transformer sizing presentation.pdf

8/9/2019 Transformer Sizing Presentation.pdf

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2013 ElectriCitiesElectric Utility

Webinar Series

Sizing of

Transformers

Calculationof Loads


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Demand and Demand Factor

Demand The value of electrical power required for a

particular load. Expressed in kW.

Usually averaged over a 15-minute or 30-minute period for billing

purposes.

Peak Demand The maximum Demand of the whole

system at any one time.


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Demand Factor The Peak Demand of the whole

system compared to the connected Demand of thesystem.

Expressed as a percentage or a ratio less than 1.


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Example - Average Home

Typical Electrical Loads:

Water Heater 4,500 watts (4.5 kW)

Range / Oven 8,000 watts (8.0 kW)

Central Air Conditioner 6,000 watts (6.0 kW)

Clothes Dryer 5,000 watts (5.0 kW)

Dishwasher 2,000 watts (2.0 kW) Lighting, Fans, Appliances, Other 7,500 watts (7.5 kW)

Connected Demand = 33 kW

Peak Demand = 18 kW

Demand Factor = 18 kW / 33 kW = 0.545


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Residential Transformer Loading

Diversity Method

Information Required

Average Square Footage of the Homes

Number of Homes with Gas and Electric Heat

Information from Tables Peak kW Demand for the size of the home.

Add all of the Peak Demands for the number of homes connected to thetransformer.

Apply the appropriate Diversity Factor to the connected Peak Demand

for the number of homes. Use the calculated Demand to determine the size of the transformer

based on the Maximum kVA Loading.


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Example Diversity Method

5 Homes - All Electric; between 1,500 and 1,800 square feet.

Step 1 Determine the Connected Peak kW for each home.

Electric

Summer

Electric

Winter

Gas

Summer

Gas

Winter

kW for 1,200 s.f. 13 15 8 8

kW for 1,500 s.f. 15 18 10 10

kW for 1,800 s.f. 16 20 11 11

kW for 2,400 s.f. 18 21 12 11

kW for 3,000 s.f. 21 26 14 11


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Peak kW per Home = 20 kW winter; 16 kW summer

Step 2 Add the Peak Demands for all of the homes.

20 kW x 5 homes = 100 kW Winter Peak Demand

16 kW x 5 homes = 80 kW Summer Peak Demand

Step 3 Apply the Diversity Factor to the Connected Peak Demand.

Number of Customers 1 2 3 4 5 6 7 8 9 10

Diversity Factor 100% 90% 75% 65% 63% 62% 61% 61% 61% 61%

Number of Customers 11 12 13 14 15 16 17 18 19 20

Diversity Factor 60% 59% 58% 58% 57% 57% 56% 54% 54% 54%


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Diversity Factor for 5 homes = 63%

Step 4 Calculate the Peak Load on the Transformer.

100 kW x 63% = 63 kW; 80 kW x 63% = 51 kW

Step 5 Determine the Transformer Size based on the Maximum kVA

Loading Table.

Transformer

Size

Summer

140%

Winter

160%

Transformer

Size

Summer

140%

Winter

160%

10 14 16 50 70 80

15 21 24 75 105 120

25 35 40 100 140 160

37.5 53 60 167 234 267


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Step 4 Calculate the Peak Load on the Transformer.

Winter Demand = 100 kW x 63% = 63 kW

Summer Demand = 80 kW x 63% = 51 kW

Step 5 Determine the Transformer Size based on the Maximum kVALoading Table.

For a 37.5 kVA Transformer:

Winter 160% Loading = 60 kW

Summer 140% Loading = 53 kW For a 50 kVA Transformer:


Summer 140% Loading = 70 kW


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Demand and Coincidence Factor

Coincidence Factor A ratio of the average running kW

load to the connected kW load on a utility transformer

based on the number of customers connected to the

transformer.

Expressed as a percentage or a ratio less than 1.

Usually will decrease as the number of connected customers

increases.

Applied by the use of tables.


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Coincidence Factor Method

Information Required Highest Summer kWh and Winter kWh Usage (from billing records).

Number of Homes Connected to the Transformer.

