This report is a short description of our one week internship carried out as a component of the BE programme. The internship was carried out within the company Vignesh Vidyuth Controls in 2016. Since we are interested in transformers and their manufacturing process, the work was concentrated on manufacture, design, and mainly testing of the produced transformers.

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BMS INSTITUTE OF TECHNOLOGY
Bengaluru 560064
(Affiliated to VTU)
INTERNSHIP PROJECT REPORT
(As A Part Of Academic Studies)
ON
TRANSFORMER MANUFACTURING
Under The Guidance Of
Mr . Madhusudhan
Internship guide - Manjunath
Submitted By
Amaresh 1BY13EE005
M N Harshan Gowda 1BY13EE019
Manjunath R D 1BY13EE021
Niteesh Shanbog 1BY13EE026
Nagesh L 1BY13EE025
Prashanth C 1BY13EE032

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SL.NO TOPICS PAGE NO
1) Introduction 4
2) Company description 5-8
3) Departments of the company
 Procurement department
 Stores department
 Design and technical
support department
 Production department
 Testing department
 Quality control
9-17
4) Transformers
 Principle
 Classification
 Parts
 Transformers manufactured by
VVC
18-29
5) Tasks performed
 Manufacturing process
 Winding section
 Core section
 Assembly section
 Inspection of quality
 Testing of transformers
30-47
6) Testing of transformers
A. Routine test
a) Parametric
b) Dielectric
B. Type test
a) Temperature rise test
48-85

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b) Impulse test
C. Special test
7) Reflections
 Experience
 Technical outcomes
 Non technical
8) Conclusions 87
9) References

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Introduction
This report is a short description of our one week internship carried out as a
component of the BE programme. The internship was carried out within the
company Vignesh Vidyuth Controls in 2016. Since we are interested in
transformers and their manufacturing process, the work was concentrated
on manufacture, design, and mainly testing of the produced transformers.
At the beginning of the internship we formulated several learning goals,
which we wanted to achieve:
 to understand the functioning and working conditions of a transformer
manufacturing industry
 to see what is like to work in a professional environment;
 to see if this kind of work is a possibility for my future career;
 to better use and understand my academic knowledge in a practical
environment
 to see what skills and knowledge I still need to work in a professional
environment;
 To learn about the organizing of a manufacturing industry (planning,
preparation, permissions etc.)
 to learn about various testing methodologies
 to get fieldwork experience/collect data in an environment unknown
for me;
 to get experience in working in with persons who are intimately
associated with the industry;
 to enhance my communication skills
 to build a network.
This internship report contains my activities that have contributed to achieve
a number of my stated goals. In the following chapter a description of the
organization Vignesh Vidyuth Controls and their activities is given. After this
a reflection on my functioning, and the learning goals achieved during the
internship are described. Finally I give a conclusion on the internship
experience according to my learning goals.

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ABOUT THE COMPANY
1.1 INTRODUCTION
I take pleasure in introducing “Vignesh Vidutyh Controls (VVC)” as one
of the leading manufacturer of TRANSFORMERS for over 6 years with special
experience in repairing and manufacturing of Distribution Transformer.
VVC offers better latest computerized design, assured quality, quicker
delivery, economical price and total prompt service right from initial stage
to commissioning and thereafter to ensure smooth and economical
working.
To ensure reliability most of the vital components are manufactured
in our work-shop for which necessary manufacturing infrastructure
exists including an excellent testing room, dust free air conditioned
winding room, vacuum filtering, fabrication department, painting, and
assembly department etc. All the “VIGNESH VIDYUTH CONTROLS
(vvc)” products are manufactured to conform relevant ISO 9001-2008
certified and our quality plan and are engineered to perfectly meet
provided specifications and each product is backed by a
comprehensive service, which includes application, assistance, layout
plan and preinstallation advice. VVC is well positioned to provide its
customers with technology-driven, value-added solutions, leveraging a broad
product portfolio on the one hand, and enhancing the entire value-chain
quality, delivery, and services on the other hand.
We have our own manufacturing facility located at Peenya, spread across
4,000 sq. ft., equipped with the latest machinery, testing equipment and
well supported by competent and experienced engineers. Our constant
endeavor is to deliver consistent quality & reliability at competitive pricing.

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Vignesh Vidyuth Controls is the pioneer manufacturer of distribution
transformers in karnataka. The company has always been contributing
towards the advancement and development of the engineering sector by
introducing a range of quality electrical equipments.
The internship basically revolved around the distribution transformer
manufacturing, and maintenance. This report stated a very brief review of
what we have seen and learnt during my internship.
I have mentioned all these as I have made an internship as according to the
schedule. This report will give its reader knowledge about the Vignesh
Vidyuth Control and power division especially about transformer unit.
1.2 HISTRORY OF ORGANANIZATION
Established in 2000, Vignesh Vidyuth controls company is one of the
leading manufacturers of Distribution Transformers in Bangalore.
Vignesh Vidyuth controls company was established in the year 2000
basically for the purpose of repairing of existing transformers and later in the
year 2008 started the process of manufacturing the distribution
transformers. And then till now the company is producing the effective and
quality distribution transformers to contribute towards the society.
1.3 ORGANIZATION STRUCTURE
1.4 PRODUCTS MANUFACTURED
 Manufacturing of Distribution Transformer: A distribution
transformer or service transformer is a transformer that provides the

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final voltage transformation in the electric power distribution system,
stepping down the voltage used in the distribution lines to the level
used by the customer.
 Distribution Transformers
 Pole Mounted Substation
Rated Output (KVA):25,63,100, 160, 200, 250.
 Indoor mounted transformer
Rated Output (KVA):25, 63, 100, 160,200, 250,315,400,500,630,750
,1000,1250,1500 ut to 2000 KVA.
1.5 SERVICES OFFERED BY COMPANY
 Repairing of existing Transformers: Vignesh Vidyuth Controls will
also undertake the repairing of existing transformers from various
customers for any different types of faults that results in distribution
transformers.
1.6 NUMBER OF PEOPLE WORKING IN ORGANIZATION
SL NO CATEGORY TOTAL
1 Engineers 2

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2 Supervisors 2
3 Accountant 1
4 Skilled 5
5 Semiskilled 2
6 Helper 2
Total 14

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ABOUT THE DEPARTMENT
2.1. DISTRIBUTION TRANSFORMERS
USAGE
Distribution transformers are used for distribution networks in urban cities,
high rise buildings, rural electrification and industrial units.Vignesh Vidyuth
Controls supplied the equipment to various applications. Vignesh Vidyuth
Controls manufacture three phase oil cooled transformers and are available
from 25KVA to 2000 KVA.
DESIGN
The windings form the vital part of the transformer. Highly sophisticated
design techniques are applied for electrical, mechanical and thermal
stability.
Helical and continuous disc type windows are made as they provide
maximum strength and short circuit withstand capabilities. The coils are
pressed before core-coil assembly to ensure proper trouble free service.
Clamping rings are placed on top and bottom of the winding to ensure high
short circuit withstands capability to the transformer.
PROCESS
The core coil assembly is gently finished and cleaned tanks and locked into
position. The assemble then goes for a controlled heating and vacuum drying
process to ensure complete removal of moisture from the assembly. At the
end of the drying process oil is filled under high level of vacuum in the
transformer and then fixing of external components and top cover assembly

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is done. Soon after the vacuum filling, the transformer will be offered for
expiry of a specified standing time.
2.2. DESIGN DEPARTMENT
The Design and Drawings Division is computerized and the company is
adopting the latest technology in designing the transformers as per the BI
Standards and customers specifications
Every transformer is individually designed to its specific requirements and
applications. The following specially-developed computer programs are used
to further ensure the reliability of the product:
 optimization of design in relation to labor and material costs, loss
evaluation and sound level,
 distribution of voltage stresses during lightning impulse and switching
surge conditions,
 behavior during short-circuit conditions,
 analysis of those areas where high electrical stresses can occur, and,
 calculations of stray losses and thermal effects
Design Team
Before being issued to the plant, new designs are reviewed by a team
consisting of representatives from Engineering, Quality Assurance,
Manufacturing, Testing, and Research and Development. Each design
remains the responsibility of the individual project engineer, who carefully
follows and checks progress throughout the manufacturing process
2.3. PURCHASE DEPARTMENT:
The main functions of the Purchase Department are defined as follows

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 Procurement of stores through indigenous and foreign sources as
required in accordance with the rules in force.
 Checking of requisitions/purchase indents.
 Selection of suppliers for issue of enquiries.
 Issuing enquiries/tenders and obtaining quotations.
 Analysing quotations and bids etc., and preparation of comparative
statement (quotation charts).
 Consultation with the Indentor for selection and approval of quotations
and with Accounts Officer for pre-audit.
 Negotiating contracts.
 Checking legal conditions of contracts. Consulting Administrative
Officer or Secretary
 Issue of Purchase Orders.
 Follow-up of purchase orders for delivery in due time
 Verification and passing of suppliers’ bills to see that payments are
made promptly.
 Correspondence and dealing with suppliers, carriers etc., regarding
shortages, rejections etc., reported by the Stores Department.
 Maintenance of purchase records.
 Maintenance of progressive expenditure statement, sub-head wise.
 Maintenance of vendor performance records/data.
 Arrangement for Insurance Surveys, as and when necessary.
 Clearance of foreign consignments.
 Keeping various Departments/Divisions informed of the progress of
their indents in case of delay in obtaining supplies.
 Serving as an information center on the materials knowledge i.e. their
prices, source of supply, specification and other allied matters.
 Development of reliable and alternate sources of supply

