STUDY OF POWER AND DISTRIBUTION

TRANSFORMERS

Mini Project Report Submitted in Partial Fulfillment of

the Requirements for the Award of Degree of

Bachelor of Technology
in
Electrical and Electronics Engineering

By

M. Abhishek (Roll No. 15B81A0203)

A. Ajay (Roll No. 15B81A0204)
Savanth Joshi (Roll No. 15B81A0260)

DEPARTMENT OF ELECTRICAL AND

ELECTRONICS ENGINEERING

CVR COLLEGE OF ENGINEERING

(An UGC Autonomous Institution, Accreditated by NBA & NAAC)
(Approved by AICTE & Govt. of Telangana and Affiliated to JNTU, Hyderabad)
Vastunagar, Mangalpalli (V), Ibrahimpatnam (M), R.R District.
Hyderabad 501 510

2018
v
 STUDY OF POWER AND DISTRIBUTION
TRANSFORMERS

Mini Project Report Submitted in Partial Fulfillment of

the Requirements for the Award of Degree of

Bachelor of Technology
in
Electrical and Electronics Engineering

By

M. Abhishek (Roll No. 15B81A0203)

A. Ajay (Roll No. 15B81A0204)
Savanth Joshi (Roll No. 15B81A0260)

DEPARTMENT OF ELECTRICAL AND

ELECTRONICS ENGINEERING

CVR COLLEGE OF ENGINEERING

(An UGC Autonomous Institution, Accreditated by NBA & NAAC)
(Approved by AICTE & Govt. of Telangana and Affiliated to JNTU, Hyderabad)
Vastunagar, Mangalpalli (V), Ibrahimpatnam (M), R.R District.
Hyderabad 501 510

2018
 CVR COLLEGE OF ENGINEERING
(An UGC Autonomous Institution, Accreditated by NBA & NAAC)
(Approved by AICTE & Govt. of Telangana and Affiliated to JNTU, Hyderabad)
Vastunagar, Mangalpalli (V), Ibrahimpatnam (M), R.R District.
Hyderabad 501 510

Department of Electrical and Electronics Engineering

Certificate

This is to certify that this Mini Project Report entitled “STUDY OF

DISTRIBUTION TRANSFORMERS” by M. Abhishek (Roll No. 15B81A0203),
A. Ajay (Roll No. 15b81A0204) and Savanth Joshi (Roll No. 15B81A0260),
submitted in partial fulfillment of the requirement for the degree of Bachelor of
Technology in Electrical and Electronics Engineering of the CVR College of
Engineering, Hyderabad, during the academic year of 2018-19, is a bonafide record
of the work carried out under our guidance and supervision.
The results embodied in this report have not been submitted to any other
University or Institution for the award of any degree or diploma.

Dr. S. Venkateshwarlu Dr. S. Venkateshwarlu

(Professor & HOD,EEE) (Professor & HOD, EEE)
(Project Guide)
 ACKNOWLEDGEMENT

We are thankful to Dr. Raghava Cherabuddi, Ph.D. Chairman, CVR College of

Engineering for providing the best-of-class infrastructure, lab facilities and top-class faculty
and providing us the best possible education.

We are highly indebted and grateful to Dr. S. Venkateshwarlu, M.Tech, Ph.D.,

Professor and Head of Electrical and Electronics Engineering Department whose kind
co-operation and valuable suggestion helped us in launching our project successfully.

We take the opportunity to express our deep sense of gratitude to our guide Dr. S.
Venkateshwarlu, M.Tech, Ph.D., Professor and Head of Electrical and Electronics
Engineering Department CVR College of Engineering, Hyderabad and for his continual
guidance, constant encouragement, discussion and unceasing enthusiasm. We consider our
self-privileged to have worked under his guidance.

We would like to express our heart full thanks to our Mini Project Coordinator
Dr.A.S.S.Murugan, M.E., Ph.D., Associate Professors, EEE for their valuable suggestions
which helped us to finish our project in a good manner.

Our sincere thanks to all faculty members & staff of Electrical and Electronics
Engineering for their constant encouragement, caring words, constructive criticism and
suggestions towards the completion of this work successfully.

We are highly indebted to the parents and family members, whose sincere prayers,
best wishes, moral support and encouragement have a constant source of assurance, guidance,
strength and inspiration to us.

Last but not least we thank Almighty for his grace enabling us to complete this work
on time.

vi
 ABSTRACT
A transformer is a device that transfers electrical energy from one circuit to another by
magnetic coupling without requiring relative motion between its parts. It comprises three
coupled windings and a core to concentrate magnetic flux. An alternating voltage applied to
one winding creates a time-varying magnetic flux in the core, which induces a voltage in the
other windings. Varying the relative number of turns between primary and secondary
windings determines the ratio of the input and output voltages. By transforming electrical
power to a high voltage, low current form and back again, the transformer greatly reduces
energy losses and so enables the economic transmission of power over long distances. It has
thus shaped the electricity supply industry, permitting generation to be located remotely from
points of demand. Amongst the simplest of electrical machines, the transformer is also one of
the most efficient, with large units attaining performances in excess of 99.75%.
 TABLE OF CONTENTS

PAGE
CHAPTER NO: TITLE:
NO.
ABSTRACT vii

LIST OF FIGURES ix

1
1 INTRODUCTION

1.1Types of Transformers 1
2 BASICS OF TRANSFORMERS 7
2.1 Introduction of Industry 7
2.2 Types of Transformers produced 8
3 CONSTRUCTION AND ASSEMBLY OF 9
TRANSFORMERS
3.1 Different Parts of a Transformer 9
3.2 Assembly of Transformers 21

4 TESTING OF A TRANSFORMER 27
4.1 Different methods of testing a Transformer s 27

5 CONCLUSION 35

6 REFERENCES 36

LIST OF FIGURES
FIGURE NO. TITLE PAGE
NO.

