WHAT IS
TURBO GENERATOR /ALTERNATOR
Alternator
is a part of A generator. It operates on the same principle of faradays law of
electromagnetic induction .it also consist of armature winding and magnetic
field, here armature winding mounted on rotating element is called stator and
field winding is called rotor.
Working Principle
of Generator
In case of DC generator armature is rotate and
field winding is stationary similarly in ac generator or alternator armature
winding mounted on stationary element called as stator and field winding is
rotating is called rotor.
It works on the principle of faradays law of electromagnetic induction .
Whenever s conductor rotates in the
magnetic field emf is induced in that conductor.
According to Fleming left hand
rule The thumb finger indicate direction
of motion and fore finger indicate magnetic lines of force produced flux and
middle finger indicates direction of current.
It consist of stator and rotor, stator consist frame ,winding
, magnetic core similarly rotor consist of rotor shaft and winding.
Construction Details
Of Turbo Generator
Turbo Generator consist main parts a stationary part called stator and a rotating part called rotor.
STATOR
Stator
Frame- It consist stator core
and winding and provide
ventilation for cooling media.
End
Shields- End shields of stator frame are closed by fabricated cover and
provides passing dust for gas suction and discharge the rotor fan.
Stator
core- It is made up cold rolled grain oriented material and lamination
generally 0.35mm to 0.5mm insulated by class B & F .varnish temperature 135
degree to 155 degree.
Stator
Winding-It is made up Copper the out put of 3 phase is bring out
through stator winding.
Gas
Cooler-It is located on the
casing of the generator for cooling the
H2 gas normally 4 gas cooler is used .
Terminal
Bushing-the 3 phase is carried out from winding and fed to transformer through terminal
bushing.
ROTOR
Corona Shield-The main purpose of corona shield to
avoid don’t exhaust flux in the out of generator.
Rotor
Shaft-It is made up Chromium
Nickel Polybednium Vandanium steel material it consist fan rotor winding and retaining ring etc.
Rotor
Winding-It is placed milled slots on rotor shaft.
Rotor Retaining Ring- The over hang portion of rotor winding is held in
position.
Slip
Ring-It is used to fed the excitation current in the rotor winding for
production of emf.
Cooling
Fan-Two number of fan are provided both end of generator for gas circulation
inside the machine.
Sealing System Of
Generator
Seal are required to prevent leakage hydrogen gas and air
in the shaft of rotor . the rotor shaft ends of the hydrogen cooled generator
are brought out of the gas tight and pressure resistance .Therefore to prevent
leakage of H2 gas in outer and air enter into the generator and air ingress
into generator through the shaft gland. To prevent those activities by
providing radial and axial seal oil
filling at high pressure than H2 So no H2 will escape from generator housing
.shaft seal supply same oil for the journal bearing of generator and turbine.
Mainly two type of shaft
sealing are used
1> Ring
type sealing system
2> Thrust
type sealing
RING TYPE SEALING
This type of sealing consist of two short bearing in the
form of bronze rings fitting closely to rotor shaft and apart from each other
by a spring. Oil is forced between 2
ring oil is passed in both axial direction along the shaft and radially flow
between the ring . The radial position of the ring balance by a spring and oil
pressure. The axial movement is prevent
by the stoppers provided in the upper part of housing ,here a small clearance
are provided between shaft and rings on H2 and air side.
THRUST TYPE SEALING
This typr of sealing consist of small shaft bearing held by oil pressure or a
spring a collar m/c on the rotor shaft.
The seal consist of an end face liner having grooves in the face , a high
pressure oil is give to chamber where flow to
radial grooves then axial grooves due to centrifugal action between
collar and liner space and shaft. The
gap between collar and seal line adjust by spring compression for more oil flows to air side than
to H2 side of the shaft. Oil flow of air
side pressure is higher than the press of oil flow in H2 side ,So oil enter
into generator absorb 6.5% .hydrogen
carry out of the generator and 10 %
air by volume absorb carried out the generator.
