1. BHARAT HEAVY ELECTRICALS LIMITED
VOCATIONAL TRAINING REPORT
7th
- 26th
May, 2018
Under the Guidance of:
Mr. Rahul Singh
Dy. Manager
Technical Service Department, BHEL Bhopal
Submitted by:
Simarjot Singh Kalsi
6th
Semester, BE- Electronics and Instrumentation
IET DAVV, Indore
Enrollment no: DE15244, Token no VT/2018/86
2. BHARAT HEAVY ELECTRICALS LIMITED
CERTIFICATE
This is to certify that Mr. Simarjot Singh Kalsi, student of 6th semester B.E.
Electronics and Instrumentation, Institute of Engineering and Technology has
successfully completed his Vocational Training at BHEL, Bhopal for 3 weeks
from 07.05.2017 to 26.05.2017. He has completed the whole training as per the
training report submitted by him.
Under the Guidance of:-
RAHUL SINGH
Dy. Manager,
Technical Service Dept.(T.S.D.)
BHEL Bhopal
3. ACKNOWLEDGEMENT
I would sincerely like to express my gratitude towards BHEL Bhopal, for
providing me the opportunity of pursuing my vocational training in this
renowned industry and endowing me with this unparallel experience and deep
understanding of a wide array of processes and manufacturing methods taking
place in different workshops of the industry.
I would also like to thank my Training Guide Mr. Rahul Singh whose guidance
and motivation went a long way in my understanding of different sections of the
industry. Furthermore, I would like to thank my institute, IET DAVV for giving
me opportunity of visiting the industry and increasing my practical
knowledgebase.
Simarjot Singh Kalsi,
6th semester, B.E. E&I
IET DAVV Indore
4. DECLARATION
I, Simarjot Singh Kalsi, student of 6th semester of Bachelor of Engineering,
Department of Electronics and Instrumentation, Institute of Engineering and
Technology, Devi Ahilya Vishwa Vidyalay Indore, hereby certify that this
Report of Vocational Training carried out at BHEL Bhopal is an original work
of mine under the guidance of the experienced mentor Mr. Rahul Singh. It is
not based on or reproduced from any existing work of some other person,
undertaken at any other time or for any other purpose, and has not been
submitted anywhere else at any time. It is based upon my individual observation
and work experience.
6. 1
BHEL - AN OVERVIEW
Bharat Heavy Electricals Limited (BHEL) owned and founded by the Government of
India, is an engineering and manufacturing company based in New Delhi, India. Established
in 1964, BHEL is India's largest power generation equipment manufacturer. The company
has been earning profit continuously since 1971-72 and paying dividends uninterruptedly
since 1976-77. It has been granted the prestigious Maharatna (big gem) status in 2013 by
Government of India for its outstanding performance. The elite list of Maharatna contains
another 6 behemoth PSU companies of India. BHEL was established in 1964 when 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 equipments for all sectors,
including transmission, transportation, oil and gas and other allied industries. In 1991, it was
converted into a public limited company. By the end of 1996, the company had handed over
100 Electric Locomotives to Indian Railway and installed 250 Hydro-sets across India.
ITS OPERATION:- BHEL is engaged in the design, engineering, manufacturing,
construction, testing, commissioning and servicing of a wide range of products, systems and
services for the core sectors of the economy, viz. power, transmission, industry,
transportation, renewable energy, oil & gas and defense. It has a 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 allowing it to execute more than 150
projects at sites across India and abroad. The company has established the capability to
deliver 20,000 MW p.a. of power equipment to address the growing demand for power
generation equipment. BHEL has retained its market leadership position during 2015-16 with
74% market share in the Power Sector. An improved focus on project execution enabled
BHEL record its highest ever commissioning/synchronization of 15059 MW of power plants
in domestic and international markets in 2015-16, marking a 59% increase over 2014-15.
With the all-time high commissioning of 15000 MW in a single year 2015-16, BHEL has
exceeded 170 GW installed base of power generating equipments. It also has been exporting
its 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
7. 2
cumulative overseas installed capacity of BHEL manufactured power plants exceeds 9,000
MW across 21 countries 6 including Malaysia, Oman, Iraq, UAE, Bhutan, Egypt and New
Zealand. Their physical exports range from turnkey projects to after sales services.
