Sahil BHEL JHANSI assembely of power transformer

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BHARAT HEAVY ELECTRICALS LIMITED JHANSI
SUMMER VOCATIONAL TRAINING - 2019
ROTATIONAL REPORT AND PROJECT REPORT ON POWER
TRANSFORMER
SUBMITTED TO: UNDER GUIDANCE OF:
Dr. Mohammad Aftab Alam Mr. PRABHAT KATIYAR
DEPUTY MANAGER (H.R.D.) Sr. ENGINEER (TRM BAY-9)
BHEL, JHANSI BHEL, JHANSI
COORDINATOR: SUBMITTED BY:
Mr. Ashok Agnihotri SAHIL KUMAR CHAUHAN
HRDC B.Tech 3rd YEAR
BHEL JHANSI ELECTRICAL ENGINEERING
NIT ARUNACHAL PRADESH

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ACKNOWLEDGEMENT
I am highly thankful and indebted to B.H.E.L. engineers and technicalstaffs for
providing me vital and valuable information about the different facts of an
industrial management system and their respective departments thus helping me to
gain an overall idea about the working of organization.
I am also thankful to Mr. Prabhat Katiyar (Sr.Engineer) for giving their
precious time and helping me in understanding various theoretical and practical
aspects of my project on Power Transformer and their processes of manufacturing,
assembly and testing, under whose supervision I accomplished my project.
I would like to thank my parents and friends who have been a constant source of
encouragement & inspiration during my studies & have always provided me
supportin every walk of life.

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PREFACE
At very outset of the prologue it becomes imperative to insist that vocational
training is an integral part of engineering curriculum. Training allows us to gain an
insight into the practical aspects ofthe various topics, with which we come across
while pursuing our B.Tech i.e. summer training gives us practical implementation
of various topics we already have learned and will learn in near future. Summer
training always emphasizes on logic and common sense instead of theoretical
aspects of subject.
On my part, I pursued five weeks training at B.H.E.L. Jhansi. The training
involved a study of various departments of the organization as per the time
logically scheduled and well planned given to me.
The rotation in various departments was necessary in order to get an overall idea
about the working of the organization.

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TABLE OF CONTENT
TITLE PAGE No.
1. About BHEL, Jhansi………………………………………………....6
2. ROTATION REPORT
i. Store.......................................................................................................10
ii. Fabrication…………………………………………………………….10
iii. Bay 3…………………………………………………………………..12
iv. Bay 4…………………………………………………………………..14
v. Bay 5…………………………………………………………………..15
vi. Bay 6…………………………………………………………………..16
vii. Bay 7…………………………………………………………………..16
viii. Bay 8…………………………………………………………………..17
ix. Bay 9 ………………………………………………………………….18
x. Transformer Engineering ……………………………………………...20
xi. Technology………………………………………………………….....21
xii. Transformer Commercial……………………………………………...23
xiii. Locomotive Production..………………………………………………24
xiv. Locomotive Manufacturing……………………………………………26
xv. Locomotive Commercial………………………………………………26
xvi. Central Quality Services ……………………………………………...27
xvii. Testing…………………………………………………………………27
xviii. Work Engineering Services……………………………………………29
3. REPORT ON ASSEMBLY POWER TRANSFORMER
i. Introduction………………………………………………………..32
ii. Principles of transformers…………………………………..……..32
iii. Core assembly…………………………………….………..……...33

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iv. Coil assembly………………………………………….…………..35
v. TG (Terminal Gear Mounting)…………………………………….36
vi. Processing P2 …..………………………………………………….37
vii. Servicing…………………………………………………..………..38
viii. Tanking…………………………………………………….….……38
ix. Processing P3……………………………………………….…....…39
x. Case Fitting ………………………………………………..…….…41
xi. Testing………………………………………………………..…….49
xii. Dispatch………………………………………………………….....60
4. CONCLUSION………………………………………………………...61
5. REFERENCES………………………………………………………...63

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BHARAT HEAVY ELECTRICALS LIMITED JHANSI
A BRIEF INTRODUCTION
By the end of the fifth year plan, it was envisaged by the planning commission that
the demand for the power transformer would raise in the coming years. Anticipating
the country’s requirement, in 1974, BHEL started a new plant in Jhansi which would
manufacture power and other type of transformer in addition to the capacity
available at BHEL in Bhopal. The Bhopal plant was engaged in the manufacture
transformers of large rating and Jhansi unit would concentrate on power
transformers, instrument transformers, traction transformers for railway etc.
This unit of Jhansi was established around 14 km from the city on the N.H. No 26
on Jhansi Lalitpur road. It is called second-generation plant of BHEL set up in 1974
at an estimated cost of Rs 16.22 crores inclusive of Rs 2.1 crores for township. Its
foundation was laid by late Mrs. Indira Gandhi the prime minister on 9th Jan. 1974.
The commercial production of the unit began in 1976-77 with an output of Rs 53
lacs since then there has been no looking back for BHEL Jhansi
The plant of BHEL is equipped with most modern manufacturing processingand
testing facilities for the manufacture of power, special transformer and instrument
transformer, Diesel shunting locomotives and AC/DC locomotives. The layout of
the plant is well streamlined to enable smoothmaterial flow from the raw material
stages to the finished goods. All the feeder bays have been laid perpendicular to the
main assembly bay and in each feeder bay raw material smoothly gets converted to
sub-assemblies, which after inspection are sent to main assembly bay.
The raw material that are produced for manufacture are used only after thorough
material testing in the testing lab and with strict quality checks at various stages of
productions. This unit of BHEL is basically engaged in the productionand
manufacturing of various types of transformers and capacities with the growing
competition in the transformer section, in 1985-86 it under took the re-powering of
DESL, but it tookthe complete year for the manufacturing to begin. In 1987-88,
BHEL has progressed a step further in under taking the production of AC
locomotives, and subsequently it manufacturing AC/DC locomotives also.

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PRODUCT PROFILE OF B.H.E.L., JHANSI

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LAYOUT PLAN OF B.H.E.L. JHANSI

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MAIN AIM OF ROTATION
Main aim behind the rotation of various departments is that one can understand the working of
each and every department and to see that how people (workers, middle level executives, top
officials) work in corporate environment.
Main departments of BHEL Jhansi are
 Production
 Administration
PRODUCTION UNIT DEPARTMENTS
1. STORE :
It is one of the prime departments of material management department.
There are separate stores for different type of material in the BHEL.
There are three sections in store:
 Control Receiving Section
 Custody Section
 Scrap Disposal Section
Function: Whatever material coming from anywhere as ordered by the purchase
department first comes to central store. The supervisor looks upon all documents and
decide whether material is to be unloaded at central store or at shop directly. A list of
material coming in store is prepared and Quality Control people are called for inspection.
If material is found as per standard, SRV (Store Receipt Voucher) is issued for each
material. A total of 08 SRV’s are prepared. If material is accepted, it is shifted to custody
stores i.e. storing agency. If rejected, material is shifted to rejection store and vendor is
informed either to replace material or to collect it. Some materials like Silicon oil,
Transformer oil, insulating material etc. are directly stored in the Bays. Scraps are also
sold through unit by a MATERIAL SCRAP TRADING –DELHI.
2. FABRICATION :
Fabrication is nothing but production. This shop deals with the manufacturing of
transformer and locomotive components such as tanks, plates and nuts & bolts. It
comprises of three bays i.e. Bay 0, Bay 1 and Bay 2.

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A. Bay 0 :
This is the preparation shop where according to the required drawings, the cutting of
different components of different materials is done. This section has the following
machines:
 Planer machine – It is used to reduce thickness.
 Cutting machine --
o CNC/ANCFlame cutting machine – To cut complicated shaft using Oxy-Acetylene
flame. This is fully computerized. The figure is loaded into a computer attached to
machine and flame thus cuts figure accordingly.
o Pantograph flame cutting machine.
o PUG cutting
 Shearing machine- This machine is used for metal sheet. Cutting range is 2mm to 6mm.
Suitable materials for use are Al, Standard steel and Cu. It cuts material just like a scissor.
 Bending machine- It is hydraulically operated. Pressure gauge is used to the pressure
applied. Stopping mechanism is used by screw to prevent the collapsing the ram blade with
bending grooves (Die). It consists of a ramp (tool) for up-down motion and the item is
leveled on it.
 Rolling machine- It is used to reduce the roll thickness. The roller material used is High
Carbon Steel. It consists of 3 rollers. The upper roller is fixed whereas below 2 rollers are
used for up-down motion.
 Flattening machine-It is used to straighten or to flattening the job using power hammer.
 Radial Drilling machine- In this machine, tool can move radically. In this cutting oil
mixed with water is used to cool the drill tool is used as the material to be drilled. Different
drill tools are used from 2mm to 100mm dia.
 Nibbling machine- Used for straight cutting, circle cutting, nibbling, circular and square
punching.
 Plano milling machine
 Plasma cutting machine- Used for non-ferrous metals.
 Hydraulic guillotine shear- used to cut the sheet which has maximum cross-sectional area
(3200*13 sq. mm).
 Pacific hydraulic shear &pressure- hydraulically operated machine to cut the sheet of
different thickness.
 Hydraulic power press- has the capacity of 100 tones and used for flattening the objects.
 Butler machine- for facing, taping and shot cutting.
B. BAY 1 :
It is an assembly shop where different parts of tank come from bay 0. Here welding
processes are used for assembly, after which a rough surface is obtained which is
eliminated by Grinder operating at 1200 rpm.

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C. BAY 2 :
It is also assembly shop dealing with making different object mentioned below:
 Tank assembly
 Tank cover assembly
 End frame assembly.
 Foot assembly.
 Cross feed assembly.
 Core clamp assembly.
 Pin and pad assembly.
The various process of assembly are fitting, welding and testing.
Fitting- In this section different components are fit according to the drawing requirements.
Welding- In this section welding of the different components of different drawings by permanent
joints is done.
Testing- When the tank for transformers is completed, then we check the leakage tests on the
tanks. There are two types of leakage tests, which we have to perform on the transformer.
 AT(Air leakage test) - In this, tank is filled by compress compressed air and dip the tank
in the shop solution for bubble test.
 VT(Deflection test)
Shot blasting-It is firing of small materials with the high pressure of 7 kg i.e. acid picking. It is
done on different parts of jobs for removing carbon layer from its surface before painting.
Painting- After shot blasting, the tank is painted for corrosion resistant.
After assembly some tests known as NON DESTRUCTIVE TESTS are carried out. They are:
 Ultrasonic Tests: To detect the welding fault on the CRO at the fault place high amplitude
waves are obtained.
 Die Penetration Tests:Red solution is put at the welding & then cleaned. After some time
white solution is put. Appearance of a red spot indicates a fault at the welding.
 Magnetic Crack Detection: Magnetic field is created & then iron powder is put at the
welding. Sticking of the iron powder in the welding indicated as a fault.
 X-Ray Test-It is same as human testing and fault is seen in X-ray film.
3. TRM BAY – 3 :
It is split in two parts, half consists of machine shop and the other half consists of winding of dry
type transformer.
Here are basically three sections:
1. Machine Section.
2. Copper Section.
3. Tooling Section.

