Answer Posted / krishnendran

kVA is the unit for apparent power. Apparent power consists
of active and reactive power. Active power is the share of
the apparent power which transmits energy from the source
(generator) to the user. Reactive power is the share of the
apparent power which represents a useless oscillation of
energy from the source to the user and back again. It
occurs when on account of some »inertia« in the system
there is a phase shift between voltage and current. This
means that the current does not change polarity synchronous
with the voltage. But the heat generated in a winding as
well as the eddy current losses generated in a transformer
core depend on the current only, regardless of whether it
aligns with the voltage or not. Therefore the heat is
always proportional to the square of the current amplitude,
irrespective of the phase angle (the shift between voltage
and current). So a transformer has to be rated (and
selected) by apparent power.

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