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INTRODUCTION
BHARAT HEAVY ELECTRICALS LIMITED
BHEL is an integrated power plant equipment manufacturer and one of the largest engineering and
manufacturing company in India in terms of turnover. It was established in 1956, ushering in the
indigenous Heavy Electrical Equipment industry in India - a dream that has been more than
realized with a well recognized track record of performance. The company has been earning profits
continuously since 1971-72 and paying dividends since 1976-77.
Government of India ( ministry of Heavy Industries and Public enterprises) has granted the status
of MAHARATNA to Bharat Heavy Electricals Limited on 1st Feb 2013.
It has been engaged in the design, engineering, manufacturing, construction, testing,
commissioning and servicing of a wide range of products and services for the core sectors of the
economy, viz. Power, Transmission, Industry, Transportation (Railway), Renewable Energy, Oil
,Gas and Defense .
BHEL have a share of 59% in India’s total installed generating capacity contributing 69% (approx.)
to the total power generated from utility sets (excluding non-conventional capacity) as of March 31,
2012.
Vision
A Global Engineering Enterprise providing Solutions
for better tomorrow
Mission
Providing sustainable business solutions in the
fields of Energy, Industry & Infrastructure
Values
Governance, Respect, Excellence, Loyalty, Integrity,
Commitment, Innovation, Team Work
BHEL’s greatest strength is the highly skilled and committed workforce of 49,390 employees.
Every employee is given an equal opportunity to develop himself/herself and grow in his/her
career. Continuous training and retraining, career planning, a positive work culture and

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participative style of management. All these have engendered development of a committed and
motivated workforce setting new benchmarks in terms of productivity, quality and responsiveness.
BHEL BHOPAL
Bharat Heavy Electricals Ltd was set up in 1956 at Bhopal. There are around 10,000 employees at
the BHEL plant in Bhopal being a vital part of BHEL on a whole.
Fig 1. Arial view of B.H.E.L. Bhopal
Bharat Heavy Electricals ltd .Bhopal is situated near Piplani which is a nowadays covers large part
of Bhopal. BHEL TOWN, Bhopal is a suburb of Bhopal, Madhya Pradesh. This has developed
like other BHEL townships after Indian public sector engineering company BHEL started its
operations here. It is spread over an area of around 20 km2 and provides facilities like, parks,
community halls, library, shopping centers, banks, post offices etc.. The company has been earning
profits continuously since 1971-72 and paying dividends uninterruptedly since 1976-77. In
recognition of its consistent high performance, BHEL has been conferred with the 'Maharatna'
status by the Government of India on 1st February 2013. It is now one among seven Maharatna
PSUs.With a widespread network of 17 manufacturing units, 2 repair units, 4 regional offices, 8
service centers, 8 overseas offices, 15 regional centers, 7 joint ventures, and infrastructure to
execute more than 150 project sites across India and abroad, BHEL provides products, systems and
services to customers efficiently and at competitive prices. The company has established capability
to deliver 20,000 MW p.a. of power equipment to address the growing demand for power
generation equipment. With an export presence in more than 76 countries, BHEL is truly India’s
industrial ambassador to the world.

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The high level of quality & reliability of our products is due to adherence to international standards
by acquiring and adapting some of the best technologies from leading companies in the world
including General Electric Company, Alston SA, Siemens AG and Mitsubishi Heavy Industries
Ltd., together with technologies developed in our own R&D centers.
Most of BHEL’s manufacturing units and other entities have been accredited to Quality
Management Systems (ISO 9001:2008), Environmental Management Systems (ISO 14001:2004)
and Occupational Health & Safety Management Systems (OHSAS 18001:2007).
BHEL has:-
 Added more than 1, 24,000 MW to the country's installed power generating capacity so
far.
 Supplied over 25000 Motors with Drive Control System to power projects,
Petrochemicals Refineries, Steel, Aluminium, Fertilizer, Cement plant, etc.
 Supplied Traction electrics and AC/DC locos over 12000 kms Railway network.
 Supplied over one million Values to Power Plants and other Industries.
 BHEL has retained its market leadership position during 2013-14 with 72% market
share in the Power Sector, even while operating in a difficult business environment.
Improved focus on project execution enabled BHEL record highest ever
commissioning/synchronization of 13,452 MW of power plants in domestic and
international markets in 2013-14, marking a 30% increase over 2012-13.
BHEL has been exporting our power and industry segment products and services for over 40 years.
BHEL's global references are spread across over 76 countries across all the six continents of the
world. The cumulative overseas installed capacity of BHEL manufactured power plants exceeds
9,000 MW across 21 countries including Malaysia, Oman, Iraq, the UAE, Bhutan, Egypt and New
Zealand. Our physical exports range from turnkey projects to after sales services.
In the world power scene BHEL ranks among the top ten manufacturers of power plant equipment,
spectrum of products and services offered, it is right on top. BHEL’s greatest strength is its highly
skilled and committed workforce of 48,399 employees. Every employee is given an equal
opportunity to develop himself/herself and grow in his/her career. Continuous training and
retraining, career planning, a positive work culture and participative style of management - all these
have engendered development of a committed and motivated workforce setting new benchmarks in
terms of productivity, quality and responsiveness. BHEL business operations cater to core sectors
of the Indian Economy like Power, Industry, Transportation, Transmission, and Defense.