SEAL OIL SUPPLY SYSTEM
The same seal oil is used the bearing lubrication
system but seal oil and lub oil are kept
separate and it consist-
Oil injector
Two seal oil pump (1 ac and 1 dc)
Two cooler
Two filters
Differential pressure regulator
Oil check pipe
Pressure oil regulator
Damper tank
Hydraulic seal tank
1-Supply Of oil to shaft seal
The oil to the shaft seal is
supplied from two different source
,During operation oil take governing system at 20ksc and oil taken lubrication
system of 1.5ksc these two oil injector
are diffused through oil injector.
2-Oil Pump
Pump are used for back up
purposes if failure of oil injector ,then oil supply by two 100% oil pumps one
stand by and other emergency ,stand by pump is driven by 415v AC motor and emergency pump is driven by 220v DC
motor ,hence there are electrically interlock. If the pressure is drop below
1.5ksc stand by pump detected due electrically interlock and run up and supply
oil
3-Oil Cooler
Mainly two oil cooler are
used to cooling the oil one stand by and another is running they are
electrically interlock ,water cooling at ambient temperature condition at
maximum 5ksc
4-OIl Filter
Oil filter are located after
oil cooler , this filter are screen type and prevent filtering foreign particle
entering the oil flow and damage of bearing and shaft seal material 2 filter
are used in service.
5-Differential Pressure
Regulator
In order to maintain
constant pressure between oil and H2 a
differential pressure regulator are used, it maintain pressure of oil at a higher than hydrogen pressure is
0.5 to 0.9ksc.
6-Thrust oil regulator
The output pressure thrust
oil regulator is 1ksc to 2.2ksc , It provide for holding the shaft seal ring
against a collar on rotor in case of thrust type of sealing.
7-Damper Tank
The oil after differential
pressure regulator enter into damper tank then goes to sealing, it provide
supply the oil to the shaft during transit condition to changeover injector
supply to the pump supply vice versa ,it is located 6mt centre of generator due to pressure remain
constant between H2 and oil. A magnetic float
indicator are used to give signals during low and emergency oil level tank.
8-Hydrogen recovery and
Hydrogen seal tank
Air and hydrogen are soluble
in lube oil at operating temp to extend 10% and 6.5% thus solubility increases with temp increases
.in operating condition oil flow towards the H2 seal while the oil flow to the air side take up air. If
the oil and contaminant within air and H2 is circulated through the seal it
gradually remove hydrogen and released air into it ,so the purity of H2 is reduced so
it will necessary to release to vent of the generator housing to atmosphere and
admit fresh H2 to maintain purity. To avoid oil contaminant with H2 and oil
contaminant in air , Separate remove the
contaminant and keep the oil and hydrogen in the system, The oil contaminant
with hydrogen is drained to a tank through
U’’ shape loop pipe service as sealing to prevent gas circulation
through this tank caused by pressure difference produced by both end of the generator this tank is called
Hydrogen seal tank . That time oil drain
from H2 side of the shaft seal and at sometime escape of H2 from generator
casing through the oil drain pipe line.
COOLING SYSTEM OF
GENERATOR
Cooling required in
generator for dissipating as heat through stator and rotor bodies. Cooling
medium should be air, hydrogen water etc. Generally for cooling the generator
stator and rotor body used Demineralised water
and H2 gas , For cooling of the exciter we used air medium. Hydrogen cooling
provided for rotor winding and core which are cooling through H2 passing
through it slots , So maximum temperature rise for cooling medium is 90 degree
in rotor and RTD(Resistance Temperature Detector) is used for measuring the
different point of temperature of variation in resistance . Stator water
cooling provide stator core and stator winding through its slots so maximum
temperature allow for cooling medium is 80 degree and Thermocouple are used to
measure temperature.
Air
Cooling Turbo Generator
Heat generated in stator
winding and core the temperature drop from winding to core across slots
insulation will proportional to insulation thickness. There is a temperature
drop between air and rotor surface this is called heat barrier this can be
reduced by increasing turbulence and heat transfer coefficient of coolant. The
temperature drop between teeth and rotor
surface is decreased because of increased turbulence this air cooling is used
of capacity up to 60 megawatt.