4.1 Temperature rise vs time curve 33

 Chapter 1

INTRODUCTION

A transformer is a static electrical device that transfers electrical energy between two
or more circuits through electromagnetic induction. A varying current in one coil of the
transformer produces a varying magnetic field, which in turn induces a varying electromotive
force (emf) or "voltage" in a second coil. Power can be transferred between the two coils,
without a metallic connection between the two circuits. Faraday's law of induction discovered
in 1831 described this effect. Transformers are used to increase or decrease the alternating
voltages in electric power applications.

Since the invention of the first constant-potential transformer in 1885, transformers

have become essential for the transmission, distribution, and utilization of alternating current
electrical energy.[3] A wide range of transformer designs is encountered in electronic and
electric power applications. Transformers range in size from RF transformers less than a
cubic centimeter in volume to units interconnecting the power grid weighing hundreds of
tons.

1.1 Types of Transformers

There are several transformer types used in the electrical power system for different
purposes, like in power generation, distribution and transmission and utilization of electrical
power. The transformers are classified based on voltage levels, Core medium used, winding
arrangements, use and installation place, etc. Here we discuss different types of transformers
are the step up and step down Transformer, Distribution Transformer, Potential Transformer,
Power Transformer, 1-ϕ and 3-ϕ transformer, Auto transformer, etc.

1
Transformers Based on Voltage Levels:

These are the most commonly used transformer types for all the applications.
Depends upon the voltage ratios from primary to secondary windings, the transformers are
classified as step-up and step-down transformers.

Step-Up Transformer:

As the name states that, the secondary voltage is stepped up with a ratio compared to
primary voltage. This can be achieved by increasing the number of windings in the secondary
than the primary windings as shown in the figure. In power plant, this transformer is used as
connecting transformer of the generator to the grid.

Step-up
Transformer

Step-Down Transformer:

It used to step down the voltage level from lower to higher level at secondary side as
shown below so that it is called as a step-down transformer. The winding turns more on the
primary side than the secondary side.

Step-Down
Transformer

2
 In distribution networks, the step-down transformer is commonly used to convert the
high grid voltage to low voltage that can be used for home appliances.

Transformer Based on the Core Medium Used:

Based on the medium placed between the primary and secondary winding the
transformers are classified as Air core and Iron core.

Air Core Transformer:

Both the primary and secondary windings are wound on a non-magnetic strip where
the flux linkage between primary and secondary windings is through the air.

Compared to iron core the mutual inductance is less in air core, i.e. the reluctance
offered to the generated flux is high in the air medium. But the hysteresis and eddy current
losses are completely eliminated in air-core type transformer.

Air Core Transformer

Iron Core Transformer:

Both the primary and secondary windings are wound on multiple iron plate bunch
which provide a perfect linkage path to the generated flux. It offers less reluctance to the
linkage flux due to the conductive and magnetic property of the iron. These are widely used
transformers in which the efficiency is high compared to the air core type transformer.

3
 Iron Core
Transformer

Transformers Based on Winding Arrangement:

AutoTransformer:

Standard transformers have primary and secondary windings placed in two different
directions, but in autotransformer windings, the primary and the secondary windings are
connected to each other in series both physically and magnetically as shown in the figure
below.

Auto Transformer

4
 On a single common coil which forms both primary and secondary winding in which
voltage is varied according to the position of secondary tapping on the body of the coil
windings.

Transformers Based on Usage:

According to the necessity, these are classified as the power transformer, distribution
transformer measuring transformer, and protection transformer.

Power Transformer:

The power transformers are big in size. They are suitable for high voltage (greater
than 33KV) power transfer applications. It used in power generation stations and
Transmission substation. It has high insulation level.

Power Transformer

Distribution Transformer:

In order to distribute the power generated from the power generation plant to remote
locations, these transformers are used. Basically, it is used for the distribution of electrical
energy at low voltage is less than 33KV in industrial purpose and 440v-220v in domestic
purpose.

5
  It works at low efficiency at 50-70%
 Small size
 Easy installation
 Low magnetic losses
 It is not always fully loaded

Distribution Transformer

Measurement Transformer:

Used to measure the electrical quantity like voltage, current, power, etc. These are
classified as potential transformers, current transformers etc.

Current Transformer

6
Protection Transformers:

This type of transformers is used in component protection purpose. The major

difference between measuring transformers and protection transformers is the accuracy that
means that the protection transformers should be accurate as compared to measuring
transformers.

7
 Chapter 2
2.1 Introduction of industry

VVE TRANSFORMERS PVT. LTD. (formerly known as Vinod Vikram Electronics)

an ISO 9001:2008 certified company was established in 1995 in one of the prime Industrial
Estate of Gandhinagar, Balanagar, Hyderabad. Later on the company established another
facility in one of the biggest industrial estates nearby Hyderabad city, where the
manufacturing units are located in a spacious setup equipped with world class Infrastructure,
high end machineries, EOT Cranes, Vacuum, ovens and testing facilities.

VVE Transformers is one of the leading transformer manufacturers in the country that
have been serving the industrial establishments and other end users since two decades. The
transformers manufactured by VVE of superior quality and high graded with state-of-theart
manufacturing facilities and are widely used in different sectors. The prime concern that
underlies the core philosophy of VVE Transformers is to offer cost and time effective
solutions and to cater the needs of the end user in accordance with the changing preferences
and demands in the market.