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PRODUCTS OF BHEL
Power Utilization
AC Motors & Alternators
Transportation
Transportation Equipment
Power Generation
Hydro Turbines
Hydro Generators
Heat Exchangers
Excitation Control Equipment
Steam Turbines
Miscellaneous
Oil Rigs
Fabrication
Power Transmission
Transformer
Switchgear
On-Load Tap Changer
Large Current Rectifiers
Control & Relay Panels
Renovation & Maintenance
Thermal Power Stations

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TRANSFORMER
POWER TRANSFORMER
A Transformer is static device used for transforming power from one circuit to another without
changing frequency.
The range of power transformers in B.H.E.L. covers from low voltage medium power transformer
to extra large power transformer of 1500 MVA bank in 765 kV class & HVDC converter
transformers of 1500 MVA banks in ± 500 kV DC . Product range also include Shunt Reactor upto
150 MVAR in 400 kV class and 330 MVAR in 765 kV class .
MAIN PARTS OF TRANSFORMER
 CORE
 WINDING
 TANK
which are made separately. Winding and core are assembled in tank and ON load / OFF load
tap changer is provided.

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CORE:
BHEL make transformers having Core type configuration. Core is built up from cold rolled
grain oriented silicon alloy steel of the best magnetic properties. CNC machine is used for
slitting ; cropping / mitering operations with perfect control un burr level . A computer program
determines no. of steps, position of the oil ducts and the sizes of the laminations for optimum
cross section of core and yokes. In order to achieve a further reduction of iron loss laminations
are mitered and core bolts for clamping are kept to a minimum . Bolts in core legs are
completely eliminated by using resin bonded glass tape for binding or by using skin stressed
cylinder.
 Magnetic circuits are of generally 3 limb construction in single phase transformer in
which only center limb is wound and the outer limbs provide return path for main flux.
 In three phase transformer 5 limb construction is used which has 2 vertical return limbs
and 3 main cores.
ConstructionalFeatures
The type of transformer core construction depends on the technical particulars of the transformer
and transport considerations. In general it is preferable to accommodate the windings of all the
three phases in a single core frame. Three phase transformers are economical over a bank of three
single-phase transformers. Another important advantage of three-phase transformer cores is that
component of the third and its multiple harmonics of mmf cancel each other, consequently the
secondary voltage wave shape are free from distortions due to the third harmonics in mmf.
However, if the three-phase ratings are large enough and difficult to transport, one has no choice
but to go for single-phase transformer units. For single-phase and three-phase transformers, the
cores can be broadly classed as:
 Single-phase three-limbed core
 Single-phase two-limbed core
 Three-phase three-limb core
 Three-phase five-limbed core

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Fig. Stepped Core Construction
(a) Single-phase Three-limbed Core
The windings are placed around the central limb, also known as main limb. The main magnetic flux
generated in the central limb gets divided into two parallel return paths provided by the yokes and
auxiliary limbs. For the same magnetic flux density as that in the main limb, the auxiliary limbs and
the yokes need to have the cross section only half of the main limb. This type of transformer core is
generally preferred for single-phase transformer, as this is more economical than two limbed
construction discussed below
(b) Single-phase Two-limbed core
Sometimes the single-phase power ratings of transformers are so large that if the windings
of full power ratings were to be placed on the central limb, its width would become too large to be
transported. To mitigate such difficulties the windings are split into two parts and placed around

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two separate limbs. Here the cross-sectional area of the legs (limbs) and the yokes are identical.
Consequently these cores are bulkier than the single phase three-limbed arrangements. Also the
percentage leakage reactance for this type of core construction is comparatively higher due to
distributed nature of the windings in the two limbs separately .
(c)Three-phase Three-limbed cores
This type of core is generally used for three-phase power transformer of small and medium power
ratings. Each phase of the winding is placed around one leg. For each phase of magnetic flux
appearing in a limb, the yokes and the remaining two limbs provide the return path. If the phase
fluxes are denoted as ØA, ØB, ØC, their summation at any instant of time is identically zero, which
can be mathematically stated as ØA + ØB + ØC = 0. In this type of construction, all the legs and the
yokes have identical cross section.

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(d) Three-phase Five-limbed cores
For large rating power transformers, cores have to be built in large diameters. In case of three-phase
three-limbed cores, the yokes have the same diameter as the limbs. In case of large diameter cores,
the overall core height will go up leading to transport problem. For such cases the yoke cross-
sections (and consequently yoke heights) are reduced by approximately 40% or more and auxiliary
paths for the magnetic flux are provided through auxiliary yokes and limbs. The cross-section and
the height of the auxiliary yokes and limbs are lower than that of the main yokes.
Fig. Three Phase Three Limb & Three Phase Five Limb Core
Flow Chart of Core Formation
Slittingof core steel rolls
to required width on
slittingmachines.
Croppingand mitringto
the required
dimensions.
Hole punchingin the
laminationswhere
required.
Assembly of insulation
between clamp
plate/end frame & core
laminations.
Lying of clamp plates
and end frame and its
levelling.
Stackingof laminations
of different sizeto the
required thickness.
Preparation of oil duct in
core,Core building.
Clampingof core after
assembly of the top end
frame and Tightening of
core.
Lifting of core by use of a
cradle,and carryingout
isolation checks after
treatment of insulation
items.

