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ASSESSINGTHE IMPACTS OF NONLINEAR LOADS ON POWERQUALITY IN COMMERCIAL BUILDINGS- AN OVERVIEWKarl JohnsonElectric Power Research Institute

Palo Alto, California

ABSTRACTIn this paper, the characteristics and effects ofnonlinear loads in the commercial off ce environmentare discussed. The interaction of distorted currentsdrawn by these loads with the power system aredescribed. An approximate approach for evaluatingpotential problems within a given facility is discussed,which, while not rigorous, can be used as a screeningtool by building designers and engineers to makeassessments of the potential for harmonic problems.

INTRODUCTIONOver the past decade, rapid advances in powersemiconductor device technology coupled with theincreasing capabilities and decreasing costs of digitalelectronic control have brought about major changes inthe fundamental character of electrical utilizationequipment. Power conversion equipment has beenused for decades, but previously was restricted tospecialized applications, such as mainframe computerpower supplies, dc motor drives, or high-power AC/DCrectifiers. Continuing improvements in both price andperformance of power semiconductor devices areexpanding markets for power conversion equipment ofall sizes.A major drawback of previous and present powerconversion echnologies is the generation of significantamounts of harmonic current distortion. Nonlinearitiesintroduced by power semiconductor switching in the

Robert ZavadilElectrotek Concepts, Inc.Knoxville, Tennessee

power conversion equipment are responsible for theharmonically-distorted currents that this type ofequipment will draw from an undistorted voltagesupply.Harmonic currents are generally undesirable; theirinteraction with the ac power supply system can resultin unacceptable levels of harmonic voltage distortion,overloaded phase and neutral conductors in branchdistribution circuits, overheating of transformers andmotors, or interference with the operation of sensitiveelectronic equipment.Most harmonic problems due to power conversionequipment had previously been confined to certainapplication areas in the industrial sector. The use ofhigh-power static conversion equipment has beencommon place for a number of decades in certainindustries. The explosion in personal computertechnology in the 1980's was an early indication thatthe commercial environment was no longer immune toproblems associated with significant penetrations ofnon-linear oad.Nonlinear load growth in the commercial sector isbeing driven by two major stimuli: the push towardoffice automation, with ever-increasing use ofelectronic-computer based equipment, and energyefficiency and conservation efforts, which are majorincentives for the application of equipment based onpower electronic technology. The combination ofthese separate initiatives is resulting in lower electricaldemands per square foot of office space for buildingservices such as lighting and HVAC, with increasedreceptacle loads as personal computers, printers,copiers, fax machines, etc. are finding their way ontonearly every desk.For the purposes of this discussion, it will be assumedthat all non-linear oads in a commercial facility can beplaced into one of three general categories:

Electronic Power Supplies. This categoryincludes virtually all modern equipment used inthe office environment. Personal computers,printers, typewriters, copiers, etc. all contain

@7803-O453-5/91$1.008 1991IEEE 1863

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power supplies with similar characteristics.These loads are typically single phase, and fedfrom 120 v supplies.Fluorescent Liahtinq. The characteristics offluorescent lighting loads vary substantially,depending on the type of ballast used.Conventional magnetic ballasts are rapidly beingreplaced by electronic ballasts, which offerhigher efficiencies, lower weight, less noise, andpotentially greater control flexibility. Harmonicgeneration characteristics of electronic ballastvary over a considerable range. Fluorescentlighting systems can be supplied at either 120 or480 v (277 v line-to-neutral) systems. Highervoltage applications are quite common in bothcommercial and industrial acilities.Adjustable Speed Drives (ASDs) for HVAC.Induction motors which were traditionally usedfor chillers and fan drives are being replacedwith adjustable speed motor drives (ASDs). Thevariable nature of HVAC loads makes them wellsuited for ASDs because of large potentialenergy savings. These loads are typically three-phase, and may be connected directly to the480 v supply, or through isolation transformersand line inductors.