VVE Transformers have a reputation of maintaining top notch quality management

system within the organization in designing, manufacturing, supplying and servicing
transformers. Stringent quality assurance plans are strictly followed to formulate and monitor
the capability of the internal work flow system and to ensure the delivery of quality products
and services.

The VVE Transformers Pvt. Ltd. has served a number of esteemed, satisfied clients
and have stood in time to earn an acclaimed name and position for its quality products and
timely service. The firm has successfully completed projects from government and private
organizations and have been able to carve out a niche in the market as one of the most
renowned transformer manufacturers.

8
2.2 Types of Transformers produced:
 Distribution Transformers up to 5MVA in 33KV & 11KV Class
 Power Transformers up to 10MVA in 33KV Class
 Dry Type Transformers with Encloser
 Unitized / Compact Sub-Station for Oil immersed and Dry Type
 Lighting Transformers
 Isolating Transformers
 Furnace Transformers
 Converter Duty Transformers
 3-Phase CSP Type-16KVA to 315KVA (11000/433-250V)
 1-Phase CSP Type-10KVA to 25KVA (11000/250V, 11000/3/250V)
 Star Rated Transformers up to 200KVA Distribution Transformers as per BEE
Standards

9
 Chapter 3
CONSTRUCTION OF TRANSFORMERS

3.1 Different parts of a transformer

These are the basic components of a transformer.

1. Laminated core
2. Windings
3. Insulating materials
4. Transformer oil
5. Tap changer
6. Oil Conservator
7. Breather
8. Cooling tubes
9. Buchholz Relay
10. Explosion vent

Of the above, laminated soft iron core, windings and insulating material are the primary
parts and are present in all transformers, whereas the rest can be seen only in transformers
having a capacity of more than 100KVA.

Laminated core:

In alternating current (AC) devices they cause energy losses, called core losses, due to
hysteresis and eddy currents in applications such as transformers and inductors. "Soft"
magnetic materials with low coercivity and hysteresis, such as silicon steel, or ferrite, are
usually used in cores.
In order to reduce the eddy current losses mentioned above, most low frequency
power transformers and inductors use laminated cores, made of stacks of thin sheets of
silicon steel. A small addition of silicon to iron (around 3%) results in a dramatic increase of
the resistivity of the metal, up to four times higher.[citation needed] The higher resistivity
reduces the eddy currents, so silicon steel is used in transformer cores. Further increase in
silicon concentration impairs the steel's mechanical properties, causing difficulties for rolling
due to brittleness.

10
 Among the two types of silicon steel, grain-oriented (GO) and grain non-oriented
(GNO), GO is most desirable for magnetic cores. It is anisotropic, offering better magnetic
properties than GNO in one direction. As the magnetic field in inductor and transformer cores
is always along the same direction, it is an advantage to use grain oriented steel in the
preferred orientation. Rotating machines, where the direction of the magnetic field can
change, gain no benefit from grain-oriented steel.
Laminated magnetic cores are made of stacks of thin iron sheets coated with an
insulating layer, lying as much as possible parallel with the lines of flux. The layers of
insulation serve as a barrier to eddy currents, so eddy currents can only flow in narrow loops
within the thickness of each single lamination. Since the current in an eddy current loop is
proportional to the area of the loop, this prevents most of the current from flowing, reducing
eddy currents to a very small level. Since power dissipated is proportional to the square of the
current, breaking a large core into narrow laminations reduces the power losses drastically.

The transformers magnetic core is built up from cold rolled and precisely grained
magnetic steel laminations. Hi-B grade & laser scribed laminations are used to decline the
load fluctuations and noise levels. These are leveled at an angle of 45 degrees. The core leg &
yoke laminations are deposited in mitered joints. The function is primarily used for easy
passage of magnetic flux, avoid no load losses, hot spots, and to maintain low noise level.

11
Windings:

In both types of transformer core design, the magnetic flux linking the primary and
secondary windings travels entirely within the core with no loss of magnetic flux through air.
In the core type transformer construction, one half of each winding is wrapped around each
leg (or limb) of the transformers magnetic circuit as shown above.

The coils are not arranged with the primary winding on one leg and the secondary on
the other but instead half of the primary winding and half of the secondary winding are
placed one over the other concentrically on each leg in order to increase magnetic coupling
allowing practically all of the magnetic lines of force go through both the primary and
secondary windings at the same time. However, with this type of transformer construction, a
small percentage of the magnetic lines of force flow outside of the core, and this is called
“leakage flux”.

Shell type transformer cores overcome this leakage flux as both the primary and
secondary windings are wound on the same centre leg or limb which has twice the cross-
sectional area of the two outer limbs. The advantage here is that the magnetic flux has two
closed magnetic paths to flow around external to the coils on both left and right hand sides
before returning back to the central coils.

12
 This means that the magnetic flux circulating around the outer limbs of this type of
transformer construction is equal to Φ/2. As the magnetic flux has a closed path around the
coils, this has the advantage of decreasing core losses and increasing overall efficiency.

But you may be wondering as to how the primary and secondary windings are wound
around these laminated iron or steel cores for this types of transformer constructions. The
coils are firstly wound on a former which has a cylindrical, rectangular or oval type cross
section to suit the construction of the laminated core. In both the shell and core type
transformer constructions, in order to mount the coil windings, the individual laminations are
stamped or punched out from larger steel sheets and formed into strips of thin steel
resembling the letters “E”s, “L”s, “U”s and “I”s as shown below.

These lamination stampings when connected together form the required core shape.
For example, two “E” stampings plus two end closing “I” stampings to give an E-I core
forming one element of a standard shell-type transformer core. These individual laminations
are tightly butted together during the transformers construction to reduce the reluctance of the
air gap at the joints producing a highly saturated magnetic flux density.

