Basics of Transformer Ratings

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Transformers are rated using several methods. Two common ratings of transformers include the:
(a) KVA rating
(b) Impedance rating

KVA Rating
KVA ratings of transformers are obtained by simply multiplying the current times the voltage. The result is a rating in VA or volt amperes. Small transformers are rated in VA. As size increases, the ratings are adjusted accordingly to KVA (kilovolt amperes) or MVA (megavolt amperes)
1KVA = 1,000VA or volt amperes
1MVA = 1,000,000VA
Power transformers are defined as transformers rated 500 kVA and larger. Transformers smaller than 500 kVA are generally called distribution transformers.


Impedance Rating
The impedance of a transformer is the total opposition offered to an alternating current. When referring to impedance of a transformer, it is the equivalent impedance that is meant. To determine the equivalent impedance of a transformer, one of the windings, usually the low voltage winding is short circuited while just enough voltage is applied to the other winding to create full load current in the short circuited winding. This voltage is known as the impedance voltage.

Impedance rating is a characteristic of large power and distribution transformers. The transformer impedance value is often given in percent on the nameplate
It means that the voltage drop due to the impedance is expressed as a percent of rated voltage.
For example, suppose the impedance rating of a 2400V/240V transformer with full load current of 90 amps is given as 3%. 
Voltage drop due to impedance with low voltage winding short circuited 
= 0.03 X 2400 = 72volts
This means there would be a 72-volt drop in the high-voltage winding at full load due to losses in the windings and core. Only 1 or 2% of the losses are due to the core; about 98% are due to the winding impedance.


We can also calculate the actual impedance value on the high voltage side in ohms:

Where:
Z = impedance in ohms
V = impedance voltage drop
I =  Rated full load current in the primary of transformer

Now V = 72V, I = 90Amp
Therefore Z = 72/90 = 0.8Ω