The nature of harmonic interactions within buildingswith strictly office-type loads and services differs fromthe harmonic analysis which might be required for anindustrial facility. In industrial facilities, sources ofharmonic distortion tend to be large and concentrated,as would be the case with high-power rectifiers or largeadjustable speed drives. They also tend to be three-phase loads. While the equipment discussed in thispaper is also common to industrial facilities, effects areusually overshadowed by those of the concentratedloads. Industrial facilities are also more energy-intensive, which implies that the service capacity isalso large, and loads such as lighting, or HVACconstitute a small percentage of the total facilitydemand. In commercial environs, however, thesepercentages can be dramatically higher, which is therationale or this discussion.

COMMERCIALUILDINGNONLINEAROAD CHARACTERISTICSWith regard to the assessment of harmonic impacts,the most important characteristic of a load is thewaveshape of the current drawn from a sinusoidalvoltage supply. Because the shunt impedancepresented by other loads within the facility is large in

comparison to the internal impedance of the powersystem, a nonlinear load appears as a current sourceat harmonic frequencies, assuming that harmonicdistortion of the voltage is less than 10% or so.Approximate calculations of harmonic effects, includingharmonic voltage distortion, harmonic loading ofneutral conductors, and transformer derating can beperformed with basic harmonic spectrum informationfor the various classes of nonlinear loads.The classes of nonlinear loads discussed here -electronic power supplies, electronic ballasts forfluorescent lighting, and small ASDs for HVAC - havean important similarity which further simplifiescomputations: the power converters all connect to theac power system through a single- or three-phasediode bridge rectifier.The diode bridge rectifier is a workhorse in powerelectronics, and is used primarily to convert ac powersystem voltages to dc. The topology is simple,requires no control, and is low-cost and robust. Asignificant disadvantage is poor true power factor andhigh harmonic distortion of the input currents.However, since to this point there are practically noeconomic penalties associated with these twocharacteristic deficiencies, it is widely used in both lowand high-power electronic equipment.The terminal characteristics of the diode bridge rectifiervary somewhat across the three classes of nonlinearloads in commercial buildings. The followingparagraphs describe the input current characteristics ofeach class, and how they might impact facility-wideevaluations of harmonic effects.Electronic Power Supply Terminal CharacteristicsSingle-phase electronic power supplies are becoming adominant load in commercial buildings. Almost allelectronic power supplies contain a diode bridgerectifier, which supply dc power to large dc filtercapacitors. The rectified, filter dc potential is thenelectronically regulated by a switch-mode DC/DCconverter before it is passed to microelectroniccircuitry. The combination of diode-bridge rectifier anddc filter capacitor results in a current waveform withhigh peak values. The peaks are centered areapproximately centered around the peak of the acterminal voltage. Input current from the ac power linecontains high amounts of odd harmonic components,with the largest components occurring at the 3rd,5the, 7the harmonic frequencies. A group of electronicpower supplies fed from the same branch distributioncircuit will appear much like a single, large load. Anobservable effect in the input current to an aggregate

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power supply load is some reduction in harmoniccontent at higher frequencies, with nearly ariihmeticaddition of harmonics at lower frequencies. The effecton the input current waveform is a broadening of the"pulse" width. Figure 1 shows current waveform andharmonic spectrum from a branch feeder servingmultiple computer workstations.

