ing electrode (building steel)should be as short as pos-sible: in the words of theCode, “as near as practicableto and preferably in thesame area...”

• The neutral and groundshould be connected at apoint on the transformer neu-tral bus. Although permitted,it is not advisable to makethe N-G bond at the mainpanel, in order to maintainthe segregation of normal re-turn currents and any groundcurrents. This point at thetransformer is the only pointon the system where N-Gshould be bonded.

480V

208Y/120V

Neutral

Grounding electrode nearby,preferably structural metal

Figure 3.4 Transformer grounding.


SolutionsThere are a number of solutionsfor transformer-related PQproblems:• Install additional distribution

transformers (SeparatelyDerived Systems)

• Derate transformers• Install K-rated transformers• Used forced air cooling

1. Separately DerivedSystem (SDS)The distribution transformer isthe supply for a SeparatelyDerived System (SDS), a termwhich is defined in the NEC(Article 100). The key idea isthat the secondary of this trans-former is the new source ofpower for all its downstreamloads: this is a powerful conceptin developing a PQ distributionsystem. The SDS accomplishesseveral important objectives, allbeneficial for PQ:• It establishes a new voltage

reference. Transformers havetaps which allow the second-ary voltage to be stepped upor down to compensate forany voltage drop on thefeeders.

• It lowers source impedanceby decreasing, sometimesdrastically, the distancebetween the load and thesource. The potential for volt-age disturbances, notablysags, is minimized.

• It achieves isolation. Sincethere is no electrical connec-tion, only magnetic coupling,between the primary andsecondary, the SDS isolatesits loads from the rest of theelectrical system. To extendthis isolation to high fre-quency disturbances, spe-cially constructed “isolationtransformers” provide ashield between the primaryand secondary to shunt RF(radio frequency) noise toground. Otherwise, the ca-pacitive coupling betweenprimary and secondarywould tend to pass thesehigh-frequency signals rightthrough.

• A new ground reference isestablished. Part of the defi-nition of the SDS is that it“has no direct electrical con-nection, including a solidlyconnected grounded circuitconductor, to supply conduc-tors originating in anothersystem.” (NEC 100) The op-portunity exists to segregatethe subsystem served by theSDS from ground loops andground noise upstream fromthe SDS, and vice versa.

2. K-rated transformers

Harmonics cause heating intransformers, at a greater ratethan the equivalent fundamen-tal currents would. This is be-cause of their higher frequency.There are three heating effectsin transformers that increasewith frequency:

• Hysteresis. When steel ismagnetized, magnetic dipolesall line up, so that the Northpoles all point one way, theSouth poles the other. Thesepoles switch with the polarityof the applied current. Thehigher the frequency, themore often the switchingoccurs, and, in a processanalogous to the effects offriction, heat losses increase.

• Eddy currents. Alternatingmagnetic fields create local-ized whirlpools of currentthat create heat loss. Thiseffect increases as a squareof the frequency. For example,a 3rd harmonic current willhave nine times the heatingeffect as the same current atthe fundamental.

• Skin effect. As frequencyincreases, electrons migrateto the outer surface of theconductor. More electrons areusing less space, so the effec-tive impedance of the con-ductor has increased; at thehigher frequency, the con-ductor behaves as if it were alower gauge, lower ampacity,higher impedance wire.

The industry has respondedwith two general solutions tothe effects of harmonics ontransformers: install a K-factorrated transformer or derate astandard transformer. Let’s lookat pros and cons of the K-factorapproach first. K-factor is acalculation based on the rms

Figure 3.5 Typical K-factor in commercialbuilding.


00

20

40

60

80

100

20 40 60 80 100

Transformer Capacity (%)After Derating forElectronic Load

Switched-Mode Power Supply Load (% of Overall Load)

value, %HD (harmonic distor-tion) of the harmonic currents,and the square of the harmonicorder (number). It is not neces-sary to actually perform the cal-culation because a harmonicanalyzer will do that for you.The important thing to under-stand is that the harmonic orderis squared in the equation andthat is precisely where thehigh- frequency heating effects,like eddy current losses, aretaken into account.

K-rated transformers are de-signed to minimize and accom-modate the heating effects ofharmonics. K-rated transformersdo not eliminate harmonics(unless additional elements likefilters are added). They accom-modate harmonics with tech-niques such as the use ofa number of smaller, parallelwindings instead of a singlelarge winding: this gives moreskin for the electrons to travelon. The primary delta windingis up-sized to tolerate the circu-lating third harmonic currentswithout overheating. The neu-tral on the secondary is alsoup-sized for third harmonics(typically sized at twice thephase ampacity).

Application issues withK-factor transformersK-rated transformers have beenwidely applied, but there arecertain issues with them. Manyconsultants do not see the needfor using transformers with arating higher than K-13 al-though K-20 and higher mightbe supplied as part of an inte-grated Power Distribution Unit(PDU). Also, early applicationssometimes overlooked the factthat K-rated transformers nec-essarily have a lower internalimpedance. Whereas a standardtransformer has an impedancetypically in the 5-6% range,K-rated transformers can goas low as 2-3% (lower as theK-rating increases). In retrofitsituations, where a standardtransformer is being replaced bya K-rated transformer of equiva-lent kVA, this may require newshort circuit calculations andre-sizing of the secondaryovercurrent protective devices.

3. Derating standardtransformersSome facilities managers usea 50% derating as a rule-of-thumb for their transformersserving single-phase, predomi-nantly nonlinear loads. Thismeans that a 150 kVA trans-former would only supply 75kVA of load. The derating curve,taken from IEEE 1100-1992(Emerald Book), shows that atransformer with 60% of itsloads consisting of SMPS(switched-mode power sup-plies), which is certainly pos-sible in a commercial officebuilding, should in fact bederated by 50%.

The following is an acceptedmethod for calculating trans-former derating for single-phaseloads only. It is based on thevery reasonable assumptionthat in single-phase circuits, thethird harmonic will predominateand cause the distorted currentwaveform to look predictablypeaked.

Use a true-rms meter to makethese current measurements:1. Measure rms and peak cur-

rent of each secondaryphase. (Peak refers to the in-stantaneous peak, not to theinrush or “peak load” rmscurrent).

2. Find the arithmetic averageof the three rms readings andthe three peak currents anduse this average in step 3(if the load is essentiallybalanced, this step is notnecessary).

3. Calculate Xformer HarmonicDerating Factor:

xHDF = (1.414 * IRMS) / I PEAK

4. Or, since the ratio of Peak/RMS is defined as CrestFactor, this equation canbe rewritten as:xHDF = 1.414 / CFIf your test instrument hasthe capability, measure theCF of each phase directly. Ifthe load is unbalanced, findthe average of the threephases and use the averagein the above formula.

Since a sine wave currentwaveform has a CF=1.414, itwill have an xHDF=1; there willbe no derating. The more the3rd harmonic, the higher thepeak, the higher the CF. If theCF were 2.0, then thexHDF=1.414 / 2 =.71. A CF=3gives us an xHDF =.47. A wavewith CF=3 is about as badlydistorted a current waveformas you can expect to see ona single-phase distributiontransformer.(Caution: This method does not apply totransformers feeding three-phase loads,where harmonics other than the third tend topredominate and CF is not useful as a simplepredictor of the amount of distortion. Acalculation for three-phase loads is availablein ANSI/IEEE C57.110. However, there is somecontroversy about this calculation since it mayunderestimate the mechanical resonantvibrations that harmonics can cause, and thataccelerate transformer wear above andbeyond the effects of heat alone.)