Transformer core laminations are usually stacked alternately to each other to produce
an overlapping joint with more lamination pairs being added to make up the correct core
thickness. This alternate stacking of the laminations also gives the transformer the advantage
of reduced flux leakage and iron losses. E-I core laminated transformer construction is mostly

13
used in isolation transformers, step-up and step-down transformers as well as auto
transformers.

Insulating Materials:

Insulation is one of the most important qualities that a transformer has, which is
responsible for providing better performance when the transformer is operated. In fact, the
durability and stability of a transformer highly depend upon the proper utilization of the
insulation materials present within it. This means that other than metallic components like
silicon steel and copper, there are also other important insulating materials used in power
transformers for better functioning. You must get in touch with a reliable transformer
manufacturer in India like Miracle Electronics so that you can rest assured that the
transformers you buy will be of top-class quality with the best materials, both metallic and
insulating, installed within. Now, let us take a look at the list of insulating materials present in
a transformer, along with their uses and applications.

Insulating oil:

Insulating oil is one of the most important components within a transformer that acts
as both an electrical insulator and coolant to dissipate heat losses. This oil is seen in 11kV or
higher voltage transformers, placed in the tank where the transformer core is placed. It is not
required in low voltage transformers as the heat dissipation in such transformers is very low.
Insulating oil is obtained by fractional distillation and subsequent treatment of crude
petroleum.

Insulating paper:

14
 Insulating paper is made up of vegetable fibers that are felted together to form a sheet.
The fibers are obtained from plants like cotton, straw, hemp, manila, and coniferous trees.
When this insulating paper is put into the insulating oil under vacuum, it attains extremely
high electric strength.

Insulating tape:

As the name itself says, insulating tape is used for various taping purposes. These are
available in a variety of forms like cotton tapes, woven tapes, glass woven tapes, and phenol
laminated paper base sheet. These tapes are used in areas where high strength is required.
They are also used for banding of transformer cores.

Pressboard:

Used in the electrical, mechanical, and thermal designing of transformers, the

pressboard is also made up of vegetable fibers that contain huge amounts of cellulose. These
pressboards can be moulded into any shape to be used in transformers; the most commonly
seen shapes being angle rings and caps.

Wood-based laminates:

Wood-based laminates are made from selected veneers that are obtained from various
timbers. The veneers are first dried, and then are partially/completely saturated naturally.
Such laminates are used in areas that require higher mechanical and lower electric strength.
They are used in making a variety of components like coil clamping rings, cores, yokes, and
supports.

All these insulation materials used in transformers are based on their temperature
withstanding capacity. They are classified into classes A, B, C, E, F, H, and Y. Let us
understand all these classes in detail.

 Class A materials are those whose maximum hot spot temperature is 105°C. These
include cotton, silk, and paper for impregnation.

15
  Class B materials are those whose maximum hot spot temperature is 130°C. These
include glass fiber, mica, and asbestos with suitable impregnation or coating
substance.
 Class C materials are those whose maximum hot spot temperature is 180°C. These
include glass, mica, asbestos, porcelain, and quartz with or without an inorganic
binder.
 Class E materials are those whose maximum hot spot temperature is 120°C. These
include wire and enamel.
 Class F materials are those whose maximum hot spot temperature is 155°C. These
include glass, mica, and asbestos with suitable binding impregnation or coating
substances.
 Class H materials are those whose maximum hot spot temperature is 180°C. These are
a combination of materials like glass fiber, asbestos, and mica suitable bonded
together.
 Class Y materials are those whose maximum hot spot temperature is 90°C. These
include cotton, silk, paper, and wood without impregnation.


Transformer oil:

Transformer oil's primary functions are to insulate and cool a transformer. It must
therefore have high dielectric strength, thermal conductivity, and chemical stability, and must
keep these properties when held at high temperatures for extended periods.[1] Typical
specifications are: flash point 140 °C or greater, pour point −30 °C or lower, dielectric
breakdown voltage 28 kV (RMS) or greater.

To improve cooling of large power transformers, the oil-filled tank may have external
radiators through which the oil circulates by natural convection. Power transformers with
capacities of thousands of kVA may also have cooling fans, oil pumps, and even oil-to-water
heat exchangers.

Power transformers undergo prolonged drying processes, using electrical self-heating,

the application of a vacuum, or both to ensure that the transformer is completely free of water
vapor before the insulating oil is introduced. This helps prevent corona formation and
subsequent electrical breakdown under load.

16
Oil filled transformers with a conservator (oil reservoir) may have a gas detector relay
(Buchholz relay). These safety devices detect the buildup of gas inside the transformer due to
corona discharge, overheating, or an internal electric arc. On a slow accumulation of gas, or
rapid pressure rise, these devices can trip a protective circuit breaker to remove power from
the transformer. Transformers without conservators are usually equipped with sudden
pressure relays, which perform a similar function as the Buchholz relay.

Mineral oil is generally effective as a transformer oil, but it has some serious
disadvantages, of which the worst is its high flammability. If a transformer leaks mineral oil,
it can easily start a fire. Fire codes often require that transformers inside buildings use a less
flammable liquid, or the use of dry-type transformers with no liquid at all. Mineral oil is also
an environmental contaminant, and its insulating properties are rapidly degraded by even
small amounts of water.

Pentaerythritol tetra fatty acid natural and synthetic esters have emerged as an
increasingly common mineral oil alternative, especially in high-fire-risk applications such as
indoors or offshore, due to their low volatility and high fire point, which can be over 300 °C.
[4]
They also have a lower pour point, greater moisture tolerance, and improved function at
high temperatures, and they are non-toxic and readily biodegradable. Silicone or
fluorocarbon-based oils, which are even less flammable, are also used, but they are more
expensive than esters, and less biodegradable.

