case study of radial overhead feeder performance at 12[1].5 and 35.4 kv (trans)

8/8/2019 Case Study of Radial Overhead Feeder Performance at 12[1].5 and 35.4 kV (Trans)

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696 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 26, NO. 4, JULYIAUGUST 1990

Case Study of Radial Overhead FeederPerformance at 12.5 and 34.5 kV

Abstract-The electric performance and the economics of four sup-ply options for a specific 12.5-kV radial distribution feeder were exam-ined. Options included upgrading to 34.5 kV and/or sectionalizing.Feeder losses, voltage regulation, and system voltage dips due to feederfaults were analyzed. It was found that distribution transformer losseswere a significant comp onent in the total feed er loss and an importantfactor in the loss comparison of the two voltage levels. In this case,sectionalizing the existing feeder at 12.5 kV was the preferred optionbased upon cost, system voltage dip performance and ease of back feed.

INTRODUCTIONREEN MOUNTAIN Power Company (GMP) faces someG nique considerations in deciding whether to upgrade itsexisting 12.5-kV distribution feeders at 12.5 kV or at 34.5kV. In addition to the factors related directly to distribution

feeders, i.e., losses, voltage drop, and ampacity, GMP mustconsider the voltage dips on the local 115 kV transmissionsystem caused by distribution feeder faults. In some cases,they must also decide whether to build lines as 34.5 kV sub-transmission or as 34.5-kV distribution.

Utility interest in a higher voltage class for distribution hasbeen based primarily upon improvement in capacity, loss re-duction, and improvement in voltage profile [l], [2]. This pa-per reports upon a comparison of the economic and electricalperformance factors on one specific overhead radial distribu-tion feeder that is in operation at 12.5 kV, but whose presentload and growth require that it be upgraded. The comparisonstudy is specific to one feeder with respect to details but givesindications that are useful with respect to upgrades of otherfeeders and to new construction.

The approach of studying a specific feeder was taken inpreference to a study of generic options because the authorsbelieve that upgrading decisions are inevitably dependent uponlocal system and feeder characteristics. Such factors as theavailability of alternate supply points, the short-circuit strengthof the supply points, feeder topology, feeder construction,load locations and sectionalization points are all unique toeach feeder and absolutely determine the study outcome.

Paper ICPSD 89-46, approved by the Rural Electric Power Committee ofthe IEEE Industry Applications Society for presentation at the 1989 IEEERural Electric Power Technical Conference, Colorado Springs, CO, April30-May 2. Manuscript released for publication December 19, 1989.R. E . Clayton and J . M. Undrill are with Electric Power Consultants, Inc.,133 Saratoga Road, Scotia, NY 12302.E. L. Shlatz is with the Green Mountain Power Company, 25 South Moun-tain Drive, South Burlington, V T 05402.IEEE Log Number 9035423.

The study examined the economics and electrical perfor-mance of several feeder upgrading options for a particularfeeder on the GMP system with respect to

1) feeder losses,2) feeder voltage regulation,3) capital costs and cost of losses,4) voltage dips on the local 115-kV transmission system

caused by feeder faults.The 115-kV transmission system in the Green Mountain

service area steps down to a networked 34.5-kV subtransmis-sion system from which radial 12.5- and 34.5-kV distributionfeeders are supplied. The magnitude of voltage dip experi-enced on the 115-kV system during distribution feeder faultshas been a concern due the possibility of an adverse impacton sensitive load.

Specifically excluded from the scope of the study were theexamination of networked versus radial distribution systems,radial supply to new load areas, and a comparison of factors inthe coordination of fault protection at the two voltage levels.Green Mountain Power presently performs live line mainte-nance at 12.5 kV but not at 34.5 kV. The impact of this prac-tice on the operation and performance of the options also wasnot considered in the comparison of the two voltage levels.Finally, calculation of probable customer interruption outagerates was not undertaken because, while numerical probabil-ity based indicators of reliability are certainly important, itwas felt that data on component outage rates was not of suffi-cient quality or comprehensiveness to make such calculationsuseful [3].

