Fundamental of Trans

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POWER TRANSFORMERS AND SHUNTREACTORS PRINCIPLES

AND CONCEPT


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A transformer is an electromagneticdevice used for transfer of energy

from one circuit to another withmagnetic field as coupling device.

The energy is always transferred at thesame frequency but usually withchanges in voltage and current


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There are two types of transformers on

the basis of construction of winding andcore

1) Core type

2) Shell type

In core type the windings surround the core.

In shell type the core surrounds the windings


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Core of the shell type transformer


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Electromotive force (emf) in a transformer

Faradays law states that

1) Whenever there is a change in magnetic field with respectto coil a voltage is induced

2) The magnitude of induced emf in the coil

by electomagnetic induction is directly proportional to the

time rate of change of flux linkage

E = N * (d / dt)

Where N= No of turns in the coil

and

(d / dt ) =Rate of change of flux


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Inductance of a transformer coil

a r N

L= -------------------l

Where,

a = area of cross- section of the coil

- 7 = Absolute permeability =4 * * 10 Henry/m

r = Relative permeability

500 1000 for transformer core depending on flux

density

1 for all other space and all non -magnetic material


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Emf equation in a transformer

Let,

N1 = No. of turns in primary

N2 = No. of turns in secondarym = Maximum flux in the core (Webers)

f = Frequency of A.C.input.

As shown in Fig.1,flux increases from zero valueto maximum value ( m ) in one quarter of acycle i.e. 1/4* f

m

Average rate of change of flux= --------- = 4 f m

1/4f


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Average emf / turn = 4 * f * m

R.M.S. Value

------------------ = 1.1Average Value

RMS emf = 1.1 *4 * f * m

= 4.4 * f * m

If there are N1 turns in primary then

E1= 4.4 * N1* f * m

For Secondary,

E2 = 4.4 * N2* f * m

Thus ,

E1/E2 = N1/N2


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Losses in transformer

Losses in transformer

1)No load losses2) Full load losses

No load losses comprise the

i hysteresis lossesii eddy current losses

iii copper losses

When a.c. is applied to the coil a magnetizing current flows in the

coil and hence a magnetizing force H is applied to the magnetic fieldWhen flux density (B) is plotted against (H) a hysteris loop is formed


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Copper losses on no load is very small

since it is dependent on the magnetizing current.however at higher load the copper losses in the

winding increases and depends on the square of

the current

2

Losses = I *R

Thus the only increase in losses from no load to

full load is the copper loss


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/

R2 =R2 * (N1/N2)

X2 = X2 * (N1/N2)

/Total Res. = R1+R2

/

Total Ind = X1+ X2


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,


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low


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Shunt ReactorsWhy a shunt reactor is required in a power system?

When a line is switched on and is on no- load thereceiving end voltage is higher than sending end .

This phenomena is known as Ferranti effect

This affects the insulation of connected equipmentparticularly the transformer. In order to keep the

receiving end voltage within a specified value whenthe line is on no-load or light load the shunt reactorsare connected

Shunt reactors differ from transformer since it usesone winding per phase and that magnetic circuit has

a gap. The magnetic energy is kept in the reactorAir gap which separates the individual ironpackets.The air gaps are made of stiff material likeceramic.


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420 kV class shunt reactors (major Technical param.)

50 MVAR,63 MVAR,80 MVAR and 125 MVAR

System fault level : 40 KAConnection : star with neutral brought

Insulation level:

a) Lightning impulse(1.2 /50 sec) : 1300 kVp

b) Switching surge impulse (20/500 sec) : 1050 kVp

c) Maximum temperature rise at rated load : 50 deg C

d) Neutral Insulation level

Impulse withstand Voltage : 550 kVp

Power frequency voltage : 230 kVp


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Fundamental of Trans