ESTABLISHMENT AND DEVELOPMENT STAGES:
Established in 1960s under the Indo-Soviet Agreements of 1959 and 1960 in the area
of Scientific, Technical and Industrial Cooperation.
DRR – prepared in 1963-64, construction started from October '63
Initial production started from January, 1967.
Major construction / commissioning completed by 1971-72 as per original DPR
scope.
Stamping Unit added later during 1968 to 1972.
Annual Manufacturing capacity for Thermal sets was expanded from 1500 MW
to3500 MW under LSTG. Project during 1979-85 (Sets up to 500 MW, extensible to
1000/1300 MW unit sizes with marginal addition in facilities with the collaboration of
M/s KWU-Siemens, Germany.
Motor manufacturing technology updated with Siemens collaboration during1984-87.
Facilities being modernized continually through Replacements / Reconditioning-
Retrofitting, Technological / operational balancing.
VISION:
World-class, innovative, competitive and profitable engineering enterprise providing total
business solutions.
MISSION:
The leading Indian engineering enterprise providing quality products systems and services in
the fields of energy, transportation, infrastructure and other potential areas.
VALUES:
Meeting commitments made to external and internal customers.
Foster learning creativity and speed of response.
Respect for dignity and potential of individuals.
Loyalty and pride in the company.
Team playing
Zeal to excel.
Integrity and fairness in all matters.
8. 3
VARIOUS BHEL PRODUCTS
Thermal power Plants Nuclear power Plants
Gas based power Plants Hydro power Plants
DG power Plants Boilers (steam generator)
Boiler Auxiliaries Gas generator
Hydro generator Steam turbine
Gas turbine Hydro turbine
Transformer Switchgear
Boiler drum Piping System
Soot Blowers Seamless Steel Tubes
Condenser s and Heat exchangers Pumps
Desalination and Water treatment plants Automation and Control systems
Power electronics Transmission system control
Solar photo voltaic Software system solutions
Capacitors Bushings
Electrical machines DC, AC heavy duty Motors
Compressors Control gears
Traction motors Control panels
9. 4
TRANSFORMERS
A transformer is a static electrical device that transfers electrical energy between two or
more circuits through electromagnetic induction. A varying current in one coil of the
transformer produces a varying magnetic field, which in turn induces a varying electromotive
force (emf) or "voltage" in a second coil. Power can be transferred between the two coils,
without a metallic connection between the two circuits. Faraday's law of induction discovered
in 1831 described this effect. Transformers are used to increase or decrease the alternating
voltages in electric power applications.. In its simplest form transformer consists of two
conducting coils having mutual inductance. In an ideal case it is assumed that all the flux
linked with the primary winding is also linked with the secondary winding. But, in practice it
is impossible to realize this condition as magnetic field cannot be confined. The greater
portion of flux flows in the core while a small portion called leakage flux links one or the
other winding. Depending on the particular application and type of connection, a transformer
may have additional windings apart from the two conventional windings.
The working principle of transformer is defined by the Faraday's law of electromagnetic
induction. Mutual induction between two or more winding is responsible for transformation
action in an electrical transformer.
According to Faraday's laws, "Rate of change of flux linkage with respect to time is directly
proportional to the induced EMF in a conductor or coil".
E=dΦ/dt
Lenz's law is a consequence of conservation of energy applied to electromagnetic induction.
It was formulated by Heinrich Lenz in 1833. While Faraday's law tells us the magnitude of
the EMF produced, Lenz's law tells us the direction that current will flow. It states that the
direction is always such that it will oppose the change in flux which produced it. This means
that any magnetic field produced by an induced current will be in the opposite direction to the
change in the original field.
Lenz's law is typically incorporated into Faraday's law with a minus sign, the inclusion of
which allows the same coordinate system to be used for both the flux and EMF. The result is
sometimes called the Faraday-Lenz law,
E= - dΦ/dt
If have one winding which is supplied by an alternating electrical source. The alternating
current through the winding produces a continually changing flux or alternating flux that
surrounds the winding. If any other winding is brought nearer to the previous one, some
portion of this flux will link with the second. As this flux is continually changing in its
amplitude and direction, there must be a change in flux linkage in the second winding or coil.
