Issue of Harm-Inject From Utility-Integ PV Sys (IEEE Trans,1988)

8/6/2019 Issue of Harm-Inject From Utility-Integ PV Sys (IEEE Trans,1988)

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IEEE Transactions on Energy Conversion, Vol. 3, No . 3, September 1988 507THE ISSUE OF HARMONIC INJECTION FROM

UTILITY INTEGRATED PHOTOVOLTAIC SYSTEMSPART 1: THE HARMONIC SOURCE*John Stevens, Member, IEEESandia National Laboratories

Albuquer que, New Mexico

ABSTRACTHarmonic injection is a growing concern to elec-

tric utility companies. One potential source of har-monics is utility intertie d photovo ltaic (PV) systems.This paper investigates the role of the PV system as aharmonic injector and discusses the various aspects ofPV power converter design which impact harmonic injec-tion. Small, residential-sized units, if greatlyproliferated, are the greatest source of concern toutilitie s, as larger systems are typically designed tothe interconnected utility's specification. Tech-niques are availabl e to limit the harmoni c output ofthese smaller units without resorting to bulky, expen-sive filtering.

INTRODUCTIONIn order for photovoltaic generated energy to be a

significant contributor to U . S . energy requirements,PV systems must be capable of being interconnectedwith the existing electric utility systems. There aretechnical questions to be addressed in such an inter-connection. None of the answers to these questionsare expected to make or break PV as a viable energysource, but the answers may impact the method of uti-lizing PV, or the mode of deploying PV arrays, andultimately affect the energy value.In an effort to understand these questions andtheir answers, the National Photovoltaic Program hasdeveloped a significant data base addressing PV/utility interface issues, and continues to do so.This data base includes results of research from sev-eral universities and utilities. This paper willdiscuss these efforts as they pertain to harmonicinjection from the photovoltaic power conditioningsubsystem (PCS) and draw conclusions where appropri-ate.

HARMON SThe topic of harmoni c injection into utility sys-

tems isn't unique to photovoltaics. Harmonic injec-tion is becoming more common with every new piece ofelectronic or magnetic equipment which is connected tothe utility grid. Concurr ently, sensitivity to har-monics is increasing with more devices depende nt on* This work supported by the U.S. Department ofEnergy, Photovoltaic Energy Systems Division.

87 S# 429-4 A paper recommended and approvedby the IEEE Power Generation Committee of the IEEEPower Engineering Society for presentation at theIEEE/PES 1987 Summer Meeting, San Franc isco,Cali forn ia, .July 12 - 17, 1987. Yanuscript sub-mitted February 2, 1987; made available for printing?fay 11, 1987.

sensing zero-crossings (which can be appear to shiftbecause of harmonic s) for control of internalswitching. Additionally, utilities are installingmore shunt capacitors to optimize distribution sys-tems, which increases the probability of harmonicproblems occuring because of the increased harmonicimpedan ce created by parallel resonance.

Harmoni cs are usually injected into a utility inthe form of current harmonics. With the notableexception of telephone interference, problems arecaused by harmonic voltaee being impressed at theterminal s of the affected equipment. (A discussi on ofharmonics and telephone interference is included as anappendix to [ l ] . ) If the voltage waveform at theterminals of the equipment of interest has little orno distorti on, there will be no harmonic impac t on theequipment, no matter what level of harmonic currentmight be present. Thi s is because the utility systemimpedanc e, which is in parallel with the load imped-ance, is significantly lower than load impedance(unless the load happens to be a capacitor), providinga path for the harmonic current away from the load.

The problems arise when shunt capacitors are con-nected to distribution systems. These capacitors,when added to a system which already has shunt induc-tance in the form of load, can form parallel resonanceat various frequencies. Many times such an arrange-ment on a distribution line creates resonance in thethird- to ninth-harmonic range, which can be particu-larly troublesome as these are frequencies whichalready tend to be prevalen t in utility systems. Thelower harmonics also tend to have more energy thanhigher harmonics, thus making them more damaging.

Parallel resonance is manifest by a sharp increasein impedance at the resonant frequency, which trans-lates into increased voltage distortion from a givenlevel of harmonic current. This current may becausing no problem at other, non-resonant, points onthe system.

