KWU Turbine System LPBP

TURBINE SYSTEM

Prepared by: K.V. Vidyanandan Sr. Manager (EDC) NTPC-Singrauli

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STEAM TURBINE: GENERAL DESCRIPTION The 210 MW KWU turbine is condensing, tandem compounded, reheat type and single shaft 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. The turbine rotors are rigidly coupled with each other and with generator rotor. HP turbine has throttle control. The steam is admitted through two combined stop and control valves. The lines leading from HPT exhaust to reheater have got two cold reheat swing check NRVs. The steam from reheater has got two cold reheat swing check NRVs. The steam from reheater is admitted to IP turbine through two combined stop and 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, LPT and front rows of LPT have inverted T roots and are shrouded. The last stages of LPT are twisted; drop forged moving blades with fir-tree roots. Highly stressed guide blades of 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, a double wedged journal bearing at the front and combined thrust/journal bearing adjacent to front IP rotor coupling. IP and LP rotors have self-adjusting circular journal bearings. The bearing pedestals of LPT are fixed on base plates where as HPT front and rear 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 EDC-Singrauli

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expansion is not restricted. HP & IP casings with their bearing pedestals move forward from 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 any axial/radial flanges. This kind of design prevents any mass accumulation and thermal stresses. Also perfect rotational symmetry permits moderate wall thickness of nearly equal strength at all sections. The inner casing is axially split and kinematically supported by outer casing. It carries the guide blades. The space between casings is filled with the main steam. Because of low differential pressure, flanges and connecting bolts are smaller in size. Barrel design facilitates flexibility of operation in the form of short start-up times and higher rate of load changes even at high steam temperature conditions.

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IP TURBINE

1. TURBINE ROTOR 2. OUTER CASING 3. OUTER CASING

4. INNER CASING 5. 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 an inner casing axially split, kinematically supported and carrying the guide blades. The hot reheat steam enters the inner casing through top and bottom centre. Arrangement of 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 of rigid 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 is supported at four points on longitudinal beams. Two rings carrying the last guide blade rows are also attached to the middle shell. Refer to Figure.

1. OUTER CASING 2. OUTER SHELL 3. INNER SHELL

4. INNER SHELL 5. 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 movements perpendicular to machine axis are prevented by fitted keys. Bearing housings are connected to HP-IP casings by guides, which ensure central position of casings while axially 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 not restricted. The outer casing of LP turbine expands from its fixed points towards generator. Bellows expansion couplings take the differences in expansion between the outer casing and fixed bearing housing. Hence HPT rotor & casing expands towards bearing no (1) while IPT rotor expands towards generator. The LPT rotor expands towards generator. The magnitude of this expansion is reduced by the amount by which the thrust bearing is moved in the opposite direction due to IPT casing expansion.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

HP FRONT PEDESTAL HP REAR PEDESTAL LP FRONT PEDESTAL LP REAR PEDESTAL HPT OUTER CASING IPT OUTER CASING LPT OUTER CASING HP FRONT PEDESTAL BASE PLATE HP REAR PEDESTAL BASE PLATE LP FRONT PEDESTAL ANCHOR POINT

11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

LP REAR PEDESTAL ANCHOR POINT LP OUTER CASING ANCHOR POINT HPT INNER CASING IPT INNER CASING LP INNER OUTER CASING LP INNER OUTER CASING HP INNER CASING ANCHOR POINT IP INNER CASING ANCHOR POINT LP INNER –OUTER CASING ANCHOR POINT 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 movements perpendicular to machine axis are prevented by fitted keys. Bearing housings are connected to HP-IP casings by guides, which ensure central position of casings while axially 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 not restricted. The outer casing of LP turbine expands from its fixed points towards generator. Bellows expansion couplings take the differences in expansion between the outer casing and fixed bearing housing. Hence HPT rotor & casing expands towards bearing no (1) while IPT rotor expands towards generator. The LPT rotor expands towards generator. The magnitude of this expansion is reduced by the amount by which the thrust bearing is moved in the opposite direction due to IPT casing expansion. 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 oil passes through oil cooler (2 nos.) before can be supplied to the bearing. Under emergency, 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 to prevent 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 stopped automatically by means of SGC oil sub-group of automatic control system. The various logics 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 entire turbine with lubricating oil and control oil, which is connected to the governing rack.

1. 2. 3. 4. 5. 6. 7. 8.

Threaded ring Pump casing, upper Journal Bearing Oil pipe Bearing bushing Seal ring Impeller Feather key

9. 10. 11. 12. 13. 14. 15. 16.

Feather key Journal + Thrust Brg Ring Vent pipe Oil inlet vessel Hyd. Speed Xter Oil line Turbine shaft

17. 18. 19. 20. 21. 22. 23.

Coupling Elect. Speed Xter Permanent Magnet Pump shaft Spacer sleeve Pump casing, lower Oil tube

TURBINE TURNING GEAR The turbine is equipped with a hydraulic turning gear assembly comprising two rows of moving blades mounted on the coupling between IP and LP rotors. The oil under pressure supplied by the AOP strikes against the hydraulic turbine blades and rotates the shaft at 110 rpm (220 rpm under full vacuum condition). In addition, provisions for manual barring in the event of failure of hydraulic turning gear, have also been made. A gear, machined of the turning gear wheel, engages with a Ratchet & 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 electro2

hydraulically controlled seal steam pressure control valve. A pressure of 0.01 Kg/cm (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 steam pressure 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 seal steam pressure control valve. The last stage leak-off of all shaft seals is sent to the gland 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 at full load.

: 147.1 kg/cm2. 535 oC.

HRH pressure/ temp at full load.

: 34.23 kg/cm2. 535 oC.

Permissible SH / RH temp variations.

o : 543 C. (Long time value but keeping within annual mean 535oC.) : 549 oC. (400 hours per annum) o : 536 C. (80 hours per annum & max. 15 min in individual case) : 76 mm Hg with CW inlet temp 33 oC.

Condenser pressure. STEAM TEMPERATURE

80-hr/ annum maximum. per 15 min., in individual cases oC

Rated value Annual mean value

Long time value keeping 400h within annual annum mean value

oC

oC

oC

Initial steam

535

543

549

563

IPT SV Inlet

535

543

549

563

HPT exhaust

343

359

Extraction 6

343

359

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500 + (special 425 case) 500 + (special 425 case) 11

Extraction 5

433

438

473

Extraction 4

316

326

366

Extraction 3

200

211

255

Extraction 2

107

127

167

Extraction 1

62

82

127

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 turbine is immediately reloaded or the boiler immediately reduced to minimum load if no-load operation is maintained. Permissible differential temperature - No time limitation between parallel steam supply lines - Short time period

: :

17 K. 28 K.

In the hottest line the limitations indicated for initial steam and reheat temperature must not be exceeded. Turbine Extractions (Pressure/ Temperature) at 200 MW Extraction Pres. (bar)

Temp.