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The Purchasing team handles all purchase requirements for the following
areas
 Production materials
 Non-production materials
 Capital equipment
 Engineering prototypes
 First article samples
 Services
 Contracts / Agreements
 Returns
 Invoice discrepancies
 Vendor-managed inventory
The major raw-materials required for manufacture of good quality of
transformers are as follows: -
 Silicon Steel Laminations - Cold Rolled Grain Oriented Steel.
 Electrical Grade Double Paper Covered Copper/Aluminium Conductors.
 Transformer Oil
 Insulating Materials like, Insulating Paper, Press Boards,
Porcelain Insulators, Varnish, Paper Tubes, Varnish & Paints etc.,
 Bushing Metal Parts and various other MS Items.
 Various other fittings like Oil Gauges,
 Silica gel Breathers, etc.
 Mild Steel Tanks
2.4. QUALITY CONTROL & TESTING DEPARTMENT
This Division will monitor the various manufacturing activities, up to final
testing of transformers to ensure that the transformers coming out will
conform to the Indian standards and customers requirement. The various
inspection procedures adopted are as under:

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 Inspection of raw-materials received from various vendors.
 Stage inspection at various levels in manufacturing shops.
 Final Testing of the transformers as per Indian Standards.
 Inspection at the dispatch area before dispatch to ensure that
 the transformers are fitted with all accessories etc.,
The various procedures and tests conducted are as per the Indian Standards
which are quite elaborative. This department is headed by qualified and
experienced engineers and is having a team of qualified personnel at various
stages and they will be independent from production department to ensure
that the quality parameters are complied with, at various stages of
manufacture.
The raw materials are tested to ensure that the quality is up to the
recommended standards , these tests are done to check the durability,
strength and the of the material.
The raw materials that are being tested are Oil , Aluminum conductors,
bushings ,press boards, Kraft paper and cork sheet.
OIL:
Three tests are carried out for oil by using BDV test set and resistivity/tanδ
test set.
 Breakdown voltage (BDV) test
 Resistivity test
 Tanδ test
DPC ALUMINIUM CONDUCTOR:
 Lapping test
 Tensile strength test

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BUSHINGS:
 BDV test
 Porosity test
PRESS BOARDS:
 BDV test
 Tensile strength test
KRAFT PAPER:
 Tensile strength test
CORK SHEET:
 Tensile strength test
TRANSFORMER TESTING:
Testing is an important activity in the manufacture of a transformer.The
basic testing requirements and testing codes are set accprding to the IS
(Indian Standard) standards. There are seven vital tests that are performed
on a transformer before it is being dispatched.
The various tests performed are
 High Voltage (HV) test
 Double the voltage-double the frequency(DVDF) test
 Short Circuit (SC)test
 Open Circuit(OC) test
 Turns ratio test
 Resistance test
 Meggar test.

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2.4. LIST OF TESTING EQUIPMENTS
Sl. No. DECRIPTION QUANTITY
01 3-Phase, 2-Wattmeter Method Testing of penal
Board
01 No
02 3-phase turns ratio meter 01 No
03 3- Phase Insulation tester-2.5kv(Meager) 02 No’s
04 High Voltage Kit-60KV 1 No
05 Oil Test kit (BDV Kit) 01 No
06 Micro ohm Meter 01 No
07 Muttering Tong Test 01 No
08 Phase Sequence meter 01 No
09 Double Voltage Double Frequency meter (DVDF) 01 No
10 Screw Gauge & Slide Calipers 01 No
11 Power Anal laser meter 01 No
12 Caliper 01 No

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2.5. LIST OF MATERIAL SUPPLIERS:
Sl.
No.
Name of the material Name of the Suppliers
01 Aluminum Wire & strips Geepee Electricals, Bangalore.
02 Copper Wire & strips Shreegee Insulated Conductors,
Bangalore
03 Aluminum & Copper Wire
& strips
Southren Wire & strips,
Bangalore
04 Aluminum & Copper Wire
& strips
Quality Insulated Conductors,
Bangalore
05 Aluminum & Copper Wire Sree Durga ParameshwriMetal
Products, Bangalore
06 Aluminum & Copper Wire Padmavathi Extrusions,
Bangalore
07 Aluminum Wires Swatik Wire Industries, Hubli.
08 CRGO Lamination (Core) F.S Enterprises, Bangalore
09 CRGO Lamination (Core) Vijayalakshmi Transcore,
Bangalore
10 CRGO Lamination (Core) Sil core Industries, Bangalore
11 CRGO Lamination (Core) Shine Enterprises, Bangalore
12 CRGO Lamination (Core) AJ Transcore, Bangalore
13 CRGO Lamination (Core) Monarch Transcore, pune
14 MS, Channels, Flats etc Steel-N- Scrap, Bangalore
15 MS, Channels, Flats etc Varun Steels, Bangalore
16 MS, Channels, Flats etc Shiva Ferric pvt ltd, Bangalore
17 Insulation material Sri Venkatadri Press Boards ,
Hydrabad

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18 Insulation material Pragathi Agenceses, Bangalore
19 Brass Metal parts Sujatha Engineering & allied
Industries
20 Brass Metal parts Vishal Enterprises, Bangalore
21 Transil Oil (Transformer
Oil)
Gokul Industries, Bangalore
22 Transil Oil (Transformer
Oil)
Chaitra Enterprises, Bangalore
23 Radiators Mallikarjuna Engineering,
Bangalore
24 Fabricated Tanks Sri Vinayaka Industries,
Bangalore
25 Fabricated Tanks Balaji Fab tech, Bangalore
26 Tap Switches Sujatha Engineering & allied
industries, Bangalore

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TRANSFORMER
INTRODUCTION
A Transformer is a static machine.
The word 'transformer' comes from the word 'transform'.
Transformer is not an energy conversion device,but it is a device that
changes AC electrical power at one voltage level to AC electrical power at
another voltage level,through the action of magnetic field ,without change in
frequency.
It can be either to step-up or step-down.
PRINCIPLE OF OPERATION OF A TRANSFORMER
As soon as the primary winding is connected to the single –phase as supply,
an ac current starts flowing through it.
The ac primary current produces an alternating flux in the core.
Most of this changing flux gets linked with the secondary winding through
the core.
The varying flux will induce voltage into the secondary winding according to
the faraday's laws of electromagnetic induction.
Thus due to primary current, there is an induced voltage in the secondary
winding due to mutual induction. Hence the emf induced in the secondary is
called as the mutually induced emf.
THE PRINCIPLE PARTS OF A TRANSFORMER AND ITS FUNCTIONS
ARE:
 The core, which makes a path for the magnetic flux.
 The primary coil, which receives energy from the ac source.
 The secondary coil, which receives energy from the primary winding and
delivers it to the load.
 The enclosure, which protects the transformer from dirt, moisture, and
mechanical damage.

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CONSTRUCTION AND WORKING OF A SINGLE PHASE TRANSFORMER
The construction of a single- phase transformer is as shown in Fig. It
consists of two highly inductive coils (windings) wound on an iron or steel
core. The winding (Coil) connected to the ac supply is called as primary
winding whereas the other one is called as the secondary winding. The ac
supply is connected to the primary winding where the load is connected to
the secondary winding. The primary and secondary winding are isolated from
each other as well as from the iron core thus there is absolutely no physical
connection between the primary and secondary windings.

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CONSTRUCTION OF A TRANSFORMER
The
most important parts of a transformer are the windings (coils) and the core.
However for the large capacity transformers, some other parts such as
suitable tank, conservator, bushings, breather, explosion vent etc. are also
used along with the core and windings. The construction of a large single
phase transformer is shown in Fig.
LAMINATED STEEL CORE
The material used for the construction of the transformer core is silicon
steel. It is used for its high permeability and low magnetic reluctance. Due
to this the magnetic field produced in the core is very strong. The core is in
the form of stacks of laminated thin steels sheet which are electrically
isolated from each other. The laminations are typically 0.35 mm thick.

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The core is
assembled in such a way, that the assembly provides a continuous path for
the magnetic flux, with a minimum air gap.In order to reduce magnetizing
current,the interleaving at the lamination joints should be done with utmost
care.The gaps between laminations must not be greater than 1-2mm.
CRGO steel sheets with an approximate silicon content of 3% is typically
used for magnetic circuits of trnasformers.CRGO steel has the following
advantages :
1. Magnetic induction is maximumand the loop of BH curve is large
2. Core loss during no load operation of the transformer is low.
3. Reactive power input at no load operation of the transformer is low.
4. Good mechanical properties
WINDINGS OF THE TRANSFORMER
we have shown the primary and
secondary windings to be on two different limbs of the core. But such an
arrangement is made practically, then a part of the flux produced in the core

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will not be linked to the secondary windings at all. This is called as the
leakage flux. In order to avoid this, the primary and secondary windings are
mounted on the same limb of the core as shown in Fig
TRANSFORMER TANK
The whole assembly of large size
transformer is placed in a sheet metal tank. Inside the tank the assembly of
the transformer is immersed in oil which acts as an insulator as well as a
coolant. The oil will take out the heart produced by the transformer windings
and core and transformer it to the surface of the transformer tank.
While designing tanks for transformers, a large number of factors have to be
considered.These factors include keeping the weigth ,stray load losses and
cost a minimum.
The tanks should be strong enough to whithstand stresses produced by
jacking and lifting.The size of the tank must be large enough to accomodate
cores,windings,internal connection and also must give the the requisite
clearance between the windings and the walls.
FUNCTION OF OIL TANK
One of the most important factors which determines the life and satisfactory
operation of a transformer is the oil in which it is immersed.The construction
of the transformer should be such that the heat generated at the core and at
the windings should be removed efficiently. Moreover, in order to avoid the
insulation deterioration, the moisture should not be allowed to creep into the
insulation. Both these objectives can be achieved by immersing the built up
transformer in a closed tank filled with noninflammable insulating oil called

