KWU Turbine System LPBP

7/31/2019 KWU Turbine System LPBP

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TURBINE SYSTEM

Prepared by:

K.V. VidyanandanSr. Manager (EDC)

NTPC-Singrauli


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STEAM TURBINE: GENERAL

DESCRIPTION

The 210 MW KWU turbine is condensing, tandem compounded, reheat type and singleshaft machine. In has separate high pressure, intermediate and low-pressure parts.The HP part is a single cylinder and IP & LP parts are double flow cylinders. Theturbine rotors are rigidly coupled with each other and with generator rotor.

HP turbine has throttle control. The steam is admitted through two combined stop andcontrol valves. The lines leading from HPT exhaust to reheater have got two cold reheatswing check NRVs. The steam from reheater has got two cold reheat swing checkNRVs. The steam from reheater is admitted to IP turbine through two combined stopand control valves. Two crossover pipes connect IP and LP cylinder.

210 MW KWU TURBINE

Blading

The entire turbine is provided with reaction blading. The moving blades of HPT, LPTand front rows of LPT have inverted T roots and are shrouded. The last stages of LPTare twisted; drop forged moving blades with fir-tree roots. Highly stressed guide bladesof HPT and IPT have inverted T roots. The other guide blades have inverted L-roots

with riveted shrouding.Bearings

The TG unit is mounted on six bearings HPT rotor is mounted on two bearings, adouble wedged journal bearing at the front and combined thrust/journal bearingadjacent to front IP rotor coupling. IP and LP rotors have self-adjusting circular journalbearings. The bearing pedestals of LPT are fixed on base plates where as HPT front andrear bearing pedestals are free to move axially.

Pedestals at machine level support the brackets at the sides of HPT. In axial direction,HP & IP parts are connected with the pedestals by means of a casing guide. Radial


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expansion is not restricted. HP & IP casings with their bearing pedestals move forwardfrom LPT front pedestal on thermal expansion.

HP TURBINE

1. TURBINE ROTOR

2. OUTER SEAL RING

3. BARREL CASING

4. GUIDE BLADE CARRIER

5. THREADED RING

6. CASING COVER

HP TURBINE SECTIONAL VIEW

HP Turbine is of double cylinder construction. Outer casing is barrel type without anyaxial/radial flanges. This kind of design prevents any mass accumulation and thermalstresses. Also perfect rotational symmetry permits moderate wall thickness of nearlyequal strength at all sections. The inner casing is axially split and kinematicallysupported by outer casing. It carries the guide blades. The space between casings isfilled with the main steam. Because of low differential pressure, flanges and connectingbolts are smaller in size. Barrel design facilitates flexibility of operation in the form ofshort start-up times and higher rate of load changes even at high steam temperatureconditions.


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IP TURBINE

1. TURBINE ROTOR2. OUTER CASING

3. OUTER CASING

4. INNER CASING5. INNER CASING

6. EXTRACTION NOZZLE

7. INLET NOZZLE

IP TURBINE SECTIONAL VIEW

IP Turbine is of double flow construction. Attached to axially split out casing is aninner casing axially split, kinematically supported and carrying the guide blades. Thehot reheat steam enters the inner casing through top and bottom centre. Arrangementof inner casing confines high inlet steam condition to admission breach of the casing.The joint of outer casing is subjected to lower pressure/temperature at the exhaust.Refer to Figure.


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LP TURBINE

Double flow LP turbine is of three-shell design. All shells are axially split and are ofrigid welded construction. The inner shell taking the first rows of guide blades is

attached kinematically in the middle shell. Independent of outer shell, middle shell issupported at four points on longitudinal beams. Two rings carrying the last guideblade rows are also attached to the middle shell. Refer to Figure.

1. OUTER CASING2. OUTER SHELL

3. INNER SHELL

4. INNER SHELL5. OUTER SHELL

6. DIFFUSER

7. OUTER CASING

LP TURBINE SECTIONAL VIEW

Fixed Points (Turbine Expansions)

a. Bearing housing between IP and LP

b. Rear bearing housing of LP turbine

c. Longitudinal beam of LP turbine

d.Thrust bearing.


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Front/rear housing of HPT can slide on base plates. Any lateral movementsperpendicular to machine axis are prevented by fitted keys. Bearing housings areconnected to HP-IP casings by guides, which ensure central position of casings whileaxially expanding and moving.

The LPT casing is located in centre area of longitudinal beam by fitted keys cast in thefoundation cross beams. Axial movements are not restricted. The outer casing of LPturbine expands from its fixed points towards generator. Bellows expansion couplingstake the differences in expansion between the outer casing and fixed bearing housing.Hence HPT rotor & casing expands towards bearing no (1) while IPT rotor expandstowards generator. The LPT rotor expands towards generator. The magnitude of thisexpansion is reduced by the amount by which the thrust bearing is moved in theopposite direction due to IPT casing expansion.

1. HP FRONT PEDESTAL2. HP REAR PEDESTAL3. LP FRONT PEDESTAL4. LP REAR PEDESTAL5. HPT OUTER CASING6. IPT OUTER CASING7. LPT OUTER CASING8. HP FRONT PEDESTAL BASE PLATE9. HP REAR PEDESTAL BASE PLATE10. LP FRONT PEDESTAL ANCHOR POINT

11. LP REAR PEDESTAL ANCHOR POINT12. LP OUTER CASING ANCHOR POINT13. HPT INNER CASING14. IPT INNER CASING15. LP INNER OUTER CASING16. LP INNER OUTER CASING17. HP INNER CASING ANCHOR POINT18. IP INNER CASING ANCHOR POINT19. LP INNER OUTER CASING ANCHOR POINT20. LP INNER INNER CASING ANCHOR POINT

TURBINE ANCHOR POINTS AND EXPANSIONS


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Front/rear housing of HPT can slide on base plates. Any lateral movementsperpendicular to machine axis are prevented by fitted keys. Bearing housings areconnected to HP-IP casings by guides, which ensure central position of casings whileaxially expanding and moving. The LPT casing is located in centre area of longitudinal

beam by fitted keys cast in the foundation cross beams. Axial movements are notrestricted. The outer casing of LP turbine expands from its fixed points towardsgenerator. Bellows expansion couplings take the differences in expansion between theouter casing and fixed bearing housing. Hence HPT rotor & casing expands towardsbearing no (1) while IPT rotor expands towards generator. The LPT rotor expandstowards generator. The magnitude of this expansion is reduced by the amount bywhich the thrust bearing is moved in the opposite direction due to IPT casingexpansion.

Turbine Oil Supply

In the 200MW KWU turbines, single oil is used for lubrication of bearings, control oil

for governing and hydraulic turbine turning gear. During start-ups, auxiliary oil pump(2 Nos.) supplies the control oil. Once the turbine speed crosses 90% of rated speed,the main oil pump (MOP) takes over. It draws oil from main oil tank. The lubricating oilpasses through oil cooler (2 nos.) before can be supplied to the bearing. Underemergency, a DC oil pump can supply lub oil. Before the turbine is turned or barred,the Jacking Oil Pump (2 nos.) supplies high-pressure oil to jack-up the TG shaft toprevent boundary lubrication in bearing. Refer to the figure.

TURBINE LUBRICATING OIL SYSTEM

The oil systems and related sub-loop controls (SLCs) can be started or stoppedautomatically by means of SGC oil sub-group of automatic control system. The variouslogics and SLCs under SGC oil are given in the ATRS section.


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MAIN OIL PUMP

The main oil pump is situated in the front bearing pedestal and supplies the entireturbine with lubricating oil and control oil, which is connected to the governing rack.

1. Threaded ring2. Pump casing, upper3. Journal Bearing4. Oil pipe5. Bearing bushing6. Seal ring7. Impeller8. Feather key

9. Feather key10. Journal + Thrust Brg11. Ring12. Vent pipe13. Oil inlet vessel14. Hyd. Speed Xter15. Oil line16. Turbine shaft

17. Coupling18. Elect. Speed Xter19. Permanent Magnet20. Pump shaft21. Spacer sleeve22. Pump casing, lower23. Oil tube

TURBINE TURNING GEAR

The turbine is equipped with a hydraulic turning gear assembly comprising two rowsof moving blades mounted on the coupling between IP and LP rotors. The oil underpressure supplied by the AOP strikes against the hydraulic turbine blades and rotatesthe shaft at 110 rpm (220 rpm under full vacuum condition).

In addition, provisions for manual barring in the event of failure of hydraulic turninggear, have also been made. A gear, machined of the turning gear wheel, engages with aRatchet & Pawl arrangement operated by a lever and bar attachment.