A 12.5-kV feeder (2802) in the Green Mountain Powerservice territory was chosen as the test system based upon theimmediacy of the need for action, the range of options avail-able for uprating, and the difficulty of supplying a relativelyremote load center under existing conditions. The 34.5112.5-kV transformer feeding 2802 has a 3750/5250 kVA OA/FOArating and a current peak loading of 5184 kVA. The 2802feeder has a main feeder length of 8.1 mi and a maximumdistance from source to load of 11.0 mi. The gross feederlength, including laterals, is approximately 50 mi and it isone of Green Mountain Power's longer feeders. The load islargely residential but has a commercial/industrial load centerapproximately 7 mi from the present source.

Feeder 2802 has 121 node points where construction-typechanges or branches are connected, 112 line sections, and909 customer services. Fig. l(a ) shows 28G2 as it is presently

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CLAYTON et al.: CASE STUDY OF RADIAL OVERHEAD FEEDER PERFORMANCE

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II 12.5 KV ' , 31.5KVFig. 1 . Feeder configurations. (a ) Option I , single source-all 12. 5 kV. (b)Option 11, single source-all 34 .5 kV. (c) Op tion 111,sectionalized-all 12 .5 kV. (d) Option IV, sectionalized- 12334.5 kV .

supplied at 12.5 kV from the 34.5 kV subtransmission networkat Charlotte. All of the options studied are described in thefollowing and are shown in Fig. l(a)-(d):

option ZV: split 2 8 6 2 , feeding the eastern section froman alternative 34.5-kV source and leaving the westernsection fed from its original source. (The western section

option Z feeder 28G2 at 12.5 kV, without changing thepresent configuration (This was studied for the hypotheti-cal condition of zero load growth on the existing feeder);option ZZ: feeder 28G2 at 34 . 5 kV , without changing thepresent configuration;option IIZ: split 2 8 6 2 , feeding the eastern section froman alternative 34.5-kV source and leaving the westernsection fed from its original source; (both the easternand western sections are operated at 12.5 kV. This optionrequires construction of a 34.5 kV subtransmission lineand construction of a 34.5112.5-kV substation.)

is operated at 12.5 kV and the eastern section is operatedat 34 . 5 kV. This option requires construction of a 34.5-kV subtransmission line.)

Other options for uprating that were considered but dis-carded as impractical because of economics included over-building the existing 2 8 6 2 feeder with an express 12.5-kVfeeder and tapping into a local 115-kV line.

THEORETICALACKGROUNDDistribution losses occur on the feeder itself, in the distri-

bution transformers feeding the load and in the secondaries.


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Since secondary losses would be common to both feeder volt-age levels, it was decided to simulate only feeder and distri-bution transformer losses. The distribution transformers weremodeled with core and winding losses because both can besignificant in a loss comparison [4].The computer programused for the loss study was limited to 200 three-phase nodesand 40 transformers. Therefore, to simulate the loss effectof distribution transformers, it was necessary to reduce thenumber of customer service points by developing an equiv-alent load model. This section describes how the equivalentload model was developed and applies the approach to severalhypothetical cases to gain insight into feeder performance.

Assuming that load is uniformly distributed along a feedersection of length L with R the feeder resistance per unit lengthand I the total feeder current, the current at any point on thefeeder is then

i , =1(1 - x / L )where x is zero at the source and L at the feeder end. Thefeeder loss is given by

PL

total loss =1 i:R d x= 1 2 R l L 1 - x / L +x2 /L2)dx=12RL/3.

Therefore, the same loss produced by the distributed loadwould be produced by lumping all of the load at a point onethird of the way down the feeder section. A similar analysis ofvoltage drop on a feeder without shunt capacitive compensa-tion shows that concentrating the load halfway down a feedersection gives the same feeder-end voltage drop as would beproduced with a uniformly distributed load.

Application of this approach to several cases at the twodifferent voltage levels and for different feeder lengths is il-lustrated in Fig. 2 . These cases were selected to simulate, inan idealized way, the effect of sectionalizing and/or upgradingon the losses and voltage drop in feeder 2 8 6 2 .

It is assumed in these cases that the same conductor is usedat both voltage levels. Comparison of the first two optionsshows the clear advantage of 34.5 kV over 12.5 kV based onfeeder losses and resistive voltage drop. The ratio of losses forthe first two options is approximately 0.13, in favor of 34.5kV. The last two options show the same result but with muchless absolute difference in losses and voltage drop. Compari-son of the third option with the second shows that losses andvoltage drop obtained by splitting the feeder at 12.5 kV arestill greater than those with a single feeder at 34.5 kV , buthave been considerably reduced, resulting in a ratio of lossesin the order of 0.50 instead of 0.13. Therefore, Fig. 2 illus-trates that, while the higher voltage level does have a strongadvantage in a comparison of feeders of the same length, theadvantage is greatly diminished when alternative supply pointsmake sectionalization an option.