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WINDING
Windings form the electrical circuit of a transformer. Their construction should ensure
safety under normal and faulty conditions. The windings must be electrically and mechanically
strong to withstand both over-voltages under transient surges, mechanical stress during short circuit
and should not attain temperatures beyond the limit under rated and overload conditions. For core-
type transformers, the windings are cylindrical, and are arranged concentrically. Circular coils offer
the greatest resistance to the radial component of electromagnetic forces, since this is the shape
which any coil will tend to assume under short circuit stresses.
Winding Conductor
The shape of the winding conductor in power transformers is usually rectangular in order to utilize
the available space effectively. Even in smaller transformers for distribution purposes where the
necessary conductor cross section easily can be obtained by means of a small circular wire, this
wire is often flattened on two sides to increase the space factor in the core window. With increasing
conductor area, the conductor must be divided into two or more parallel conductor elements in
order to reduce the eddy current losses in the winding and ease the winding work. Strands may be
insulated either by paper lapping or by an enamel lacquer. The matter is mechanically soft. In order
to withstand the short circuit forces it is sometimes necessary to increase the strength of the
material by means of a cold working. In large power transformers the mechanical forces during
short circuit current have often more influence on the winding dimensions then thermal aspects and
loss considerations. Generally two types of conductors are used for winding:
Paper Insulated Copper Conductor (PICC)
In PICCs the strands (Copper conductors) have a lapping of paper insulation. The paper lapping is
built up of thin paper strips, a few centimeters wide, wound around and along the strand as
indicated in figure. The paper is lapped in several layers to obtain the necessary total thickness set
by the electrical and mechanical stresses.

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Continuously Transposed Copper Conductor (CTC)
Special kind of winding conductor is the ‘Continuously Transposed Cable (CTC)’. This cable is
built up of two layers of enamel lacquer insulated strands arranged axially upon each other as
shown in figure. By transposing the outer strain of one layer to the next layer with a regular pitch
and applying common outer insulation a continuous transposed cable is achieved.
When traversing the same flux for a whole transposition cycle, all strands loops receive the same
induced voltage, and circulating currents between the strands are avoided. Transposition of strands
must also be made in windings with conventional conductors to avoid circulating currents. If
necessary for increased mechanical strength, the strands are covered with the epoxy glue, which
cures during processing the winding. For lower voltages a netting around the transposed cable is
used to keep the strands together. For higher voltages insulation paper covers the cable.
Distributed Cross-overWindings
These windings are suitable for currents not exceeding about 20A. They comprise wires of circular
cross-section and are used for HV windings in small transformers in the distribution range. A
number of such coils are joined in series, spaced with blocks which provide insulation as well as
duct for cooling.
Spiral Winding
This type of winding is normally used up to 33 kV and low current ratings. Strip conductors
are wound closely in the axial direction without any radial ducts between turns. Spiral coils are
normally wound on a Bakelite or pressboard cylinder.

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Though normally the conductors are wound on the flat side, sometimes they are wound on the edge.
However, the thickness of the conductor should be sufficient compared to its width, so that the
winding remains twist-free.
Helical Winding
This type of winding is used in low-voltage and high-current ratings. A number of conductors are
used in parallel to form one turn. The turns are wound in a helix along the axial direction and each
turn is separated from the next by a duct. Helical coils may be single-layer or double layer or multi-
layer, if the number of turns are more.
Unless transposed, the conductors within a coil do not have the same length and same flux
embracing and therefore have unequal impedance, resulting in eddy losses due to circulating
current between the conductors in parallel. To reduce these eddy losses, the helical windings are
provided with transposition of the conductors which equalize the impedances of the parallel
conductors.
Continuous Disk Winding
This type of winding is used for voltage between 33 and 132 kV and medium current ratings.
These coils consist of a number of sections placed in the axial direction, with ducts between them.
Each section is a flat coil, having more than one turn, while each turn itself may comprise one or
more conductors (usually not more than four or five), in parallel.
Helical coil (Single layer) Helical coil (Double Layer)
Layer LLayer Layer layer)

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The sections are connected in series, but without any joints between them. This is achieved by a
special method of winding. It is not necessary to provide a cylindrical former for these coils, as
these are self-supporting. Each disc is mechanically strong and exhibits good withstand of axial
forces. Another particular advantage of these coils is that, each section can have either integral or
fractional number of turns (for example 4 turns per section).
INTERLEAVED DISC WINDING
A disadvantage with the continuous disc winding is that their strength against impulse voltages is
not adequate for voltages above, say, 145 kV class. The impulse voltage withstand behavior of disc
coils can be increased if the turns are interleaved in such a fashion that two adjacent conductors
belong to two different turns. Figure shows such a winding in which interleaving has been done in
each pair of discs. It will be noticed that it is necessary to have 2n conductors in hand for winding
when n in the number of conductors in parallel. Conductors of turns 8 and 9 are joined by brazing.
A cross-over is given at the bottom of the disc.
Apart from interleaving between every double-disc, it is also possible to have more number of discs
(say four) in each interleaved group. This gives further improved behavior against impulse voltage,
though there are concomitant increased complexities.
Interleaved windings require more skill and labor than plain continuous disc windings. Sometimes
a part of the winding is interleaved while the remaining part is plain disc, so as to combine the
advantages of better impulse withstand at the high voltage end of the winding and reasonable labor
cost for the winding as a whole. These are known as partially interleaved windings.
(2 discs per group)
(4 discs per group)

