Application Note

Troubleshootingpower harmonics

Basic troubleshooting usingmultimeters and current clamps

A mystery is occurring intoday’s office buildingsand manufacturing plants.Transformers supplyingseemingly average loadsare overheating. Neutralconductors in balancedcircuits are overheatingfrom excessive loads.Circuit breakers aretripping for no apparentreason. Yet the standardtroubleshooting proce-dures show everything tobe normal. So what’s theproblem?

In one word—harmonics.

New technology, newchallengesHarmonics are the byproducts ofmodern electronics. They areespecially prevalent whereverthere are large numbers of per-sonal computers, adjustablespeed drives, and other types ofequipment that draw current inshort pulses.

RECURRENT

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A 100V DC PROBE*105ms/DIV SINGLE

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Figure 1. Near sine wave

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Figure 2A. Distorted current waveform

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Figure 2B. Distorted voltage waveform

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

This equipment is designedto draw current only during acontrolled portion of the incom-ing voltage waveform. Whilethis dramatically improves effi-ciency, it causes harmonics inthe load current. And thatcauses overheated transformersand neutrals, as well as trippedcircuit breakers.

If you were to listen to an or-dinary 60-cycle power line,you’d hear a monotone hum.When harmonics are present,you hear a different tune, richwith high notes. The problem iseven more evident when youlook at the waveform. A normal60-cycle power line voltage ap-pears on the oscilloscope as anear sine wave (Figure 1). Whenharmonics are present, thewaveform is distorted (Figure 2Aand 2B). These waves are de-scribed as non-sinusoidal. Thevoltage and current waveformsare no longer simply related-hence the term “non-linear.”

Getting to the rootof the problemFinding the problem is relativelyeasy once you know what tolook for and where to look. Har-monics symptoms are usuallyanything but subtle. This appli-cation note provides some basicpointers on how to find har-monics and some suggestions ofways to address the problemsthey create.

2 Fluke Corporation Troubleshooting power harmonics

Sources of harmonics

Defining the problemHarmonics are currents or volt-ages with frequencies that areinteger multiples of the funda-mental power frequency. Forexample, if the fundamental fre-quency is 60 Hz, then the sec-ond harmonic is 120 Hz, thethird is 180 Hz, etc.

Harmonics are created bynon-linear loads that draw cur-rent in abrupt pulses ratherthan in a smooth sinusoidalmanner. These pulses cause dis-torted current wave shapeswhich in turn cause harmoniccurrents to flow back into otherparts of the power system.

The equipment can draw thecurrent down to a regulatedlower limit. Typically, beforereaching that limit, the capacitoris recharged to the peak in thenext half cycle of the sine wave.This process is repeated overand over. The capacitor basi-cally draws a pulse of currentonly during the peak of thewave. During the rest of thewave, when the voltage is be-low the capacitor residual, thecapacitor draws no current.

The diode/capacitor powersupplies found in office equip-ment are typically single-phase,non-linear loads (Figure 3A).In industrial plants, the mostcommon causes of harmoniccurrents are three-phase,non-linear loads which includeelectronic motor drives, anduninterruptible power supplies(UPS) (Figure 3B).

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Figure 3B. Three-phase, non-linear loadcurrent waveform

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Figure 3A. Single-phase, non-linear loadcurrent waveform

The inside storyThis phenomenon is especiallyprevalent with equipment thathas diode-capacitor inputpower supplies; i.e., personalcomputers, printers and medicaltest equipment.

Electrically what happens isthe incoming ac voltage is dioderectified and is then used tocharge a large capacitor. After afew cycles, the capacitor ischarged to the peak voltage ofthe sine wave (e.g., 170 V for a120 V ac line). The electronicequipment then draws currentfrom this high dc voltage topower the rest of the circuit.

Voltage harmonicsThe power line itself can be an indi-rect source of voltage harmonics.