The foregoing analysis ignored the effect of distributiontransformer core losses which, in addition to having a sig-

OPTION CONFIGURATI ON FEEDER TOTAL VOLTAGEVOLTAGE LOSS DROP(KV) (PU) (PU)I 0 : 4 12.5 0.0666 0.1000I1 0 ; I 34.5 0.0086 0.0130

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II14- 12.5/12.5 0.0167 0.0250IV o ? O , ' ' T = 34.5/34.5 0.00 22 0.0032

Assumptions: Feeder load/line section - 0.5 puFeeder resistance/line section- 0.100 pu @ 12.5 KV(0.013 pu @ 34.5 KV)Fig. 2. Example of feeder loss and voltage drop.

nificant magnitude, have a loss factor of unity and certainlywill affect a loss comparison of the two voltage levels. Ques-tions about the effects of transformer core losses, nonuniformload distribution, different line and substation costs at the twovoltage levels, and the impact of feeder faults, can only beanswered by a specific case study. A case study of GMP ' s2 8 6 2 feeder is described in the following sections.

FEEDER28G2 SIMULATIONA multiphase feeder program was used to representaccurately unbalanced conditions such as load unbalance and

single/two-phase laterals. All main feeder sections wererepresented as three-phase branches and all lateral feeder sec-tions were represented as single or two-phase branches, asappropriate. All construction types were modeled by specificimpedance data and node points were placed at all construc-tion changes as well as at branch points. The load points werelocated one third of the way down each section representinga construction type.

The installed distribution transformer capacity on each sec-tion representing a construction type was totaled by phase.Branches (one per phase) were attached at each load pointto simulate the impedance of the total installed transformercapacity on that phase. The impedance ( r + j x ) of each trans-former branch was calculated in inverse proportion to its in-stalled capacity.

Loads were represented by a constant real and reactivepower characteristic and reactive power characteristic and lo-cated on the low side of the transformer branches. The magni-tude of each lumped load was set equal to the installed trans-former capacity on the line section times an appropriate capac-ity factor. Load growth was simulated by appropriate changesin transformer installed capacity and related factors.

Core losses are strongly dependent upon voltage [ 5 ] ,vary-ing approximately as the square of the voltage over the voltagerange of interest and were represented at each load bus by aconstant resistance load with a base magnitude proportionalto its installed transformer capacity.

The Thevenin short-circuit source impedance was calcu-lated for each option from a full-scale transmission systempositive and zero sequence simulation.

LOADANALYSISFig.3 shows a typical load cycle for the 2 8 6 2 feeder. A nu-

merical integration of this load cycle resulted in the following


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CLAYTON et al . : CASE STUDY OF RADIAL OVERHEAD FEEDER PERFORMANCE 69 92862 AVERAGE HA> AN3 J J N E ,OA:3;464 E l - - -7

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Fig. 3 Typical load cycle for feeder 28G2

values:load factor = 0.79loss factor = 0.65.

The loss factor was assumed to be constant for all stages ofgrowth considered in the study. Analysis of the historical peakloads on the 28G2 feeder showed that growth could be repre-sented as

where po = 4200 kVA and n is the number of years from thebase year. This base load growth model was supplemented byadditional future loading to give the high and medium growthscenarios shown in Fig. 4.

The high load growth scenario will exaggerate the absoluteloss differential between the 12.5- and 34.5-kV options (ihfavor of the 34.5-kV options). To avoid biasing the results infavor of 34.5 kV, it was decided to simulate both the mediumand high growth scenarios for all study options. Only the highload growth results are reported in this paper.

The year 1991 was selected as the starting year for the eco-nomic analysis based upon an estimate of the earliest possibledate for completion of the construction necessary for each op-tion. An economic line life of 18 years was assumed for theanalysis and three stages were selected to simulate the highand medium growth scenarios. The feeder loading in years1994, 2000, and 2006 were chosen to represent the loading ineach stage.