4. Forced air coolingIf heat is the problem, cooling isthe solution. Break out the fan,turn it on the transformer anduse forced air cooling. Someexperienced hands figure that’sworth 20-30% on the up side.In any case, it can only help.

Figure 3-6 Transformer derating curve (IEEE 1100-1992).


Electrical noise is the result ofmore or less random electricalsignals getting coupled into cir-cuits where they are unwanted,i.e., where they disrupt informa-tion-carrying signals. Noise oc-curs on both power and signalcircuits, but generally speaking,it becomes a problem when itgets on signal circuits. Signaland data circuits are particularlyvulnerable to noise becausethey operate at fast speeds andwith low voltage levels. Thelower the signal voltage, theless the amplitude of the noisevoltage that can be tolerated.The signal-to-noise ratio de-scribes how much noise a cir-cuit can tolerate before the validinformation, the signal, becomescorrupted.

Noise is one of the more mys-terious subjects in PQ, especiallysince it must be consideredwith its equally mysterioustwin, grounding. To lessen themystery, there are two keyconcepts to understand:

• The first is that electricaleffects do not require directconnection (such as throughcopper conductors) to occur.For an electrician who’s beentrained to size, install andtest wiring, this may not beintuitive. Yet think of light-ning, or of the primary andsecondary of an isolationtransformer, or of the an-tenna to your radio: there’sno direct, hard-wired con-nection, but somehow com-plete electrical circuits arestill happening. The sameelectrical rules-of-behaviorare in operation for noisecoupling, as will beexplained below.

• The second concept is thatwe can no longer stay in therealm of 60 Hz. One of thebenefits of 60 Hz is that it’sa low enough frequency thatpower circuits can be treated(almost) like dc circuits; inother words, basic Ohm’sLaw will get you most placesyou need to go. But when itcomes to noise, we need tokeep in mind that signal cir-cuits occur at high frequen-cies, that noise is typicallya broad spectrum of frequen-cies, and that we need toconsider the frequency-de-pendent behavior of potentialsources of noise.

Coupling mechanismsThere are four basic mecha-nisms of noise coupling. It paysto understand them and howthey differ one from the otherbecause a lot of the trouble-shooter’s job will be to identifywhich coupling effect is domi-nant in a particular situation.

1. Capacitive couplingThis is often referred to aselectrostatic noise and is avoltage-based effect. Lightningdischarge is just an extremeexample. Any conductors sepa-rated by an insulating material(including air) constitute a ca-pacitor—in other words, capaci-tance is an inseparable part ofany circuit. The potential forcapacitive coupling increases asfrequency increases (capacitivereactance, which can bethought of as the resistance tocapacitive coupling, decreaseswith frequency, as can be seenin the formula: XC = 1/ 2π fC).

2. Inductive couplingThis is magnetic-coupled noiseand is a current-based effect.Every conductor with currentflowing through it has an asso-ciated magnetic field. A chang-ing current can induce currentin another circuit, even if thatcircuit is a single loop; in otherwords, the source circuit acts asa transformer primary with thevictim circuit being the second-ary. The inductive couplingeffect increases with the follow-ing factors: (1) larger currentflow, (2) faster rate of change ofcurrent, (3) proximity of the twoconductors (primary and sec-ondary) and (4) the more theadjacent conductor resembles acoil (round diameter as opposedto flat, or coiled as opposed tostraight).

Here are some examples ofhow inductive coupling cancause noise in power circuits:

Section 4Electrical Noise and Transients

Figure 4.1 Lower voltage, faster signals increase sensitivity to noise.

20 - 30Vlogic signal

3 - 5Vlogic signal

Noise

Noise


• A transient surge, especiallyif it occurs on a high-energycircuit, causes a very fastchange in current which cancouple into an adjacent con-ductor. Lightning surges are aworst case, but commonswitching transients or arc-ing can do the same thing.

• If feeder cables are posi-tioned such that there is anet magnetic field, then cur-rents can be induced intoground cables that share theraceway.

• It is well known that signalwires and power conductorsshould not be laid parallel toeach other in the same race-way, which would maximizetheir inductive coupling, butinstead be separated andcrossed at right angles whennecessary. Input and outputcables should also be isolatedfrom each other in the samemanner.

Magnetic fields are isolated byeffective shielding. The materialused must be capable of con-ducting magnetic fields (ferrousmaterial as opposed to copper).The reason that a dedicated cir-cuit (hot, neutral, ground)

should be run in its own metalconduit when possible is that isin effect magnetically shieldedto minimize inductive couplingeffects.

Both inductive and capacitivecoupling are referred to as nearfield effects, since they domi-nate at short distances and dis-tance decreases their couplingeffects. This helps explain oneof the mysteries of noise—howslight physical repositioning ofwiring can have such majoreffects on coupled noise.

3. Conducted noiseWhile all coupled noise ends upas conducted noise, this term isgenerally used to refer to noisethat is coupled by a direct, gal-vanic (metallic) connection. In-cluded in this category arecircuits that have shared con-ductors (such as shared neutralsor grounds). Conducted noisecould be high frequency, butmay also be 60 Hz.

These are some commonexamples of connections thatput objectionable noise currentsdirectly onto the ground:• Sub-panels with extra N-G

bonds

• Receptacles miswired withN and G switched

• Equipment with internal solidstate protective devices thathave shorted from lineor neutral to ground, or thathave not failed but have nor-mal leakage current. Thisleakage current is limited byUL to 3.5 mA for plug-con-nected equipment, but thereis no limit for permanentlywired equipment with poten-tially much higher leakagecurrents. (Leakage currentsare easy to identify becausethey will disappear when thedevice is turned off).

• Another common example isthe so-called isolated groundrod. When it is at a differentearth potential than thesource grounding electrode,a ground loop current occurs.This is still conducted noise,even though the direct con-nection is through the earth.

• Datacom connections thatprovide a metallic path fromone terminal to another canalso conduct noise. In thecase of single-ended, unbal-anced connections (RS-232),the connection to terminalground is made at each endof the cable. This offers apath for ground currents ifthe equipment at each endis referenced to a differentpower source with a differentground.

4. RFI (Radio FrequencyInterference)RFI ranges from 10 kHz to the10s of MHz (and higher). Atthese frequencies, lengths ofwire start acting like transmit-ting and receiving antennas.The culprit circuit acts as atransmitter and the victim circuitis acting as a receiving antenna.RFI, like the other couplingmechanisms, is a fact of life, butit can be controlled (not withoutsome thought and effort, how-ever).

Figure 4.2 Noise coupling. Ground noise measured as ø-G or N-G noise.

Ø - Ø

Ø - G N - GNoise Coupling

Ground

ØA

ØB

N

ØC


RFI noise reduction employsa number of strategies:• Fiber optic cable, of course,

is immune to electrical noise.• Shielded cabling (such as

coax cables) attempts tobreak the coupling betweenthe noise and signal.