The harmonic current drawn bynon-linear loads acts in an Ohm’s lawrelationship with the source imped-ance of the supplying transformer toproduce voltage harmonics. Sourceimpedance includes the supplyingtransformer and branch circuit com-ponents. For example, a 10 A har-monic current being drawn from asource impedance of 0.1 Ω will gen-erate a harmonic voltage of 1.0 V.

Any loads sharing a transformer ora branch circuit with a heavy har-monic load can be affected by thevoltage harmonics generated.

The personal computer can be af-fected by voltage harmonics. The per-formance of the diode/capacitorpower supply is critically dependenton the magnitude of the peak voltage.Voltage harmonics can cause “flattopping” of the voltage waveformlowering the peak voltage (see Figure2B). In severe cases, the computermay reset due to insufficient peakvoltage.

In the industrial environment, theinduction motor and power factor cor-rection capacitors can also be seri-ously affected by voltage harmonics.

Power correction capacitors canform a resonant circuit with the in-ductive parts of a power distributionsystem. If the resonant frequency isnear that of the harmonic voltage, theresultant harmonic current can in-crease substantially, overloading thecapacitors and blowing the capacitorfuses. Fortunately, the capacitor fail-ure detunes the circuit and the reso-nance disappears.

Troubleshooting power harmonics Fluke Corporation 3

Effects of harmonic currents

Symptoms of harmonics usuallyshow up in the power distribu-tion equipment that supportsthe non-linear loads. There aretwo basic types of non-linearloads: single-phase and three-phase. Single-phase, non-linearloads are prevalent in offices,while three-phase loads arewidespread in industrial plants.Each component of the powerdistribution system manifeststhe effects of harmonics a littledifferently, yet all are subject todamage and inefficient perfor-mance if not designed to handleelectronic loads.

Neutral conductorsIn a three-phase, four-wiresystem, neutral conductors canbe severely affected by non-linear loads connected to the120 V branch circuits. Undernormal conditions for a bal-anced linear load, the funda-mental 60 Hz portion of thephase currents will cancel inthe neutral conductor.

In a four-wire system withsingle-phase, non-linear loads,certain odd-numbered harmon-ics called triplens — odd mul-tiples of the third harmonic: 3rd,9th, 15th, etc — do not cancel,but rather add together in theneutral conductor. In systemswith many single-phase, non-linear loads, the neutral currentcan actually exceed the phasecurrent. The danger here is ex-cessive overheating because,unlike phase conductors, thereare no circuit breakers in theneutral conductor to limit thecurrent.

Excessive current in theneutral conductor can alsocause higher-than-normal volt-age drops between the neutralconductor and ground at the120 V outlet.

Circuit breakersCommon thermal-magnetic cir-cuit breakers use a bi-metallictrip mechanism that responds tothe heating effect of the circuitcurrent. They are designed torespond to the true-rms value ofthe current waveform and willtrip when the trip mechanismgets too hot. This type of breakerhas a good chance of protectingagainst harmonic current over-loads.

A peak-sensing, electronictrip circuit breaker responds tothe peak of current waveform.As a result, it won’t always re-spond properly to harmonic cur-rents. Since the peak of the har-monic current is usually higherthan normal, this type of circuitbreaker may trip prematurely ata low current. If the peak islower than normal, the breakermay fail to trip when it should.

Bus bars and connectinglugsNeutral bus bars and connectinglugs are sized to carry the fullvalue of the rated phase current.They can become overloadedwhen the neutral conductors areoverloaded with the additionalsum of the triplen harmonics.

Electrical panelsPanels that are designed tocarry 60 Hz currents can becomemechanically resonant to themagnetic fields generated byhigher frequency harmoniccurrents. When this happens,the panel vibrates and emits abuzzing sound at the harmonicfrequencies.

TelecommunicationsTelecommunications systemsoften give you the first clue to aharmonics problem because thecable can be run right next topower cables. To minimize theinductive interference fromphase currents, telecommunica-tions cables are run closer tothe neutral wire.

Triplens in the neutral con-ductor commonly cause induc-tive interference, which can beheard on a phone line. This isoften the first indication of aharmonics problem and givesyou a head start in detectingthe problem before it causesmajor damage.