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a. MACHINE SECTION :
The operations to form small components of Power & Traction Transformers are done in section.
The shop consists of following machines:
1. Lathe machine
 Central lathe machine- It consist one tailstock, headstock low part of tailstock is fixed &
tailstock spindle is moving. On this machine facing, turning & threading is done. It is used
for light work and its range is 20mm to 200mm.
 Turret lathe-Its function is same as central lathe but it is used for mass production. Here
turret lathe is used in presence of tailstock because turret lathe contains many tailstocks.
It is used for heavy duty and its range is 250mm to 300 mm.
 Capstone lathe-It is belt drive. Used for light work and its range is 20mm to 150mm.
2. Radial arm drilling machine-It is used for drilling & boring.
3. Horizontal boring machine-It is computerized and used for making bore, facing etc.
4. Milling machine:
 Horizontal milling machine – It is used for making gear and cutting operations.
 Vertical milling machine – By this machine facing, cutting & T slot cutting is done
5. Grinder:
 Tool cutter grinder- Used for cutting tools and also grind them.
 Hydraulic surface grinder- It consist of magnetic platform. Cooling oils are used as
coolant.
 Vertical grinder machine- Used for grinding purpose.
6. Hydraulic power press- Used for straightening the material. The capacity of this machine is
25 tons.
7. Resistance brazing machine- used for overlap connections.
8. Bend saw machine- Bit is used for cutting circular object. In this machine blade width is inch
and thickness is ¾ inch.
9. Electric furnace-Used for heating the object.
10. Hydraulic punching machine-Used for making small pieces of material for desire purpose.
Range of this machine is 12mm to 250mm.
11. Hydraulic shearing machine-Used for cutting the material in range between 12mm to
25mm.
b. COPPER SECTION :
This part is only with copper cutting, bending, tinning etc. Machine used are as listed below.
Tube slitting machine-This machine is developed here & is used for cutting the tube along its
length & across its diameter. Its blade thickness is 3 mm.
Shearing machine-It is operated hydraulically & its blade has V-shape & a thickness of 15 mm.
Die and punching machine-It is also hydraulically operated & has a die & punch for making
holes.

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Hydraulic bending machine-It is used for bending job up to 90 degree.
Fly press machine- It is used to press the job. It is operated mechanically by a wheel, which is
on the top of the machine.
Bend saw machine -This machine is used for cutting job having small thickness. It is circularly
operated blade, around 1.5 m long.
Water cooled brazing machine-It contain two carbon brushes. The sheet is put along with a
sulfas sheet & the carbon brushes are heated. A lap joint is formed between the sheets as the
sulfas sheet melts.
Linking belt machine- It creates a smooth surfaces.
Hydraulic press machine-To press the job.
Solder pot machine-It has a pot that contains solder. Solder has a composition of 60% Zn &
40% Pb.
c. TOOLING SECTION :
In this section the servicing of tools is done.
Blade shape machine-It sharpens the blade using a circular diamond cutter. Blade of CNC,
cropping line machine is sharpened here.
Mini surface grinder machine-It serves grinding purposes. It has a grinding wheel made of
Aluminium Oxide.
Tool & surface grinding machine-This is specially used to grind the tool used in Bay 7.
Drill grinding machine-To grind the drills.
4. TRM BAY 4 :
It is the power transformer winding section.
TYPES OF WINDING
1) Reverse section winding.
2) Helical winding
3) Spiral winding.
4) Interleaved winding.
5) Half Section winding
There are four types of coils fixed in a transformer.
1) Low voltage coil (LV)
2) High voltage coil (HV)
3) Tertiary coil.
4) Tap coil.

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The type of winding depends on job requirement also the width and thickness of the conductors
are designed particulars & are decided by design department. Conductors used for winding is in
the form of very long strips wound on a spool, the conductor is covered by cellulose paper for
insulation.
For winding, first the mould of diameter equal to inner diameter of required coil is made. The
specifications of coils are given in drawing. The diameter of is adjustable as its body is made up
of wooden sections that interlock with each other. Interlocking can be increased or decreased to
adjust the inner diameter of coil.
The Moulds are of following types:
1. Belly types.
2. Link types.
3. Cone types.
5. TRM BAY 5 :
It is core and punch section. The lamination used in power, dry, ESP transformer etc. for making
core is cut in this section.
CRGO (Cold rolled grain oriented) silicon steel is used for lamination, which is imported in India
from Japan, Korea, U.K. or Germany. It is available in 0.27 & 0.28 mm thick sheets, 1 m wide &
measured in Kg. The sheets are coated with very thin layer of insulating material called “carlites”
to avoid short circuiting.
Core is the basic requirements of transformer in this bay various type of lamination of core is
made, they are:
a) Side leg- this lamination is at the extreme ends of the core which stand vertical. They are
isosceles trapezium in shape with angle of 45o
.
b) Central leg-This as the name suggests is central vertical portion of the core. It is a diamond
shaped with vertical edges longer. At the narrower edge one side is kept longer than the
other for making a well fixed joint; the angles are kept 450
.
c) Yoke- The horizontal lamination of the core is called yoke. They are also isosceles
trapezium in shape with angles of 450
.
For the purpose of cutting & punching the core three machines are installed in shop.
1. Slitting machine: It is used to cut CRGO sheets in different width. It has a circular cutter
whose position can be changed as per the requirement.
2. CNC cropping line pneumatic: It contains only one blade, which can rotate 90 degree
about the sheet. It is operated pneumatically.
CNC cropping line hydraulic: It is all so used to cut the CRGO Sheet. It contains two
blades, one is fixed and the other rotates 90 degree above the sheet. It is operated
hydraulically. M4 quality sheet 0.23-0.33 mm thickness is used

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6. TRM BAY 6 :
4 different types of Transformers are manufactured in this section -
i. 3-phase Freight loco Traction Transformer
ii. 1-phase Freight loco Traction Transformer
iii. 3-phase ACEMU
iv. 1-phase ACEMU
Single – traction transformer for AC locomotives is assembled in this section. These Freight
locomotive transformers are used where there is frequent change in speed. They are basically used
in locomotives. In this bay core winding & all the assembly & testing of transformer is done.
These three phase transformer for ACEMU are also manufactured in this section. The supply
lines for this transformer are of 25 KV & power of the transformer is 6500 KVA.
The tap changer of rectifier transformer is also assembled in this bay. Rectified transformer is
used in big furnace like the thermal power stations/plants (TPP).
7. TRM BAY 7 :
This is the insulation shop. Various types of insulation which are to be used in
transformers are prepared in this bay.
MATERIALS USED:
1) AWWW: All Wood Water Washed press paper. The paper is 0.2-0.5 mm thick cellulose
paper & is wound on the conductors for insulation.
2) Pre-compressed board: This is widely used for general insulation & separation of
conductors in the forms of block.
3) Press board: This is used for separation of coils e.g. L.V. from H.V. It is up to 38 mm
thick.
Transformer
Core
CRGO sheet

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4) UDEL: Undemnified Electrical Laminated Wood or Perm wood. This is special type of
plywood made for insulation purposes.
5) Fiber glass: This is a resin material & is used in fire prone areas (Used in DTT)
6) Bakelite: Size 4mm to 25 mm.
7) Gasket: It is used for protection against leakage.
8) Silicon rubber: It is used for dry type transformer.
9) Perma wood: Size 4mm to 25mm.
10) N.B.C. (Naporeniom Bonded Cork) sheets: Size 3mm, 6mm 10mm and 12mm.
The machine used for shaping the insulation material are-
1) Cylindrical machines.
2) Circle cutting machine- Used for cutting the circular objects
3) Scarping machine (used for taper cutting) & Lines in machines (to remove burrs).
4) Punching Press and Drilling machine.
5) Bench saw (specially for OD)
6) Jig saw (specially for ID)
7) Circular saw
8) Rolling machines-Used to roll the press board.
8. TRM BAY 8 :
Basically two types of transformers are assembled in this bay:
1) Electrostatic Precipitator (ESP)
2) Instrument Transformers
a) Current Transformer (CT) up to 400kV class.
b) Electromotive Voltage Transformer (EMVT) up to 220kV class.
INSTRUMENT TRANSFORMER:
These are used for measurement. Actual measurement is done by measuring instruments but
these transformers serve the purpose of stepping down the voltage to protect the measuring
instrument. They are used in AC system for measurement of current voltage and energy and can
also be used for measuring power factor, frequency and for indication of synchronism. They find
application in protection of power system and for the operation of over voltage, over current,
earth fault and various other types of relays.
In power lines current & voltage handled is very large & therefore direct measurement are not
possible as these current & voltage are for too large for any other of reasonable size & cost. The
solution lies in steeping down the current & voltage with instrument transformers that would be
mattered with instruments of moderate size. The transformers are also used for protective purpose.
Body:
 The main body is a bushing which also acts as insulators in which the winding is placed.

