Question

Why is the transformer rated in kVA, not in KW?

Answer 1

Hint: Iron loss on voltage and copper loss of a transformer depends on current. Hence, total transformer heat loss depends on volt-ampere (VA) and independent of phase angle among voltage and current i.e., it is independent of power factor. That is why the rating of the transformer is in kVA and not in kW.

Complete step-by-step solution:
Transformers only transfer the energy from one circuit to another without altering the value of power and frequency. In different words, it can only step up or step down the amount of current and voltage while the energy and frequency would remain the same. An available date on the transformer nameplate is marked for further details, such as rating in VA, single-phase / three-phase (power or configuration transformer), step up / step down, connection, etc. When manufacturers invent a transformer, they have no idea which load will be joined to the Transformer. The load may be resistive (R), inductive (L), capacitive (C), or mixed load (R, L, and C). It means, there would be different power factors at the secondary (load) side on different kinds of connected loads depending on R, L, and C. This way, they go for VA instead of W in the case of Transformer.
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.

Note:Now the temperature rises because of the losses. The copper losses, which vary with the load depend on the square of the current, and slight core loss, which depends on practically constant supply voltage, is also present. The loss will be the same as long as the current magnitude is identical irrespective of the power factor of the load current.