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transformer oil. In order to increase the cooling surface exposed to
ambient, tubes or fins are provided on the outside of tank walls.
The transformer oil has two prime functions:
1. To create an acceptable level of insulation in conjunction with insulated
conductors and coils.
2. To provide a cooling medium capable of extracting quantites of heat
without deterioration as an insulating medium.
Transformer oil tends to deteriorate in service,but this tendancy can be
greatly reduced by paying attention to transformer operating conditions and
to oil itself when this is shown to be necessary as the result of regular tests.
The fig below shows the oil filteration tank:
CONSERVATOR
In large transformers, some empty space is always provided above the oil
level. This space is essential for letting the oil to expand or contract due to
the temperature changes. When the oil temperature increase, it expands
and the air will be expelled out from the conservator. Whereas when the oil
cools, it contracts and the outside air gets sucked inside the conservator.
This process is called as the breathing of the transformer. However, the

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outside air which has being drawn in can have the moisture content. When
such an air comes in contact with the oil, the oil will absorb the moisture
content and loses its insulating properties, to some extent. This can be
prevented by using a conservator. The conservator is a cylindrical shaped air
tight metal drum placed on the transformer tank. The conservator is
connected to the tank by a pipe. The oil level in the conservator is such that,
always some empty space is available above the oil. Due to the use of
conservator, the main tank will be always full with oil and the surface of oil
in the tank will not be exposed directly to the air.
With the use of conservators,interchange of oil between conservatorsand
main tank as a result of temperature change is slow.Sludge formation is
considerably reduced and whatever sludge is formed remains in the
conservator and there is no sludge formation in the main tank.This is a great
improvement over the ordinary tank with air space above the oil.
The fig below shows conservator of a transformers
BREATHER
The apparatus through which breathing of the transformer take place is
called as “Breather”. The air goes in or out through the breather. To reduce
the moisture content of this air, some drying agent (material that absorbs

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moisture) such as silica gel or calcium chloride is used in the breather. The
dust particles present in the air are also removed by the breather.
The breather consists of a small container connected to the vent pipe and
contains a dehydrating material like silca gel crystals impregnated with
cobalt chloride.The material is blue is dry and a whitish pink when damp.The
colour can be observed through a glass window provided in front of the
container.

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BUCCHOLZ RELAY
There is a pipe connecting the tank and conservator. On this pipe a
protective is called Buccholz is mounted. When the transformer is about to
be faulty and draw large currents, the oil becomes very hot and
decomposes. During this process different types of gases are liberated. The
Buccholz relay get operated by these gases and given an alarm to the
operator. If the fault continues to persist, they the relay will trip off the main
circuit breaker to protect the transformer.

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Temperature Indicators
The most obvious indicator of transformer temperature is the temperature of
the hot oil.The oil temperature is measured by a dial type type
thermometer.The bulb of the thermometer is mounted in the oil and the dial
is mounted outside the tank.
Winding temperature indicator is a thermometer witha bulb.The
thermometer is immersed in oil and the bulb is heated by heaters which
carry a current proportional to the winding current.Therefore ,the reading of
the thermometer is an analogue indication of winding temperature.
EXPLOSION VENT
The explosion vent or relief value is the bent up pipe fitted on the main
tank. The explosion vent consists of a glass diaphragm or aluminum foil.
When the transformer becomes faculty, the cooling oil will get decomposed
and various types of gasses are liberated. If the gas pressure reaches a
certain level then the diaphragm in the explosion vent will burst to release
the pressure. This will save the main tank from getting damaged.The relief
device must be above the level of oil in the conservator in order to prevent
to overflow of oil in case the device operates.

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TRANSFORMERS MANUFACTURED BY VVC
USAGE
Distribution transformers are used for distribution networks in urban cities,
high rise buildings, rural electrification and industrial units. The usage and
the ambient conditions vary widely as vvc makes transformer are being
supplied to varies places. Vvc can proudly claim to have supplied the
equipment to varies applications. VVC manufactures three phase oil cool
transformers and are available from 25kva to 2500kva and special
transformers.
the company has supplied more than 1000 numbers of transformers to
varies turn key projects, GESCOM, HESCOM, KIADB, KSSIDC, ISRO, PWD,
KHB, KPHC, Private layouts, Educational institutes, Builders, Developers etc
within a short of 5 years.

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DESIGN
The windings from the most vital parts of transformers. Highly sophisticated
design techniques are applied for electrical, mechanical and thermal
stability.
Helical and continuous disc type windows are made as they provide
maximum strength and short circuit withstand capabilities. In case of
multiple radial wires continuous transpositions are done to eliminate inter
strand circulating currents.
The coils are pressed before the core coil assembly to ensure proper trouble
free service. Clamping rings are placed on top and bottom of winding to
ensure high short circuit withstand capability to the transformer.

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Tasks Performed
Our internship was completely focused on assembly and testing of
transformers. We participated and observed the process of assembling a
transformer from scratch with the procured items and participated in the
testing process of the transformers. In the next paragraphs each activity is
discussed in more detail. A time schedule of the activities during my
internship is given:
 Manufacturing process
The core coil assembly is gently lowered into the finished and clean
tanks and logged into position. The assembly then goes for a
controlled heating and vacuum drying process to ensure complete
removal of moisture from the assembly . At the of the drying process
oil is filled under high level of vacuum in the transformer and then
fixing of external components and top cover assembly is done . soon
after the vacuum filling, the transformer will be offered for expiry of a
specified standing time
 Winding department
In the first day of the internship we saw the winding department of the
company. The winding department is responsible for getting the
windings of the transformers ready. The windings are prepared using
aluminium strips for LV coil and copper strips for HV coil.
Two types of windings was shown to us
1. Cylindrical winding
These windings are layered type and use either rectangular or
round type conductors.
2. Helical windings
These windings have its turns wound in an axial direction along a
scre line with an inclination corresponding to the height of a
conductor and an oil duct between turns

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CONSTRUCTION OF WINDINGS
The windings consist of the current-carrying conductors wound around
the sections of the core, and these must be properly insulated, supported
and cooled to withstand operational and test conditions.
The terms winding and coil are used interchangeably in this
discussion. Copper and aluminum are the primary materials used as
conductors in power-transformer windings.
While aluminum is lighter and generally less expensive than copper, a
larger cross section of aluminum conductor must be used to carry a
current with similar performance as copper. Copper has higher mechanical
strength and is used almost exclusively in all but the smaller size ranges,
where aluminum conductors may be perfectly acceptable.
In cases where extreme forces are encountered, materials such as silver-
bearing copper can be used for even greater strength.
The conductors used in power transformers are typically stranded with a
rectangular cross section, although some transformers at the lowest
ratings may use sheet or foil conductors. Multiple strands can be wound in
parallel and joined together at the ends ofthe winding, in which case it is
necessary to transpose the strands at various points throughout the
winding to prevent circulating currents around the loop(s) created by
joining the strands at the ends.
Individual strands may be subjected to differences in the flux field due to
their respective positions within the winding, which create differences in
voltages between the strands and drive circulating currents through the
conductor loops.

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Figure 1 – Continuously transposed cable (CTC)
Proper transposition ofthe strands cancels out these voltage differences and
eliminates or greatly reduces the circulating currents. A variation ofthis
technique,involving many rectangular conductor strands combined into a
cable, is called continuously transposed cable (CTC), as shown
in Figure 1.
In core-form transformers,the windings are usually arranged concentrically
around the core leg, as illustrated in Figure 2, which shows a winding
being lowered over another winding already on the core leg of a three-
phase transformer.
A schematic of coils arranged in this three-phase application was also
shown in Figure 1 (article ‘Power Transformer Construction – Core’).
Shell-form transformers use a similar concentric arrangement or an inter-
leaved arrangement, as illustrated in the schematic Figure 3 and the
photograph in Figure 7.
Figure 2 – Concentric arrangement,
outer coil being lowered onto core leg over top of inner coil

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Figure 3 – Example of stacking (interleaved)
arrangement of windings in shell-form construction
With an interleaved arrangement, individual coils are stacked, separated by
insulating barriers and cooling ducts. The coils are typically connected with
the inside of one coil connected to the inside of an adjacent coil and,
similarly, the outside of one coil connected to the outside of an adjacent
coil. Sets of coils are assembled into groups, which then form the primary
or secondary winding.
When considering concentric windings, it is generally understood that
circular windings have inherently higher mechanical strength than
rectangular windings, whereas rectangular coils can have lower associated
material and labor costs.
Rectangular windings permit a more efficient use of space, but their use is
limited to small power transformers and the lower range of medium-power
transformers, where the internal forces are not extremely high. As the
rating increases, the forces significantly increase, and there is need for
added strength in the windings, so circular coils, or shell-form construction
are used.

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In some special cases, elliptically shaped windings are used.
Concentric coils are typically wound over cylinders with spacers attached so
as to form a duct between the conductors and the cylinder. The flow of
liquid through the windings can be based solely on natural convection, or
the flow can be somewhat controlled through the use of strategically placed
barriers within the winding.
Figures 4 and 5 show winding arrangements comparing nondirected and
directed flow. This concept is sometimes referred to as guided liquid
flow.
Figure 4 – Nondirected flow
A variety of different types of windings have been used in power
transformers through the years. Coils can be wound in an upright, vertical
orientation, as is necessary with larger, heavier coils; or they can be
wound horizontally and placed upright upon completion.
As mentioned previously, the type of winding depends on the transformer
rating as well as the core construction. Several of the more
common winding types are discussed below.