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HYDRAULIC BARRING GEAR AND MECHANICAL BARRING GEAR

TURBINE GLAND SEALING

Turbine shaft glands are sealed with auxiliary steam supplied by an electro-

hydraulically controlled seal steam pressure control valve. A pressure of 0.01 Kg/cm2

(g) is maintained in the seals. Above a load of 80 MW the turbine becomes self-sealing.The leak off steam from HPT/IPT glands is used for sealing LPT glands. The steampressure in the header is then maintained constant by means of a leak-off control

valve, which is also controlled by the same electro-hydraulic controller, controlling sealsteam pressure control valve. The last stage leak-off of all shaft seals is sent to thegland steam cooler for regenerative feed heating. Refer the Figure.


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TURBINE SEAL STEAM SYSTEM


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TURBINE SPECIFICATIONS

Type:Three cylinders reheat condensing turbine having:

i. Single flow HP turbine with 25 reaction stages.

ii. Double flow IP turbine with 20 reaction stages per flow.

iii. Double flow LP turbines with 8 reaction stages per flow.

Rated Parameters

Nominal rating : 210 MW

Peak loading (without HP heaters) : 229 MW

Rated speed. : 3000 RPM

Main steam flow at full load(With HP heaters in service).

: 630 tons/hr.

Main steam pressure/ temperature atfull load.

: 147.1 kg/cm2. 535 oC.

HRH pressure/ temp at full load. : 34.23 kg/cm2. 535 oC.

Permissible SH / RH temp variations.

:543 oC. (Long time value but keeping

within annual mean 535oC.)

: 549 oC. (400 hours per annum)

: 536oC. (80 hours per annum & max.

15 min in individual case)

Condenser pressure. : 76 mm Hg with CW inlet temp 33 oC.

STEAM TEMPERATURE

Rated valueAnnual

mean value

Long timevalue keepingwithin annual

mean value

400h per

annum

80-hr/annummaximum.15 min., in

individualcases

oC oC oC oC

Initial steam 535 543 549 563

IPT SV Inlet 535 543 549 563

HPT exhaust 343 359500 + (specialcase)

425

Extraction 6 343 359500 + (specialcase)

425


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Extraction 5 433 438 473





LPT exhaust 49 70 100 70

* Long-time operation: Upper limit value permissible without time limit

Valid only for the no-load period with high reheat pressure after trip-out from full-load

operation. For the individual case approx. 15 min. Provision for this is that the turbineis immediately reloaded or the boiler immediately reduced to minimum load if no-loadoperation is maintained.

Permissible differential temperaturebetween parallel steam supply lines

- No time limitation- Short time period

::

17 K.28 K.

In the hottest line the limitations indicated for initial steam and reheat temperaturemust not be exceeded.

Turbine Extractions (Pressure/ Temperature) at 200 MW

ExtractionPres. (bar)

Temp. 0 C.

1. Extraction No. 6 (from HPT exhaust) 39.23 343

2. Extraction No. 5 (from 11 the stage IPT) 16.75 433

3. Extraction No. 4 (from IPT exhaust) 7.06 136

4. Extraction No. 3 (from 3rd stage LPT) 2.37 200

5. Extraction No. 2 (from 5th stage LPT) 0.858 107

6. Extraction No. 1 (from 7th stage LPT) 0.216 62


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Alarm and Limiting Values of some Important Parameters

Parameters Alarm value Limit value

HPT Diff. Expansion. +4.5 mm +5.5 mm

- 2.5 mm - 3.5 mm

IPT Diff. Expansion. +5.0 mm + 6.0 mm

-2.0 mm - 3.0 mm

LPT Diff. Expansion. +25.0 mm +30.0 mm

-5.0 mm - 7.0 mm

HPT exhaust casing temperature 480 oC 500 oC

LPT outer casing metal temperature 90 oC 110 oC

Metal temp diff. between upper & lower casing(HPT front middle, IPT front, rear). +/- 30

oC +/- 45 oC

Turbine Bearing Metal Temperature

Maxm Oil Temperature before coolers 76 oC

Whose normal operating temp is 75 oC 90 oC 120 oC

Whose normal operating temp is 85 oC 100 oC 120 oC

Turbine bearing housing vibration 35 microns 45 microns

Turbine absolute shaft vibration 30 microns 200 microns

Condenser vacuum (absolute) 120 mm Hg 200 mm Hg

Turbine axial shift 0.3 mm 0.6 mm

Turbine over speed 51.5 Hz 55.5 Hz


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TURBINE GOVERNING SYSTEM

In order to maintain the synchronous speed under changing load/grid or steam

conditions, the KWU turbine supplied by BHEL at NTPC Korba is equipped withelectro-hydraulic governor; fully backed-up by a hydraulic governor. The measuringand processing of electrical signal offer the advantages such as flexibility, dynamicstability and simple representation of complicated functional systems. The integrationof electrical and hydraulic system is an excellent combination with followingadvantages:

Exact load-frequency droop with high sensitivity. Avoids over speeding of turbine during load throw offs. Adjustment of droop in fine steps, even during on-load operation.

Elements of Governing System

The main elements of the governing system and the brief description of their functionsare as follows:

Remote trip solenoids (RTS). Main trip valves (Turbine trip gear). Starting and Load limit device. Speeder Gear (Hydraulic Governor). Aux. follow-up piston valves. Hydraulic amplifier. Follow-up piston valves. Electro-Hydraulic Converter (EHC). Sequence trimming device. Solenoids for load shedding relay. Test valve. Extraction valve relay. Oil shutoff valve. Hydraulic protective devices.


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REMOTE TRIP SOLENOIDS (RTS)

The remote trip solenoid operated valves are two in number and form a part of turbineprotection circuit. During the normal operation of the turbine, these solenoids remain

de-energised. In this condition, the control oil from the governing rack is free to passthrough them to the main trip valves. The solenoids gets energised whenever anyelectrical trip command is initiated or turbine is tripped manually from local or UCB.Under energised condition the down stream oil supply after the remote trip solenoidsgets connected to drain and the upstream will be blocked. By resetting Unit TripRelays (UTR) from UCB, these solenoids can be reset. Refer to Figure.

REMOTE TRIP SOLENOIDS

MAIN TRIP VALVES

The main trip valves (two in numbers) are the main trip gear of the turbine protectivecircuit. All turbine tripping take place through these valves. The control oil fromremote trip solenoids is supplied to them.

Under normal conditions, this oil flows into two different circuits, called as the Trip Oiland Auxiliary Trip Oil. The Trip Oil is supplied to the Stop Valves (of HP Turbine and IPTurbine), Auxiliary Secondary Oil circuit and Secondary Oil circuits. The AuxiliaryTrip Oil flows in a closed loop formed by main trip valves and turbine hydraulicprotective devices (Over Speed trip device, Low Vacuum trip device and Thrust Bearingtrip device).

The construction of main trip valves is such that when aux. trip oil pressure isadequate, it holds the valves' spools in open condition against the spring force.Whenever control oil pressure drops or any of the hydraulic protective devices areactuated, the main trip valves are tripped. Under tripped condition, trip oil pressure isdrained rapidly through the main valves; closing turbine stop and control valves. Referto the figure below.


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MAIN TRIP VALVES

STARTING AND LOAD LIMIT DEVICE

The starting and load limit device is used for resetting the turbine after tripping, foropening the stop valves and releasing the control valves for opening. The startingdevice consists of a pilot valve that can be operated either manually by means of ahand wheel or by means of a motor from remote. It has got port connections with thecontrol oil, start-up oil and auxiliary start-up oil circuits. The starting device can

mechanically act upon the hydraulic governor bellows by means of a lever and linkarrangement.

Before start-up, the pilot valve is brought to its bottom limit position by reducing thestarting device to 0% position. This causes the hydraulic governor bellows to becompressed thus blocking the build-up of secondary oil pressure. This is known ascontrol valve close position. With the valve in the bottom limit position (starting device= 0%) control oil flows into the auxiliary start-up circuit (to reset trip gear andprotective devices) and into the start-up oil circuit (to reset turbine stop valves). Abuild-up of oil pressure in these circuits can be observed, while bringing the startingdevice to zero position. When the pilot valve i.e. the starting device position is raised,the start-up oil and auxiliary start-up oil circuits are drained. This opens the stop


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valves; ESVs open at 42% and IVs open at 56% positions of the starting device.Further raising of the starting device release hydraulic governor bellows which is inequilibrium with hydraulic governor's spring tension and primary oil pressure (turbinespeed), and raises the aux. sec. oil pressure; closing the aux. follow-up drains of

hydraulic governor.

STARTING DEVICE ACTING ON SPEEDER GEAR


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SPEEDER GEAR

The speeder gear is an assembly of a bellow and a spring, the tension of which can beadjusted manually from UCB by an electric motor or locally by a hand wheel. The

bellow compression depends upon the position of the starting device and the speedergear position, which alters the spring tension on the top of the bellow. The bellow isalso subjected to the primary oil pressure, which is the feedback signal for actualturbine speed. The zero position of speeder gear corresponds to 2800 rpm i.e.hydraulic governor comes into action after 2800 RPM. The bellow and spring assemblyis rigidly linked to the sleeves of the auxiliary follow-up piston valves. The position ofthe sleeve changes with the equilibrium position of the bellow.