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 It has a top and bottom chamber.
 The top chamber is the cylindrical tank of mild steel. It has terminals for connection of HV
coils.
a) Current transformer: It is a step down transformer. The body is divided in to three parts
– top chamber, bushing, bottom chamber. Top chamber is the cylindrical tank of mild
steel. It has terminal for connection of HV coil. It also has oil window to indicate the oil
level. Below it is bushing which houses the winding and also act as insulator. It has
several folds or rain sheds to provide specific electric field distribution and long leakage
path. Some bushings are cylindrical while modern ones are conical, as the amount of oil
porcelain used is reduced without any undesirable effects. Bottom chamber houses the
secondary winding. There is also a connection box to which the connection of low voltage
coil is made.
b) Voltage Transformer: It is also an step down transformer and outer construction is same as
that of CT. the difference is only in winding.
ESP TRANSFORMER:
The Electrostatic Precipitator transformer is used for environmental application. It is used to filter
in a suspended charge particle in the waste gases of an industry. They are of particular use in
thermal power stations and the ash is used in cement industry.
The ESP is a single-phase transformer. It has a primary and secondary. The core is laminated and
is made up of CRGOS. It is a step up transformer. An AC reactor is connected in series with
primary coil. The output of the transformer must be DC this is obtained by rectifying AC using a
bridge rectifier (bridge rectifier is a combination of several hundred diodes). A radio frequency
choke (RF choke) is connected in series with the DC output for the protection of the secondary
circuit and filter circuit. The output is chosen negative because the particles are positively charged.
The DC output from the secondary is given to a set of plates (electrodes) arrange one after the
others. Impurity particles being positively charged stick to these plates, which can be jerked off.
For this a network of plates has to be setup all across the plant. This is very costly process in
comparison with the transformer cost. A relive vent is also provided to prevent the transformer
from bursting it higher pressure develops, inside it. It is the weakest point in the transformer body.
An oil temperature indicator and the secondary supply spark detector are also provided.
One side of the transformer output is taken and other side has a ‘marshalling box’ which is the
control box of the transformer.
ESP is of two types.
1. Si oil type.
2. Transformer oil type.
It is also known as Rectifier Transformer as it converts AC into DC.
9. TRM BAY 9:

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In this bay, power transformers are assembled. After taking input from different bays 0-8,
assemblies are done.
Power Transformer is used to step up & step down voltages at generating & sub-stations. There
are various ratings 11 KV, 22 KV etc. manufactured, they are:
a) Generator Transformer
b) System Transformer
c) Auto Transformer
A transformer in a process of assemblage is called a job. The design of the transformer is done by
the design department & is unique for each job; depends on the requirement of customer. The
design department provides drawing to the assembly shop, which assembles it accordingly.
The steps involved in assembly are:
A. Core Building: It is made of cold rolled grain oriented steel CRGO. The punch core is sent
to this shop from core punching shop. Here it is assembled with the help of drawing a set
of 4 laminations is called a packet. The vertical portion of a core is called a leg. The
horizontal one is caller a yoke. Packets of both are interlinked. It is undesirable to keep the
X section of core circular to provide low reluctance part without air space. A perfect circle
cannot be made so the core is steeped to achieve a near circle. Wherever the spaces are left,
are fitted with thin wooden rod. After core building the end frames are bolted. The bolts
are insulated from the core.
B. Core Lifting: The core is lifted by a crane & is placed vertical. The rest of the assembly is
done on the core in this portion.
C. Unlacing & Core Coil Assembly: The yoke of this core is removed using crane. Bottom
insulation in form of 50 mm thick UDEL sheets is placed. PCB & press board are also used
for filling the gap & to provide a good base for the coil to rest. The coil are then lowered
primary, secondary, tertiary & tap in that sequences.
The following tests are done during relacing:
Megger’s test
Ratio test
High voltage
Testing at this stage is called pre-testing.
D. Replacing & End-frame mounting: After lowering a coil the top insulation similar to the
bottom one is provided. The removed yoke is placed end-frame bolted back into its
position. The connections are then made as per drawing. All the conductors are insulated
using crepe paper. Brazing copper makes the connections. For brazing sulfas is used.
E. H.V.T.G. & L.V.T.G.: Terminals gears are accessories provided at high voltage and low
voltage. Main device used is tap changer. Tap changers can be on-load or off-load. In off-
load type the supply has to be tripped, then the tapings changes but in on-load type the
tapings can be changed while the supply. .The upper portion of the OLTC contains
mechanism by which tapping is changed .There is switch which changes tap in very small
time (Microseconds). But there is a possibility of sparking. To get rid of it, OLTC is filled
with oil. The bottom part houses the terminals and the mechanism, which makes automatic
connections. The terminals are made of thick Al strops.
F. Vapour Phasing & Oil Soaking: It is well known fact that water (impure) is a good
conductor electricity. Therefore, moisture presence in transformer will affect insulation;

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the process of moisture removal from transformer is called vapour phasing. The job is put
in dummy tank and place in a vacuum vessel. It is an airtight chamber with heating
facilities. A solvent vessel is released is the chamber which enters all transformer parts
and insulations. It absorbs water rapidly. The job is heated in vacuum. All the solvent
vapour is sucked out with moisture and if not taken care of, may burst the job.
After moisture removal tank is filled with transformer oil & soaked for at least three hours,
so that every part gets wet with oil.
The job remains in vessel for three days during phasing. It is then taken out of the vessel
& also out of the dummy tank.
G. Final Servicing & Tanking: After taking the job out of dummy tank all the parts
retightened any other defects are rectified and the job is retimed in mild steel tank. After
this the tanking oil is filled.
H. Case Fitting: The accessories are fixed and final touches given the job. The accessories
included tank cover, fixing bushing, fixing valves etc. The terminals are marked and R and
D (rating and diagram) plate is fixed by bolting and not riveting because it may require
maintenance while opening and closing the tank. The bottom chamber is mild steel tank
with a steel frame attached to its base for earthing.
10. TRE (TRANSFORMER ENGINEERING):
The transformers manufactured in BHEL Jhansi range from the 100MVA to 315MVA and up to
400kV.The various transformers manufactured in this unit are:
Power Transformer
 Generation transformer
 System transformer
 Auto transformer
Special Transformer
 Freight loco transformer
 ESP transformer
 Dry Type Transformer
All above types are oil cooled except dry type, which is air cooled.
The designing of all these transformers is done by the TRE department according to the
specifications of the customers’ i.e.
1. The input/output voltage.
2. The KVA rating.
3. The weight of iron and copper.
The basic design factors are:
1. The amount of copper and iron losses.
2. The rise in temperature of coils.
3. The ambient condition.

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The designer has to keep rise between the design factors and selects the optimization of the core
yoke winding etc.
Functioning of TRE
a) Tendering/Submission of offer with EFC
i. Study of customer specifications
ii. Designing
iii. Preparation of material sheet
iv. EFC(Estimated Factory Cost)
v. Guaranteed technical particulars with tender drawings
vi. Type test reports of similar equipment’s
vii. Deviation/clarification etc.
b) Contract execution
i. Designing
ii. Electrical Specification
iii. Guaranteed technical particulars*
iv. Testing schedule*
v. Customer Drawings*
vi. Components Drawings etc.
vii. Indenting of Materials
viii. Manufacturing information to shops
ix. Shipping instruction for transportation
*Approval required from customer
Basically there are 4 sections of the TRE department -
a. Estimation
b. Tendering
c. Design
d. Drawing Office
The Estimation section calculates all ratings related to power transformer, number of jobs to be
prepared, etc. As per the customer requirement, the Tendering section decides materials required
and estimates the cost involved in manufacturing. Using T146 software for power transformer
actual designing takes place considering actual amount of material and scrap. The designing of
core windings, type of tank assembly, etc. all work under guidelines. The drawing office section
issues all drawings of related customer demand are issued to various shops and accordingly final
product is made.
11. TECHNOLOGY:
This department analysis the changes taking place in the world and suggest changes and upgrades
accordingly. This section mainly deals with continuous modification in the operations to be
performed for the completion of the job. It gets the PSR (Performa of Specification & routine
Sheet) from PPC.
This is very important department because the product must not get obsolete in the market
otherwise they will be rejected by the customer.

[23]
This section gives the sequence of operations, time for operations, no. of labors etc., according to
the given standards; it can be modify the above things to obtain best results.
FUNCTIONS:
Technology functions can be classified as:
 Processing Sequence: The sequence of process of manufacturing is decided for timely and
economic completion of the job.
 Operation Time Estimate: It includes incentive scheme management.
 Allowed Operation Time: It includes incentive amount.
 Facilities Identification: It includes looking for new equipment or plant or tools to increase
productivity.
 Special Process Certification: special processes are the ones requiring expertise for
example identifying errors, cracks, air bubbles in the welding.
 Special Tools Requirement: special tools are allotted, if possible, when required else the
design has to be reconsidered.
 Productivity Projects compilation: It includes the initial analysis of the problem and their
appropriate solution to enhance productivity
Recently it has modified the operation of crimping in which it advised the use of tungsten carbide
thus reducing the work time to three hours instead of 27 hours with HSS; this also relieved the
workers from maintenance of different dies for different jobs.
The principle of working is that
“IF YOU DO NOT MAKE THE CHANGES IN YOUR COMPANY, THE CUSTOMER
WILL CHANGE YOU”.
BUS DUCT:
Bus ducts are conductors for high power application. They are used in power connections over
150 MW. The bus ducts consist of a mild steel casing and Aluminium conductors held within the
casing with electro-porcelain insulators. The bus ducts are installed with hot air blowing fans to
keep the conductors moisture free at the time of operation. Two types of bus ducts are
manufactured:
 Segregated Bus Ducts (SBD)
 Isolated Bus Ducts (IBD)
In segregated bus ducts, one casing is divided into three chambers through which Aluminium
conductors for three phases pass.
In isolated bus ducts, each phase is provided with its own casing. The bus ducts also have
cubicles in which circuits and controls are installed.
The mild steel casings of bus ducts are mostly procured as pre-fabricated items. Aluminium bars
are also main raw material for the manufacturing of bus ducts.

[24]
12. TRC (TRANSFORMER COMMERCIAL):
The objective of this department is to interact with the customers. It brings out tenders & notices
& also responds to them. It is department that places the contracts of building the transformers &
after delivery further interacts with the customer regarding faults, this department does failure &
maintenance. All such snags are reported to them & they forward the information to the
concerning department.
The works of the commercial department are:
 Tenders and Notices.
 Interaction with design department
 Place of work
 Approximate cost of work
 Earnest money
 Place and time where document can be seen.
 Amount if any to be paid for such document.
Tenders and notices: The department response to the tenders calls of
companies/organization, which requires transformers. Contracts are bagged through also
notices. Before inviting tenders it must be sure that BHEL is ready to undertake the
contract and before full knowledge of scope of work is essential.
The main functioning of this department is divided in 3 parts -
a) Tendering
b) Execution
c) Closing
The Tendering section bring orders from different sectors of market viz. power sector,
industry sector, etc. After bringing of order it comes under execution section which ensures
timely delivery of order. The contract execution group ensures to fulfill all the terms on
which they have agreed with the customer. At last, contract closing group measures
performance. Also meetings with customer are conducted and bank guarantee and money for
completing job is asked from customer.
LOCOMOTIVE DEPARTMENT
This unit was started in 1985.
A locomotive is a rail vehicle that provides the motive power for a train.
“LOCO” means from a place.
“MOTIVE” means causing motion.
A locomotive has no payload capacity of its own. It is used to move a train.
This department of Jhansi consists of two sections the first is manufacturing & other is design.
The diesel, AC, AC/DC locomotives are manufactured here.
1. THE DIESEL LOCOMOTIVE:
Salient features:
a) Flat bed under frame.