Tap Changer:

The purpose of a tap changer is to regulate the output voltage of a transformer. It does
this by altering the number of turns in one winding and thereby changing the turns ratio of the
transformer. There are two types of transformer tap changers: an on-load tap changer (OLTC)
and a deenergised tap changer (DETC). Note that not all transformers have tap changers.

An OLTC varies the transformer ratio while the transformer is energized and carrying
load. The switching principle uses the “make before break” contact concept. An adjacent tap
is bridged before breaking contact with the load carrying tap for the purpose of transferring
load from one tap to the other without interrupting or appreciably changing the load current.
While in a bridging position (i.e., contact is made with two taps), some form of impedance
(resistive or reactive) is present to limit circulating current. A high speed resistive type OLTC

17
uses a resistor pair to absorb energy and does not use the bridging position as a service
position. A reactive type OLTC uses a reactor that is designed for continuous loading, e.g., a
preventative autotransformer, and therefore uses the bridging position as a service position.

Here are two primary OLTC designs. A diverter design, used for higher voltages and
power, has both a tap selector and a separate diverter switch (also called arcing switch). The
switching arc may occur in oil or may be contained in a vacuum bottle. A non-diverter design,
used for lower voltage ratings, simply uses a so-called selector switch (also called arcing tap
switch) that combines the functions of a diverter switch and tap selector.

A DETC is a tap changer that cannot be moved while the transformer is energized. It
often has 5 positions (A,B,C,D,E, or 1,2,3,4,5). If a DETC is not exercised on a regular basis,
there is increased risk that the DETC will not make properly when next moved.

Tap changers have historically been one of the top causes of transformer failures
(Cigre_WG 12-05 “An international survey on failures in large power transformers in
service“, Electra No. 88, 1983, and ANSI/IEEE, 1985). Faults in OLTC’s can be classified as
dielectric failures (oil quality or clearance related), thermal failures (due to coking or crimp
problems), or mechanical failures (contact wear and misalignment, limit switches, sheared
pins on the linkage that operates the reversing switch, lubrication problems, etc).

Oil Conservator:

This is a cylindrical tank mounted on supporting structure on the roof the transformer
main tank. The main function of conservator tank of transformer is to provide adequate space
for expansion of oil inside the transformer.

When transformer is loaded and when ambient temperature rises, the volume of oil
inside transformer increases. A conservator tank of transformer provides adequate space to
this expandedtransformeroil.It also acts as a reservoir for transformer insulating oil.

This is a cylindrical shaped oil container closed from both ends. One large inspection
cover is provided on either side of the container to facilitate maintenance and cleaning inside
of the conservator.
Conservator pipe, i.e. pipe comes from main transformer tank, is projected inside the
conservator from bottom portion. Head of the conservator pipe inside the conservator is

18
provided with a cap. This pipe is projected as well as provided with a cap because this design
prevent oil sludge and sediment to enter into main tank from conservator. Generally silica gel
breather fixing pipe enters into the conservator from top. If it enters from bottom, it should be
projected well above the level of oil inside the conservator. This arrangement ensure that oil
does not enter the silica gel breather even at highest operating level.

Breather:
When load on transformer increases or when the transformer under full load, the
insulating oil of the transformer gets heated up, expands and gets expel out in to the
conservator tank present at the top of the power transformer and subsequently pushes the dry
air out of the conservator tank through the silica gel breather. This process is called breathing
out of the transformer. When the oil cools down, air from the atmosphere is drawn in to the
transformer. This is called breathing in of the transformer.
Most of the power generation companies use silica gel breathers fitted to the
conservator of oil filled transformers. The purpose of these silica gel breathers is to absorb
the moisture in the air sucked in by the transformer during the breathing process.

19
During the breathing process, the incoming air may consist of
moisture and dirt which should be removed in order to prevent any
damage. Hence the air is made to pass through the silica gel
breather, which will absorb the moisture in the air and ensures that
only dry air enters in to the transformer. Silica gel in the breather
will be blue when installed and they turn to pink colour when they
absorb moisture which indicates the crystals should be replaced.
These breathers also have an oil cup fitted with, so that the dust
particles get settled in the cup.

Thus Silica gel breathers provide an economic and efficient means of controlling the level of
moisture entering the conservator tank during the breathing process.

Cooling Tubes:
Cooling of a transformer is the process of dissipation of heat developed in the
transformer to the surroundings. The losses occurring in the transformer are converted into
heat which increases the temperature of the windings and the core. In order to dissipate the
heat generated cooling should be done.
There are two ways of cooling the transformer:
 First, the coolant circulating inside the transformer transfers the heat from the
windings and the core entirely to the tank walls and then it is dissipated to the
surrounding medium
 Second, along with the first technique, the heat can also be transferred by coolants
inside the transformer.
The choice of method used depends on the size, type of applications and the working
conditions.

The coolants used in the transformer are air and oil. In dry type transformer air
coolant is used and in oil immersed one, oil is user. In the first said, the heat generated is
conducted across the core and windings and is dissipated from the outer surface of the core
and windings to the surrounding air. In the next, heat is transferred to the oil surrounding the
core and windings and it is conducted to the walls of the transformer tank. Finally the heat is
transferred to the surround air by radiation and convection.