SPEEDER GEAR


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HYDRAULIC SPEED TRANSMITTER

The hydraulic speedtransmitter runs in the MOP

bearing and operates on theprinciple of a centrifugalpump. The variation ofpressure in the discharge lineis proportional to the squareof the machine speed. Thisprimary oil pressure acts asthe control impulse for thehydraulic speed governor.The transmitter is suppliedwith control oil via an oilreservoir. An annular groove

in the speed transmitterensures that its inside isalways covered with a thinlayer of oil to maintain auniform initial pressure.Excess oil drains into thebearing pedestal.

CURVE SHOWING TURBINE SPEED Vs PRIMARY OIL


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AUXILIARY FOLLOW-UP PISTON VALVES

Two Auxiliary Follow-uppistons are connected in

parallel and the trip oil issupplied to them throughorifice. The sleeves of thesevalves are attached to thespeeder gear bellow link. Theposition of the sleevedetermines the draining rateof trip oil through the ports.Accordingly the trip oilpressure downstream ofthese valves changes. Oildownstream of auxiliaryfollow-up pistons circuit istermed as AUXILIARYSECONDARY OIL. Hence,aux. follow-up piston valvescan be said to controlauxiliary secondary oilpressure.

SEQUENCE TRIMMING DEVICE

The function of the sequence trimming device or HP/IP TRIM DEVICE is to prevent anyexcessive HP turbine exhaust temperature due to churning. It changes response of

main and reheat control valves. When the reheat pressure is more than 32 Kg/cm2

andload less than 20% the IP turbine tends to get loaded more than HP turbine. The steamflow through HP turbine tends to fall to very minimum, causing a lot of churning andexcessive exhaust temperature. The trim device operates at this moment trimming theIP turbine control valve. The control valves of HPT open more to maintain flow ofsteam, reducing the HPT exhaust temperature.

It consists of a spring-loaded piston assembly, which is supported by control oilpressure from beneath, under normal conditions. The control oil is supplied via an

energised solenoid valve. When the turbine loads is less then 40 MW and hot reheatpressure is more than 32 kg/cm

2the solenoid valve gets de-energised cutting out the

control oil supply to the trim device.

The trim device trips under spring pressure. The trim device is connected to the follow-up piston valves of IP control valves by means of a lever. Upon tripping, the trim devicealters the spring tension of follow-up pistons of IP pistons control valves, draining thesecondary oil. The IP control valves openings are trimmed down.


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HYDRAULIC AMPLIFIER

Hydraulic Amplifier consists of a pilot valve and an amplifier piston. The position of thepilot valve spool depends upon the aux. secondary oil pressure. Depending upon the

pilot spool position, the control oil is admitted either to the top or the bottom of theamplifier piston. The other side of amplifier is connected to the drain. The movementsof the amplifier piston are transformed into rotation of a Camshaft through a pistonrod and a lever assembly. A feedback linkage mechanism stabilises the system for oneparticular aux. secondary oil pressure.

1. Amplifier piston2. Follow-up piston3. Sleeve4. Shaft5. Lever6. Feedback lever7. Pilot valve8. Compression spring9. Adjusting screw

a : Control oilb : Secondary oil

b1 : Aux. Sec oil

c : Return oil

HYDRAULIC AMPLIFIER

SOLENOIDS FOR LOAD SHEDDING RELAY

A pair of solenoid valves has been incorporated in the IP Sec oil line on control valvesand Aux Sec. oil line, in order to prevent the turbine from reaching high speed in theevent of sudden turbine load throw-off. The control valves are operated (closed) by theload-shedding relay when the rate of load reduction exceeds a certain value. Thesolenoid drains the IPCV secondary oil directly. Direct draining of IP Sec oil circuitcauses the reheat valves to close without any significant delay. The HP control valvesare closed due to draining of aux. secondary oil before the hydraulic amplifier, by thesecond solenoid valve. The extraction stops valves controlled by IP secondary oil actingthrough extraction valves relays also get closed. After an adjustable time delay (approx.2 seconds) the solenoid valves are re-closed and secondary oil pressure correspondingto reduce load builds-up in the HP and IP turbine secondary oil lines.


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FOLLOW-UP PISTON VALVES

The trip oil is supplied to the follow up piston valves through orifices and flows in the

secondary oil piping to control valves. The secondary oil pressure depends uponposition of sleeves of follow-up piston valves; which determines the amount of drainageof trip oil.

FOLLOW-UP PISTON VALVES

There are in all twelve follow-up piston valves. Six of them are associated withhydraulic amplifier and six of them with EHC in the governing system.

The follow-up piston valves constitute a minimum value gate for both the governors.This means the governor with lower reference set point, is effectively in control. This isalso termed as HYDRAULIC MINIMUM SELECTION of governors.

The drain port openings of follow-up pistons of hydraulic amplifier depends onauxiliary secondary oil pressure, upstream of aux. follow-up pistons; and that ofelectro hydraulic converter, on the piston of pilot spool valve of the elector-hydraulicconverter (i.e. EHC output).


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TEST VALVEEach of the HP and IP stop

valves' servomotors receivestrip oil through theirassociated test valves. The testvalves have got port openingsfor trip oil as well as start-upoil. The test valves facilitatesupply of trip oil pressurebeneath the servomotor disc.(Stop valve open condition,under normal operation). Forthe purpose of resetting stopvalves after a tripping, start-up oil pressure is supplied tothe associated test valves,which moves their spooldownwards against the springforce. In their bottom mostposition the trip oil pressurestarts building up above thestop valve servomotor pistonwhile the trip oil beneath thedisc gets connected to drain.When start-up oil pressure is

reduced the test valve movesup draining trip oil above theservomotor piston andbuilding the trip oil pressurebelow the disc, thus openingthe stop valve. A hand wheelis also provided for manualoperation of test valves.

1. Bolt2. Hand wheel3. Spindle4. Cover5. Oil Seal6. Bushing7. O-ring8. Valve Cover9. Valve Body10.Trip Oil11.Piston sleeve

12. Trip Oil13. Piston valve14. Spring plate15. Spring16. Spacer17. Bottom cover18. Trip oil19. Drain20. Trip oil21. Startup oil

EXTRACTION N.R.VS AND EXTRACTION VALVE RELAY

Four pair of swing check valves are provided in the extraction lines to the feed heaters(LP Heaters No: 2,3, Deaerator and HPH No: 5) to prevent back flow of condensedsteam into the turbine from heaters on account of high levels in the heaters. There aretwo NRVs provided in each of these extraction lines and is force closing type. Boththese valves are free-swinging check type, however the first valve is equipped with anactuator. In case of flow reversals, both the valves are closed automatically. Theactuator assists the fast closing of the first valve.

The mechanical design of force-closed valves is such that they are brought into free-swinging position by means of trip oil. They are open as soon as differential pressure issufficient. If the trip oil pressure falls, the spring force closes the valve when steampressure either falls or is lowered (reduced load).


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The extraction valve relay, its changeover valve and its solenoid valve control the tripoil to each of the actuators of force closing type valves. Extraction valve relay actuatesthe FCNRVs in proportion to secondary oil pressure. By suitable adjustment of itsspring, the secondary oil pressure at which the FCNRVs will be released for openingcan be set. However, swing check FCNRVs will also open without the release action,also if the steam pressure is more than the spring force. But in this case the pressureloss shall be more leading to loss of efficiency.

In case of turbine trip or sudden load reduction, by energising the associated solenoidvalve, draining of trip oil pressure through extraction valve relay assists closingmovements of FCNRVs. In both the cases the actuator is devoid of trip oil and itsspring force closes the NRV. Extraction (4) FCNRV solenoid is also energisedadditionally by lower differential pressure in the extraction line.


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b : Control Oil c : Return Oil

b1 : Secondary Oil x : Trip Oil

b2 : Secondary Oil x1 : Trip Oil

COLD REHEAT SWING CHECK VALVE

Two numbers of swing check valves are provided on the CRH lines from which thesteam is drawn for HPH-6. Their pilot valves via their rotary servomotor in proportionto secondary oil pressure operate the CRH NRVs. They open out fully when maincontrol valves open up corresponding to 5-10% of maximum turbine out-put. Only

when the control valves are closed to this threshold again, the NRVs return into steamflow by the hydraulic actuator, so that when the steam flow ceases in the normaldirection, they are closed by the torque of rotary servomotor. Even when the pressureof secondary oil has not built up sufficiently, NRVs can be opened up like safety valveswhen the upstream pressure rises above the downstream side pressure by one bar.


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VACUUM BREAKER

The function of the vacuum breakers is to cause an increase in condenser pressure

by conducting atmospheric air into the condenser together with the steam flowing

from the LP Bypass. When the pressure in the condenser increases, the ventilationof the turbine balding is increased, which causes the turboset to slow down so that

the running down time of the turboset and the time needed for passing through

critical speeds are shortened.