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

+5.0 mm

+ 6.0 mm

-2.0 mm

- 3.0 mm

+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

IPT Diff. Expansion. LPT Diff. Expansion.

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

+/- 45 oC

Turbine Bearing Metal Temperature 76 oC

Maxm Oil Temperature before coolers 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 with electro-hydraulic governor; fully backed-up by a hydraulic governor. The measuring and processing of electrical signal offer the advantages such as flexibility, dynamic stability and simple representation of complicated functional systems. The integration of electrical and hydraulic system is an excellent combination with following advantages: •

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 functions are 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 turbine protection 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 pass through them to the main trip valves. The solenoids gets energised whenever any electrical 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 solenoids gets connected to drain and the upstream will be blocked. By resetting Unit Trip Relays (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 protective circuit. All turbine tripping take place through these valves. The control oil from remote trip solenoids is supplied to them. Under normal conditions, this oil flows into two different circuits, called as the Trip Oil and Auxiliary Trip Oil. The Trip Oil is supplied to the Stop Valves (of HP Turbine and IP Turbine), Auxiliary Secondary Oil circuit and Secondary Oil circuits. The Auxiliary Trip Oil flows in a closed loop formed by main trip valves and turbine hydraulic protective devices (Over Speed trip device, Low Vacuum trip device and Thrust Bearing trip device). The construction of main trip valves is such that when aux. trip oil pressure is adequate, it holds the valves' spools in open condition against the spring force. Whenever control oil pressure drops or any of the hydraulic protective devices are actuated, the main trip valves are tripped. Under tripped condition, trip oil pressure is drained rapidly through the main valves; closing turbine stop and control valves. Refer to 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, for opening the stop valves and releasing the control valves for opening. The starting device consists of a pilot valve that can be operated either manually by means of a hand wheel or by means of a motor from remote. It has got port connections with the control 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 link arrangement. Before start-up, the pilot valve is brought to its bottom limit position by reducing the starting device to 0% position. This causes the hydraulic governor bellows to be compressed thus blocking the build-up of secondary oil pressure. This is known as control 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 and protective devices) and into the start-up oil circuit (to reset turbine stop valves). A build-up of oil pressure in these circuits can be observed, while bringing the starting device 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 EDC-Singrauli

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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 in equilibrium with hydraulic governor's spring tension and primary oil pressure (turbine speed), and raises the aux. sec. oil pressure; closing the aux. follow-up drains of hydraulic governor.

STARTING DEVICE ACTING ON SPEEDER GEAR EDC-Singrauli

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SPEEDER GEAR The speeder gear is an assembly of a bellow and a spring, the tension of which can be adjusted 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 speeder gear position, which alters the spring tension on the top of the bellow. The bellow is also subjected to the primary oil pressure, which is the feedback signal for actual turbine 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 assembly is rigidly linked to the sleeves of the auxiliary follow-up piston valves. The position of the sleeve changes with the equilibrium position of the bellow.

SPEEDER GEAR

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HYDRAULIC SPEED TRANSMITTER The hydraulic speed transmitter runs in the MOP bearing and operates on the principle of a centrifugal pump. The variation of pressure in the discharge line is proportional to the square of the machine speed. This primary oil pressure acts as the control impulse for the hydraulic speed governor. The transmitter is supplied with control oil via an oil reservoir. An annular groove in the speed transmitter ensures that its inside is always covered with a thin layer of oil to maintain a uniform initial pressure. Excess oil drains into the bearing pedestal.

CURVE SHOWING TURBINE SPEED Vs PRIMARY OIL PRESSURE

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AUXILIARY FOLLOW-UP PISTON VALVES Two Auxiliary Follow-up pistons are connected in parallel and the trip oil is supplied to them through orifice. The sleeves of these valves are attached to the speeder gear bellow link. The position of the sleeve determines the draining rate of trip oil through the ports. Accordingly the trip oil pressure downstream of these valves changes. Oil downstream of auxiliary follow-up pistons circuit is termed as AUXILIARY SECONDARY OIL. Hence, aux. follow-up piston valves can be said to control auxiliary secondary oil pressure.

SEQUENCE TRIMMING DEVICE The function of the sequence trimming device or HP/IP TRIM DEVICE is to prevent any excessive HP turbine exhaust temperature due to churning. It changes response of 2 main and reheat control valves. When the reheat pressure is more than 32 Kg/cm and load less than 20% the IP turbine tends to get loaded more than HP turbine. The steam flow through HP turbine tends to fall to very minimum, causing a lot of churning and excessive exhaust temperature. The trim device operates at this moment trimming the IP turbine control valve. The control valves of HPT open more to maintain flow of steam, reducing the HPT exhaust temperature. It consists of a spring-loaded piston assembly, which is supported by control oil pressure 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 reheat 2 pressure is more than 32 kg/cm the 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 followup piston valves of IP control valves by means of a lever. Upon tripping, the trim device alters the spring tension of follow-up pistons of IP pistons control valves, draining the secondary 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 the pilot 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 the amplifier piston. The other side of amplifier is connected to the drain. The movements of the amplifier piston are transformed into rotation of a Camshaft through a piston rod and a lever assembly. A feedback linkage mechanism stabilises the system for one particular aux. secondary oil pressure.

1. 2. 3. 4. 5. 6. 7. 8. 9.

Amplifier piston Follow-up piston Sleeve Shaft Lever Feedback lever Pilot valve Compression spring Adjusting screw

a : Control oil b : 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 valves and Aux Sec. oil line, in order to prevent the turbine from reaching high speed in the event of sudden turbine load throw-off. The control valves are operated (closed) by the load-shedding relay when the rate of load reduction exceeds a certain value. The solenoid drains the IPCV secondary oil directly. Direct draining of IP Sec oil circuit causes the reheat valves to close without any significant delay. The HP control valves are closed due to draining of aux. secondary oil before the hydraulic amplifier, by the second solenoid valve. The extraction stops valves controlled by IP secondary oil acting through extraction valves relays also get closed. After an adjustable time delay (approx. 2 seconds) the solenoid valves are re-closed and secondary oil pressure corresponding to 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 upon position of sleeves of follow-up piston valves; which determines the amount of drainage of trip oil.

