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A distribution transformer will provide service to one or more custom-ers�EachcustomerwillhaveademandcurvesimilartothatofFigure2�2�
However,thepeaksandvalleysandmaximumdemandswillbedifferentfor
eachcustomer�Figures2�3through2�5givethedemandcurvesforthethree
additionalcustomersconnectedtothesamedistributiontransformer�
The load curves for the four customers show that each customer has
its unique loading characteristic� The customers’ individual maximum
kW demand occurs at different times of the day� Customer #3 is the only
0 1 2 3 4 5 6 7 8
0:15 2:00 3:45 5:30 7:15 9:00 10:45 12:30 14:15 16:00 17:45 19:30 21:15 23:00 Time of day
15 min kW demand
FIGURE 2.3
Twenty-fourhourdemandcurveforcustomer#2�
0 1 2 3 4 5 6 7 8
0:15 2:00 3:45 5:30 7:15 9:00 10:45 12:30 14:15 16:00 17:45 19:30 21:15 23:00 Time of day
15 min kW demand
FIGURE 2.4
Twenty-fourhourdemandcurveforcustomer#3�
customerwhowillhaveahighloadfactor�Asummaryofindividualloads
isgiveninTable2�1�ThefourcustomersgiveninTable2�1demonstratethat
thereisgreatdiversitybetweentheirloads�
2.3.1 Diversified Demand
Itisassumedthatthesamedistributiontransformerservesthefourcustom-ersdiscussedpreviously�Thesumofthefour15kWdemandsforeachtime
intervalisthe“diversifieddemand”forthegroupinthattimeinterval,and
inthiscase,thedistributiontransformer�The15mindiversifiedkWdemand
ofthetransformerforthedayisshowninFigure2�6�Noteinthisfigurehow
thedemandcurveisbeginningtosmoothout�Therearenotasmanysignifi-cantchangesasseenbysomeoftheindividualcustomercurves�
2.3.2 Maximum Diversified Demand
ThetransformerdemandcurveofFigure2�6demonstrateshowthecombined
customerloadsbegintosmoothouttheextremechangesoftheindividualloads�
Forthetransformer,the15minkWdemandexceeds16kWtwice�The greater
0 1 2 3 4 5 6 7 8
0:15 2:00 3:45 5:30 7:15 9:00 10:45 12:30 14:15 16:00 17:45 19:30 21:15 23:00 Time of day
15 min kW demand
FIGURE 2.5
Twenty-fourhourdemandcurveforcustomer#4�
TABLE 2.1
IndividualCustomerLoadCharacteristics
Customer #1 Customer #2 Customer #3 Customer #4
Energyusage(kWh) 58�57 36�46 95�64 42�75
MaximumkWdemand 6�18 6�82 4�93 7�05
TimeofmaximumkW
demand 13:15 11:30 6:45 20:30
AveragekWdemand 2�44 1�52 3�98 1�78
Loadfactor 0�40 0�22 0�81 0�25
oftheseisthe“15minmaximumdiversifiedkWdemand”ofthetransformer�
Itoccursat17:30andhasavalueof16�16kW�Notethatthismaximumdemand
doesnotoccuratthesametimeasanyoneoftheindividualdemandsnoris
thismaximumdemandthesumoftheindividualmaximumdemands�
2.3.3 Load Duration Curve
A“loaddurationcurve”canbedevelopedforthetransformerservingthe
fourcustomers�Sortinginadescendingorder,thekWdemandofthetrans-formerdevelopstheloaddurationcurveshowninFigure2�7�
0 2 4 6 8 10 12 14 16 18
0:15 2:00 3:45 5:30 7:15 9:00 10:45 12:30 14:15 16:00 17:45 19:30 21:15 23:00 Time of day
15 min kW demand
FIGURE 2.6
Transformerdiversifieddemandcurve�
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
1 8 15 22 29 36 43 50 57 64 71 78 85 92
Percent of time
15 min kW demand
FIGURE 2.7
Transformerloaddurationcurve�
Theloaddurationcurveplotsthe15minkWdemandversusthepercent
oftimethetransformeroperatesatorabovethespecifickWdemand�For
example, the load duration curve shows the transformer operates with a
15minkWdemandof12kWorgreater22%ofthetime�Thiscurvecanbe
usedtodeterminewhetherornotatransformerneedstobereplaceddueto
anoverloadingcondition�
2.3.4 Maximum Noncoincident Demand
The“15minmaximumnoncoincidentkWdemand”forthedayisthesumof
theindividualcustomer15minmaximumkWdemands�Forthetransformer
inquestion,thesumoftheindividualmaximumsis
Max noncoincident demand._ _ =6 18 6 82 4 93 7 05 24 98 kW. + . + . + . = . (2�4)
2.3.5 Diversity Factor
Bydefinition,diversityfactor(DF)istheratioofthemaximumnoncoinci-dentdemandofagroupofcustomerstothemaximumdiversifieddemand
ofthegroup�Withreferencetothetransformerservingfourcustomers,the
DFforthefourcustomerswouldbe
Diversity factor Maximum noncoincident demand Maximum diver
_ _ _
= _
ssified demand_ .