According to Faraday's law of electromagnetic induction, there must be an EMF induced in
the second. If the circuit of the later winding is closed, there must be a current flowing
10. 5
through it. This is the simplest form of an electrical power transformer, and this is the most
basic of working principle of transformer.
Types of Transformers:
Transformers can be categorized in different ways, depending upon their purpose, use,
construction etc. The types of transformer are as follows,
Step Up Transformer and Step Down Transformer - Generally used for stepping
up and down the voltage level of power in transmission and distribution power system
network.
Three Phase Transformer and Single Phase Transformer - Former is generally
used in three phase power system as it is cost effective than latter. But when size
matters, it is preferable to use a bank of three single phase transformer as it is easier to
transport than one single three phase transformer unit.
Electrical Power Transformer, Distribution Transformer and Instrument
Transformer - Power transformers are generally used in transmission network for
stepping up or down the voltage level. It operates mainly during high or peak loads
and has maximum efficiency at or near full load. Distribution transformer steps down
the voltage for distribution purpose to domestic or commercial users. It has good
voltage regulation and operates 24 hrs a day with maximum efficiency at 50% of full
load.
Two Winding Transformer and Auto Transformer - Former is generally used
where ratio between high voltage and low voltage is greater than 2. It is cost effective
to use latter where the ratio between high voltage and low voltage is less than 2.
Outdoor Transformer and Indoor Transformer - Transformers that are designed
for installing at outdoor are outdoor transformers and transformers designed for
installing at indoor are indoor transformers.
Oil Cooled and Dry Type Transformer - In oil cooled transformer the cooling
medium is transformer oil whereas the dry type transformer is air cooled.
Core type, Shell type and Berry type transformer - In core type transformer it has
two vertical legs or limbs with two horizontal sections named yoke. Core is
rectangular in shape with a common magnetic circuit. Cylindrical coils (HV and
LV) are placed on both the limbs.
Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils
are placed on the central limb. Double magnetic circuit is present.
Berry type transformer: The core looks like spokes of wheels. Tightly fitted metal
sheet tanks are used for housing this type of transformer with transformer oil filled
inside.
11. 6
Components of a Transformer
Core : The core acts as support to the winding in the transformer. It also provides a low
reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to
reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends
on such as factors voltage, current, and frequency. The diameter of the transformer core is
directly proportional to copper loss and is inversely proportional to iron loss. If the diameter
of the core is decreased, the weight of the steel in the core is reduced, which leads to less core
loss of the transformer and the copper loss increase. When the diameter of the core is
increased, the vice versa occurs
Winding: Two sets of winding are made over the transformer core and are insulated from
each other. Winding consists of several turns of copper conductors bundled together, and
connected in series.
Within the input/output supply classification, winding are further categorized:
1. Primary winding - These are the winding to which the input voltage is applied.
2. Secondary winding - These are the winding to which the output voltage is applied.
Within the voltage range classification, winding are further categorized:
1. High voltage winding - It is made of copper conductor. The number of turns made shall
be the multiple of the number of turns in the low voltage winding. The conductor used
will be thinner than that of the low voltage winding.
2. Low voltage winding - It consists of fewer number of turns than the high voltage
winding. It is made of thick copper conductors. This is because the current in the low
voltage winding is higher than that of high voltage winding.
Input supply to the transformers can be applied from either low voltage (LV) or high voltage
(HV) winding based on the requirement.
Insulating Materials: Insulating paper and cardboard are used in transformers to isolate
primary and secondary winding from each other and from the transformer core. Transformer
oil is another insulating material. Transformer oil performs two important functions: in
addition to insulating function, it can also cool the core and coil assembly. The transformer's
core and winding must be completely immersed in the oil. Normally, hydrocarbon mineral
oils are used as transformer oil. Oil contamination is a serious problem because
contamination robs the oil of its dielectric properties and renders it useless as an insulating
medium.