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Figure 5 – Directed flow
Pancake Windings
Several types of windings are commonly referred to as “pancake” windings
due to the arrangement of conductors into discs. However, the term most
often refers to a coil type that is used almost exclusively in shell-form
transformers.
The conductors are wound around a rectangular form, with the widest
face of the conductor oriented either horizontally or vertically. Figure
6 illustrates how these coils are typically wound. This type of winding lends
itself to the interleaved arrangement previously discussed (Figure 7).
Figure 6 – Pancake winding during
winding process

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Figure 7 – Stacked pancake
windings
Layer (Barrel) Windings
Layer (barrel) windings are among the simplest of windings in that the
insulated conductors are wound directly next to each other around the
cylinder and spacers.
Several layers can be wound on top of one another, with the layers
separated by solid insulation,ducts,or a combination. Several strands can
be wound in parallel ifthe current magnitude so dictates.
Variations of this winding are often used for applications such as tap
windings used inload-tap-changing (LTC) transformers and for tertiary
windings used for,among other things,third-harmonic suppression.
Figure 8 shows a layer winding during assembly that will be used as a
regulating winding in an LTC transformer.

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Figure 8 – Layer windings (single
layer with two strands wound in parallel)
Helical Windings
Helical windings are also referred to as screw or spiral windings, with
each term accurately characterizing the coil’s construction.
A helical winding consists of a few to more than 100 insulated strands
wound in parallel continuously along the length of the cylinder, with
spacers inserted between adjacent turns or discs and suitable
transpositions included to minimize circulating currents between parallel
strands.
Figure 9 – Helical winding during assembly
The manner of construction is such that the coil resembles a
corkscrew. Figure 9 shows a helical winding during the winding process.
Helical windings are used for the higher-current applications
frequently encountered in the lower-voltage classes.

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Disc Windings
A disc winding can involve a single strand or several strands of
insulated conductors wound in a series of parallel discs of horizontal
orientation, with the discs connected at either the inside or outside as
a crossover point. Each disc comprises multiple turns wound over other
turns, with the crossovers alternating between inside and outside.
Figure 10 – Basic disc winding
layout
Figure 11 – Disc winding inner and outer
crossovers
Figure 10 outlines the basic concept, and Figure 11 shows typical
crossovers during the winding process.

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Most windings of 25-kV class and above used in core-form transformers are
disc type. Given the high voltages involved in test and operation,
particular attention is required to avoid high stresses between discs and
turns near the end of the winding when subjected to transient voltage
surges.
Numerous techniques have been developed to ensure an acceptable voltage
distribution along the winding under these conditions.
 Core Section
The second day of the internship was spent in studying the functioning
of the preparation of the core section.The core is made of laminated
sheets of CRGO (Cold Rolled Grain Oriented) Silicon Steel sheets which
are cut into the required shapes.All the sheets are arranged together
to form the core.We looked at how the core is assembled.
The core is responsible for iron losses in the transformer.Iron losses
mainly consist of hysteresis loss and eddy current loss.It is to reduce
this loss that we use laminated sheets instead off a solid core section.
The core in VVC is made using CRGO steel.The advantages of using
CRGO steel is that the hysteresis loss due to this material is very low
and it also has high resistivity.Also the sheets used must have very
low thickness and there must not be in burfs i.e, roughness in the
edges.Also the material selected must not rust easily.
The cores assembled int his way are kept in position by side plates
bolted together at intervals along the limbs and yoke.Holes are
punched out in the laminations in orderto accommodate the
bolts.These bolts which pass throufght the cores must be insulated
both from side plates and laminations.
The transformers assembled in Vignesh Vidyuth Controls have ONAN
(Oil Natural Air Natural) mechanism of cooling.This method uses the
ambient air as the cooling medium .The natural circulation of
surrounding air is utilized to carry away the heat generated by natural
convection.In addition to this,the transformer is immersed in oil and
the heat generated in cores and the windings is passed on to oil by
conduction.

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Construction of Core
Purpose of Transformer Core
In an electrical power transformer, there are primary, secondary and may be
tertiary windings. The performance of a transformer mainly depends upon
the flux linkages between these windings. For efficient flux linking between
these windings, one low reluctance magnetic path common to all windings
should be provided in the transformer. This low reluctance magnetic path in
transformer is known as core of transformer.
Material for Transformer Core
The main problem with transformer core is, its hysteresis loss and eddy
current loss in transformer. Hysteresis loss in transformer mainly depends
upon its core materials. It is found that, a small quantity of silicon alloyed
with low carbon content steel produces material for transformer core, which
has low hysteresis loss and high permeability. Because of increasing demand
of power, it is required to further reduce the core losses and for that,
another technique is employed on steel, which is known as cold rolling. This
technique arranges the orientation of grain in ferromagnetic steel in the
direction of rolling. The core steel which has under gone through both the
silicon alloying and cold rolling treatments is commonly known as CRGOS or
Cold Rolled Grain Oriented Silicon Steel. This material is now universally
used for manufacturing transformer core. Although this material has low
specific iron loss but still; it has some disadvantages, like, it is susceptible to
increase loss due to flux flow in direction other than grain orientation and it
also susceptible to impaired performance due to impact of bending and
blanking the cutting CRGOS sheet. Both the surfaces of the sheet are
provided with an insulating of oxide coating.
Optimum Design of Cross – Section of Transformer Core
The maximum flux density of CRGO steel is about 1.9 Tesla. Means the steel
becomes saturated at the flux density 1.9 Tesla. One important criteria for
the design of transformer core, is that, it must not be saturated during the

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transformer’s normal operation mode. Voltages of transformer depend upon
its total magnetizing flux. Total magnetizing flux through core is nothing but
the product of flux density and cross – sectional area of the core. Hence, flux
density of a core can be controlled by adjusting the cross sectional area of
the core during its design. The ideal shape of cross-section of a transformer
core is circular. For making perfect circular cross section, each and every
successive lamination steel sheet should be cut in different dimension and
size. This is absolutely uneconomical for practical manufacturing. In reality,
manufacturers use different groups or packets of predefined number of same
dimension lamination sheets. The group or packet is a block of laminated
sheets with a predefined optimum height (thickness). The core is an
assembly of these blocks in such a successive manner as per their size from
core central line, that it gives an optimum circular shape of the cross-
section. Such typical cross-section is shown in the figure below. Oil ducts are
needed for cooling the core. Cooling ducts are necessary because hot-spot
temperature may rise dangerously high and their number depends on the
core diameter and materials that get used for core. In addition to that,
clamp plates made of steel are needed on either sides of the core to clamp
the lamination. The steel sheet lamination blocks, oil ducts, and clamping
plates; all should lie within the peripheral of optimum core circle. The net
sectional area is calculated from the dimensions of various packets and
allowance is made for the space lost between lamination (known as stacking
factor) for which steel sheet of 0.28 mm thickness with insulation coating is
approximately 0.96. Area is also deducted for oil ducts. The ratio of net
cross sectional area of core to the gross cross - sectional area inside the
imaginary peripheral circle is known as Utilization factor of transformer core.
Increasing numbers of steps improve the Utilization factor but at the same
time, it increases manufacturing cost. Optimum numbers of steps are
between 6 (for smaller diameter) to 15 (larger diameter)’

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Manufacturing of Transformer Core
During core manufacturing in factory some factors are taken into
consideration,
1. Higher reliability.
2. Reduction in iron loss in transformer and magnetizing current.
3. Lowering material cost and labor cost.
4. abatement of noise levels.
Quality checking is necessary at every step of manufacturing to ensure
quality and reliability. The steel sheet must be tested for ensuring the
specific core loss or iron loss values. The lamination should be properly
checked and inspected visually, rusty and bend lamination should be
rejected. For reducing the transformer noises, the lamination should be
tightly clamped together and punch holes should be avoided as far as
possible to minimize cross flux iron losses. The air gap at the joint of limbs
and yokes should be reduced as much as possible for allowing maximum
smooth conducting paths for magnetizing current.
Corner Jointing of Limbs with Yokes
Core losses in transformer happen mainly due to,
1. Magnetic flux flow along the direction of the grain orientation,
2. Magnetic flux flow perpendicular to the direction of the grain
orientation, this is also known as cross grain iron losses. The cross grain loss
mainly occurs in the zones of corner jointing of limbs with yokes and it can

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be controlled to some extent by applying special corner jointing techniques.
There are normally two types of joints used in transformer core,
1. Interleaved joints
2. Mited joints
Interleaved Joints in Transformer Core
Interleaved joint in transformer core is the simplest form of joints. This joint
is shown in the figure. The flux leaves and enters at the joint in
perpendicular to grain orientation. Hence cross grain losses are high in this
type of joints. But considering the low manufacturing cost, it is preferable to
use in small rating transformer.
Mitred Joints in Transformer Core
Here the lamination's are cut at 45°. The limbs and yoke lamination edges
are placed face to face at the Mitred joints in transformer core. Here the flux
enters and leaves the lamination, gets smooth path in the direction of its
flow; hence, cross grain loss is minimum here. However it involves extra
manufacturing cost but it is preferable to use in electrical power transformer
where loss minimization is the main criteria in designing of transformer core.

