The transformer is not made to consume power. Anything that consumes power is rated at KW whereas transformer and generator are made to provide power to the consumer therefore it is rated in KVA.
Now, you may argue that the transformer also consumes power. Yes, it does, but it consumes power in form of the losses in the winding and the core.
There are two types of losses in a transformer:
1. Copper Losses
2. Iron Losses or Core Losses or Insulation Losses
Copper losses ( I²R) depend on the current which passes through transformer winding while Iron Losses or Core Losses or Insulation Losses depend on Voltage. However, the losses do not depend on the power factor.
Hence, to determine the complete power consumption of the transformer we need both voltage and current but not the angle between them.
That’s why the Transformer Rating is expressed in kVA, Not in kW.
