BHEL, Bhopal Transformer, Bushing and Insulatoin

1. University Teaching Department,
Rajasthan Technical University,
Kota.
Presentation
on
Vocational Training
at
BHEL, Bhopal.
Submitted to: Submitted by:
Dr. Dinesh Birla Sir Divyansh Gupta
Ashok Kumar Sharma Sir Batch EE2 (13/029)

2. Bharat Heavy Electricals Limited (BHEL)
• BHEL owned by the Government of India, is a power plant equipment
manufacturer and operates as an engineering and manufacturing
company of India with the headquarters situated at New Delhi.
• It is India’s largest manufacturing and engineering company of its own
kind. It has been granted the prestigious MAHARATNA (big gem) status
by the Government of India for its spectacular performance.

3. Bharat Heavy Electricals Limited (BHEL)
• BHEL was established in 1964, Heavy Electricals (India) Limited was
merged with BHEL in 1974. In 1982, it entered into power equipment, to
reduce its dependence on the power sector. It developed the capability to
produce a variety of electrical, electronic and mechanical equipment for
all sectors, including transmission, transportation, oil and gas and other
allied industries.

4. BHEL, Bhopal
BHEL Bhopal is an integrated power plant equipment manufacturer and one of the
largest engineering companies in India in terms of turnover. It was established in 1964,
ushering in the indigenous Heavy Electrical Equipment industry in India. The company
has been earning profits continuously since 1971-72.
BHEL TOWN, Bhopal is a suburb of Bhopal, Madhya Pradesh. It is spread over an area of
around 20 km2 and provides facilities like, parks, community halls, library, shopping
centres, banks, post offices etc. Besides, free health services is extended to all the
employees through 350 bedded (inclusive of 50 floating beds) Kasturba Hospital and
chain of dispensaries.

5. Transformer
Traction
Motor OLTCSwitchgear
Induction
Motor
Alternator Turbine
BHEL, Bhopal

6. TRANSFORMER
Introduction

7. 1. The term transformer comes from the word ‘transform’ which has its
literal meaning as a change by transformation. A transformer is a static
device which transforms the voltage or passes the electrical energy from
one electrical circuit to the other electrical circuit through a common
magnetic circuit based on the principle of mutual induction without a
change in frequency.

8. 2. A varying current in the transformer's primary winding creates a
varying magnetic flux in the transformer core and a varying field impinging
on the transformer's secondary winding. This varying magnetic field at the
secondary winding induces a varying electromotive force (EMF) or voltage in
the secondary winding due to electromagnetic induction.

9. A transformer has two main parts:
CORE WINDING
 Core of the transformer is usually made up of thin laminations of CRGO (cold-
rolled grain oriented) silicon steel. Grain oriented steel is most desirable for the
magnetic cores because it is anisotropic providing better magnetic properties
than GNO(grain not oriented) in one direction.

10.  The concept behind using thin laminations is to reduce the amounts of
eddy currents flowing, as these laminations doesn’t provide a smooth
regular path to flow. Silicon steel is preferred because it has low hysteresis
and it is a “soft” magnetic material. It has high magnetic permeability too.
 EDDY CURRENTS: If the core is electrically conductive then the
magnetization induces circulating loops of currents in it. These are called
as Eddy currents, due to electromagnetic induction.

11. HYSTERESIS: Sometimes, it is highly undesirable if the core still retains
the magnetization after the excitation is removed. This particular
property is called as Hysteresis which can cause energy losses. When the
magnetic field of the core changes, the magnetization of the core changes
by expansion and contraction of the magnetic domains inside the
material. This change in the configuration causes losses termed as
Hysteresis losses. These losses increase with increase in frequency.

12. The winding of the transformer can be broadly divided into two:
Low voltage High voltage
In transformer design, LV winding will be wound close to the core of the
transformer because the insulation provided between the LV winding and
core of the transformer is quite less compared to the insulation provided
between the HV winding and the core of the transformer which results in
less cost for insulation and reduction in the size of the transformer for
same MVA rating. Hence due to this reason LV winding is placed near to
the core of the transformer.

13.  On the core of the transformer insulation is placed and LV winding is
wound. Once again insulation is placed between the LV winding and HV
winding.
 The windings are made of up of copper wires which are insulated from
each other so that the current travels throughout every turn. Insulation is
made up of oil impregnated paper and chunks (T-shape) of pressboards.
These T-shaped pressboards are inserted in order to create space between
two turns intended to flourish the cooling effects by oil circulation.

