TRANSFORMERS
Transformer Tests
 The transformer tests are performed to determine
the circuit constants, efficiency and voltage
regulation
 We distinguish between two typical tests:
- Open Circuit Test (O.C test / No load test)
- Short Circuit Test (S.C test/Impedance test)
 These tests are economical and convenient
 These tests furnish the result without actually
loading the transformer
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Open-circuit test
 secondary winding(usually the high voltage side) is
open-circuited, and its primary winding is connected to
a full-rated line voltage at rated frequency.
 A voltmeter V, an ammeter A and a wattmeter W are
connected in the low voltage (LV) side
 Since the secondary is open circuited, a very small
current I0 (usually 2 to 5% of rated current) called noload current, flows in the primary.
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Open-circuit test
 Power loss in the transformer is due to core loss and
negligible copper loss, I2R in the primary winding
 There is no I2R loss in the secondary since it is open and
Is = 0.
 The readings of the instruments are as follows:
Ammeter reading = no-load current I0
Voltmeter reading = primary rated voltage Vs
Wattmeter reading = iron or core loss Pi
 From these measured values components of the no-load
equivalent circuit can be determined as follows:
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Open-circuit test
(b) No-load power factor
(a) Core loss
(c) Active component of I0
(e) Core loss resistance R0
(d) Reactive component of I0
(f) Magnetising reactance X0
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Short-circuit Test
 secondary terminals (usually the low voltage side) are
short circuited by a thick conductor or through an
ammeter.
 An ammeter, a voltmeter and a wattmeter are connected
on the high-voltage side.
 The input voltage is gradually adjusted until full load
flows.
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Short-circuit Test
The readings of the instruments in the
short-circuit test are as follows:
Ammeter reading = full-load primary current,Isc
Voltmeter reading = short circuit voltage,Vsc
Wattmeter reading = full-load copper losses, Pcfl
The output voltage Vs is zero because of
the short circuit.
Consequently, the whole primary voltage
is used to supply the voltage drop in the
total impedance Z1e referred to the
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Short-circuit Test
 If cosΦsc = power factor at short circuit then:
Pc = V1scI1sccosΦsc
Psc  I sc2 Re
Equivalent resistance referred to the primary:
Psc
Re  2
I sc
Vsc
Ze 
I sc
:Equivalent impedance referred to the primary
Equivalent reactance referred to the primary:
X e  Z e2  Re2
Re
cossc 
Ze
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Transformer Ratings
Transformers are rated to supply a given
output in
Volt Amps
or
VA
at a specified frequency and terminal
voltage.
They are NOT RATED in Watts because
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Transformer losses
 The transformer losses are classified into electrical losses
(copper losses) and Magnetic losses (Iron losses).
 Copper losses occur in both the primary and secondary
windings.
Pcu  I R1  I R2
2
1
2
2
 Magnetic losses are divided into eddy current losses and
hysteresis losses.
Pcore  Peddy  Physterises
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Copper losses (Pcu)
Varies with load current, Pcu=I2R
Produces HEAT.
Created by resistance of windings.
Short Circuit Test supplies copper losses.
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Cu Losses (W)
Copper Losses
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
110
% Load
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Core loss or iron loss
They are independent of the load i.e. remain
constant with loading of the transformer.
Produces HEAT.
They are determined by Open Circuit Test.
Eddy current loss is minimised by using
laminations.
Hysteresis loss minimised by using silicon
steel.
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Transformer efficiency
Power
In
Some Power
is used to:
Power
Out
Overcome
Copper
Losses
Overcome
Iron
Losses
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Efficiency
Ratio between Input power and Output Power
η
Output Power
Input Power
Input  Output  Losses
Output Power
η
Output Power  Losses
Input Power  Losses
η
Input Power
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Efficiency
Efficiency is normally expressed as a percentage:
Output Power
η% 
 100
Input Power
VS I S cos 

