In a Transformer, iron loss on voltage and copper loss of transformer depends on current. Hence, total transformer heat loss depends on volt-ampere (VA) and is independent of phase angle among voltage and current i.e., it is independent of power factor. Any transformer introduces core losses and copper losses. Core losses are dependent on the input voltage. Copper losses are dependent on the current running through the winding.
Hence, total losses depend on voltage in joining to current but not upon power factor. Thus, the rating of the transformer is done in kVA, not in kW.
The second reason is when the transformer is designed the manufacturer does not know which type of load will be connected in the future. And the power factor depends upon the load. Since kVA is fixed and kW depends on the power factor.