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TRANSPOSITION
(a) For helical windings, usually three transpositions are provided. The complete transposition
[fig.1 (a)] is provided in the middle of the windings. Two partial transpositions are provided, one at
25% of turns [Fig. 1 (b)] and the other at 75% of turns [Fig 1 (c)]. In complete transposition, each
conductor position is varied symmetrically, relative to the middle point, whereas in partial
transpositions, two halves of parallel conductors are interchanged in the positions: the upper half
becomes the lower, and vice versa. Such a transposition needs extra space in the height of coil.
Figure.1Transpositions in helical winding
(a) Complete transposition (b) and (c) Partial transposition
(b)With a multi-start helical winding, the transposition can be achieved by using rotary
transposition. Figure. 2 shows transposition in a double-start helical winding. By this arrangement,
every conductor occupies every position by turn and thereby complete equalization of impedance is
possible. Also, there is no need for extra space in coil height.
(b)
Figure.2Rotarytransposition for double star helical winding
1
12
1
3
4 9
5 8
6 7
2 1
3
4
5
6 9
7 8
3 2
4 1
5
6
7
8 9
1
2 9
3 8
4 7
5 6
2
1
11
12
10
12
11
10
12
11
10
12 11
10

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(c) For disc windings having more than one conductor in parallel. Transposition is made between
the conductors by changing their mutual position at each crossover from one section to another
(Fig. 3)
In power transformers, generally, winding can be divided according to their voltage ratings and
performance viz. HV, LV and Tap windings.
High Voltage Winding Disk type
Large no. of turns
Low current density
Comparatively small conductor cross section
Connected to core
Low Voltage Winding Single layer helical type
Less no. of turns
High current density
Large conductor cross section
Nearest to the core
Tap Winding Interleaved helical type
Generally connected in series with High Voltage Winding (Except
in GTs)
Various coils are provided for tapings to regulate voltage to
± 10 % or more
Figure.3 Transposition at each cross-over in continuous disc winding

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CORE PUNCH
Ferrous
Material
Machine
Component
AssembledCore
CRGO Lamination
Preparation
To Next Process
B.O.Insulation
Component
In House Insulation
component
COREBUILDING
Fabrication
Component

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CORE MANUFACTURING PROCESS
CRGO Imported
Slitting Machine
Cropping Machine
Stacking/Arranging
Core Building
Clamping
Lifting
Tapping
Setting(Jobs)
Curing
Test(2 KV/10KV)
SHIFT TO
ASSEMBLY/TANK

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CRGO (cold rolled grain oriented) Imported from:
1) Nippon Steel Corporation, Yawata Works (Japan)
2) VIZ-Steel Ltd., Yekaterinburg (Russia)
3) POSCO
Slitting Machine (Sequence of operation):
- Drawing / Q plan
- Size / Grade CRGO
- Burr Level 20 micron
- Steel width within tolerance
- Every 500m width check Burr Gauge
- Scrap and Buckling
Cropping Machine (Sequence of operation):
- Revised drawing / QA Plan checked
- Every 100 sheet parameter check
After completion of assembly of core including curing of resin glass tape, 10 KV AC test
between
- Core and End-Frame
- Core and Yoke-Bolts
- End-Frame and Yoke-Bolts
INSULATION SHOP
Insulating Material
Sr. No. Material Applications
1. Transformer Oil
(Mineral Oil, PXE)
Liquid dielectric and coolant
2. Craft paper Layer winding Insulation
Covering copper conductor and transposed copper conductor
3. Creep Kraft paper Insulation of winding lead and shield
4. Press Paper Backing paper for axial cooling duct
5. Press Board Angle ring, Cap, lead out, insulating end of winding ,
Cylinder, Barrier, Washer, Yoke, top and bottom coil
clamping ring
6. Wood and laminated
wood
Cleat , core/ yoke clamp, Wedge block
7. Insulation Tap Tapping and bending of transformer cores
8. Phenolic laminated paper
base sheet
Terminal gear support and cleat, gap filter in reactor, tap
changer components
9. Phenolic laminated
cotton fabric sheet
Terminal board, for making core duct, support and cleat

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Coil Winding
Coil Pre-Heating:
- 100 oC (min 95 oC oven temperature)
- Duration 3 Hours
Types of Winding:
- Disc and Interleaved
- Helical, Spiral and Inter-Wound Helical
Copper Conductors:
- PICC - Bunched and Glued
- CTC
- Glued
Coil Assembly And Power Assembly
PICC/CTC
Moulded comp.
B.O. Insul.Comp.
Coil
Winding Wound Coil
To Process
In houseinsu.comp
WOUND COIL
Moulded coil
Coil
Assembly
In house
insul.comp
B.O.Insu .comp
To Process 4
Assembled Coil

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BUSHING

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INTRODUCTION
In electrical power, a bushing is an insulated device that allows an electrical conductor to pass
safely through a (usually) earthed conducting barrier such as the wall of a transformer or a circuit
breaker. In its simplest form, a bushing consists of a central conductor embedded in a cylindrical
insulation material having a radial thickness enough to withstand the high voltage.
A bushing has to :
(a) Carry the full load current.
(b) Provide electrical insulation to the conductor for working voltage and for various over-voltages
that occur during service.
(c) Provide support against various mechanical forces.
CLASSIFICATION OF BUSHINGS
Bushings are classified according to the following factors:
APPLICATION OR UTILITY
(A) ALTERNATOR BUSHING
AC generators require bushings up to 33 kV, but 22 kV, is more usual. With modern alternators,
current ratings up to 20,000 Amp are required.
(B) BUSHINGS FOR SWITCHGEAR
In the switchgear, bushings are to carry the conductors through the tank wall, and support the
switch contacts.
(C) TRANSFORMER BUSHINGS

