08-10-2012, 01:48 PM
MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR AIR COOLED TURBO GENERATOR BY V.P.I PROCESS
MANUFACTURING AND DESIGN.pdf (Size: 1.32 MB / Downloads: 88)
PROFILE OF B.H.E.L.
Bharat Heavy Electrical Limited (BHEL) is today the largest engineering
enterprise of India with an excellent track record of performance. Its first plant
was set up at Bhopal in 1956 under technical collaboration with M/s. AEI, UK
followed by three more major plants at Haridwar, Hyderabad and Tiruchirapalli
with Russian and Czechoslovak assistance.
These plants have been at the core of BHEL’s efforts to grow and
diversify and become India’s leading engineering company. The company now
has 14 manufacturing divisions, 8 service centres and 4 power sector regional
centres, besides project sites spread all over India and abroad and also regional
operations divisions in various state capitals in India for providing quick service
to customers.
BHEL manufactures over 180 products and meets the needs of coresectors
like power, industry, transmission, transportation (including railways),
defence, telecommunications, oil business, etc. Products of BHEL make have
established an enviable reputation for high quality and reliability.
BHEL has installed equipment for over 62,000 MW of power generationfor
Utilities, Captive and Industrial users. Supplied 2,00,000 MVA transformer
capacity and sustained equipment operating in Transmission & Distribution
network up to 400kV – AC & DC, Supplied over 25,000 Motors with Drive Control
System Power projects. Petrochemicals, Refineries, Steel, Aluminium, Fertiliser,
Cement plants etc., supplied Traction electric and AC/DC Locos to power over
12,000 Km Railway network.
Supplied over one million Valves to Power Plants and other Industries.
This is due to the emphasis placed all along on designing, engineering and
manufacturing to international standards by acquiring and assimilating some of
the best technologies in the world from leading companies in USA, Europe and
Japan, together with technologies from its-own R & D centres BHEL has acquired
ISO 9000 certification for its operations and has also adopted the concepts of
Total Quality Management (TQM).
Introduction
Electrical insulating materials are defined as materials that offer a
large resistance to the flow of current and for that reason they are used to keep
the current in its proper path i.e. along the conductor. Insulation is the heart of
the generator. Since generator principle is based on the induction of e.m.f in a
conductor when placed in a varying magnetic field. There should be proper
insulation between the magnetic field and the conductors. For smaller capacities
of few KW, the insulation may not affect more on the performance of the
generator but for larger capacities of few MW (>100MW) the optimisation of
insulation is an inevitable task, moreover the thickness of insulation should be
on par with the level of the voltage, also non homogenic insulation provisions
may lead to deterioration where it is thin and prone to hazardous short circuits,
also the insulating materials applied to the conductors are required to be flexible
and have high specific (dielectric) strength and ability to withstand unlimited
cycles of heating and cooling.
Keeping this in view among other insulating materials like solids
gases etc liquid dielectrics are playing a major role in heavy electrical equipment
where they can embedded deep into the micro pores and provide better
insulating properties. Where as solid di-electrics provide better insulation with
lower thickness and with greater mechanical strength. So the process of
insulation design which has the added advantage of both solid and liquid
dielectrics would be a superior process of insulation design. One such process
which has all the above qualities is the VPI (vacuum pressurised impregnation)
process and has proven to be the best process till date.
Drawbacks of Early VPI Process:
DR. MEYER brought the VPI system with the collaboration of WESTING
HOUSE in the year 1956. It has been used for many years as a basic process for
thorough filling of all interstices in insulated components, especially high voltage
stator coils and bars. Prior to development of thermosetting resins, the widely
used insulation system for 6.6kv and higher voltages was a VPI system in which,
Bitumen Bonded Mica Flake Tape is used as main ground insulation. The
bitumen is heated up to about 180°C to obtain low viscosity which aids thorough
impregnation.
Advantage of present resin poor VPI process:
VPI is a process, which is a step above the conventional vacuum
system. VPI includes pressure in addition to vacuum, thus assuring good
penetration of the varnish in the coil. The result is improved mechanical strength
and electrical properties. With the improved penetration, a void free coil is
achieved as well as giving greater mechanical strength. With the superior
varnish distribution, the temperature gradient is also reduced and therefore,
there is a lower hot spot rise compared to the average rise.
In order to minimise the overall cost of the machine & to reduce
the time cycle of the insulation system vacuum pressure Impregnated System is
used. The stator coils are taped with porous resin poor mica tapes before
inserting in the slots of cage stator, subsequently wounded stator is subjected to
VPI process, in which first the stator is vacuum dried and then impregnated in
resin bath under pressure of Nitrogen gas.
ROTOR WINDING AND RETAINING RINGS:
The rotor winding consisting of several coils, which are inserted
into the slots and series connected such that two coils groups from one pole.
Each coil consists of several connected turns, each of which consists of two half
turns which are connected by brazing in the end section. The individual turns of
the coils are insulated against each other, the layer insulation L-shaped strips of
lamination epoxy glass fibre with nomax filler are used for slot insulation. The
slot wedges are made of high electrical conductivity material and thus act as
damper winding. At their ends the slots wedges are short circuited through the
rotor body.
The centrifugal forces of the rotor end winding are contained by
single piece of non magnetic high strengthen steel in order to reduce stray
losses, each retaining rings with its shrinks fitted insert ring is shrunk into the
rotor body in an overhang position. The retaining rings are secured in the axial
position by a snap ring.
EXCITATION SYSTEM:
In all industrial applications, the electrical power demand is ever
increasing. This automatically demands for the design, development and
construction of increasingly large capacity Synchronous generators. These
generators should be highly reliable in operation to meet the demand. This calls
for a reliable and sophisticated mode of excitation system.
When the first a.c generators were introducing a natural choice for the
supply of field systems was the DC exciter. DC exciter has the capability for
equal voltage output of either polarity, which helps in improving the generator
transient performance. DC exciters, how ever, could not be adopted for large
ratings because of the problems in the design commutator and brush gear,
which is economically unattractive. Of –course, the problems are not uncommon
in power stations but Of the environment with sulphur vapours, acidic fumes as
in the cases of petrochemical and fertilizer industries, exposure of DC exciter.
This adds to the problem of design.
PERMANENT MAGNET GENERATOR AND AVR:
This system is highly reliable with least maintenance and is ideally suitable
for gas driven generators.
The static excitation system was developed contemporarily as an
alternative to brush less excitation system. This system was successfully
adapted to medium and large capacity Turbo generators. Though the system
offers very good transient performance, the problems associated with slip rings
and brush gear system are still present.
This system consists of rectifier transformer, thyristor converts, field
breaker and AVR. This system is ideally suitable where fast response is called for.
The system is flexible in operation and needs very little maintenance.
Thus, each excitation system has its own advantages and disadvantages.
The selection of system is influenced by the transient response required, nature
of pollution and pollution level in the power plant and cost of equipment.