FOLLOW-UP PISTON VALVES

There are in all twelve follow-up piston valves. Six of them are associated with hydraulic 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 is also termed as HYDRAULIC MINIMUM SELECTION of governors. The drain port openings of follow-up pistons of hydraulic amplifier depends on auxiliary secondary oil pressure, upstream of aux. follow-up pistons; and that of electro hydraulic converter, on the piston of pilot spool valve of the elector-hydraulic converter (i.e. EHC output).

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TEST VALVE

1. Bolt 2. Hand wheel 3. Spindle 4. Cover 5. Oil Seal 6. Bushing 7. O-ring 8. Valve Cover 9. Valve Body 10. Trip Oil 11. Piston sleeve

12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

EXTRACTION N.R.VS AND

Trip Oil Piston valve Spring plate Spring Spacer Bottom cover Trip oil Drain Trip oil Startup oil

Each of the HP and IP stop valves' servomotors receives trip oil through their associated test valves. The test valves have got port openings for trip oil as well as start-up oil. The test valves facilitate supply of trip oil pressure beneath the servomotor disc. (Stop valve open condition, under normal operation). For the purpose of resetting stop valves after a tripping, startup oil pressure is supplied to the associated test valves, which moves their spool downwards against the spring force. In their bottom most position the trip oil pressure starts building up above the stop valve servomotor piston while the trip oil beneath the disc gets connected to drain. When start-up oil pressure is reduced the test valve moves up draining trip oil above the servomotor piston and building the trip oil pressure below the disc, thus opening the stop valve. A hand wheel is also provided for manual operation of test valves.

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 condensed steam into the turbine from heaters on account of high levels in the heaters. There are two NRVs provided in each of these extraction lines and is force closing type. Both these valves are free-swinging check type, however the first valve is equipped with an actuator. In case of flow reversals, both the valves are closed automatically. The actuator assists the fast closing of the first valve. The mechanical design of force-closed valves is such that they are brought into freeswinging position by means of trip oil. They are open as soon as differential pressure is sufficient. If the trip oil pressure falls, the spring force closes the valve when steam pressure either falls or is lowered (reduced load). EDC-Singrauli

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The extraction valve relay, its changeover valve and its solenoid valve control the trip oil to each of the actuators of force closing type valves. Extraction valve relay actuates the FCNRVs in proportion to secondary oil pressure. By suitable adjustment of its spring, the secondary oil pressure at which the FCNRVs will be released for opening can 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 pressure loss shall be more leading to loss of efficiency. In case of turbine trip or sudden load reduction, by energising the associated solenoid valve, draining of trip oil pressure through extraction valve relay assists closing movements of FCNRVs. In both the cases the actuator is devoid of trip oil and its spring force closes the NRV. Extraction (4) FCNRV solenoid is also energised additionally by lower differential pressure in the extraction line.

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c

:

Return Oil

Secondary Oil

x

:

Trip Oil

Secondary Oil

x1

:

Trip Oil

b

:

Control Oil

b1

:

b2

:

COLD REHEAT SWING CHECK VALVE Two numbers of swing check valves are provided on the CRH lines from which the steam is drawn for HPH-6. Their pilot valves via their rotary servomotor in proportion to secondary oil pressure operate the CRH NRVs. They open out fully when main control 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 steam flow by the hydraulic actuator, so that when the steam flow ceases in the normal direction, they are closed by the torque of rotary servomotor. Even when the pressure of secondary oil has not built up sufficiently, NRVs can be opened up like safety valves when 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 ventilation of 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 and control equipments) together with power from other plants through process of synchronization 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 major responsibility of load dispatchers. When a Turbo-generator is connected to grid, the speed of each machine in the grid remains same to all other machines connected to the grid. When an increase of load is required, more steam is admitted by opening/controlling the steam control valves. A basic understanding of turbine speed governors 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 using theoretical information, figures and descriptions of governing systems. All turbines are equipped with speed governors. The purpose of the governor is to sense the instantaneous speed of the turbine in revolutions per minute, and to transmit a signal to the turbine control valves to open or close and maintain the desired 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 is increased on the generator, the turbine speed tends to slow down. The speed governor spins slower (control arm moves toward “LOW” position), which results in the control mechanism in increasing steam flow to the turbine (control valve opens). The governors therefore control the steam supply to the turbine as well as ensure maximum safety of the 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-generators connected 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 selfcompensating to changes in load demand.

•

The governor enables the turbine-generators not connected together, in a grid, 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, in instances of losing its’ electrical loads.

Turbine Governor System type-1 EDC-Singrauli

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Governors of the turbines basically control the steam flow to the turbine. The governor usually takes the form of spring-loaded weights mounted on a shaft assembly that is driven 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 force and this movement is dependent upon the speed of the turbine shaft. The movement of the weights is arranged to operate on oil relay valve and this valve through an oil pressure relay system, opens or closes valves that admit steam to the turbine. When an increase of load is required, more steam is admitted to the turbine by opening the steam valves.

Simple turbine governor type-2 The governor (A) is driven from the turbine shaft. An arm pivoted at (B) has attached to it, the governor weights and a moveable sleeve (C). Sleeve (C) is connected to a floating lever (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 causing sleeve 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 the power piston. Due to this operation, the steam valve will move towards the closed position, thus admitting less steam to the machine. During installation and also afterwards, the governor springs are adjusted periodically, so as to keep the range at which the governor operates between limits. Loading on the machine is done/carried out by operating the hand wheel (K) thus opening the steam valve. The hand wheel (K) is normally on remote operation from the EDC-Singrauli

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

The percentage of control valve opening on each turbine depends upon the electrical output 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 is increased adequately. Since with more customer load on the system, the frequency tends to decrease then the governors on all the system turbines need to operate (to open) the control valves to admit more steam to Turbine and allow the system to supply the extra load. Mechanical –Hydraulic System Block Diagram: The speed acts on the radial spring governor, this in turn, affects the hydraulic relay and also, the anticipatory derivative system (acceleration component). Local or remote adjustment on the speeder gear output is algebraically summed to act with the speed component, thus the gain that is also regulated by local adjustment of governor reputation through the pilot oil regulating valve, passes through a minimum selector that has been provided with another signal of locally/remotely controlled load limiting device; minimum signal thus obtained from here is acted upon the Auxiliary and main relays 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 feedback signal of S.V pressure, vacuum unloading gear and anti-motoring device act on check valve and also for differential pressure switching (it compares the minimum selector O/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. EDC-Singrauli

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The hydraulic oil used in the governor system is at a pressure up to 20 Bar. Better control 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 the Fire Resistant Fluid (F.R.F) system and thus the pressures can be increased without the 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 associated with steam supply to the turbine. Emergency governors are normally set to operate instantaneously if turbine speed reaches 110% of rated (3300 rpm on a two pole turbine generator) or higher speeds. The emergency governor shuts off the steam supply in the event of rotor speed increasing by more than 10% above its normal speed. A sliding bolt or an eccentric ring is attached to the shaft. These are held in position by means of a retaining spring. The bolt or the ring flies out of the normal position .In doing so, it operates a trip and releases the relay oil pressure, which is holding the emergency, valve open. The emergency valve then shuts off the steam supply.

