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IEEE Transactions on Power Delivery, Vol. 14, No. 4, October 1999 1555

COM PARISON BE TWE EN I.E.E.E. AND CIGRE AMPACITY STANDARDS

Neil P. Schmidt, Senior Member, IEEEPacific Gas Electric Company

San Francisco, C A 94105

Abstract: Bolh l,E,:.E. and CIGRt havc published industrystandards Cor calc ulatin g thc ampaciiy of overhrad conductors.Although these t w o standards use the same basic heal balanceconcept, they usc difSercnt appriiaches to calculate ainpacity ratings.As a result of thcse diSCercnces. thc ampacity ra tin g cak ukdted byeach mcthod may vary by almost 10 dcpcnding on theenvironmenlal conditions being considered. This paperlooks at thcdiSSerent approaches used to calculate indiv idua l heal bdldncc terms,at lhe ovcrall impacl oSlhesc tcrms on the ampacity rating. andatLhc scnsitivityoithesc approaches to varinus in put parameters. Theuscr of these standzirds should he aware of thesc variations incalculated ratings and, i fthey consider these variations significant.thz uscr inectls lo sc l cc t a singlc approach Cor rating thcir linesbasedon thcir undcrstanding oC these difScrcnt approaches and of thcpuhlishcd cxpcrimcntdl research supportingeach method.

has to understand the differences and the lim itations o f eachmethod. This paper helps in this understanding by lo oking atthe different approaches used to calculate individual heatbalance terms, at the overall impact of these terms on theampacity rating, and at the sensitivity o f these differentapproaches to various inpu t parameters. The paper does notattempt to establish w hich o f the two methods is morcaccurate. Users must make this determination based theirunderstanding of these different approaches and of thepublished exp erimciital research supporting each method.

11. - I J E AT B A L A N C E T E R M S

A . Heal Balance Equalion

Keywords: Conductors, current, EHV lrdnsinission lincs, powerdistributio n lines. powcr transmission lincs. resistance heating. solarheatinglcooling.

The foundation of l.E.E.E. and CIGRE me thod s is asteady state heat balance concept that all heat supplied to theconductor is balanced by the licat dissipated. The generalequation describing this basic concept is:. - I N T R O D U C T I O N

Both the IEEE Standard for Calculating the CurrenlTemperafure Relationship of Bare Overhead Conductors [I]and CIGREs Thermul Behavior cf Overhead Conductors [ 2 ]each present diffcrent methodologies for calculating thesteady state ampacity of bare overhcad conductors. A ltho ughthese two industly standards use the same basic heat balanceconcept, their approach to the calculationof the heat balanceterms i s different. As a result of these differences, theampacity rating calculated by each method may vaiy byalmost 10% depending on the environmental conditionsbeing considered.

PI t P,,, + P, + Pf = P, + P, + P ,

where:PI = Joule HeatingP, = Magnetic HeatingP, = Solar HeatingPf = Coronal leat ingP, = Convective CoolingP, = Radiative CoolingP = Evaporative CoolingUnits = Watts per u nit area

A person wishing to dctermine the ampacity ratin g of an Although both methodsuse the same heat balance concept,overhead lin e should be aware of th e different standards and each metho d uses a different approach, often based onthe pcrson needs to select whic h of the two standards to use different published experimental rcsearch, to calculatefor h i s or hcr calculations. To inake this selection, the user indiv idua l heat balance terms.

The I.E.E.E. method simplifies the above heat balanceby eliminating hree terms that often have little

i w a c t on the determination of ampacity ratings. The threeeliminated terms are M agnetic Heating., Corona H eating and

PE-749-PWRD-0-06-1997 A paper recommended and approved bythe IEEE Transmission and Distribution Committeeof the IEEE PowerEngineering Society for publication n theIEEE Transactions on PowerDelivew. Manuscrint submitted December 9. 1996: made available for~ ~~ -printingJune9 , ~997:~~ Evaporative Cooling. CIGRE on the other hand includes

these terms in i ts heat balance form ula although tw oof them,Corona Heating and Evaporative Cooling, are gcnerally notused in ampacity rating calculations.

I t should be noted that all I.E.E.E. formulas arc inBritishunits and therefore the I.E.E.E. heat balance terms are inWatts/root2 while the CIGREs SI Unit formulas are inWatts/meter2. The different units used in the two methodscan be easily converted from one choice o f units to the other.