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HYDRAULIC AND ELECTRO-HYDRAULIC GOVERNING OF TURBINES

Power produced by any power plant is sent out on utility grid (Transmission line andcontrol equipments) together with power from other plants through process ofsynchronization with the grid and to distribution systems and then to the consumer.Control of system frequency on the grid or interconnected grid/pool is a majorresponsibility of load dispatchers. When a Turbo-generator is connected to grid, thespeed of each machine in the grid remains same to all other machines connected to thegrid. When an increase of load is required, more steam is admitted byopening/controlling the steam control valves. A basic understanding of turbine speedgovernors is necessary to maintain the central control of system parameters like speed,frequency, load, system voltages etc.

In the paragraphs that follow, the turbine governing has been explained usingtheoretical information, figures and descriptions of governing systems.

All turbines are equipped with speed governors. The purpose of the governor is tosense the instantaneous speed of the turbine in revolutions per minute, and totransmit a signal to the turbine control valves to open or close and maintain thedesired speed. Most governors do not hold absolutely constant speed as load changes,but are designed to permit the speed to drop as the load is increased. As load isincreased on the generator, the turbine speed tends to slow down. The speed governorspins slower (control arm moves toward LOW position), which results in the controlmechanism in increasing steam flow to the turbine (control valve opens). The governorstherefore control the steam supply to the turbine as well as ensure maximum safety ofthe machine and to the operating people when the turbine is on load.

Basically, the governors perform functions such as: -

Parallel operation/working of machines with other turbine-generatorsconnected together in a grid.

Output of each individual unit is controllable due to governing actions. The governor enables the electrical grid system to be to some extent self-

compensating to changes in load demand.

The governor enables the turbine-generators not connected together, in agrid, run as single unit. (Before synchronisation), and also enables speed of

turbine, kept under control.

The governor controls the rise in speed of all turbines irrespective of duty, ininstances of losing its electrical loads.

Turbine Governor System type-1


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Governors of the turbines basically control the steam flow to the turbine. The governorusually takes the form of spring-loaded weights mounted on a shaft assembly that isdriven by a worm & worm wheel from end of the H.P. shaft.

The weights, which are held by springs, tend to move outwards due to centrifugal forceand this movement is dependent upon the speed of the turbine shaft. The movement ofthe weights is arranged to operate on oil relay valve and this valve through an oilpressure relay system, opens or closes valves that admit steam to the turbine. Whenan increase of load is required, more steam is admitted to the turbine by opening thesteam valves.

Simple turbine governor type-2

The governor (A) is driven from the turbine shaft. An arm pivoted at (B) has attached toit, the governor weights and a moveable sleeve (C). Sleeve (C) is connected to a floatinglever (D) to which is attached the spindle (E) of the pilot relay valve and the spindle (F)of the main steam valve.

If the turbine shaft speed increases, the governor weight will move outwards causingsleeve C to lift; this also tilts floating lever (D). These movements uncover the port (G)of the pilot valve thereby allowing oil pressure to act on the top of the power piston (H).At the same time port (I) in the pilot valve, allows oil to drain from the bottom (J) of thepower piston. Due to this operation, the steam valve will move towards the closedposition, thus admitting less steam to the machine. During installation and alsoafterwards, the governor springs are adjusted periodically, so as to keep the range atwhich the governor operates between limits.

Loading on the machine is done/carried out by operating the hand wheel (K) thusopening the steam valve. The hand wheel (K) is normally on remote operation from the


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control panel by means of a reversible motor known as the speeder motor. Suchgovernors do not use the electro-hydraulic governors, which control the operation byelectrical interfacing units i.e. the electro-hydraulic converter. For detailed working ofGovernor, the drawing as shown below should be referred.

The percentage of control valve opening on each turbine depends upon the electricaloutput from that individual T.G, and in turn the entire system at the same speed(frequency). The system frequency decreases, as more electrical load is required. To

regain the previous frequency/speed, the amount of fuel fed to the steam generator isincreased adequately. Since with more customer load on the system, the frequencytends to decrease then the governors on all the system turbines need to operate (toopen) the control valves to admit more steam to Turbine and allow the system tosupply the extra load.

Mechanical Hydraulic System Block Diagram:

The speed acts on the radial spring governor, this in turn, affects the hydraulic relayand also, the anticipatory derivative system (acceleration component). Local or remoteadjustment on the speeder gear output is algebraically summed to act with the speedcomponent, thus the gain that is also regulated by local adjustment of governorreputation through the pilot oil regulating valve, passes through a minimum selectorthat has been provided with another signal of locally/remotely controlled load limitingdevice; minimum signal thus obtained from here is acted upon the Auxiliary and mainrelays of governor valves of H.P and I.P control valves and the pressure switching &relaying that effects to operate the release and bled steam check valve. The feedbacksignal of S.V pressure, vacuum unloading gear and anti-motoring device act on checkvalve and also for differential pressure switching (it compares the minimum selectorO/P as explained above); this forms the speeder gear runback as the feedback also.H.P and I.P control valves position are derived for valve offset adjustments.

The figure below shows the block diagram of mechanical-hydraulic system.


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The hydraulic oil used in the governor system is at a pressure up to 20 Bar. Bettercontrol can be achieved by increasing this pressure (more than 35 Kg/cm2 pressure)

but this leads to leaks and fires. For this reason some turbines in use today utilize theFire Resistant Fluid (F.R.F) system and thus the pressures can be increased withoutthe risk of fires.

Turbine bearings are lubricated with oil at between 0.3 and l.4 bar pressure depending

upon the make and type of machine. A high-pressure oil pump normally supplies this

oil and then pressure of oil is reduced as above.

Emergency governors (often referred as the Over speed Governor): -

The emergency governor is the final line of defense to protect the turbine from

dangerous over speeds. This device, when actuated rapidly closes all valves associatedwith steam supply to the turbine. Emergency governors are normally set to operateinstantaneously if turbine speed reaches 110% of rated (3300 rpm on a two poleturbine generator) or higher speeds. The emergency governor shuts off the steamsupply in the event of rotor speed increasing by more than 10% above its normalspeed. A sliding bolt or an eccentric ring is attached to the shaft. These are held inposition by means of a retaining spring.

The bolt or the ring flies out of the normal position .In doing so, it operates a trip andreleases the relay oil pressure, which is holding the emergency, valve open. Theemergency valve then shuts off the steam supply.


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The emergency governor is tested at periods by deliberately over-speeding the machinewhen the load has been taken off. Each of the twin bolts or rings is operated in turn.The one not being tested is made inoperative by a selector lever.

Droop of Turbo-generators:

Speed regulations of turbine also called the Droop, (or the proportional band), isdefined as the amount of speed change from no load to full load divided by the ratedspeed. Turbine Droop can be set in turbines either mechanically or electrically (In KWUturbines the provision of droop is made to range from 2.5% to 8.0% and to match thegrid frequency, chosen setting is 5%). If the governor speed regulation is required to beset at 5% then for a 3000 rpm turbine, the control valves will be open wide at a speedof 2925 rpm or 2 % below 3000 rpm. And likewise in other side of 50 Hzfrequencies, the control valves will be fully closed, at a speed of 3075 rpm, or 2 %above 3000 rpm. The droop setting in electronic system of EHG has been incorporatedin a module connected in series which receives input as the load

controller/comparator forming the error (MV-DV), and the droopcorrected/incorporated signal is fed to the final load controller module of the loadcontrol loop.

The amount of the inherent decrease in speed from no load to full load is called speedregulation, droop, or proportional band. The Droop is necessary in the control systemin order to sense a change in speed and thus to reposition the valves. In KWU turbine(of SSTPS droop is set at 5%, i.e. = 2.5% from 3000 rpm, or 50 Hz frequency), thedroop is set such that a biased zone is maintained from 3000 r.p.m to 3075 rpm.Beyond this speed until 3225 rpm, the droop gets affected automatically for unloading.


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Most grids operate automatically, to sense a change in system frequency as load goesup or down and to provide continuous signal to the controlled generating units inorder to maintain the desired 50 Hz system frequencies. If the cost of generation atgiven moment on the grid is such that a load of 100 MW should be generated by that

unit, that is the load that the automatic control will attempt to maintain The frequencybias of all controlling turbine generators on the grid is added up to determine thesystem frequency bias.

In order to view the economical loading on the sets connected in parallel an example ofa single unit can be considered for understanding the cost controlled situation. If thecost of generation at given moment on the grid is such that a load of 100MW should begenerated by that unit is the load that the automatic control will attempt to maintain.The frequency bias of all controlling turbine generators on the grid is added up, todetermine the system frequency bias.

Our single unit example was being cost controlled to provide 100MW and it went to104MW when system frequency dropped 1/10th of a cycle. With a 0 bias setting, assoon as the load increased to 104MW, the cost control would close the control valves torestore 100MW. At this point, the cost control is acting to oppose frequency correctionback to 50 Hz.