The measured power factor at the feeder source at the his-toric peak load of 5184 kVA was 0.99. Accounting for 1900kVA of shunt capacitor compensation on the feeder gave a

raw load power factor of 0.89, which was assumed for allindividual loads in the study.

TRANSFORMERNALYSISThe 909 distribution transformers on feeder 28G2 rangedin rating from 5 to 25 kVA, with several larger commer-

cial/industrial loads. It was impractical to represent each ofthese transformers individually, and therefore, it was neces-sary to develop an equivalent transformer as follows:

total installed transformer capacity on 2862 =total number of single-phase loads on 2862 = 909installed capacity per load x 0 kVA.

The total transformer capacity lumped at any load bus wassimulated as n 20-kVA equivalent transformers connectedin parallel. It was necessary to calculate the impedance andcore loss of the equivalent 20-kVA transformer. Fortunately,GM P received bids on 12.5- and 34.5-kV pole- and pad-mounted distribution transformers in 1987. These data givevalues for winding and core losses for various manufacturersand ratings and allowed core and winding losses for the equiv-alent 20-kVA transformer to be estimated as shown in TableI. The real part of the transformer impedance was calculatedbased upon the value of winding loss and the imaginary partestimated from the GE Distribution Data Book [ 5 ] .

It is interesting to note that both the core and the windinglosses at the two voltage levels are almost equal. In fact, trans-formers can be designed for a wide range of loss levels. Theoptimum point in the trade-off between capital cost and thecost of losses is determined by the relative weight assigned bythe purchaser to capital and operating costs. The equivalentvalues calculated from the GMP bid data base are substantiallythe same as typical data values found, for example, in [ 5 ] .

Analysis of the same data base for pole- and pad-mounted

18 336 kVA

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 26 , NO. 4, JULYIAUGUST 1990KV A LOADIYC11000 I10000 -9000 -

4000 I I I I I I I 1 I I 11988 1998

YEARFig. 4. Load growth scenarios for feeder 28G2

TABLE IEQUIVALENTO - K V A RANSFORMERHARACTERISTICS

200B

Voltage Core Loss Winding Lossa Impedance(kV) (W) (W) (P.U.)12.47 59.95 327.7 0.0164 + 0.013534.50 55.35 311.1 0.0156 + 0.0165

aAt rated load.distribution transformers gave the following values for trans-former base prices:

20 kVA transformer at 12.47 kV = $24.5/kVA20 kVA transformer at 34.50 kV = $292/kVA.The effect of load growth was simulated by increasing in-

stalled transformer capacity at the load buses in proportion tothe load growth and thereby keeping the transformer capacityfactors for each phase at a constant value.

PERFORMANCEF OPTIONSThe voltage profile and losses in the feeder for the four op-

tions were evaluated at various feeder loading levels. The lossresults are summarized in Tables I1 and 111. Table I1 showsresults for hypothetical new construction of feeder 2 8 6 2 withzero load growth. Table I11 shows results for the various up-rating options for the high load growth scenario. All casesmet the i -V (on a 120-V base) voltage criterion withoutany shunt capacitor additions.

Table I1shows loss results for options I and 11. Feeder lossesin each case were totaled into transformer (winding and core)and line (main and lateral) components. As expected, linelosses at 34.5 kV are less than at 12.5 kV by approximatelythe square of the voltage ratio ({12.5/34.5}2 = 0.13). Allcases show that the majority of feeder line losses occur on themain and also that transformer losses are significant. Inclu-sion of the transformer losses in the simulation changes theratio of the total losses in the two cases from 0 . 1 2 to 0 . 3 9 ,thereby reducing the advantage of the higher voltage level.The loss ratio will be even closer to unity at lower loadinglevels because core losses are independent of load level andwill dominate at light load.

Table I11 shows the loss results for the high load growthscenario for upgrading options 11, 111, and IV for the years

TABLE I1LOSSESOR NE WCONSTRUCTIONPTIONS-ZEROOA DGROWTH1988 LOADINGLosses (kW)Option FeederVoltage Load Transformer Line TotalOption (kW) (MW ) Winding Core M ain Lateral

I 12.5 5.05 26.08 57.23 177.92 7.55 268.78I1 34.5 4.89 24.48 58.97 21.74 0.49 105.68

1994, 2000, and 2006. The west and east section lossesshown for the sectionalized options I11 and IV are added to-gether to allow a loss comparison with each other and withthe unsectionalized option 11.