• Balanced circuits (such astwisted pair) don’t break thecoupling, but instead takeadvantage of the fact thatthe RFI will be coupled intoboth conductors (signal andreturn). This noise (calledCommon Mode noise) is thensubtracted, while the signalis retained. In effect, the bal-anced circuit creates a highimpedance for the couplednoise.

• Another example of thehigh-impedance-to-noiseapproach is the use of RFchokes. Whether used withdata or power cables, RFchokes can offer effectivehigh-frequency impedance(XL increases with frequency).

• A low-impedance path canbe used to shunt away thenoise. This is the principlebehind filtering and the useof decoupling caps (low im-pedance to high frequency,but open at power line fre-quencies). But a sometimes-overlooked, yet critical, as-pect is that the ground pathand plane must be capableof handling high-frequencycurrents. High-frequencygrounding techniques areused to accomplish this. TheSRG (Signal Reference Grid),first developed for raisedfloor computer room installa-tions, is an effective solution.It is essentially an equipo-tential ground plane at high-frequency. (For further infor-mation on high-frequencygrounding, see the refer-ences listed on the back page.)

Signal GroundingTo understand the importance of“clean” signal grounds, let’s dis-cuss the distinction betweenDifferential Mode (DM) vs. Com-mon Mode (CM) signals. Imag-ine a basic two-wire circuit:supply and return. Any currentthat circulates or any voltageread across a load betweenthe two wires is called DM(the terms normal mode, trans-verse mode and signal modeare also used). The DM signal istypically the desired signal (justlike 120V at a receptacle).Imagine a third conductor, typi-cally a grounding conductor.Any current that flows nowthrough the two original con-ductors and returns on this thirdconductor is common to both ofthe original conductors. The CMcurrent is the noise that thegenuine signal has to overcome.CM is all that extra traffic on thehighway. It could have gottenthere through any of the cou-pling mechanisms, such asmagnetic field coupling atpower line frequency or RFI athigher frequencies. The point isto control or minimize theseground or CM currents, to makelife easier for the DM currents.

MeasurementCM currents can be measuredwith current clamps using thezero-sequence technique. Theclamp circles the signal pair (or,in a three-phase circuit, allthree-phase conductors and theneutral, if any). If signal and re-turn current are equal, theirequal and opposite magneticfields cancel. Any current readmust be common mode; in otherwords, any current read is cur-rent that is not returning on thesignal wires, but via a groundpath. This technique applies tosignal as well as power conduc-tors. For fundamental currents, aClampMeter or DMM + clampwould suffice, but for higher fre-quencies, a high bandwidth in-strument like the Fluke 43Power Quality Analyzer orScopeMeter should be usedwith a clamp accessory.

A Matter of Life and Death

Sometimes PQ troubleshootingis a matter of life and death.

Dave was the on-site fieldengineer at the hospital. Oneday he got a call from a veryconcerned nurse in the ER. Oneof their patients had died. Butas upsetting as that was, itwasn’t the main source of con-cern. What was really unusualwas that this particular corpsehad a heartbeat.

Dave soon arrived at thescene. A quick glance told himthat the dead had not comeback to life. The problem layelsewhere. The nurses pointedout what they had seen, a sig-nal on the EKG indicating aheartbeat. But there was some-thing unusual about this signal(above and beyond the fact thatit seemed to be coming from adead body). He noticed that thesignal was a 60 Hz sine wave(slightly flat-topped). A furtherlook at the signal wires told him

that they had been laid parallelto the power cord. The couplingbetween signal and powerwires caused the 60 Hz “Heart-beat” on the EKG machine.The moral of the story is toalways isolate the signal andpower conductors—before itbecomes a matter of lifeand death.


Transients

Transients should be distin-guished from surges. Surges area special case of high-energytransient which result fromlightning strikes (see section 5,“PQ Troubleshooting of Light-ning Protection Systems”). Volt-age transients are lower energyevents, typically caused byequipment switching.

They are harmful in anumber of ways:• They deteriorate solid state

components. Sometimes asingle high energy transientwill puncture a solid statejunction, sometimes repetitivelow energy transients will ac-complish the same thing. Forexample, transients whichexceed the PIV (peak inversevoltage) rating of diodes area common cause of diodefailure.

• Their high-frequency compo-nent (fast rise times) causethem to be capacitivelycoupled into adjoining con-ductors. If those conductorsare carrying digital logic, thatlogic will get trashed. Tran-sients also couple acrosstransformer windings unlessspecial shielding is provided.Fortunately this same highfrequency component causestransients to be relativelylocalized, since they aredamped (attenuated) by theimpedance of the conductors(inductive reactance increaseswith frequency).

• Utility capacitor switchingtransients are an example ofa commonly-occurring high-energy transient (still by nomeans in the class of light-ning) that can affect loads atall levels of the distributionsystem. They are a wellknown cause of nuisancetripping of ASDs: they haveenough energy to drive atransient current into the dclink of the drive and causean overvoltage trip.

Transients can be categorizedby waveform. The first categoryis “impulsive” transients, com-monly called “spikes,” becausea high-frequency spike pro-trudes from the waveform. Thecap switching transient, on theother hand, is an “oscillatory”transient because a ringingwaveform rides on and distortsthe normal waveform.It is lower frequency, buthigher energy.

CausesTransients are unavoidable.They are created by the fastswitching of relatively high cur-rents. For example, an inductiveload like a motor will create akickback spike when it is turnedoff. In fact, removing a Wiggy (asolenoid voltage tester) from ahigh-energy circuit can create aspike of thousands of volts! Acapacitor, on the other hand,creates a momentary short cir-cuit when it’s turned on. Afterthis sudden collapse of the ap-plied voltage, the voltage re-bounds and an oscillating waveoccurs. Not all transients are thesame, but as a general state-ment, load switching causestransients.

In offices, the laser copier/printer is a well-recognized“bad guy” on the office branchcircuit. It requires an internalheater to kick in whenever it isused and every 30 seconds orso when it is not used. Thisconstant switching has twoeffects: the current surge or in-rush can cause repetitive volt-age sags; the rapid changes incurrent also generate transientsthat can affect other loads onthe same branch.

Measurement and recordingTransients can be captured byDSOs (Digital Storage Oscillo-scopes). The Fluke 43 PQAnalyzer, which includes DSOfunctions, has the ability tocapture, store and subsequentlydisplay up to 40 transientwaveforms. Events are taggedwith time and date stamps (realtime stamps). The VR101S Volt-age Event Recorder will alsocapture transients at the recep-tacle. Peak voltage and real timestamps are provided.

Figure 4.3 Fluke 43 can capture and save upto 40 transients.

Cursor moves to display peak Min/Max values.