125.0FI/DIU U E R T I C nL 3 . 3 M S x D I U H O R I ZFundamental arps 58 5 A r m sFundanental freq 60 0 H zHARM PCT PHASE HARM PCT PHASE---- ------ ----- ---- --____ _ _ _ _ _FUND 100 0% -37- 2nd 0 2% 6 5 -3rd 6 5 7% -97' 4t h 0 4% -72:5th 37 7% -166.7t h 12 7% 113' 8th 0 3% 112'9t h 4 4% -46' 10 th11th 5 3% -158' 12 th 0 1% 142'1 3 t h 2 5% 92' 14th 0 1% 65'1 5 t h 1 9% -51' 16th1 7 t h 1 8% -151' 18th1 9 t h 1 1% 84. 20th21st 0 6% -41- 22nd23rd 0 8% -148- 24th25th 0 4% 6 4 - 26th2 7 t h 0 2% -25: 28 th2 9 t h 0 2% -122 30th31st 0 2% 102' 32 nd33rd 0 2% 56' 34 th

6 t h 0 4% -154

'Igure 1. Input current to aggregate load of computer workstations,:urrent waveform and harmonic spectrum.

Electronic Ballasts for Fluorescent LightingSystemsElectronic ballasts also contain diode bridge rectifiercircuits which supply a dc voltage to an inverter, whichfeeds the fluorescent lamp. Dc filter capacitors aresmaller than those found in electronic power supplies.This in combination with series inductance on the ac-side of the rectifier for limiting conducted EM1generation reduces input current distortion.Commercially-available electronic ballasts exhibit awide range of harmonic current generationcharacteristics, from a low of about 10%THD to a highof about 50% THD. Harmonics generated by a singletype of electronic ballast add almost arithmetically inbranch distribution circuits, as evidenced by Figure 2.If a single type of ballast is used almost exclusivelywithin a facility, the calculation of total harmoniccurrent generation from the fluorescent lighting load isrelatively easy. If many types of ballasts are used,assumptions about the aggregate harmonic generationcharacteristics must be made.

Fundanental amps:Fundanental frea:HARM PCT PHA SEFUND 100.0% -124'3r d 19.9% - 1 4 4 '5th 7.4% 62'7th 3.2% -39'

---- - _ _ _ _ _ _---9th 2.42 -171'11th 1.8% 111'13th 0 8% 17'15th 0 4% -93:17th 0 1% -16419th 0. 2% -99:21st 0 1% 16023rd 0.1% 86'25th27th 0.1% 161'29th31st

1 5 . 2 A rms6 0 . 0 HzHARM PCT PHASE2n d 0.2% 136'4t h6th8t h10th12th14th16th18th20th2 nd24th26th28th30th32nd 0 1% 156'

---- -_-___

:lgure 2. Current and harmonic spectrum from branch feeder servingtlectronic ballasts for fluorescent lighting.

4SDs for HVAC ApplicationsSmall ASDs used in HVAC applications also contain adiode bridge rectifier and dc filter capacitor. The dcvoltage is supplied to a pulse-width modulated (PWM)inverter, which in turn provides variable frequency,variable voltage to an ac induction motor. Theoperational characteristics of the input rectifier aresimilar to those found in electronic power supplies orelectronic ballasts, but the three-phase connectionresults in two current "pulses" during each half-cycle ofac voltage. The effect on the harmonic generationunder balanced conditions is the elimination of the thirdharmonic component. A typical current waveform 2ndharmonic spectrum are shown in Figure3.

2 5 . 0 W D I U U E RT I CR L 3 . 3 MS f DI U HORIZ.PHASE A CURRENT SPECTRUH 12 29 46 PMFundanental amps 6 6 A rm sFundanental freq 6 0 0 HzHARM PC T PHASE HARM FCT PHASEFUND 100 0% -14. 2nd 3 8% -85:3r d 8 5% -114' 4th 3 5% -1035th 79 5% 145' 6th 0 3% 25'7th 66 0% 124: 8t h 2 5% 55'9th 2 7% 11 10th 1 7% 68:11th 36 0% -92' 12th 1 2% 13213th 21 8% -118' 14th 1 2% 156'15th 2 4% 22' 16th 0 3% -136'17th 10 4% -23' 18 th 0 8% -92'19th 8 0% -79' 20th 0 9% -117'21st 1 4% 131' 22nd 0 5% -105'23rd 6 7% 39' 24th25th 4 5% -2' 26th 0 3% -12'27th 0 9% 143' 28t h 0 2% 76'29t h 3 7% 83' 30 th 0 3% 42'31st 3 1% 29: 32n d 0 4% 10'33rd 0 4% -110 34th 0 1% 31'

