TRANSFORMERS

Shripranav R

Page 1
 S.No. CONTENTS PG.No.
1 Introduction 3
2 Principle of operation of transformers 3
3 Why do we use high voltages? 5
4 Who invented the electric transformers? 7
5 Components of transformer 8
6 Buchholz Relay 10
7 Ideal Transformer 11
8 Step up & Step down transformer 12
9 Turns Ratio 12
10 Losses in a transformer 13
11 Voltage regulation of the transformer 16
12 Classification of transformer 17
13 Tests conducted in a transformer 18
14 Single phase transformer 19
15 Three-phase transformer connections 20
16 Wye and Delta connections 20
17 Three-Phase Connections Using Single-Phase 21
Transformers
18 Paralleling Three-Phase Transformers 23
19 Why are they used in the power system? 27
20 Guidelines For Energy Conservation Opportunities 28
21 References 29

Page 2
INTRODUCTION
An electric transformer is a piece of equipment that is
designed to change the magnitude of AC voltage in a circuit,
without altering the frequency, and at a minimum power loss.
Power is delivered from its input side to its output side by the
process of electromagnetic induction.

They are used to transmit power generated at a remote

location to the consumer efficiently at the required voltage.
They are available in various sizes and ratings from those huge
ones in a substation to those tiny ones in an electronic board.

THE PRINCIPLE OF OPERATION

Transformers work on the principle of mutual inductance
and Faraday’s law of electromagnetic induction. The flow of
an alternating current through a coil produces an alternating
magnetic field. When another coil is brought in contact with
the alternating magnetic field, voltage is induced in that coil.

Page 3
According to Faraday’s law, the magnitude of the induced
voltage depends on the rate of change of magnetic flux linking
the second coil and the number of turns.
ε =-N dΦ/dt
When it comes to transformers, the rate of change of magnetic
flux between the coils is almost the same. Therefore, the
induced voltage depends on the number of turns of the coils.
Hence, the higher the number of turns of the coil, the larger
will be the induced voltage.
A typical transformer is made up of a set of primary and
secondary coils wound over a laminated soft iron core. The
iron core acts as a low reluctance path for the flow of magnetic
flux.

When an alternating voltage is applied to the primary coil of

the transformer, an alternating flux is generated which links
the secondary coil through the laminated iron core and
induces an alternating voltage in it. The magnitude of the
induced voltage depends on the ratio between the number of

Page 4
turns of the primary coil and the number of turns of the
secondary coil.

WHY DO WE USE HIGH VOLTAGES?

Your first question is probably this: if our homes and offices are
using photocopiers, computers, washing machines, and
electric shavers rated at 110–250 volts, why don't power
stations simply transmit electricity at that voltage? Why do
they use such high voltages? To explain that, we need to know
a little about how electricity travels.
As electricity flows down a metal wire, the electrons that carry
its energy jiggle through the metal structure, bashing and
crashing about and generally wasting energy like unruly
schoolchildren running down a corridor. That's why wires get
hot when electricity flows through them (something that's
very useful in electric toasters and other appliances that
use heating elements). It turns out that the higher the voltage
electricity you use, and the lower the current, the less energy
is wasted in this way. So, the electricity that comes from power

Page 5
plants is sent down the wires at extremely high voltages to
save energy.

But there's another reason too. Industrial plants have huge

factory machines that are much bigger and more energy-
hungry than anything you have at home. The energy an
appliance uses is directly related (proportional) to the voltage
it uses. So, instead of running on 110–250 volts, power-hungry
machines might use 10,000–30,000 volts. Smaller factories
and machine shops may need supplies of 400 volts or so. In
other words, different electricity users need different voltages.
It makes sense to ship high-voltage electricity from the power
station and then transform it to lower voltages when it reaches
its various destinations. (Even so, centralized power stations
are still very inefficient. About two thirds of the energy that
arrives at a power plant, in the form of raw fuel, is wasted in
the plant itself and on the journey to your home.

