Dore chakravarthy

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THERMAL POWER STATION

Steam turbine generator unit (TG unit) [[1]]

The steam turbine generator units are provided with auxiliary systems to make it work not only satisfactorily but also safely too. The following auxiliary systems are generally provided by various manufacturers depending on the size of unit and construction details. The list herein shows what is normally provided for large units, of the order of about 200MW, depending on the type of construction of the main components.

The simple line diagram given below also indicates the different auxiliary systems.(to be included ) (Refers to Makers: GE (USA)[2]and BHEL (India)[[[Media:http://66.39.99.160/bhel/sitemap.htm]]]-TG units in INDIA.)

1. Bearing oil system

2. Barring gear system

3. Auxiliary oil system

4. Generator seal oil system

5. Generator hydrogen cooling system

6. Generator stator water cooling system

7. Generator excitation system

8. Generator High Voltage system

9. Monitoring and alarm system for the TG unit

1) Bearing oil system

The TG unit being rotating equipment has generally a fairly heavy and large diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. Further they should offer less resistance (friction) for rotation. Therefore the shaft is provided with number of bearings, corresponding housings and supports, depending on the construction adopted by each manufacturer. The bearings shells, in which the shaft rotates, are lined with material with low friction, like Babbitt.

To make the friction between shaft and bearing surface as much less as possible and to keep the heat generated indicated by the temperature rise of the bearing oil, at the bearings to minimum to avoid melting of bearing material, oil lubrication is used. Therefore bearing oil system with pumps, monitoring and control equipment are incorporated.

The sketch shows a typical oil system. Normally the oil system is such that the TG unit can be started on its own without any assistance from grid or other units.

One fairly large Oil Tank is provided for not only storage of oil but also for locating the oil pumps needed. The oil pumps are generally Vertical type, motor on top of the tank and pump themselves submerged, to make pumps more reliable. Since there are other systems requiring this oil, this oil tank is made as a common oil tank catering other oil requirements as well.

The bearing oil itself has requirements for normal operation, emergency operations and shut down conditions. For normal operation of the TG unit the oil pump is driven by the turbine shaft itself, taking suction from the oil tank For starting of the TG unit, an Auxiliary Oil Pump (AOP) is provided. In emergencies when AOP is not available, a DC driven oil pump is provided in such a way that it takes over automatically when the lube oil pressure falls down to a particular level due to any reason whatsoever. However the DC oil pump cannot cater for the hydraulic system. Hence DC pump is only for lubrication and seals, when the AOP and main oil pumps are not available due to any reason whatsoever. The unit cannot be started in this condition. Some manufacturers provide therefore TWO Auxiliary Oil Pumps.

The bearing oil not only acts as a lubricant but also as a cooling medium. The hot oil coming out of the bearings generally drains to a sump below the bearing housing. Each of the pipes coming out is connected to a drain pipe header leading to the oil tank inside, where it falls on a strainer basket to remove any foreign particles. The drain pipe header sometimes is enclosed in another casing for fire protection purposes.

An Auxiliary Oil Pump (AOP) is provided for start up operation. Main oil pump driven by the turbine shaft takes over when the TG shaft comes to near about full speed, automatically. In case of emergency, due to any tripping of TG unit (with AOP not coming up,) the DC oil pump will start to supply oil to bearings at a predetermined fall of lube oil pressure. The auxiliary and main oil pumps also supply the Hydraulic oil for governing control but the DC oil pump is only for Bearings and generator seals.

In general all oil pumps submerged in lube oil inside the tank have positive suction to increase their reliability.

2) Barring gear

This is the term normally used for the complete mechanism provided for the rotation of the Turbine Generator shaft at a very low RPM (Revolutions per Minute) after the unit stoppage due to any reason. The requirement for this is very important for all capacities of turbine generator units. The driving motor supply is therefore made from a reliable source.In very small units of capacities of the order of about 500 KW and below, the mechanism may be hand operated to change the shaft position by 180 degrees frequently.

The necessity for this requirement requires careful study. Once the unit is tripped (i.e. Turbine stop valve is closed cutting off steam to Turbine) due to any reason, the driving force of steam to rotate the turbine is not available. Hence the TG unit speed starts going down, known also as coasting down. When it reaches zero speed, i.e. dead stop, and allowed to be at dead stop position for some time there is a tendency for the turbine shaft to deflect or bend, detected only by monitoring meters such as eccentricity meter on the operators’ console.

This deflection is because of the heat inside the turbine casing tries to get concentrated on the top half the casing as compared to bottom half casing, thus making the top half portion of the shaft hotter than the bottom half. The shaft therefore warps or bends by millionth of inches, not perceptible to human eyes when casing is opened and seen. But this small amount of shat deflection is enough to cause vibrations and wreck the whole TG unit when the huge mass starts spinning. Therefore in TG units the shaft is not allowed to come to a dead stop, but automatically taken over by a mechanism known as turning gear or barring gear at a particular set low speed.

If the unit has to be taken down for major maintenance requiring inspection of turbine bearings etc., then barring gear has to be kept in service till the temperatures of the casings, as seen on the operators’ console is sufficiently low as prescribed by the manufacturers.

All the necessary monitoring instruments such as turbine casing temperatures, shaft eccentricity meters, vibration indicators etc. with alarms for various operating limits, are provided at operators’ console, in addition to starting and stopping controls.