[25]
b) All pneumatic valves provides in single panel.
c) All electrical Equipment’s provided in single panel.
d) Improved filtration system.
e) Brush less traction alternators.
f) Fault display on control desk with alarm.
g) Simple driving procedure.
h) Automatic wheel slip detection & correction.
i) Multiple unit operation up to three locomotives.
j) Air & vacuum brakes.
2. THE AC LOCOMOTIVE
Salient features:
a) Operate on 25 KV AC Single-phase lines.
b) Driving cab at both ends.
c) Corridors on both sides for maintenance.
d) All pneumatic valves at one place.
e) Automatic wheel slip detection & correction.
f) Multiple unit operation up to three locomotives.
g) Fault display on driver’s desk.
h) VCB in AC circuit.
i) Air & Vacuum brakes.
3. THE AC/DC LOCOMOTIVE
Salient features:
a) Designed to operate both in 1500 V DC & 25 KV AC lines.
b) Driving cab at both ends.
c) High adhesion bogie.
d) Corridors on both sides for maintenance.
e) All pneumatic valves at one place.
f) Automatic wheel slip detection & correction.
g) Multiple unit operation up to three locomotives.
h) Fault display on driver’s desk.
i) Static inverter for auxiliary supply.
j) FRP control desk.
k) VCB in AC circuit.
l) Air & Vacuum brakes.
m) Air dryer for brake system.
13. LOCOMOTIVE PRODUCTION (LMP):
There are following products are manufactured at Loco shops
 Alternating Current Locomotive (ac Loco)
 WAG-5H
 AC./D.C. Loco
 WCAM-2P
 WCAM-3
W-broad gauge A-running in AC mode

[26]
C-running in DC mode G-hauling goods train
P-hauling passenger train M-hauling passenger & goods train
 Diesel Electric Locomotive Shunting (DESL)
 350 HP
 700 HP
 Single Power Pack (SPP): One 700 HP m/c is made as a single
 Unit. It is a meter gauge locomotive
 Twin Power Pack (TPP): 2 350HP m/cs are combined in 1 engine & can be operated
individually or in combination depending on the load.
 450 HP
 1400 HP
 1150 HP
 1350 HP
 2600 HP
 1150 HP and 1350 HP DESL s are non-standard locomotives and are modified versions
of 1400 HP DESL based on requirement of customer.
Under mention are the new non-conventional products designed and developed for Indian
Railways based on their requirement.
 OHE (Overhead electric) recording and testing cars
 UTV(Utility vehicle )
 RRV(Rail cum road vehicle)
 DETV( Diesel electric tower car)
 BPRV(Battery power road vehicle)
 BCM(Blast cleaning machine)
 200 T Well wagon for BHEL Haridwar
 Metro Rake-Kolkata Metro Railways

[27]
14. LMM (LOCOMOTIVE MANUFACTURING):
This section deals with manufacturing of locomotives. The main parts of the
locomotive are -
 Under frame: The frame on which a locomotive is built
 Super structure: The body of locomotive is called superstructure or Shell and is made of
sheet of Mild steel
 DC motor
 Alternator
 Compressor: When air pressure is 5kg/cm2, it will lift up pantograph.
 Flower
 B.D.Pane : It will control and power low power equipment’s.
 Static Rectifier-MSR : converts 1-phase AC supply to 110V DC to charge battery.
 Static Converter-SC : converts 1-phase 1000V AC supply from overhead extension
(OHE) to 3-phase 440V AC supply .
 Exchanger
 Blower-MVMT : It will suck air through 3 ducts. These ducts are directly connected to
Traction motor for cooling it.
 ATFEX : It is braking transformer.
 Bogie-The wheel arrangement of a loco is called a bogie. A bogie essentially contains
1-wheel axle arrangement
2-Suspension
3-Brake rigging
Traction transformer: It is fixed on under frame and gets supply from an overhead line by
equipment called pantograph. The type of pantograph depends on supply. This transformer steps
down voltage and is fitted with a tap changer. Different taps are taken from it for operating
different equipment. One tap is taken and is rectified into DC using MSR and is fed to the DC
motor.
Railways has two types of power supplies – 25 KV , 1 Phase ,50hz AC and1500 V DC
An AC/DC loco is able to work on both of these supplies. For e.g. WCAM-3.
15. LMC (LOCOMOTIVE COMMERCIAL):
This department is divided into 3 sections-

[28]
a) Tendering
b) Execution
c) Service after sales
The Tendering section Looks upon requirements of customer, estimate cost of project and
other information of tender. On receipt of tender forms formal tender enquiries are issued to
engineering dept., production planning and control, central dispatch cell. If tender is technically
acceptable then offer is submitted to customer. Offer includes technical, commercial details and
other terms and conditions. On opening of technical bid the commercial bid of technically
qualified tenderers is opened and order is placed on lowest value tender.
The Contract Execution section on receipt of purchase order issue internal work orders for
execution of work as per the contract. To maintain the key dates of the contract internal meetings
with concerned departments are held on regular basis to monitor the progress. After the supply of
equipment’s and other materials and billing the major responsibility of this section is to collect
the payment from customer. After the dispatch of full material and collection of full payment and
fulfilling all other requirements, the contract is formally closed by this section.
The after sales service section looks upon 2 types of work-
a) Within Warranty period – free of cost service and replacement of material on account of
failure of material due to faulty material or bad workmanship.
b) Beyond Warranty – services and material on chargeable basis.
16. CQX (CENTRAL QUALITY SERVICES) :
First we get acquainted with a few terms concerning this department.
Quality: It is the extent to which products and services satisfy the customer needs.
Quality Assurance: All those plants and systematic action necessary to provide adequate
confidence that a product or service will satisfy the given requirement is called quality
assurance.
Quality control: The operational techniques and activities that are used to fulfill
requirement for quality are quality controlled.
Quality Inspection: Activities such as measuring, testing, gauging one or more
characteristics of a product or service and comparing these with specified requirements to
determine conformity are termed as Quality Inspection.
17. TESTING :
In this shop testing on the transformer is carried out in one section and for loco in other section.
In transformer testing section there are for MG sets. The electrical specifications of the entire test
are given. These tests are done on demand of customer on transformer manufactured.

[29]
Here basically there are basically following tests conducted on power transformers:
1. ROUTINE TEST- It is conducted on all transformers. They are:
a) Ratio test: To determine the Voltage Transformation ratio on each tapping between HV and
LV.
b) Vector Group: To verify the internal connection of the coils (Windings) and the connection
to the terminal.
c) Winding Resistance measurement: To check the healthiness of various joints, internal
connection of the coil and connection to the terminals/Bushings.
d) Magnetizing Current: To measure No load current at low voltage (Supply voltage)
e) Magnetic Balance: To measure flux distribution in each winding by exciting (by applying
voltage) one winding only.
f) Insulation Resistance measurement: To check the healthiness of the insulations provided
on each winding in turn to all other windings, core and frame or tank.
g) Separate source test: To check the healthiness of the insulation of each winding in turn to
all other windings, core and frame or tank with applies single phase voltage.
h) No-load loss measurement: To calculate the Power consumption during No load condition
of the Transformer itself.
i) Load loss & impedance measurement: To calculate the Power consumption during full
Load condition of the Transformer itself.
j) Induced overvoltage test: To verify the A.C. voltage withstands strength of each line
terminal and it’s connected winding to earth and other windings, withstand strength between
phases and along the winding under test.
k) 02 KV core isolation test: To check the isolation of core.
2. TYPE TEST- These are to be conducted only on one unit of same design.
a) Temperature rise test: To observe the maximum temperature when the transformer is
running on continuous full load.
b) Impulse test: To verify the A.C. voltage withstands strength of each line terminal and it’s
connected winding to earth and other windings, withstand strength between phases and along
the winding under test. This test is conducted at a voltage even higher than induced overvoltage.
c) Auxiliary loss test: To measure the power taken by cooling gear like Fans & pumps.
d) Acoustic Noise level measurement: To measure average sound level generated by the
Transformer when energized at rated voltage and rated frequency at no load.
e) Zero sequence impedance measurement: To calculate the impedance when all three
phase are symmetric.
f) Short time current test (STC test)
Tests conducted on INSTRUMENT TRANSFORMERS are as follows-
1. ROUTINE TEST
a) Polarity test:
 Instrument used: Polarity meter analog multimeter.
One of the winding is supplied with 1.5V D.C. supply and the other is connected to ammeter.
If the direction of the deflection is correct implies the connections are correct else it is wrongly
connected.
b) Accuracy test:
It is the test for checking the turn ratio steps:

[30]
 A standard transformer primary is connected across the primary of the job.
 As the no. of turns of the secondary transformer is known the no. of turns of secondary
of job is calculated.
 The ratio is taken and the max permissible error should be not more than that specified
by the design.
 Even the phase angle is checked for this max permissible limit.
c) Inter turn insulation test:
Checks for the insulation of the transformer
 Current is given to primary and secondary is open circuited.
 Either of the rated primary current or the 4.5KV peak secondary voltages whichever
appears first is allowed to withstand for 1 min.
 Then if the insulation can withstand then it is said to be okay.
d) Winding resistance:
Error in winding resistance appears if the conductors of different length are used if the
conductors are joined in between to check this winding resistance is checked and if it
appears then the internal points of connections is changed.
e) One minute power frequency (dry) withstand test-It is the high voltage test used to
check the insulations on primary and secondary. It depends upon the line voltage system
for primary. And for secondary it is 3 or 5 kVrms as applicable.
f) Tan d test-It is conducted to justify the quality of insulation. Its limit is 0.005 at Um/√3
.
g) Partial discharge test- It is used to justify the manufacturing process. Its limit is 10 pc
at 1.1Um/√3.
2. TYPE TESTS
a) Temperature rise test
b) Short time current test (STC test)-It is conducted only on CT.
c) Short time withstand capability test- It is conducted only on VT.
d) Impulse test
One minute power frequency wet withstand test- It is conducted in rainy conditions to
check the external insulations.
18. WE&S (WORK ENGINEERING AND SERVICES) :
As the name suggest this section deals with services & maintenance. It has following sections:
a) Plant equipment: This has electronics & elect/mech. Maintenance.
b) Services: This section deals with air, steam & Power equipment’s.
c) Telephone Exchange.
d) Township Electrical Maintenance.
e) W.E. & S Planning.
This sections deals with stores & new machines procurement & other general things. There are
three maintenance centers at bay-2, substation 1 & Loco. This section is also responsible for Power
distribution in B.H.E.L.
The power distribution is of two types:
1) HT Power Distribution: This is at 11 KV, OCB are used for protection. There are four
substations for this distribution.