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The emergency governor is tested at periods by deliberately over-speeding the machine when 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), is defined as the amount of speed change from no load to full load divided by the rated speed. Turbine Droop can be set in turbines either mechanically or electrically (In KWU turbines the provision of droop is made to range from 2.5% to 8.0% and to match the grid frequency, chosen setting is 5%). If the governor speed regulation is required to be set at 5% then for a 3000 rpm turbine, the control valves will be open wide at a speed of 2925 rpm or 2½ % below 3000 rpm. And likewise in other side of 50 Hz frequencies, the control valves will be fully closed, at a speed of 3075 rpm, or 2½ % above 3000 rpm. The droop setting in electronic system of EHG has been incorporated in a module connected in series which receives input as the load controller/comparator forming the error (MV-DV), and the droop corrected/incorporated signal is fed to the final load controller module of the load control loop. The amount of the inherent decrease in speed from no load to full load is called speed regulation, droop, or proportional band. The Droop is necessary in the control system in 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), the droop 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 goes up or down and to provide continuous signal to the controlled generating units in order to maintain the desired 50 Hz system frequencies. If the cost of generation at given 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 frequency bias of all controlling turbine generators on the grid is added up to determine the system frequency bias. In order to view the economical loading on the sets connected in parallel an example of a single unit can be considered for understanding the cost controlled situation. If the cost of generation at given moment on the grid is such that a load of 100MW should be generated 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, to determine the system frequency bias. Our single unit example was being cost controlled to provide 100MW and it went to 104MW when system frequency dropped 1/10th of a cycle. With a 0 bias setting, as soon as the load increased to 104MW, the cost control would close the control valves to restore 100MW. At this point, the cost control is acting to oppose frequency correction back 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 frequency drops to 49.9 Hz, the cost signal representing desired generation from this unit changes from 100 MW to 104 MW, under the added influence of frequency bias. If we can again assume that the turbine governor would again have picked up 4 MW, no control 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 dispatcher must manually direct an increase or decrease in generation from the units under his control, in order to restore system frequency to 50 Hz. In this case, the dispatcher “corrects” system frequency in order to provide the correct frequency on a 24-hour basis. This is usually done fairly close to midnight of each day. Instrumentation will advice him how far above or below 50 Hz the system has been operating for the past 24 hours. Knowing his system frequency bias, the dispatcher can then order more or less load to be generated for a given period in order to restore system frequency to an average of 50 Hz for the past 24 hours. This phenomenon is particularly important for controlling system frequency specially in view of controlling power generation with ABT.

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Transient speed rise (TSR): When load rejection takes place, 8-

settling down to steady state value

6-

TSR gives the % speed rise on full

4-

load throw-off

2-

TSR

speed shoots up temporarily before

O-

Steady state Time

Steady State Regulation:

nmax.

It is defined as the Ratio of % speed

nmin.

change (from no load to Full load) to the nominal rated speed. %Regulation=100x(nmax–

min)/nnom

0%

Load

100%

Load Frequency Control is shown in the figure below; it shows the single turbogenerator system supplying an isolated load. Main component are; 1. Fly ball Speed governor system 2. Hydraulic Amplifier 3. Linkage Mechanism 4. Speed changer Increase in frequency f causes the fly balls to move outwards so that B moves downwards by a proportional amount k2’ f. The net movement of C is therefore yC = k1 kC PC + k2 f and movement D, yD= k3 yC + k4 yE. The movement yD depending upon its sign opens one of the ports of the pilot valve admitting highpressure oil into the cylinder thus moving the main piston & opening the steam valve by yE.

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In KWU turbines, the stop valve & control valve (one set) share a common body. The piston of the servomotor is subjected to disc spring force in the close direction and Hydraulic pressure in the opening direction. Hydraulic Governor controls the steam supply by operating the control valves. The fluid pressure under the piston determines the position of the valve; this is controlled by pilot valve of the turbine governor & the secondary fluid oil system. Electro-Hydraulic Governor (EHG) Electro-Hydraulic Governor (EHG) works in parallel with Hydraulic governor at all times of requirements. Basically the Electro-Hydraulic Converter (EHC) is the connecting element between the electrical and hydraulic parts of the turbine governing control 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 operational reliability, 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 various electronic / selector modules configured in four modes of controls: • •

Admission Control mode, Speed Control mode, EDC-Singrauli

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• •

Load Control mode Pressure Control mode.

The Hydraulic governor and the EHG system have been designed such that the governor with lower set point takes over or assumes the system control, as such normally, the set point of the Hydraulic Governor must be set above that of the Electro-Hydraulic Governor when EHG is effective. In cases, when EHG fails to cause shut-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 the EHG set point and it causes increase in small load when the control is transferred to Hydraulic-Governor. The tracking device is either switched on or off manually but when EHG failure or turbine trip occurs, the tracking device is switched off automatically 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 signals and large positioning forces are generated in control valves. The electrical signal from governor control circuit operates the sleeve and pilot valve spool; this regulates the trip fluid drain. Under steady state condition pilot is at central position; in deflected position, the control oil is admitted above or below the amplifier piston. The motion of the amplifier piston is transmitted via a lever to a camshaft, which actuates the sleeves of 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 Common for all three modes of EHG, and it supplies the operating current for driving the plunger coil. The Position controller loop uses a PID control mode for processing outputs 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 for opening of the HP /IP Control Valve. The reference signal therefore works in reverse manner (rise in the coil current for low reference condition). By using two Nos of differential transformer (housed in EHC), feedback signal from the valve lift is derived to ensure proper stationing of plunger spool. Whenever current through the plunger coil gets interrupted or the electrical feedback circuit gets faulted, the reference value of the Hydraulic controller determines the actual 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 as transformed is quite large. The figure below gives various connections and modules used 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 at times of over-speeding of turbine (during load throw-off situations), the input to the minimum selector: MIN–1: takes care of transient condition of the load throw-off and is EDC-Singrauli