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Transformers require terminal bushings for both primary and secondary windings. In some cases, a
high voltage cable is directly connected to the transformer via an oil filled cable box. A bushing
then provides the connection between the cable box and transformer winding.
(D) WALL OR ROOF BUSHING
In recent years, many sub-stations for 132 kV and above, in unfavorable situations have been put
inside a building. For such applications wall/roof bushings are used.
(E) LOCO BUSHINGS
These bushings are used in freight loco and AC EMU transformers for the traction application.
NON-CONDENSER AND CONDENSER BUSHINGS
(A) NON-CONDENSER BUSHING
In its simplest form, a bushing would be a cylinder of insulating material, porcelain, glass resin, etc.
with the radial clearance and axial clearance to suit the electric strengths. The voltage is not
distributed evenly through the material, or along its length. As the rated voltage increases, the
dimensions required become so large that this form of bushing is not a practical proposition. The
concentration of stress in the insulation and on its surface may give rise to partial discharge. This
type of bushing is commonly used as low-voltage bushings for large generator transformers.
(B) CONDENSER BUSHING
The condenser bushings is made by inserting very fine layers of metallic foil into the paper during
the winding process. The inserted conductive foils produce a capacitive effect which dissipates the
electrical energy more evenly through the insulated paper and reduces the electrical field stress
between the energized conductor and any earthed material.

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BUSHING DESIGN
All materials carrying an electric charge generate an electric field. When an energized conductor is
near any material at earth potential it can cause very high field strengths to be formed. As the
strength of the electric field increases, leakage paths may develop within the insulation. If the
energy of the leakage path overcomes the dielectric strength of the insulation ,it may puncture the
insulation and allow the leakage current to flow through the shortest path through the earthed
material toward the earth causing burning and arcing.
The design of bushing must involve following considerations :
(A) AIR-END CLEARANCE
The air-end clearance has to be sufficient to meet the specified over-voltage tests. It is also
determined by the creepage distance, and the proportion of it that is protected from the rain. Having
determined the air-end length, the air-end dimension of the internal condenser can be determined. It
is not necessary to grade 100%. Internal grading of 70% or less will give adequate surface grading
for large bushings.
(B) OIL-END CLEARANCE
As internal breakdown unlike air flashover, is more severe, specifications, therefore, demand an
internal breakdown with a sufficient margin (about 15%) above the air withstand value. Both power
frequency, and impulse voltage withstand tests have been used to specify this characteristic.
(C) NUMBER OF CONDENSER LAYERS

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The number of partial condensers is so chosen that the test voltage of each partial condenser should
be between 10 kV to 15 kV. If more foils are introduced, it will cause too many folds and weaken
the bushing. Also, will be air introduced in the folds, complicating the manufacture of bushing of
high voltage class.
(D) LENGTH OF EARTH LAYER
The length of the earth layer of a bushing is usually determined by the accommodation required for
current-transformers, or by mounting considerations, though in some cases it may be allowed to
assume its optimum dimension in relation to the radial dimensions. The ratio of Length of first foil
(L1) and Length of nth foil (Ln) may be taken between 3 to 4. This ratio is denoted by a.
(E) RADIAL GRADIENTS AND DIAMETERS
The radial gradient is limited for avoiding damage by discharges at the power-frequency test
voltages, whether one minute or instantaneous. If the ratio of the earth layer diameter to that of the
conductor𝑟𝑛 𝑟𝑜⁄ , is denoted by 𝛽, the stresses at the HV end and the earth voltage end will be equal,
if the product of 𝛼and 𝛽is unity. However, it is not always possible to achieve this value. Hence
𝛼and 𝛽can vary from 0.8 to 1.2 if 𝛼, 𝛽=1, then Ln.Dn = L1.D0
(F) EQUIPOTENTIAL LAYER POSITION
After determining the dimensions of the inner and outer layers of the condenser, the position of the
other layers can be calculate. The basis of the design of the condenser bushing is generally equal
partial capacitances, which mean equal voltage on them and equal axial spacing between the ends
of layers.
INSULATING MATERIAL
Porcelain insulation:
A basic porcelain bushing is a hollow porcelain shape that fits through a hole in a wall or metal
case, allowing a conductor to pass through its center, and connect at both ends to the other
equipments. The inside of these bushings is often filled with oil to provide additional insulation and
used up to 36 kV
Paper insulation:
The insulating material of bushing windings is usually paper-based with the following most
common types:
(A) SYNTHETIC RESIN BONDED PAPER (SRBP)