0885-8977/99/$10,00 1997 IEEE

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B. Solar Heating Equations

I) Solar Position: A key part of the calculation of SolarHeating is determination of the suns position with respect tothe conductor. Th e position of the sun isa function of theSolar Declination or height of the sun due to the day of theyear, the Hour Angle of the sun due to the time of day, and

the Latitude of the line. The I.E.E.E. document contains anAltitude and Azimuth table that is used to determine theposition of the sun. Since the I.E.E.E. solar position tableisbased on a specific day of the year (June10 and July 3), atime period between O:OO AM and : O O PM, and on alocation in the Northern hemisphere, its application isrestricted to these environmental boundaries. The CIGR Emethod on the other hand uses formulas to calculate theposition of the sun. These formulas can he applied any timeor day of the year and at any latitude. The CIC Rk method forcalculating the solar position is therefore much more flexiblethan the I.E.E.E. method. The So lar Altitude and Azimuthvalues calculated by either method, within I.E.E .E.senvironmental restrictions, are vely similar.

2) Solar Intensify: I.E .E. E. only considers direct radiationin its Solar Intensity calculations. In contrast, CIGREconsiders direct radiation, reflected radiation and defuseradiation. I.E.E.E.s method varies the solar intensitydepending on the type of atmosphere, clear or industrial,where industrial air quality has a lower solar intensity thanclear air. C E R E approach is different in that it doesntadjust for air quality but does make adjustments for the typeof ground surface, a term needed for the reflected radiationcalculation. The I.E.E.E. method usesa Solar Intensity Tableto determine the intensity while C E R E uses a formula tocalculate this term.

The overall impact of these different approachesis thatCIGRks method generally calculates a Solar Heating termslightly higher (-l0- 15% ) than I.E.E.E. If desired by theuser, the solar reflectance term can be adjusted to make thetwo methods calculate essentially the same Solar Heatingvalues.

Both methods increase the solar intensity for altitudeabove sea level. CIGRE calculatesa larger intensity increasefor altitude during the Summer months than the Winter.I.E.E .E.s solar intensity increase for altitude doe s notaccount for seasonal variations. The overall impact is thatCIG RE s method increases the solar intensity at a slightlyhigher rate than I.E.E.E. for the Summer months but

increases the intensity ata lower rate for the Winter months.C Convective Cooling Equations

Convective Cooling is the cooling effect from air flowaround the conductor. Both methods evaluate both forced airflow, i.e. from wind movement, and natural convection, i.e.without wind. The largest of these two cooling effects is then

used for the Convective Cooling term in the hea balanceequation.

I.E.E.E. uses a tabular method to determine the airviscosity, air density and thermal conductivity whil CIGRI?uses formulas lo determine these teims that arc then used forconvective cooling calculations. Th e effectof these different

methods is that, at wind speeds less than 5 f p s C E R Ecalculates a slightly higher value for Co nvective CO ( ing thandoes I.E.E.E. and at higher wind speeds I.E.E.E. ca culatesahigher value. Ihe difference for the Convective Coolingterm between the two methods is less than4% at all windspeeds.

There is a much m ore pronounc cd variation bel ween thetwo methods at different wind angles. At wind angl:s greaterthan I O degrees, CIGRE consistently calculates valres up to7% highcr, than I.E.E.E. At wind angle less than 1 degreesor almost parallel to the line, I.E.E.E. calculates a highervalue. At 0 degree attack angle, I.E.E.E. calxlates aConvective Cooling term 18 higher than CIGRE.

D. Radiative Cooling Equation.s

Radiation heat loss, i.e., heat emitted directly from theconductor, is usually a small fraction of the total iea t loss,especially comparcd to forccd convection. Both methods useformulas to calculate the radiation heatloss and the resultingvalues are almost identical.

E. Mag netic Heating .Joule Heating Eyziation

For nonferrous conductors, CIGRE computes the JouleHeating or 12R effect in the sam e manor as thi I.E.E .E.method. For ferrous conductors CIGR E adjusts the Joule

Heating term to account for Magnetic Heating and skinaffects. This extra Cl CR k heating term resultsin overallreduction in the ampacity rating on ferrous conductors, due toMagne tic H eating and skin affect, typically betw :en0-3depending on the number of wire layers and lhe ampacityrating being evaluated.

