TURBO-ALTERNATOR GOVERNING SYSTEMS

FORTY-FIRST CONFERENCE TURBO-ALTERNATOR GOVERNING SYSTEMS BY C. W. HAYES and A. C. VALENTINE W. H. Allen Sons & Co. Ltd., Bedford, England. (Subsidiary of Amalgamated Power Engineering Ltd.) All sugar mills use large quantities of low-pressure steam and electrical power. A back-pressure turbo-alternator provides both these services from a single steam supply. High-pressure steam expands in the turbine down to the process pressure. The kinetic energy of expansion is converted into electrical power and the exhaust steam gives up latent heat to the process. This combined cycle has a lower operating cost than purchased power and low-pressure boilers. An ideal back-pressure turbo-alternator installation is conditional upon the power demand always corresponding to the output obtainable from the process steam flow. Such an ideal is never realized in a sugar mill, and a steam pressure reducing valve is installed in parallel with the turbine to make up any deficiency in the process steam requirements. Some sets run in parallel with the public electricity supply system, either importing or exporting power, to provide a heatlpower balance. The need for a governing system When a turbo-alternator supplies power to the mill electric motors, it must maintain a sensibly constant frequency, i.e., rotational speed. If the electrical load, inlet steam valve opening and steam conditions all remain unchanged, the turbo-alternator speed would remain constant. However, should the load increase, the shaft speed would decrease, because the turbine would not then receive sufficient steam. More steam is required to match the load before the speed will return to normal. Conyersely, when the load decreases, less steam is required. Speed governor gear changes the steam valve opening automatically and maintains a nominally constant rotational speed, irrespective of changes of electrical load. Early simple hydraulic systems The simplest form of speed governor controls the opening of a single large throttle valve which passes the total steam flow to the turbine. This type of governor is quite satisfactory for a turbine which will operate continuously at, or only slightly below, its maximum continuous rating. To obtain maximum efficiency at partial steam flows, the turbine nozzle inlet pressure must be maintained as high as possible, i.e., avoiding an execessive pressure drop across the throttle valve. A more complex system with automatic multiple throttle valves achieves this effect. In the 1950's, steam consumption was not of prime importance for the l MW to 2.5 MW sets then supplied to cane-sugar mills, due to the

1 24 FORM-FIRST CONFERENCE 1974 Fwn contwn OIL runr EWLRCENCY CoVLRNOn, Fig. I-A simple direct-acting hydraulic speed governor system controlling a single throttle valve. availability of bagasse as boiler fuel and the need for simple turbines. They are fitted with a single large throttle valve and a measure of nozzle control is achieved by hand valves, which are set to suit the load. Figure l shows the simple direct-acting hydraulic speed governing system employed by some of these turbines. An oil relay is incorporated to increase the power of the governor to move the throttle valve against the steam pressure, while the governor itself, which only has to move the oil relay pilot valve, can be made sensitive and conveniently small. The inlet steam passes through an emergency trip steam valve which is held fully open by the high-pressure oil and then through a single throttle valve under the control of the centrifugal speed governor. Relay oil, at a pressure of 420 kpa, leaks past the top of the pilot valve beat into the throttle valve relay cylinder and also back past the bottom of the pilot valve beat to drain. The position of the pilot valve beat relative to the port determines the valve opening. When the electrical load increases, the turbine speed decreases and the pilot valve moves downwards, increasing the oil pressure under the relay piston, and the valve opens wider to admit more steam to the turbine. This increases the speed, and the pilot valve moves upwards again until a new position of equilibrium is reached with the turbine carrying the increased load. Conversely, for a decrease in electrical load, the governor reduces the steam valve opening. Speed variation while the turbine is running, either locally by hand or remotely by an electric motor, is achieved by adjusting the position of the pilot valve sleeve. Moving the sleeve downwards closes the steam valve further, and the turbine slows down. Similarly, moving the sleeve upwards increases the speed.