Real-time stamp. Date:hr:min:sec


500

400

300

200

140

120110100908070

40

0

Applicable to 120, 120/208, and120/240 nominal voltages

110V

90V

1µs 1ms 3ms 20ms 0.5s 10s Steadystate

0.001c 0.01c 0.1c 0.5c 1c 10c 100c 1000c

Duration of disturbance in cycles (c) and seconds (s)

Perc

ent

of n

omin

al v

olta

ge (R

MS

or p

eak

equi

vale

nt)

Voltage-Toleranceenvelope

Voltage susceptibility profile

The new ITIC profile (Informa-tion Technology Industry Coun-cil) is based on extensiveresearch and updates theCBEMA curve. The CBEMAcurve (Computer BusinessEquipment Manufacturers Asso-ciation, now ITIC) was the origi-nal voltage susceptibility profilefor manufacturers of computersand other sensitive equipment.Similar curves are being devel-oped for 230V/50Hz equipmentand for adjustable speed drives.Sensitive equipment should beable to survive events inside thecurve. Events outside of thecurve could require additional

power conditioning equipmentor other remedial action. A ma-jor change in ITIC is that theride-through times for outagesas well as the tolerance for sagshave both been increased.The field troubleshooter mustkeep in mind that the profilesare recommendations and thata particular piece of equipmentmay or may not match the pro-file. Having said that, the pro-files are still useful because,when recorded events are plot-ted against them, they give ageneral idea of the voltagequality at a particular site.

Transient voltage surgesuppressors (TVSS)Fortunately, transient protectionis not expensive. Virtually allelectronic equipment has (orshould have) some level of pro-tection built in. One commonly-used protective component isthe MOV (metal oxide varistor)which clips the excess voltage.

TVSS are applied to provideadditional transient protection.TVSS are low voltage (600V)devices and are tested and cer-tified to UL 1449. UL 1449 ratesTVSS devices by Grade, Classand Mode. As an example, thehighest rating for a TVSS wouldbe Grade A (6000V, 3000A),Class 1 (let-through voltage of330V max) and Mode 1 (L-Nsuppression). The proper ratingshould be chosen based on theload’s protection needs:• A lower Grade might result in

a TVSS that lasts one year in-stead of ten years. The solidstate components in a TVSSwill themselves deteriorate asthey keep on taking hits fromtransients.

• A lower Class might permit toomuch let-through voltage thatcould damage the load. Class1 is recommended for switchmode power supplies.

• A Mode 2 device would passtransients to ground, wherethey could disrupt electroniccircuit operation.

Figure 4.4 ITIC Curve.


Lightning protection plays avital part in the overall powerquality of an installation. Light-ning occurrence varies by geog-raphy, with Florida being thelightning capital of the U.S.Lightning does not have toscore a direct hit to be disrup-tive. It has so much energy thatit couples surges into conduc-tors, both those exposed to airand those buried in the ground.

Basic lightning protection hastwo main requirements:

1. Effective groundingA low impedance of the ground-ing electrode system to earth isimportant. But, equally impor-tant is that all parts of thegrounding system be bondedtogether: all ground electrodesare bonded (and extraneousground rods removed), structuralsteel is tied to service entranceground, all grounding connec-tions are tight and free of corro-sion, etc. This minimizes thephenomenon called “transferredearth potential,” where largesurge currents create large volt-age differences between twoground points with differentimpedances to earth. This samegrounding practice is importantfor performance reasons, as ittends to minimize ground loopcurrents that circulate in anattempt to equalize groundpotentials.

2. Surge arrestorsA surge arrestor “is a protectivedevice for limiting surge volt-ages by discharging or bypass-ing surge current...” (NEC 280).Since the surge current is by-passed to ground, surge arres-tors are only as effective as thegrounding system.

Surge arrestors are sized forthe location where they are in-stalled. Three categories are de-fined (ANSI/IEEE C62.41-1991).

Section 5Lightning Protection

A surge arrestor at an outsideinstallation is closest to thelightning event and must ab-sorb the most energy. This isconsidered a Category C loca-tion (corresponding to CAT IV inIEC 61010). Category B refers tofeeders and distribution panels(equivalent to CAT III in IEC61010), and Category A refers toreceptacle connected surge ar-restors (equivalent to CAT II).

Surge arrestor or TVSSA surge arrestor is there toprotect the insulation and, ulti-mately, prevent failures thatcould lead to fires. It is notnecessarily designed to protectsensitive equipment. That’s thejob of the TVSS (transient volt-age surge suppressor).

Table 5.1 Inspection of lightning protection system.

Check Look for Reason

Surge arrestors • Installed at main service • Lightning is high energypanel, subpanels and and needs multilevelcritical equipment. protection.

• To minimize high • Lightning has high ffrequency impedance, components. Shorter leadsleads should be short, have less XL and lesswith no bends. impedance at high f.

Grounding electrode • Grounding electrode • Ensure low impedanceconductors at service connections are not loose ground to minimizeentrance or at SDS or corroded. potential to ground with

• Grounding conductor lightning induced surges.should not be coiled or • Minimize impedance tohave unnecessary bends. high frequency

components of lightning.

Grounding electrode bonding All grounding electrodes Prevent difference in earthshould be effectively bonded potential between electrodestogether (<0.1Ω). in event of lightning.

Separately driven Electrode and equipment Same as above—entire(“isolated”) electrode ground should both be tied grounding system should be

to building steel, and an equipotential groundthereby to the service plane for lightning.entrance ground.

Datacom cabling that runs Surge arrestors on datacom Datacom cabling runbetween buildings cabling or use of fiber optic between buildings can be a

cables. path for surge currents, dueto differences betweenbuilding earth potentials.

Lightning protection is covered in a number of standards and codes, including:NEC: Articles 250 and 280National Fire Protection Association: NFPA 780Lightning Protection Institute: LPI-175UL-96 and UL-96A

Figure 5.1. Surge arrestors installed at service, panel, load.

MainService

SubPanel

SurgeArrestor

SurgeArrestor

SurgeArrestor

Load


About two-thirds of the electricpower in the U.S. is consumedby motors, with industrialthree-phase motors above 5 HP(7 kW) being by far the bulk ofthat load. They are linear loadsand therefore don’t contributeto harmonics. They are, how-ever, the major contributor toreduced Displacement PowerFactor, which is a measurementof the effective use of systemcapacity.

Measurements

1. Voltage unbalanceVoltage unbalance should notexceed 1-2% (unless the motoris lightly loaded). Why such asmall number? Voltage unbal-ance has a very large effect oncurrent unbalance, in the neigh-borhood of 8:1. In other words,a voltage unbalance of 1% cancause current unbalance of 8%.Current unbalance will causethe motor to draw more currentthan it otherwise would. This inturn causes more heat and heatis the enemy of motor life, sinceit deteriorates the winding insu-lation.

2. Voltage %THD andharmonic spectrumVoltage THD should not exceed5% on any phase. If the voltagedistortion on any phase is ex-cessive, it can cause current un-balance. The usual culprit is the5th harmonic and therefore theharmonic spectrum should beexamined for the 5th in particu-lar. The 5th is a negative se-quence harmonic which createscounter-torque in the motor. Amotor fed by a voltage withhigh 5th harmonic content willtend to draw more current thanotherwise. This is a major prob-lem when across-the-line orsoft-start motors share the samebus with ASDs.

Section 6Polyphase Induction Motors

3. Current unbalanceTo find current unbalance, mea-sure amps in all three phases.Do the same calculation as forvoltage unbalance. In general,current unbalance should notexceed 10%. However, unbal-ance can usually be tolerated ifthe high leg reading doesn’t ex-ceed the nameplate FLA (FullLoad Amps) and SF (ServiceFactor). The FLA and ServiceFactor are available on the mo-tor nameplate. If the voltage un-balance and the voltage THDare within limits, high currentunbalance can be an indicationof motor problems, such asdamaged winding insulation oruneven air gaps.