x100%
PCu  Pcore  VS I S cos 
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Behavior of magnetic losses and efficiency with loading
1.4
Losses (W)
1.2
1.0
η%
Cu Losses
Fe Losses
97.00
0.8
0.6
η%
0.4
0.2
0.0
96.00
0
10
20
30
40
50
60
70
80
90
100
110
% Load
Fe = Cu =Max η
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Condition for maximum
efficiency
 It is observed that the efficiency is low at small loads
and reaches a maximum value for a certain load.
 The efficiency then decreases as the load is
increased.
 The efficiency of the transformer for a given power
factor is always maximum when the variable copper
loss is equal to the constant iron(core) loss:
PFe = PCu =Max η
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All Day Efficiency
 The primary of a distribution transformer is connected to
the line for 24 hours a day.
 Thus the core losses occur for the whole day whereas
copper losses occur only when the transformer is on
load.
 Distribution transformers operate well below the rated
power output for most of the time.
 Performance of the distribution transformer is more
appropriately represented by all day efficiency.
 The all-day efficiency of a transformer is defined as the
ratio of total energy output for a certain period to the total
energy input for the same period.
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All day efficiency
The energy efficiency can be calculated for a specified period.
When the energy efficiency is calculated for a 24 hours it is
called the all-day efficiency.
out put in watts
Ordinary commercial efficiency 
input in watts
output in kWh
All  day  efficiency :all day 
( for 24 hours)
Input in kWh
All day efficiency is always less than the commercial efficiency
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Autotransformer
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Autotransformer
Sometimes it is desirable to change voltage
level only by a small amount.
This may be due to small increase in voltage
drop that occur in a power system with long
lines.
In such cases it is very expensive to hire a
two full winding transformer, however a
special transformer called: ”auto-transformer”
can be used.
 Auto transformers are a special type, since
they have no electrical isolation between the
primary and secondary windings.
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Autotransformer
 The voltage across the common winding is called the
common voltage VC, and the current through this coil is
called the common current IC.
 The voltage across the series winding is called a series
voltage VSE, and the current through that coil is called a
series current ISE.

-The
A
Series
winding
following can be

B
deduced for an

Common
Autotransformer:
winding
C
Circuit diagram of Autotransformer
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Types of Autotransformers
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Types of Autotransformers
If the output from the auto transformer can be
varied via a moveable tapping, as shown below,
it is also known as a variac (variable ac supply)
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Autotransformers
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Advantages of autotransformer
 Uses less winding material than two winding
transformer.
 Smaller in size and cheaper than two winding
transformer of the same output.
 Has higher efficiency.
 Has better voltage regulation.
 Can deliver variable voltage, if a sliding contact
is used.
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Disadvantages of autotransformer
There is no electrical isolation between the
high-voltage and low-voltage sides. In
case of open circuit in the common
winding, the full primary voltage would be
applied on the secondary side. This high
voltage
would
damage
equipment
connected to the secondary.
Effective impedance of a autotransformer
is smaller compared to two winding
transformer.
The
reduced
internal
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Application of auto-transformer
Interconnection of power systems of
different voltages
Boosting of supply voltage by a small
amount in distribution systems to
compensate the for voltage drop.
For starting induction and synchronous
motors , if equipped with a number of
tappings.
Used as a VARIAC (variable a.c.) in
laboratory.
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Three phase Transformers
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Construction
The majority of the power generation/distribution systems in the world are 3phase systems.
The transformers for such circuits can be constructed either as a 3-phase bank of
independent identical transformers (can be replaced independently) or as a single
transformer wound on a single 3-legged core (lighter, cheaper, more efficient).
Three -phase transformer bank
Three phase transformer wound on
single 3 legged core
Three-phase transformer cores
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Three-Phase Transformers
C
D
E
B
F
F
A
Three-phase transformer.
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Primary and Secondary
Connections
 The primary (input) side of a three-phase
transformer can be connected in a wye or
delta configuration.
 The secondary (output) side of a three-phase
transformer can also be connected in a wye or
delta configuration.
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Primary and Secondary
Connections
This allows four basic connection patterns:
 Wye-Wye
 Delta-Delta
 Wye-Delta
 Delta-Wye
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Wye-wye connection
Wye – wye (Y – Y)
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Wye-delta connection
Wye-delta three-phase connection schematic.
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Delta-wye connection
Delta-wye three-phase connection schematic.
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Delta-delta connection
Delta – delta (Δ – Δ)
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Winding identification
Winding identification.
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Special transformers

Two types of special purpose transformers used
in power systems for taking measurements.
– Potential Transformer
– Current Transformer
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Standard symbols for instrument
transformers.
The standard secondary values for full-scale meter readings are also
shown
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Instrument transformers
(a) Voltage transformer
(b) Current transformer
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APPLICATION
OF
TRANSFORMERS
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Electrical Power System
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Distribution transformers
Single phase transformer
Three phase transformer
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Step up Transformer
Step up transformer at power plant
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END OF TOPIC!!
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