Information from Tables

Determine the kW Demand for the home based on kWh usage.

Add all of the Peak Demands for the customers connected to thetransformer.

Apply the appropriate Coincidence Factor to the connected PeakDemand.

Use the calculated Demand to determine the size of the transformerbased on the Maximum kVA Loading.


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Example Coincidence Factor Method

3 Homes

Step 1 Determine the Summer and Winter Peak kWh Usage for each

home (listed in table below)

Summer kWh Winter kWh

Customer # 1 2,224 853

Customer # 2 2,734 1,274

Customer # 3 1,849 1,283


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Step 2 Determine the kW Demand for each home.

Step 3 Add the kW Demands.

Summer Winter

kWh kW Demand kWh kW Demand

Customer #1 2,224 11.57 853 7.47

Customer #2 2,734 13.16 1,274 9.97

Customer #3 1,849 10.29 1,283 10.27

Totals 35.0 27.7


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Step 3 Add the kW Demands. Summer Demand = 35.0 kW

Winter Demand = 27.7 kW

Step 4 Determine the Coincidence Factor for the number of customers

from the table.

Number of

Customers

Coincidence

Factor

1 1.00

2 0.85

3 0.74

4 0.66

5 0.61


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Summer Winter

Coincidence

Factor

Total kW

Demand

Transformer

kW Demand

Coincidence

Factor

Total kW

Demand

Transformer

kW Demand

0.74 35.0 kW 25.9 kW 0.74 27.7 kW 20.5 kW



Step 4 Determine the Coincidence Factor for the number of customersfrom the table.

Coincidence Factor = 0.74

Step 5 Multiply the total kW Demand by the Coincidence Factor for

Summer and Winter loads. Summer Transformer Demand = 35.0 kW x 0.74 = 25.9 kW

Winter Transformer Demand = 27.7 kW x 0.74 = 20.5 kW


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Step 6 The answer yields the peak 15-minute demand on thetransformer.

Summer Transformer Demand = 35.0 kW x 0.74 = 25.9 kW


Apply the two kW demands in step 5 to the Maximum kVA Loading table for

transformers.

Transformer

Size

Summer

140%

Winter

160%

Transformer

Size

Summer

140%

Winter

160%

10 14 16 50 70 80

15 21 24 75 105 120

25 35 40 100 140 160

37.5 53 60 167 234 267


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Determine the Transformer Size based on the Maximum kVA LoadingTable.

Summer Transformer Demand = 25.9 kW

Winter Transformer Demand = 20.5 kW

For a 15 kVA Transformer:







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Example Adding a New Customer

Step 1 - Determine the Additional Load on the Transformer based onthe Diversity Method Tables.

New 2,200 square foot Home, Total Electric.

Summer Peak = 18 kW

Winter Peak = 21 kW

Step 2 - Add kW for the New Customer to the Total Connected Load of

the Existing Customers


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Step 2 - Add kW for the New Customer to the Total Connected Load ofthe Existing Customers

Summer Winter

kWh kW Demand kWh kW Demand

Customer #1 2,224 11.57 853 7.47

Customer #2 2,734 13.16 1,274 9.97

Customer #3 1,849 10.29 1,283 10.27

New Customer - - - 18 - - - 21

Totals 53.0 48.7


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Step 3 Add the kW Demands. Summer Demand = 53.0 kW

Winter Demand = 48.7 kW

Step 4 Determine the Coincidence Factor for the number of customers

from the table.

Number of

Customers

Coincidence

Factor

1 1.00

2 0.85

3 0.74

4 0.66

5 0.61


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Summer Winter

Coincidence

Factor

Total kW

Demand

Transformer

kW Demand

Coincidence

Factor

Total kW

Demand

Transformer

kW Demand

0.66 53.0 kW 35.0 kW 0.66 48.7 kW 32.1 kW



Step 4 Determine the Coincidence Factor for the number of customersfrom the table.

Coincidence Factor = 0.66

Step 5 Multiply the total kW Demand by the Coincidence Factor for

Summer and Winter loads. Summer Transformer Demand = 53.0 kW x 0.66 = 35.0 kW



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Step 6 The answer yields the peak 15-minute demand on thetransformer.