[31]
2) LT Distribution: This is for the auxiliary in each shop & other section of B.H.E.L. It uses
OCB/OVC/BHEL BHOPAL, 800 KVA 415V Transformer & ACB (English Electro).
This department looks after the commissioning and maintenance of all the machinery used in the
factory. It also has 3 two-stage air compressors for supplying compressed air to the various bays.
The department has 03 different divisions:
 Electrical Engg.
 Electronics Engg.
 Mechanical Engg.
ELECTRICAL ENGINEERING:
This division looks after all the electrical machinery and power distribution of the factory. Snags
detected in the system are immediately reported to this deppt. by the concerning deppt. WE&S
takes prompt action to rectify it.
The factory has a feeder of 11KV .The total load sanctioned for the factory is 2500MVA. But
the maximum demand reaches the range of 1700-2000 MVA.
Here are various sub-stations (SS) inside the factory, for distribution of power to different
sections.
SS -1 Supplies Bay-6 to Bay –9
SS -3 Supplies Bay 1to Bay-4
SS -4 Supplies Boiler and loco plant
SS -5 Supplies Bay -5
SS -6 Supplies Administrative building

[32]
REPORT ON
ASSEMBLY OF
POWER
TRANSFORMER

[34]
INTRODUCTION:
A transformer is a static machine used for transforming power from one circuit to another
without changing frequency. This is very basic definition of transformer. Transformer generally
used in transmission network is normally known as Power Transformer. The term 'Power
Transformers’ refers to the transformers used between the generator and the distribution circuits,
and these are usually rated at 500 Kva and above. Power systems typically consist of a large
number of generation locations, distribution points, and interconnections within the system or with
nearby systems, such as a neighbouring utility. The complexity of the system leads to a variety of
transmission and distribution voltages. Power transformers must be used at each of these points
where there is a transition between voltage levels.
Power transformers are selected based on the application, with the emphasis toward custom design
being more apparent than larger the unit. Power transformers are available for step-up operation,
primarily used at the generator and referred to as generator step-up (GSU) transformers; step-down
operation, mainly used end to feed distribution circuits and to connect grids operating at different
voltage levels through interconnecting transformers. Power transformers are available as single-
phase or three-phase apparatus.
Power transformers have been loosely grouped into three market segments based on size ranges.
These three segments are:
1. Small power transformers 500 to 7500 kVA
2. Medium power transformers 7500 to 100 MVA
3. Large power transformers 100 MVA and above.
HISTORY:
The History of transformer commenced in the year of 1880. In the year of 1950 400KV electrical
power transformer first introduced in high voltage electrical power system. In the early 1970s
unit rating as large as 1100MVA were produced and 800KV and even higher KV class
Transformers were manufactured in year of 1980.
WORKING PRINCIPLE:
The working principle of transformer is very simple. It depends upon Faraday's law of
electromagnetic induction. Actually mutual induction between two or more winding is
responsible for transformation action in an electrical transformer.
In its simplest form, a transformer consists of two conducting coils having a mutual inductance.
The primary is the winding which receives electric power, and the secondary is the one which
may deliver it. The coils are wound on a laminated core of magnetic material.
The physical basis of a transformer is mutual inductance between two circuits linked by a
common magnetic flux through a path of low reluctance.
The two coils possess high mutual inductance. If one coil is connected to a source of alternating
voltage, an alternating flux is set up in the laminated core, most of which is linked up with the
other coil in which it produces mutually induced vemf (electromotive force) according to
Faraday's laws electromagnetic induction, i.e.
e=𝑴
𝒅𝒊
𝒅𝒕
Where, e = induced emf
M = mutual inductance

[35]
If the second circuit is closed, a current flows in it and so electric energy is transferred (entirely
magnetically) from the first coil (primary winding) to the second coil (secondary winding).
 CORE ASSEMBLY
Core building from the finished lamination sheets is done in horizontal position on specially raised
platforms. The lamination sheets are susceptible to mechanical stresses of bending, twisting,
impact, etc. A lot of care is exercised while handling and normally two persons are needed to hold
the two ends of the laminations at the time of laying.
At first the clamp plates and end frame structure of one side of the core assembly are laid out.
Guide pins are used at suitable positions for maintaining the proper alignments during core
building process. Oil ducts are formed by sticking strips on lamination and put in position as
required.
For each packet, the laminations are manufactured in two different lengths and these sets are laid
out alternately, keeping at a time two to four laminations together. The two alternate arrangements
provide overlapping at the corner joints and when the lamination packets are clamped together,
these overlapping edges provide sufficient mechanical strength in holding the edges in tight grip.
After laying out the complete laminations, the clamp plates, and end frame structure of the other
side are laid out and the entire core-end frame structure is properly secured through bolts and steel
bands at a number of positions.
The platform on which the core building takes place is of special design and the core-end frame
assembly can be raised to the vertical position along with the platform which serves as a cradle.
Subsequently the platform is disengaged.
In this process, the core assembly is spared from the mechanical strain of lifting and raising in the
vertical position. Small-size cores can however be built up without these special platforms.
Steel bands used for tightening the laminations is only a temporary arrangement and are later
removed, otherwise these will form short circuited turns.
Two commonly used methods of holding the leg laminations together is their clamping by either
resiglass tape or using skin stressed Bakelite cylinders.
In case of resiglass tapes, these are tightly wound around the legs at specified pitch and cured by
heating. The tape shrinks after heating and provides a firm grip. The tensile strength of resiglass
tapes is even higher than that of steel tapes.
In the case of core legs tightened by skin stressed cylinders (base cylinder of innermost coil), these
are lowered from the top and the steel bands and cut progressively. Wooden wedges are inserted
along the packet corners and hammered down, so that the enveloping Bakelite cylinder and the leg
laminations are fitting tightly against each other.

[36]
Conventionally, the core is assembled along with all the yokes, and after assembly the top yokes
are unlaced after removing the top-end frames for the purpose of lowering the windings. This takes
a lot of labour and manufacturing time. The latest development is to assemble the core without top
yokes and insert the top yokes after lowering all the windings in the core leg.
Fig.13. Core assembly process

[37]
 CORE-COIL ASSEMBLY
After the unlacing of core is done i.e., the top yoke is removed, the core is made to stand erect
and then the coils are mounted on the core which may have been wound in any of the previous
manners i.e. spiral, helical etc.
The coils as specified in the design may be of following types:
1. L.V COIL: This is known as low voltage coil. These coils are often referred to as the primary coil
for stepdown transformer. Thesecoilare made in order to allow the flow of largecurrent through
it and thus the cross sectional area of the conductor used in this kind of coil is larger and the
numbers of turns per conductor are few, also less number of conductors is used in L.V coil.
2. H.V COIL: This is known as the high voltage coil. Thesecoils are often referred as the secondary
coil of a step down transformer. These are made in order to allow high voltages and hence small
amount of current through it. So the conductors used are smaller in size and number of turns per
conductor is more in numbers. Also the number of conductor in this kind of coil increases.
3. T.V COIL: This is known as the tertiary voltage coil.
4. M.V COIL: This is known as the medium voltage coil.
As required or specified in the design at the bottom of the core an insulator circular in shape is
provided with blocks made of wood attached on it . This is known as the block washer assembly
the wood attached on the wad man insulator material serves ducts, which help in circulation of
transformer oil and thus better cooling of transformer is achieved. The core is also given a
surrounding of alayer of wad man insulating material on which spacers are provided which serves
the purpose of creating ducts for oil circulation as well as it gives support to the coil wounded on
it. Generally the L.V coil is mounted as the first layer after the spacers on insulation material
thereafter the coil is again shielded with the insulating material and the spacers on which the H.V
coil is mounted and this way the process is carried on based on the design.

[38]
After the L.Vand H.V coilare mounted on the core, the top of the core where the mounting
ends again the layer of wad man insulation material the block washer assembly is provided. Now
the job is taken for relacing.
This sheet is nothing but arecord of various data's to be provided during the core coilassembly.
The following information's are achieved in this sheet:
 Terminal gear assembly
After relacing of the top yoke, the preparation of the Terminal Gear Assembly done as described
below:
(a) Cutting of the leads as required.
(b) Crimping/brazing of the leads with cables.
(c) Brazing of bus bars.
(d) Fixing of different cleats.
(e) Crimping/brazing of cables with terminal lugs.
(f) Mounting of the tap changer/tap switch.
(g) Preparation of HV line lead.
In this stage, connections between phases to form the required vector group, tapping lead
connections, line and neutral leads formation, etc., are completed.
Low-voltage connections are done on one side of the winding and are designated as LV terminal
gear. On the opposite side, high-voltage connections are done and are designated as HV terminal
gear. Medium voltage leads (in system or autotransformer are taken out on LV side and tapping
connections on either LV or HV side depending upon design layout. Generally in generator
transformer, a three-phase on-lad or off-circuit tap-changer is mounted on one end and in case
of autotransformer three single-phase tap changers are mounted in front of the windings. Tap

