Friday, 30 June 2023

Sizing of Transformer Capacity

Sizing of Transformer capacity is based on the quantity and type of the load.


Transformer

TRANSFORMER

 Step 1:

  • Calculate the total connected load from the given load list.
  • Add 10% or Whatever the requirement in percentage of load will add up in future, with the total connected load.
  • Calculate maximum Demand based on the demand factor (Individually or Totally).
  • Convert calculated maximum demand in KW to KVA at given power factor
  • Apply de-rating factor for transformer (90% Efficiency) - Calculate peak demand in KVA with consideration of De-rating factor.
  • Consider Maximum Demand calculated by above procedure as Solution - A


Example : Power Factor – 0.89, Overall Demand Factor – 0.9, Motor Efficiency – 90%, Star-Delta Starter – 0.87 Pf, Tr Derating Factor – 85%


S.No

Name of the load

Load in (KW)

Demand Factor

1

Lighting

200

0.7

2

HVAC

750

0.7

3

Plumbing

150

0.4

4

Lift

60

0.8

5

Fire Fighting

15

 

6

External Lighting

25

0.3

7

Others

65

0.3

 

Total

1265

 












Total Connected Load as per the given list = 1265 KW

Future consideration of 15%                       = 1265 + 15%

                                                                    = 1454.75 KW

Total Maximum Demand (in KW)                = 1454.75 x 0.9

                                                                    = 1309.28 KW

Total Maximum Demand (in KVA)               = 1309.28 / Power Factor

                                                                    =  1309.28 / 0.89

                                                                    = 1471.10 KVA

Maximum Demand after Derating factor     = 1471.10 / 0.85

                                                                    = 1730.70 KVA

Consider the above Demand 1730.70 as Solution - A

Step 2:


  • Check the largest capacity of the motor to be started.
  • Calculate the full load (in KVA) at given power factor and efficiency
  • Calculate the starting current of the motor.
         1. If its starting method is through DOL Starter then starting current of motor will be 600% of the full load current or 6 times of its capacity (we can use DOL starter if the load is below 7.5 HP).

         2. If its starting method is through star-delta Starter then starting current of the motor will be 300% of the full load current or 3 times of its capacity (we can use star-delta starter if the load is above 7.5 HP).

  • To arrive the base load of the transformer, subtract the actual KVA of motor with the peak demand.

  • To arrive the load on transformer during starting of motor, add the base load of transformer with starting KVA of motor
  • Refer the standards for over load factor of transformer to arrive rated continuous capacity of transformer capacity to meet starting rating. Compare the value with rated continuous capacity with rated factor.

From the Same Example :


Highest capacity motor is 280 KW

Load in KVA                                 = 280 / 0.87

                                                    = 321.84 KVA

Load in KVA with 90% Efficiency = 321.84 / 0.90

                                                     = 357.6 KVA

Motor is started using Star-Delta Starter. Starting current will be 300 % of Full-load current or 3 times of full-load

Full load of motor                          =357.6 x 3 

                                                      = 1072.8 KVA

Base Load on Transformer (without largest motor) = Total maximum demand with motor – Motor Full (in KVA)

                                                = 1471.10 - 357.6 KVA

                                                = 1113.5

Load on Transformer during starting time of motor = base load + load of largest motor during starting

                                                = 1113.5 + 1072.8

                                                = 2186.3 KVA

Overload factor of the transformer to meet starting duty is 1.5

Maximum Demand (with starting duty consideration) = 2186.3 / 1.5

                                                                                    = 1457.5 KVA

Consider it as solution B

When compare solution A with B, Solution A is higher. So we take solution A which is 1730.70 KVA.

TRANSFORMER SIZE AVAILABLE IN MARKET (KVA)

25

63

100

150

200

250

315

400

500

630

800

1250

1600

2000

2500

 

 

 

 

 



      



Select the nearest capacity of the transformer capacity which should be higher than the Solution A. 

From the table, We can select 2000KVA Transformer.

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