TransformerCommercial buildings commonlyhave a 208/120 V transformerin a delta-wye configuration.These transformers commonlyfeed receptacles in a commer-cial building. Single-phase,non-linear loads connected tothe receptacles produce triplenharmonics, which add up in theneutral. When this neutral cur-rent reaches the transformer, itis reflected into the delta pri-mary winding where it causesoverheating and transformerfailures.

Another transformer problemresults from core loss and cop-per loss. Transformers are nor-mally rated for a 60 Hz phasecurrent load only. Higher fre-quency harmonic currents causeincreased core loss due to eddycurrents and hysteresis, result-ing in more heating than wouldoccur at the same 60 Hz current.

01

02

03

120

V B

ranc

h Ci

rcui

ts

208/480 Volt Transformer

SecondaryPrimary

Neutral

A

B

C

4 Fluke Corporation Troubleshooting power harmonics

These heating effects demandthat transformers be derated forharmonic loads or replaced withspecially designed transformers.

GeneratorsStandby generators are subjectto the same kind of overheatingproblems as transformers. Be-cause they provide emergencybackup for harmonic producingloads such as data processingequipment, they are often evenmore vulnerable. In addition tooverheating, certain types ofharmonics produce distortion atthe zero crossing of the currentwaveform, which causes inter-ference and instability for thegenerator’s control circuits.

Classification of harmonicsEach harmonic has a name, frequency and sequence. The sequence refers tophasor rotation with respect to the fundamental (F), i.e., in an induction motor,a positive sequence harmonic would generate a magnetic field that rotated inthe same direction as the fundamental. A negative sequence harmonic wouldrotate in the reverse direction. The first nine harmonics along with their effectsare listed below:

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

Frequency 60 120 180 240 300 360 420 480 540

Sequence + — 0 + — 0 + — 0

*Even harmonics disappear when waves are symmetrical (typical for electrical circuits)

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

Positive Forward Heating of conductors, circuit breakers, etc.

Negative Reverse Heating as above plus motor problems

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

**Zero sequence harmonics (odd multiples of the 3rd) are called “Triplens” (3rd, 9th, 15th, 21st, etc.)

A harmonic survey will give youa good idea if you have a prob-lem and where it is located. Hereare a few guidelines to follow.1. Load inventory. Make a

walking tour of the facilityand take a look at the typesof equipment in use. If youhave a lot of personal com-puters and printers, adjust-able speed motors, solid-stateheater controls, and certaintypes of fluorescent lighting,there’s a good chance thatharmonics are present.

2. Transformer heat check.Locate the transformers feed-ing those non-linear loadsand check for excessive heat-ing. Also make sure the cool-ing vents are unobstructed.

3. Transformer secondarycurrent. Use a true-rmsdigital clamp to checktransformer currents.• Verify that the voltage rat-

ings for the clamp meterare adequate for the trans-former being tested.

• Measure and record thetransformer secondary cur-rents in each phase and inthe neutral (if used).

• Calculate the kVA deliveredto the load and compare it

Finding harmonics

to the nameplate rating.(If harmonic currents arepresent, the transformer canoverheat even if the kVAdelivered is less than thenameplate rating.)

• If the transformer secondaryis a four-wire system,compare the measuredneutral current to the valuepredicted from imbalancein the phase currents. Ifthe neutral current is unex-pectedly high, triplenharmonics are likely andthe transformer may needto be derated.

• Measure the frequency ofthe neutral current. 180 Hzwould be a typical readingfor a neutral current con-sisting of mostly thirdharmonic.

4. Sub-Panel neutral currentcheck. Survey the sub-panelsthat feed harmonic loads.Measure the current in eachbranch neutral and comparethe measured value to therated capacity for the wiresize used. Check the neutralbus bar and feeder connec-tions for heating or discolora-tion. A non-contact infraredthermometer, like the Fluke

61, is useful for detectingexcessive overheating on busbars and connections.