The importance of the correct simulation of transformerlosses is again emphasized by the loss results shown in Table111. The loss values shown in Tables I1 and 111 are used tocalculate the cost of losses for each option as reported uponin the following section.

ECONOMICOMPARISONF OPTIONSTables IV and V summarize the present worth of revenue

requirements (PWRR) for the various options. Table IV showsan economic comparison of an idealized feeder based upon28G2 as if it were newly constructed in 1991 at 12.5 kV(option I) and at 34.5 kV (option 11). Both cases assume noload growth on the feeder and use appropriate loss values fromTable 11. Energy losses were calculated with a loss factor of0 . 6 5 for the feederltransformer winding losses and with a lossfactor of 1 .O for the transformer core losses. The capital costswere based upon local figures.

Comparison of the two options in Table IV show that thepresent worth of revenue required for the capital costs of thetwo voltage levels are almost identical. This is not surprisinggiven that the 12.5-kV option has higher substation costs butlower line and distribution transformer costs when comparedto 34.5 kV. The 34.5-kV PWRR of losses are 44%of the 12.5-kV losses. This comparison is somewhat biased in favor of12.5 kV because the higher voltage has the capability to servea much greater load area than the 12.5-kV option. However,based upon the 2 8 6 2 feeder conditions covered by Table IV ,the 34.5-kV option has only a 5% PWRR cost advantage overthe 12.5-kV option.

Table V shows the results of the economic analysis for up-rating the existing feeder to options 11, 111, and IV , for thehigh load growth scenario. Table V shows that option I11 hasthe lowest cost of all options and option I1 had the highest.The advantages of loss reduction at 34.5 kV for options I1and IV were far outweighed by their higher direct costs ofuprating when compared to the sunk costs of option 111.

It is interesting to compare the PWRR of losses of optionsI1 and 111. Option I1 losses (with the original 28G2 feederbeing fed at 34.5 kV) are 6 8 . 4 % of option I11 losses (withthe original 28G2 feeder split and both halves fed at 12.5 kV)for the high load growth scenario and 71 % for the mediumgrowth scenario. A comparison of feeder losses on feedersthat are identical except for voltage level would result in 34.5-kV feeder losses being approximately 13% of 12.5-kV feeder


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CLAYTON et a l . : CASE STUDY OF RADIAL OVERHEAD FEEDER PERFORMANCE 701TABLE I11LOSSESOR UPRATINCPTIONS-HIGH OAD ROWTH

Losses (kW)Option FeederVoltage Load Transformer Line TotalOption (kV) (MW) Winding Core Main LateralStage 1: 1994 LoadingI1 34.5 7.47 41.66 78.51 52.54 2.15 174.86

111-WEST 12.5 3.03 16.77 33.93 40.43 37.39 128.52111-EAST 12.5 4.53 30.45 41.34 17.23 41.14 130.16Il l Totals 7.56 47.22 75.27 57.6 6 78.53 258.68IV -WEST 12.5 3.03 16.77 33.93 40.43 37.39 128.52IV-EAST 34.5 4.47 29.93 40.15 0.69 0.94 71.71IV Totals 7.50 46.70 74.08 41.12 38.33 200.23I1 34.5 8.92 49.13 93.45 76.53 3.72 222.83111-WEST 12.5 3.62 19.61 41.17 54.97 53.08 168.8312.5 5.42 35.34 50.58 24.85 59.27 170.04I l l Totals 9.04 54.95 91.75 79.82 112.35 338.87IV-WEST 12.5 3.62 19.61 41.17 54.97 53.08 168.83IV-EAST 34.5 5.33 34.61 49.30 ~1.01 1.33 86.25

8.95 54.22 90.47 ' 55.98 54.41 255.08V Totals3.98 266.511 34.5 10.12 55.65 106.87 100.01

111-WEST 12.5 4.23 23.84 45.44 81.00 74.14 224.42111-EAST 12.5 6.05 38.73 57.40 31.19 79.77 207.09I11 Totals 10.28 62.57 102.84 112.19 153.91 431.51IV-WEST 12.5 4.23 23.84 45.44 81.00 74.14 224.42IV-EAST 34.5 5.95 37.74 56.16 1.28 1.67 96.85IV Totals 10.18 61.58 101.60 82.28 75.81 321.27