Summer Transformer Demand = 53.0 kW x 0.66 = 35.0 kW


Apply the two kW demands in step 5 to the Maximum kVA Loading table for

transformers.

Transformer

Size

Summer

140%

Winter

160%

Transformer

Size

Summer

140%

Winter

160%

10 14 16 50 70 80

15 21 24 75 105 120

25 35 40 100 140 160

37.5 53 60 167 234 267


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Square Footage Table Method

Very Useful in Sizing and Laying out Transformers in NewSubdivisions


Average Square Footage of Homes in the Section of the Subdivision

Number of Homes that are Total Electric and that have Gas Heat.

Number of Homes proposed to connect to each transformer location.


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Square Footage Table Method

Information from Tables The Number of Total Electric Customers.

The Number of Gas Heat Customers

The Number of Homes that will be connected at the Transformer

Location. Use the Table to Determine the Transformer Size for the particular

Location


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Example Square Footage Table Method

3 Homes - Gas Heat; between 1,500 and 1,800 square feet.

4 Homes - Total Electric; between 1,500 and 1,800 square feet.


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Example Square Footage Table Method

3 Homes; Gas Heat; go down from 3 Gas Customers

4 Homes; Total Electric; go right from 4 Electric Customers

Table recommends 50 kVA transformer.


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Diversity Method - Useful when sizing transformers fornew customer load.

Can be used with a variety of sizes of homes located within thesame subdivision.

Useful when there is a mix of Total Electric and Gas Heathomes.

Square Footage Table Method - Useful when sizingtransformers for new customer load. Can only be used when the sizes of homes to be served from

the service transformer are relatively the same size.

Useful when there is a mix of Total Electric and Gas Heathomes.


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Coincidence Factor Method - Useful when sizing

transformers for existing loads or adding new load to

existing loads. Can be used with only available customer billing information.

Easily accommodates the addition of new residential loads to

existing service transformer locations.


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Commercial Transformer Loading

Watts per Square Foot Method


Type of Business or Institution.

Total square footage of the facility.

Information from Tables

By using the Average Watts per Square Foot tables the average kW

Demand of the facility can be estimated.


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% of Connected Load Method

Information Required Type of Business or Institution.

Total connected electrical load of the facility.

Information from Tables By using the % of Connected Load tables the average kW Demand of

the facility can be estimated.


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Example Watts per Square Foot Method

Grocery Store

Winter = 10.1 watts per square foot

For structures with electric heat.

Summer = 10.4 watts per square foot Size of the Facility = 40,000 square feet

Average Demand:

Winter = 40,000 x 10.1 = 404,000 watts = 404 kW

Summer = 40,000 x 10.4 = 416,000 watts = 416 kW


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Example % of Connected Load Method

Grocery Store

Total Connected Load = 920 kW

Of the total 920 kW, 250 kW is Winter Heating.

Average Demand: Winter = 920 kW x 45% = 414 kW

Summer = (920 kW 250 kW) = 670 kW x 61% = 409 kW


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Comparison of Two Methods

Grocery Store


Winter = 404 kW

Summer = 416 kW % of Connected Load Method

Winter = 414 kW

Summer = 409 kW

The calculated Peak kW Demand for the Grocery Store should bearound 400 kW.

These calculations would then be used to determine the

Transformer Size.


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Application of Maximum kVA Loading on Three PhaseTransformers for Commercial Applications

Transformer

Size

Summer

120%

Winter

140%

Transformer

Size

Summer

120%

Winter

140%

150 180 190 750 900 1,050

225 270 315 1,000 1,200 1,400

300 360 420 1,500 1,800 2,100

500 600 700 2,500 3,000 3,500

% of Connected Load Method

Winter = 414 kW

Summer = 409 kW


Winter = 404 kW

Summer = 416 kW


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Determine the Transformer Size based on the Maximum kVALoading Table.

Maximum Average Transformer Demand Summer = 416 kW

Winter = 414 kW


Summer 120% Loading = 270 kW Winter 140% Loading = 315 kW








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Another Method Engineering Loading Data

Use the supplied electrical loading data supplied by the buildingelectrical plans supplied by the buildings engineer.