[39]
changers are supported from end frame during terminal gear assembly. All leads, i.e. line and
neutral leads of low-voltage, medium-voltage and high-voltage windings, tapping leads, etc., are
laid out and connected using different types of joints (i.e. bolted, crimped, soldered or brazed)
and insulated for the required insulation level. Leads are properly supported by cleats mounted
on end frames.
The clearances between various leads, coil to leads, leads to end frame and other parts are
maintained and checked.
The connections available in this stage are either of the following two types or a combination of
these two types:
a. Star Connection: Also called Wyes winding. Each phase terminal connects to one end of
a winding and other end of a winding connects to other at a central point, so that the
configuration resembles a capitalletter Y. The central point may or may not be connected
outside of the transformer.
b. DeltaConnection:Alsocalledmesh winding. In delta connection the bottom position lead
of one coil is joined to the top or starting position lead of the second coil and the bottom
lead of second coil are joined to top lead of the third coil and the top lead of the first coil
is connected to bottom lead of third coil. In H.V. side the H.V. main lead is taken out and
the various tap leads are then joined to OLTC (On Load Tap Changer) through conductors.
The conductors to the tap changer can be observed in the H.V. marked as (3-14) in
numbers these are then connected to the tap changer. All the leads are properly brazed
for accurate connections so that same amount of current flows through each conductor
and the ratio can be achieved. In L.V. side the bottom leads of the L.V. coils are joined
together to form neutral. Whereas the top lead positions are taken out for respective 3-
Ø connection.
 PROCESSING-P2 (VAPOUR PHASE AND OIL IMPREGNATION)
The second drying out is commenced when the core and windings are placed. The job is put in
dummy type shell and then in a vacuum vessel. It is an air tight chamber with heating facilities.
The job is heated in vapour-phase heating systemin which a liquid, such as white spirit, is heated
and admitted to the transformer tank under low pressure as vapour. This condenses on the core
and windings, and as it does so it releases its latent heat of vaporisation. It is necessary to ensure
that the insulation does not exceed a temperature of about 130°C to prevent ageing damage:
when this temperature is reached, the white spirit and water vapour is pumped off. Finally, a
vacuum equivalent of between 0.2 and 0.5 mbar absolute pressure is applied to complete the

[40]
removal of all air and vapours. During this phase, it is necessary to supply further heat to provide
the latent heat of vaporisation; this is usually done by heating coils in an autoclave, or by
circulating hot air around the tank within the dry-out oven.
Vacuum is applied when the initial reduction in the rate of change of these parameters is
noted: the ability to achieve and maintain the required vacuum, coupled with a reduction and
leveling out of the quantity of water removed and supported by the indication given by
monitoring of the above parameters, will confirm that the required dryness is being reached. For
a vapour phase drying system, since it could be dangerous to monitor electrical parameters,
drying termination is identified by monitoring water condensate in the vacuum pumping system.
After moisture removal tank is filled with transformer oil and soaked for at least three
hours, so that every part gets wet with oil.
 SERVICING
Drying out of insulation is accompanied by significant shrinkage, so it is usual practice for a large
transformer to be immediately following initial oil impregnation to allow for retightening of all
windings, as well as cleats and clamps on all leads and insulation materials. This operation is
carried out as quickly as possible in order to reduce the time for which windings are exposed to
the atmosphere.
 TANKING
The transformer tank provides the containment of the core and windings and for the dielectric
fluid. It must withstand the forces imposed on it during transport.
Transformer tanks are almost invariably constructed of welded boiler plate to BS 4630. The tank
must have a removal cover so that access can be obtained for the installation and future removal.
The cover is fastened by a flange around the tank. The cover is normally inclined to the
horizontal at about 1°, so that it will not collect the rain water.
The job is finally put into a tank in which various points are taken care of such as:
 The proper tank dimensions is achieved as specified in the design.
 Weld leakagetest is performed on the tank to check that whether any kind of leakage is
present in the tank or not.
 Vacuum and pressure test is performed on the tank to check its endurance.

[41]
The accessories are fixed and final touches given to job. The accessories include tank cover,
fixing bushing, fixing valves etc. The terminal are marked R&D (rating and drawing) plate is
fixed by bolting.
 PROCESSING-P3 CONVENTIONAL (HEATING/VACUUM)
AND OIL FILLING UNDER VACUUM
On returning the core and windings to the tank, the manufacturer will probably have a rule which
says that vacuum should be reapplied for a particular time, before refilling with hot, filtered,
degassed oil. The final drying out is commenced when the core and windings are placed are fitted
into their tank, all main connections made, and the tank placed in an oven and connected to the
drying system. The tapping switch may be fitted at this stage, or later, depending on the ability of
the tapping switch components to withstand the drying process.
BASIC PRINCIPLES OF DRYING
Cellulosic insulation is dried by creating conditions in which water vapour pressure (WVP) around
the insulation is less than that in the insulation. The vapour pressure in the insulation is increased
by heating the insulation and the vapour pressure around the insulation is decreased by removing
water vapour. Fig.3. shows pressure plotted against humidity and temperature, from which it is
seen that a 20 ºC temperature rise increases the internal pressure by more than 100% (a factor of
two). Basically one should aim at achieving the highest processing temperature consistent with the
type and ageing properties of the insulation. The upper temperature limit is set by the maximum
permissible drying temperature for paper, i.e. about 110 ºC (or 130 ºC in an oxygen-free
atmosphere).

[42]
Fig.23. Partial insulation water vapour pressure v. temperature, for various insulation moisture contents.
Drying efficiency also depends on diffusion coefficient of the insulation material. This coefficient
is dependent on the material to be dried, its temperature, pressure and moisture content.
Two basic processes are adopted for drying of transformers.
(a) Conventional vacuum drying
(b) Vapour phase drying
IMPORTANCE OF OIL FOR THE LIFE OF POWER TRANSFORMER
A transformer’s life expectancy is based on a number of factors, the most important of
which is the quality of its insulation system over time. The oil used in power transformers is
particularly susceptible to moisture and its insulating value is seriously reduced when even small
amounts of water are present. In addition to this, the oil’s insulating quality and performance as a
cooling medium can be reduced by oxidization as well. It is, therefore, extremely important that
the design of the transformer be such that it impedes the contact of the insulation system to the
outside atmosphere, which contains both moisture and oxygen. Since the oil in the transformer
will expand and contract with temperature and load, a number of systems have been developed to
help preserve the overall insulation quality of the transformer. These designs include open style,
sealed tank, conservator style, and automatic gas pressure.
At BHEL Jhansi we mostly follow the conservator style. The conservator- or expansion-
type design has the main tank completely filled with oil and a smaller expansion tank positioned
above the main tank, with about 5 to 10 per cent the volume of the main. As the oil expands and
contracts with temperature and load, the atmosphere moves in and out through a uni-directional
moisture removing breather. Only a small surface area of the oil in the expansion tank has exposure
to the atmosphere and the expansion tank is designed in such a way so that if moisture should get
in, it remains trapped in the expansion tank and cannot be exposed to the paper/wood insulation
and clamping system of the core and coils. This is the most cost-effective design in higher MVA
units and also offers the easiest versatility in shipping because the main tank is totally immersed
in oil with no top head space.

[43]
 FINAL TANKING/CASE FITTING
The job is finally put into a tank in which various points are taken care of such as:
1. Dimension: the proper tank dimension is achieved as specified in the design.
2. Weld leakage test: this test is performed on the tank to check that whether any kind of leakage
is present in the tank or not.
3. Vacuum and pressure tests are per formed on the tank to check its endurance.
The accessories are fixed and final touch is given to job. The accessories include tank cover, fixing
bushing, fixing valves etc. The terminals are marked R&D (rating and drawing) plate is fixed by
bolting.
The bottom
chamber is mild steel tank with a steel frame attached to its base for earthling the following points
are taken care of in the process of CASE FITTING:
1. CT connections:
The current transformer leads are properly connected and connection is provided at top of the tank
for user.
2. Feed positions:
The job is put into the tank and it is locked to the feeds or shoes provide at the bottom of the tank,
also same kind of locking system is provided at the top of tank so that the job remains static during
transportation.
3. Bushing and Electrical clearance:
The required clearance is obtained as specified in design.
4. MD unit alignment:

[44]
This is the motor driving unit alignment. The main function of this unit is to operate the tap
changer. Using this unit the tap changer can be operated manually and required voltage can be
achieved.
Before commencement of final works tests, the transformer is then usually left to stand
for several days to allow the oil to permeate the insulation fully and any remaining air bubbles
to become absorbed by the oil.
*TANKS AND ANCILLARY EQUIPMENT
 TANKS
The tank is manufactured by forming and welding steel plate to be used as a container for holding
the core and coil assembly together with insulating oil. The base and the shroud, over which a
cover is sometimes bolted. These parts are manufactured in steel plates assembled together via
weld beads. The tank is provided internally with devices usually made of wood for fixing the
magnetic circuit and the windings. In addition, the tank is designed to withstand a total vacuum
during the treatment process. Sealing between the base and shroud is provided by weld beads. The
other openings are sealed with oil-resistant synthetic rubber joints, whose compression is limited
by steel stops. Finally the tank is designed to withstand the application of the internal overpressure
specified, without permanent deformation.
 CONSERVATOR

[45]
The tank is equipped with an expansion reservoir (conservator) which allows for the expansion of
the oil during operation. The conservator is designed to hold a total vacuum and may be equipped
with a rubber membrane preventing direct contact between the oil and the air.
 HANDLING DEVICES
Various parts of the tank are provided with the following arrangements for handling the
Transformer.
- Four locations (under the base) intended to accommodate bidirectional roller boxes for
displacement on rails.
-Four pull rings (on two sides of the base)
-Four jacking pads (under the base)
-Tank Earthing terminals:
The tank is provided with Earthing terminals for Earthing the various metal parts of the
Transformer at one point. The magnetic circuit is earthed via a special external terminal.
 VALVES
The Transformers are provided with sealed valves, sealing joints, locking devices and position
indicators.
The Transformers usually include:
- Two isolating valves for the "Buchholz" relay.
- One drainage and filtering valve located below the tank.
- One isolating valve per radiator or per cooler.
- One conservator drainage and filtering valve.
And when there is an on-load adjuster:
- Two isolating valves for the protection relay.