5. Receptacle neutral-to-ground voltage check. Neu-tral overloading in receptaclebranch circuits can sometimesbe detected by measuring theneutral-to-ground voltage atthe receptacle.• Measure the voltage when

the loads are on. Two voltsor less is about normal.Higher voltages can indi-cate trouble depending onthe length of the run, qual-ity of connections, etc.Measure the frequency. Afrequency of 180 Hz wouldsuggest a strong presenceof harmonics, while 60 Hzwould suggest that thephases are out of balance.Pay special attention tounder carpet wiring andmod-ular office panels withintegrated wiring that usesa neutral shared by three-phase conductors.

• Because the typical loadsin these two areas arecomputer and officemachines, they are oftentrouble spots for over-loaded neutrals.

Troubleshooting power harmonics Fluke Corporation 5

To determine whether you havea harmonics problem you needto measure the true-rms valueand the instantaneous peakvalue of the wave shape. Forthis, you need either a clampmeter like the Fluke 335, 336or 337 or a multimeter likethose in the Fluke 170 and180 Series that makes true-rmsmeasurements.

“True-rms” refers to theroot-mean-square, or equivalentheating value of a current orvoltage wave shape. “True”distinguishes the measurementfrom those taken by “averageresponding” meters. The vastmajority of low-cost, portablemeters are average responding.These instruments give correctreadings for pure sine wavesonly and will typically read lowby as much as 50 percent whenconfronted with a distortedcurrent waveform. True-rmsmeters give correct readingsfor any wave shape within theinstrument’s crest factor andbandwidth specifications.

Troubleshooting tools

Crest factorThe crest factor of a waveformis the ratio of the peak value tothe rms value. For a sine wave,the crest factor is 1.414. A true-rms meter will have a crest fac-tor specification. This spec re-lates to the level of peaking thatcan be measured without errors.

A quality true-rms handhelddigital multimeter has a crestfactor of 3.0 at full scale. This ismore than adequate for mostpower distribution measure-ments. At half scale the crestfactor is double. For example,the Fluke 187 DMM has a crestfactor spec of up to 3.0 whenmeasuring 400 V ac and a crestfactor of up to 6.0 when mea-suring 200 V ac.

Using a true-rms meter witha “Peak” function — like theFluke 187 — the crest factor canbe easily calculated. A crest fac-tor other than 1.414 indicatesthe presence of harmonics. Intypical single-phase cases, thegreater the difference from1.414, the higher the harmoniccontent. For voltage harmonics,the typical crest factor is below1.414; i.e., a “flat top” waveform.For single-phase current har-monics, the typical crest factoris much higher than 1.414.

Three-phase current wave-forms often exhibit a “doublehump” waveform, therefore thecrest factor comparison methodshould not be applied to three-phase load current.

After you’ve determined thatharmonics are present, you canmake a more in-depth analysisof the situation with a harmonicanalyzer such as the Fluke 43BPower Quality Analyzer.

Meter Measuring Sine Wave Square Wave Distorted WaveType Circuit Response* Response* Response*

Average RectifiedCorrect 10% High Up to 50% LowResponding Average x 1.1

True-rms RMS Calculatingconverter.

Correct Correct CorrectCalculatesheating value.

*Within multimeter’s bandwidth and crest factor specifications

Multimeter performance comparisonaverage responding vs. true-rms

6 Fluke Corporation Troubleshooting power harmonics

The following are suggestionsof ways to address some typicalharmonics problems. Beforetaking any such measures youshould call a power qualityexpert to analyze the problemand design a plan tailored toyour specific situation.

In overloaded neutralsIn a three-phase, four-wire sys-tem, the 60 Hz portion of theneutral current can be mini-mized by balancing the loadsin each phase. The triplen har-monic neutral current can bereduced by adding harmonicfilters at the load. If neither ofthese solutions is practical,you can pull in extra neutrals —ideally one neutral for eachphase — or you can install anoversized neutral shared bythree phase conductors.

