Lifting trunnions are usually made detachable. The design of the welded joints is carefully con- trolled to avoid the presence of unfused lands wherever possible. The main welds have to be leak-tight against hydrogen at 4 bar, which is a very exacting require- ment. The complete casing may be too large to be stress-relieved in an annealing oven, in which case it must be assumed that stresses up to yield. In one design, the casing is con- structed in two halves, which are stress-relieved before being welded together.
The end shields are thick circular fabricated steel plates, ribbed to withstand the casing pressure with minimal axial deflection.
They house the shaft seal stationary components and, in some designs, the out- board bearing. The casing assembly must be designed to avoid resonances in the range of these exciting frequencies. Drains are arranged so that any oil or water col- lecting in the bottom of the casing is piped to liquid leakage detectors, which initiate an alarm. Electrical heaters are fitted in the lower half of the casing to maintain dry conditions during outages. The casing is bolted down to the supporting steel- work on packing plates which are machined after trial erection to provide the correct alignment.
Axial and transverse keys prevent subsequent movement. The weight of the casing, complete with core frame, coolers and water, is up to tonnes. Even though the. In some stations, most of the generator and exciter losses are transferred into the boiler feedwater system by using condensate in the heat exchangers. While such an arrangement can be economic, there is a penalty in the form of added complication, and the most modern stations do!
Even at the rated pressure 4 or 5 bar and with the allowable level of gaseous impurities, it is still only half as dense as air at normal temperature and pressure NTP.
The large loss due to the gas being churned by the rotor, and to its circulation through the fans and cooling passages, is minimised by the use of hydrogen as a coolant. The heat transfer of hydrogen is up to twice that of air in similar conditions, though, as with all gases, it increases with increasing pressure. Together with the several times higher thermal con- Cooling systems ductivity and specific heat of hydrogen, the effect is that heat removal from heated surfaces is up to ten times more effective, resulting in lower tem- peratures.
Coolers can also be considerably smaller. The use of hydrogen imposes the need for herme- tic sealing and condition control, which helps to ensure that the original electrical clearances are maintained. More importantly: the degradation of insulation by oxidation processes cannot occur in a hydrogen atmosphere.
The disadvantages are: Since concentrations of from to of hy- drogen in air are explosive, hydrogen must not be allowed to escape from the stator casing and its associated pipework in significant quantities and become trapped in potentially explosive pockets. The casing and end shields have to be of rugged construction and leak proof, demanding meticulous welding techniques.
Penetrations such as the rotor shafts, and all outgoing connections, must be posi- tively sealed, the former requiring a sophisticated sealing system. A comprehensive gas control system is required. For generators rated much above I 00 MW, air cool- ing is not practical; more than half the total loss would be due to fan and rotor windage. At and MW, hydrogen pressures of 4 or 5 bar are economic; higher pressures than this have little or no advantage.
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The only practical alternative at these ratings is complete cooling including the rotor, which has not been adopted in the UK, and only rarely elsewhere, because of leakage pro- blems at the very high water pressures produced by the rotation. The usual method is to use carbon dioxide as a buffer between the two other gases, in a process known as scavenging, or simply gassing-up and degassing.
Carbon dioxide, stored as a liquid under pressure, is expanded to a suitably low pressure above atmos- pheric. It is also heated, because the expansion causes it to cool and it would otherwise freeze. With the rotor stationary, C0 2 is fed into the bottom of the stator casing through a long perforated pipe, and because it is more dense than air it displaces air from the top via the hydrogen inlet distribution pipe to atmosphere outside the station.
Some mixing of gases occurs at the interface.
A gas analyser is used to The generator monitor the proportion of C0 2 in the gas passing to atmosphere; when tllis is sufficiently high, the C0 2 inlet is closed see Fig' 6. High purity hydrogen from a central storage tank or electrolytic! Being very much lighter, it displaces the C0 2 from the bottom of the casings via the C0 2 pipe to atmosphere, again with some degree of mixing.
When the proportion of C0 2 in the vent is low enough, the proportion of air left in the casing will be very low, and if the casing is then pressurised with hydrogen to its pp- erating pressure say 4 bar , the proportion of air will be reduced to a quarter of this low value. The complete process normally occupies a few hours. Separate procedures are followed to ensure that other components, such as tanks, are properly scav- enged, so that dangerous mixtures do not occur.
The reverse of the foregoing procedure, using C0 2 and then dry compressed air, is followed to remove hydrogen from the machine for inspection or for a prolonged outage.
In one design of MW generator, air is removed from the casing by drawing a vacuum, using the pump normally used to degas the seal oil. The shaft seals are arranged to seal effectively under this unusual operating condition. When the vacuum is as low as can be achieved, hydrogen is admitted, the resulting purity when pressurised being sufficiently high. Normally, hydrogen purity remains high, since air cannot leak into the pressurised system.
