Question: Why transformer rating is expressed in kVA instead of kW or kVAR?
Answer: At the time of manufacturing, the nature of load to be connected is not known. (i.e it is resistive, reactive, R, RL, RC, RLC or of any other type). Therefore it is beneficial to express power in terms of voltage and amps. This V & A represents the overall power delivered to any sort of load.
The power supplied to a resistive load is the product of voltage times current flowing through it.
In case of ac loads, the case is not same. Here the load is the sum of resistance and reactance. The power delivered to an AC load is neither real nor reactive, it is the apparent power which is expressed in terms of kVA.
Furthermore, two types of losses exist in Transformers:
- Copper loess → They depend on current
- Iron losses → They depend on the voltage
Since these losses depend on V & I only, a transformer is rated in terms of VA.