20
Based on the coolant used the cooling methods can be classified into:
1. Air cooling
2. Oil and Air cooling
3. Oil and Water cooling
1. Air cooling (Dry type transformers)
 Air Natural(AN)
 Air Blast (AB)
2. Oil cooling (Oil immersed transformers)
 Oil Natural Air Natural (ONAN)
 Oil Natural Air Forced (ONAF)
 Oil Forced Air Natural (OFAN)
 Oil Forced Air Forced (OFAF)

Buchholz relay:
Buchholz relay is a type of oil and gas actuated protection relay universally used on
all oil immersed transformers having rating more than 500 kVA. Buchholz relay is not
provided in relays having rating below 500 kVA from the point of view of economic
considerations.
Buchholz relay is used for the protection of transformers from the faults occurring
inside the transformer. Short circuit faults such as inter turn faults, incipient winding faults,
and core faults may occur due to the impulse breakdown of the insulating oil or simply the
transformer oil. Buchholz relay will sense such faults and closes the alarm circuit.
Buchholz relay can be used in the transformers having the conservators only. It is
placed in the pipe connecting the conservator and the transformer tank. It consists of an oil
filled chamber. Two hinged floats, one at the top of the chamber and the other at the bottom
of the chamber which accompanies a mercury switch each is present in the oil filled chamber.
The mercury switch on the upper float is connected to an external alarm circuit and the
mercury switch on the lower is connected to an external trip circuit.

During the occurrence of severe faults such as phase to earth faults and faults in tap
changing gear, the amount of volume of gas evolves will be large and the float at the bottom
of the chamber is tilted and the trip circuit is closed. This trip circuit will operate the circuit
breaker and isolates the transformer.
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 Buchholz relay operates during three conditions:
1. Whenever gas bubbles are formed inside the transformer due to severe fault.
2. Whenever the level of transformer oil falls.
3. Whenever transformer oil flows rapidly from the conservation tank to the main or from the
main tank to the conservation tank.

 Buchholz relay indicates inter turn faults and faults due to heating of core and helps in
the avoidance of severe faults.
 Nature and severity of fault can be determined without dismantling the transformer by
testing the air samples.


It can sense the faults occurring below the oil level only. The relay is slow and has a
minimum operating range of 0.1second and an average operating range of 0.2 seconds.

Explosion Vent:
The explosion vent is used to expel boiling oil in the transformer during heavy internal
faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes
out of the vent. The level of the explosion vent is normally maintained above the level of the
conservatory tank.

3.2 Assembly of a Transformer:

Copper and silver alloyed copper magnet wire—or continuously transposed copper cable
—is used for the winding conductors on all power transformers. Continuously transposed
copper cable is used to minimize losses and hot-spot temperatures and to produce a more
compact winding with improved short-circuit performance.

All windings are circular, concentric type and provide maximum through-fault withstand
capability. High voltage and low voltage windings use a continuous-disc or helical winding
design. This construction provides maximum strength and short-circuit withstand capability,
increased predictability and lower hot-spot temperatures for loading and overloading.

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 Multiple conductor continuous disc and helical windings are transposed throughout the
winding to minimize circulating current losses. State-of-the-art design techniques are used to
provide maximum impulse strength in the windings and to minimize voltage stresses. Special
design consideration is also given to the line end discs to control voltage stress distribution.

Ampere-Turn balancing techniques are used to minimize radial leakage flux and to
minimize axial short-circuit forces. In order to ensure short circuit performance, windings are
manufactured to meet stringent design tolerances for coil winding electrical height, tap
locations and spread section locations.

Cooling ducts are formed between discs in the continuous-disc and helical windings by
keyed radial spacers made of special high-density pressboard insulation. These spacers are
aligned in column to provide axial support for the windings and high short-circuit strength.

All windings are manufactured in a clean winding environment. This isolated “factory
within a factory” is humidity and temperature controlled 24 hours a day with controlled
access to minimize contamination.

Cores are manufactured from high permeability-grade, domain-refined “H” grade, cold-
rolled grain-oriented silicon steel (“M” grade steel is used in some applications).
Annealing all core steel after slitting provides optimal loss performance.

Core designs utilize a multiple step circular cross section with fully mitered joints.
Laminations cut to length on special high-speed, computer-controlled, automatic shears to
high dimensional accuracy ensure tight-fitting joints with minimal gaps to minimize core
loss, exciting current and sound levels.

Insulating the core from the frame and connecting to ground at only one point
prevents any accumulation of static charges. Grounding at a single point also eliminates
circulating currents and associated combustible gas generation. The grounding strap is
brought out to a convenient location adjacent to an access hole opening on the cover or
through a bushing on the tank cover to facilitate testing the core insulation.

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 After stacking, a two-part epoxy is applied to bind the core legs together, then
banding is installed to form a rigid structure. Sturdy steel end frames provide a complete core
structure of high mechanical strength to withstand heavy stresses during shipment or under
short-circuit conditions without distortion of the core or windings.

After the cores are banded together and uprighted and the coils have been wound,
processed, pressed and sized, its time to put them together in a process called “landing the
coils.” Each limb, or leg, of the core will have 2 to 5 windings landed on it after which the
entire core and coil assembly goes through a thorough cleaning and careful inspection
process before proceeding to a pressing operation that aids the final assembly of the core.

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 Once the assembly is under pressure, it is cleaned and inspected again and then put
through a process called top yoking, where top yoke steel is assembled to the limbs within
very close tolerances to ensure no core loss issues on the test floor. Upon completing the top
yoking, the unit is “pulled in.” This “pulling in” procedure includes tightening the core
clamps onto the steel, tightening up the yoke bands and adding all the extra insulation
required in the design. Precise, pre-calculated methods of clamping the core and coil
assembly together assure a positive clamping pressure on the coils at every point and provide
maximum through-fault protection no matter how dry the transformer may become while in
service.