Conservator: The conservator conserves the transformer oil. It is an airtight, metallic,
cylindrical drum that is fitted above the transformer. The conservator tank is vented to the
atmosphere at the top, and the normal oil level is approximately in the middle of the
12. 7
conservator to allow the oil to expand and contract as the temperature varies. The conservator
is connected to the main tank inside the transformer, which is completely filled with
transformer oil through a pipeline.
Breather: The breather controls the moisture level in the transformer. Moisture can arise
when temperature variations cause expansion and contraction of the insulating oil, which then
causes the pressure to change inside the conservator. Pressure changes are balanced by a flow
of atmospheric air in and out of the conservator, which is how moisture can enter the system.
If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to
internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.
The transformer's breather is a cylindrical container that is filled with silica gel. When the
atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by
the silica crystals. The breather acts like an air filter for the transformer and controls the
moisture level inside a transformer. It is connected to the end of breather pipe.
Tap Changer: The output voltage of transformers vary according to its input voltage and the
load. During loaded conditions, the voltage on the output terminal decreases, whereas during
off-load conditions the output voltage increases. In order to balance the voltage variations,
tap changers are used. Tap changers can be either on-load tap changers or off-load tap
changers. In an on-load tap changer, the tapping can be changed without isolating the
transformer from the supply. In an off-load tap changer, it is done after disconnecting the
transformer. Automatic tap changers are also available.
Cooling Tubes: Cooling tubes are used to cool the transformer oil. The transformer oil is
circulated through the cooling tubes. The circulation of the oil may either be natural or
forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises
to the top and the cold oil sinks downward. Thus the oil naturally circulates through the tubes.
In forced circulation, an external pump is used to circulate the oil.
Buchholz Relay: The Buchholz Relay is a protective device container housed over the
connecting pipe from the main tank to the conservator tank. It is used to sense the faults
occurring inside the transformer. It is a simple relay that is operated by the gases emitted
during the decomposition of transformer oil during internal faults. It helps in sensing and
protecting the transformer from internal faults.
Explosion Vent: The explosion vent is used to expel boiling oil in the transformer during
heavy internal faults in order to avoid the explosion of the transformer. During heavy faults,
the oil rushes out of the vent. The level of the explosion vent is normally maintained above
the level of the conservatory tank.
Bushing: It is an insulating structure, including a through conductor or providing a central
passage for such a conductor, with provision for mounting a barrier, conducting or otherwise,
for the purpose of insulating the conductor from the barrier and conducting current from one
13. 8
side of the barrier to the other. 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.
POWER TRANSFORMER
1. The tub
2. The lid
3. The expansion vessel or conservator
4. The oil level gauge
5. The Buchholz relay
6. The spider or piping to the Buchholz
7. The load switch
8. The motor drive of the tap changer
9. Drive shaft for tap changer
10.High voltage (HV) bushing connects the internal HV coil with the external HV grid
11.High voltage bushing current transformers for measurement and protection
12.Low voltage (LV) bushing connects LV coil to LV grid
13.Low voltage current transformers .
14.Bushing voltage-transformer for metering the current through the passing bushing
15.Core
16.Yoke of the core
17.Limbs connect the yokes and hold them up.
18.Coils
19.Internal wiring between coils and tap changer.
20.Oil release valve
21.Vacuum valve
14. 9
TRANSFORMER MANUFACTURING AT BHEL
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 includes
Shunt Reactor up to 150 MVAR in 400 kV class and 330 MVAR in 765 kV class.
The manufacturing process of the transformer starts with the manufacturing of core. The core
acts as the supporting structure of the transformer, that is why it is manufactured in the first
place.
CORE MANUFACTURING:
The core of the transformer is manufactured using a special type of steel called C.R.G.O.
steel. The grains(crystals) of CRGO(cold rolled grain oriented steel) are aligned in the
direction of rolling. When the magnetic flux passes through the it, the magnetic domains
(region in grains with aligned magnetic moment) get aligned in one direction causing
minimum resistant path to flux causing lesser hysteresis loss, furthermore, high silicon
content leads to high resistance decreasing eddy currents, so overall Core losses get reduced.
WINDING MANUFACTURING:
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
15. 10
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.
