The rating of transformer is given in terms of its kVA, HV/LV voltage and phase rotation (for three phase transformer). To calculate the rating of the transformer for both single phase and three phase, the load required to be fed and voltage at which load is to be fed must be known or should be calculated. In addition to this, the voltage of source to should also be known. The power factor of load may be assumed to be 0.85.
Suppose the load requirement of a utility to be fed is 85 kW, then the kVA rating of transformer should be (85/0.85 = 100). Again, if the source voltage is 11 kV and load is to be fed by 415 V, then the rating of transformer will be 100 kVA, 11 kV / 451 V.
But if we know the voltage and corresponding current in HV / LV side of Transformer, then we can calculate the rating of transformer by using the simple formulas as below:
For single phase transformer: VA Rating = V x I
For three phase transformer: VA Rating = √3V x I
But, we know, the ratings of the transformers are always done in kVA, and both the above formulas are not expressed in kVA. So, we can write both the equations as:
Rating of single phase transformer in kVA:
= ( V X I ) / 1000 ………..(1)
Rating of three phase transformer in kVA:
= (√3V X I ) / 1000 ……..(2)
The above calculation of transformer rating may also be verified by looking at the name-plate of the transformer, where the values of different parameters are given. For example,
A nameplate of three phase transformer has the following parameters:
Transformer rating = 100 kV
Primary voltage or H.V.= 11 kV Primary current= 5.25 A
Secondary voltage or L.V.= 415 volts Secondary current= 139 A
(It must be noted that a name plate have many more parameters like phase rotation, tap positions with voltage etc.)
Now, if we calculate the rating by using first two equations, we get
kVA rating = 11 x 5.25 = 57.75 kVA ; for HV side
kVA Rating = 415 x 139 = 57.6 kVA ; for LV side
But, the rated value of transformer kVA given in the nameplate was 100 kVA. This means our calculation is wrong somewhere, and that is; we took into consideration the formula of single phase transformer and not of three phase transformer. So now, if we calculate by using the formula of eq. (2), we get
kVA rating = √3 x 11 x 5.25 = 100 kVA for HV side.
kVA rating = √3 x 415 x139= 99.99 kVA for LV side.
It can be seen that, kVA rating as calculated for HV and LV side are equal. This is as per the working principle of Transformer. Thus, the correct value of the required kVA is obtained. The formula for kVA rating of single phase transformer should be used when we are dealing with single phase transformer. This way, we can calculate the rating of the transformer.