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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
8/10/2019 Transformer Unit 1.pdf
http://slidepdf.com/reader/full/transformer-unit-1pdf 2/27
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
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• U1- Ideal single phase transformer
3
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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
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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
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Transformers are integral part of any modern power system
DistributionDistribution
220kV ‐750kV
rans ormerrans ormer
TransmissionTransmission15 kV ‐ 25kV TransformerTransformer
‐erv ceerv ceTransformerTransformer
6
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8/10/2019 Transformer Unit 1.pdf
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Transmission Transformer
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8/10/2019 Transformer Unit 1.pdf
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Distribution Transformer
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8/10/2019 Transformer Unit 1.pdf
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8/10/2019 Transformer Unit 1.pdf
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Service Transformer
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8/10/2019 Transformer Unit 1.pdf
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Low power Transformer
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8/10/2019 Transformer Unit 1.pdf
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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 .
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Steel sheet laminations are used as core materials
14
ore type e type
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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%
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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
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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
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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
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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.
8/10/2019 Transformer Unit 1.pdf
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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.
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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
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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
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Equivalent circuit of an ideal transformer
1 N
2 N
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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.
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Single ‐Phase, Ideal Transformer RatingsI1 I
Apparent Power + +
2 KVA 120/240 V
N2N1
‐
V1
‐
V2
Primary Vo tage Secondary Voltage
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• 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
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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
27
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