actually rating of the machine (kva or kw) depends upon the
power factor since the load power factor to which
transformer is supplying power is not known , it may be
capacitive ,inductive , or resistive that is why its rating
is in kva and not in kw
The unit of voltage in Volts
The unit of current in Amperes
When the energy transfer from primary ciruit to secondary
circuit power is constant. there is no involvement of power
factor. Due to this transformer rating in KVA not in KW
As we know Cu loss depends on current & iron loss depends
on voltage and not affected by power fector hence always
transformer ratting defined in KVA not in Kw.
Since Transformer is a Static device and it is having two
losses namely core loss and Cu loss. The core loss depends
on the Voltage (V) only, where Cu loss depends on Current
(I) only.So for this purpose the transformer always called
its rating in apparent power only(VA).
Because transformenr is used for converting voltage level
only and nothing therefore its rating is in KVA i.e. KV x
Amp It gives us rating/capacity of transformer how power it
can handle.
The equipments not meant for consuming power are rated in
KVA units & those consuming power are rated in KW.
The instruments which converts energy from one form to
another, rated in KW like motor etc which keeps same form
of energy will be rated in KVA like Transformer,stabilizer
etc
The leading/lagging power factor has
magnetizing/demagnetizing effect on armature reaction in a
rotating electrical machine. Hence the losses depends on
the load power factor. Unity PF has neither magnetizing nor
de-magnetizing effect.
Whereas in a transformer, the power factor doesn't play any
role in machine losses i.e. Cu & core losses are
independent of power factor.
Hence the rating of transformer is always given in KVA and
the rating of generators is given in kW at certain pf i.e.
usually at 0.8 lag.
bcoz ..cu loss depends the current.iron loss depends the
voltage..so that total loss of transformer is depends
voltage,current...so the transformer ratings in KVA..
Transformer is a static device having two type of losses
such as ironlossed and cupper losess. since iron losses
depends upon voltage(V) and cupper lossed depends upon on
the curret(I) so rating of Transformer is in KVA
because trasformer is connected to supply resistive lode
inductive lode &capaitive load .the power factor for those
load is different (lag or leding )it worked apperent power.
the unit of apperent power is volt-aamper.that why
transformer rated in kva.
(its my knowledge)
The X-mer o/p is limited by heating and hence by the losses
in the X-mer. There are two kinds of losses in the X-mer:
copper losses (ohmic losses) and iron losses. ohmic losses
(I*I*R) depend upon current while core losses depend upon X-
mer voltage and are almost unaffected by X-mer P.F.. Hence
the rated o/p is expressed in VA (V*I) or in KVA and not in
KW.
The copper loss of a Transformer depends on current i.e.
I^2 R and the iron loss depends on voltage. Hence the total
losses depends on volt-ampere (VA) and not on phase angle
between voltage & current.
Therefor the losses in a transformer are independent of
power factor which is cosine of angle between voltage &
current.
That's why the rating of transformers is in KVA and not in
kW.
Now coming to Efficiency of a transformer: Yes it depends
on power factor of load. Because the
Efficiency = Output/Input
= (Input - Losses) / Input
= 1 - (Losses / Input)
= 1 - [Losses / (Vs * Is * Cos phi + Losses)]
Efficiency is inverted V curve (Y-scale) and the peak of
curve appears between 50-60% load (X-scale)for various load
power factors. Efficiency increases as the power factor
increases.
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.
we know
output=Input-Losess. 1
and the losess in transformer are two types ohmic, core
losess and these are depends on the Voltage and secondary
current , independent of P.F.
so the rating of losess is in VI not VICOSPIE.From equation
1 losess in KVA so rating of T.F also in KVA.
Eg.if any machine connected at noload, the output put is
zero, because of no current from below equation.
VICOSPIE.
But in T.F there will be some power cosumption inform of
loss.i.e. core loss.
because Transformer has Two winding.It becomes very hot
while in running condition.So a considerable amount of heat
loss occured.So the loss in Kw may vary.So Kw rating is
avoided and Kva rating is given in Transformer nameplate.
Rating of the equipment connected to T/F depends upon the
power factor since the load power factor to which
T/F is supplying power is not known , it may be any type
of load (capacitive ,resistive , or inductive) that is why
its rating is mentioned in KVA
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.
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.
pls tell about gas insulated substation?which gas is used
for that substation?above which rating we are used this
substation?how we take maintenance in that substation?it
means substation fully shutdown or live condition?if live
condition means how?pls clear this things
along with your userid / name. ThanQ
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