Pre-assembled cleat and lead structures (wood frames with the insulated cable and,
oftentimes, a de-energized tap changer assembled to them) are now attached to the assembly.
All connections are crimped and wrapped according to engineering specifications, with each
crimp signed by the operator for quality auditing purposes; special care is taken when
wrapping crimps to minimize dielectric stresses. Once all connections are made, the assembly
is ratioed and tested again, inspected and then released to Vapor Phase.

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Vacuum Drying Plant:

The transformer shall be free from humidity traces the vacuum drying process ensures
total removal of moisture that exists in the active part. The process is made by effective
combination of heating in vacuum cycle.

Transformer Oil Filtration:

The oil filtration is one of the major parts and the oil is out sourced from the approved
suppliers. The oil further undergoes for filtration process before filling in to the
transformer tank. The moisture is removed by heating under suitable vacuum condition.

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Final Assembly:

Final Assembly begins with the removal of the core and coil assembly from vapor
phase. All internal assemblies are re-pressed to an engineering-specified pressure to
ensure tightness now that excess moisture has been removed. Metal and fiber nuts are
tightened or torqued as required and electrical clearances verified again.
Core and coil assembly is placed inside its tank, tank is filled with oil to stop exposure
to air, a cover assembly added and bushing connections made. Once the cover is welded
and leak checked, galvanized radiators are assembled to the tank, the unit is drained and
the transformer is vacuum filled for final testing. During this same time, the control box
and any other customer-specified monitors and components are installed and wired. All
external welded assemblies are mounted to ensure correct fit to the tank and removed
before the finished transformer moves on for factory acceptance testing.

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 Chapter 4
Testing of transformers
 Measurement of voltage ratio and check of phase displacement.
 Measurement of winding resistance.
 Measurement of insulation resistance.
 Measurement of no load loss and no load current.
 Measurement of load loss and impedance voltage with calculation of stray loss copper
loss at 750C.
 Calculation of efficiency and regulation.
 Separate source voltage withstand test. Induced over voltage withstand test.
 Magnetic balance test.
 Temperature rise test.

Measurement of winding resistance:

Winding resistance measurements are an important diagnostic tool for assessing
possible damage to transformers resulting from poor design, assembly, handling, unfavorable
environments, overloading or poor maintenance.

The main purpose of this test is to check for gross differences between windings and
for opens in the connections. Measuring the resistance of transformer windings assures that
each circuit is wired properly and that all connections are tight.

Winding resistance in transformers will change due to shorted turns, loose

connections, or deteriorating contacts in tap changers. Regardless of the configuration, the
resistance measurements are normally made phase-to-phase and the readings are compared
with each other to determine if they are acceptable.

Transformer winding resistance measurements are obtained by passing a known DC

current through the winding under test and measuring the voltage drop across each terminal
(Ohm's Law). Modern test equipment for this purposes utilizes a Kelvin bridge to achieve
results; you might think of a winding resistance test set as a very large low-resistance
ohmmeter (DLRO).

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Measurement of Insulation Resistance:
Insulation resistance tests are made to determine insulation resistance from individual
windings to ground or between individual windings. Insulation resistance tests are commonly
measured directly in megohms or may be calculated from measurements of applied voltage
and leakage current.

The recommended practice in measuring insulation resistance is to always ground the

tank (and the core). Short circuit each winding of the transformer at the bushing
terminals. Resistance measurements are then made between each winding and all other
windings grounded.

Insul
ation resistance testing: HV – Earth and HV – LV
Transformer windings are never left floating for insulation resistance measurements.
Solidly grounded winding must have the ground removed in order to measure the
insulation resistance of the winding grounded. If the ground cannot be removed, as in the
case of some windings with solidly grounded neutrals, the insulation resistance of the
winding cannot be measured. Treat it as part of the grounded section of the circuit.
We need to test winding to winding and winding to ground ( E ).For three phase
transformers, We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta
transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye
transformers.
The IR value of transformers are influenced by
 Surface condition of the terminal bushing

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  Quality of oil
 Quality of winding insulation
 Temperature of oil
 Duration of application and value of test voltage


Measurement of No Load Loss and No load current in transformer:

The no-load losses are very much related to the operational performance of a
transformer. As long as the transformer is operated, these losses occur. For this reason, no
load losses are very important for operational economy. No-load losses are also used in
the heating test.
The no-load loss and current measurements of a transformer are made while one of
the windings (usually the HV winding) is kept open and the other winding is supplied at
the rated voltage and frequency. During this test the no-load current (Io) and the no-load
losses (Po) are measured.
The measured losses depend heavily on the applied voltage waveform and frequency.
For this reason, the waveform of the voltage should be very sinusoidal and at rated
frequency.
Normally, the measurements are made while the supply voltage is increased at equal
intervals from 90% to 115% of the transformer rated voltage (Un) and this way the values
at the rated voltage can also be found.

The no-load losses of a transformer are grouped in three main topics:

1. Iron losses at the core of the transformer,
2. Dielectric losses at the insulating material and
3. The copper losses due to no-load current.
The last two of them are very small in value and can be ignored.
So, only the iron losses are considered in determining the no-load losses.

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Induced over voltage withstand test:

 The aim of this test is to check the insulation both between phases and between turns
of the windings and also the insulation between the input terminals of the graded
insulation windings and earth
 During test, normally the test voltage is applied to the low voltage winding.
Meanwhile HV windings should be keeping open and earthed from a common point.
 Since the test voltage will be much higher than the transformer’s rated voltage, the
test frequency should not be less than twice the rated frequency value, in order to
avoid oversaturation of the transformer core.
 The test shall start with a voltage lower than 1/3 the full test voltage and it shall be
quickly increased up to desired value.
 The test voltage can either be measured on a voltage divider connected to the HV
terminal or on a voltage transformer and voltmeter which have been set together with
this voltage divider at the LV side. Another method is to measure the test voltage with
a peak-value measuring instrument at the measuring-tap end of the capacitor type
bushing (if any).
 Test period which should not be less than 15 seconds.
 It is calculated according ,Test period=120 seconds x ( Rated frequency / Test
frequency )
 The duration of the test shall be 60 second.
 The test is accepted to be successful if no surges, voltage collapses or extreme
increases in the current have occurred.