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sufficient to check the turbine from over speeding. (During sudden load throw-off, over speeding of turbine is effected and since 10.5 V is generated by a potentiometer that gets algebraically summated with hrnc then it outputs voltage which is less than that of the speed/load signal as selected from the MAX-1:) The signal from the Minimum selector: MIN–1: passes through another Minimum selector: MIN–2: that receives the Pressure Controller output (hrPrc) signal as explained in pressure controller loop. Finally through the last minimum selector: MIN– 2:, the control signal connects the Admission Valve (spool) Controller loop which outputs 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 in Operational condition. The hydraulic speed control eqpt and the start up eqpt of the hydraulic controller are set in upper end position. The actual speed is sensed since turbine already is in barring gear and by slow rising of speed reference the speed controller works /is in service; the turbine speed is then brought up situation for synchronising TG with grid using speed controller. Operation under load Load controller can be taken in service after turbine is synchronised to control load in quick response and high linearity either as per LDC/AFDC or using various modes/sub loops explained in Load control. Frequency change is selected via the integral action load controller to corresponding droop values and a sensitivity of 5 Milli-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 the speed controller has assumed control of TG set, the unit can be disconnected from the grid. Load shedding In case of load shedding i.e. sudden separation of the generator, from the grid, the output signal of the load controller is immediately reduced to value below that of speed controller. Consequently due to minimum selection, the speed controller assumes control and returns turbine back to the set reference speed. This reference speed practically coincides with the rated speed, since the speed controller 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 EDC-Singrauli

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valve /control valve and keeps the turbo set speed from rising along the droop characteristic. An additional effect is the reduction of the speed oscillations. In case of automatic reclosing of the generator CB, the reduction of the load controller output signal 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 slightly above synchronous speed, the control action in speed control mode quickly reduces the turbine speed very close to synchronous speed) During load shedding with subsequent operation of the TG set in an isolated grid situation, (The speed controller assumes continuous TG set control in such situations)

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 (in this situation lowering of nR gets slowed down.

The reference nR is varied in the range of 0-3600 rpm and for minute operation during synchronizing, above the speed of 2800 rpm; a reducing gear lowers the speed of the motorized potentiometer to ¼ rate for exact speed adjustment. The speed reference (nR) cannot be raised when follow-up condition exists and dn/dt is less than monitoring rather in this condition raising or lowering phenomena of nR gets slowed down when the speed reference (nR) is less than 2800. Two indicators have been provided in UCB panel 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 the speed reference nR considerably. During start-up of turbine this nRTD allows rising of the turbine speed at the highest permissible rate consistent with the conservative operation as decided by the TSE computed margin signal introduced between a DC amplifiers. 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 optimized during commissioning. Fast mode or the stop action facility, modify the final nR .The output nRTD of the integrator module, is transmitted to the speed controller and displayed 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 of 120 rpm and is effected (switched automatically) if load controller is operative, final load reference (hrpc) is more than final speed reference (hrnc) by 10% and frequency is between 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 the follow-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 cause change in speed reference (nR ). ‘Blocking ‘ or the ‘Stop nRTD ‘ of the speed signal is generated by an AND module in conditions i) speed >2850rpm, ii) nR is more than nRTD by 300 rpm and iii) an OR ed output of many conditions as given below:1. TSE influence gets faulted (goes out of order or switched off) or EHC fault condition appears AND turbine speed is more than 2950rpm. . 2. During the transition of control from electric to Hydraulic, the speed reference signal becomes less than actual speed and if is more than 50 rpm, i.e. (nR - nact ) < 50 rpm.; 3. If nR > nRTD; pressure controller is in action OR Generator breaker not ON. This Block signal stops the integration (further) function of time dependent speed reference integrator, it blocks the already generated nRTD , and thus the speed controller input signal remains stay-put during stop action. During rolling of the turbine, if between the speeds of 600-2820 rpm the rate of speed rise is very low i.e. less than 100 rpm per minute, then the dn/dt is less than monitoring 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 monitoring alarms the operator and takes care of low acceleration rate in turbine during rolling by suitable output from the speed reference setting module, and at the critical speeds (between 600-2829 rpm) of the turbine. The dn/dt is less than monitoring is derived from an AND gate module, its conditions are 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 a feedback signal of dn/dt + 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 also comprises 'Arming' and 'Disarming' features to facilitate re-start of turbine, under low main steam temperature conditions. Over Speed Trip Device Two hydraulically operated over speed trips are provided to protect the turbine against over speeding in the event of load coincident with failure of speed governor.

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OVER SPEED TRIP DEVICE

1. 2. 3. 4. 5.

Bearing pedestal Spindle Spring Piston Piston body

6. 7. 8. 9. 10.

Spring Pawl Over speed trip bolt Shaft journal Limit switch

c: Return Oil u: Auxiliary Stratup Oil x: Auxiliary Trip Oil

When the preset over speed is reached, the eccentric fly bolt activates the piston and limit switch via a pawl. This connects the auxiliary trip oil to drain thereby depressurising it. The loss of auxiliary trip medium pressure causes the main trip valve 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 tension pushes 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 6 downwards. The pressure beneath valve is thereby dispersed and the auxiliary trip medium circuit is connected to drain. The resultant depressurisation of the auxiliary trip oil actuates main trip valves MAX51 AA 005 and MAX51 AA 006 thereby closing all turbine valves. The electrical tripping on low vacuum occurs through a pressure switch on the vacuum line to mechanical hydraulic low vacuum trip device also at the same condenser pressure. When turbine is started up again, this pressure switch is interlocked against a second pressure switch, which monitors this condition and prevents 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 bearing pedestal and, if a fault occurs, to depressurize the auxiliary trip medium and thus the trip oil in the shortest possible time, thereby tripping the turbine. 1. Compression spring 2. Bearing pedestal 3. Piston 4. Valve body 5. Turbine shaft 6. Pawl 7. Torsion spring 8. Piston 9. Compression spring 10. Limit switch 11. Knob a: c: u: x:

Test Oil Return Oil Aux. Startup Oil 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 to pawl of the thrust-bearing trip. If the axial displacement of the shaft exceeds the permissible limit, the cams engage pawl, which releases a piston to depressurise the auxiliary trip oil and at the same time to actuate limit switch. Electrical tripping of turbine is achieved by fire protection along with closure/stoppage of total control oil supply to turbine governing system by tripping the emergency stop valve on the control oil line. The fire protection trip is achieved by manual Pushbutton in UCB or automatically by very low MOT level (- 150 mm below the normal working level 'O'). Please refer to the associated logics at the end of this chapter. Also fire protection-1 (automatic actuation) gets bypassed if the barring gear valve is 'not closed'.

