Answer / princealekota

Since transformer is a static there will not be any
rotational losses it have only iron loss depend upon
voltage and copper loss depend upon current

for AC supply systems like generator, transformer rating is
done in KVA because of inductance present in AC . we don't
know load is resitive or inductive

so we will take load KW=KVA,

power = voltage*current*cos(pi)
if load isinductive cos(pi) have some value less than 1
if load is resistive cos(pi)is 1

Answer / karthik

Basically any rotating machine depends up on power factor
and other rotational losses. Transformer is not a rotating
machine and transformer losses depends on only Iron losses &
Copper losses. Iron losses depends on voltage and Copper
losses depends on current. these losses not depends on power
factor. So Transformer rating measured in KVA.

Answer / purushotham

In transfromer the core loss depends on volatage and cu
loss depnds on current so the both are voltage and current
(VA) so the tranformer rating is in Kva.

Answer / pradyumna chandra panda

Why the motors are rated in KW and transformers are rated in
KVA?
Ans . Motor is a consuming device it consume the power
(wt) so it is rated in KW. But the
Transformer is a static device which transfers
the power (Apparent power) from the
Generating point to the user. Apparent power is
the product of voltage and current
I.e. Increase voltage with decrease current and
decrease voltage with increase
current. Power factor of the load does not
affect apparent power it affect the Active
Power.
Active power is the real power which the transformer
gives. Transformer should not be rated on the base of
Active power (KW) which is variable because the power factor
of the load is not knows before which to be connected across
the transformer.

Reactive power Apparent power (k)
KVA
VI sin Q k
Kw







Q q


Active power VI cos Q
Active power is equal to apparent power at resistivity load
where power factor is unit at cos Q is equal to zero degree.
Active power is decrease power where cos Q (not fi) is
increase
Therefore in simple we can say due to unknown power factor
transformer and generator are rated in KVA.
Hint – we know cos FI is the value of cos Q i.e. apparent
power/ active power

Answer / s saranraj(sri durgadevi polyt

Both the losses of the transformer is independent of the
power factor (cos phi), which means a transformer would not
make any change in the power factor, since both the losses
viz copper loss(depends on current) and iron loss(depends
on voltage) are independent of power factor, that is why a
Transformers rating is not on kW, but on KVA
(or)
because,ironloss is depends in voltage and cu_loss is
depends in current.so the transformer ratings in kva

Answer / vignesh

Thumb Rule - all AC power sources - Generators,
Transformers, UPS etc. are rated in KVA and all loads are
rated in Watt or Kilowatt..
AND hence there is no need of power factor.

KVA=V X A (FRO SINGLE PHASE TRANSFORMER)
KVA = 1.732 X V XA (FOR THREE PHASE TRANSFORMER)

Answer / m. jamshed

As transformers are the power source and every power source
is deigned for specifid value of current and voltage.

The amount of current in A/C circuits is greatly affected
by p.f.as if we take two examples for the same kw with
different p.f we will get different amount of current that
is high current for low p.f and vice versa.

For the transformers we do not know about the load p.f,so
it will be rated in kva to determined the exact value of
current it can provide.

Answer / manikanta..{munna} geethanjali

load is something which dissipates energy..!! in case of
transformer , ideally a transformer is power source
device...and but this acts as load practically bcoz of
losses in it...!!!

coming 2 d rating any load is alwayz rated in kw..!! here
in a transformer the losses are due 2 copper and iron which
depend on current and voltage respectively...there fore v
rate the transformer in v*i !!!! and they is no use of
including powerfactor.

Answer / aryan

Since transformer is a static there will not be any
rotational losses it have only iron loss depend upon
voltage and copper loss depend upon current

for AC supply systems like generator, transformer rating is
done in KVA because of inductance present in AC . we don't
know load is resitive or inductive

it is independent there fore it is measured in kva

Answer / hitesh g. dobariya

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.

on 22.03.2015 at 17:15hrs Y phase 66kV class Power Trf-2 Current transformer has flashed over and oil came out. The failed CT has been replaced on 24.03.2015 at 13:15hrs and the Trf has been charged in presence of RT Hassan after conducting the pre-commissioning tests. On the same day at 17:00hrs the commissioned CT has again flashed over and immediately the Trf  has been isolated from power supply. It is confirmed the secondary is not opened and checked the earth integrity and is found ok. Here what would be the reason for the CT failure? 

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