why transformer or generator rated in kva and not in kvar?

Answer 1

Because for a given transformer, its kVA rating will be a constant, but it's kVAR (as well, its kW) rating will depend on the phase angle that the load causes.

For example, let's say you have a 1kVA transformer. If the load is purely resistive, the voltage and current will be in phase, and you will be able to apply load up to 1kW, but in such a case, you will draw 0kVAR even tho the transformer is loaded 100%. For a completely inductive or capacitive load, you may draw 1kVAR and again the transformer is 100% loaded but the load is 0kW.

And let's say your load was half inductive and half resistive. The phase angle between voltage and current would be 45 degrees, and in that case, although you still may load it with 1kVA, that 1kVA would be split equally between kW and kVA (0.707 of each, I think??? Could be wrong, I'll look it up later).

Look at it this way: Ignoring the core, the allowable current thru a transformer is a function of the cross-sectional area of the conductor whereas the allowable voltage is a function of the insulation between the windings. The phase angle has nothing to do with either, therefore the kVA is the same, no matter whether the load is inductive, capacitive, or resistive.

Answer 2

KVAR (KiloVoltAmpsReactive) is a measure of quadrature current, not power.

HTH ☺

Doug