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FIRE PROTECTION-1 CHANNEL-1

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FIRE PROTECTION-1 CHANNEL-2

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FIRE PROTECTION-2 CHANNEL-1

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FIRE PROTECTION-2 CHANNEL-2

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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 between turbine overhauls and less frequent shutdowns. This warrants testing of equipments and protection devices at regular intervals, during normal operation. The steam stop valves and control valves along with all the protective devices on the turbine must be always maintained in serviceable condition for the safety and reliability. The stop and control valves can be tested manually from the location but this test does not cover all components involved in a tripping. Also, manual testing always poses a risk of mal-operation on the part of the operator, which might result in loss 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 incorporated which ensures that the testing does not cause any unintentional shutdown and also provides 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 during ATT. Only its pretest is carried out without any faults i.e. if the substitute circuit is healthy, 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 of trip oil pressure due to actuation of any protective device, is the criteria for the satisfactory functioning of devices. During testing, general alarm or the cause of tripping is also initiated so that this part of alarm annunciation system also gets tested. Also, during testing, two electrically formed 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. EDC-Singrauli

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Each sub-group contains the device and all associated transmission elements for initiation 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 main trip valves. Whenever any tripping condition (hydraulic/electrical) occurs, the protective device concerned is actuated. It drains the control/aux. trip oil, closing the main 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 of test solenoid valves and the changeover valve. This control oil in trip circuit prevents any actual tripping of the machine. However, all alarm/annunciation are activates as in 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 are tested for their healthiness; the turbine is fully protected against any inadvertent tripping during ATT. If any fault is present; further testing is inhibited. During preliminary 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 to establish hydraulic test circuit (substitute circuit). The hydraulic test circuit is responsible for the supply of control oil in trip oil circuits. The test solenoids valves are again energised building up the control oil pressure upstream of changeover valve. At this moment another solenoid (SVX) gets energised, draining control oil and creating differential pressure across the changeover valve, it assumes upper (test) position and annunciation is flashed to this effect. With changeover valve in its test position, control oil flows in the trip oil piping. After successful establishment of hydraulic test circuit command 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 the exception of remote trip solenoids provides the necessary signal to actuate protective devices. The protective device under test operates and drains the aux. trip oil. Turbine trip gear (main trip valves) is closed after trip oil pressure drains and associated alarms flash. Reset Programme The resetting programme automatically starts after the main test is over. The reset solenoid valves energise and supply control oil in aux. start-up oil circuit to reset main trip 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 be built-up and monitored. If oil pressure is satisfactory, reset solenoids along with test solenoid valves and SVX get de-energised, deactivating hydraulic test circuit and resetting circuit. TESTING OF PROTECTIVE DEVICES The main trip valves and remote trip solenoid valves have already been discussed in previous 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 springs whose tensions can be adjusted corresponding to 110% - 111% over speed. When over speed occurs, the fly weights (bolts) fly out due to centrifugal force and strike against the pawl and valves, draining aux. trip oil pressure and tripping the turbine.

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HYDRAULIC TEST SIGNAL TRANSMITTER (HTT) FOR OVER SPEED TRIP DEVICE

During ATT, the associated hydraulic

test signal transmitter

I. II. III. IV. V.

Control Oil Test Oil Aux. Trip Oil Aux. Startup Oil Drain Oil

1. 2. 3. 4.

Limit switch (normal) Limit switch (test) Valve for Test Oil Actuator

(HTT) becomes 'on'; spool valve slowly moves down to gradually build-up test oil (control) pressure beneath the flyweights. At pre-defined test oil pressure fly weight one and two operate to actuate individual pawl and spool arrangements bringing in the associated alarm. For resetting, spool moves-up and when test oil pressure is fully drained, aux. start up oil (control oil from 'reset' solenoids) pressure resets the devices to their normal position.

Low Vacuum Trip Device With deterioration of vacuum, pressure builds-up over the diaphragm, the spool valve move down, causing valve also to move toward lower position. The aux. trip oil pressure drains, tripping main trip valves and the turbine stop/control valves. During ATT, after hydraulic test circuit is established, the HTT (Hydraulic Test signal Transmitter) gets energised and connects the space above diaphragm to atmospheric pressure through an orifice. The device operates, bringing in the associated alarm. As soon as reset programme starts, HTT is de-energised and vacuum trip device is automatically reset, Field adjustment facilities and checks have been provided when turbine is stationary and there is no vacuum in the condenser. Thrust Bearing Trip Device This device operates in case of excessive axial shift ( >0.6 mm) or excessive thrust pad wear. Two rows of tripping cams on the shaft engage with the pawl) under high axial shift condition. Valves spool moves up draining aux. trip oil and tripping the trip gear and turbine. During ATT, associated ATT solenoid is energised and test oil pressure is supplied to test piston valve. The piston rod actuates the pawl and spool valve assembly, bringing in the associated alarms. During resetting, HTT is de-energised and aux. start-up oil (control oil) resets the device back into normal position.

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AUTOMATIC TESTING OF STOP/CONTROL VALVES The combined stop/control valves are final control elements of the turbine governing system. They must be maintained in absolutely workable condition for safety and reliability of turbine. All the four stop and control valve assemblies are tested individually.

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During ATT of stop/control valves, they are actually closed. In order to prevent large fluctuations of initial pressure or load on the machine, it is essential that Electro Hydraulic Governor is in service and machine load is less than 160 MW and load controller is 'ACTIVE'.

As soon as test programme is initiated, the positioner motor of control valve servomotor's pilot starts. The control oil supply pressure beneath the servomotor piston drops and control valve starts closing. After the control valve is fully closed, command goes to energise solenoid valve (1). The trip oil pressure drains beneath the disc of stop valve servomotor piston. The stop valve closes. After the stop valve is closed, automatically a command goes to energise another solenoid (2). This supplies trip oil to the test valve such that test valve moves down gradually to admit trip oil pressure above the servomotor piston. As soon as piston sits on the disc, there is a sudden rise in trip oil pressure, which is sensed by pressure switches. After these solenoids (1) & (2) de-energise, the test valve moves up to admit trip oil beneath the disc and connecting the space on top of the piston to drain. This pressure difference causes the stop valve to open. Once the stop valve is opened, next command goes to the positioner motor to move in reverse direction; opening the control valve. All along this test, the other control valves are operated by the governor, so as to keep the load and pressure reasonably constant. Should any turbine trip occur during the test, all solenoids are de-energised and tripping takes place in the usual manner.