~ __ __ __ ~ ~

~ __ ~ ~ ~Stage 2: 2000 Loading

Ill-EAST~~ __ __ ~ ~

~ ____ __Stage 3: 2006 Loading

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TABLE IV1991 Present Worth of Revenue Requirem ents(New Construction Options-Zero Load Growth)

Option PWRRVoltage Capital Losses TotalOption (kV) (k$) (k$) (k$)I 12.5 6153.61a 727.44 6881.06I1 34.5 6197.61b 320.20 6517.81

"New 34.5/12.5-kV 7.5110.5 MVA substation and new 28G2 12.5-kVbNew 34.5-kV sw itching station and new 28 02 34.5-kV feeder.feeder.

TABLE V19 91 PRESENT WORTH OF R E V E N U EEQUIREMENTS (UPRATING OPT IO NS -H IG H OAD RO W TH )~~ Option PWRRVoltage Capital Losses TotalOption (kV) (k$) (W (k$)

I1 34.5 7624.43" 635.48 8259.90I11 12.5/12.5 1292.52 929.16 2221.68IV 12.5134.5 241 6.06' 723.89 3139.95aUprate 28G2 to 34.5 kV.'Extend 34.5-kV subtransmission plus new 34 .5 12.5-kV 7.5 / 10.5-MVA substation.'Uprate 286 2 Eastern section to 34.5 kV . Extend 34.5 kV subtransmission plus new 34.5-kVswitching station.

losses. The theoretical analysis suggested that this apparentadvantage for the higher voltage would be offset by the effectof sectionalizing the lower voltage line and by distribution

Similar economic results were obtained for the medium loadgrowth scenario. Therefore, it is clear that option I11 is thepreferred economic choice.

transformer core losses; this is confirmed by the foregoingresult. SYSTEM OLTAGELUCTUATIONIt is believed that reasonable variations in the capital costs,

cost of losses and economic factors used in the foregoing cal-culations would not affect the economic ranking of the optionsbecause of the wide differences in the resulting total costs.

Much of the GMP system, including the area under dis-cussion, is remote from major generation and, as a result,distribution faults can cause voltage dips of significant magni-tude on the transmission system. Concern about transmission


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Fig 5 . Flicker curvessystem voltage dips is heightened because industrial customersare connected to the local 115-kV transmission and 34.5-kVsubtransmission systems with loads that are particularly sen-sitive to voltage. While it must be accepted that transmissionline faults will disrupt all feeders in the area, it is most impor-tant that distribution feeder faults, which are more frequent,should not disturb the transmission system beyond the toler-ance of connected load.Voltage Tolerance Characteristics

Feeder reclosers characteristically will have one to two fastopenlclose operations and one to two slow opedclose oper-ations to clear transient faults (lightning, tree contact) or toallow fuses to blow and/or downline sectionalizers to open toclear permanent faults. If the fault is not cleared in the deadtime between reclosures then each reclosure will reapply thefault to the system and the system will experience a voltagedip for the period between reclosing and opening. The deadtime could be 15-45 s, giving a repetition frequency of 4-1.33times per minute.

Voltage dip caused by feeder faults is not flicker in thehistorical sense where long term repetitive voltage fluctua-tions caused by arc welding, for example, cause annoyancebecause of perceptible changes in the light output of incandes-cent lamps [5], [6]. Fig. 5[5] shows a border line of irrita-tion flicker curve as a function of percent voltage fluctuationand frequency of occurrence. However, without statistics onthe frequency, number of reclosures and the time relationshipbetween feeder faults, it is impossible to reach any conclu-sion on the impact of feeder faults on flicker. This is because,while voltage dip can be defined from a short-circuit analysis,the frequency of systemwide voltage dips caused by feederfaults is undefined.