Load changes , as occur continually on a utilityfeeder, have an important impact on parallel reso-nance. As the parallel resista nce is decreas ed (thatis, resisti ve load is increased) the maPnitude of theresonant impedance is decreased, thus decreasingvoltage distortion. Also, as the parallel inductanceis decreas ed (again, inducti ve load is increased) theresonant freauencY increases. These two factors cancombine in such a manner that, while a lightly loadedfeeder may have resonance problems, as the normaldaily increase in load occurs, the problems disappear.This is fortuitous when considering harmonic outputfrom a photovoltaic system, as the PV system outputtends to incr ease at the same time loads increase.

m PV SYS[l%%lPhotovoltaic generating systems have a few pecu-

liarities as compared to rotating generators. The PVarray, wnich produces dc energy, is intertied to theutility via a solid state device, the power condi-tioning subsyste m, or PCS. The PCS performs threebasic function s, a dc interface which monitors the PVarray and respond s in a manner to maximize power

0885-8969/88/09oO-0507$01 .MO 1988 IEEE


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508production, an inverter which converts dc to ac, andan ac interface which monitors the utility to maintainsteady state compatibilit y and respond appropriatelyto utility system dynamics. In this document we areonly interested in those aspects of the PCS whichimpact harmonics. This includes the inversion sectionas well a s the output circuitry.

The P V PCSA P V PCS is simply one more device which may add

to the harmonics in the local electrical system. Thewaveshaping method in the inverter section of a PCS isthe prime determinant of the harmonic content of thePCS output. If the PCS output is unacceptably high inharmonics, then the output can be filtered to achieveany desired harmonic specification. There are severalimportant points relative to harmonics which should beborn in mind when comparing inverter designs s o thatthe proper perspective is maintained. These pointsare

Commutation methodSwitching frequencyOutput filteringPCS power ratingThese topics are all interrelated, making it dif-ficult to address one without discussing the impact onothers. The driving force for making these trade-off

decisions is economic. For example, if harmoniclimitation is necessary, then filtering, commutationmethod and sw itching frequency all affect power ratingas follows: the availability of switching devices(power rating and s witching speed) fo r self-commutatedunits and the cost of filtering for line-commutatedunits dictate that self-commutated PC S switched at4kHz or greater are most economical for single-phaseapplications up to the lOkW size. Similarly self-commutated PC S switched at high frequency are bestfor three-phase applications up to the lOOkW sizerang e, above which a filtered line-commutated twelve-pulse unit shows superior economics. These generalstatements ch ange wit h the rapidly changing powerelectronics industry, but the type of impact remainsthe same.

Commutation Met od. Line-commutated PCS areswitched by the line current passing through ze ro, andare thus inherently more simple and less costly thanforce-commutated units which rely on internal cir-cuitry t o maintain frequency con trol and initiateswitching. The comment is often heard that line-commutated P CS have higher harmonic content thanforce-commuta ted units. While this may be a fact ofcommercially available hardware, there is no technicalreason that it must be true. Line commutation simplymeans that the PCS output bridge is switched frompositive to negative conduction when the utility cur-rent passes through zero, thus reversing the following(lagging) inverter current. This same function isaccomplished in a force- or self-commutated PCS by aninternal signal to the same switches. A force-commu-tated PCS with 60 Hz switching frequency will have thesame current harmonic content as a line-commutatedunit (which also is switched at 60Hz). A s far asharmonics are concerned, the importance of force-com-mutation is the ability to select a switching fre-quency, as will be discussed in the next section.There is one difference between 60Hz switchingwith a line-commutated ver sus a force-commutated unit.The line-commutated unit, while switching at currentzero, is not in phase with the voltage z ero and thus ashort, or commutatio n notc h, appears in the voltage

waveform. This commutation notch contributes toharmonic as well as non-harmonic distortion. Distor-tion from the commutation notch is not of the samemagnitude as that characterist ic of the PCS switchingscheme. Commutation notch appears to play an insig-nificant role in PCS harmonics [ 2 ] although it is ofconcern as a source of noise and stress in the powersystem. Force commutation, besides providingswitching frequency flexibility, provides notch-freeoperation along with providing control over the outp utcurrentfvoltage phase relation by controlling theswitching angle , thus controlli ng the power factor.