-__-__--_ _---- ---- ______ -----

Figure 3. Input current and harmonic spectrum for adjustable-speedmotor drive used in HV AC systems.1865

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EFFECTS F HARMONICS ONPOWER SYSTEM EQUIPMENT AND LOADS

Harmonic distortion of power system voltages orcurrents is always undesirable. While deliveringalmost no useful work, harmonic components add tosystem losses by creating additional heating in powersystem equipment. High levels of harmonic voltagedistortion can cause equipment misoperation or failure.Voice communication circuits may be adverselyimpacted. Of economic importance to both customersand the utility are the negative effects of harmonics onsystem capacity. Transformer and motor derating dueto harmonic currents and voltages increases thecapital cost of equipment for a given application.A simplified one-line electrical diagram for a typicalcommercial facility is shown in Figure 4. Locations ofthe three types of nonlinear loads described areindicated. Electronic loads are supplied through a480/208 V step-down transformer. Deratingrequirements for these transformers and the neutralconductors in these circuits must be evaluatedcarefully. Fluorescent lighting in this example is at 277V, and fed directly from the 480 V main. ASDs in theHVAC system are also connected directly to the 480 Vsupply.

Uti'ity 1T ''

Figure 4. One-line electrical diagram of commercial facility showingtypical locations of nonlinear loads.

Harmonic Voltage DistortionVoltages produced by utility generators are remarkablyfree of harmonic distortion. At the same time, ambientlevels of harmonic voltage distortion are commonlydetectable at utilization levels. The cause of thesebackground levels of harmonic distortion at utilizationand distribution system levels can be attributed todistorted currents drawn by nonlinear customer loads.While a rigorous analysis of harmonic distortion ofpower system voltages by nonlinear devices isextremely complex, the approximate analysis in which

nonlinear loads are considered to be sources ofharmonic currents greatly simplifies computation butstil l produces useful results.Using the current source approximation, harmonicvoltage at any point in the power system will be afunction of the current flow and the internal or short-circuit impedance of the power system at eachharmonic frequency. Total harmonic distortion (THD)of the voltage can be determined from superposition ofthe results of computations at each frequency. Atcustomer facilities, a simple representation of thefacility and the power system as shown in Figure 4 isgenerally sufficient for this analysis.

The short-circuit impedance at the service entrance incommercial buildings will be dominated by the internalimpedance of the supply transformer. If utility short-circuit data is unavailable, an approximate of powersystem impedance can be made from supplytransformer nameplate data. Unless power factorcorrection capacitors are present, the impedance ofthe power system at harmonic frequencies can becalculated as:

kV2xh = z,(Pu)x h = hZ,(%) (in % on

transformer base)

Harmonic voltage distortion at the service entrance isthe product of the harmonic current components in themain and the corresponding harmonic impedance.One of the complicating factors in the application of theapproximate calculation method to commercial facilitiesis the determination of the harmonic current whichresults from a large number of nonlinear loads. Anassumption that the harmonic currents from allnonlinear loads will add arithmetically can produceoverly-conservative results.An estimate of total harmonic current at the serviceentrance can be made using the generalcharacteristics of the three classes of nonlinear loadsdescribed in the previous section, with attention to thelocation of these loads within the facility. If smalldifferences in current waveshapes between equipmentin each of the categories is neglected, the determiningthe maximum harmonic current injection becomesmuch easier. The process is further simplified if the

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building distribution circuits are segregated by types ofload, which is the case in most facilities. Maximumharmonic current flow on a branch feeder can beestimated as the product of the harmonic currentinjection from a single device and the total number ofdevices on that feeder. The simple computationignores variations among the harmonic currentsignature of loads within each nonlinear category andload diversity.