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TESTING SCHEDULE All important turbine components must be tested at regular intervals. The operating reliability and availability can only meet the high requirements if testing is undertaken at the scheduled times, as recommended below. Testing Intervals Tests are scheduled according to the following Testing Interval Categories. 1. Testing Interval 0

Fortnightly

2. Testing Interval I

Quarterly

3. Testing Interval II

Six-monthly

4. Testing Interval III

Annually

5. Testing Interval IV

After operation interruptions more than 12 month

6. Testing Interval V

After or during overhauls

Testing Interval Category 0 applies to devices, which can be tested automatically without interrupting operation. The tables show the allocation of the Testing Interval Categories to the test. Controller System / Device

Test

Test Conditions

Operation Turbine controller Load shedding relay Bypass controller Pressure controller Oil temp controller

Function Adjustment

Test Interval 0

I

II

III

x

x

x X*

Standstill

x

x x

Load rejection Operation

Function

V

Load Rejection Standstill

Function

IV

x X*

x

x

Load rejection

x X*

Adjustment Standstill

x

x

Function

Operation

x

x

x

Function

Operation

x

x

x

X*: Recommended; not required by manufacturer

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Sub loop Control of Pumps System / Device

Oil Pumps

Test

Test Conditions

Test Interval O

I

II

Function

Shutdown

x

Start-up Pressure

Operation

x

III

IV

V

x

x

Valves System / Device

Test

Test Interval

Test Conditions

O

I

II

III

Standstill Stop Valves

Freedom of movement Leak Test

Operation

Freedom of movement Leak Test

V

x

x

x

x

x

x

x

x

x

x

x

x

x

Operation ATT x Shutdown Start-up

/

x

Standstill Control Valves

IV

Operation

x

Operation ATT x Shutdown / Start-up

LP Bypass Control Valves

Freedom of movement

Standstill

Extraction Valves

Freedom of movement

Operation

Safety valves

Actuating valves

Operation/ Standstill

Vacuum breaker

Function

Standstill

x

x

x x

x

x

IV

V

x

x

x

x

Protection and Safety Devices System / Device

Test

Main Trip Valves (Gear) Function Remote Trip Solenoids

Function

EDC-Singrauli

Test Conditions

Test Interval O

Standstill Operation ATT Standstill Operation ATT

I

II

III

x

x

79

Over Speed Trips

Function Actuating value

Hydraulic Function Low Vacuum Actuating Trip value Electrical Function Low Vacuum Actuating Trip value

Rated speed ATT Standstill Operation ATT

x

Function

x x

LP Bypass Condenser Protection

Function Actuating value

Reverse Power Protection Fire Protection

x

x x

Standstill

ATT

x

x

Standstill

Thrust Bearing Trip

x x

x

x

Standstill

x

x

Function

Shutdown

x

x

Function

Shutdown

x

x

Safety Function Devices for Actuating Reverse Flow value Low Lub Oil Pressure Protection Device

Over speed after load operation Rated speed

Function Actuating value

Protection Devices

Too high Steam Pr.

Function Actuating value

Too low Steam Pr.

Function Actuating value

EDC-Singrauli

Operation

x

x

Standstill

x

x

x

Standstill

x

x

Standstill

x

x

80

Alarms and Measuring Devices System / Device

Alarms for all system Digital Signal Transmitter

Test Conditions

Test

Function

Operation

Actuating Value

Standstill

Function Actuating Value

Standstill

Speed

Test Interval O

I

II

III

IV

V

x x

x

Operation

x

x

x

Measuring Devices

Pressure

x

x

Temperature

x

x

x

x

Expansion Vibration Oil Level

Accuracy of indication

Standstill

Valve Position

x

x

x

x

x

x

x

x

x

Measurement of Important Operating Parameters System / Device

Steam Temperature. Steam Flow Internal Efficiency Condenser Leak Tightness Bearing Metal Temp.

Test Conditions

Test Interval

O

Long Term Monitoring

Steam Pressure

Test

I

II

III

IV

V

x x x x x Operation

x

x

Expansion

x

x

Vibration

x

x

Oil Levels

x

x

Oil Pressure

x

x

Oil Temperature

x

x

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LOW PRESSURE (LP) BYPASS SYSTEM Low Pressure bypass system enables to establish an alternative pass for dumping the steam from reheater outlet directly into condenser at suitable steam parameters. The controls for LP BYPASS system are essentially a combination of electrical and hydraulic system. Electro-hydraulic converter provides the necessary link between hydraulic actuators and the electrical system. The control of LP bypass system is hooked up by the same control, which is used for turbine governing system. The LP bypass valves are two in number. The double shut-off arrangement separates the reheater from the condenser during normal operation. In addition to these, two steam pressure control valves, four injection water valves are provided for de-superheating purposes. This injection water is taken from condensate extraction pump discharge.

LP BYPASS SYSTEM

Set Point Formation Two set points, the fixed set point and the variable set point are formed for the LP Bypass controller, the effective set point under any set of operating conditions being the greater of the two. The fixed set point can be set manually from the control panel to a point between 0 120 % of the maximum LP Bypass pressure with the aid of a motorised set point adjuster. It can also be regulated automatically by means of the 'Automatic Control EDC-Singrauli

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Interface' during the start-up phase and is normally used to set the lower limit for pressure set point. The pressure upstream of the H.P. blading, required for reference variable set point formation, is measured by a pressure transducer and transmitted to a matching amplifier which sets the characteristic for the reference variable as a function of the pressure upstream of H.P. blading i.e. throttle pressure.

LPBP EHC POSITION

Vs VARIOUS VALVE OPENING

PRESSURE CONTROL FOR LP BYPASS SYSTEM The reheat steam pressure before interceptor valve is the controlling variable for the LP bypass system. Control of this parameter can be done in the 'MANUAL' mode by changing the electro-hydraulic controller (EHC) output as required by means of the OPEN/CLOSE push buttons located on the control module. In the 'auto' mode, the controller matches the hot reheat pressure with the effective set point (either FIXED or VARIABLE) by modulating the LP Bypass control valves as necessary. A tracking controller is provided so that the control mode (manual or auto) not in service automatically follows the effective controller. This facilitates bump less changeover, between the modes. But when charging over from 'MANUAL' to 'AUTO' care must be taken for matching the set point and actual value, otherwise, a jerk in the system will be felt due to the error present (which the AUTO controller tries to bring to zero).