The absolute value and duration of voltage dips, indepen-dent of frequency of occurrence, are of critical concern to

closely controlled industrial process equipment and comput-ers. Even a single dip of quite brief duration can cause unac-ceptable disruptions of customer operations. Fig. 6 shows arepresentative voltage tolerance envelope developed as a de-sign goal for electronic equipment [7]. It shows that voltagedips of up to 13% can be tolerated for periods of 120 cycles orless. The tolerance associated with feeder fault clearing timesof 15-45 cycles are voltage dips of approximately 20-15%,respectively. This is a general guide and specific items ofequipment may be more sensitive.VoltageDip

Given the remoteness of the GM P system from major volt-age supporting generation, it was necessary to model a widearea of the interconnected transmission system to calculatevoltage dips accurately at key 115-kV transmission buses fordistribution feeder faults. Short-circuit calculations were madewith a transmission system model covering the main transmis-sion of New England and New York and with a detailed repre-sentation of the 115-kV transmission and 69, 46 , and 34.5 kVsubtransmission systems in Vermont. The 28G2 feeder mainline was represented at 12.5 and 34.5 kV to allow considera-tion of feeder faults at 1and 7 mi from the source at Charlotte.Table VI shows the voltage dips produced at the Essex 115-kV bus by faults on the 28G2 feeder at both voltage levels.Table VI also shows the effect of distribution feeder faults onfeeders connected to the 34.5-kV subtransmission at severalother system locations as shown in Fig. 1. The Middlesexfeeder location is 26 mi from Essex but close to the Middle-sex 115/34.5-kV transformer interconnection and the Essex19 feeder location is at the low side of the Essex 115/34.5-kVtransformer interconnection.

The voltage dips on the 115-kV system are generally moresevere for three-phase faults than single-phase faults becauseof strong zero-sequence ground paths provided by delta-wye

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Fig. 6. Typical design goals for power co nscious computer manufacturers

TABLE VISUMMARY F FAULT ASESFeeder Voltage Dip at Essex 115-kV Bus (% )Feeder Voltage Fault @ 1 mi Fault @ 7 miCase Option Location (kV) 16 3 6 16 3 6

1 I Charlotte 12.5 0.98 1 .39 0.55 0.642 I1 Charlotte 34.5 4 .29 6.70 2.33 3 .933 I Middlesex 12.5 0.61 0 .90 0 . 3 3 0 . 3 84 I1 Middlesex 34.5 8.70 10.96 2.11 3 .425 I Essex 19 12.5 1 .39 1.98 0 . 7 9 0 . 9 06 I1 Essex 19 34.5 23.10 22.51 5 . 9 4 7 . 5 8

transformers between the 34.5- and 115-kV systems. The 115-kV voltage dips for faults on the 12.5-kV feeders are muchless than for corresponding faults on 34.5-kV feeders becauseof their higher impedance as seen from the 115-kV systemand the impedance of 34.Y12.5 kV transformers. Most of thecases indicate'that voltage dips are within the typical designgoals shown in Fig. 6, but case 6 suggests that 34.5-kV dis-tribution would be a cause of concern if single-phase or three-phase feeder faults occur with any regularity. The frequencyof three-phase and single-phase distribution feeder faults inVermont depends upon the route of the feeder through forest,field or suburban areas, upon the currency of tree trimming,and upon other design factors. However, it is clear that theuse of 34.5-kV distribution in Vermont makes transmissionvoltage dips a concern, while 12.5-kV distribution faults arenot of concern with respect to transmission voltage dips.

CONCLUSIONThe relative economic merits of 34.5- versus 12.5-kV dis-

tribution are strongly affected by recognition of distributiontransformer winding and core losses. Transformer losses forthe new construction options in this study were 31 % of thetotal feeder losses at 12.5 kV and 79% of the total at 34.5 kV.Ignoring the effect of transformer losses will bias the resultsin favor of the 34.5-kV voltage level.

An economic comparison of capital costs and the cost oflosses for new construction based upon 2802 at 12.5 kV and at34.5 kV shows that the PWRR of capital costs in the two casesare almost equal. This is due to the trade-off between higher

substation costs but lower line and distribution transformercosts at 12.5 kV. The PWRR of 34.5 kV losses are 44%of those at 12.5 kV. Overall, the 34.5-kV voltage level onlyshows a 5% cost advantage over 12.5 kV for this application.A 5% difference in cost is probably less than the accuracyof the input data and, therefore, no clear economic advantagecan be demonstrated for either voltage level based upon thiscomparison.