Further, let us review the frequency effects and the frequency bias on a particular unit,if it has been set to 4 MW per 0.1 Hz deviations. As soon as the system frequencydrops to 49.9 Hz, the cost signal representing desired generation from this unitchanges from 100 MW to 104 MW, under the added influence of frequency bias. If wecan again assume that the turbine governor would again have picked up 4 MW, nocontrol action occurs to reduce generation back to 100 MW and system frequency

should return to 50 Hz.

Of course, if no automatic load frequency control is being used, then the dispatchermust manually direct an increase or decrease in generation from the units under hiscontrol, in order to restore system frequency to 50 Hz. In this case, the dispatchercorrects system frequency in order to provide the correct frequency on a 24-hourbasis. This is usually done fairly close to midnight of each day. Instrumentation willadvice him how far above or below 50 Hz the system has been operating for the past 24hours. Knowing his system frequency bias, the dispatcher can then order more or lessload to be generated for a given period in order to restore system frequency to anaverage of 50 Hz for the past 24 hours. This phenomenon is particularly important forcontrolling system frequency specially in view of controlling power generation with

ABT.


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Transient speed rise (TSR):

When load rejection takes place,

speed shoots up temporarily before

settling down to steady state value

TSR gives the % speed rise on full

load throw-off

8-

6-

4-

2-

O- Steady state

Time

Steady State Regulation:

It is defined as the Ratio of % speed

change (from no load to Full load) to

the nominal rated speed.

%Regulation=100x(nmaxmin)/nnom

nmax.

nmin.

0% Load 100%

Load Frequency Control is shown in the figure below; it shows the single turbo-generator system supplying an isolated load. Main component are;

1. Fly ball Speed governor system2. Hydraulic Amplifier3. Linkage Mechanism4. Speed changer

Increase in frequency f causes the fly balls to move outwards so that B movesdownwards by a proportional amount k2f. The net movement of C is therefore yC =k1 kC PC + k2 fand movement D, yD= k3 yC + k4 yE. The movementyDdepending upon its sign opens one of the ports of the pilot valve admitting high-

pressure oil into the cylinder thus moving the main piston & opening the steam valvebyyE.

TSR


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In KWU turbines, the stop valve & control valve (one set) share a common body. Thepiston of the servomotor is subjected to disc spring force in the close direction andHydraulic pressure in the opening direction. Hydraulic Governor controls the steamsupply by operating the control valves. The fluid pressure under the piston determinesthe position of the valve; this is controlled by pilot valve of the turbine governor & thesecondary fluid oil system.

Electro-Hydraulic Governor (EHG)

Electro-Hydraulic Governor (EHG) works in parallel with Hydraulic governor at alltimes of requirements. Basically the Electro-Hydraulic Converter (EHC) is theconnecting element between the electrical and hydraulic parts of the turbine governingcontrol system for carrying out the Electro-Hydraulic Governing of the turbine.

The Electro-Hydraulic Governor (EHG) is beneficial in:-

Offering the flexibility, dynamic stability, dependability, excellent operationalreliability, Low transients and steady-state speed deviations at all instances.

Maintaining exact load frequency droop with high sensitivity. Providing reliable operation at times of grid isolation conditions. Operating the turbo-generator safely in conjunction with TSE.

In KWU turbines, Electro-Hydraulic Governing has been achieved through variouselectronic / selector modules configured in four modes of controls:

Admission Control mode, Speed Control mode,


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Load Control mode Pressure Controlmode.

The Hydraulic governor and the EHG system have been designed such that thegovernor with lower set point takes over or assumes the system control, as suchnormally, the set point of the Hydraulic Governor must be set above that of theElectro-Hydraulic Governor when EHG is effective. In cases, when EHG fails to causeshut-off, the set point that is, affected is that of Hydraulic Governor.

In such situations the Tracking Device provides a revised set point of 5-10% above theEHG set point and it causes increase in small load when the control is transferred toHydraulic-Governor. The tracking device is either switched on or off manually butwhen EHG failure or turbine trip occurs, the tracking device is switched offautomatically thus tracking under faulted operation mode is prevented or prohibited.More details on tracking actions are covered in the follow-up circuits of the speed/load

control modes.

Electro Hydraulic Converter details:

Electro Hydraulic Converter converts the electrical signal in to the hydraulic signalsand large positioning forces are generated in control valves. The electrical signal fromgovernor control circuit operates the sleeve and pilot valve spool; this regulates the tripfluid drain. Under steady state condition pilot is at central position; in deflectedposition, the control oil is admitted above or below the amplifier piston. The motion ofthe amplifier piston is transmitted via a lever to a camshaft, which actuates the sleevesof follow-up piston valves, causing secondary oil pressure to change. The speed, load,and pressure signals are measured and converted into conditioned signal in electronic

modules.

Admission Valve (spool) Controller


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Admission Valve (spool) Controller also referred as the position controller is Commonfor all three modes of EHG, and it supplies the operating current for driving theplunger coil. The Position controller loop uses a PID control mode for processingoutputs that provide the driving current signal to the plunger and regulate the oil

drains of HP/IP control valves (CV) ; thereby it controls steam supply into the turbine.

The current in the plunger coil is increased for closing the HP /IP CV and vice versa foropening of the HP /IP Control Valve. The reference signal therefore works in reversemanner (rise in the coil current for low reference condition). By using two Nos ofdifferential transformer (housed in EHC), feedback signal from the valve lift is derivedto ensure proper stationing of plunger spool.

Whenever current through the plunger coil gets interrupted or the electrical feedbackcircuit gets faulted, the reference value of the Hydraulic controller determines theactual valve position. Although the force to the plunger coil and to the control sleeve is,considerably smaller, but the regulating signal to the secondary auxiliary oil flow astransformed is quite large. The figure below gives various connections and modulesused in EHG.

Control Transfer of various controllers:

Three selectors have been used for specific functioning Speed controller output (hrnc)and the load controller output (hrpc) are passed through a Maximum selector (MAX-1)and the selected signal passes to a minimum selector (MIN-1) in such a fashion that attimes of over-speeding of turbine (during load throw-off situations), the input to theminimum selector: MIN1: takes care of transient condition of the load throw-off and is


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sufficient to check the turbine from over speeding. (During sudden load throw-off, overspeeding of turbine is effected and since 10.5 V is generated by a potentiometer thatgets algebraically summated with hrnc then it outputs voltage which is less than thatof the speed/load signal as selected from the MAX-1:)

The signal from the Minimum selector: MIN1: passes through another Minimumselector: MIN2: that receives the Pressure Controller output (hrPrc) signal asexplained in pressure controller loop. Finally through the last minimum selector: MIN2:, the control signal connects the Admission Valve (spool) Controller loop whichoutputs the driving current for the EHC plunger coil.

Operation of EHG in various modes

Start-up

Switching the supplies ON and setting the speed/load setter to zero puts the EHG inOperational condition. The hydraulic speed control eqpt and the start up eqpt of thehydraulic controller are set in upper end position. The actual speed is sensed sinceturbine already is in barring gear and by slow rising of speed reference the speedcontroller works /is in service; the turbine speed is then brought up situation forsynchronising TG with grid using speed controller.

Operation under load

Load controller can be taken in service after turbine is synchronised to control load inquick response and high linearity either as per LDC/AFDC or using variousmodes/sub loops explained in Load control. Frequency change is selected via the

integral action load controller to corresponding droop values and a sensitivity of 5Milli-Hz is obtained which meets the operational requirements of the present day large

grid. The output signal of the speed controllers is automatically matched to the output

signal of load controller from rated power on down to station load. The speed controller

then remains in standby mode only and stands ready to provide station load in of load

shading.

Shutdown

During normal shut down operation, the load controller is set to zero value. After thespeed controller has assumed control of TG set, the unit can be disconnected from thegrid.

Load shedding

In case of load shedding i.e. sudden separation of the generator, from the grid, theoutput signal of the load controller is immediately reduced to value below that of speedcontroller. Consequently due to minimum selection, the speed controller assumescontrol and returns turbine back to the set reference speed.

This reference speed practically coincides with the rated speed, since the speedcontroller is set to provide the station load during the start of operation under load.This provision improves the dynamic response of the closing of the main steam stop


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valve /control valve and keeps the turbo set speed from rising along the droopcharacteristic. An additional effect is the reduction of the speed oscillations. In case ofautomatic reclosing of the generator CB, the reduction of the load controller outputsignal below the speed controller output signal below the speed controller output

signal is cancelled and the initially selected load level restored.

Speed Control Mode

Speed Control Mode works during

Rolling (start-up or shut-down of the turbine), Speeding up of turbine until synchronisation, For effecting block loading & full loading of TG set at exceptional emergency

situations House-loading operation during fast load throw-off

For Controlling the TG set during rapid/large frequency fluctuations.