m g Frea- can be an economic trade-offagainst filtering Either technique can be utilizedto reduce harmonic output, and each has an associatedcost penalty. Since the PCS output is actually aseries of discrete steps, the smaller these steps,that is the higher the swit ching frequency, the closerthe approximation to a sinewave. Additionally, higherswitching frequency implies higher frequency charac-teristic harmonics with lower harmonic power, bothcontributing to simplifying any filtering require-ments. High-frequency switching isn t all goodthough. It requires a force-commutated unit which isinherently more complex than a line-commutated unitbecause of the requirement for switching controlcircuitry. Also there are losses associated withswitching, which can result in high frequency deviceshaving a bit lower efficiency than those with lowerswitching frequencies [ 3 ] .Today the question of switching frequency is mostimportant to single phase, and therefore small, PCS aswill be discussed under power rating . Switchingfrequency may become important to larger, three-phaseunits in the future as manufacturers begin to takeadvantage of new devi ces which are capable of handlinglarge amount s of power w hile switching at higherfrequencies. This will enable them to economicallymanufacture high-frequency units in larg er, three-phase ratin gs, thus saving the cost of filtering.Traditional three-phase units (as are normally foundtoday) are line-commutate d, which implies low-fre-quency, requiring the use of an output filter inapplications where limiting harmonic injection isimportant. A three-phase PCS can be switched as atwelve-pulse devi ce [ 4 ] which allows relat ively inex-pensive filtering, and this is tod ay s technique ofchoice. Today a filtered twelve-pulse unit in anysize larger than about lOOkW is cheaper in terms offirst cost plus the value of energy lost than acomparably sized high-freq uency PCS. On the otherhand, filtering a single-phase PCS isn t as attractiveas filtering a three-phase unit since, as mentionedpreviously, the lower order harmonics are more costlyto filter, requiring a relatively large and expensivefilter.

Power Rat ag. Powe r rating in itself has nothingto do with harmonic output on a percent distortionbasis. The impact of power rating is based onswitching device availability (speed and powerhandling abilities). At low power requirements, highspeed devices such as transistors are readily avail-able, making high-frequency switching feasible. Con-sider that any utility intertied equipment of lessthan about lOkW is usually single phase. A P V systemof less than lOkW will also normally be single phase.The most economic al units to build, whether single orthree phase, are line commutated. For a single-phaseunit , this means a switching frequency of 60Hz is thehighest available. From the previous discussion thecost of filterin g the single-phase unit will begreater, in terms of $/kW, than for filtering atwelve-pulse uni t.

For a system large enough to be three-phase, aline-commutated twelve-pulse PCS with appropriatefiltering will give good results. However for single-phase systems, a low-frequency system will have sub-

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stantial harmonics. Measurements at Sandia Labora-tories have shown a 6kW low-frequency single-phase PCSto vary between 7.8% and 32% current THD at full ratedoutput, depending on both PV array voltage and utilityvoltage [ 5 1 . Figure 1 is a graph of the harmonicoutput of this same unit as measured at Arizona StateUniversity (as part of a Sandia contract with the SaltRiver Project) which demonstrates the trend very well.On the other hand, a high-frequency, single-phase 4kWPCS has shown current THD ranging between one andthree percent in testing at Sandia [6]. Figure 2displays harmonic output of this unit as measured byASU under the same Salt River Project contract. Itshould be noted that the harmonic output of a PCS isnot only dependent on the individual design, but onother factors such as ac and dc voltage levels and theutility's ambient harmonic level. This is seen in thedifference between the results reported by Sandia andASU. The important message is the general trend.Both sets of measurements show significantly higherharmonic content from the low-frequency unit than fromthe high-frequency unit.

Some ComDarative ResultsA comparison of harmonic output from high-

frequency and low-frequency PCS's with severalhousehold appliances was performed by Alabama SolarEnergy Center as shown in figure 3 [ 7 1 . The first twodevices on the graph are high-frequency PV PCS's. Itcan be noted that their actual harmonic output is onthe same order as a black & white television set. Thethird item on the graph is a low-frequency, line-commutated PV PCS. The harmonic output current ofthis unit is substantially higher than the first two