Current measurement willalso find single-phasing. If athree-phase motor loses aphase (perhaps caused by ablown fuse or loose connection),it may still try to run singlephase off the remaining twophases. Since the motor actslike a constant power device, itwill simply draw additional cur-rent in an attempt to providesufficient torque. A voltage mea-surement alone will not neces-sarily find this condition, sincevoltage is induced by the twopowered windings into thenon-powered winding.

Part II: Three-Phase Loads

Example of voltage unbalance calculation:%VUNBALANCE = Max deviation from average x 100%

Average (of three phases)1. Make three phase-to-phase measurements:

A-B = 475V A-C = 471V B-C = 470V2. Find the average: (475+471+470) ÷ 3 = 472V3. Find the maximum deviation from the average:

This occurs on the A-B phase: 475V-472V = 3V4. Divide maximum deviation by average to find

% unbalance: 3V/472V <1%

Voltage unbalance can be caused by severe load unbalancebut it could just as easily be caused by loose connectionsand worn contacts.

Table 6.1 Measurements at the motor.


1. Voltage unbalance Unbalance <1% 43, 41B, 87

2. Voltage %THD %THD <5% 43, 41B

3. Current unbalance Unbalance <10% 43, 41B87 w/80i-400

Single phasing No current on one phase.(extreme currentunbalance)

4. Loading • Nameplate data on FLA1 and SF2: SameCurrent < (FLA x SF).

• Overloading or extreme underloading.80% of rated load is optimal.

5. Inrush current • Inrush causing voltage sag. 43• Inrush causing nuisance trips.

6. Power factor • Low Displacement PF. 43, 41B• Large difference between DPF and PF

(Total Power Factor) indicating harmonics.

1 FLA = Full Load Amps2 SF = Service Factor(If the FLA = 100A and the SF=1.15, the motor can be run at 115A continuously.)


4. LoadingMeasure current draw of themotor. If the motor is at or nearits FLA rating (times the ServiceFactor multiplier), it will bemore sensitive to the additionalheating from harmonics, as wellas current unbalance. A motorthat is only lightly loaded isusually safe from overheating.On the other hand, its efficiencyand DPF are both less than op-timal. Most motors reach maxi-mum efficiency at 60%-80% offull load rating. DisplacementPower Factor is maximum atrated load (including S.F.) anddrops off, especially at less than80% of rated load. This leads tothe conclusion that, to the de-gree a motor load is constantand predictable, 80% of ratedload is the most efficient oper-ating range.

5. InrushMotors which are startedacross-the-line (as opposed tothose using soft-starts or drives)draw a current inrush, alsocalled locked rotor current. Thisinrush tapers off to normal run-ning current as the motor comesup to speed.• Older motors draw an inrush

of typically 500-600% of therunning current. Newer en-ergy efficient designs drawbrief inrushes as high as1200% of running current, adirect result of the lower im-pedances which help makethem more energy efficientin the first place.

• High torque, high HP motorloads require proportionallyhigher inrush.

• Motor loads started at thesame time will have a cumu-lative inrush.

Another source of inrush is UPSand ASD systems with diodeconverters. They draw inrushcurrent as their cap banks firstcharge.

Effects of inrush current1. Inrush causes voltage sags if

the source voltage is not stiffenough:• Relays and contactor coils

might drop out (typically,the sag would have to getas bad as about 70% ofnormal line voltage); or, ifthey hold in, their contactsmight chatter (especially ifthe additional load causesa long-term undervoltage).

• Control circuits might resetor lock up (at 90% andbelow).

• Drives might trip off-line(undervoltage trip).

2. High peak demand periods,which may cause higherutility bills.

3. Cycling loads can cause peri-odic sags, which might showup as flickering lights.

4. If the motor is required tostart up a high torque load,the inrush can be relativelyprolonged (e.g., 10 to 20seconds or more) and thiscan cause nuisance trippingas the overload heaters tripthe motor starter.

6. Power FactorTo size PF correction capacitors,it is necessary to measure theDPF (Displacement PF) andActive Power consumption (kW)of the motor load. Measurementof the DPF and kW of a three-phase induction motor is ex-plained in the sidebar on thenext page. These measurementsassume that the motor voltageand current is balanced. There-fore, before undertaking PF cor-rection, first make sure thatvoltage and current unbalanceare within limits. Either problemcan shorten motor life andshould take priority over DPFcorrection.

Flattopping

Total harmonic distortion < 5%

Figure 6.1. Voltage distortion.

OneOpenFuse

L1 L2 L3

T1 T3

T2

M

Single-phasing

Figure 6.2. Single-phasing.

Figure 6.3. Inrush current (Model 43).


Measuring Displacement Power Factoron 3-phase Induction Motors

Select which of the two methods to use based onthe transformer configuration supplying the mo-tor. Either method will give the same results.Method #1 is for the grounded-Y source. It issimple and can be applied in most situations,since virtually all of the low voltage motors incommercial and light industrial facilities are fedfrom a grounded-Y source. Method #2 is for float-ing sources sometimes found in heavy industrialfacilities.

Method #1: Grounded-Y sourceTo check if the source is grounded-Y, measurevoltage of each phase to ground. If the readingsare equal, you can use this measurement method.Set-up:1. Clamp the current probe on any phase (with

arrow on clamp pointing towards the motor).2. Attach the red voltage probe to the same

phase and the black probe to ground (not toanother phase).

Active Power: Read kW and multiply by 3:kWMOTOR = 3 * kWDisplacement PF: Read DPF.(Not necessary to measure kVA)

Method #2: Three-wire SourceWith floating-Y, floating-delta or grounded-delta,the voltage will be different for at least one ofthe phase-to-ground readings. (For the floatingsource, the phase-to-ground voltage is unpredictable,since it depends on phase to ground capacitance).Method #2 is known as the Two-Wattmeter method.Set-up:1. Connect the black voltage probe to any phase.2. Connect the red voltage probe and the clamp

(arrow towards the load) together on a secondphase. Record kW1.

3. Move red probe and clamp to the third phase(do not move the black probe). Record kW2.Record kVA (kVA of either phase will be moreor less equal if the current unbalance is withinlimits).

Active Power: KwMOTOR = kW1 + kW2 (If eitherkW reading is negative, as might happen on avery lightly loaded motor, it would be subtractedinstead of added)Apparent Power: kVAMOTOR = kVA * 1.73.DPF = kWMOTOR / kVAMOTOR

Example (Two-Wattmeter):Measurements:kW1 = + 1.52kW2 = + 1.74kVA = 2.41Calculations:KwMOTOR = kW1 + kW2 = (+ 1.52) + (+1.74) = 3.26 kWkVAMOTOR = kVA * 1.73. = (2.41) (1.73) = 4.17 kVADPF = KwMOTOR / kVAMOTOR = 3.26 / 4.17 = 0.78

black

red

Load

kWTOTAL = kW1 + kW2

kVATOTAL = kVA1 • 3

PF = kWTOTAL

kVATOTAL

kVAkW1

Load

kW2

Power Factor Using the Two-Wattmeter Method:

red

red

blackblack


AC ASDs can be both a sourceand a victim of poor PQ (see“Measurement of AdjustableSpeed Drives with FlukeMeters,” document numberG0416UEN, for more informationon drive troubleshooting).