Paper insulated copper conductor(PICC) is used in windings of transformers. 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. The paper is
lapped in several layers to obtain the necessary total thickness set by the electrical and
mechanical stresses.
CORE AND COIL ASSEMBLY:
A part of the transformer manufacturing process, the core and coil assembly aspect plays a
significant role where the core assembly is vertically placed where the foot plate touches the
ground and the top yoke is removed. The limbs of the core are tightly wrapped with cotton
tape and then varnished during the manufacturing and even repairing process.
1. First, the individual windings are assembled one over the other to form the entire
phase assembly.
2. The radial gaps between the windings are subdivided by means of solid transformer
board barriers.
3. Stress rings and angle rings are placed on top and bottom of the windings to achieve
a contoured end insulation design for optimal control of the oil gaps and creepage
stresses.
4. The complete phase assemblies are then carefully lowered over the separate core
legs and solidly packed towards the core to assure optimal short circuit capability.
5. The top core yoke is then repacked and the complete core and coil assembly is
clamped.
6. The lead exits (if applicable) and the lead supports and beams are installed. All
winding connections and tap lead connections to the tap changes are made before
drying the complete core and coil assembly in the vapor phase oven.
TANK FABRICATION AND FITTINGS:
The tanks are made of high quality steel and can withstand vacuum and pressure test as
specified in IS as well as by the customers. All welds are checked ensuring 100 % leak proof
seems and mechanical strength. All tanks are pressure tested before tanking the active part.
The Pressed steel radiators are used to dissipate heat generated at rated load. The fin height
and length are calculated according to the rating of transformers as well as customers'
specifications. The fins can be plain or embossed. The radiators are fitted variably according
to the rating of transformer. For smaller rating radiators are directed welded to the main tank
while for higher rating detachable type radiators are provided with valves to facilitate during
transportation and handling at site. The tanks are fabricated from MS plates and are welded
16. 11
construction. They are tested at a pressure of 0.35 Kg./Sq. cm. for oil leakage output and they
are normally welded directly to the tank. However, transformers can be supplied with
detachable radiators.
TESTS ON TRANSFORMER:
The following tests are generally performed on the transformer:
Routine tests:
Measurement of winding resistance.
Measurement of voltage ratio, polarity and check of voltage vector relationship.
Measurement of no-load loss and excitation current.
Measurement of short-circuit impedance and load loss.
Measurement of insulation resistance.
Switching impulse voltage withstand test.
Lightning impulse voltage withstand test.
Separate-source voltage withstand test.
Induced ac over voltage withstand test with partial discharge measurement.
Magnetic circuit (isolation) test.
Type tests
Temperature rise test.
Measurement of power taken by water pumps.
Dissolved gas analysis (DGA) of oil filled in the transformer.
17. 12
BUSHING MANUFACTURING
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.
(d) Acts as an external safety device.
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: 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.
18. 13
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 are 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.
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): 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.
19. 14
(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.
Typical Bushing Assembly
20. 15
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: Bushings for outdoor applications are 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.
21. 16
CALIBRATION
Every measuring instrument is subject to ageing as a result of mechanical, chemical or
thermal stress and thus delivers measured values that change over time. This cannot be
prevented, but it can be detected in good time by calibration. In the process of calibration, the
displayed value of the measuring instrument is compared with the measuring result of a
different measuring device which is known to function correctly and accurately and which
itself has been made to coincide directly or indirectly with a national (or international)
reference instrument (standard). Usually the accuracy of the standard equipment is ten times
more than the measuring instrument which has to be calibrated.
The formal definition of calibration by the International Bureau of Weights and
Measures (BIPM) is the following: "Operation that, under specified conditions, in a first step,
establishes a relation between the quantity values with measurement uncertainties provided
by measurement standards and corresponding indications with associated measurement
uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses
this information to establish a relation for obtaining a measurement result from an
indication".
Measurement of dimensions can’t be perfect and reliable unless and until measuring
instruments are calibrated accurately. Thus, calibration plays a vital role in maintaining
quality control. Calibration of measuring instruments is not only an advantage to any
company but it is a necessity for every manufacturing industry.