In new construction, undercarpet wiring and modularoffice partitions wiring shouldbe specified with individualneutrals and possibly an iso-lated ground separate from thesafety ground.

Derating transformersOne way to protect a trans-former from harmonics is to limitthe amount of load placed on it.This is called “derating” thetransformer. The most rigorousderating method is described inANSI/IEEE standard C57.110-1986. It is somewhat impracticalbecause it requires extensiveloss data from the transformermanufacturer plus a completeharmonic spectrum of the loadcurrent.

The Computer & BusinessEquipment ManufacturersAssociation has recommended asecond method that involvesseveral straightforward mea-surements that you can getwith commonly available testequipment. It appears to givereasonable results for 208/120V receptacle transformers thatsupply low frequency oddharmonics (third, fifth, seventh)commonly generated by com-puters and office machinesoperating from single-phasebranch circuits.

Derating factorTo determine the derating factor for the transformer, take the peak andtrue-rms current measurements for the three phase conductors. If thephases are not balanced, average the three measurements and plug thatvalue into the following formula:

HDF = Harmonic derating factor= (1.414)(true-rms phase current)

(Instantaneous peak phase current)

This formula generates a value between 0 and 1.0, typicallybetween 0.5 and 0.9. If the phase currents are purely sinusoidal(undistorted) the instantaneous peaks are 1.414 times thetrue-rms value and the derating factor is 1.0. If that is the caseno derating is required.

However, with harmonics present the transformer rating is the product ofthe nameplate kVA rating times the HDF.

kVA derated = (HDF) x (kVA nameplate)

For example: 208/120 Y transformer rated at 225 kVA:

Conductor True-rms Instantaneousname current amps peak current01 410 A 804 A02 445 A 892 A03 435 A 828 A

I phase avg. = 410 + 445 + 435 = 430 A

3

I pk avg. = 804 + 892 + 828 = 841 A

3

HDF = (1.414) (430) = 72.3 %

841

The results indicate that with the level of harmonics present the transformershould be derated to 72.3 % of its rating to prevent overheating.

Load currents were measuredwith a Fluke Model 87 and an80i-600 ac current probe toproduce the following results:

Solving the problem

Troubleshooting power harmonics Fluke Corporation 7

Work safely

The high voltage and currentpresent in electrical power sys-tems can cause serious injury ordeath by electrocution. Conse-quently, only trained, experi-enced electricians who haveknowledge of electrical systemsin general and the equipmentunder test should perform test-ing and modification of electri-cal systems.

Fluke cannot anticipate allpossible precautions that youmust take when performing themeasurements described here.At a minimum, however, youshould:• Use appropriate safety equip-

ment such as safety glasses,insulating gloves, insulatingmats, etc.

• Be sure that all power hasbeen turned off, locked out,and tagged in any situationwhere you will be in direct

contact with circuit compo-nents. Be certain that thepower can’t be turned on byanyone but you.

• Read and understand all ofthe applicable manuals beforeusing the application informa-tion in this application note.Take special note of all safetyprecautions and warnings inthe instruction manuals.This article is a general guide

to understanding harmonics. Itis not intended to substitute forthe services of a professionalelectrical systems consultant.Before you take any measures todiagnose or address your poten-tial harmonics problems, youshould have your operationthoroughly analyzed by a pro-fessional electrical engineer.

SituationA modern office building dedi-cated primarily to computersoftware development con-tained a large number of per-sonal computers and other elec-tronic office equipment. Theseelectronic loads were fed by a120/208 V transformer config-ured with a delta primary and awye secondary. The PCs werefairly well distributed through-out the building, except for onelarge room that contained sev-eral machines. The PCs in thisroom, used exclusively for test-ing, were served by severalbranch circuits.

The transformer and mainswitch gear were located in aground floor electrical room. In-spection of this room immedi-ately revealed two symptoms ofhigh harmonic currents:• The transformer was gener-

ating a substantial amount ofheat.

