Why transformer rating is shown in KVA?

Transformers are rated in {VA, kVA, MVA etc.} due to flows of active and reactive power through transformer. In case of transformer we have active power losses as consequence of existence inside resistance of windings (primary and secondary) and existence of active losses of ferromagnetic core and other side we have reactive power losses as consequence of existence losses of magnetic flux (primary and secondary) and existence of reactive power losses of ferromagnetic core...

[VA]=sqrt(sqr[W]+sqr[VAr])

transformer is rated in kVA by the manufacturer to inform users about the maximum power (voltage and current) that support it, the reason for not rating it in KW is that the active power (kW) is depend on the loads (lignting, machines..)

Not necessarily shown in kVA, always show their apparent power, where their units are Volt-Ampere (VA), depends on the magnitude of it if you are in kilos (k), megabytes (M), etc.. But it is not relevant. We indicate how big is the load at which the transformer is connected. Greetings.

As kVA=1.732 Vx I ,where V is fixed current carried by conductor(winding) is proportional to kVA. As thermal limit corresponds I, kVA is selected for rating

Transformer is neither source nor load. It is basically is a transformation medium. it's rating is not affected by the power factor of the load or the source, it is supplying.
Rather, it's rating is affected by magnitude of voltage and current supplied by the transformer.

Transformer nameplate rating indicate the KVA, Primary Voltage,Secondary Voltage, Impedance, Taps, Insulation, Winding Connection, Polarity, Etc....

Why it is shown in KVA? Why KVA? Because KVA is the capacity of the transformer, it is the most predominantly used identification of the transformer. Same analogy with a storage Tank. The first question they ask is how many gallons? or how many liters? Gallons or liters is the capacity of a Tank.

As opposed to kW - because the transformers "cares" only about the magnitude of the current without regard to its phase angle - the current generates heat in the windings as "I square x R" (R of the windings).

The transformer does not have a definitive power factor hence should not be identified in KW capacity, it will be a misnomer. The resultant load the transformer serves determines the resultant power factor. Thus the transformer capacity shall be identified / labeled in KVA. not KW

Dear, not only to Transformers, it is recommended to indicate the KVA rating also for all Electrical Equipment since the KVA is the appearant Power and remains constant from which Active & reactive powers can be extracted & are constitute the KVA's two components. The values of these two components (Active/reactive power) depends on the nature of the load i.e the value of the power factor of the connected loads - whether are resistive, inductive, capacitive or in combination -

KW = KVA Cos (Fi).

KVA rating remains constant regardless to the nature of the connected load, while KW as well as the Power factor varies as the case may be. Hence KVA is used as the decleared rating for the Electrical equipment. Loads can be defined in KW.

For DG sets kW rating should match that of the driving engine,so they specify kVA @ 0.8pf..

Yes correct .. electrical equipment is basically expresed in kVA.. (That defines the size independent on cos phi).
When it is transformed into mechanical power (e.g. rotating) then it is (of course always) expressed in kW(m)

KVA is the unit for apparent power.
Apparent power comprise active and reactive power
It is often helpful to think of an extreme example: Imagine a use case where the only and exclusive load is a static var compensator .Would the load then be zero because the active power is zero? Most certainly not.

Most electrical equipment with windings like generators and transformers have electrical design parameters which are based on the voltage-insulating capacity and current-carrying capacity of their windings. Regardless of the magnitude of the real power (W) and reactive power (VAr) that are being carried or delivered, the limiting factor will always be the voltage-insulating capacity (V) and current-carrying capacity of their windings. Thus, it is more logical to rate electrical equipment with windings in VA, kVA or MVA.
Electric motors are an exception because motor design parameters are based on the real power (hp, W, or kW) output at a given rotational speed or rpm of the motor shaft.

Dear ;

First, the power of all Sources " Transformers, Generators, UPS " is defined by the value of the " Apparent Power " where the unit is " VA, or kVA, or MVA ", and the main reasons to use the Apparent Power are :
1- The Power Factor value is an important specification of any load, and this value is not the same for all loads.
2- When the Source’s Power is defined by kW, we should define also the relative Power Factor, to be able to calculate and know if this source can supply load or loads have different Power Factors values.
So, The “ Apparent Power ” is like a general value.

By the way, an important point should be taken into consideration, that :
- Only the Apparent Power of any Transformer should be defined, but for " Generators & UPS " the manufactures define the value of the Apparent Power with a relative " Power Factor " value, that help us to calculate the maximum value of Active Power that can be supplied by these sources.