1 974 FORM-FIRST CONFERENCE 125 As the continuous power rating of the turbines increased, so the throttle valves increased in size, requiring a larger volume of control oil. To prevent the turbine over-speeding in the event of a sudden load reduction, a dump valve is fitted to drain the relay oil from the throttle valve cylinder faster than it could drain back past the governor pilot valve beat. Relay oil pressure from the top beat of the pilot valve actuates the dump valve. An overspeed trip-ring actuates an emergency trip-valve which connects the high-pressure relay oil acting under the emergency steam valve piston to drain so that the valve shuts under spring action. Simultaneously, the oil is drained from the throttle valve relay cylinder so this valve closes, again under spring action. Fig. 2-Assembly of throttle and emergency steam valves showing the oil servo-cylinder which amplifies the speed governor movement and improves transient response.

126 FORTY-FIRST CONFERENCE 1974 Addition of an oil-servo system As the turbine rating and throttle valve size increased still further, it became impossible to pass sufficient oil through the speed governor pilot valve beats to operate the throttle valve with the required rate of response and stability. Therefore, in the early 1960's, an oil servo-cylinder and an accumulator were added to the system to amplify the speed governor action and improve transietn response (see Figure 2). The speed governor itself responds to load changes in the same way as described for Figure l, but the modulated oil pressure signal passes to the servo-cylinder mounted on the throttle valve relay cylinder. On load increases, the servo pilot valve allows oil, at a pressure of 420 kpa from the accumulator, to pass into the throttle valve relay cylinder to open the valve further. Simultaneously, a feedback linkage returns it to its original position, cutting off further oil flow. The system is again in equilibrium, with the throttle valve in a new position, until again disturbed by a movement of the control governor. Conversely, on load decrease, the servo pilot valve allows oil to drain from the throttle valve relay cylinder so that the valve closes further. This governor system is very sensitive and has a quick response time. In some cases of parallel operation with a public electricity supply system having a frequency which, far from being constant, varies cyclically for + one cycle, the resulting repeated turbo-alternator speed changes caused the governing to become unstable. A stabilizer was fitted to the throttle valve feed-back spring arrangement, and this restored governor stability. It is pointed out, however, that this design of speed governor system fitted to turbines in other parts of the world, where there is no electrical system frequency problem, is perfectly stable. Speed droop and parallel operation It is felt that a few words at this point about speed droop and parallel operation with a public electricity supply system would be of interest. A governor is said to be stable if, for every turbine rotor speed, the governor is able to take up a definite position. If the governor spring rate matched the increase in centrifugal force of the governor weights for each unit change in weight radius resulting from a speed change, the system would hunt wildly from steam valve shut to valve wide open, without ever reaching a balance. To overcome this situation, the spring force must increase faster than the weight force as the speed rises. When the rotor speed increases, centrifugal force of the governor weights momentarily exceeds the spring force, so that they move outwards, moving the pilot valve to decrease the steam valve opening. Reduced steam flow limits the speed increase, so the spring force again balances the centrifugal force and the governor takes up a new position of equilibrium at a higher speed. The converse applies for a speed decrease. The speed variation from no-load to full-load is called the governor droop or regulation. This is usually set to four per cent, but can be greater when required. To adjust the speed droop of some centrifugal governors involves fitting weight springs having a different rate. The speed governor itself can be made stable with a relatively small speed droop. Unfortunately, a stable governor does not ensure a stable speed governing system. The complete system has a chain of stability involving additional factors, such as the time lag before the steam throttle