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 Assembly department
In the third day of our internship we studied the assembly model
followed in Vignesh Vidyuth Controls for the manufacture of
transformers.The assembly section of the company follows the
following model of operation:
i. Core Coil Assembly:
The HV and LV coils prepared by the winding department and
the core section prepared by the core department are assembled
together.The width and thickness of the coils taking into account
the width of the paper insulation used was calculated.A distance
of 15mm is left between the core and the coil.Oil sheet is used to
provide insulation between HV and LV windings.For good design
the height of the coil must atleast 15mm smaller than the height
of the core.Wooden blocks are also used for better support and
also to provide insulation between the core and the MH channel
Care is taken to see that each part of the transformer i.e,
windings,core , MH channel and the tank are insulated from each
other.For this insulation paper,oil sheet as well as wooden blocks
are used.
ii. Before connection test
This test is performed to check the turns ratio if the transformer.
A TTR (Transformer Turns Ratio) Meter is used.The transformers
manufactured in Vignesh Vidyuth Controls are designed to have
44 turns.A tolerance level of 0.5% is acceptable.
We used the TTR meter to physically verify the turns ratio in the
transforemer.
iii. HT and LT connection
The star and delta connection is made on the three phase
winding winding.The wires are soldrerd together.Brazing powder
is used for soldering.Also unlike conventional soldering
process,liquid flux is used.
The voltage of power networks supplied supplied by transformers
can be controlled by changing the ratio of transformation of the
transformers.The change in ratio of transformation can be
affected by providing tapings on the transformer windings.The
transformers manufactured in Vignesh Vidyuth Controls are off

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circuit tap changing type.In this tyoe, the tappings are changed
by disconnecting the transformer from the supply.
iv. After connection test
This test is similar to BCT test.The turns ratio is checked with
TTR meter.The turns ratio for tapping is=n the HT winding is
found using TTR meter.The turns ratio in the HT winding is
varied from +2.5% to -10%
We tested this in a three phase transformer using the TTR
meter.The tested values are
First tap – 45.1
Second tap – 44
Third tap – 42.9
Fourth tap – 41.8
Fifth tap – 40.7
Sixth tap – 39.6
v. Furnace
The prepared transformer is placed in a furnace for 48 to 72
hours depending upon the kVA of the transformer.This is to
remove any moisture in the transformer.The furnace is heated to
upto 120C.The furnace is supplied with a 4 wire,3 phase heating
system.It also contains a fan in order to circulate the heat more
efficiently.
vi. Cover plate assembly
The transformer which is heated in the oven should be inserted
into the tank within 5-6 hours in order to prevent the entry of
moisture.
vii. Pre tanking section
The transformer tank should be inspected before inserting the
core coil assembly.Megger test should be conducted on the
transformer to check the insulation between HV and LV winding
and ground.
With the exception of individual laminations and core bolts,all
internal metal parts of the transformer require earthing.Due
care must be taken in the design of the earthing system to avoid
multiple paths which may inititate partial discharges because of
the circulating currents inducing relatively high voltage across
high impedance sections of an earth plate.
viii. Oil Filling Section

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The transformer must be filled with oil which is a good insulator
as well as a coolant.Paraffin based and Naphtha based mineral
oils are used for this purpose.A oil filteration unit is used to
recycle and reuse used oil.
ix. Paint
The oil tank is applied with varnish as a base for the paint to be
used.The transformer is then painted using paint spray units.
x. Finishing
The transformer is then fitted with breather,tap changing switch
and other external devices.
 Inspection of quality
Care is taken during design, manufacturing at each stage to ensure
trouble free operations and fulfilling customer requirements, the
company standard manufacturing includes 25kva to 2500kva in 11,22
and 33kv voltage classes. Special voltages and ratings are also
tailored as per customer requirements. Each distribution transformers
are dried under vacuum to consolidate the insulation to withstand of
art temperature indicators to match it with the best practices.
Transformers with 500kva and above can be supplied with protective
accessories as per customer requirements as bucholz relay, magnetic
oil level guage with low level alarm, OTI and WTI alarm and trip
contacts.
 Testing of Transformers
In the last two days of the internship,we conducted various tests to
ascertain the suitability of the transformer for operation in the real
world.The tests we performed were:
1) Turns ratio of all taps
By this we determined the turns ratio in the transformer
and we copared the values with specification
2) No Load Test

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By conducting this test,iron losses in the core was found
3) Load loss test
The copper losses of the transformer were determined at
various loads.From this data,we extrapolated the losses at
ambient temperature as well as at 75C.
4) Winding resistance test
The equivalent resistance of the HV and LV winding are
measured and the theoretical values of copper losses can
be calculated and then compared with practical values.
5) Induced Overvoltage test
Here the transformer is supplied with double voltage and
double frequency of the rated values.If the transformer
can withstand these conditions for 1 minute,the
transformer is then deemed to be suitable for operation.
6) Insulation resistance test of HV and LV windings
The insulation resistance of HV to ground, LV to
ground,and LV to HV are measured using a megger and
the values are found to be extremely high.This ensures
proper insulation between all the components in the
transformer and helps to avoid short circuit.
7) Magnetic Balance Test
This test was conducted on the transformer and there was
no imbalance in the circuit.
8) Vector Group Test
Vector group test of thransformer is to ensure the
customer specified vector group of transformer.For DYN11
transformer it was found that LV is leading HV with 30
degree.

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TESTING OF TRANSFORMERS
For confirming the specifications and performances of an electrical power
transformer it has to go through numbers of testing procedures. Some tests
are done at manufacturer premises before delivering the transformer. Mainly
two types of transformer testing are done at manufacturer premises- type
test of transformer and routine test of transformer. In addition to that
some transformer tests are also carried out at the consumer site before
commissioning and also periodically in regular & emergency basis through
out its service life.
Type of Transformer Testing
1. Routine tests
2. Type tests
3. Special tests
Routine test
1. Transformer winding resistance measurement.
2. Transformer ratio test.
3. Transformer vector group test.
4. Measurement of impedance voltage/short circuit impedance (principal
tap) and load loss (Short circuit test).
5. Measurement of no load loss and current (Open circuit test)
6. Measurement of insulation resistance.
7. Dielectric tests of transformer.
8. Tests on on-load tap-changer.
9. Oil pressure test on transformer to check against leakages past joints
and gaskets.

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That means Routine tests of transformer include all the type tests except
temperature rise and vacuum tests. The oil pressure test on transformer to
check against leakages past joints and gaskets is included.
Dielectric test
The dielectric test of transformer is generally performed in two different
steps, likewise, separate source voltage withstand test and induced voltage
withstand test of transformer, which we have discussed one by one below.
 Purpose of the test:
 Dielectric test is conducted on a medium used for insulation
purpose (oil in this case) to check the quality of the test oil.
 Circuit diagram:
Figure 1 BDV TEST INSTRUMENT

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 Procedure:
 The pot is rinsed thoroughly with the test oil.
 The oil that is to be tested is filled in a pot fitted with two
electrodes.The electrodes are placed at a distance of 2.5mm.
Figure 2 ELECTRODES AND THE POT
 Then the pot is placed in a kit connecting the electrodes to
supply ranging from (0 – 260kV) through auto transformer.
 The voltage level is slowly varied by auto transformer.
 At a particular voltage level the oil medium breaks down
resulting in a flash of spark.
 The voltage level is noted down and the procedure is repeated
two to three times.
 The value of the voltage level is noted. This value is the
breakdown voltage of the oil.
 Conclusion:
The lesser the breakdown voltage poorer the quality of oil.

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DVDF (Double Voltage And Double
Frequency Test)
 Purpose: To test the withstand capability of the transformer.
 Circuit diagram:
 About :
 It is a popular test conducted on transformer.
 When alternating current is applied to primary winding of the
transformer ,it draws magnetizing current.

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 This in turn produces alternating flux in the core of the
transformer.
 This flux links both primary and secondary windings and due to
this alternating emf is E=4.44∅fN volts
 When double voltage is applied withot doubling the frequency
the core starts to heat up abnormally.
 This disturbs the magnetizing properties of the core
permanently.
 induced across both windings.
 Procedure:
 In this test double the rated voltage an double the rated
frequency is applied.
 The withstand capabilities of the trtansformer is observed for a
minute.
 If the transformer fails to withstand then the currents start to
increase in the windings.
 The supply must be switched off to prevent further increasing .
Insulation Resistance Test (IRT) On
Transformer
 Purpose:
 Irt is conducted to check insulation resistance across
HV winding and Earth and across HV and LV winding.
 Circuit diagram:

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 Circuit diagram:
 Circuit diagram:
 Circuit diagram:
 Explaination:
 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. Insulation resistance
testing: HV – Earth and HV – LV 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.

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 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.
 Procedure:
 Isolate the equipment, apply working grounds to all incoming and
outgoing cables and disconnect all incoming and outgoing cables
from the transformer bushing terminals connections.
 Disconnected cables should have sufficient clearance from the
switchgear terminals greater that the phase spacing distance. Use
nylon rope to hold cable away from incoming and outgoing terminals
as required.
 Ensure the transformer tank and core is grounded.
 Disconnect all lightning arresters, fan system, meter or low voltage
control systems that are connected to the transformer winding.
 Short circuit all winding terminals of the same voltage level together.
 Perform a 1 minute resistance measurements between each winding
group to the other windings and ground. 6. Remove all shorting leads
after completion of all test.
 Conclusion:
 The insulation resistance across HV - Earth and HV-LV is
determined and verified.
 HV to Ground resistance was found to be 5 mega ohms.LV to
Ground resistance was found to be 5 mega ohms.LV to HV was
found to be 5 mega ohms.

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Separate Source Power Frequency Test
 Purpose:
 This dielectric test is intended to check the the ability of main
insulation to earth and between winding.
 Circuit diagram:
 Procedure:
 All three line terminals of the winding to be tested are
connected together.
 Other winding terminals which are not under test and also tank
of the transformer should be connected to earth.

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 Then 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 break down in the dielectric of the
insulation occurs during test.
 In this transformer testing, the peak value of voltage is
measured, that is why the capacitor voltage divider with digital
peak voltmeter is employed as shown in the diagram above.
The peal value multiplied by 0.707 (1/√2) is the test voltage.
Nominal system
voltage rating
for equipment
Highest system
voltage rating
for equipment
Rated short duration
power frequency withstand
voltage
415V 1.1 KV 3 KV
11 KV 12 KV 28 KV
33 KV 36 KV 70 KV
132 KV 145 KV 230 / 275 KV
220 KV 245 KV 360 / 395 KV
400 KV 420 KV 570 / 630 KV
Winding with graded insulation, which has neutral intended for direct earthing,
is tested at 38KV
 Conclusion:
 The withstanding capability of the windings for separate
source power frequency is determined.