ASDs as Victim LoadsAlthough ASDs are usuallydepicted as the culprit in thePQ scenario, there are waysin which they can be a victimload as well.Capacitor switching transientsHigh-energy (relatively low-frequency) transients that arecharacteristic of utility capacitorswitching can pass through theservice transformer, feeders,and converter front-end of thedrive directly to the dc link bus,where it will often cause a dclink overvoltage trip. Input di-odes could also be blown outby these transients.

Voltage distortionIf high-voltage distortion showsup as excessive flat-topping, itwill prevent dc link capacitorsfrom charging fully and willdiminish the ride-throughcapability of the drive. Thus avoltage sag which would notnormally affect a drive willcause the drive to trip onundervoltage.Improper grounding will affectthe internal control circuits ofthe drive, with unpredictableresults.

ASDs as Culprit LoadsA drive can definitely be a “cul-prit load” and have a major im-pact on system PQ. But beforewe talk of problems, let’s put ina good word for the positive ef-fects of drives on PQ. First of all,they offer built-in soft-start ca-pabilities. This means there will

Section 7PQ Troubleshooting of Adjustable Speed Drives

be no inrush current and novoltage sag effect on the rest ofthe system. Secondly, if thedrive is of the PWM type, with adiode converter front-end, theDisplacement Power Factor ishigh (commonly >95% at ratedload) and more or less constantthroughout the range. Thismeans that drives can reduceenergy usage and correct forDPF at the same time. It’s agood thing too, because drivesand PF correction capacitorsdon’t mix (see “Power SystemResonance,” page 28). Caps arevulnerable to the higher fre-quency harmonic currents gen-erated by drives, since theirimpedance decreases as fre-quency increases.

The type of drive has a majorimpact on the PQ symptoms, be-cause of the different converterdesigns (converters or rectifiersturn ac to dc and are the firststage of the drive). There aretwo major types of converterdesign.

SCR Convertor with VoltageSource Inverter/VariableVoltage Inverter (VSI/VVI)DrivesCommonly called six-stepdrives, they use SCRs (Silicon -Controlled Rectifiers) in theirconverter front-ends (the fol-lowing discussion applies toCSI, Current Source Inverterdrives, which also use SCRs).VSI and CSI drive designstended to be applied on largerdrives (>100HP). SCR converterscontrol the dc link voltage byswitching on (or “gating”) cur-rent flow for a portion of the ap-plied sine wave and switchingoff at the zero-crossing points.Unlike diodes, SCRs requirecontrol circuits for gate firing.

Table 7.1 Line-side measurements on ASDs.


Voltage waveform • Voltage notching (SCR converters) 43• Flat-topping

Harmonic spectrum Harmonic orders and amplitudes, before and 43, 41Bafter filter application

Displacement PF For PWM drives, DPF should remain high even 43, 41Bat low speeds (it will typically decrease slightly)

Voltage unbalance Less than drive manufacturer specs, or current 43, 41Boverload trips can result. The drive may have ahigher limit for unbalance than the motor.

Figure 7.1 Voltage Source Invertor (VSI) ASD

InductionMotor

M

LineReactors


For the SCR converter, there arethree main issues that affectline-side PQ:• Commutation notches. SCR

switching or commutation issuch that there are brief mo-ments when two phases willboth be “ON.” This causeswhat is in effect a momen-tary short circuit that tendsto collapse the line voltage.This shows up as “notches”on the voltage waveform.These notches cause bothhigh V-THD and transients.The solution is to place a re-actor coil or isolation trans-former in series with thedrive’s front end to clean upboth problems.

• Displacement Power Factordeclines as drive speed de-creases. This is not as seriousa problem as it sounds, be-cause the power requirementof the drive-motor-load de-creases even more.

• Harmonic currents, typicallythe 5th and 7th, are gener-ated by VSI drives.

Diode Convertor withPulse Width Modulation(PWM) DrivesThe other and more commonconverter design uses diodesand is used in the PWM drive.The diodes require no switchingcontrol circuitry. One of themain trends in the industry hasbeen the proliferation of PWMdrives, mainly due to the con-tinued development of fast-switching, efficient IGBTs(Insulated Gate Bipolar Transis-tors) used in the inverter sectionof the drive (inverters turn dc toac). For all practical purposes,PWM drives are the industrystandard.

For an in-depth discussion ofthe efffects of PWM-IGBT driveson motors, see “Measurement ofASDs with Fluke Meters,” docu-ment number G0416UEN.

For the diode converter, themain PQ issue is harmonics. Theactual harmonic orders beinggenerated depend on the num-ber of diodes in the front end.For three-phase conversion, aminimum set of six diodes is re-

quired. This “six-pulse” con-verter will generate 5th and 7thharmonics. If a 12-pulse con-verter were used, the 11th and13th harmonics will be gener-ated instead of the 5th and6th—and, very importantly, forthe same load, the amplitude ofthe 11th and 13th would beconsiderably less than the 5thand 6th. Therefore, the THDwould be less. The vast majorityof drives, however, are six-pulsePWM style, which is one reasonwe see so much 5th harmonicon the system.

Harmonics solutionsThere are a number of solutionsto mitigating drive-generatedharmonics:Harmonic trap filters (Fig. 7.5)These are typically LC networksconnected in parallel at thesource of the harmonics (inother words, at the drive input).They are tuned to just belowthe 5th harmonic (typically 280Hz) and will tend to sink both5th and much of the 7th har-monic. Obviously, they must besized to the harmonic-generat-ing load.Phase-shift transformersThis can be as simple as adelta-wye transformer feedingone drive(s) and a delta-deltafeeding another drive(s). Thereis a 30 degree phase shift effectbetween these two configura-tions, which effectively resultsin cancellation of harmonics atthe closest upstream PCC (Pointof Common Coupling). The can-cellation effect is optimal whenboth loads are more or lessequal.

Figure 7.2 Voltage Notching.

Figure 7.3 Typical ASD Harmonic Spectrum

Figure 7.4. Pulse Width Modulated (PWM) ASD.


12-pulse converterIf the delta-wye/delta-delta arepackaged together (delta pri-mary, delta and wye secondary)and each secondary feeds oneof two paralleled six-pulse con-verters, a 12-pulse front-end iscreated with all the benefitsmentioned above. 18-pulsedesigns are also available. Be-cause of the extra cost, this typeof solution tends to only getused on high HP loads.Active filtersThis relatively new technologyis based on an elegant concept–using power electronics to solvethe problems created by powerelectronics. It senses the instan-taneous ac sine wave; it thenactively cancels the harmonicsit detects by generating equaland opposite polarity harmon-ics, thus recreating the sinewave. Commercial packagesmight provide voltage regula-tion as well.

Active PF CorrectionAnother recent solution is formanufacturers to offer converterfront ends using fast switchingtechnology that generates aminimum amount of harmonicsand has near unity power factor(both Total PF and DPF).

There is room for discussionon which approach to harmonicmitigation might prove most ef-fective and economical in a par-ticular situation. However, whatis often overlooked by the end-user, and what should be clearfrom the information in this sec-tion, is that the total cost of adrive system should includeboth the cost of the drive itselfand the harmonic mitigation(whether part of the drive orinstalled separately).