Stator Water Cooling
Generally in 210MW power plant DM water is used for
stator cooling by passes DM water
through the slots of the stator. Main bushing and phase connection stator winding is cooling by absorbing of
heat of water through radiation. The cooling water being in direct contact with
the high voltage winding an conductivity of the order of 0.5 to 1 micro-Ω cm. The
stator winding cooled by closed circulation
of low conductivity of DM water through the hollow rectangular conductor which
form the cooling water system for higher rating 210MW.
Two 100% duty AC motor driven centrifugal pump one working and other standby
.They are inter lock to each other for circulating cooling water from the
expansion tank to stator slots. Two 100% cooler are provided for cooling when
temperature increase above 45 degree.
The 100% duty water filter are provided after cooler one working and other
stand by ,Here water filter is mesh type filters. magnetic filter are provided
for catch the magnetic particle circle through water.
The following of water towards winding is measured by 3 flow indicator
which connected to and orifice plate . 200 to 300 mm of Hg MMWC inside the
expansion tank maintain. The purity of DM water is measured in specific
resistance falls below 75KΩ cm.
The cooling water primary cooling absorb heat loss designed as primary
water . The water flows inlet header at turbine end to out let header at
exciter end inside generator housing. When water is passed the hollow
conductor of stator winding absorbs heat
from them reach primary water tank.
H2 gas diffuser into primary water circuit through Teflon house liberated by
degassing action in the primary water tank , this tank is again taken into
cooling by pumps through primary water coolers.
The heat absorbed by the primary water in generator stator winding is
dissipated to the secondary coolant in these coolers.
Hydrogen Cooling
First we have purging the line by Co2 fill which hydrogen gas is circulated, H2 gas circulate 3.5ksc in rotor slot .Mainly
H2 gas is used for cooling the rotor
winding and core.
This gas is used to less wind age and friction loss. Reduced size of machine
and increase life and output of machine
4 hydrogen gas cooler installed in the
turbine side turbo generator. Hydrogen and air mixture between 5/95% , 75/25%, are explosive and hence
normally a 95/5% to 98/2% is maintained.
The gas cooler consist cooling tubes made of brass with coil copper wire wound to increases the
surface area of cooling water tubes
clarified water is used for cooling hydrogen gas .This water flows across the
cooling tubes and the hydrogen flows across the cooling tubes and it comes in
contact with external surface of the cooling water tubes and
the hydrogen is cooled.
The
hydrogen cooling system takes the supply of gas in main holes into the
generator housing through two headers, one at the top of generator and other is bottom of generator .The gas
main hole are connected to gas generation station.
During purging out operation Co2 is heavier than hydrogen this Co2 is
charged through the bottom header while H2 is forced out through the top header
it cannot contact with air inside the generator from any explosive mixture . So
during filling in H2 gas it charged through top header and Co2 is forced out
from bottom header.
SYNCHRONIZING PROCESS
OF GENERATOR
Check all
parameter before synchronizing of generator are
-Check all the parameter of generator are healthy.
-Stator cooling water flow more than 21t/Hr .
-Stator cooling water temperature 40 degree.
-Cold end hydrogen temperature less than 40 degree.
-Seal oil differential pressure between 1.2 ksc to 1.5 ksc.
-Generator stator water cooling system water conductivity is less than 5 mm
of water column.
-Hydrogen and stator water cooler are charged for cooling and clarified water
pressure to hydrogen and stator water cooler low.
-Turbine RPM is more than 2990 RPM then closing the bus side isolator from
switchyard control room.
-Closing transformer side isolator from switchyard control room.
-Excitation system is control manually or automatically by remote.
-Switch on the synchroscope to check mode.
-Incoming and running voltage &
frequency start showing on their
corresponding meter.
-Synchroscope needle start rotating at proportional difference to generator and
grid frequency.
-Closed the field circuit breaker and giving impulse for 5 seconds. And releasing
the switch when indicating lamp show then closing the field breaker glow.
-Adjust the turbine speed to match generator frequency with grid.