. . . .. .-150A75.OA/DIV VERTICAL 3.3MS/DIV HORIZ

Harmonic Number135791113

Phase Sequence0++0

+

order. Sequences for harmonic currents frombalanced nonlinear oads are indicated n Table 1.

I-5OA- ' ' ' ' ' ' . '25.OA/DIV VERTI CAL 3.3MS/DIV HORIZ

Table 1:

Phase shifts created by delta-wye transformers cansometimes reduce the total harmonic current at theservice entrance. Vector addition of estimatedharmonic currents in each branch distribution circuitwould result in a less-conservative estimate of totalharmonic current at the service entrance.

Harmonic Currents in Neutral ConductorsAnother well-known consequence of the characteristicsof nonlinear loads is harmonic current loading of 3-phase 4-wire circuit neutral conductors. Neutralcurrent loading in three-phase circuits with linear loadsis simply a function of the load balance among thethree phases. In circuits with nonlinear loads,however, zero-sequence harmonic currents (See Table1) will add arithmetically in the neutral conductors ofthree-phase circuits. The third harmonic is usually thelargest harmonic component in single-phase electronicpower supplies or electronic ballasts.The root-mean-square (rms) value of the currentflowing in power system conductors is responsible forlosses and heating, and conductor ampacity is basedon rms value. For distorted currents, the rms valuecan be calculated rom the harmonic components as:

Irms4112+132+ 152+l,2+. .Triplen harmonic currents are not a problem in three-phase nonlinear loads, such as ASDs, since thetopology of the diode bridge rectifier circuit and theeffective line-to-line connection at the input prevent

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In+,, = ~(31,)2+(31,)2+(31 5)2+ . =31,For electronic power supplies, where 13 0.7 I1

--n-rms 3 (0.7) 1.711-rms = ,=2 =

For electronic ballasts, on the other hand, with I, =0.31 ,

In-rms 3 ( 0 . 3 )11-rms = 4 ~ 2o.8--

True Power Factor and Transformer DeratingPower factor is defined as the ratio of real power toapparent power. With strictly sinusoidal currents andvoltages, this ratio can be expressed as the phasedisplacement between voltage and current signals. Ifother frequency components are present in the voltageandor current signals, the traditional electrical utilitydefinition of power factor must be modified to accountfor this distortion. More generally, power factor isdefined as:

11IrmsPower Factor = -cos())

This quantity is sometimes called True Power Factor,to distinguish it from the utility definition, which isactually part of the true power factor definition andknown as Displacement Power Factor.The loads described in this paper all exhibit pooruncorrected true power factor. However, displacementpower factors for these loads are usually above 0.95,and in most cases, closer to 1.0. It is important todistinguish between true power factor anddisplacement power factor, since the traditionalcorrective measure for poor "power factor", theaddition of shunt capacitors, will not improve the truepower factor of this type of load. In fact, shuntcapacitors will actually decrease true power factor bydriving the displacement power factor well to the"leading" side. Electric utilities that have power factorincentives or penalty clauses in their rate structuresand metering equipment for this purpose are actuallymeasuring displacement power factor.For diode bridge rectifier loads, true power factor canbe expressed as a function of total harmonic distortionin the current waveform, assuming that harmonicdistortion of the voltage at the terminals is low (below

