Transformer Unit 1.pdf

8/10/2019 Transformer Unit 1.pdf

http://slidepdf.com/reader/full/transformer-unit-1pdf 1/27

‐

Dr. Prabodh BajpaiAssociate Professor

Indian Institute of Technology Kharagpur

1



M2 ‐ Transformer• U1‐ Ideal single phase transformer

• Ana ysis o i ea trans ormer • No load phasor diagram

• Equivalent circuit of an ideal transformer • ‐

• Taking finite permeability of core into account • Takin core loss into account • Taking winding resistances and leakage flux into

account • Equiva ent circuit o an practica trans ormer



• U1- Ideal single phase transformer

3



What is Transformer ? • An electromagnetic device used for transfer of

with a magnetic field as coupling device.

– Based on

construction y , y

– Based on connection

,

,

– Based on application, , , ,

matching



Why do we need transformers?

• Increase voltage of generator’s output – Transmit hi h ower at low current – Reduce cost of transmission system

• Create electrical isolation

• Match load impedance• Filters



Transformers are integral part of any modern power system

DistributionDistribution

220kV ‐750kV

rans ormerrans ormer

TransmissionTransmission15 kV ‐ 25kV TransformerTransformer

‐erv ceerv ceTransformerTransformer

6





Transmission Transformer

8



Distribution Transformer

9



10



Service Transformer

11



Low power Transformer

12



Principle of operation • A transformer in its simplest form will consist of a rectangular

laminated magnetic structure on which two coils of different .

• The winding to which A.C. voltage is impressed is called the

load is connected is called the secondary of the transformer .



Steel sheet laminations are used as core materials

14

ore type e type



Ideal Transformer

• Primary and secondary windings has no resistance. – No resistive voltage drop, no resistive heat loss

• All the flux produced by the primary links the secondary winding – No leakage flux, i.e. coefficient of coupling unity and – no reactive voltage drop in winding.

• Permeability μ r of the core is infinitely large. – Very small (or zero) current (or mmf) required to establish flux in the

core.

neglected. – Power transfer efficiency is 100%



Core flux gets fixed by voltage & frequency

• we app y a vo age a requency , ow much current will be drawn by the coil

• .

f AN B

44.4max

• Corresponding to this Bmax obtain the value of H from the B‐H characteristic

• the required value of the current l H I max

max



Analysis of ideal transformer

• A sinusoidal voltage v 1 in primary winding causes ac current producing

• At no load, very small primary current

flux ɸ in time phase with current •

drawn and • ’

11 eV , ,

primary e 1 as well as secondary coils

e 2 proportional to N1 and N2

, load current i 2 will produce

flux in direction opposite respec ve y o mu ua ux.

• Demagnetizing AT of

secon ary are

us

near y

neutralized by the increase in primary AT

1 1 1 2 2 2 I N F I N F

17



Flux is common for both the primary and secondary coils, it is

d

customary to take flux phasor as the reference wt t sinmax

wt wN dt

cosmax1

111

ns an aneous n uce vo age n pr marywinding balances the applied voltage

Instantaneous induced voltage inwt wN ev cosmax222

Magnitudes of the rms induced voltages

max1max11

44.42

44.42

N N E

fN fN E

No load Phasor Diagram18



Basic Analysis: Voltage

d i1 i2

2

11

2

1

N N d dt

t et e

N1e 1 + _N2 e2+

_2 dt

a N E 2

1

2

1

2

2

1

1

N N • Volts/turn is constant • Voltages are in phase (no phase shift)• Voltage magnitudes vary with turns ratio.



Transformer under loaded condition

21

**2211

*

Phasor Diagram under load condition

1

2

1

2*2

1

N E I a N I 1

2

2

12211 I N I N

• Currents are in phase.



Basic Analysis: Reflected impedance

Flux

I 1 I 2Load

N 1

E 1 N 2

E 2Source

Z load

Primary Secondary

E

2 I load



Basic Anal sis:

Reflected

im edance

' Z E 1Source 1

I E Z ' load

Primary 2

' N I E Z 212 N I E Z load '

1' N Z Z load load

22

21 N N

oaoa

2

22



Equivalent circuit of an ideal transformer

1 N

2 N



Dot convention• In mutually coupled coils, dots are put at the proper

terminals of the primary and secondary merely to n cat ve t e status o po ar t es o t e vo tages.

• Dot indicates instantaneous positive polarity of AC voltage induced in both the primary and secondary

windings• Dot terminals will have at any point of time identical

polarities.

• Dots can be found out by doing some simple tests such as olarit test or d.c kick test.



Single ‐Phase, Ideal Transformer RatingsI1 I

Apparent Power + +

2 KVA 120/240 V

N2N1

‐

V1

‐

V2

Primary Vo tage Secondary Voltage



• Rated voltage: The device can continuously

operate at

the

rated

voltage

without

being

damaged due to insulation failure

• operate at the rated current without being



Exam leTransformer rating:

2

2 kVA, 240/120 VCompute the currents

N2N1V1 V2

kVA I V I V S 22211

‐ ‐

A67.16V120

kVA2

22

V

S I

A33.8kVA2

1 S

I 1

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Transformer Unit 1.pdf