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In SRBP bushings, one side of the paper is film coated with synthetic resin which is cylindrically
wound under heat and pressure inserting conducting layers at appropriate intervals. However, use
of SRBP bushings is limited to voltages around 72.5 kV
There is also the danger of thermal instability of insulation produced by the dielectric loss of the
resins. The SRBP insulation is essentially a laminate of resin and paper which is prone to cracking.
Moreover, paper itself will include air which will cause partial discharges even at low levels of
electrical stress.
(B) OIL IMPREGNATED PAPER (OIP)
OIP insulation is widely used in bushing and instrument transformers up to the highest service
voltages. In the manufacturing process, the Kraft paper tape or sheet is wound onto the conductor.
Aluminum layers are inserted in predetermined positions to build up a stress-controlling condenser
insulator. The condenser layer may be closer together, allowing higher radial stress to be used. The
bushing is fully assembled before being vacuum impregnated in order to contain the oil.
(C) RESIN IMPREGNATED PAPER (RIP)
RIP bushings are wound in a similar manner as OIP. The raw paper insulation is then kept in a
casting tool inside an auto-clave. A strictly controlled process of heat and vacuum is used to dry the
paper prior to impregnation with epoxy resin.
CONSTRUCTIONAL DETAILS AND MAIN PARTS OF BUSHING
CORE
The core of bushing consists of a hollow or solid metallic tube, over which high grade electrical
Kraft paper is wound. For condenser cores, conducting layers of metallic foil are introduced at
predetermined diameters to make uniform distribution of electrical stress. The winding of the
condenser core is done in a dust-free chamber. The core is then processed; this comprises of drying
in a high degree of vacuum (0.005mm), and then impregnating with high quality, filtered and de-
gassed transformer oil.
PORCELAIN

27. 27 | P a g e
Bushing for outdoor applications is fitted with hollow porcelain insulators. The OIP bushings are
provide with insulators, both at air and oil ends, thus forming an insulating envelope, and the
intervening space may be filled with an insulating liquid or another insulating medium. The
function of an insulator is to resist flash over in adverse conditions. This is determined by.
 The profile of the dielectric.
 The mounting arrangement of the insulator, i.e., vertical, horizontal, or inclined.
 The properties of the surface, i.e., hydrophobic nature, toughness etc.
TOP CAP
This is a metallic housing for the spring pack. It serves as an in-built oil conservator to cater for oil
expansion, and has an oil level indicator. In many cases, it also serves the purpose of a corona
shield.
MOUNTING FLANGE
This is used for mounting the bushing on an earth barrier, such as a transformer tank or a wall. It
may have the provisions for following:
 CT accommodation length
 Rating plate giving the rating and identification details of bushing.
 Test tap
 Oil drain plug for sampling of oil
 Air release plug
The design of the flange and top cap is such as to minimize the loss due to hysteresis and eddy
current effects. When heavy currents are being carried, this loss raises the temperature of the flange
and top cap to a noticeable extent. For heavy currents, ordinary cast iron material cannot be used,
hence non-magnetic materials such as stainless steel or aluminum are used.
TEST TAP
The test tap is provided for measurement of the power factor and capacitance of the bushing during
testing and service. The test tap is connected via a tapping lead to the last condenser foil of the core
within the bushing. During normal service, this tapping is electrically connected to the mounting
flange through a self-grounding arrangement.

28. 28 | P a g e
TESTING OF BUSHING
To prove the design and quality of manufacture, bushings are subjected to type tests and routing
tests as given in IS 2099 and IEC 137. Where the type tests are done to prove design features of
bushing, routine tests check the quality of individual bushing.
TAN DELTA AND CAPACITANCE MEASUREMENT
This test is probably the most universally applied test for all types of condenser bushings. The
bushing is set up as in service connected to one arm of the Schering Bridge. The voltage is applied
in increasing steps, up to the rated voltage. Capacitance and tan delta values are recorded for each
voltage. (For bushings, power factor and tan delta values may be regarded as identical). Tan delta
indicates the degree of processing of the condenser core in OIP bushings and also the moisture
content.
DRY POWER FREQUENCY VOLTAGE WITHSTAND TEST
This is the most common routine test used for various classes of electrical equipment. A specified
power frequency voltage is applied for one minute. Under wet conditions, the power frequency test
is a type test, and is done only for bushings having rating 300 kV and below.
PARTIAL DISCHARGE TEST
Partial discharges are the localized electrical discharges within the insulation system, restricted to
only one part of the dielectric material, thus only partially bridging the electrodes.
They are caused due to:
 Voids and cavities present in the solid and liquid dielectrics.
 Surface discharges that appear at the boundary of different dielectrics.
 Corona discharges, if strongly non-homogenous fields are present.
The continuous impact of discharges in solid dielectrics form discharge channels called TREEING.
Every discharge event deteriorates the material by the energy of impact of high electrode ions,
causing many types of chemical transformation.

29. 29 | P a g e
IMPULSE VOLTAGE WITHSTAND TEST
Lightning impulse is a type test, and is applicable for all types of bushings. The bushing is
subjected to 15 full wave impulses of positive polarity, followed by15 full wave impulses of
negative polarity of the standard waveform 1.2/50μs.
For bushings of rated voltage equal to or greater than 300 kV, the switching impulse voltage test is
applicable. The impulse tests simulate more closely than the power frequency withstand test, the
over-voltages likely to be seen by the bushing in the service.
THERMAL STABILITY TEST
The theory of thermal stability states that at an elevated operating temperature, stable thermal
equilibrium is assured only when a maximum value of the sustained voltage characteristic of a
particular bushing is not exceeded. The magnitude of this voltage serves as a representative
measure of the thermal stability. The dimension of the body has no role to play. It has been
considered unnecessary to specify thermal stability tests for OIP bushings owing to low dielectric
loss. However, it is necessary in large bushings (greater than 300 kV) with high current, to pay
attention in the design to the dissipation of the conductor losses which may be several times the
dielectric loss.
INSTALLATION
 Before installation the bushings shall be wiped clean of dust and dirt. The porcelain surface
shall be inspected for any scratches or cracks on it. Oil level in the bushing shall be noted
and all joints thoroughly inspected for any leakage.
 The insulated stress relieving shield shall be processed before fitting on the transformer
bushings. The shield be hot air dried to remove moisture in insulation and then vacuum oil
impregnated and shall be kept under oil in sealed containers till the time of installation. The
shield shall be fixed to the bushings with minimum exposure time.
 The draw-lead or draw terminal with transformer lead jointed at the level of bushing fixing
flange, shall be pulled up using a lifting eye through the center tube while the bushing is
inserting into the transformer tank.