Page 6
WHO INVENTED THE ELECTRIC TRANSFORMERS?
The most primitive version was invented by Faraday. In 1884,
three Hungarian engineers, Károly Zipernowsky, Ottó Bláthy,
and Miksa Déri, designed the first high-efficiency called ZND
transformer. It led to development of new and highly efficient
modern design. The first three-phase transformer was
designed by Mikhail Dolivo-Dobrovolsky.

Page 7
COMPONENTS IN A TRANSFORMER

Irrespective of design types, the following are the major

components of a transformer.

Page 8
 • Core
• Winding
• Insulation
• Conservator
• Transformer oil
• Buchholz Relay
CORE

A core is a structure over which primary and secondary are

wound. It supports the windings as well as provides a low
reluctance path for the magnetic flux linking primary and
secondary winding. It is made up of high-permeability silicon
steel lamination to reduce core losses. The core must be
designed in such a way as to minimize the eddy current and
hysteresis losses.
WINDING

Electric transformers have two sets of windings, a low-tension

winding, and a high-tension winding. Several turns of copper
conductors bundled together to form a winding. The size of
copper conductors depends on the load current. Most of the
time windings are referred to as primary winding and
secondary winding.
Normally the winding to which the input voltage is connected
is known as the primary winding and the winding to which the
load is connected is known as the secondary winding.
Sometimes, based on the level of voltage applied to them, they
are referred to as high-tension winding and low-tension
windings

Page 9
INSULATION

Windings are insulated from each other and the core.

Insulation failures might result in short circuits and can cause
severe damage. Hence greater care is taken on the insulation
part during the design phase. Varnish, kraft paper, Cotton
cellulose, and Pressboard are the most widely used winding
insulation materials.
TRANSFORMER OIL

In oil-immersed transformers, the oil serves the dual purpose

of insulation and cooling. It has a high breakdown voltage, high
resistivity, and high dielectric strength. It extracts heat from
the windings and core and helps in reducing losses and
improves efficiency.
BUCHHOLZ RELAY

Buchholz relay is a protection device used to protect a

transformer from internal faults such as short circuits,
overheating, and oil leakages. It is an oil-actuated relay used
to sense the faults occurring in the windings and core and
trigger the alarm circuit and trips the circuit breaker and
interrupts the power supply.

Page 10
IDEAL TRANSFORMER
An ideal transformer is an imaginary device, having zero
losses, infinite permeability to magnetic flux, and 100%
efficiency. Since the same amount of flux is linking the primary
and the secondary winding, the ratio of applied voltage
(Vprimary) and induced voltage (Vsecondary) must be proportional
to the ratio of number of turns in the primary to the number
of turns (Nprimary) in the secondary winding (Nsecondary).
Vprimary / Vsecondary = Nprimary / Nsecondary
In an Ideal transformer, input power is equal to output power.
Vprimary / Vsecondary = Isecondary / Iprimary

In a real transformer, the voltage induced per turn is given by

the following equation:

E/N = K.Φm.f

where K is a constant, Φm is the maximum value of total flux

in Webers linking that turn and f is the supply frequency in
hertz.

Page 11
STEP-UP & STEP-DOWN TRANSFORMER
In step-up transformers, the secondary winding has more
turns than the primary winding. Also, the voltage at the
secondary shall be higher than the primary voltage (depending
on the turn ratio). They are used to increase transmission
voltage to reduce transmission losses. They can be found in
generating stations and are commonly known as power
transformers.
In a step-down transformer, the number of turns of coil in the
secondary winding is lesser than on the primary side, and
hence the voltage. They are used to reduce the voltage at the
distribution side of the power system.
TURNS RATIO
Turns ratio ‘n’, is a number denoting the ratio of the number of
turns of the conductor in the primary coil to that of the
secondary coil. The transformer ratio is also known as the
voltage transformation ratio. This tells about the voltage

Page 12
available at the secondary terminals for an applied primary
voltage.