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Separate Source Voltage Withstand Test:

 All the terminals of the winding under test should be connected together and the
voltage should be applied.
 The secondary windings of bushing type current transformers should be connected
together and earthed. The current should be stable during test and no surges should
occur.
 A single phase power frequency voltage of shape approximately sinusoidal is applied
for 60 seconds to the terminals of the winding under test.
 The test shall be performed on all the windings one by one.
 The test is successful if no breakdown in the dielectric of the insulation occurs during
test.
 During the Separate source AC withstand voltage test, the frequency of the test
voltage should be equal to the transformer’s rated frequency or should be not less than
80% of this frequency. In this way, 60 Hz transformers can also be tested at 50 Hz.
The shape of the voltage should be single phase and sinusoidal as far as possible.
 This test is applied to the star point (neutral point) of uniform insulated windings and
gradual (non-uniform) insulation windings. Every point of the winding which test
voltage has been applied is accepted to be tested with this voltage.
 The test voltage is measured with the help of a voltage divider. The test voltage
should be read from voltmeter as peak value divided by2. Test period is 1 minute.

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Magnetic balance test:
The Magnetic Balance test is conducted on Transformers to identify inter turn faults
and magnetic imbalance. The magnetic balance test is usually done on the star side of a
transformer. This indicates that the transformer is magnetically balanced. If there is any
inter-turn short circuit that may result in the sum of the two voltages not being equal to
the applied voltage. The Magnetic balance test is only an indicative test for the
transformer. Its results are not absolute. It needs to be used in conjunction with other tests.
One of the important precommissioning tests normally conducted on a transformer is the
core-balance test, also known as the magnetic balance test.
Magnetic Balance test is carried out by applying a low single phase voltage at 50 or
60 Hz to each H.V winding in succession and measuring all the other voltages both on
high voltage and low voltage side . Under ideal conditions, the results on each limb
conform to a particular pattern, depending upon the magnetic lengths to be traversed by
the flux the reluctance of the various portions and the division of the flux in the return
paths provided by the remaining two limbs.
Any inter-turn fault in one limb may cause the so called magnetic balance to be upset.
But the chances of such an inter-turn fault occurring during transit (as the transformers
are tested for their inter-turn strength at the factory) are nil, unless a serious accident
causes the windings to be damaged.
Another contributory factor can be the failure of the core bolts, both in the yoke and
in the limbs. While there is no possibility of testing the limb-bolts for failure of the limb-
bolt insulation once the coils are lowered, such a fault should show up durng factory tests.
The recent trend towards binding the cores with resin-bonded fibre-glass tapes at once
gives two advantages that the failure of a magnetic circuit due to limb bolts is fully
eliminated and the resilient fibre-glass tapes give much needed flexibility, when the core
becomes hot during service conditions. The chances of such a core bolt failure
(mechanical failure of the core bolt insulation or clamp to bolt insulation) occurring
during transit are almost nil unless the entire unit is subjected to violent jerks during
transit.

Temperature rise test:

In this test we check whether the temperature rising limit of transformer winding and
oil as per specification or not.
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 First the LV winding of the transformer is short circuited.
Then one thermometer is placed in a pocket in transformer top cover. Other two
thermometers are placed at the inlet and outlet of the cooler bank respectively.
The voltage of such value is applied to the HV winding that power input is equal to no
load losses plus load losses corrected to a reference temperature of 75oC.
The total losses are measured by three wattmeters method.

During the test, hourly readings of top oil temperature are taken from the thermometer
already placed in the pocket of top cover.
Hourly readings of the thermometers placed at inlet and outlet of the cooler bank are
also noted to calculate the mean temperature of the oil.
Ambient temperature is measured by means of thermometer placed around the
transformer at three or four points situated at a distance of 1 to 2 meter from and half-way
up the cooling surface of the transformer.
Temperature rise test for top oil of transformer should be continued until the top oil
temperature has reached an approximate steady value that means testing would be
continued until the temperature increment of the top oil becomes less than 3 oC in one
hour. This steady value of top oil is determined as final temperature rise of transformer
insulating oil.
There is another method of determination of oil temperature. Here the test in allowed
to be continued until the top oil temperature rise does not vary more than 1 oC per hour for
four consecutive hours.
The least reading is taken
as final temperature rise
of the oil.
During temperature rise test
for top oil of transformer we
make the LV winding short
circuited and apply voltage
to the HV winding. So for
full load rated current flows
in the transformer, the supply
voltage required will much
less than rated transformer
34
voltage. We know that core loss of a transformer depends upon voltage. So there will not be
any considerable core loss occurs in the transformer during test. But for getting actual
temperature rise of the oil in a transformer, we have to compensate the lack of core losses by
additional copper loss in the transformer. For supplying this total losses, transformer draws
current from the source much more than its rated value for transformer.

35
 CONCLUSION

Working at VVE transformers, we studied about design, various stages and materials
used in transformer construction, assembling of transformer parts and cooling mechanism in
oil cooled transformers.

We have learnt about various methods of testing of transformers and the design and
working principles of distribution and power transformers.

We have learnt about the ways of constructing a transformer based on its use,
placement of transformer and its ratings.

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 REFERENCES

1.Dr P. S. Bimbhra “ Electrical Machinery” 7th Edition Khanna Publishers, 2003

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