True-rms test tools are necessary for accuratemeasurements of distorted waveforms. For moreinformation, see the Fluke application note WhyTrue-rms, document number B0294A.

A comparison of average-responding and true-rms multimeters

Multimeter ReadingWaveform Description Average-sensing True-rms

DMM DMM

Sine wave Correct Correct

Square wave(flat-top voltage) 10% high Correct

Current to singlephase diode rectifier 40% low Correct

Current to 3 phasediode rectifier 5-30% low Correct

Line Load

Figure 7.5 Harmonic trap filter.

Why True-rms


Power system resonanceHot vibes can result when harmonics and capacitors get together

Is it possible to install “PowerFactor Correction Capacitors”and have PF get worse? It cer-tainly is, and a starting place tounderstanding this puzzle liesin the distinction between Dis-placement PF (DPF) and TotalPower Factor (PF). The penaltyfor not understanding the differ-ence can be blown capacitorsand wasted investment.

Total PF and Displacement PFare the same in one basicsense: they are the ratio of RealPower to Apparent Power, orWatts to VA. DPF is the classicconcept of power factor. It canbe considered as the power fac-tor at the fundamental fre-quency. Total Power Factor,abbreviated to Power Factor(PF), now includes the effectsof fundamental and of harmoniccurrents (it is also referred to asTrue PF or Distortion PF,Fig. 7.7). It follows that with thepresence of harmonics, PF is al-ways lower than DPF and isalso a more accurate descriptionof total system efficiency thanDPF alone.

Strictly speaking, the term“Power Factor” refers to Total PF,but in practice can also be usedto refer to DPF. Needless to say,this introduces some confusioninto discussions of power factor.You have to be clear which oneyou’re talking about.

Displacement PowerFactorLower DPF is caused by motorloads which introduce the needfor Reactive Power (Volt-AmpReactive or VARs). The systemhas to have the capacity, mea-sured in Volt-Amps (VA) to sup-ply both VARs and Watts. Themore VARs needed, the largerthe VA requirement and thesmaller the DPF. The cost ofVARs is accounted for in apower factor penalty charge.Utilities often levy additionalcharges for DPF below a certainlevel; the actual number varieswidely, but typical numbers are0.90 to 0.95.

To reduce VARs caused bymotor loads, power factor cor-rection capacitors are installed.Upstream system capacity, bothin the plant and at the utilitylevel, is released and availablefor other uses. (Fig. 7.6)

Historically, this has been thegist of the PF story: a relativelywell-known problem with arelatively straightforwardsolution.

Harmonics and CapacitorsHarmonics have had a dramaticimpact on our approach toPower Factor correction. Themotor and capacitor loads de-scribed above are all linear andfor all practical purposes gener-ate no harmonics. Non-linearloads such as ASDS, on theother hand, do generate har-monic currents.

Take a plant which is step-by-step putting adjustablespeed drives on its motor loads.ASDs generate significant har-monic currents (5th and 7th onsix-pulse converter drives). Sud-denly the fuses on existing PFcorrection caps start blowing.Since these are three-phasecaps, only one of the three fusesmight blow. Now you’ve got un-balanced currents, possibly un-balanced voltages. Theelectrician replaces the fuses.They blow again. He puts inlarger fuses. Now the fuses sur-vive, but the capacitor blows.He replaces the capacitor. Samething happens. What’s goingon? Harmonics are higher fre-quency currents. The higher thefrequency, the lower the imped-ance of a cap (XC = 1/ 2πfC).The cap acts like a sink forharmonic currents.

After: PF = 100%Before: PF = 42%

1/6 HP Motor

Active165 Watts

Reactive360 VAR

3.3A

1/6 HP Motor

Active165 Watts

Reactive360 VAR

1.4A

Capacitor60 µF

Figure 7.6 Capacitor corrects Displacement Power Factor (DPF).


A. System Diagram B. Equivalent Circuit

HarmonicsourceXC

XTXT

XS

XS

XC

kvar(nonwork producing)

VA

kW(work producing)

Harmonics(nonworkproducing)

Power System ResonanceIn a worst-case scenario, theinductive reactance (XL) of thetransformer and the capacitivereactance (XC) of the PF correc-tion cap form a parallel resonantcircuit: XL = XC at a resonant fre-quency which is the same as orclose to a harmonic frequency.The harmonic current generatedby the load excites the circuitinto oscillation. Currents thencirculate within this circuit

Figure 7.8 Resonant circuit when XC = (XT + XS)

Figure 7.7 Total Power Factor increases with harmonics.

which are many times greaterthan the exciting current. Thisso-called “tank circuit” canseverely damage equipment,and it will also cause a dropin power factor. Perversely, thisresonant condition often ap-pears only when the systemis lightly loaded, because thedamping effect of resistive loadsis removed. In other words, wehave what the audio buffs calla “high Q” circuit. (Fig. 7.8)

Imagine coming to work ona Monday and seeing the insu-lation on your cables melted off.How can this happen over aweekend when there washardly any load on the system?Has Ohm’s Law been overruled?Not quite. Your power systemjust spent the weekend tankedout on the Harmonics. It wasquite a party, but now comesthe clean-up.

Start with HarmonicsMitigationThe correct solution path startswith measuring and mitigatingthe harmonics generated by thedrives. Harmonic trap filterswould generally be called for.These trap filters are installedlocally on the line side of thedrive. Their effect is very muchlike the traditional PF correctioncap, in two senses: they reduceDPF as well as PF, and also theylocalize the circulation of theproblem harmonics (generallythe 5th). Harmonics mitigationand traditional DPF correctionshould be addressed as onesystems issue. In other words,manage Total PF, not just DPF.


Lighting loads are a major loadfor many large facilities. Evalu-ating these circuits is importantfor both energy conservationand power quality. Keep inmind that commercial lightingloads are wired single phase,with the loads connected fromphase to neutral. Typically, thephase-to-phase voltage is480V, with the phase-to-neu-tral voltage at 277V. Measure-ments must be taken at thelighting panel, one phase at atime, since power consumptionand Power Factor could vary oneach phase.

Table 8.1 Measurements on commercial lighting loads.


1. Power Balance among three phases. 43, 41Bconsumption (kW)

2. Power Factor Magnetic ballast will have low DPF. Electronic 43, 41B(DPF and PF) ballast may have low total PF, although new

generations of ballast often have harmonicmitigation built-in.

3. Total Harmonic Current %THD <20% is desirable. 43, 41BDistortion (%THD)

4. Voltage Stability Unstable voltage can cause lights to flicker. 43

Section 8Troubleshooting Commercial Lighting Loads

1. Power consumptionExcessive phase unbalance cancause voltage unbalance, whichin turn can affect three-phasemotor loads.

2. Power FactorBallast with low PF might havelower cost-of-purchase buthigher cost-of-operation.

3. Total Harmonic DistortionCurrent THD should be consid-ered when selecting ballast, es-pecially if there is a possibilityof transformer overloading.

4. Voltage stabilityThe sags and swells mode ofthe Fluke 43 is especially usefulfor recording repetitive voltagesags which can show up asflickering lights. Both currentand voltage are monitored si-multaneously. This helps us totell if sags are downstream ofthe measuring point (load-related) or upstream (source-related). For example, if voltagesags while current swells, adownstream current inrushlikely caused the sag. If bothvoltage and current sag, someevent upstream causedthe sags.