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Parametric test
It includes the following tests:
Load Loss And Impedence Test On
Transformer
 Purpose:
 The purpose of a load loss test is to determine the series
branch parameters of the equivalent circuit of a real
transformer.
 Circuit diagram:
 Explaination:
 The test is conducted on the highvoltage (HV) side of the
transformer where the lowvoltage (LV) side or the secondary is
short circuited.
 A wattmeter is connected to the primary. An ammeter is
connected in series with the primary winding. A voltmeter is

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optional since the applied voltage is the same as the voltmeter
reading.
 Rated voltage is applied at primary.The supply voltage required
to circulate rated current through the transformer is usually
very small and is of the order of a few percent of the nominal
voltage and this 5% voltage is applied across primary.
 The core lossesare very small because applied voltage is only a
few percentage of the nominal voltage and hence can be
neglected. Thus the wattmeter reading measures only the full
load copper loss.
 Procedure:
 The connection diagram for short circuit test on
transformer is shown in the figure.
 A voltmeter, wattmeter, and an ammeter are connected in
HV side of the transformer as shown.
 The voltage at rated frequency is applied to that HV side
with the help of a variac of variable ratio auto transformer.
 . Voltage is applied to the HV side and increased from the
zero until the ammeter reading equals the rated current.
 All the readings are taken at this rated current.
 Calculations:
 Let us consider wattmeter reading is Psc .
 Where Re is equivalent resistance of transformer. If, Ze is
equivalent impedance of transformer

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 . Therefore, if equivalent reactance of transformer is Xe
 These values are referred to the HV side of transformer as
because the test is conduced on HV side of transformer. These
values could easily be referred to LV side by dividing these
values with square of transformation ratio.
Observations
At 100%loading ,
I= 1.31A
V= 503 v
P= 352.3 W
Power factor= 0.46
At 50% loading,
I= 0.65A
V=250V
P=86W
Open Circuit Test On Transformer
 Purpose:
 To calculate the various parameters of the transformer.
Circuit Diagram:

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 Explaination:
 The ammeter reading gives the no load current Ie . As no
load current Ie is quite small compared to rated current of
the transformer, the voltage drops due to this current that
can be taken as negligible.
 Since, voltmeter reading V1 can be considered equal to
secondary induced voltage of the transformer, the input
power during test is indicated by wattmeter reading. As the
transformer is open circuited, there is no output, hence the
input power here consists of core losses in transformer and
copper loss in transformer during no load condition.
 But as said earlier, the no load current in the transformer is
quite small compared to full load current, so copper loss due
to the small no load current can be neglected.
 Hence, the wattmeter reading can be taken as equal to core
losses in transformer.

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 Procedure:
 The connection diagram for open circuit test on transformer is
shown in the figure.
 A voltmeter, wattmeter, and an ammeter are connected in LV
side of the transformer as shown.
 The voltage at rated frequency is applied to that LV side with
the help of a variac of variable ratio auto transformer.
 The HV side of the transformer is kept open. Now with the
help of variac, applied voltage gets slowly increased until the
voltmeter gives reading equal to the rated voltage of the LV
side.
 After reaching at rated LV side voltage, all three instruments
reading (Voltmeter, Ammeter and Wattmeter readings) are
recorded.
 Further calculations are carried out using suitable formulae.
 Calculations:
 Where Rm is shunt branch resistance of transformer. If, Zm is
shunt branch impedance of transformer.
 Therefore, if shunt branch reactance of transformer is Xm,
Observations
V=403v
I=0.66A
P=82W
F=49.97Hz Power factor= 0.162

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WINDING RESISTANCE TEST
Transformer winding resistance measurement is carried out as a type test,
routine test and also as a field test. In the factory, it helps in determining
the following :
1. Calculation of the I2R losses in transformer.
2. Calculation of winding temperature at the end of temperature rise test of
transformer.
3. As a benchmark for assessing possible damages in the field.
It is done at site in order to check for abnormalities due to loose
connections, broken strands of conductor, high contact resistance in tap
changers, high voltage leads and bushings.
Procedure of Transformer Winding Resistance Measurement
For star connected winding, the resistance shall be measured between the
line and neutral terminal. For star connected auto-transformers the
resistance of the HV side is measured between HV terminal and IV terminal,
then between IV terminal and the neutral. For delta connected windings,
measurement of winding resistance shall be done between pairs of line
terminals. As in delta connection the resistance of individual winding can not
be measured separately, the resistance per winding shall be calculated as
per the following formula:
Resistance per winding = 1.5 x Measured value
The resistance is measured at ambient temperature and then converted to
resistance at 75˚C for all practical purposes of comparison with specified
design values, previous results and diagnostics. Winding Resistance at
standard temperature of 75° C
Rt = Winding resistance at temperature t. t = Winding temperature.
Generally transformer windings are immersed in insulation liquid and
covered with paper insulation, hence it is impossible to measure the actual
winding temperature in a de-energizing transformer at time of transformer

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winding resistance measurement. An approximation is developed to calculate
temperature of winding at that condition, as follows
Temperature of winding = Average temperature of insulating oil.
(Average temperature of insulating oil should be taken 3 to 8 hours after de-
energizing transformer and when the difference between top & bottom oil
temperatures becomes less than 5° C.) The resistance can be measured by
simple voltmeter ammeter method, Kelvin Bridge meter or automatic
winding resistance measurement kit. (ohm meter, preferably 25 Amps kit)
Caution for voltmeter ammeter method: Current shall not exceed 15% of the
rated current of the winding. Large values may cause inaccuracy by heating
the winding and thereby changing its temperature and resistance. NB: -
Measurement of winding resistance of transformer shall be carried out at
each tap.
Current Voltage Method of Measurement of Winding Resistance
The transformer winding resistances can be measured by current voltage
method. In this method of measurement of winding resistance, the test
current is injected to the winding and corresponding voltage drop across the
winding is measured.
By applying simple Ohm's law i.e. Rx = V ⁄ I, one can easily determine the
value of resistance.
Procedure of Current Voltage Method of Measurement of Winding Resistance
1. Before measurement the transformer should be kept in OFF condition
without excitation at least for 3 to 4 hours. During this time the winding
temperature will become equal to its oil temperature.
2. Measurement is done with D.C.
3. To minimize observation errors, polarity of the core magnetization shall
be kept constant during all resistance readings.
4. Voltmeter leads shall be independent of the current leads to protect it
from high voltages which may occur during switching on and off the
current circuit.

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5. The readings shall be taken after the current and voltage have reached
steady state values. In some cases this may take several minutes
depending upon the winding impedance.
6. The test current shall not exceed 15% of the rated current of the
winding. Large values may cause inaccuracy by heating the winding and
thereby changing its resistance.
7. For expressing resistance, the corresponding temperature of the winding
at the time of measurement must be mentioned along with resistance
value. As we said earlier that after remaining in switch off condition for 3
to 4 hours, the winding temperature would become equal to oil
temperature. The oil temperature at the time of testing is taken as the
average of top oil and bottom oil temperatures of transformer.
1. For star connected three phase winding, the resistance per phase would
be half of measured resistance between two line terminals of the
transformer.
2. For delta connected three phase winding, the resistance per phase would
be 0.67 times of measured resistance between two line terminals of the
transformer.
3. This current voltage method of measurement of winding resistance of
transformer should be repeated for each pair of line terminals of winding
at every tap position.

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TURNS AND VOLTAGE RATIO TEST
Test Purpose:
 Turns Ratio Test / Voltage Ratio Test are done in Transformer to find out
Open Circuited turns, Short Circuited turns in Transformer winding.
 The voltage ratio is equal to the turn’s ratio in a transformer
(V1/V2=N1/N2). Using this principle, the turn’s ratio is measured with the
help of a turn’s ratio meter. If it is correct , then the voltage ratio is
assumed to be correct
 This test should be made for any new high-voltage power transformer at
the time it is being installed.
 With use of Turns Ratio meter (TTR), turns Ratio between HV & LV
windings at various taps to be measured & recorded.
 The turn’s ratio is measure of the RMS voltage applied to the primary
terminals to the RMS Voltage measured at the secondary terminals.
 R= Np / Ns
 Where,
 R=Voltage ratio
 Np=Number of turns at primary winding.
 Ns= Number of turns at secondary Winding.
 The voltage ratio shall be measured on each tapping in the no-load
condition.
Test Instruments:
 Turns Ratio meter (TTR) to energies the transformer from a low-voltage
supply and measure the HV and LV voltages.
 Wheatstone Bridge Circuit
Method No1 Turns Ratio Testing:
Test Procedure:
 Transformer Turns Ratio Meter (TTR):

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 Transformer ratio test can be done by Transformer Turns Ratio (TTR)
Meter. It has in built power supply, with the voltages commonly used
being very low, such as 8, 10 V and 50 Hz.
 The HV and LV windings of one phase of a transformer (i.e. R-Y & r-n) are
connected to the instrument, and the internal bridge elements are varied
to produce a null indication on the detector.
 Values are recorded at each tap in case of tapped windings and then
compared to calculated ratio at the same tap.
 The ratio meter gives accuracy of 0.1 per cent over a ratio range up to
1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of
the windings of the transformer under test are balanced against the
voltages developed across the fixed and variable resistors of the ratio
meter.
 Adjustment of the calibrated variable resistor until zero deflection is
obtained on the galvanometer then gives the ratio to unity of the
transformer windings from the ratio of the resistors.
 Bridge Circuit:
 A phase voltage is applied to the one of the windings by means of a
bridge circuit and the ratio of induced voltage is measured at the bridge.
The accuracy of the measuring instrument is < 0.1 %
 This theoretical turn ratio is adjusted on the transformer turn ratio tested
or TTR by the adjustable
transformer as shown in the figure above and it should be changed until a
balance occurs in the percentage error indicator. The reading on this
indicator implies the deviation of measured turn ratio from expected turn
ratio in percentage.
 Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
 % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected
Turns Ration
 Out-of-tolerance, ratio test of transformer can be due to shorted turns,
especially if there is an associated high excitation current.