Regulating during Over-speeding;(When the speed of the TG set rises slightlyabove synchronous speed, the control action in speed control mode quicklyreduces the turbine speed very close to synchronous speed)

During load shedding with subsequent operation of the TG set in an isolatedgrid situation, (The speed controller assumes continuous TG set control in suchsituations)

Speed reference signal (nR) is varied (In the range of 0-3600 rpm):

Manually by Raise/Lower push buttons (using motorized potentiometer, By the synchronizer (when selected) or By follow up signal (explained separately). The speed reference (nR) can not be

raised when follow-up condition exists and dn/dt is less than monitoring (inthis situation lowering of nR gets slowed down.

The reference nRis varied in the range of 0-3600rpm and for minute operation duringsynchronizing, above the speed of 2800 rpm; a reducing gear lowers the speed of themotorized potentiometer to rate for exact speed adjustment. The speed reference (nR)cannot be raised when follow-up condition exists and dn/dt is less thanmonitoringrather in this condition raising or lowering phenomena ofnR gets slowed down whenthe speed reference (nR) is less than 2800. Two indicators have been provided in UCBpanel for monitoring speeds; of narrow range (2700-3300) and wide range (0-3600).

The Time-dependent speed reference signal ( nRTD )

The Time-dependent speed reference signal (nRTD) also referred as nR lim.influences thespeed reference nR considerably. During start-up of turbine this nRTD allows rising ofthe turbine speed at the highest permissible rate consistent with the conservativeoperation as decided by the TSE computed margin signal introduced between a DCamplifiers. The Integrator module performs this function rising with time like a ramp.

The slope of the integrator ramp can be adjusted over a wide range and is optimizedduring commissioning. Fast mode or the stop action facility, modify the final nR .Theoutput nRTD of the integrator module, is transmitted to the speed controller anddisplayed on the desk in the range 0-3600 rpm.


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Quantum of Follow-up signal is the difference between actual speed (nact) and offset of120 rpm and is effected (switched automatically) if load controller is operative, finalload reference (hrpc) is more than final speed reference (hrnc) by 10% and frequency isbetween 49-51 Hz OR if turbine is tripped (time elapsed) and speed reference (nr) is

equal to actual speed (nact) minus 60 rpm. During follow up, the quantum of thefollow-up signal is derived from the actual less the off-set (60-120 rpm) speed reference(nR) and difference is further added or subtracted as per the magnitude to causechange in speed reference (nR).

Blocking or the Stop nRTD of the speed signal is generated by an AND module inconditions i) speed >2850rpm, ii) nR is more than nRTD by 300 rpm and iii) an OR edoutput of many conditions as given below:-

1.TSE influence gets faulted (goes out of order or switched off) or EHC faultcondition appears AND turbine speed is more than 2950rpm. .

2. During the transition of control from electric to Hydraulic, the speed referencesignal becomes less than actual speed and if is more than 50 rpm, i.e. (nR- nact )< 50 rpm.;

3. IfnR > nRTD; pressure controller is in action OR Generator breaker not ON.This Block signal stops the integration (further) function of time dependent speedreference integrator, it blocks the already generated nRTD , and thus the speedcontroller input signal remains stay-put during stop action.

During rolling of the turbine, if between the speeds of 600-2820 rpm the rate of speedrise is very low i.e. less than 100 rpm per minute, then the dn/dt is less thanmonitoring signal appears to alarm the operator; it also blocks any further rise in

speed and brings back the speed reference to 600 rpm. dn/dt less than monitoringalarms the operator and takes care of low acceleration rate in turbine during rolling bysuitable output from the speed reference setting module, and at the critical speeds(between 600-2829 rpm) of the turbine.

The dn/dt is less thanmonitoring is derived from an AND gate module, its conditionsare i) nR is more than 600 rpm, ii) nact is less than 2850 rpm, iii) MSV is open (>0%),

vi) speed controller is selected & in action, v) Generator breaker is not on and afeedback signal ofdn/dt


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The difference of actual speed and time dependent speed signals (nact - nRTD) form theinput error of the Speed controller which outputs control signal (in the path asexplained in selection section) through the selection modules for driving the EHC andfinally establishing the EHG.

ACTUAL SPEED MEASUREMENT / FORMATION

The speed controller is poportional+derivative (P-D) action controller, with slopingcharacteristics. During steady-state condition, the speed controller outputs for betterload sharing by more nos. of turbo-generators connected in the grid as compared withpurely mechanical and Hydraulic Governor run T-G sets. Due to proportional control,


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a control error (off-set) is obvious in the speed reference but it does not matter much.An identical speed at synchronising point is possible to be achieved due to a Pre-feedfunction of pressure.

No load correction of speed is achieved by a feed forward signal that is obtained fromBoiler pressure controller during synchronising the T.G. set.

Load Control Mode

Load reference value pR is generated by means of a reference value setter module asdescribed in speed control mode and is derived manually by the operator adjusted(lower/raise) values by means of a remote driven motorized potentiometer, Load

controller is switched ON for bringing load controller in service., it can also be variedby the Automatic dispatch control (When switched ON ) ; it is also termed as pR ADS,and is basically the MW demand generated by the Coordinated Master Controlloop(CMC). The load demand signal is restricted within upper and lower MW loadinglimits as detailed in CMC loop description.

ADC influence ON appears when there is NO ADC fault if is selected. In case if, CMCor ADC gets faulted, it is automatically switched OFF; at this situation, matching orthe follow up is automatically taking place and loading of the TG set is subsequentlymade in standby basis.


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Load reference pRas obtained from the potentiometer in voltage signal, is fed to thehigh gain amplifier whose gain is adjusted by the rate of loading dp/dt or the loadmargins (In order to load the turbo-generator at highest possible rate consistent topermissible level of thermal stress, the upper/lower load margins are computed by

TSE, explained separately.). The Load gradient (load rate dp/dt.) is selected either byOn/Off push button or by follow-up command, for inclusion and it either modifies thehigh gain amplifier slope in both negative and positive sides of the amplifier or the TSEcomputed margins as explained above modify the high gain amplifier throughminimum selector. Upper release margin can result reduction of generated power andlower release margin can result unloading.

Time dependent reference signal also referred as pRLIM:

The Time dependent reference signal also referred as pRLIM is generated through ahigh gain amplifier and an integrator functioning in fast, normal and stop modes. Itfollows the ramp characteristic. The proportional leg of the response of the pRLIMcan beadjusted between 0-20% of MCR power of the TG Set. The response of the pRLIM ispurely integral, if the rate of rise of the pRLIM is limited to the load gradient selected,and at this situation the proportional channel is switched off.

This pRLIM rises during start-up at a rate (Mw/min) selected through load gradientsetter until final value. The pRLIM module is continuously allowing matching of theactual power output as long as the generator breaker is open (not synchronized) andensures smooth transition of speed (during start-up) to load controller (aftersynchronization).

The characteristic of pRLIM is linear 0-10v rising in 04 minutes. And it (pRLIM) acts

directly on the load controller without any intermediate control device. At conditionswhen TG is not synchronised, power error (Pr-Pact) signal governs the follow-up/tracking as explained below.

Tracking or the follow-up conditions in the load controller:

When the generator C.B. is on but the speed controller has taken over (due toconditions of follow-up) and speed controller remains in action until load controllersignal (pr) < (nr) of speed controller; then tracking gets released as soon as (pr) = (nr)and

When load shedding is less than station auxiliary power (p act < station load) and Mw

error reaches to more than 5 % or the generator circuit breaker is not made on; theload controller output, tracks to speed signal.

Stop signal in load control:

A Blocking or STOP command gets initiated at conditions shown below then theintegrator stops further integrating and pRLIM (the load demand) remains steady untilthe blocking signals are cleared or restored. The block conditions are met at conditionsas given below: -


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TSE switch is ON (selected) and it goes out of order (got faulted); it is enabled againafter the TSE is reset and it becomes O.k..

The load reference signal has been raised and the pressure controller goes/switchesto limit pressure mode of operation.

The STOP set point binary condition is/gets introduced.A highly and sensitive linear response with respect to power grid frequency is effected

by having the additional load reference componentpRf (this can be set as low aspossibly up to 5 milli Hz); it can be included for operating the load controller withfrequency influence included in the system.

This frequency influence was being excluded in the system sometimes in nineties,because the units were been operated at very high differentials of frequencies whenfrequency used to rise to approx 52 Hz at off-peak hours (in night hours) or

reduce/decline as low as 47.5 Hz at peak load hours. But now due to insistence byLoad Dispatch Center (LDC) to regulate the grid frequency at very tight margin and inorder to run systems on ABT mode, the frequency influence inclusion have becomemandatory. This is being referred as FGMO operation of units.

The load reference thus derived is fed to a minimum selector, which also is fed with theload limiter output. In order to restrict loading, Load limiter is preselected; the Loadlimit value (due to plant conditions) can be adjusted by the load limiter potentiometersand can also be seen on control desk. Even the reduction in grid frequency cannotcause the TG set to exceed the preset power level due to load limiting.