Some air may, however, be released from the shaft seal oil flowing into the casing hydrogen space. Replacement hydrogen to make up for leakage is usually sufficient to maintain the required purity. The purity monitor and the gas analyser can be calibrated with pure gases from the piped supplies. A check on the purity is also possible by monitoring the differential pressure developed by the fans, which responds markedly to the change in density produced by air impurity.
A pressure sensitive valve admits hydrogen from the bus main if the casing pressure falls below a pre- determined level, while a spring-loaded relief valve is set to release hydrogen to the outside atmosphere if the pressure becomes excessive. It is important that these two 1 pressures are not set so close that wastage occurs, particularly as the gas temperature and pressure changes when on-load cycling. Monitoring of the hydrogen consumption is a recently introduced feature on some units see Fig 6.
The temperature of the hydrogen is normally moni- Chapter 6 tored by several thermocouples, whose readings should be averaged, at the inlets to and outlets from the hydrogen coolers. Typically, hydrogen is circulated at 30 m 3 Is which, with a full-load loss input of about kW, results in a temperature rise of the order of 30C. The cooled gas should not be hotter than 40C, so the temperature of the gas entering the coolers should not exceed 70C. Water cannot normally leak into the casing from the stator winding water circuit or the hydrogen coolers, since the water pressure is lower than the gas pressure in both circuits.
It can be released from the shaft seal oil, particularly if the oil is untreated turbine lubricating oil which has picked.
It is important that the mois- ture content of the casing hydrogen be kept low enough to prevent condensation occurring on the coldest component, which may be the water cooled winding. The differential pressure is used to circulate a flow of hydrogen continuously through a dryer, typically of the twin-tower type, using activated alu- mina, with automatic changeover and regeneration.
A motor-driven blower maintains the flow through the rotor when the rotor is not running at speed see Fig 6. Continuous monitoring of the humidity of the cas- ing gas is provided by means of a hygrometer. The maximum permissible dewpoint is not less than 20C below the cold gas temperature, measured at casing pressure. It is important that this caveat is observed, particularly if the dewpoint is being compared with that of a sample drawn from the casing and measured at atmospheric pressure. Hydrogen is circulated by the fans through the stator core and end wiQdings, the precise paths being different in different designs.
The rotor acts virtually as its own fan, hydrogen being drawn through the windings and exhausted into the airgap, again dif- ferently in different designs. The hydrogen removes the electrical loss in the rotor winding, the 'iron loss' in the stator core, the windage loss produced by the rotor and fans, and most of the electrical losses gen- erated in the frame and end winding structures. Because it is impractical to ensure that potentially explosive mixtures of hydrogen and air never occur in the small bore instrumentation pipework, those instruments and devices containing electrical circuits in contact with the gas, such as katharometers, must be intrinsically safe in such mixtures.
This means that a sudden break in an electrical circuit must not be capable of providing enough spark energy to ignite the gas. Zone 2, in which explosive mixtures are unlikely to occur and, if they do will only exist for a short time, covers :l,strumentation as previously noted; the hydrogen dryer and blower, the detraining tanks, and the in- terior of the control cubicle to which hydrogen is piped. Also classified as Zone 2 are the areas into which hydrogen may leak, through gaskets, seals, etc.
Sources of ignition are not located in such areas. It is, however, virtually impossible to eliminate some potential igni- tion sources, such as the rotating shaft rubbing an oil scraper ring, or sparking at brushgear.
Another potential source of ignition occurs where currents are induced in pipework loops, as may be the case when pipes are routed near to main con- nections. Here, flanged joints are insulated to break the possible current path. If a serious rupture occurs, e. Hydrogen has been used universally for 50 years for high speed generator cooling, and incidents such as this have been very rare. The meticulous attention to safety precautions both in design and operation have been largely responsible for this good record.
What- ever their design, they are located in the end shields, and are inboard of the bearings. Two types of seal have been commonly used: the thrust seal and the journal seal.
Turbine lubricating oil is fed to a central circum- ferential groove in the white-metalled face of the seal ring, at a pressure controlled to be greater than that of the casing hydrogen. Most of the oil flows outwards over the thrust face and drains into a well. This oil can release entrained air and water at this point, thus contami- nating the casing hydrogen, as noted earlier, and it is therefore important that the inward oil flow is small. The seal ring is attached to a housing which must be free to move axially to accommodate the 30 mm or so of axial movement imposed on the shaft by thermal expansion of all the coupled rotors down- stream from the turbine thrust bearing, as they pass from cold to hot conditions.