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 Open turns in HV winding will indicate very low exciting current and no
output voltage since open turns in HV winding causes no excitation
current in the winding means no flux hence no induced voltage.
 But open turn in LV winding causes, low fluctuating LV voltage but normal
excitation current in HV winding. Hence open turns in LV winding will be
indicated by normal levels of exciting current, but very low levels of
unstable output voltage.
 The turn ratio test of transformer also detects high resistance connections
in the lead circuitry or high contact resistance in tap changers by higher
excitation current and a difficulty in balancing the bridge.
Test Caution:
 Disconnect all transformer terminals from line or load.
 Neutrals directly grounded to the grid can remain connected
Method No 2 Voltage Ratio Testing:
 This test is done to check both the transformer voltage ratio and tap
changer.
 When “Turns Ratio meter” is not available, Voltage Ratio Test is done at
various tap position by applying 3 phases LT (415V) supply on HT side of
Power transformer. In order to obtain the required accuracy it is usual to
use a ratio meter rather than to energies the transformer from a low-
voltage supply and measure the HV and LV voltages.
 At Various taps applied voltage and Resultant voltages LV side between
various Phases and phases& neutral measured with precision voltmeter &
noted.
Test Procedure:
 With 415 V applied on high voltage side, measure the voltage between all
phases on the low voltage side for every tap position.
 First, the tap changer of transformer is kept in the lowest position and LV
terminals are kept open.
 Then apply 3-phase 415 V supply on HV terminals. Measure the voltages
applied on each phase (Phase-Phase) on HV and induced voltages at LV
terminals simultaneously.
 After measuring the voltages at HV and LV terminals, the tap changer of
transformer should be raised by one position and repeat test.
 Repeat the same for each of the tap position separately.
 At other taps values will be as per the percentage raise or lower at the
respective tap positions.
 In case of Delta/Star transformers the ratio measure between RY-rn, YB-
yn and BR-bn.

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 Being Delta/Star transformers the voltage ratio between HV winding and
LV winding in each phase limb at normal tap is 33 KV OR 33x√3 = 5.196
,11 KV / √3 11
 At higher taps (i-e high voltage steps) less number of turns is in circuit
than normal. Hence ratio values increase by a value equal to.5.196 +
{5.196 x (no. of steps above normal) x (% rise per each tap)} 100
 Similarly for lower taps than normal the ratio is equal to 5.196 – {5.196 x
(no. of steps above normal) x (% rise per each tap)}100
Test Acceptance Criteria:
 Range of measured ratio shall be equal to the calculated ratio ±0.5%.
 Phase displacement is identical to approved arrangement and
transformer’s nameplate.
 The IEEE standard (IEEE Standard 62) states that when rated voltage is
applied to one winding of the transformer, all other rated voltages at no
load shall be correct within one half of one percent of the nameplate
readings. It also states that all tap voltages shall be correct to the nearest
turn if the volts per turn exceed one half of one percent desired voltage
.The ratio test verifies that these conditions are met.
 The IEC60076-1 standard defines the permissible deviation of the actual
to declared ratio
 Principal tapping for a specified first winding pair: the lesser ±0.5% of the
declared voltage ratio
 or 0.1 times the actual short circuit impedance. Other taps on the first
winding pair and other winding pair must be agreed upon, and must be
lower than the smaller of the two values stated above.
 Measurements are typically made by applying a known low voltage across
the high voltage winding so that the induced voltage on the secondary is
lower, thereby reducing hazards while performing the test .For three
phase delta/wye or wye/delta transformer, a three phase equivalency test
is performed, i.e. the test is performed across corresponding single
winding.
Test can detect:
 Shorted turns or open circuits in the windings.
 Incorrect winding connections ,and other internal faults or defects in tap
changer
POLARITY/VECTOR GROUP TEST
Purpose of Test:
 The vector group of transformer is an essential property for successful
parallel operation of transformers. Hence every electrical power
transformer must undergo through vector group test of transformer at

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factory site for ensuring the customer specified vector group of
transformer.
Test Instruments:
 Ratio meter.
 Volt Meter. A Ratio meter may not always be available and this is usually
the case on site so that the polarity may be checked by voltmeter.
Test Circuit Diagram:
Test Procedure:
 The primary and secondary windings are connected together at one point.
 Connect neutral point of star connected winding with earth.
 Low-voltage three-phase supply (415 V) is then applied to the HV
terminals.
 Voltage measurements are then taken between various pairs of terminals
as indicated in the diagram and the readings obtained should be the
phasor sum of the separate voltages of each winding under consideration.
Condition:(HV side R-Y-B-N and LV Side r-y-b-n)
 R and r should be shorted.
 Apply 415 Volt to R-Y-B
 Measure Voltage between Following Phase and Satisfy Following Condition
Vector Group Satisfied Following Condition
Dyn1
Rb=Rn+Bn
Bb=By
Yy<Yb
Dyn11
Ry=Rn+Yn
Yb=Yy
Bb<By
Ynd1 RN=Ry+Yn

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By=Yy
Yy<Yb
Ynyn0
Bb=Yy
Bn=Yn
RN=Rn+Nn
B.TYPE TESTS
INTRODUTION:-
Basically type tests are done to prove that the transformer meets customer’s
specifications and design expectations, the transformer has to go through different
testing procedures in manufacturer premises. Some transformer tests are carried
out for confirming the basic design expectation of that transformer. These tests are
done mainly in a prototype unit not in all manufactured units in a lot. Type test of
transformer confirms main and basic design criteria of a production lot.
There are two tests under TYPE TESTS of the transformer namely:-
1) Temperature rise test
2) Impulse test
1) TEMPERATURE RISE TEST
Temperature rise test of Transformer is included in type test of transformer. In
this test we check whether the temperature rising limit of transformer winding and oil
as per specification or not.
Note: this test is done for both oil and transformer windings to check their
temperature rising limits.
Temperature Rise Test for Oil of Transformer

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1. First the LV winding of the transformer is short circuited.
2. 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.
3. 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 75°C.
4. The total losses are measured by three watt-meters method.
5. During the test, hourly readings of oil temperature are taken from the
thermometer already placed in the pocket of top cover.
6. 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.
7. 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.
8. 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

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of the oil becomes less than 3°C in one hour. This steady value of oil is
determined as final temperature rise of transformer insulating oil.
9. There is another method of determination of oil temperature. Here the
test in allowed to be continued until the oil temperature rise does not
vary more than 1°C per hour for four consecutive hours. The least
reading is taken as final temperature rise of the oil.

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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 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.
Temperature rise limits of transformer when it is oil immersed, given in the
table below
Temperature rise limit
for air as
cooling medium
Temperature rise limit
for water as
cooling medium
Condition
Winding 55oC 60oC
When oil circulatio

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60oC 65oC
When oil circulatio
Top Oil
50oC 55oC
When transformer
equipped with con
45oC 50oC
When transformer
nor equipped with
Winding Temperature Rise Test on Transformer
1. After completion of temperature rise test for top oil of transformer the
current is reduced to its rated value for transformer and is maintained
for one hour.
2. After one hour the supply is switch off and short circuit and supply
connection to the HV side and short circuit connection to the LV side
are opened.
3. But, the fans and pumps are kept running (if any).

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4. Then winding resistances are measured quickly.
5. But there is always a minimum 3 to 4 minutes time gap between first
measurement of resistance and the instant of switching off the
transformer, which cannot be avoided.
6. Then the resistances are measured at the same 3 to 4 minutes time
intervals over a period of 15 minutes.
7. Graph of hot resistance versus time is plotted, from which winding
resistance (R2) at the instant of shut down can be extrapolated.
8. From this value, θ2, the winding temperature at the instant of shut
down can be determined by the formula given below-
Where, R1 is the cold resistance of the winding at temperature t1.

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For determining winding temperature rise we have to apply the above
discussed indirect method. That means hot winding resistance is measured
and determined first and then from that value we have to calculate the
winding temperature rise, by applying resistance temperature relation
formula. This is because unlike oil the winding of transformer is not
accessible for external temperature measurement.
2)IMPULSE TEST
Lighting is a common phenomenon in transmission lines because of their tall
height. This lightning stroke on the line conductor causes impulse voltage.
The terminal equipment of transmission line such as power transformer then
experiences this lightning impulse voltages. Again during all kind of online
switching operation in the system, there will be switching impulses occur in
the network. The magnitude of the switching impulses may be about 3.5
times the system voltage.
Insulation is one of the most important constituents of a transformer. Any
weakness in the insulation may cause failure of transformer. To ensure the
effectiveness of the insulation system of a transformer, it must confirm the
dielectric test. But the power frequency withstand test alone cannot be
adequate to demonstrate the dielectric strength of a transformer. That is
why impulse test of transformer performed on it. Both lightning impulse test
and switching impulse test are included in this category of testing.
The purpose of the impulse voltage test is to secure that the transformer
insulation withstand the lightning overvoltage which may occur in service.