The output signal from the load limiter at the minimum selector in the form of the pR ,

that is the sum of all reference values acting on the load controller as reference signalor the desired value.

Actual Load signal is acquired threefold by means of the load measuring device andtransmitted to the controller comparator module but in case the signals of the threeparallel cannels deviate by more than 5% an alarm ACTUAL LOAD SIGNAL FAULTYalong with group alarm of Turbine Controller Faulty appears.

The difference of the actual measured power signal (pact) and the pRform the input ofthe load controller that outputs control signal and passes through a selection modulefor driving the EHC as explained in the admission control and selection diagram.

Load Controller consists of two plug-in modules first one to accommodate isolated griddetection and the second to accommodate dynamic loading of the generators & tohousing the tracking module. Load Controller is a proportional (P) + integral (I)controller to take care of small changes of load in Proportional mode and large changesin Integral mode operation. Due to this addition, the response of the controller isproportional for small changes of the load reference value but for the large changes ofreference value proportional plus integral mode refines the system operation.

In order to effect smooth transition from speed controller to Load Controller (Generatorbreaker open condition i.e. turbine not synchronised with grid) pR is comparedcontinuously with pact and control signal is matched ensuring bump less switching.


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During the time the speed controller is in control for start-up, shut-down or no loadoperation of TG set, the starting time constant of the TG set remains in dominatingwhereas during synchronised operation, the transfer functions of generator and thepower grid become of vital importance for controller optimisation.

Pressure Control Mode

The pressure controller controls the turbine load with respect to the main steamthrottle pressure and prevents the large pressure drop during fast loading (Quick loadincrease). The actual steam throttle pressure is measured in turbine area and pressurereference is derived from CMC loop, after comparison the deviated control signal (hRprc)is fed to the Proportional +Integral (PI) action type Pressure controller and its finaloutput is fed to the minimum selector-2 as described earlier in speed controller andload controller loops.

The Pressure Controller functions in two modes of operation: Initial pressure mode Limit pressure mode

Initial Pressure Mode:

In Initial pressure mode of operation, constant initial pressure (turbine inlet throttlepressure) is maintained and acts in proportional to pressure setting by minimizing thepressure error (Actual-Ref) even up to zero value. The power delivered by the TG set isdetermined by the boiler capability up to a maximum of power level as set by loadcontroller; increase of load above this is blocked thus, because it is connected to a

minimum selector. The difference pressure pbetween the reference and actual value

is controlled up to a value of 10 Kg/cm2 which is equal to the pressure drop of steamflow from boiler to the turbine control valve; it therefore ensures natural differentialpressure of the steam flowing from boiler to the turbine. A preset potentiometerequivalent to this pressure generates negative voltage to the controller input and it

biases the pressure differential p thus in the controller.

Limit pressure mode:

Limit pressure mode uses the boiler storage capacity and is effected either by push

button or gets automatically selected as soon as the pressure deviates to 10 Kg/cm2

from normal running pressure to operate the controller in Limit pressure mode. This

deviation of 10 kg/cm2 pressure signal already subtracted in the in the input of

Pressure controller, as described in the initial pressure mode, gets neutralized by

automatic switching.

Introducing the Limit Pressure Operation is therefore possible to regulate boilerpressure beyond a pre-set pressure of main steam and load in small or quickvariations, and is controlled until pre-set pressure is reached that is not possible innormal frequency based load control.

In fact the normal p from boiler to turbine (as explained in initial pressure mode) isnot persisting either due to increase in pressure at turbine side due to load throw,


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vacuum drop, extractions closure etc or due to pressure drop in boiler side by non-availability/reduced loading of mills, of coal feeders or any other causes. At this

situation due to equating/reducing of differential pressure p , an alarm is generatedso as to warn/alarm the operator of the discrepancy. When Limit pressure engagedalarm appears, the stop signal in the load control loop is also generated for blockingthe pRLIM signal from increasing/reducing.

All the three controllers are operative in such a manner that the governing of T.G isensured full proof and speeding or loading of the T.G. is best maintained as per thepressure in the system and Boiler or turbine follow mode is achieved with fullreliability and safety.

Co-ordinated Master Control (CMC) ensures co-ordination between Boiler & Turbine.The Co-ordinated Master Control (500 Mw) block diagram has been given below, wefind that the Unit master receives load demand signal from load dispatcher (ALC). AGNI computer/SPCM is provided with the system to decide target value Z0, Run Backload limits & load rate required for proper generation, Boiler master controller, Turbineload set-point etc., through which the CMC is ensured. The load demand signal asgenerated in CMC, for turbine control reaches to point D of EHG block diagram (referthe load control mode) and is switched for inclusion to operate the EHG incoordination with grid dispatch ADC demand. Boiler Follow or Turbine follow modes


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are decided by switching suitably and loaded TG operation is achieved as explained indetails of CMC mode of integrated control in C&I , ACS section.


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KWUGoverning(Simplifiedschematic)


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TURBINE PROTECTION SYSTEM

Turbine protection system performs to cover the following functions: -

a. Protection of turbine from inadmissible operating conditions.

b. In case of plant failures, protection against subsequent damages.

c. It restricts occurring failures to minimum.

Standard turbine protection system comprises the following:

Mechanical/hydraulic turbine protection. Electrical turbine protections.

BLOCK DIAGRAM OF TURBINE PROTECTION AND ATT


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Mechanical Hydraulic Turbine Protection

The design of mechanical hydraul ic protection equipment is in accordancewith hydraulic break current principle and consists of following:

a. Two manual trip devices (main trip valves)

b. Two speed monitors (over speed trip device)

c. One hydraulic low vacuum device

d. Two solenoid valves for trip initiation (remote solenoid valves)

As explained earlier, turbine stop and control valves are tripped to close position if thetrip oil pressure is reduced below the minimum value. The main trip valves allow rapiddraining of trip oil in case they are operated either manually or automatically by thereduction of aux. trip oil pressure. Aux. trip oil pressure can be drained because ofactuation of hydraulic low vacuum trip device, over speed trip device or thrust bearingtrip device. The principle of functioning of individual hydraulic trip devices is explainedin details under the chapter of Automatic Turbine Testing System.

Remote trip solenoids act as interfaces between mechanical hydraulic and electro-hydraulic protection equipment of turbine. Upon receiving the electrical trip command,the solenoids get energised and close the valves. Thus control oil supply to main trip

values is cut off leading to their closure.

Electrical Hydraulic Turbine Protection

Electrical turbine trip equipments comprise two-channel redundancy and function onoperating current principle. All electrical trip criteria act on the two remote tripsolenoid valves to energise the solenoids.

The electro-hydraulic turbine protection equipment features -

-Two solenoid operated valves for trip initiation (Remote trip solenoids).-Emergency trip contactor cabinet containing trip channels 1 and 2-Monitors with signal conditioning-One substitute channel to ensure uninterrupted transmission of eventualturbine trip signals during testing by ATT.

The remote trip solenoids (RTS) have already been described. Operation of any onechannel causes energising both solenoid-operated valves leading to turbine tripeventually. Transmitters that cause a trip in the case of any electrical tripping signalare conditioned and monitored via binary signal conditioning of the ATT system or viathe central analog/binary signal conditioning.


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TURBINE PROTECTION FOR 200MW KWU SETS


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Turbine Trip Actuation Circuits

The turbine protection system is sub divided into two parts:

a. Protective circuits for the standard turbine protection equipments or criteria.b. Protective criteria from other areas.

Standard criteria are specified by the turbine manufacturer and are responsible for fullprotection of turbine under various specific conditions, which are:

1. Manual tripping devices (Turbine trip gear local operating lever)

2. Speed monitors (over speed trip devices)

3. Thrust bearing trip device

4. Hydraulic low vacuum trip device

5. Electrical low vacuum trip device

6. Lub oil pressure protection

7. Fire protection

8. Manual turbine tripping (electrical UCB switch)

Protection criteria from other areas are as follows:

Boiler trip (MFR) Boiler drum level very high ( > + 225 mm wcl ) Main steam temperature trip ( < 480 o C ) Trip from functional group control (ATRS shut-down programme) Generator trip

Like low vacuum tripping (electrical) the low steam temperature protection alsocomprises 'Arming' and 'Disarming' features to facilitate re-start of turbine, under lowmain steam temperature conditions.

Over Speed Trip Device

Two hydraulically operated over speed trips are provided to protect the turbine againstover speeding in the event of load coincident with failure of speed governor.


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OVER SPEED TRIP DEVICE

1. Bearing pedestal2. Spindle3. Spring4. Piston5. Piston body

6. Spring7. Pawl8. Over speed trip bolt9. Shaft journal10. Limit switch

c: Return Oil

u: Auxiliary Stratup Oilx: Auxiliary Trip Oil

When the preset over speed is reached, the eccentric fly bolt activates the piston andlimit switch via a pawl. This connects the auxiliary trip oil to drain therebydepressurising it. The loss of auxiliary trip medium pressure causes the main tripvalve to drop, which in turn causes the trip oil pressure to collapse.