The advantages of calibration are accuracy in performing manufacturing operations, reduced
inspection, and ensured quality products by reducing errors in measurement.
TRACEABILITY AND CALIBRATION HIERARCHY:
To be able to compare measuring results, they must be “traceable” to a national or
international standard via a chain of comparative measurements. To this end, the displayed
values of the measuring instrument used or a measurement standard are compared over one or
several stages to this standard. At each of these stages, calibration with a standard previously
calibrated with a higher-ranking standard is carried out. In accordance with the ranking order
of the standards – from the working standard or factory standard and the reference standard to
the national standard – the calibration bodies have a calibration hierarchy. This ranges from
the in-house calibration laboratory to the accredited calibration laboratory and to the national
metrological institute.
22. 17
Traceability And Calibration Hierarchy
IMPORTANT TERMS IN CALIBRATION:
1.Precision (i.e., repeatability):
Instrumental precision is often defined as the spread of values obtained with repeated
measurements on a given specimen. It is generally assumed that the number of repeated
measurements is large, that the spread of values obtained is due to random causes, and that
randomness results in a Gaussian or “normal’ distribution of measurement data about a mean
value. If these assumptions are true, a multiple of the root-mean-square of the measured
deviations about this mean can be taken as a measure of the instrumental precision
appropriate for that given specimen and for the conditions under which it was measured.
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Basically, precision is a measure of repeatability of a measurement with some things held
constant and, perhaps, other things inadvertently or intentionally allowed to vary.
2. Accuracy (i.e., correctness of mean value):
Accuracy is defined as the correctness of a measurement or of the mean of repeated
measurements. Unfortunately, there are usually many potential sources of nonrandom
systematic errors that affect the mean of repeated measurements. Since these systematic
errors remain constant from measurement to measurement, they cannot be reduced by
averaging the results of repeated measurements. Therefore, systematic errors can lead to
significantly incorrect measurement results regardless of the precision of the instrument used.
Therefore, good precision is a necessary condition for good accuracy, but not a sufficient
condition, The concept of correctness assumes that there is some agreed upon standard which
can be used to determine the correctness of a measurement. The desired accuracy may be
achieved only if the instrument being calibrated is sufficiently precise, if the standard of
comparison is calibrated with sufficient accuracy, and if the specimens of interest exactly
match the standard of comparison in all-important ways. One method of using standards is to
prepare a calibration curve using a set of standards with a range that includes the desired
range of that parameter of interest. Note, however, that it is not good practice to extrapolate
this curve outside the range of the standards used in the calibration. Assuming that the
standard itself has been prepared with sufficient accuracy, calibration is essentially a
measurement of the systematic error of the instrument being calibrated. This calibration can
never be more accurate than the standard used and, in general, the calibration will be inferior
to the standard because of the inevitable imprecision of the measurements made during the
calibration procedure. Another way of looking at this is to consider the instrument in question
to be a comparator that compares the unknown to a standard. Therefore, it requires a high
quality comparison standard and a high precision instrument (comparator) to give a high
quality result.
Accuracy of an instrument can be determined using following ways:
1. Percentage of reading.
2. Percentage of free scale range/deflection.
3. Percentage of digits.
4. Percentage of Units.
5. Combination of all four predefined ways.
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3.Reproducibility (Of Results Of Measurements):
Closeness of the agreement between the results of measurements of the same measurand
carried out under changed conditions of measurement.
1. A valid statement of reproducibility requires specification of the conditions changed.
2. The changed conditions may include: - principle of measurement, - method of
measurement, - observer, - measuring instrument, - reference standard, - location, -
conditions of use, - time.
3. Reproducibility may be expressed quantitatively in terms of the dispersion characteristics
of the results.
4. Results are here usually understood to be corrected results
4.Uncertainty Of Measurement:
In metrology, measurement uncertainty is a non-negative parameter characterizing
the dispersion of the values attributed to a measured quantity. All measurements are subject
to uncertainty and a measurement result is complete only when it is accompanied by a
statement of the associated uncertainty. By international agreement, this uncertainty has a
probabilistic basis and reflects incomplete knowledge of the quantity value.