Information will include total connected and diversified load

information. This information and calculations used in the engineers

determination are based on National Electric Code load tables, not

on Utility practices and history.

Be Cautious Check the calculations for yourself to satisfy yourself

that you are installing the correct size transformer so serve the load.


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TOTAL CONNECTED ELECTRICAL LOAD

DESCRIPTIONAMPS AT 277/480, 3 PHASE, 4 WIRE

KVAL-1 L-2 L-3

INTERIOR LIGHTS 4.0 8.0 4.0 4.0

EXTERNAL LIGHTS 4.0 - - 1.0

RECEPTACLES 16.0 16.0 12.0 12.0

AHU FAN & UNIT HEATER FANS 2.0 2.0 4.0 2.0

EXHAUST FANS 6.0 1.0 2.0 3.0

ELECTRIC HEAT - 17.0 17.0 10.0

* COOLING 9.0 - 9.0 5.0 *

WATER HEATERS 10.0 10.0 - 5.0

SHOP EQUIPMENT 7.0 7.0 7.0 6.0

MISCELLANEOUS 2.0 - 2.0 1.0

TOTAL 51.0 61.0 48.0 44.0

* NOT CALCULATED IN TOTAL


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School Maintenance Building

44 kVA Connected Load

3,000 Square Foot Facility

Key Electrical Components Derated for Average Demand: Receptacles: 12 kVA derate to 5% = 0.6 kVA Lighting: 5 kVA derate to 85% = 4.25 kVA Water Heating: 5 kVA derate to 25% = 1.25 kVA Fans: 5 kVA derate to 40% = 2 kVA Heating: 10 kVA derate to 80% = 8 kVA Shop Equipment & Misc: 7 kVA derate to 35% = 2.45 kVA

Average Demand = 18.55 kVA

I d t i l / L C i l


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Industrial / Large Commercial

Transformer Loading Engineered Loading Data

Panel Schedules

Actual Diversified Load Tables

Watts per Square Foot Tables

Usage History from Other Electric Utilities (Other Cities, Co-Op, Investor Owned, and other similar facilities on the same system)

I d t i l / L C i l


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Transformer Loading

I d t i l / L C i l


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Transformer Loading Areas of Consideration in Sizing Transformers for High

Load Factor Customer

High load factor customers use a large amount of energy (kWh)

for extended periods of time.

24-hours per day, 7-days per week department store is high load

factor usage as compared 10-hour per day, 6-days per week

local merchant which has a lower load factor.



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Load Factor Customer (continued)

Due to the extended energy usage service transformers do not

have a chance to cool down from operating at their nameplate

rating.

Normal distribution class transformers are not designed to

operate at their nameplate rating for extended periods of time.



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Load Factor Customer (continued)

To account for the high load factor usage transformers to serve

this class of customer need to be oversized by 20% to 30% or

specified as a substation class transformer.

Substation class transformers can operate at their nameplate

rating for extended periods without excess heating of the core.

Delta Secondary


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Delta Secondary

(High-Leg) Transformer SizingTwo-Transformer vs. Three-Transformer Bank Sizing

Two-Transformer Banks

Two-Transformers Banks work well for Single Phase Loads and

relatively small Three Phase Loads.

Two-Transformer Banks are more stable than Three-Transformer

Banks because the Neutral on the Primary Side of the bank isGrounded.

Three-Transformer Banks

Three-Transformer Banks are well suited for Single Phase Loads

and Large Three Phase Loads.

Delta Secondary


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Delta Secondary

(High-Leg) Transformer Sizing


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Secondary Conductor Sizing

Example 4/0 Aluminum Service Drop; 150 feet in length; 25 kVA of

load. kVA x Feet = 3,750


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Future Webinar Discussion

Future Webinar Topics

Future Webinar Frequency

Webinar Instructors

2013 El t iCiti


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2013 ElectriCities

Electric Utility

Webinar Series

Sizing of

Transformers

Calculationof Loads

transformer sizing presentation.pdf

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