Low Vacuum Trip Device


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In the hydraulic low vacuum trip device, a compression spring set to a specific tensionpushes downwards against diaphragm, the topside of which is subject to the vacuum.If the vacuum is too weak to counteract the spring tension, the spring moves valve 6downwards. The pressure beneath valve is thereby dispersed and the auxiliary trip

medium circuit is connected to drain. The resultant depressurisation of the auxiliarytrip oil actuates main trip valves MAX51 AA 005 and MAX51 AA 006 thereby closing allturbine valves.

The electrical tripping on low vacuum occurs through a pressure switch on thevacuum line to mechanical hydraulic low vacuum trip device also at the samecondenser pressure. When turbine is started up again, this pressure switch isinterlocked against a second pressure switch, which monitors this condition andprevents continuation of tripping initiation when condenser pressure is high.

Thrust Bearing Trip Device

The function of the thrust bearing trip is to monitor the shaft position in the bearingpedestal and, if a fault occurs, to depressurize the auxiliary trip medium and thus thetrip oil in the shortest possible time, thereby tripping the turbine.

1. Compressionspring

2. Bearing pedestal3. Piston4. Valve body5. Turbine shaft6. Pawl7. Torsion spring8. Piston9. Compression

spring10.Limit switch11.Knoba: Test Oilc: Return Oil

u: Aux. Startup Oilx: Aux. Trip Oil

The two rows of tripping cams, which are arranged on opposite sides of turbine shaft,have a specific clearance, equivalent to the permissible shaft displacement, relative topawl of the thrust-bearing trip. If the axial displacement of the shaft exceeds thepermissible limit, the cams engage pawl, which releases a piston to depressurise theauxiliary trip oil and at the same time to actuate limit switch.

Electrical tripping of turbine is achieved by fire protection along with closure/stoppageof total control oil supply to turbine governing system by tripping the emergency stopvalve on the control oil line. The fire protection trip is achieved by manual Pushbuttonin UCB or automatically by very low MOT level (- 150 mm below the normal workinglevel 'O'). Please refer to the associated logics at the end of this chapter. Also fireprotection-1 (automatic actuation) gets bypassed if the barring gear valve is 'notclosed'.


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FIRE PROTECTION-1 CHANNEL-1


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FIRE PROTECTION OIL TANK LEVEL MONITOR


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AUTOMATIC TURBINE TESTING (ATT)

INTRODUCTION

Under the present crunch of power crisis, the economy dictates long intervals betweenturbine overhauls and less frequent shutdowns. This warrants testing of equipmentsand protection devices at regular intervals, during normal operation.

The steam stop valves and control valves along with all the protective devices on theturbine must be always maintained in serviceable condition for the safety andreliability. The stop and control valves can be tested manually from the location butthis test does not cover all components involved in a tripping. Also, manual testingalways poses a risk of mal-operation on the part of the operator, which might result inloss of generation or damage to machine components.

These disadvantages are fully avoided with the Automatic Turbine Test.

A fully automatic sequence for testing all the safety devices has been incorporatedwhich ensures that the testing does not cause any unintentional shutdown and alsoprovides full protection to turbine during testing.

SALIENT FEATURES

The Automatic Turbine Tester is distinguishable by following features:

Individual testing of each protective device and stop/control valve assembly.

Automatic functional protective substitute devices that protect turbine duringATT.

Only its pretest is carried out without any faults i.e. if the substitute circuit ishealthy, the main test begins.

Monitoring of all programme steps for execution within a predefined time. Interruption if the running time of any programme step is exceeded or if tripping

is initiated.

Automatic re-setting of test programme after a fault Full protection of turbine provided by special test safety devices.

Automatic Turbine Testing extends into trip oil piping network where total reduction oftrip oil pressure due to actuation of any protective device, is the criteria for thesatisfactory functioning of devices.

During testing, general alarm or the cause of tripping is also initiated so that this partof alarm annunciation system also gets tested. Also, during testing, two electricallyformed values of 3300 rpm take over protection of turbine against over speed.

The testing system or ATT is sub divided in two functional sub-groups.


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Each sub-group contains the device and all associated transmission elements forinitiation of a trip.

AUTOMATIC TESTING OF PROTECTIVE DEVICES

ATT sub group for protective devices covers the following devices.

1. Remote trip solenoid-1.2. Remote trip solenoid-2.3. Over speed trip device.4. Hydraulic low vacuum trip device.5.Thrust bearing trip device.

During normal operation, protective devices act on the stop/control valves via the maintrip valves. Whenever any tripping condition (hydraulic/electrical) occurs, theprotective device concerned is actuated. It drains the control/aux. trip oil, closing themain trip valves. The closure of main Trip Gear drains the trip oil, causing

stop/control valves to close.

During testing, trip oil circuit is isolated and changed over to control oil by means oftest solenoid valves and the changeover valve. This control oil in trip circuit preventsany actual tripping of the machine. However, all alarm/annunciation are activates asin case of an actual tripping. Refer Fig.

ATT for protective devices broadly incorporates the following sub programmes:

a. Preliminary test programme.

b. Hydraulic test circuit establishment.

c. Main test programme.

d. Reset programme.


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ATT SAFETY DEVICES

Preliminary Test

In preliminary test programme, the substitute circuit elements and the circuit aretested for their healthiness; the turbine is fully protected against any inadvertenttripping during ATT. If any fault is present; further testing is inhibited. Duringpreliminary test, following steps are performed.


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Test solenoids (TSX) become energised. Build-up of control oil pressure upstream of changeover valve is monitored. Test solenoids de-energised one by one & drop of control oil pressure is monitored. If all steps are executed within a specified time period pre-test is said to be

successfully.

Hydraulic Test Circuit Establishment

If no fault is present during preliminary test; command is automatically given toestablish hydraulic test circuit (substitute circuit). The hydraulic test circuit isresponsible for the supply of control oil in trip oil circuits. The test solenoids valves areagain energised building up the control oil pressure upstream of changeover valve. Atthis moment another solenoid (SVX) gets energised, draining control oil and creatingdifferential pressure across the changeover valve, it assumes upper (test) position andannunciation is flashed to this effect. With changeover valve in its test position, controloil flows in the trip oil piping. After successful establishment of hydraulic test circuitcommand goes to initiate the main test, in which individual devices can be checked.


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Main Test

During main test programme, the associated hydraulic test signal transmitter with theexception of remote trip solenoids provides the necessary signal to actuate protective

devices. The protective device under test operates and drains the aux. trip oil. Turbinetrip gear (main trip valves) is closed after trip oil pressure drains and associatedalarms flash.

Reset Programme

The resetting programme automatically starts after the main test is over. The resetsolenoid valves energise and supply control oil in aux. start-up oil circuit to reset maintrip valves and protective devices, which have tripped from their normal positions.Once they return to their normal position, trip oil and aux. trip oil pressure can bebuilt-up and monitored. If oil pressure is satisfactory, reset solenoids along with testsolenoid valves and SVX get de-energised, deactivating hydraulic test circuit andresetting circuit.

TESTING OF PROTECTIVE DEVICES

The main trip valves and remote trip solenoid valves have already been discussed inprevious chapters; hence the remaining ones will be taken up here.

Over speed Trip Device

Trip consists of two eccentric bolts fitted on the shaft with centre of gravity displaced

from the shaft axis. They are held in position against centrifugal force by springswhose tensions can be adjusted corresponding to 110% - 111% over speed. When overspeed occurs, the fly weights (bolts) fly out due to centrifugal force and strike againstthe pawl and valves, draining aux. trip oil pressure and tripping the turbine.


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HYDRAULIC TEST SIGNALTRANSMITTER (HTT) FOROVER SPEED TRIP DEVICE

During ATT, the

associated hydraulictest signal transmitter(HTT) becomes 'on'; spoolvalve slowly moves downto gradually build-up testoil (control) pressurebeneath the flyweights.At pre-defined test oilpressure fly weight oneand two operate to

actuate individual pawland spool arrangementsbringing in theassociated alarm. For re-setting, spool moves-upand when test oilpressure is fully drained,aux. start up oil (controloil from 'reset' solenoids)pressure resets thedevices to their normal

I. Control OilII. Test OilIII. Aux. Trip OilIV. Aux. Startup OilV. Drain Oil

1. Limit switch (normal)2. Limit switch (test)3. Valve for Test Oil4. Actuator

Low Vacuum Trip Device

With deterioration of vacuum, pressure builds-up over the diaphragm, the spool valvemove down, causing valve also to move toward lower position. The aux. trip oil pressuredrains, tripping main trip valves and the turbine stop/control valves. During ATT, afterhydraulic test circuit is established, the HTT (Hydraulic Test sign

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