1974 FORTY-FIRST CONFERENCE 127 valve moves in response to a speed change, the volume of steam entrained between the throttle valve and the nozzles, the inertia of the turbine rotor, gears and alternator, excitation control, and so on, all of which affect the time lag before the speed recovers after a load change. When a privately-owned generating set operates in parallel with a public electricity supply system, it is incapable of influencing the frequency of the system, and its shaft speed will follow the system frequency. Because the shaft speed cannot change, the speed governor becomes a load governor or, more literally, a steam flow governor. Therefore, once the governor is set and assuming a constant system frequency, the public supply system will accept all variations in the electrical power demand. To enable the turbine to accept more or less load, the governor spring controlling force must be adjusted by the speeder gear. See Figure 3, lines "A", "B", "C" and "D", which represent different speeder gear settings for a governor having a four per cent droop. If the public supply frequency falls, the turbine speed governor responds to the fall in speed, automatically opening the throttle valve wider, and the turbine accepts a larger share of the load, e.g., from line "F" to line "G" relative to line "B"; the load increases from 75 to 95 per cent. Conversely, if the frequency rises to line "E", the turbine load decreases to 55 per cent. If the governor is set to line "A", i.e., full-load at 100 per cent of rated frequency, and the frequency falls to line "G", the alternator would be overloaded 20 per cent, provided sufficient steam can pass through the nozzles. The turbine driver can counteract this automatic procedure by adjusting the speeder gear. The overload condition can be avoided by fitting a positive stop to limit the governor travel in the direction which opens the throttle valve. This may be necessary to avoid overloading the alternator and gear. It will be appreciated that if the public supply frequency fluctuates rapidly, the turbine driver will find it difficult to control the position. This subject is discussed further below under the system employing a Woodward speed governor. Governor performance and steam conditions. If the turbine inlet steam conditions are increased, the available heat drop per kp of steam is greater. Therefore, the turbine needs less steam of reduced specific volume to generate full-load. This has the effect of reducing the throttle valve travel and also the speed droop from full-load to no-load. The slope of the lines "A" to "D" in Figure 3 is reduced and the effect on the load carried by the turbine due to changes in public supply system frequency is magnified. This effect can be corrected by fitting a smaller diameter throttle valve which would restore the valve travel and governor movement. Alternatively, the no-load to full-load speed droop can be adjusted. Mechanical governor system Turbo-alternators currently on order and being supplied to the sugar industry, in the 3 MW to 6 MW power bracket with relatively low inlet steam conditions, are being fitted with a mechanical governor system incorporating a Woodward speed governor and control valve actuator (see Figure 4). Due to the increased steam volume flows involved, a single throttle valve relay cylinder would become very large if operated by 420 kpa oil.

1 28 FORTY-FIRST CONFERENCE 1974 PER CENT OF RATED SPEED OR FREQUENCY Fig. 3-Chart showing change of kilowatt load carried by a back-pressure turbo-alternator, having normal speed governor gear and operating in parallel with the public supply system, due to variation of speeder gear setting or system frequency. The speed governor has a mechanical output, which is connected to two sequentially-opening throttle valves through an actuator, which uses oil at a pressure of 1700 kpa to amplify the governor movement, and a fulcrum lever arrangement. This system reduces the oil quantity required and gives a quicker response to changes. The speed governor is driven from the turbine rotor. It is a centrifugal flyweight type which operates through a hydraulic servo-mechanism

1974 FORTY-FIRST CONFERENCE 129 to produce the mechanical output. The hydraulic system is self-contained within the governor casing. The output is connected to the actuator floating lever which controls the amplifying servo system. A rotor speed decrease, due to additional load, moves the pilot valve upwards to admit oil to the top of the servo-piston. This moves downwards against its return-spring and, through the mechanical fulcrum lever, opens the throttle valve wider. The extra steam flow increases the speed and the corresponding governor output movement causes the floating lever to move the pilot valve downwards until it relaps the control port and the system is again in equilibrium at a new speed. The converse operation applies for a speed increase. A useful feature of this speed governor is the external knob which permits speed-droop adjustment over a range of 0-10 per cent without changing any parts. When the turbine is running in parallel with a public electricity supply system and a one per cent frequency change occurs with the turbine governor set for four per cent droop, the load change on the turbine would be 25 per cent. If this is inconveniently large, the droop setting can readily be widened to say eight per cent, thus halving the load variation. Of course, the droop setting should be returned to normal as soon as the tie-line is opened, otherwise the turbo-alternator speed rise on loss of load would be excessive. This would cause the machine to trip unless the overspeed trip is set high. Whatever type of mechanical speed governor is fitted, if the public electricity system frequency varies, load transfer will take place, the value of which is determined only by the frequency variation and the turbine governor droop setting. However, if the governor can be set-up out of the way so that it does not respond to speed changes, the load would not be affected by public supply frequency changes. This speed governor has an external load-limit knob which permits this arrangement. After putting the desired load on the turbo-alternator with the speeder gear, the load-limit knob is set down to this value and the speed-adjusting knob wound up slightly. Then, if the system frequency falls, the loadlimit stop prevents the turbo-alternator taking on extra load, while if the system frequency rises, load cannot be shed provided the speed remains below that previously set-up on the governor. TO EWERCEUCY EMERGENCY VALVE MECHAUICAL LlUK TO CTUATE PllOl VALVE COIITROL VALVE OIL FROM CONTROL 011 PUMP CONTROL VALYt I OF2 Fig. 4-A mechanical speed governor system operating two throttle valves through an actuator, which amplifies the governor movement, and a fulcrum lever system.