F. Corona Heating Equation

CIGRE considers Corona Healing in its hert balanceequation although it is only a theoretical inclus on in theformula. Since Corona Heatingis only signific ant hen thereare high surfac e voltagc gradients, a condition t iat wouldnormally only occur during heavy precipitation and high

wind on a well designed line, this heating term is usuallymore than offset by the high convec tive and e aporativecooling terms also occurring at these same metiorologicalperiods. The Cor ona Heating term is therefore generallyignored in determining ampacity ratings.

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the other is a co oling term, the net effect of these differencescancel each other and the important value, the ampacityrating, is essentially the sa me.

C. WindSpeedEffect s

Of all variables, wind speed has the most influence on thedetermination o f the ampacity rating. The two calculationmethods produce slightly different ampacity ratings forvarying wind speeds as shown in Fig. 1. The maximumampacity deviation for the wide range of winds speedsevaluated is at 6 fps where there is a 29 Ampere or 2.4%differencc in the ampacity values with the I.E.E.E. methodcalculating a higher rating. (Note: The "Basc Case" ishighlighted 011 Fig. 1-6 to give a comparison refcrcncc point.)

Solar tlctting I 4.31 Wifl I 4.96 W/R I +13.2%Magnetic lleating N A I Ignored ICnrona Heating 1 N i h 1 Ignored 1Convective Cooling I 25.0 WIfl I 26.0 W/fl I t 3 . 9

~~~

Radiative Cooling I 7.5 W/fl I 7 .5 Wif l I 0.0%Evap. Cooling I NIA I Ignored Ihrnpaci lyRating I 9 9 2 A m p i I 99RArnpr I +0.6%

Tahlc I . asc Case Comparison Resolls

G Evaporative Cooling

Of the two methods, only C E R E considers EvaporativeCooling in it's heat balance formula. Evaporative Cooling isgenerally not significant from air horn water vapor or fromwater droplets flowing around the con ductor but can be whenthe conductor is entirely wetted. CIGRE states that, ingeneral, Evaporative Cooling can be ignored. The CIGREdocume nt also does not provide a method for calculating thisterm.

111. - AMPACITY RATING COMPARISONS

A . Base Case Scenario

'The following base case scenario was de veloped tocompare the ampacity ratings calculated by I.E.E.E. andofthe CIGRE mcthods. Later in this section, the sensitivity ofvarious input parame ters are also examined. Thesecomparisons show the overall net impactof all the heatbalance terms calculated by each method.

For a base case cxample, the ampacity ratingof a 795kcmil 26/7 ACSR (Drake) conductoris evaluatcd. This is acommon conductor used in the United States and a conductor

often used in technical investigations and comparisons. Thefollowing assumptions are usedin the base case calculations:

Amhient Temperature:Wind Speed:Wind DirectionLatitude:Azimuth o r Conductor,Atmosphcrc:Solar Heating:Diltitse Solar Radiation:Emissivity:Asnrplivily:Elcv. above Sea I x v c l :Oruond Surfacc 'Type:'Time of Day:Time of Year:Man. Conductor 'Temp.:

40 C2 fpsPcrpcndicularto l i n e3 0 "90

ClearOn

0 (Ignored)0.50.50 FcetIlrhan11'00 amJunc 10imoc

B. Base Case Comparison

The results of the base case sce nario arc shown in Table1In the base casc example the U G R E method calculatesslightly higher values for both Solar Heating and Convec tiveCooling. But, because one of the terms is a heating term and

D. Wind Direction Efects

Fig. 2 shows the variation of ampacity ratings versuschanges in wind direction. With the exc eptionof a winddirection of less than I O degrees, the t w o methods arerelatively consistent with the largest variation occurring at aline anglc of approximately80 degrees where the differencein ampacity ratings is 19 Amperes or I.8%, For wind attack

700 ~~~, ~ , ,. ~0 1 2 3 4 5 6 7 8

Wind Spced (fps)

Fig. I Wind Spccd Er ect s Comparison

O0 i

~~ ~~~ ~,o i n 20 3 40 50 60 70 80 9n

WitidDirccticiii 90 Ucg = I'crpendicular)

Fig. Wind Direction Erfccts Conipariron

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angles less than I O degrees the two methods deviate slightlybecause the I.E.E.E. method calculates a constant ratingbetween 0 and I O degrees while the CIGRe methodcalculates varying ratings. The maximum variance for winddirection occurs a1 0 degrees or a parallel wind with adifference of64 Amperes or 8.5%.

E. Ambient Temperature Effects

Ambient temperature is another important factor indeterm ination of ampa city ratings. Fig..3 compares theampacity ratings as a function o f changes in ambie nt