14. NOTE: use of plastic is also an option for the insulation but it is highly
undesirable as it has lower oil absorbing capacity as well as it is expensive
compared to pressboards.
The windings are clothed on the core of transformer in two different ways:
1. Horizontal (tap and low voltage)
2. Vertical (common and high voltage)
Different transformers have different types of windings. For instance, auto-
transformers (used at sub-stations) has four types of windings.
1. Common winding
2. Low Voltage (tertiary winding)
3. High Voltage (series winding)

15. 4. Tap winding
Power transformer (used at generating stations) has usually three types of
windings.
1. High voltage
2. Low voltage
3. Tap winding

16. Methods of winding
Disc type Layer type
The number of concentric circles is the governing principle of the method of
winding to be used. If we have only one concentric circle throughout the winding
then it is termed as Layer type whereas if the number of concentric circles are
more than one then it is termed as Disc type.
For better understanding, let’s take a fixed size transformer. By the convention, the
low voltage winding is placed first in layer type throughout the core of length l.

17. Now, if we intend to place the high voltage winding on it, then the voltage
needs to be stepped up. Here’s the reason why the HV winding is placed in disc
type and the LV winding in layer type so that we can have the desired results
in the givens size. Otherwise the transformer size would have increased
resulting into rise in cost.

18. The types of methods are further classified into:
• Disc type
o Interleave
o Partial interleave
o Counter shield
• Layer type
o Helical
o Multi-stack
o Spiral

19. BUSHINGS
Introduction

20. In electric engineering, a bushing is an insulated device that allows an
electrical conductor to pass safely through a grounded conducting barrier
such as the case of a transformer.
 The main functions of the bushings are as follows:
1. It helps for the external safety of the device.
2. It protects the transformer from lightning surges by bypassing the
current to the ground.
3. It saves the device from unexpected high voltages.
WHYBUSHINGS??

21. As we know that transformer has two windings inside the core. And we need
a further connection from these windings outside for further use, so here
bushings come into play.
The primary reason behind using bushings is the cooling phenomenon. Usage
of other things results into exposition of the core to the air(breakdown
strength of 30kV/cm) which is highly undesirable.
CONSTRUCTION
The bushing is made up of well-polished porcelain material, usually found
in grey or maroon colour as per the ISO standards.

22. The top cap plays the role of supporting the line
cable as well as it indicates the oil level.
The wavy shape is to maximize surface path
length and minimize surface leakage, corona,
and eventual arcing from exposure to year-
round weather conditions, dust, air pollution
etc.
They are a bit inclined so that in rainy season
the water particles doesn’t accumulate over the
shed.

23. Peeping into, the bushing consists of a long
copper rod which is wrapped with alternate
layers of Al(aluminium) and paper(OIP) to carry
the current.
A bushing consists of all three i.e., a capacitor
(between paper and Al), conductor (copper) and
an insulator (porcelain).
The bushings are spaced so that they don’t
develop charge within themselves and get short
circuited because if it happens so, the
transformer will face huge damage. So all the
three bushings i.e. R, Y & B (U, V & W) are
placed at some angles from each other.

24. The bushing is filled with oil for the cooling purpose as well as more
heat is dissipated in oil. This oil circulates between the porcelain body
and the copper rod clothed with Al and paper.
The end of the bushing consists of a steel ring/plate to seal pack the
end .
The voltage is not allowed to fall drastically instead it is lowered step
by step so that till it reaches test tap, its value is approximately
2kV/cm. This is the reason why we use paper and Al alternatively
forming capacitance foils.
The bottom end of the bushing is inserted in the transformer for
connecting it to the windings with the help of tarade(mounting
flange).

25. ON LOAD TAP
CHANGER(OLTC)
Introduction

26. A tap changer is a connection point selection mechanism along a
power transformer winding that allows a variable number of turns to be
selected in discrete steps.
The voltage is changed by changing the voltage ratio in steps. The
transformer is equipped with tap windings whose tapping are connected
with the tap changer of OLTC.
It is always attached to the HV side of the transformer because of obvious
reasons.

27. In larger electrical power transformer(generating stations), for
uninterrupted voltage regulation, on load tap changer is required. If the
demand rises unexpectedly, then the transformer should be able to adapt
the changes. In order to do so the generating transformers are equipped
with OLTC which allows a variable number of turns to be selected in
discrete steps.
To reduce arcing at the contacts, the tap changer operates in a chamber
filled with insulating transformer oil, or inside a vessel filled with
pressurised SF6 gas.
An OLTC has four main parts. They are discussed below.