[46]
- One refilling valve for the on-load tap-changer.
- One drain plug for the tap-changer compartment.
 THE DEHYDRATING BREATHER
The dehydrating breather is provided at the entrance of the conservator of oil immersed equipment
such as Transformers and reactors.
The conservator governs the breathing action of the oil system on forming to the temperature
change of the equipment, and the dehydrating breather removes the moisture and dust in the air
inhaled and prevents the deterioration of the Transformer oil due to moisture absorption. The
dehydrating breather uses silica - gel as the desiccating agent and is provided with an oil pot at the
bottom to filtrate the inhaled air.
 BUSHING CONNECTIONS

[47]
Fig.18. Connections of bushings
A bushing is a means of bringing an electrical connection from the inside to the outside of the tank.
It provides the necessary insulation between the winding electrical connection and the main tank
which is at earth potential.
The bushing forms a pressure-tight barrier enabling the necessary vacuum to be drawn for the
purpose of oil impregnation of the windings. It must ensure freedom from leaks during the
operating lifetime of the transformer and be capable of maintaining electrical insulation under all
conditions such as driving rain, ice and fog and has to provide the required current-carrying path
with an acceptable temperature rise. Varying degrees of sophistication are necessary to meet these
requirements, depending on the voltage and/or current rating of the bushing.

[48]
 CABLE BOX CONNECTIONS
Cable boxes are the preferred means of making connections at 11, 6.6, 3.3 kV and 415 V in
industrial complexes, as for most other electrical plant installed in these locations. Cabling
principles are not within the scope of this volume and practices differ widely, but the following
section reviews what might be considered best practice for power transformer terminations on HV
systems having high fault levels.
Modern polymeric-insulated cables can be housed in air-insulated boxes. Such connections can be
disconnected with relative simplicity and it is not therefore necessary to provide the separate
disconnecting chamber needed for a compound-filled cable box with a paper-insulated cable. LV
line currents can occasionally be as high as 3000 A at 11 kV, for example on the station
transformers of a large power station, and, with cable current ratings limited to 600 800 A, as many
as five cables per phase can be necessary. For small transformers of 1 MVA or less on high fault
level installations it is still advantageous to use one cable per phase since generally this will restrict
faults to single phase to earth. On fuse-protected circuits at this rating three-core cables are a
possibility. Since the very rapid price rise of copper which took place in the 1960s, many power
cables are made of aluminium. The solid conductors tend to be bulkier and stiffer than their copper
counterparts and this has to be taken into account in the cable box design if aluminium-cored cables
are to be used. Each cable has its own individual gland plate so that the cable jointer can gland the
cable, maneuver it into position and connect it to the terminal. Both cable core and bushing will
usually have palm-type terminations which are connected with a single bolt. To give the jointer
some flexibility and to provide the necessary tolerances, it is desirable that the gland plate-to-
bushing terminal separation should be at least 320 mm.
The following protective devices are used so that, upon a fault development inside a Transformer,
an alarm is set off or the Transformer is disconnected from the circuit. In the event of a fault, oil
or insulations decomposes by heat, producing gas or developing an impulse oil flow.
To detect these phenomena, a Buchholz relay is installed.

[49]
 BUCCHOLZ RELAYS
The Buchholz relay is installed at the middle of the connection pipe between the Transformer tank
and the conservator. There are a 1st stage contact and a 2nd stage contact as shown in Fig. (1). the
1st stage contact is used to detect minor faults. When gas produced in the tank due to a minor fault
surfaces to accumulate in the relay chamber within a certain amount (0.3Q-0.35Q) or above, the
float lowers and closes the contact, thereby actuating the alarm device.
The 2nd stage contact is used to detect major faults. In the event of a major fault, abrupt gas
production causes pressure in the tank to flow oil into the conservator. In this case, the float is
lowered to close the contact, thereby causing the Circuit Breaker to trip or actuating the alarm
device.
 TEMPERATURE MEASURING DEVICE
Liquid Temperature Indicator (like BM SERIES Type) is used to measure oil temperature as a
standard practice. With its temperature detector installed on the tank cover and with its indicating
part installed at any position easy to observe on the front of the Transformer, the dial temperature
detector is used to measure maximum oil temperature.
The indicating part, provided with an alarm contact and a maximum temperature pointer, is of
airtight construction with moisture absorbent contained therein; thus, there is no possibility of the
glass interior collecting moisture whereby it would be difficult to observe the indicator Fig. (1&2).
Further, during remote measurement and recording of the oil temperatures, on request a search coil
can be installed which is fine copper wire wound on a bobbin used to measure temperature through
changes in its resistance.
 Winding Temperature Indicator Relay (BM SERIES)
Fig. (1). Buchholz Relay

[50]
The winding temperature indicator relay is a conventional oil temperature indicator supplemented
with an electrical heating element. The relay measures the temperature of the hottest part of the
Transformer winding. If specified, the relay can be fitted with a precision potentiometer with the
same characteristics as the search coil for remote indication.
Fig. 0 Construction of Winding Temperature Indicator Relay
 PRESSURE RELIEF DEVICE
When the gauge pressure in the tank reaches abnormally to 0.35-0.7 kg/cm.sq. The pressure relief
device starts automatically to discharge the oil. When the pressure in the tank has dropped beyond
the limit through discharging, the device is automatically reset to prevent more oil than required
from being discharged.
Fig. 1.PRESSURE RELIEF DEVICE

[51]
 METHODS OF COOLING
RADIATORS AND FANS- They are used for cooling of the oil.
Fig.19. Radiator and fans
 Testing of Power Transformer
Tests during manufacture
As part of the manufacturer’s QA system some testing will of necessity be carried out during
manufacture. These are:
Core-plate checks: Incoming core plate is checked for thickness and quality of insulation coating.
A sample of the material is cut and built up into a small loop known as an Epstein Square from
which a measurement of specific loss is made.
Core-frame insulationresistance:This is checked by Megger and by application of a 2 kV R.M.S.
test voltage on completion of erection of the core. These checks are repeated following

[52]
replacement of the top yoke after fitting the windings. A similar test is applied to any electrostatic
shield and across any insulated breaks in the core frames.
Core-loss measurement: If there are any novel features associated with a core design or if the
manufacturer has any other reason to doubt whether the guaranteed core loss will be achieved,
then this can be measured by the application of temporary turns to allow the core to be excited at
normal flux density before the windings are fitted.
Winding copper checks: If continuously transposed conductor is to be used for any of the
windings, strand-to-strand checks of the enamel insulation should be carried out directly the
conductor is received in the works.
Tank tests: The first tank of any new design should be checked for stiffness and vacuum-withstand
capability. For 132 kV transformers, a vacuum equivalent to 330 mbar absolute pressure should
be applied. This need only is held long enough to take the necessary readings and verify that the
vacuum is indeed being held for hours. After release of the vacuum, the permanent deflection of
the tank sides should be measured and should not exceed specified limits, depending on length.
Following this test, a further test for the purpose of checking mechanical withstands capability
should be carried out. Typically a pressure equivalent to 3 mbar absolute should be applied for 8
hours.
FINAL TESTING
Final works tests for a transformer fall into three categories:
Tests to prove that the transformer has been built correctly: These include ratio, polarity,
resistance, and tap change operation.
Tests to prove guarantees: -These are losses, impedance, temperature rise, noise level.
Tests to prove that the transformer will be satisfactory in service for at least
30 years: The tests in this category are the most important and the most difficult to frame: they
include all the dielectric or over voltage tests, and load current runs.
Routine tests
All transformers are subjected to the following tests:
1. Voltage ratio and polarity.

[53]
2. Winding resistance.
3. Impedance voltage, short-circuit impedance and load loss.
4. Dielectric tests.
(a) Separate source AC voltage.
(b) Induced over voltage.
5. No-load losses and current.
6. On-load tap changers, where appropriate.
Type tests
1. Temperature rise test.
2. Noise level test.
3. Lightning impulse tests.
Special tests
Special tests are tests, other than routine or type tests, agreed between manufacturer and purchaser,
for example:
1. Test with lightning impulse chopped on the tail.
2. Zero-sequence impedance on three-phase transformers.
3. Harmonics on the no-load current.
4. Power taken by fan and oil-pump motors.
Routine test
Voltage ratio and polarity test
Measurements are made on every transformer to ensure that the turns ratio of the windings, tapping
positions and winding connections are correct. The BS tolerance at no-load on the principal tapping
is:
±0.5% of the declared ratio
These measurements are usually carried out during assembly of both the core and windings, while
all the connections are accessible, and finally when the transformer is fully assembled with
terminals and tap changing mechanism. In order to obtain the required accuracy it is usual to use
a ratio meter rather than to energize the transformer from a low-voltage supply and measure the
HV and LV voltages.

[54]
Ratio meter method
The ratio meter is designed to give a measurement accuracy of 0.1% 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. This method also confirms the polarity of the windings since a zero reading
would not be obtained if one of the winding connections was reversed.
Insulation resistance test
Insulation resistance tests are carried out on all windings, core and core clamping bolts. The
standard Megger testing equipment is used, the ‘line’ terminal of which is connected to the winding
or core bolt under test. When making the test on the windings, so long as the phases are connected,
together, either by the neutral lead in the case of the star connection or the interphase connections
in the case of the delta, it is only necessary to make one connection between the Megger and the
windings.
Resistance of windings
The DC resistances of both HV and LV windings can be measured simply by the
voltmeter/ammeter method, and this information provides the data necessary to permit the
separation of I²R and eddy-current losses in the windings.
This is necessary in order that transformer performances may be calculated at any specified
temperature. The voltmeter/ammeter method is not entirely satisfactory and a more accurate
method such as measurement with the Wheatstone or Kelvin double bridge should be employed.
It is essential that the temperature of the windings is accurately measured; remembering that at test
room ambient temperature the temperature at the top of the winding can differ from the
temperature at the bottom of the winding. Care also must be taken to ensure that the direct current
circulating in the windings has settled down before measurements are made.
Measurement of no-load loss and current
The no-load loss and the no-load current shall be measured on one of the windings at rated
frequency and at a voltage corresponding to rated voltage if the test is performed on the principal
tapping or to the appropriate tapping voltage if the test is performed on another tapping. The
remaining winding or windings shall be left open-circuited and any windings which can be
connected in open delta shall have the delta closed.
Dielectric tests
The insulation of the HV and LV windings of all transformers is tested before leaving the factory.
These tests consist of:

[55]
(a) Induced over voltage withstand test;
(b) separate-source voltage withstand test
a) Separate source AC voltage: -
This test is intended to check the adequacy of main insulation to earth and between windings. The
line terminals of the windings under test are connected together & the appropriate test voltage is
applied to them, while the windings & tank are connected together to the earth. Winding with
graded insulation, which have neutral intended for direct earthing, are tested at 38 kv. The supply
voltage should be nearly sinusoidal and the peak voltage is measured from digital peak voltmeter
associated with capacitive voltage divider. The duration of test is 60 seconds.
Highest voltage for equipment (Kv) Rated short duration
(r.m.s.) power frequency withstand
Voltage (Kv)-r.m.s.
1.1 3
3.6 10
7.2 20
12 28
17.5 38
24 50
36 70
b) Induce over voltage withstand test:
The test is intended to check the inter-turn and line end insulation as well as main insulation to
earth & between windings.
In order to avoid core saturation at the test voltage, it is necessary to use a supply frequency higher
than the normal. When frequency is chosen in the range of 150-240 Hz, capacitive reactance is
reduced, and in draws significant capacitive current at test voltage, which causes heavy loading on
the generator can be reduced by connecting a variable reactor across the generator terminals.
Test duration is determined by the following formula-
Test duration in seconds=
120∗Rated frequency
Test frequency
but not less than 15 sec

[56]
The test is applied to all the non-uniformly insulated windings of the transformer. The neutral
terminal of the winding under test is earthed. For other separate windings, if they are star connected
they are earthed at the neutral and if they are delta connected they are earthed at one of the terminals
Impulse test levels
These levels are based on uniform and non-uniform insulation. Impulse voltage withstands test
levels for transformers have been standardized in BS 171. Transformers to be impulse tested are
completely erected with all fittings in position, including the bushings, so that, in addition to
applying the surge voltage to the windings, the test is applied simultaneously to all ancillary
equipment such as tap-changers, etc., together with a test on clearances between bushings and to
earth.
Impulse voltage wave shapes
A double exponential wave of the form v=V (e α t-e β t) is used for laboratory impulse tests. This
wave shape is further defined by the nominal duration of the wave front and the total time to half
value of the tail, both times being given in microseconds and measured from the start of the wave.
BS 923, the British Standard for impulse testing, defines the standard wave shape as being 1.2/50
s and gives the methods by which the duration of the front and tail can be obtained. The nominal
wave front is 1.25 times the time interval between points on the wave front at 10 and 90% of the
peak voltage. The time to half value of the wave tail is the total time taken for the impulse voltage
to rise to peak value and fall to half peak value, measured from the start. The tolerances allowed
on these values are ±30% on the wave front, and ±20% on the wave tail.

[57]
Rated transformer impulse withstand voltages:-
Highest system Lightning impulse withstand Category of winding
Voltage (kV r.m.s.) Voltage (kV peak) insulation
Dry type Oil immersed
(a) (b) (c) (d)
3.6 20 40 20 40
7.2 40 60 40 60
12 60 75 60 75
17.5 75 95 75 95
24 95 125 95 125
36 145 170 145 170 Uniform and
52 250 non-uniform
72.5 325
123 450
145 550
170 650
245 850
300 1050
362 1175 Non-uniform
420 1425
300 950
362 1050
420 1175 Non-uniform
525 1425

[58]
765 1800
Chopped wave
If an impulse voltage is applied to a piece of insulation and if a flashover or puncture occurs
causing sudden collapse of the impulse voltage, it is called a chopped impulse voltage. If chopping
takes place on the front part of the wave, it is known as front chopped wave. Again, if chopping
takes place on the front, it is specified by the peak value corresponding to the chopped value and
its nominal steepness is the rate of rise of voltage measured between the points where the voltage
is 10% & 90% respectively of the voltage at the instant of chopping.
IMPULSE GENERATOR
The production of voltage impulses is achieved by the discharge of a capacitor or number of
capacitors into a wave-forming circuit and the voltage impulse so produced is applied to the object
under test. For conducting high-voltage impulse tests a multi-stage generator is used. This consists
of a number of capacitors initially charged in parallel and discharged in series by the sequential
firing of the interstage spark gaps.
The generator consists of a capacitor C which is charged by direct current and discharged
through a sphere gap G. A resistor Rc limits the charging current while the resistors Rt and Rf
control the wave shape of the surge voltage produced by the generator. The output voltage of the
generator can be increased by adding more stages and frequently up to 12 stages are employed for
this purpose. Additional stages are shown in Fig. and as will be seen from this diagram all stages
are so arranged that the capacitors C1, C2, C3, etc. are charged in parallel. When the stage voltage
reaches the required level V the first gap G1 discharges and the voltage V is momentarily applied
to one electrode of the capacitor C2. The other electrode of C2 is immediately raised to 2V and
the second gap G2 discharges. This process is repeated throughout all stages of the generator and
if there are n stages the resultant voltage appearing at the output terminal is nV. This output is the
surge voltage which is applied to the test object.

[59]
12-stage Impulse generator having an open-circuit test voltage of 2.4 MV and store energy of 180
KJ. Each of the 12 stages has an output of 200 kV
Impulse tests on transformers
The withstand impulse voltages to be applied to a transformer under test are specified in IS: - 2026
and the test voltages are required to be applied in the following order:
1. One calibration shot at between 50 and 75% of the standard insulation
level.
2. Three subsequent full-wave shots at the standard level.
The application of voltages 1 and 2 comprises a standard impulse-type test and they are applied
successively to each line terminal of the transformer. If during any application, flashover of a
bushing gap occurs, that particular application shall be disregarded and repeated.
Where chopped waves are specified, the test sequence is as follows:
(a) One reduced full wave, at 75% of the test level.
(b) One full wave at the test level.
(c) One or more reduced chopped waves.
(d) Two 100% chopped waves.
(e) Two full waves at the test level.
For oil-immersed transformers the test voltage is normally of negative polarity since this reduces
the risk of erratic external flashover. The time interval between successive applications of voltage
should be as short as possible. These tests employ the 1.2/50 s wave shape and the chopped waves
can be obtained by setting the gap in parallel with the transformer under test.
Tolerances: The standard lightning impulse shall have a time to chopping (TC) between 2 to 6
microseconds.
Test Criteria: The absence of significant difference between voltage and current transients
recorded at reduced voltage and those recorded at full test voltage constitute evidence that the
insulation has withstood the test.
Fault detection during impulse tests
Detection of a breakdown in the major insulation of a transformer usually presents no problem as
comparison of the voltage oscillograms with that obtained during the calibration shot at reduced

[60]
voltage level gives a clear indication of this type of breakdown. The principal indications are as
follows:
1. Any change of wave shape as shown by comparison with the full-wave voltage oscillograms
taken before and after the chopped-wave shots.
2. Any difference in the chopped-wave voltage oscillograms, up to the time of chopping, by
comparison with the full-wave oscillograms.
3. The presence of a chopped wave in the oscillogram of any application of voltage for which no
external flashover was observed.
Audible noise
There is one another completely different method of fault detection known as the electro
acoustic probe, which records pressure vibrations caused by discharges in the oil when a fault
occurs. The mechanical vibration set up in the oil is detected by a microphone suspended below
the oil surface. The electrical oscillation produced by the microphone is amplified and applied to
an oscilloscope, from which a photographic record is obtained. Alternatively acoustic devices may
be attached to the external surfaces of the tank to detect these discharges.
Fault location
The location of the fault after an indication of breakdown is often a long and tedious procedure
which may involve the complete dismantling of the transformer and even then an inter turn or
interlayer fault may escape detection. Any indication of the approximate position in the winding
of the breakdown will help to reduce the time spent in locating the fault. Current oscillograms may
give an indication of this position by a burst of high-frequency oscillations or a divergence from
the ‘no-fault’ wave shape.
TEMPERATURE RISE TEST
When a test for temperature rise is specified it is necessary to measure the temperature rise of the
oil and the windings at continuous full load, and the various methods of conducting this test are as
follows:
(a) Short-circuit equivalent test;
(b) Back-to-back test;
(c) delta/delta test;
(d) open-circuit test.
Method (a)
One winding of the transformer is short-circuited and a voltage applied to the other winding of
such a value that the power input is equal to the total normal full-load losses of the transformer at

[61]
the temperature corresponding to continuous full load. Hence it is necessary first of all to measure
the iron and copper losses. As these measurements are generally taken with the transformer at
ambient temperature, the next step is to calculate the value of the copper loss at the temperature
corresponding to continuous full load.
Method (b)
In this method, known as the back-to-back (or Sumpner) test, the transformer is excited at normal
voltage and the full-load current is circulated by means of an auxiliary transformer.
Method (c)
This method, known as the delta/delta test, is applicable to single- as well as three-phase
transformers where the single-phase transformers can be connected up as a three-phase group.
Method (d)
If it happens that a transformer possesses a low ratio of copper loss to iron loss it is generally
impossible to conduct a temperature rise test by the short-circuit method. This is because the
required power input necessitates an excessive current in the windings on the supply side of the
transformer, so that a prohibitively high current density would be reached. In such cases it may be
possible to test the transformer on open circuit, the normal losses being dissipated in the iron
circuit. If a supply at a frequency considerably below the normal rated frequency of the transformer
is available, a condition may be obtained whereby the total losses are dissipated at a test voltage
and current in the neighborhood of the normal rated voltage and current of the transformer. If,
however, a lower frequency supply is not available, the transformer may be run at the normal rated
frequency with a supply voltage greater than the normal rated voltage, and of such a value that the
total losses are dissipated in the iron circuit.
The iron loss varies as the square of the voltage, the required voltage under these conditions is
given by the formula:
Normal voltage (1+
1.2∗𝐶𝑜𝑙𝑑 𝑐𝑜𝑝𝑝𝑒𝑟 𝑙𝑜𝑠𝑠
𝑁𝑜𝑟𝑚𝑎𝑙 𝐼𝑟𝑜𝑛 𝐿𝑜𝑠𝑠
)
Partial discharge test
This test is carried out on the windings of the transformer to assess the magnitude of discharges.
If the apparent measured charge exceeds 104 pC, the discharge magnitude is severe.
(a) Partial discharge in the insulation system may be caused by insufficient drying or oil
impregnation. Reprocessing or a period of rest, followed by repetition of the test, may therefore
be effective.
(b) A particular partial discharge gives rise to different values of apparent charge at different
terminals of the transformer and the comparison of simultaneous indications at different terminals
may give information about the location of the partial discharge source.

Sahil BHEL JHANSI assembely of power transformer

Sahil BHEL JHANSI assembely of power transformer

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