It could be an upstreamload like a motor on a parallelbranch circuit which drewdown the feeder voltage. Orit could be source voltage-related, for example, a lightningstrike or breaker trip/reclosureon the utility distributionsystem.

Ø1

Ø2

Ø3

N

Lighting Panel

277V Phase-to-Neutral

480V Phase-to-Phase

Figure 8.1

Figure 8.2 Fluke 43 trends voltage (top) andcurrent (bottom) simultaneously. Currentswells/inrush caused voltage sags, indicatingthat a load downstream from the measure-ment point is the cause of the disturbance.


A Lineup of Power Quality CulpritsFrom utility source to receptacle

LightningCan be extremely destructive ifproper surge protection is notinstalled. It also causes sagsand undervoltages on the utilityline if far away. If close by, itcauses swells and overvoltages.But in the final analysis, light-ning is an act of nature and notin the same category as thedamage man does to himself.Utility automaticbreaker reclosureCauses short duration sags/outages, but better than the al-ternative, 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 nearthe facility, this transient canpropagate all through thebuilding.Commercial high riseswithout enough distributiontransformersTrying to cut corners in thewrong places; running 208Vfeeder up twenty stories isnot the road to PQ.Gen-sets not sized forharmonic loadsExcessive voltage distortionaffects electronic controlcircuits. If SCR converter loadsare present, notching canaffect frequency control circuits.Applying PF correctioncapacitors without consider-ing the effects of harmonicsHarmonics and caps don’t mix.Those bulging capacitors arecrying for help.Inrush currents from hightorque motor loads startedacross-the-lineCauses voltage sags if the loadis too large or the source imped-ance too great. Staggered motorstarts can help.Undersized neutralsat panelboardIn the era of the 3rd harmonic,neutrals can easily carry asmuch current or more currentthan the phase conductor.Keeping them undersized leadsto overheated lugs, potential firehazards and high N-G voltage.

Running power and signalcables togetherThink of the signal cable as asingle-wire transformer second-ary and the power cable as theprimary. The opportunities forcoupling are endless.Loose conduit connectionsand lack of green wiregrounding conductorCauses open or high impedanceground circuit. Not good for PQor safety.Shared neutrals onbranch circuitsCauses load interaction andoverloaded neutrals.Laser printers and copierssharing branch circuits withsensitive loadsGuaranteed periodic voltagesags and switching transients.Miswired receptacles(N-G swapped)Hard to believe, but they are outthere in quantity. Guaranteed toput return currents on theground conductor and createa noisy ground.Data cables connected todifferent ground referencesat each endOuch! Shows up as voltagebetween equipment case andthe data cable connector.Hi-frequency noiseThe most effective high fre-quency grounding technique isthe installation of a Signal Ref-erence Grid (SRG).

And in a class by themselvesIsolated ground rods (below)They’re a safety hazard becausethe earth is a high impedancepath and will prevent enoughcurrent from flowing to trip thebreaker. They also cause groundloops; after all, every electronstill has to go back where itcame from. One of the greatmysteries of PQ is how somemanufacturers get away withinsisting that their equipmentwarranty is void unless an iso-lated ground rod is installed. Myauto mechanic was so inspiredby this practice that he nowwarranties every car he workson unless it’s driven.Illegal N-G bonds(Fig. 2.1, page 8)Guaranteed to put return cur-rents on ground. A commonenough problem that the Unionof PQ Consultants wants tocharge piecework rates—say, adollar for every N-G bondfound. They’d all get rich. Notonly is it a PQ problem, it’s aplumbing problem. Circulatingground currents cause corrosionof water pipes. That explainswhy you can never find theelectrician that put in those N-Gbonds—it’s all being done onthe sly by the plumbers. Guar-anteed employment, as if theyneeded it.

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

Load Current

Neutral ReturnCurrent Error

Panel

Line Line

Neutral NeutralGround Ground

SeparatelyDerivedSystem

N - GBond

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

Fluke CorporationPO Box 9090, Everett, WA USA 98206

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

For more information call:U.S.A. (800) 443-5853 orFax (425) 356-5116Europe/M-East (31 40) 2 678 200 orFax (31 40) 2 678 222Canada (905) 890-7600 orFax (905) 890-6866Other countries (425) 356-5500 orFax (425) 356-5116Web access: http://www.fluke.com

©1998 Fluke Corporation. All rights reserved.Printed in U.S.A. 9/98 B0333UEN Rev A

Printed on recycled paper.

Fluke. Keeping your worldup and running.

References andacknowledgments• EC&M Practical Guide to Qual-

ity Power for Sensitive Elec-tronic Equipment, 2nd Edition.

• Dranetz Field Handbook forPower Quality Analysis.

• Ontario Hydro Power QualityReference Guide.

• Association of EnergyEngineers: Fundamentalsof Power Quality.

• IEEE Std 1100-1992: Recom-mended Practice for Poweringand Grounding Sensitive Elec-tronic Equipment.

• EPRI, Electrotek Concepts, Inc.:Power Quality for Utility andIndustrial Applications

• Power Quality AssuranceMagazine

The minimum requirement fortest tools used in PQ trouble-shooting are:• True-rms for accurate mea-

surement with harmonicsand distorted waveforms.

• CAT III-600V or higher(CAT III-1000V) safety rating,which are appropriate formeasurements on powercircuits

A word on test tools

In addition, instruments withrecording capability, waveformdisplay and specialized mea-surements (such as harmonics,sags and swells, transient cap-ture, high frequency noise, etc.)are needed.

The following Fluke test tools are referred to in this application note.

Model Fluke 43 Fluke 41B Fluke VR101S Fluke 87 Series III Fluke 36

Test Tools Power Quality Harmonic Voltage Event Digital ClampMeterAnalyzer Analyzer Recorder Multimeter

Power kVA, kW, kVAR, PF, DPF kVA, kW, kVAR, PF, DPF

Recording TrendPlot™, PC logging PC logging 4000 voltage events

Real-Time Clock • •Harmonics To 51st harmonic To 31st harmonic True-rms volts and True-rms volts and

current current

Voltage Transients 20 nanoseconds 1 microsecond 250 microsecondwith waveform event recording Peak MIN/MAX

Sags & Swells Single cycle MIN/MAX Single cycle 100 millisecond(Voltage only) with trend event recording MIN/MAX

Sags & Swells(Simultaneous Voltage Single cycle MIN/MAXand Current)

Outages Single cycle MIN/MAX Event recording with 100 millisecondwith trend duration MIN/MAX

Documentation, FlukeView® FlukeView® 41 EventView™RS232 Computer Power Quality Software Software Software

Motor In-rush Current Waveform with cursors MIN/MAX MAX Hold

Waveform 20 MHz scope Fundamental

Noise •Peak • • •True-rms • • • •

The ABCs of DigitalMultimeter Safety Video TapeLearn how proper workprocedures and equipmentcan protect you from hazards.

Part Number: 609104

For sale at Fluke distributors,or call the appropriate phonenumber listed to the right

Understanding andManaging HarmonicsPart Number: 609096

Power Quality TroubleshootingLearn to measure, diagnose andsolve PQ problems.

Part Number: 800919 - NTSC800927 - PAL

Documents

Fluke Power Quality Troubleshooting