Station Commissioning, Volume Volume H, Third Edition: Incorporating Modern Power System Practice
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Emergency supply equipment. Mechanical plant electrical services. E - Chemistry and Metallurgy. Corrosion: feed and boiler water.
Connect with Michael Negnevitsky
Plant cleaning and inspection. Non-ferrous metals and alloys. Nonmetallic materials. Welding processes. Non-destructive testing. Environmental defects. F - Control and Instrumentation. Automation, protection and interlocks and manual controls.
Boiler and turbine instrumentation and actuators. Central control rooms. On-line computer systems. G - Station Operation and Maintenance. Performance and operation of generators. The planning and management of work. H - Station Commissioning. Principles of commissioning. Common equipment and station plant commissioning. Unit commissioning and post-commissioning activities. J- Nuclear Power Generation. Nuclear physics and basic technology. Nuclear safety.
K - EHV Transmission. Tranmission planning and development.
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.
Modern Power Station Practice
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. 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.
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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.
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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.