. .
=24 98= 16 16 1 5458
(2�5) TheideabehindtheDFisthatwhenthemaximumdemandsofthecustomers
areknown,thenthemaximumdiversifieddemandofagroupofcustomerscan
becomputed�TherewillbeadifferentvalueoftheDFfordifferentnumbersof
customers�Thevaluecomputedearlierwouldapplyforfourcustomers�Ifthere
arefivecustomers,thenaloadsurveywouldhaveto besetuptodeterminethe
DFforfivecustomers�Thisprocesswouldhavetoberepeatedforallpracti-calnumberofcustomers�Table2�2isanexampleoftheDFsforthenumberof
customersrangingfrom1upto70�Thetablewasdevelopedfromadifferent
databasethanthefourcustomersthathavebeendiscussedpreviously�Agraph
oftheDFsisshowninFigure2�8�
Notethat,inTable2�2andFigure2�8,thevalueoftheDFbasicallyleveled
outwhenthenumberofcustomersreached70�Thisisanimportantobser-vationbecauseitmeans,atleastforthesystemfromwhichtheseDFswere
determined,thattheDFwillremainconstantat3�20from70customersand
above�Inotherwordsasviewedfromthesubstation,themaximumdiversi- fieddemandofafeedercanbepredictedbycomputingthetotalnoncoin-cidentmaximumdemandofallofthecustomersservedbythefeederand
dividingby3�2�
2.3.6 Demand Factor
Thedemandfactorcanbedefinedforanindividualcustomer�Forexample,
the15minmaximumkWdemandofcustomer#1wasfoundtobe6�18kW�
Inordertodeterminethedemandfactor,thetotalconnectedloadofthecus-tomerneedstobeknown�Thetotalconnectedloadwillbethesumofthe
ratingsofalloftheelectricaldevicesatthecustomer’slocation�Assumethat
thistotalcomesto35kW,thenthedemandfactoriscomputedtobe
Demand factor Maximum demand Total connected load
_ _
_ _
. .
= =6 18=
35 0 11766 (2�6)
0 0.5 1 1.5 2 2.5 3 3.5
1 6 11 16 21 26 31 36 41 46 51 56 61 66
Number of customers
Diversity factors
FIGURE 2.8 Diversityfactors�
TABLE 2.2 DiversityFactors
N DF N DF N DF N DF N DF N DF N DF
1 1�0 11 2�67 21 2�90 31 3�05 41 3�13 51 3�15 61 3�18
2 1�60 12 2�70 22 2�92 32 3�06 42 3�13 52 3�15 62 3�18
3 1�80 13 2�74 23 2�94 33 3�08 43 3�14 53 3�16 63 3�18
4 2�10 14 2�78 24 2�96 34 3�09 44 3�14 54 3�16 64 3�19
5 2�20 15 2�80 25 2�98 35 3�10 45 3�14 55 3�16 65 3�19
6 2�30 16 2�82 26 3�00 36 3�10 46 3�14 56 3�17 66 3�19
7 2�40 17 2�84 27 3�01 37 3�11 47 3�15 57 3�17 67 3�19
8 2�55 18 2�86 28 3�02 38 3�12 48 3�15 58 3�17 68 3�19
9 2�60 19 2�88 29 3�04 39 3�12 49 3�15 59 3�18 69 3�20
10 2�65 20 2�90 30 3�05 40 3�13 50 3�15 60 3�18 70 3�20
Thedemandfactorgivesanindicationofthepercentageofelectricaldevices
thatareonwhenthemaximumdemandoccurs�Thedemandfactorcanbe
computedforanindividualcustomerbutnotforadistributiontransformer
orthetotalfeeder�
2.3.7 Utilization Factor
The utilization factor gives an indication of how well the capacity of an
electrical device is being utilized� For example, the transformer serving
the four loads is rated 15kVA� Using the 16�16kW maximum diversified
demand and assuming a power factor of 0�9, the 15min maximum kVA
demand on the transformer is computed by dividing the 16�16kW maxi- mumkWdemandbythepowerfactorandwouldbe17�96kVA�Theutiliza-tionfactoriscomputedtobe
Utilization factor Maximum kVA demand Transformer kVA ratin
_ _ _
_ _
= gg=17 96=
15. 1 197. (2�7)
2.3.8 Load Diversity
Loaddiversityisdefinedasthedifferencebetweenthenoncoincidentmaxi-mumdemandandthemaximumdiversifieddemand�Forthetransformerin
question,theloaddiversityiscomputedtobe
Load diversity_ =24 97 16 16 8 81kVA. − . = . (2�8)