The uncertainty of measurements can come from various sources; such as the reference
measurement device used for making the measurement, from environmental conditions, from
the operator making the measurements, from the procedure and from many others sources.
Shortly and simply we can say that is the “doubt” of the measurement, it tells us how good
the measurement is. Every measurement we make has some “doubt”, and we should know
how much this “doubt” is, in order to decide if the measurement is good enough for the
usage.
It is good to remember that error is not the same as uncertainty. When we compare our
device to be calibrated, against the reference standard, the error is the difference between
these two readings. But the error does not have any meaning unless we know the
uncertainty of the measurement.
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NATIONAL PHYSICAL LABORATORY OF INDIA:
The National Physical Laboratory of India, situated in New Delhi, is the measurement
standards laboratory of India. It maintains standards of SI units in India and calibrates the
national standards of weights and measures.
The standards maintained at NPL are periodically compared with standards maintained at
other National Metrological Institutes in the world as well as the BIPM in Paris. This exercise
ensures that Indian national standards are equivalent to those of the rest of the world.
Any measurement made in a country should be directly or indirectly linked to the national
standards of the country, For this purpose, a chain of laboratories are set up in
different states of the country. The weights and measures used in daily life are tested in the
laboratories and certified. It is the responsibility of the NPL to calibrate the measurement
standards in these laboratories at different levels. In this manner, the measurements made in
any part of the country are linked to the national standards and through them to the
international standards.
The weights and balances used in local markets and other areas are expected to be certified
by the Department of Weights and Measures of the local government. Working standards of
these local departments should, in turn, be calibrated against the state level standards or any
other laboratory which is entitled to do so. The state level laboratories are required to get
their standards calibrated from the NPL at the national level which is equivalent to the
international standards.
NATIONAL ACCREDITATION BOARD FOR TESTING & CALIBRATION
LABORATORIES (NABL):
National Accreditation Board for Testing & Calibration Laboratories (NABL) is an
autonomous society providing Accreditation (Recognition) of Technical competence of a
testing, calibration, medical laboratory & Proficiency testing provider (PTP) & Reference
Material Producer (RMP) for a specific scope following IEC/ISO17025:2005, ISO
15189:2012, IEC/ISO 17043:2010 & IEC/ISO 17034:2016 Standards
Accreditation is the third party attestation related to a conformity assessment body conveying
the formal demonstration of its competence to carry out specific conformity assessment task.
Conformity Assessment Body (CAB) is a body which includes Testing including medical
Laboratory, Calibration Laboratory, Proficiency Testing Provider, Certified Reference
Material Producer.
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VARIOUS CALIBRATION LABS AT BHEL BHOPAL:.
Thermal Calibration Lab: In this lab the thermal instruments like thermometers are
calibrated using Secondary Standard Platinum Resistance Thermometer (SSPRT),
Thermal Bath and Digital Thermometer.
Pressure Calibration Lab: This laboratory is used to calibrate pressure measuring
instruments such as pressure gauges. Various machines used for the purpose are Dead
weight pressure calibrator, Vacuum pressure calibrator, Digital pressure calibrator,
etc.
Electrical Calibration Lab: Here various electric measuring instruments like
Ammeters, Voltmeters, Multimeters, etc. are calibrated using Multifunction
Calibrator, 8.5 Digit Reference Multimeter.
Dimensional Calibration Lab: Here several linear measurement instruments like
Venire Calipers, Micrometers, etc. are calibrated using Slip gauges as standards.
STANDARD PROCEDURE FOR CALIBRATION:
1) Cleaning Of Instruments: Every instrument should be first cleaned thoroughly.
2) Determination Of Error: The next step is to determine the errors in the instrument by
various methods.
3) Check For Tolerable Limits: After determination of error, the error is to be compared
with the allowable tolerance.
4) Minor Change: These are made in the instrument; if possible, minimize the error in
reading indicated by the instrument.
5) Allotment Of Calibration Set Up: Each instrument is allotted the set up as per its
condition.
6) Next Calibration Date: The instruments that are allotted an active status are also given
the next calibration date as per standards.
A measuring instrument is normally allotted calibration interval based on guidelines as
given in the following table.