5%).current THD is:The expression relating true power factor to

Shown graphically:

t'Igure 6. Relationship between true power factor and total harmonicdistortion in input current for diode-bridge rectifier loads. (Voltage isassumed to be undistorted)From the graph, true power factor of electronic loads,where current THD may be on the order of 70 - 90%, is0.6 to 0.75. Electronic ballasts with current THD below35% have true power factors greater than 0.94.Another important consequence of harmonic currentdistortion from both the customer and utilityperspectives is transformer derating. Powertransformers not specially designed for non-sinusoidalload currents must be derated to account for theadditional winding eddy current losses from harmoniccurrents. Derating factors depend on both themagnitude and frequency harmonic components ofload current. A procedure for establishing transformercapability for supplying nonsinusoidal load currents isdescribed in ANSMEEE C57.110 which usestransformer nameplate data and load current harmonicspectrum information.A complete description of transformer derating isbeyond the scope of this document, and the reader isreferred to the ANSI document for further information.The results of applying the derating procedure asdescribed in the standard to electronic power supplyloads are shown in the following graph. The verticalaxis indicates the maximum transformer capability inpercent of nameplate rating for a total load consistingof linear and electronic power supply loads.Maximum transformer capability for load consisting ofelectronic power supplies only is less than 50% ofnameplate rating. Derating for a typical electronicballast load is not as severe. For an electronic ballastwith an input current waveshape as shown in Figure 2,

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the maximum transformer capability must derated toonly 85% of nameplate.

EFFECT OF NONLINEAR LOAD PENETRATIONIn the evaluation of harmonic impacts at the serviceentrance from nonlinear oads in commercial buildings,a primary consideration must be the percentage of thetotal facility load that is nonlinear. At 208/120 V panelsserved by 480 V transformers, the amount of nonlinearload served by each of these transformers must beconsidered for evaluating both harmonic voltagedistortion and transformer derating. Small amounts ofnonlinear load relative to transformer capacity will notcause harmonic problems.The load equipment described in this paper can befound in any commercial buiiding. Yet harmonicproblems are not widespread in these facilities. Thefollowing graphic shows the potential impact of threetypes electronic ballast on harmonic voltage distortionfor varying lighting load penetrations. Five percentharmonic voltage distortion is indicated as anacceptable imit.

I 10% Ballast3: 56% THD I-/

I0% 20% 4077 6W7 80% 100%

Lighting Load(%of Total)Figure 7. Voltage distortion as a function of fluorescent lighting loadpenetration for three different ballast selections (influence of othernonlinear loads within facility neglected here).

Figure 7 indicates the importance of assessingpotential harmonic impacts in the building designstage, if possible, so that more costly retrofit solutionsare unnecessary. In buildings with high percentages ofnonlinear loads, additional costs for low distortionballasts, for instance, can be easily justified.

may be some implications or commercial facilities withhigh percentages of nonlinear loads. Methods andtools for more accurate calculations of harmonicimpacts from nonlinear oads will be necessary.

SUMMARY AND CONCLUSIONSHarmonic current generation from nonlinear loads incommercial buildings continues to grow as off cesbecome more automated and energy efficientequipment with nonlinear characteristics replacesolder, less efficient linear loads. Effects of high relativelevels of harmonic current generation in commercialbuildings nclude:

harmonic voltage distortiontransformer and motor overheatingneutral conductor overloading

Reasonable estimates of the impacts of nonlinearloads can be made from generalized harmonic currentsource characteristics of three basic nonlinear types.Interactions with the electric power system can bepredicted with simplified representationsof the buildingelectrical system.As nonlinear loads in commercial facilities continue togrow, an increased emphasis on evaluating harmonicimpacts before problems occur will become important.The fundamentals described in this paper can beextended to more sophisticated computational tools,which can be used by building designers andengineers for evaluating these impacts andrecommending appropriate measures for mitigation ofharmonic-relatedproblems.

ACKNOWLEDGEMENT

Future utility harmonic distortion standards mayprovide an additional incentive for the selection ofequipment with low harmonic current distortion. Therevision to IEEE 519 is expected to include proposedlimits for harmonic current generation by utilitycustomers, with the utility being responsible or limitingharmonic voltaga distortion to specified levels.Although the standard may originally have beenintended for application to industrial facilities, there

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The work described n this paper was performed underElectric Power Research Institute (EPRI) contractsRP 2935-91 and RP 2935-1 .