An economic comparison of the three options available forthe uprating of feeder 2862 shows a clear advantage to optionI11 (with the original 28G2 feeder split and both halves fedat 12.5 kV). Options involving upgrades to 34.5 kV werenot justified economically in this case because their increasedcapital costs far outweighted their advantage in loss reduction.Option 111 has further advantages of lower 115 kV systemvoltage dip and ease of back feed should one source be lost.

Voltage dip on the 115-kV system for faults on 34.5-kVfeeders are significantly higher than for 12.5-kV feeder faults.The 115-kV system voltage dips for faults on the 34.5-kVfeeders studied could exceed current industry voltage tolerancegoals depending upon feeder location with respect to sensitive115-kV buses.

All comparisons of the two voltage levels should be an-alyzed in their specific application. Analysis of the feederconsidered herein has shown that there is no clear economicadvantage to 34.5 kV and that there may be a disadvantagewith respect to voltage dip performance. However, analysis oflonger feeders connected to a stiff transmission system couldreverse this conclusion.

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REFERENCESPanel session on 35 kV distribution update, overhead and under-groun d, IEEE Rep. 86TH0162-PWR. sponsored by the Transmissionand Distribution Committee of IEEE PES.Higher distribution voltages: Not always a panacea, Elec. World,Apr. 1988.Needed: Consistent distribution-outage data, Elec. World, July1988.D . I. H. Sun et al., Calculation of energy losses in a distribution sys-t em, IEEE Trans. Power Ap p. Syst., vol. PAS-90, no. 4 , JulyIAug.1980.Distribution Data Book, General Electric Co., no. 7-80 (5M).IEEE Distribution Subc omm ittee, Working Group on Voltage Flicker,Flicker limitations of electric utilities, ZEEE Trans. Power Ap p.Syst., PAS #85 WM 204-3, Sept. 1985.IEEE Recommended Practice for Emergency and Standby PowerSystems fo r Industrial and Commercial Applications (IEEE OrangeBook), A NSIDEEE Standard 446-1987.

Roger E. Clayton (M71-SM78) received theB.Sc. (hons) degree in heavy current engineeringand the M.Sc. degree in power system engineeringfrom the University of Aston, Birmingham, UK, in1966 and 1968, respectively.He joined the General Electric Company in 1968in the Electric Utility Engineering Operation andwas engaged in studies of power system transientsand transmission line design . He joined Power Tech-nologies, Inc., in 1972 working on line design andtransmission planning studies in the U.S. and SouthAmerica. He is now a Principal of Electric Power Consultants Incorporatedof which he cofounded in 1986.Mr. Clayton is a Registered Professional Engineer in the State of NewYork. He is a member of the Lightning and Insulator Subcommittee and theWorking Group on Estimating the Lightning Performance of TransmissionLines.

John M. Undrill (M66-SM74-F78) receivedthe honours degree in engineering and the Ph.D.degree from the University of Canterbury, NewZealand, in 1963 and 1965, respectively.He joined the General Electric Company, ElectricUtility Engineering Operation in 1966 and PowerTechnologies Incorporated in 1972. He is now aPrincipal of Electric Power Consultants Incorpo-rated which he cofounded in 1986. He has workedextensively in electric machine and power systemdynamics and on load flow, short circuit and dy-namic simulation programs.Dr. Undrill is a Registered Professional Engineer in the State of New York,the Province of Ontario, and New Zealand. He is a member of the PowerSystem Engineering Committee, of its subcommittee on Computer and An-alytical Methods, and of its Working Group on Exchange of Power SystemAnalytical Data.

Eugene L. S h a h (M78) received the B.S. andM.S. degrees in electric power engineering fromRensselaer Polytechnic Institute, Troy, NY, in 1977and 1978, respectively.He joined the Advanced System Technology Divi-sion of Westinghouse Electric Corporation in 1978as a Systems Engineer. In 1980, he joined GilbertAssociates, Inc., in Reading, PA and was assignedto their management consulting group. At Gilbert,he worked on numerous projects in generation plan-ning, computer applications, and advanced technol-ogy assessment. He is currently the Assistant Vice President of Engineeringand Electrical O perations for Green Mountain Power Corporation located inSouth Burlington, VT, responsible for the overall operation of the powergeneration and delivery system.Mr. Smith is a Licensed Professional Engineer.

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case study of radial overhead feeder performance at 12[1].5 and 35.4 kv (trans)

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