[[Image:Dores-TG_Cycle_diag1.jpg]]

3. Auxiliary oil system

This system with Auxiliary Oil Pump (AOP) is designed for supplying oil at the start up of TG unit. The oil pump is designed to supply the Hydraulic oil system consisting of Turbine stop valve in the main steam line to turbine, the governing control valves, the bearing and seal oil systems and relevant hydraulic relays and other mechanisms. This pump is stopped manually after the unit comes to about its rated speed and the turbine main shaft driven pump takes over the functions of the AOP. Some units have a stand by AOP to automatically take over if one fails during start up or defective operations. The pump casings are submerged in oil so that they have always positive suction to make them more reliable.

4. Generator seal oil system 5.Generator hydrogen [[3]] cooling system and 6) Stator cooling water system.

The Generator directly coupled to turbine does require some arrangement for cooling to dissipate the heat generated inside, depending on the size of the unit. Though in small size units it is mostly natural air cooling through air filters, in larger units generally found in power generating utilities now a day, special closed circuit cooling arrangements are incorporated. The one shown in the typical diagram has Hydrogen gas cooling in the generator casing and stator coils are hollow to take water cooling. The Hydrogen gas is used because mainly of its higher affinity for heat absorption, lower density to give less friction in a rotating medium, and being non self igniting. However it should be noted that Hydrogen gas is explosive in presence of air. The Hydrogen pressure is maintained slightly higher than atmosphere pressure say at about 1.5 Kg/cm2, to avoid outside air ingress. Due to this higher pressure than outside pressure, to avoid leakage to outside atmosphere, some form of sealing arrangement has to be provided where the shaft emerges out of the casing. This is done by providing mechanical seals round the shaft with a very small gap in-between to avoid rubbing between shaft and seal. To avoid gas leakage from this very small annular gap oil under pressure is provided in such way that part of the oil flows to inside and part to outside. The oil flowing inside prevents the Hydrogen gas leakage to atmosphere.

This oil is called seal oil and should always be present as long as gas pressure inside is above atmosphere. Some manufacturers provide Carbon rings in housing as seal and some others provide other types known generally as labyrinths. The seal oil entering the seal oil housings is therefore split in to two parts one going inside the casing and the other coming out. The coming out oil is taken to bearing drain itself. The oil going inside the casing, which is under pressure, requires some sort of seal system to remove this oil to outside of casing as no oil should accumulate inside the casing, having conductors stressed to High Voltage. Different makers have their own designs for removing through a loop seal arrangement.

Since Hydrogen gas [[4]] is explosive with atmospheric air under certain conditions, a system has to be incorporated for its handling. The first filling of Hydrogen, purging out of the same for any inspection inside, for normal make up for losses during running and to maintain desired pressure and purity continuously, a separate system is provided. Also firefighting arrangement for inside generator explosions is incorporated with monitoring and control equipment. This system includes Hydrogen gas cylinders and CO2 gas cylinders suitably located with pressure reduction stations and piping to the generator casing, and purging piping. All purging pipe connections are lead to open air at highest level to dilute the purged gases to avoid explosions. Some Power stations have Hydrogen generation plant at site to avoid long distance transport of large number of cylinders. The generator conductors are made hollow to take water for cooling to remove further heat from coils. The generator coils being at about 20 KV (KiloVolts), and water being generally conductive, some form of insulating barrier material (such as Teflon tubing) is used for interconnection of cooling water and the generator coils. The DM water of lowest conductivity and without any impurities is used for passing through the coils. The DM water used for this purpose is in a closed circuit, because of its affinity to absorb Oxygen from atmosphere and make it highly corrosive. It is also provided with its own mixed bed ion exchangers and magnetic filters to maintain highest purity and free of any foreign material, though it takes DM water from the station DM plant unit for initial charge and small make up.

7. Generator excitation system

The Generator requires DC excitation current, it being an alternate current generation. During 1960s the Turbine generators were generally provided with a separate main shaft driven DC generator, known as Exciter, and some times another exciter known as Pilot exciter is provided for better regulation of Generator output. Now a day due to further technology developments instead of DC generator, an AC generator is provided with incorporated rotating Rectifier assembly, with external electronic control of the DC output to the main generator. The heavy DC current leads for the main generator are taken directly from this AC rotating exciter. This system has more advantages and reliability. It mainly avoids the cumbersome voltage regulator provided at the control room. Even small Diesel Generator units are now a day provided with this system. The electronic controls are installed in a separate room and require very little attention.

8. Generator High Voltage system

The Generator voltage is normally 11 KV in smaller units and in bigger units it would be about 20 KV. Probably this is limited by the insulation media available and the construction methodology limitations available today. The generator HV leads are normally of large section Aluminum channels because of very high Amps. They are enclosed in Aluminum Bus Ducts (with good grounding), live channels being supported on insulators inside, as against cables used in smaller units. Further the generator HV leads are directly connected to a suitably designed Transformer for connecting to a Transmission yard High Voltage sub station, of the order of 110KV or 220KV. The necessary protection and metering devices are incorporated on the HV leads of generator. Thus the TG unit and the transformer form one unit. In smaller units, generating normally at 11KV, a breaker is provided to connect it to a common 11KV bus system in a cubicle located normally indoors.

9. Monitoring and alarm system for the TG unit

The TG units do require prechecking for start up during the first start or after a shut down for any reason whatsoever. The safety aspects and the normal procedures have to be looked into at all stages of operation. Manual intervention is also unavoidable however much the system is made automatic. In view of this necessary protection, monitoring with alarms for out of limit parameters, and auto and manual control equipment are provided on the operators’ console, both on mechanical and electrical equipment.

--Dore chakravarty 22:41, 13 August 2005 (UTC)

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Hello, friend

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