NP – Number of turns of the conductor in the primary coil

VP – Applied Primary voltage

NS – Number of turns of the conductor in the secondary coil

VS – Transformed voltage measured at the secondary

LOSSES IN TRANSFORMER
In any electrical machine, 'loss' can be defined as the
difference between input power and output power. An
electrical transformer is a static device, hence mechanical
losses (like windage or friction losses) are absent in it. A
transformer only consists of electrical losses (iron losses and
copper losses). Transformer losses are similar to losses in a DC

Page 13
machine, except that transformers do not have mechanical
losses.

Losses in transformer are explained below –

1. Core losses or Iron losses:

Eddy current loss and hysteresis loss depend upon the

magnetic properties of the material used for the
construction of core. Hence these losses are also known as
core losses or iron losses.

• Hysteresis loss in transformer: Hysteresis loss is due to

reversal of magnetization in the transformer core. This
loss depends upon the volume and grade of the iron,
frequency of magnetic reversals and value of flux density.
It can be given by, Steinmetz formula: Wh= ηBmax 1.6fV
(watts) where, η = Steinmetz hysteresis constant V =
volume of the core in m3

• Eddy current loss in transformer: In transformer, AC

current is supplied to the primary winding which sets up
alternating magnetizing flux. When this flux links with
secondary winding, it produces induced emf in it. But
some part of this flux also gets linked with other
conducting parts like steel core or iron body or the
transformer, which will result in induced emf in those
parts, causing small circulating current in them. This
current is called as eddy current. Due to these eddy
currents, some energy will be dissipated in the form of
heat.

Page 14
 2. Copper loss in transformer

Copper loss is due to ohmic resistance of the transformer

windings. Copper loss for the primary winding is I1 2R1
and for secondary winding is I2 2R2. Where, I1 and I2 are
current in primary and secondary winding respectively,
R1 and R2 are the resistances of primary and secondary
winding respectively. It is clear that Cu loss is proportional
to square of the current, and current depends on the
load. Hence copper loss in transformer varies with the
load.

Equivalent Circuit

It is a theoretical circuit that represents a transformer and its

physical behaviour. This circuit shown below represents its
electrical parameters from which losses and voltage drops can
be easily calculated.

VP – Primary voltage or applied voltage

IP – Primary current

RP – Resistance offered by the primary winding

Page 15
XP – Reactance offered by the primary winding

IC – Current component contributing to core losses

RC – Resistive component contributing to core losses

IM – Magnetizing current

XM – Magnetizing reactance

Vs – Secondary voltage or applied voltage

Is – Secondary current

Rs – Resistance offered by the secondary winding

Xs – Reactance offered by the secondary winding

Note: The above equivalent circuit is a generalized form of an

equivalent circuit for an ideal transformer with a turn’s ratio of
1:1 and without referring to either the primary or the
secondary side.

VOLTAGE REGULATION OF THE TRANSFORMER

How accurate the voltage transformation occurs in the
transformer when the load varies from no load to full load is
dictated by their voltage regulation. It is calculated using the
following formula:

Where,

Page 16
 Esec-noload – Voltage measured at the secondary at no load

Esec-fullload – Voltage measured at the secondary at full load

CLASSIFICATION OF TRANSFORMER
Transformers are available in various types, shapes, and forms.
Based on where they are used and various other parameters
such as type of supply, their application, type of construction,
cooling method, operational voltage, duty type, the shape of
the core, etc.
Classification based on the type of power supply: Three-
phase and single-phase device.
Classification based on the type of construction: Core type
and Shell type transformer.
Classification based on cooling method: Dry-type or natural
air-cooled, Oil cooled- Oil Natural Air Natural (ONAN), Oil
Natural Air Forced (ONAF), Oil Forced Air Natural (OFAN), Oil
Forced Air Forced (OFAF), Oil and Water-cooled – Oil Natural
Water Forced (ONWF), Oil Forced Water Forced (OFWF)
Classification based on purpose: Distribution, Instrument
(potential and current), isolation, grounding, radio frequency,
phase-shifting and autotransformers, and tesla coil.
TESTS CONDUCTED IN A TRANSFORMER
The following tests are conducted to ensure the proper
functioning of the device:

1. Winding resistance test.

Page 17
 2. Insulation resistance test.
3. Transformer resistance test.
4. No-load test – Open circuit test.
5. Short circuit impedance test – Short circuit test.
6. Temperature rise test.
7. Polarity checks.
8. Dielectric test for oil.
9. Noise Level tests

SINGLE-PHASE TRANSFORMER
Figure shows a typical arrangement of bringing leads out of a
single-phase distribution transformer. To provide flexibility for
connection, the secondary winding is arranged in two sections.
Each section has the same number of turns and, consequently,
the same voltage. Two primary leads (H1, H2) are brought out
from the top through porcelain bushings. Three secondary
leads (X1, X2, X3) are brought out through insulating bushings
on the side of the tank, one lead from the centre tap (neutral)
(X2) and one from each end of the secondary coil (X1 and X3).
Connections, as shown, are typical of services to homes and
small businesses. This connection provides a three-wire
service that permits adequate capacity at minimum cost. The
neutral wire (X2) (centre tap) is grounded. A 120-volt circuit is
between the neutral and each of the other leads, and a 240-
volt circuit is between the two ungrounded leads.

Page 18
THREE-PHASE TRANSFORMER CONNECTIONS

Three-phase power is attainable with one three-phase

transformer, which is constructed with three single-phase
units enclosed in the same tank or three separate single-phase
transformers. The methods of connecting windings are the
same, whether using the one three-phase transformer or
three separate single-phase transformers.

WYE AND DELTA CONNECTIONS

The two common methods of connecting three-phase
generators, motors, and transformers are shown in figure. The
method shown in at left figure is known as a delta connection,
because the diagram bears a close resemblance to the Greek
letter, called delta.
The other method, figure on right, is known as the star or wye
connection.
The wye differs from the delta connection in that it has two
phases in series. The common point “O” of the three windings

Page 19
is called the neutral because equal voltages exist between this
point and any of the three phases.
When windings are connected wye, the voltage between any
two lines will be 1.732 times the phase voltage, and the line
current will be the same as the phase current. When
transformers are connected delta, the line current will be
1.732 times the phase current, and the voltage between any
two will be the same as that of the phase voltage.

Page 20
THREE-PHASE CONNECTIONS USING SINGLE-PHASE
TRANSFORMERS
As mentioned above, single-phase transformers may be
connected to obtain three-phase power. These are found at
many Reclamation facilities, at shops, offices, and warehouses.
ANSI standard connections are illustrated below in the
following figures. There are other angular displacements that
will work but are seldom used. Do not attempt to connect
single-phase units together in any combination that does keep
the exact angular displacement on both primary and
secondary; a dangerous short circuit could be the result.
Additive and subtractive polarities can be mixed (see the
following figures). These banks also may be paralleled for
additional capacity if the rules are followed for three-phase
paralleling discussed below. When paralleling individual three-
phase units or single-phase banks to operate three phase,
angular displacements must be the same. Figure top shows
delta-delta connections. Figure bottom, shows wye-wye
connections, which are seldom used at Reclamation facilities,
due to inherent third harmonic problems. Methods of dealing
with the third harmonic problem by grounding are listed
below.
However, it is easier just to use another connection scheme
(i.e., delta-delta, wye-delta, or delta-wye [figure 20]), to avoid
this problem altogether. In addition, these schemes are much
more familiar to Reclamation personnel.

Page 21
Page 22
PARALLELING THREE-PHASE TRANSFORMERS
Two or more three-phase transformers, or two or more banks
made up of three single-phase units, can be connected in
parallel for additional capacity. In addition to requirements
listed above for single-phase transformers, phase angular

Page 23
displacements (phase rotation) between high and low voltages
must be the same for both. The requirement for identical
angular displacement must be met for paralleling any
combination of three-phase units and/or any combination of
banks made up of three single-phase units.
For delta-delta and wye-wye connections, corresponding
voltages on the high-voltage and low-voltage sides are in
phase. This is known as zero phase (angular) displacement.
Since the displacement is the same, these may be paralleled.
For delta-wye and wye-delta connections, each low-voltage
phase lags its corresponding high voltage phase by 30 degrees.
Since the lag is the same with both transformers, these may be
paralleled. A delta-delta, wye-wye transformer, or bank (both
with zero degrees displacement) cannot be paralleled with a
delta-wye or a wye-delta that has 30 degrees of displacement.
This will result in a dangerous short circuit.