28. Diverter Switch Motor Drive Selector Switch
Oil
Compartment
This switch
diverts the
resistors with
the help of a
vertical shaft
coupled with the
horizontal shaft
of the motor.
The motor is
horizontally
coupled with a
shaft which is
operated with
the help of
control panel
outside.
This switch
helps to select
the proper
resistances
according to the
demand rise/fall
as instructed by
the operator.
The diverter
switch is
situated in this
oil compartment
to avoid heating
in OLTC as well
as to get rid of
arcing due to
change in
resistances.

30. INSULATION
Introduction

31. Insulation is one of the prominent task to do while the production of
transformer. Proper and healthy insulation will ascertain long life of the
transformer as well as it will increase the efficiency of the system. The various
types of insulation used in the transformer are as follows:
1. Transformer oil
2. Pressboard
3. Insulating paper

32. TRANSFORMER OIL
A. The oil used for the insulation purposes in the transformer is called as
Transformer oil. It is normally obtained by the fractional distillation of the
crude petroleum.
B. Transformer oil has two types:
1. Paraffin based
2. Naphthalene based
C. The salient features of the transformer oil are as follows:

33. It helps to preserve the core and the winding immersed in the oil.
It prevents the direct contact of the oxygen with the cellulose present in the
paper used for insulation which would otherwise result in oxidation
It provides the liquid insulation inside the transformer.
It dissipates the heat easily i.e. it acts as a very good coolant.

34. VALIDATION TESTS
Before using the oil, it is thoroughly checked and tested as per the standards. The
various types of tests performed for validation are as follows.
1. Dielectric strength
 The breakdown voltage of oil is measured by observing at what value the
sparking takes place between the electrodes immerged in oil and separated
by a distance.

35. 2. DIELECTRIC DISSIPATION FACTOR (tan§) OF TRANSFORMER
 This test is done to determine whether the dielectric medium used has high
value of resistance or not.
 When an insulating material is placed between live part and grounded part,
current flows through it. This current is ideally leading the voltage by 90°
because of the dielectric medium.
 But in practical, as there is no perfect dielectric material present, so it leads
the voltage by an angle < 90°.
 Tangent of this angle is called as Dielectric Dissipation Factor of TX.

36. REPRESENTATION OF tan§ WITH ANGLE SOMEWHAT LESS

37. INSULATION PAPER AND PRESSBOARD
• Paper is a fabric made from vegetables fibres.
• It attains a very high value of electric strength when emerged in oil under
vacuum.
• Pressboard is a widely used material in the making of various components
in electrical, mechanical and thermal designs of transformer.
• It contains much cellulose which are there because of the vegetable fibres.
• Pressboard of approximately 6 – 8 mm thick is made and used originally.
• The most difficult insulation problem in HT transformer occur at the ends
of the windings and lead outs from the windings hence moulded
pressboards are widely used in these parts for insulation.

38. WINDING TEMPERATURE INDICATOR (WTI)
In order to detect the temperature of the transformer windings, now-a-days
optical fibre probes (OFP) are in fashion. These probes are made up of
polytetrafluoroethylene (PTFE) Teflon cables. They have a working range of
temperature from -80°C to 250°C. Prima facie, they are properly tested in the
testing machine with different parameters in range. If tested OK, then they are
used to detect temperature in working conditions.
Similarly, we also have a privilege of sensing the temperature of the cooling oil
used in transformer with the help of oil temperature indicator (OTI).

39. TRANSFORMER PROCESS MAP
INSULATION WINDING PRE-COIL ASSEMBLY
CRGO LAMINATIONS CORE COMBINING
SERVICING AND VAPOUR PHASE DRYING POWER ASSEMBLY
JOINT INSPECTION
PROCESS 2
U.H.V TESTING MECHANICAL TEST DISMANTLING & PACKING

40. RATINGS OF TRANSFORMERS
1. 315 MVA 3-ø 400/220/33 KV Auto TX.
2. 200 MVA 1-ø 21/420√3 KV Generator TX.
3. 315 MVA 1-ø 400/√3 / 213/√3 / 213 KV HVDC Converter TX.
4. 254 MVAR 1-ø 500 KV HVDC Smoothing Reactor.
5. 315 MVA 3-ø 16.5/235 KV Generator TX.
6. 80 MVAR 3-ø 420 KV Shunt Reactor.
7. 800 KV Capacitive Voltage TX.
All systems equipped up to 420 KV class.

41. THANK YOU !!