30. 30 | P a g e
Capacitor
Capacitor is an important element of power system. It is used in power network to improve the
quality of power supply by improving the power factor voltage. There are many advantages of
installing capacitor. Some of these are:
 It is 50 times cheaper than generation of equal power.
 Improvement in power factor and reduction in penalty due to lagging power factor.
 Saving of power.
APPLICATION & KVAR RATING
Capacitor Bank is designed based on the application of capacitor. For Power Factor improvement,
Shunt capacitor is used which is connected in parallel with the line. Similarly for Series Capacitors
which is used for Voltage improvement of long transmission line, the capacitor is connected in
Series. Capacitance / kVAr rating is required for designing the capacitor Bank. Capacitor units
are connected in parallel to get the required bank Capacitance.
CLASSIFICATION OF CAPACITORS
BASED ON APPLICATION
1. Shunt Capacitors for Power Factor Improvement
2. Series Capacitors for Reactive Compensation and Voltage Regulation
3. Filter Capacitors for Harmonic Filtering
4. Capacitor Voltage Transformer (CVT) for PLCC, Metering & Protection
5. Surge Capacitors for protection of Generator and Transformer Winding

31. 31 | P a g e
BASED ON PROTECTION
1. Internal Fuse
2. External Fuse
BASED ON CONFIGURATION
1. Star connected
2. Double Star connected
3. Delta connected
4. Single Phase Bridge connected
BASED ON PROTECTION
CONNECTION DIAGRAM PROPERTIES
REDUCES lagging component of circuit current, I R and
I X power losses, demand kVAr where power is
purchased (HT services), investment in system facilities
per KW of load supplied, loading on source generators
and circuits to relieve and overloaded condition or release
capacity for additional load growth.
INCREASES voltage level at load, power factor of
source generators, improves voltage regulation.
Smooth running of motors.
REDUCES the voltage drop by compensating line
reactance and reduces sending end kVAr demand by
compensating I2XL component of the line
BETTER reactive power balance and frequency, and
effective with added voltage regulation when the pf is
low, also it is most economical solution with respect to
both the steady & transient stability,
Losses are dependent on line areas and current
distribution. The proper R/X ratio giving the minimum
transmission losses during the development stages of a
power system can be easily achieved by means of
adjusting series capacitors.
The capacitors are connected in combination of reactors
and resistors to form filter capacitor. It is used for
filtering harmonics present in the system. It is generally
used in HVDC application where large amount of
harmonics are generated by Thyristor.
Capacitor voltage transformer is used for PLCC and
metering and protection of the line. Capacitor divider is a
part of CVT. Coupling capacitor is used only for PLCC
application.

32. 32 | P a g e
INTERNAL FUSE EXTERNAL FUSE
In case of internal fuse capacitor, If One
element fails, internal fuse operates and Isolates
the faulty element and the bank continues to
operate at reduced kVAr rating (Approx. 98%).
In case of external fuse capacitor units, the fuse
operates even if one element develops fault. The
series section of that branch will get short circuited
& it will lead to complete failure (0 %).

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Internal fusing is inherently superior from
Reliability point of view as the internal fuse
protects many sub section (elements)
Here fuse protects the complete unit and hence it is
less reliable.
The service & maintenance can be scheduled in
advance after alarm indication.
Non schedule maintenance is required.
Bank is compact & occupies less space. Bank is not compact & occupies more space.
Design of capacitor unit with internal fuse can
be carried to practically any unit size. There is
no theoretical upper limit of unit size.
Unit size limited as the fuse for higher kVAr rating
not available beyond 100 Amps.
Most suitable for HVDC / Filter applications.
Not suitable for HVDC / Filter application as the
element failure will completely fail the unit de
tuning will have to be carried out
RATED SYSTEM VOLTAGE
The Design of Capacitor Bank depends on Rated System Voltage. The voltage of Capacitor units is
low due to design limits (less than 20 kV). These units are connected in series to get the required
capacitor bank voltage. For very high system voltage i.e. more than 220 kV, special arrangement
for Corona Protection is required.
TYPE OF CAPACITOR BANK PROTECTION
The design of Capacitor depends on the type of Bank protection. Generally following types of Bank
protections are used.
Star
connected
The capacitor banks are connected in single star configuration. Residual
voltage transformer (RVT) is connected for capacitor bank protection.
Double Star
connected
The capacitor banks are connected in split star / Double star configuration.
Neutral current transformer (NCT) is connected to neutrals of the 2 star. Any
unbalance in capacitance due to failure of capacitor unit is detected by NCT
and Capacitor bank is automatically switched OFF.
Delta
connected
This connection is used for protection of single phase Capacitor bank. This
configuration is generally used for Series Capacitor / Filter Capacitor.
COMMON TROUBLES, CAUSES & REMEDIES