Page 24
Page 25
Wye-wye connected transformers are seldom, if ever, used to
supply plant loads or as GSU units, due to the inherent third
harmonic problems with this connection. Delta-delta, delta-
wye, and wye-delta are used extensively at Reclamation
facilities. Some rural electric associations use wye-wye
connections that may be supplying Reclamation structures in
remote areas. There are three methods to
negate the third harmonic problems found with wye-wye
connections:
1. Primary and secondary neutrals can be connected together
and grounded by one common grounding conductor.
2. Primary and secondary neutrals can be grounded
individually using two grounding conductors.

Page 26
3. The neutral of the primary can be connected back to the
neutral of the sending transformer by using the transmission
line neutral.
In making parallel connections of transformers, polarity
markings must be followed. Regardless of whether
transformers are additive or subtractive, connections of the
terminals must be made according to the markings and
according to the method of the connection (i.e., delta
or wye).

WHY ARE THEY USED IN THE POWER SYSTEM?

An electric transformer can be considered the most important
component in a power transmission and distribution network.
It performs the duty of improving transmission efficiency and
reducing losses and transmission costs by stepping down
voltages. The power station generates power at a voltage of
11kV to 28kV at 50Hz. To reduce transmission losses, the
voltage is stepped up to 220kV or more and transmitted. At the
distribution substation, it is again stepped down to 33kV or
11kV upon the requirement and supplied to industries. It is
again stepped down at the domestic consumer end to low
voltage loads of the consumer.
By stepping up the voltage, the load current flowing through
the transmission lines is reduced. Reduction in load current
results in the reduction of copper loss (I2R loss) and the size of
the conductor used for power transmission. Hence, the cost of
power transmission as well as its efficiency is improved. Hence,

Page 27
they improve the system efficiency, and reliability and reduce
power transmission costs.
GUIDELINES FOR ENERGY CONSERVATION
OPPORTUNITIES
The following points have to be considered.
1. Power factor correction for reducing copper losses.
2. System operating voltages to be observed for maintaining
near rated voltages and unbalanced to be minimized.
3. Augmented cooling and relative benefits to be seen where
applicable.
4. Possibility of switching off paralleled transformers at any low
loads.
5. Working out existing realistic losses and cost thereof. This
follows study of annual r.m.s. loading and operating losses at
operating temperature, covering harmonic loading. This is a
prerequisite for finding replacement alternatives.
6. Replacement by a low loss transformer with economic
justification, considering present and future harmonic loading
and load pattern.
7. When replacement is not justified, collection of
invited/standard low loss design data for optimum cost/rating
of transformer for future replacement or for new installation.

Page 28
CONCLUSION
An electric transformer alters AC voltage levels while
maintaining frequency, using electromagnetic induction. It
consists of primary and secondary windings, a laminated iron
core, and insulation, and can be configured as step-up or step-
down transformers. Key losses occur through core and copper
resistance. Transformers improve power transmission
efficiency by reducing current and minimizing losses. Regular
testing and maintenance help ensure optimal performance
and energy conservation.

Page 29
REFERENCES:
1. Toroidal Line Power Transformers. Power Ratings
Tripled.
2. Lane, Keith (2007) (June 2007). “The Basics of Large
Dry-Type Transformers”. EC&M. Retrieved 29
January 2013.
3. Electromechanical systems, electric machines, and
applied mechatronics By Sergey Edward Lyshevski.
4. https://www.explainthatstuff.com/transformers.ht
ml
5. https://www.rgpv.ac.in/campus/BTech_I/transfor
mer.pdf
6. https://nredcap.in/PDFs/BEE_manuals/BEE_CODE_
TRANSFORMERS.pdf
7. https://www.cedengineering.com/userfiles/E05-
013%20-
%20Introduction%20to%20Transformers%20-
%20US.pdf

Page 30
 THANK YOU

Page 31