Transformers are generally measured in KVA because it has
got nothing to do with the power-phase voltage. Its
measured against the angle between Voltage-current. And
hence KVA.

the losses in the transformer viz., core and copper losses
depend upon voltage and current respectively. as a
machine's rating is framed depending upon the heating
inturn on the losses, the transformer rating is always
specified in terms of voltage and current i.e., in terms of
kva rating.
moreover a transformer has to take any kind of load
irrespective of the power factor. it is not economical to
design a transformer to carry only one kind of load.

hence due to tje above two reasons the rating of a
transformer is given in terms of its kva rating.

KVA mean Voltage x Current
and
KW mean Voltage x Current x Power factor
The only difference is power factor
Power factor depends upon load and its value varies from
0.5 to 1

As kw is depend upon the power factor , but kva does not
depends upon the power factor mean kva of any transformer
or generator does not change with power factor . SO to know
the correct power of transformer ,we use kva as it nothing
to do with load type or power factor value

Any AC electrical equipment that has conductors have to be rated in voltage and amps, so it is KVA if more than thousand. WHY.
SIMPLE . If more than the designed amps flows through the equipment it will over heat and burn.(destroyed)
IF a 500 kw transformer is loaded at 415 v at unity pf to the max it will carry 500000/415x1.732 = 695 amps
if the same transformer is loaded at 500kw at 415 v but at 0.5 pf it will carry 500000/415x1.732x0.5 = 1391 amps
can the transformer winding take this amount of current.You see on both occasions the load was 500 kw but the pf was different and the amps was double.What will happen if the transformer is loaded at a further lower pf. should the transformer be still rated as 500kw

In transmission line the voltage value measured as volt and
current value measured as amphere and also the indicated
in kilo only .then only we measure in KVA

The Losees in the transformer are mainly 1.Core Losses
2.Copper Losses.
As we are well aware,the core losses depends on Volage and
Copper Losses on Current.

KVA means Product of Volatge & Current.
Thts what The rating of Transformer in KVA but not in KW.