34. 34 | P a g e
Symptoms Cause Remedy
Leakage of
oil
Minor transit damage, leak in
weld / terminal cap.
Apply Araldite or M seal or Mecatec
compound after thorough cleaning &
abrading by emery cloth. If leakage does not
stop replace the unit and refer to us.
Abnormal
Bulging
Capacitor oil almost leaked
out. Gas formation due to
internal arcing causing unit to
bulge or burst.
Disconnect unit and store in a covered area in
vertical condition and refer to us.
Cracking
Noise
Partial internal faults Disconnect the unit and refer to us.
PP Film Al. Foil
Element Winding
Pack Testing
DR Connection with Pack
Insulation Wrapping
Boxing
Element Testing
Internal Fuse Assy.
Box Fabrication
DR Assembly
Impregnation
Lid Welding
PXEPXE Processing
Steel
FLOW CHART FOR CAPACITOR MANUFACTURING

35. 35 | P a g e
ROUTINETEST
Sl.
No.
Test
Procedure
1. Measurement of
Capacitance.
Capacitance is measured at rated voltage.
2. Capacitor Loss tangent
(Tan δ) measurement
at rated voltage.
Capacitance is measured at rated voltage.

36. 36 | P a g e
3.
Voltage test between
terminals.
4.3 times the rated voltage in DC is applied between the
terminal for 10 sec.
4.
AC test between
terminals and
container.
An AC Voltage depending on the BIL level of the unit is
applied between shorted terminals and container for 10
Sec. This test is applicable only for units having all
terminals insulated.
5.
Test of discharge
device.
Discharge resistance shall be measured. The DR shall be
suitable to discharge to less than 50 volts from an initial
peak voltage of √2 times the rated voltage after
disconnecting from supply. The maximum discharge time
shall be 10 min
6.
Sealing test
A sealing test shall be carried out to demonstrate that the
impregnate does not leak from the capacitor. The test shall
be carried out at 80 ± 5 ° C for a period of 4 hours after
attaining the container temperature of 80 ± 5 °C.
TYPE TEST
Sl.
No.
Test
Procedure

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1.
Thermal Stability Test
a) Capacitor Units are placed inside the oven having 55 ±
2C.
b) An A.C. voltage equal to 1.2 Un is applied for 48
hours.
c) The value of tan delta measured during last 10 hours
shall not increase by 1 X 10-4 Capacitance is measured at
rated voltage.
2. Capacitor loss tangent
(Tan delta)
measurement at
elevated temperature
Capacitor Loss tangent is measured after 48 hours of
stabilization or at the end of Thermal Stability Test.
3.
AC test between
terminals and container
An AC Voltage depending on the BIL level of the unit is
applied between shorted terminals and container for 1
min. This test is applicable only for units having all
terminals insulated.
4. Lightning Impulse
voltage test between
shorted terminals and
container.
An Impulse Voltage depending on the BIL level of the
unit is applied between shorted terminals and container.
This test is applicable only for units having all terminals
insulated.
5.
Short Circuit Discharge
Test.
a) 2.5 times the rated voltage in DC is applied between
the terminals and then discharged through a gap situated
as close as possible to the capacitor. The unit shall be
subjected to give 5 such discharges within 10 min.
b) Within 5 min after this test , the unit shall be subjected
to 4.3 times the rated voltage between terminals for 10
sec.
ULTRA HIGH VOLTAGE (UHV)

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Voltages equal and above 750 kV are considered as ultra high voltages.
Developments in field of UHV by BHEL, Bhopal
 India’s 1st 1200KV,333 MVA Power Transformer is designed by BHEL, Bhopal for
National Test Station at Bina (MP).
 India’s 1st 765KV,500MVA transformer developed with indigenous technology for Power
grid Wardha(Maharashtra).
 BHEL has a set up of 1.2 million volt, 30 mA HVDC Generator(cascade).
(1) 1200 kV transformer
Rated Power – 180 MVA
Rated voltage – 1200/√3/138√3 kV
BIL : 2300 KVp/500 KVp
Total Height : 14 m
Total Weight :281 Ton
(2) 1200 Current Voltage Transformer(CVT)
Rated Voltage – 1150 kV
Rated Capacitor - 2000µf
BIL : 1200 KVp/2400KVp
No. of capacitor units : 6
Total Height : 11.2m
Total Weight : 2630 Kg
(3) 1200 KV ,333 MVA single phase Auto transformer
Rated Power : 333 MVA
Rated Voltage : 1150/√3/400/√3/33 kV
BIL : 2250 KVp/950 KVp/280 KVp
SI : 1800 KVp
PD level : <300 Pc at 1.5 um/√3
Total Height : 14.5m
Total Weight : 321 ton
UHV TEST FACILITY
 Has a capacity of testing equipments up to 1200KV

39. 39 | P a g e
 Has electromagnetic shielded hall of size 67×35×35 m & 50×27×30 m with double walled
GI construction with electrical continuity.
 Posses acoustic attenuation with a reverberation time of 3 to 3.5 seconds.
 Is provided with a low impedance grounding system.
Parts of UHV
 AC winding hall.
 Horizontal winding hall.
 Final assembly platform.
 Lifting beam & vapors phase drying plant.
 Core assembly platform.
 Vertical winding machine.
 Pressurized assembly bay.
 Core assembly platform.