Case study

• The main panel emitted anaudible buzzing sound. Thesound was not the chattercommonly associated with afaulty circuit breaker, butrather a deep resonant buzzthat indicated the mechanicalparts of the panel itself werevibrating.

Ductwork installed directly overthe transformer to carry offsome of the excess heat keptthe room temperature withinreasonable limits.

Defining the problemTransformer—Current measure-ments (see Table 1) were takenon the neutral and on eachphase of the transformer sec-ondary using both a true-rmsmultimeter and an average-re-sponding unit. A 600 A clamp-on current transformer acces-sory was connected to eachmeter to allow them to makehigh current readings. The cur-rent waveshapes are shown inFigures 4 and 5.

Table 1. Current readings at the receptacle transformer secondary

AverageTrue-rms responding Instantaneous

Conductor multimeter multimeter peak currentname (amps) (amps) (amps)

Phase 1 410 328 804

Phase 2 445 346 892

Phase 3 435 355 828

Neutral 548 537 762

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The presence of harmonicswas obvious by comparison ofphase current and neutral cur-rent measurements. As Table 1shows, the neutral current wassubstantially higher than any ofthe phase currents, even thoughthe phase currents were rela-tively well balanced. The aver-age-responding meter consis-tently took readings approxi-mately 20 percent low on all thephases. Its neutral current read-ings were only 2 percent low.

The waveforms explain thediscrepancy. The phase currentswere badly distorted by largeamounts of third harmonic cur-rent, while the neutral currentwas nearly a pure sinewave atthe third harmonic frequency.The phase current readingslisted in Table 1 demonstrateclearly why true-rms measure-ment capability is required toaccurately determine the valueof harmonic currents.

The next step was to calculatethe “harmonic derating factor” orHDF (Refer to “Derating trans-formers” section on page 6.)

The results indicated that,with the level of harmonicspresent, the transformer shouldbe derated to 72.3 percent of itsnameplate rating to preventoverheating. In this case thetransformer should be deratedto 72.3 percent of its 225 kVArating, or derated to 162.7 kVA.

The actual load was calcu-lated to be 151.3 kVA. Althoughthat figure was far less than thenameplate rating, the trans-former was operating close toits derated capacity.

Subpanel—Next a subpanelwhich supplied branch circuitsfor the 120 V receptacles wasexamined. The current in eachneutral was measured andrecorded (see Table 2).

When a marginal or over-loaded conductor was identi-fied, the associated phase cur-rents and the neutral-to-groundvoltage at the receptacle werealso measured. When a check ofneutral #6 revealed 15 A in aconductor rated for 16 A, thephase currents of the circuits(#25, #27, and #29) thatshared that neutral were alsomeasured (Table 3). Note that

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Figure 4. Phase current

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Figure 5. Neutral current

Neutral-to-groundCircuit Phase current voltage drop atnumber (amps) receptacle

25 7.8 3.75 V27 9.7 4.00 V29 13.5 8.05 V

Table 3. Phase currents and neutral-to-ground voltage for neutral #06

Neutralconductor Currentnumber (amps)

01 5.002 11.303 5.004 13.105 12.406 15.0*07 1.808 11.709 4.510 11.811 9.612 11.513 11.314 6.715 7.016 2.317 2.6

Table 2. Subpanel branch circuit neutralcurrents

each of the phase currents ofthese three branch circuits wassubstantially less than 15 A, andalso the same phase conductorshad significant neutral-to-ground voltage drops.

In the branch circuits whichhad high neutral current, therelationship between the neu-tral and the phase currents wassimilar to that of the transformersecondary. The neutral currentwas higher than any of the as-sociated phase currents. Thedanger here is that the neutralconductors could become over-loaded and not offer the warn-ing signs of tripped circuitbreakers.

Recommendations1. Refrain from adding addi-

tional loads to the receptacletransformer unless steps aretaken to reduce the level ofharmonics.

2. Pull in extra neutrals to thebranch circuits that areheavily loaded.

3. Monitor the load currentson a regular basis usingtrue-rms measuring testequipment.