temperature. The two methods are very consistent forvariations i n ainbieiit temperature with the maximumvariance occurring at 10C where the I.E.E .E. methodcalculates a lower value by 10 Amperes or 0.8%.

F. Solar Intensity Effects

The effect of the sun on the ampacity ratingis shown in

Table 2 .

The time of day has relatively small influence on solarintensiiy and therefore onlya Small influence on the ampacityrating itself. Fig. 4 compares the changes in ratings fordifferent times of the day. The two methods are veryconsistent for all times of day with the I.E.E.E. methodconsistently calculating ratings about 6 Amperes or 0.7%less.

G Elevation Effects

The effect o f elevation on the ampac ity rating is illustrated

At elevations less than 3000 feet the I.E.E.E.n Fig. 5

Solar Healing I / .E .E ,E . I C M R k I Di/ fSun On I 9 9 2 h m p s I 998Amps 1 +0.6Sun Off I 1065Amps I 10x1Amps I +1.5%

Table 2 - Solar Intensity Effect Comparison

1300 I

00 c-- ~~~~10 20 ?n 40 50

Ainbienl Tcinpcratuic DEB. l

Fig. 3 Ambient Tempernlure Effccts Comparison

I100

900 ~~~~ . ~ ~ . ~~... - ~ ~ . 4

1 0 0 0 h M I O0 AM 1 2 0 0 PM L O OPM :OO PTime of Day

Fig. 4 Time of Day Effects Comparisoii

1000

-950

$E2 900

$ 850

800 -- , , , ~ ~-ii

5,000 10,000 15.000Elevation (Feel)

Fig, 5 Elevation Effects Comparison

method consistently calculates a rating appn ,ximately6Amperes or 0.6% less than the CIGRE meth(d . As theelevation increases above 3000 feet, the differe ice steadilyincreases between the two methods to where he I.E.E.E.method is approximately I O Amperes or 1.1% I :ss at 6,000feet.

H. Conductor Size Effects

Fig. 6 compares the ampacity ratings for a w h e rangeof

500 1000 ~ S O D 2000 2500 300c 3500A AC Conductor kcmil Sine

Fig, 6 Conductor Size Effects Comparison

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used to base their ampac ity ratings. This determinationshould be based on their understanding of the differentapproaches and of the published experimental researchsupporting each method.

Another broader issue is the need to have two industrystandards. Although both CIGRE and I.E.E.E. organizations

encourage the developmentof joint standards, a comparisonbetween these two methods shows how two differentorganization? can dcvelop parallel standards using the samebasic principles but taking different approaches to theirsolution. There should be an effort to combine these twostandards into one worldwide standard.

conductor sizes. Th is comparison was made for allaluminum conductors (AAC) to avoid complications fromdifferent stecl core combinations usedin ACSR conductors.

This comparison S OWS that the two methods are veryconsistent, with a typical variations less than I % , ver therange of conductor sizes studied. I.E.E.E. calculates a

slightly lower ampacity rating for all conductor sizes lessthan 1750 kcmil and calculates a slightly higher ratings forlarger conductor sizes.

1V. - SUMMARY

Both I .E .E .E . and CIGRE usc the same general heatbalance equation methodology for determining conductorratings. I.E.E.E. has simplified this equationby eliminatingthree heat balance terms that normally have very littleinfluencc on most ampacity rating calculations. CIGREonthe other hand has maintained the more theoreticallycomplete heat balance evaluation. The overall impact of theomission of the three heat balance terms is gcnerally notsignificant.

Although the difference between ampacity ratingscalculated by the two methods was as high as8.596 in onesituation examined, the difference in the ratings for mosttypical applications were often lcss than1 . The I.E.E.E.method generally calculates slightly lower ampacity valuescxcept For high wind speeds and for wind directionscssentially parallel to thc line.

V. -CONCLUSIONS

Although the general methodologies arc very similar, theapproachcs used by I.E.E.E. and CIGRk to the calculations

are very different. I.E .E. E. relies very heavily on tabular datato determine various heat balance terms while C E R E usesclosed form equations to calculate these same terms. In somecases CIGREs equations may the source, or at least are verya similar method, to the methods used to establish theoriginal I .E.E.E. tables. The overall impact of these tablesvs. equations approaches is that CIG Rk methodis muclimore flexible and can be used over a much wider rangeofenvironmental situations. I n contrast the I.E.E.E. method isrestrictcd to the limited environmental situations consideredin the tables (See Writers tiote below).

