Iron Core
Magnetic
lines
Devic
e
Secondary
coil
Primary coil
AC
Suppl
y

Working principle
 Physical basis of a transformer is mutual induction
between two circuits linked by a common
magnetic flux.
 If one coil is connected to a source of
alternating voltage, an alternating flux is set up in
the laminated core, most of which is linked with
the other coils in which it produces mutually
induced emf

 A transformer is a static device which is used to step up
or step down voltages at constant frequency
 It consists of two coils, that are electrically isolated but
magnetically linked
 The primary coil is connected to the power source and
the secondary coil is connected to the load
 Voltage is stepped up or stepped down proportional to
turns ratio
 The turn’s ratio is the ratio between the number of
turns on the secondary (Ns)to the number of turns on
the primary (Np).

Turn’s Ratio = No. windings in Secondary
No. Windings in Primary
= Voltage in Secondary
Voltage in Primary
V s = N s
V p N p

Transformer classification
 Based on construction
 Core type
 Shell type
 Berry type
 Based on application
 Power transformer
 Distribution transformer
 Based on cooling
 Oil filled self cooled
 Oil fill water cooled
 Air blast type

Losses in a transformer
 No load losses or core losses
 Load losses or copper losses

No load losses
 No load losses remains the same irrespective of the load
connected to the transformer
 It is the power consumed to sustain the magnetic field in the transformer’s
core
 It is of two types – hysteresis loss and eddy current loss
 Hysteresis loss is the energy lost by reversing the magnetizng field in
the core as the AC changes direction in every cycle.
 Eddy current loss is a result of induced currents circulating in the core
 Hysteresis loss is minimized by using steel of high silicon content for
the core
 Eddy current loss is minimized by using very thin laminations
polished with varnish
No load loss = IL( Va / Vr ) ²

Load losses
 It is associated with load current flow in the transformer windings
 Copper loss is power lost in the primary and secondary windings of
a transformer due to the ohmic resistance of the windings
load loss = I ² R

Problem
 Find the total losses taking place in a 250 KVA
transformer operating at 60% of its rated capacity
whose No load loss = 500 W and
Full load loss = 4500 W

Problem
 Transformer Rating 5 0 0 kVA, PF is 0 . 8, No
Load Loss =3.5 kW, Full Load Loss = 4 . 5 kW
No. of
hrs
Load kW PF
6 4 0 0 0 . 8
1 0 3 0 0 0 . 7 5
4 1 0 0 0 . 8
4 0 0

How to improve the efficiency of
transformer
 By operating the transformer at optimum load
 By operating the transformers in parallel
 Voltage regulation of transformer
 At optimum loading no load loss = Full load loss
 Thus during max. efficiency no load loss = Full load loss
 No Load Loss = 1600 W, Full Load Loss = 2 845 W
 X = 100 √(No Load Loss/ Full Load Loss)
Load at max Eff = ( 1 6 0 0 / 2 8 4 5 ) 0 . 5
= 7 5 . 0 %

Parallel operation of transformer
 This is done for fluctuating loads, so that the load
can be optimized by sharing the load between the
transformers
 This way of operation provides high efficiency
 For parallel operation, both the transformers
should be technically identical and should have
the same impedance level.

Problem
 Power Required : 8 0 0 kVA
( 4 0 0 kVA x 2 )
 No of Transformers : 2
Rated Capacity : 1 2 5 0 kVA each
No Load Loss : 2 k W
Load Loss : 1 5 k W

Transformer selection
 Calculate the connected load and diversity factor
 Multiply Diversity Factor with connected load
applicable to particular industry and arrive at
kVA rating of transformers
 Diversity factor is the ratio of sum of individual
maximum demand of various equipment to the
overall maximum demand of the plant
 It will be always greater than one

Voltage regulation
 When the supply voltage changes, it causes tripping of
voltage sensitive load devices
 The voltage regulation in transformers is done by
altering the voltage transformation ratio with the help
of tapping
 There are two methods of tap changing facility
available
 Off-circuit tap changer
 On-load tap changer

System Distribution losses and
Optimization
 Relocating transformers and substations near to the
load centres
 Re-routing the feeders and cables, where the line
losses and voltage drops are higher
 Power factor improvement by incorporating capacitors
at the load end.
 Optimum loading of the transformers in the system
 Opting for low resistance All Aluminium Alloy
Conductors (AAAC) instead of conventional
Aluminium Cored Steel Reinforced (ACSR) lines

Capacity
( kVA )
C o n v e n t i o n a l A m o r p h o u s
No Load
Loss
Copper a
Loss at
No Load
Loss
Copper
Loss at FL
W
1 0 0 2 6 0 1 7 6 0 6 0 1 6 3 5
2 5 0 6 0 0 3 6 0 0 1 6 0 3 2 8 0
5 0 0 8 4 0 5 7 0 0 2 4 0 5 6 0 0
7 5 0 1 1 0 0 7 5 0 0 3 6 0 7 2 0 0
1 0 0 0 1 3 0 0 9 8 0 0 4 3 0 9 0 0 0
E N E R G Y L O S S E S C O N V E N T I O N A L
T R A N S F O R M E R V I S - A - V I S
A M O R P H O U S T R A N S F O R M E R

Energy conservation in transformer
 % Loading of Transformer
 Transformer Efficiency
 No Load Losses
 Operating Power Factor
 Power Factor improvement in capacitor installation
 On Load Tap Changer ( OLTC )
 Parallel operation of Transformers
 Idle transformer
 Separate transformer for lighting