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Performance of Impulse Test
The test is performed with standard lightning impulses of negative polarity.
The front time (T1) and the time to half-value (T2) are defined in accordance
with the standard.
Standard lightning impulse Front time T1 = 1,2 μs ± 30% Time to half-value
T2 = 50 μs ± 20%
In practice the impulse shape may deviate from the standard impulse when
testing low-voltage windings of high rated power and windings of high input
capacitance. The impulse test is performed with negative polarity voltages to
avoid erratic flashovers in the external insulation and test circuit. Waveform
adjustments are necessary for most test objects. Experience gained from
results of tests on similar units or eventual pre-calculation can give guidance
for selecting components for the wave shaping circuit.
The test sequence consists of one reference impulse (RW) at 75% of full
amplitude followed by the specified number of voltage applications at full
amplitude (FW) (according to IEC 60076-3 three full impulses). The
equipment for voltage and current signal recording consists of digital
transient recorder, monitor, computer, plotter and printer. The recordings at
the two levels can be compared directly for failure indication. For regulating
transformers one phase is tested with the on-load tap changer set for the

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rated voltage and the two other phases are tested in each of the extreme
positions.
Connection of Impulse Test
All the dielectric tests check the insulation level of the job. Impulse
generator is used to produce the specified voltage impulse wave of 1.2/50
micro seconds wave. One impulse of a reduced voltage between 50 to 75%
of the full test voltage and subsequent three impulses at full voltage.
For a three phase transformer, impulse is carried out on all three phases in
succession.
The voltage is applied on each of the line terminal in succession, keeping the
other terminals earthed. The current and voltage wave shapes are recorded
on the oscilloscope and any distortion in the wave shape is the criteria for
failure.

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C.SPECIAL TESTS
INTRODUCTION:-
Special tests of transformer is done as per customer requirement to obtain
information useful to the user during operation or maintenance of the transformer.
There are two tests under SPECIAL TESTS of the transformer namely:-
1) Unbalance/magnetizing current test
2) Magnetic balance test
3) Measurement of zero-sequence impedance test
4) Vector group test
1) UNBALANCE/MAGNETIZING CURRENT TEST
Magnetizing current test of transformer is performed to locate defects in the
magnetic core structure, shifting of windings, failure in turn to turn
insulation or problem in tap changers. These conditions change the effective
reluctance of the magnetic circuit, thus affecting the current required to
establish flux in the core.
1.First of all keep the tap changer in the lowest position and open all IV & LV
terminals.
2.Then apply three phase 415 V supply on the line terminals for three phase
transformers and single phase 230 V supply on single phase transformers.
3.Measure the supply voltage and current in each phase.
4.Now repeat the magnetizing current test of transformer test with keeping
tap changer in normal position.
And repeat the test with keeping the tap at highest position.
Generally there are two similar higher readings on two outer limb phases on
transformer core and one lower reading on the centre limb phase, in case of
three phase transformers. An agreement to within 30% of the measured
exciting current with the previous test is usually considered satisfactory. If
the measured exciting current value is 50 times higher than the value
measured during factory test, there is likelihood of a fault in the winding
which needs further analysis.

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Caution: This magnetizing current test of transformer is to be carried out
before DC resistance measurement.
2)MAGNETIC BALANCE TEST
Magnetic balance test of transformer is conducted only on three phase
transformers to check the imbalance in the magnetic circuit
Procedure of Magnetic Balance Test of Transformer
1. First keep the tap changer of transformer in normal position.
2. Now disconnect the transformer neutral from ground.
3. Then apply single phase 187 V AC supply across one of the HV winding
terminals and neutral terminal.
4. Measure the voltage in two other HV terminals in respect of neutral
terminal.
5. Repeat the test for each of the three phases.
In case of auto transformer, magnetic balance test of transformer should be
repeated for LV winding also.
There are three limbs side by side in a core of transformer. One phase
winding is wound in one limb. The voltage induced in different phases
depends upon the respective position of the limb in the core. The voltage
induced in different phases of transformer in respect to neutral terminals
given in the table below.

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1U TO 1V 1V TO 1W 1W TO 1U
187 V 164 V 18 V
82 V 187 V 81 V
21 V 158 V 187V
3)MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE TEST
The zero-phase-sequence impedance characteristics of three-phase transformers
depend upon the winding connections and, in some cases, upon the core construction.
Zero-phase-sequence impedance tests described in this standard apply only to
transformers having one or more windings with a physical neutral brought out for
external connection. In all tests, one such winding shall be excited at rated frequency
between the neutral and the three line terminals connected together. External
connection of other windings shall be as described in zero-phase-sequence impedance
for various transformer connections. Transformers with connections other than as
described in zero-phase-sequence impedance shall be tested as determined by the
individuals responsible for design and application.
The excitation voltage and current shall be established as follows:
a) If no delta connection is present on the transformer and the transformer is a three
leg core design, there is a risk of excessive tank wall heating due to the return flux
from the core going into the tank wall. To avoid this, the applied voltage should be
such that the current is no more than 20% of the base rating of the winding being
excited. This applies to both open-circuit tests and short-circuit tests.
If the transformer is a five leg core or a shell form design the zero-sequence
impedance is equal to the positive sequence impedance and the zero-sequence test is
generally not needed. However, should the test be done, the applied voltage should
not exceed 30% of the rated line-to-neutral voltage of the winding being energized for
the open-circuit test, and the phase current should not exceed its the base rated value
of the winding being excited for the short-circuit test.
b) If a delta connection is present, the applied voltage should be such that the base
rated phase current of any delta winding is not exceeded.

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The percent excitation voltage at which the tests are made shall be shown on the test
report. The time duration of the test shall be such that the thermal limits of any of the
transformer parts are not exceeded.
4)VECTOR GROUP TEST
The vector group of transformer is an essential property for successful
parallel operation of transformers. Hence every electrical power transformer
must undergo through vector group test of transformer at factory site for
ensuring the customer specified vector group of transformer.
The phase sequence or the order in which the phases reach their maximum positive
voltages, must be identical for two paralleled transformers. Otherwise, during the
cycle, each pair of phases will be short circuited. The several secondary connections
are available in respect of various primary three phase connection in the three phase
transformer. So for same primary applied three phase voltage there may be different
three phase secondary voltages with various magnitudes and phases for different
internal connection of the transformer.

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We know that, the primary and secondary coils on any one limb have induced emfs
that are in time-phase. Let's consider two transformers of same number primary turns
and the primary windings are connected in star. The secondary number of turns per
phase in both transformers are also same. But the first transformer has star
connected secondary and other transformer has delta connected secondary. If same
voltages are applied in primary of both transformers, the secondary induced emf in
each phase will be in same time-phase with that of respective primary phase, as
because the primary and secondary coils of same phase are wound on the same limb
in the core of transformer. In first transformer, as the secondary is star connected,
the secondary line voltage is √3 times of induced voltage per secondary phase coil.
But in case of second transformer, where secondary is delta connected, the line
voltage is equal to induced voltage per secondary phase coil. If we go through the
vector diagram of secondary line voltages of both transformer, we will easily find that
there will be a clear 30o angular difference between the line voltages of these
transformers. Now, if we try to run these transformers in parallel then there will be a
circulating current flows between the transformers as because there is a phase angle
difference between their secondary line voltages. This phase difference cannot be
compensated. Thus two sets of connections giving secondary voltages with a phase
displacement cannot be intended for parallel operation of transformers.
The following table gives the connections for which from the view point of
phase sequence and angular divergences, transformer can be operated
parallel. According to their vector relation, all three phase transformers are
divided into different vector group of transformer. All electrical power

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transformers of a particular vector group can easily be operated in parallel if
they fulfill other condition for parallel operation of transformers.
Procedure of Vector Group Test of Transformer
Let’s have a DYN11 transformer.
1. Connect neutral point of star connected winding with earth.
2. Join 1U of HV and 2U of LV together.
3. Apply 415 V, three phase supply to HV terminals.
4. Measure voltages between terminals 1V-2V, 1V-2W, 1W-2V and 1W-
2W that means voltages between each LV terminal and HV terminal.
For Dyn11 transformer, we will find,
1V-2W = 1V-2V
1W-2W < 1W-2V
1V-2W < 1V-1U
GROUP Connection Connection
( 30o)
Yd11
Dy11

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The vector group test of transformer for other group can also be done in
similar way.
Readings:-
1U-2U short 1U-1V=405V
1V-2V 1W-2V 1V-2W 1W-2W
393V 396V 393V 382V
1V-2V short 1V-1W=390V
1U-2U 1W-2U 1U-2W 1W-2W
395V 379V 409V 379V
1W-2W short 1U-1W=400V
1U-2U 1V-2U 1U-2V 1V-2V
385V 396V 386V 379V

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Reflection on the internship
In this chapter we reflect on the internship. Regarding our learning goals we
shortly discuss our experiences; if we have achieved our goal, and the
technical as well as the non technical outcomes of the internship
programme:
 The functioning and working conditions of a transformer
manufacturing industry
At the beginning we did not have any experience of a transformer
manufacturing industry. Although we had seen one, we now
understand better the functioning like the organization structure and
setting up the industry. Trying to operate as a industry we saw the
importance of financial support and personal capacity. The
dependence on external suppliers for various items used in the
transformer is a very important part of the industry.
 Enhancing technical skills
The academic knowledge of transformers which we had studied as a
part of our academic programme was found to be woefully short of the
practical experience. The various testing procedures was new to us
and we found them to be extremely useful in understanding their
importance in determining if a transformer is ready to be used in real
life.
 The use of skills and knowledge gained in the university
The skills and knowledge gained in my study could be put to use to
practise in our internship. Calculations of phase resistances using data
from the tests performed was done based on the academic studies we
had made over the past year.

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Conclusion
On the whole, this internship was a useful experience. We have gained new
knowledge, skills and met many new people. We achieved several of our
learning goals. We got insight into professional practices currently advocated
in the industry. We learned the different facets of working within a well
established industry. Related to our study we learned more about the
manufacture, assembly and testing of three phase distribution transformers.
Furthermore we have experienced that it is of importance that education is
objective and that we have to be aware of the industrial aspect of the topics
we study. This internship programme was not one sided, but it was a way of
sharing knowledge, ideas and opinions.
The internship was also good to find out what our strengths and weaknesses
are. This helped us to define what skills and knowledge we have to improve
in the coming time. We can confidently assert that the knowledge we gained
through this internship is sufficient to contribute towards our future
endeavours. At last this internship has given us new insights and motivation
to pursue a career in core electrical departments.