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LP BYPASS CONTROLLER

AUTOMATIC CONTROL INTERFACE DEVICE (ACI) During the start-up, it is intended to avoid a very high level of set point. For this purpose, the Automatic Control Interface Device has been introduced. For the Automatic Control Interface to come in action, it must be switched on by means of the ON/OFF push button provided on the control panel. Also the Bypass controller must be in auto mode. When the Automatic Control Interface is switched ON, it brings the fixed set point down to 3 Kg/cm2, in case the actual reheat pressure is below 3 Kg/cm2. When the actual reheat pressure exceeds 3 Kg/cm2 the ACI opens the LP Bypass control valves + 25% and they remain locked in 25% position up to a reheat pressure of 12 Kg/cm2. During this time, the fixed set point tracks the actual reheat pressure so that the output of LP Bypass "auto" Controller is zero. Once the ACI has brought the fixed set point 12 Kg/cm2, it gets automatically switched off. The fixed set point remains static at 12 Kg/cm2 and the LP Bypass controller modulates the control valve to maintain this set pressure. Any change in the reheat pressure can now be brought only by manually varying the fixed set point to the desired value. Two Stage Water Injection To prevent undue overloading of condensate pumps under normal shutdown/start-up conditions, the injection water demanded from CEPs is staggered in two stages. This arrangement opens the injection valves (INV-2, 4) via the pressure switch (LPPS), solenoid valve (SVV) & slide valve SV-2/4 when the steam pressure upstream at the expansion orifice exceeds value corresponding to 45% of maximum bypass flow. EDC-Singrauli

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Protective Closing of LP Bypass System (Condenser Back-up Protection) The LP Bypass valves will close automatically under the following conditions to prevent damage to the condenser. •

Condenser vacuum is low (> 0.4 Kg/cm2 abs)

•

Spray water pressure is low (< 10 Kg/cm2 or both condensate pumps off).

•

Condenser wall temperature is high (> 90oC).

•

The steam pressure downstream of LP BP is greater than 19 Kg/cm2.

High exhaust hood temperature will automatically switch on the exhaust hood spray water. In case of condenser wall temperature protection operation, the 'RESET BYPASS TRIP'-Pushbutton for solenoids SV-1 and SV-2 are to be depressed to reset the TRIP command. LP Bypass Control (Hydraulic) Due to difference between set and actual HRH pressure the electro-hydraulic LP bypass governor generates a proportional signal voltage in the moving coil of the converter (EHC). With increasing signal voltage, the jet pipe of the converter moves towards right and the amplifier piston (KA-08) moves down. A feedback mechanism stabilises the amplifier piston for a given voltage change. The sleeves (KA04) of follow-up piston valves (KA02/KA03) also move down increasing the signal oil Pressure of water injection Valves, there by opening them, in the beginning of control operation.

LP BYPASS CONTROL SYSTEM

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LP bypass stop valves (LPSV-1, 2) open up with a slight time delay after injection valves are opened; due to rising oil pressure in follow-up pistons KA02 (assuming piston KA07 of bypass limiting regulator is in upper position). LPBP Steam control valves (LPCV-1, 2) open up due to hydraulic feedback between actuator pistons and pilot valves (PV-1, 2).

1. 2. 3. 4. 5. 6.

Electric LP bypass governor Plunger coil measuring system Jet pipe Adjusting spring Adjusting screw Jet pipe regulator

a. Control fluid a1. Control fluid under control piston of differential pressure relay a2. Control fluid above control piston of differential pressure relay

Electro Hydraulic Converter for LP Bypass

LP bypass limiting regulator (LPLR) has priority over (EHC). As soon as condensate at required pressure is available with sufficient vacuum in condenser, its jet pipe swings to right and its piston KA07 moves to upper position. This increases the signal oil pressure in KA02 (follow-up pistons), releasing steam Stop Valves and Control Valves to open. In case of condensate water pressure low and condenser pressure high the reverse action takes place and the spring of KA02 is de-tensioned to such an extent that LP bypass valves are unable to open, Refer to Figure. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Jet pipe Jet pipe regulator Adjusting spring Adjusting screw Corrugated measuring system Adjusting spring Corrugated measuring Corrugated measuring Adjusting spring

a. Control fluid a1. Control fluid above control piston of limit pressure amplifier a2. Control fluidunder control piston of limit pressure amplifier k. Condensate from hydraulic pressure switch of injection water pressure monitor l. Vacuum signal from bypass steam piping behind bypass control valve

LP Byapss Limiting Regulator

LP BYPASS PROTECTIONS EDC-Singrauli

86

Low Vacuum Safety Device

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Capnut Adjusting csrew Cover Compression spring Diaphragm Valve Valve sleeve Casting Can Lever

a Bypass signal oil from converter a1 Signal oil to bypass valve c Oil drain l Vacuum from condenser

A low vacuum safety device is installed in the signal oil line from follow-up piston KA02 to bypass valves' pilots (PV-1, 2) and (PV-3,4) If vacuum drops below a preset value; the valve of the safety device moves downwards due to increasing pressure above it. The valve thus blocks off the signal oil line and opens the oil between itself and PV-1, 2 & 3, 4 to drain, closing the LP bypass stop and control valves. As vacuum increases, bypass operation is restored in reverse sequence when the preset vacuum has built up. Low Injection Water Pressure Protection A pressure switch (WPS) is installed in the signal oil line from KA02 to PV-1, 2 & PV-3,4 of bypass valves, to protect the condenser in the event of water injection failing. If the injection water pressure drops below a preset value, the valve of the pressure switch (WPS) moves down, blocking off the signal oil line and de-pressuring the oil between itself and PV-1, 2 & PV 3, 4. The LP bypass valves are thus closed, due to low condensate water pressure. Bypass operation is restored in the reverse sequence when injection water pressure becomes normal.

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Hood Bellos Pushrod Knife edge lever Cam shaft Compression spring Fitted key Shaft Scale Cylindrical pin Nozzle Slide valve Valve bushing Compression spring Bearing bushing Torsion spring Lever

a Control oil a1 Control oil a2 Control oil to pilot valve of bypass valve c Return flow l Injection water pressure

Low Injection Water Pressure Protection

High Condenser Wall Temperature Protection At a preset condenser wall temperature the two thermocouples mounted in steam dome opposite bypass steam inlet transmit a switching pulse to the associated solenoid valves (SOLV-1, 2). 1. Solenoid 2. Compression spring 3. Solenoid valve 4. Compression spring 5. Solenoid 6. Compression spring 7. Main valve 8. Compression spring 9. Limit switch for injection a Control oil b Signal oil to Stop and Control valve operator of bypass SV/CV c Drain

The solenoid valves block off the depressive signal oil and close bypass valves in the event of high condenser wall temperatures. The bypass valves can be opened from the control room manually only after the solenoids are manually reset after the temperature has become normal.

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