For most practical transmission linc design and operationapplications, both methods can be considered equivalent and

the difference in ampacity results are generally notsignificant, especially considering the precision of mostenvironmental input parameters. For some less typicalapplications such as high wind speed and/or parallel windspeed calculations, usersof thcse standards should be awareof the variations in the calculated ratings and, if they considerthe difference between the two methods to be significant, theuser needs to determine which of the two methods should be

VI. - REFERENCES

[ I ] IEEE Standard for Calculating llie CurrentTempcrature Relationship of Bare Ovcrhead Conductors,IEEE P738-1993.

121 CIGRE Thermal Behavior of OverheadConductors, ELECTRANo. 144, October 1992.

VII. - BIOGRAPHY

Neil Srlimidt received his Bachclor 1Electrical LhginceringDzgrec from FrcsnoSlate Univcrsity iii 1968 and ti Master ofElectrical lingincering Ucgree fromUniversitynf Santa Clara in 1974.

Hc is a Registered Professional Engineer inthe stale OC California. Me is a SeniorMcinber of l i E E and is Vice-Chairman ofIELEs Towcrs, Poles and Conduclors SuhC ~ n n y t t c e . Ile i s t h o a incmlicr if

CIGRE.

Mr. Sclimidllias 2 8 years of enperieiicc iiial l pliaser o f ransmission engineering prima rilyiii ransmission liiicdesign.Hc is currcntly the supcwisar of PG&i?s transmission sobstationenginccringgroup

WUl(1773SVOTE: The I.C.E.I sliiiiiliird i s currcnlly bcing opdatcila s partof l.l

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the reference [2] [3], which enables

consideration of the a bove factors.

Wind tunnel studies were carried out

previously to com pare different

ampacity mod els [4] where a com parison

of Nusselt vs. Reynold num ber

relationship is presented show ing good

agreem ent of empirical models presented

by d ifferent authors. A good discussion

of ac resistance and ac/dc ratio, as well

as radial conduc tor differential is alsoprovided in this reference. In the Cigrk

report [5],a probabilistic approach to

conductor thermal modeling is given

where m ost of the above mentioned

factors are also co nsidered.

In conclus ion I would like to state that

while it is important to properly evaluate

different conductor thermal rating

models, as done in this paper, it is also

important to analyze the assum ptions of

the different parameters that are used to

calculate transmission line ampacity.

Proper assumptions regarding the

meteorological variables, transmission

line terrain, route, line to g round

clearance, and conductor thermal

properties are best prepared by the

transmission line engineer by following

line design standard and by the analysis

of meteorological cond itions in the

specific geographic region [ l] .

REFERENCES

[l ] Power Line Ampacity S ystem

Theorv, Ma deline Aaalications.

Anjan K. Deb, Ph.D. dissertation.

School of Arts & Science, C olumbia

Pacific University, California, USA .

December 1998.

[2] "Object oriented expert systemestimates power line ampacity,"

Anjan K. Deb, IEEE Co mputer

Application in Powe r, volume 8 ,

number 3, 1995.

[3] "Line-rating system boosts

economical energy transfer,"M. B.

Wook, M. Choi, AnjanK. Deb,

IEEE Computer A pplication inPower, volume 8 , number3, October

1997.

[4] "Wind tunnel studiesof

transmission line conductor

temperatures,"J. F. Hall,

Anjan K. D eb, J. Savoullis, IEEE

Transactions on Power D elivery,

Vol. 3 number 2, April 1988.

[5] "Probabilistic determination of

conductor current ratings," Cigrk

WG 22.12, Electra, Num ber 164

, February 1996.

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Neil P. Schmidt:I agree with Mr. Deb that the enviroumenlalparamcler assumptions used in the ampacityformulas have a significant effect on theoutcome of the calculations. Th e intentof thepaper was to compare the IEEE and CIG&ampacity calculation methods. In making thiscomparison, the sensitivityof the two methods

to each of the environmental parameters wasevaluated. It was not the intentof this pap(:r toevaluate or assess which parameters slroul~ beused when making thcse calculations.As Mr.Deb indicates, the linc engineer needs to s':lcclthe environmental parainetcrs that bestfil thcspecific situation be ing evaluated.Manuscript receivcd April I 1998.