-Adjust the generator excitation to match the generator voltage with that of
grid.
-As synchroscope needles comes 11 ‘O’ clock position (Clockwise)the green lamp
should be ON and RED lamp goes off at 12
‘o’ clock position and indicate synchronising by glowing of lamp at generator
desk.
it taken load at 10 to 20MW.
What is excitation system and why
excitation system of generator
Excitation system is To create
strengenth magnetic field in rotor field winding of generator by giving DC
current to rotor field winding for produced emf , So that DC excitation system
is required for generation of pole ,in case of AC excitation system cannot
produced poles and cannot produced emf.
In case of generator a
additional energy is required for torque generation in generator and develop EMF
in the rotor winding in turbo generator and this additional energy called
Excitation System.
Excitation system is two type
-Static Excitation
-Brushless Excitation
Now a days static excitation is mostly
used due to renovation and modernization remove brushless excitation system .
4MW power required in DC excitation and now a days carbon brush is replaced by
using carbon-silver brush which advantages are ,Very low contact resistance,
Long life , Resistance does not changes when rotor on the stationary or steady state condition which will cause
increase in loss.
So to increase in excitation voltage 1st closed the Field Circuit
Breaker ,
2nd Increase Automatic
Voltage Regulator through Permanent Magnet Generator, 3rd Monitor
field current.
Main Component of Excitation System
Main components of excitation system are
Main exciter
Pilot Exciter(PMG)
Excitation Transformer
AVR
Thyristor Converter
Field Breaker
Automatic
Voltage Regulator-
Automatic voltage regulator is the
important part in synchronous generator , It control the output voltage of the
generator by controlling its excitation current. Thus it can control output
reactive power of the generator. It increase or decrease exciter current for
more a linear voltage and frequency , It maintain constant voltage of
alternator terminals by maintaining the excitation required for maintain the
terminal voltage of alternator with change in the load, Increasing excitation
voltage with increase in reactive volt ampere(KVAR) which decreases the power
factor. Generally voltage of generator is kept constant so rest of the
component VICOSᵩ current and COSᵩ vary according to load , Increase in
current with decrease in PF vice-versa.
Excitation Voltage V α KVAR & I
V α
1/COS ᵩ
Hence increase in reactive voltage ampere (KVAR) and increase in power factor .
Thyristor Converter--
It is a SCR device familiarise , It is used for rectification of AC voltage
fast control of DC current.
Field Circuit Breaker –
FCB is a AC or DC side of the converter, It open the field under fault
condition first absorption of field energy
it is also known as field discharge energy circuit.
BRUSHLESS EXCITATION
In
this system DC excitation is give through both
bar not in brush, Here three pilot exciter (PMG) is used to supply DC into
field winding of mechanical energy through AVR and diode,
Here diode is convert AC to DC and AVR control its excitation current after
that mechanical energy , a rotating diode is used to fed the DC supply into
rotor winding of generator through bar
,so it is called Brushless excitation.
Here silp ring eliminating brush gear
for simple and reliable and minimum operation and maintenance cost should be
less. Here stroboscope used for failure of detection. No external source
required since pilot exciter has permanent magnet field.
One additional bearing and increases in shaft length are required. This system
get activate through PMG ,ME,RD(rotating diode).
STATIC EXCITATION SYSTEM
In
this case brush and slip ring are used for excitation , Here PMG has 400HZ/sec
which supply 3 phase DC to ME through diode and AVR. Here ME supply gives to PE
supply give to ME rotor winding for production of flux and EMF this is utilised
to feed supply to rotor winding of generator through Field breaker and again
come back to ME field winding through rotating diode.
Here also a excitation transformer
which give 4MW power for DC supply and this is held on a AVR through field
flashing through diode .
Field flashing means for start up
DC excitation system is fed to field from external source like battery or
rectifier AC from station AC supply so
both flashing and regulating up 30% to 70% above 70% flashing get cut-off.
Excitation
Power Required
200/210MW 500MW
Excitation
current at full load -
2600A
310A
Excitation
Voltage at full load -
6300V
600V
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