THD VS. AC POWER O U T P U TP E R C E N T2

2120191817

161 514

13

1 21 1

1098765U

32

NORURLlZEO TO FULL OU TPU TGEUINl INVERTER POWER IN U R l l S

50 0 1 0 0 0 1 5 0 0 2000 2 5 0 0 3000 3 5 0 0 11000 11500 5000POWER

LEGENO: L I N E U 110V % I - 110V % V *-*-I 55 8 % I B-DO 255V % VFigure I . Output harmonics of low-frequency PCS

509THD ITS. AC POWER OUTPUT

L I O A M R L I Z E I T O FULL O J T P L l5 U N 5 : V E : N V E R - E i

fI:I :I *I

!!I

I

j i

00 500 1000 1500 2000 2500 3000 3500 YO00

POYERLEGENO: L lNE - 2 0 ~%I- 20 v I V ++ + Z Y O V 71a-0- 2 L IOY 7.V *--. 255Y % I &** 5% 7.v

Figure 2 . Output harmonics of high-frequency PCS

HARMONIC CURRENT :PHOTOVOLTAIC PCS VS. OCSEHOLD APPLIANCES

Figure 3. Harmonic distortion (amps) from varioussources

PCS' s. Also note that it is roughly one-third higherthan the window air condit-ioner. Although this outputseems high , there are probably many residentialfeeders where the average home has more than onewindow air conditioner with no resultant harmonicproblem, while the probability of 50% of the homesever having a PV system is low [e].

Harmonic distortion limits are usually set aspercent harmonic distortion. Although it is probablythe best approach to addressing harmonic limits, caremust be taken as illustrated in figure 4. In thisgraph percent distortion has been charted instead ofactual current. This illustrates the potential prob-lem in using percent distortion as a limiting factor.For example, if 5% third harmonic distortion were setas a limit, it would preclude the use of black & whiteTVs, personal computers and window fans.


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5 10HARMONIC DISTORTIONPHOTOVOLTAIC PCS VS. HOUSEHOLD APPLIANCES

B k l50 LEGEND n

m2 305 206p

10

0

Figure 4. Harmonic distortion (percent) from varioussources

CONCLUSIONAlthough photovoltaic generating systems have the

potential for injecting sig nificant quantities ofharmonic current into an interconnected utility, thetechnology exists to insure that this doesn't occur.In sizes where the PV power conditioner can economi-cally be a self-commutated unit switched at highfrequency, the switching pattern can be such that theoutput waveform closely resembles a sinewave with lowdistortion. If a three-phas e PC S is utilized, a line-commutated twelve-puls e inverter is easily filtered.In either case, limiting harmonics to whatever levelis required ca n be achieved.

REFERENCES[ l ] S. J. Ranade, Characteristics an Imuact of Util-

Jtv Interactive Photovoltaic Prototypes on theFeeder Serving the South est Res dential Exueri-ment Stat on, Sandia National Labs, Albuquerque,SAND86-7043 , Feb. 1987

[2] G.L. Campen, m u l t s of the Harmonics Measurementat the John F. Lone Photovoltaic H o w ,Oak Ridge National Lab. Pub. ORNL-5834, Mar. 1982

[3] B. D. Bedford and R. G. Hoft, PrinciDals of In er-ter Circuits, John Wiley and Sons, 1964, p.33

Vol.1,4 ] E. W. Kimbark, Direc t Current Tr nsmission..John Wiley and Sons, 1971, pp.67[5] W. I. Bower, Enpineerine Evaluation Summarv Reuort

Lateractive Residential Photovoltaic Power Condi-ubsvstem, Sandia National L abs, Albuquer-que, SAND87-01 93, to be published

. .. .

[ 6 ] W. I. Bower, et al, m~ i n e e rne Evaluation Sican Power Ccmiersion CorDorationModel U1 4000 U t i w e r a c t ve Res dential.

Photovoltaic Power Conditionine Subsvstem, SandiaNational Labs, Albuquerque, SAND83-2601, Jan. 1985

[ 7 ] D. B. Wallace, "PV Power Conditioner Harmonics,"Proceedings of the Joint A S M E A S 3 Solar EnergyConference (SED Seventh Annual Conference),Knoxville, TN, March 25-28, 1985.

. .

[81 D. E. Mahone, et al, Study of Ph-ltaic Resi -dential Retrofits, Sandia National Labs, Albuquer-que, SAND81-7019/1, Apr. 1982

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Issue of Harm-Inject From Utility-Integ PV Sys (IEEE Trans,1988)