130 FORTY-FIRST CONFERENCE 1974 High-pressure hydraulic governor system As the turbo-alternator rating increases further, say, beyond 6 MW, high efficiency increases in importance relative to simplicity. As mentioned earlier, this involves a different design of turbine having multiple throttle valves to control the up-stream nozzle steam pressure. A mechanical control system for more than two throttle valves becomes rather complex. These turbines are fitted with a hydraulic relay system as shown in Figure 5. To permit small throttle valve relay cylinders, provide quicker response and use less oil for the same power, this system operates on oil at a pressure of 10 000 kpa. The system includes accumulators which cope with transient demands for control oil. The system incorporates a Regulateurs Europa-type speed governor driven by an auxiliary shaft from the steam end of the turbine rotor. It is of the centrifugal fly-weight type and operates through a hydraulic servo-mechanism, the oil reservoir and pump for which are incorporated within the governor casing. The governor provides a mechanical output which is connected to an oil-pressure modulating valve. Movement of the mechanical input causes a pilot valve assembly to deliver oil at a pressure proportional to input position. Figure 6 shows the throttle valve actuator. It is similar in operation to the system shown in Figure 2, incorporating an oil-servo to operate the power piston with a feed-back linkage. The pilot-valve springs are designed so that a series of actuators open a number of throttle valves sequentially with increasing modulated oil pressure. Like the Woodward governor, this governor has a facility for externally adjusting the speed droop over a range of 0-10 per cent, without changing any parts. The emergency governor system operates on a 420 kpa oil system. The trip-valve operates a second oil trip-valve in the high-pressure oil system to ensure that the throttle valves are closed immediately the main trip-valve operates. Turbo-alternators supplied to the British Sugar Corporation in beet-sugar factories in England and to other process industries are using this system. RETURN TO FROM 10000 kpo -- CONTROL OIL IOW0 kp' FROM 420 LP. RELAY TAW OIL PUMP - TRIP SYSTEM OIL 420 k ~ a OIL PUMP VIA OIL COOLERS ---- MODULATED OIL 0-1700 kpa Fig. 5-A high-pressure oil relay governor system in which a modulator converts the speed governor mechanical movement into a proportional oil pressure which operates multi~le throttle valves.

1974 FORTY-FIRST CONFERENCE RETURN SPRING WWER PISTON Fig. 6-Sectional arrangement of a high-pressure oil throttle-valve actuator and oil-servo system. Future steam/power ratios A continuous programme of modernization and increasing capacity is taking place in most sugar mills and factories. This usually results in an increased electrical power demand in relation to the low-pressure steam demand. With back-pressure turbo-alternators, this involves either buying more power from the public electricity supply system or obtaining more power per kp of steam passing through the turbine. An increase in the ratio : supply steam pressure/exhaust steam pressure will give this increase in power. The exhaust pressure may be unalterably fixed by existing process plant. The most obvious way of increasing the pressure ratio is to raise the supply pressure if a new boiler is being contemplated. The British Sugar Corporation factories increased their boiler pressures during modernization programmes (Lanyon 1973). The pressure was first doubled to 2 200 kpa, at which a simple condensate/boiler feed water treatment system was satisfactory, but, after a decade, this was found to be only marginally adequate. Boilers are now installed at 4 400 kpa and 425 C. Their present policy is to abandon two or more turbo-alternators in favour of larger single units ranging in size from 5 MW to 10 MW. Figure 7 shows the gross improved output available per kilogram of steam at different turbine stop valve pressures when exhausting to a gauge pressure of 200 kpa, based on turbo-alternators supplied to the British Sugar Corporation. Conclusions The electrical power and steam demands in sugar mills and factories continue to increase. These can still be supplied by larger sizes of turboalternators operating either independently or in parallel with the public

132 FORTY-FIRST CONFERENCE 1974 Inlet steam Inlet steam Steam Approx. Set output pressure temp rate date kw k Pa OC kg/kwh of installation 5000 4300 405 8.4 1970 8000 3300 420 8.2 to 10,000 4300 410 8.0 1973 Table showing the reduction in typical steam rate of back pressure turbines, as supplied to the British Sugar Corporation, achieved by increasing the turbine inlet steam conditions and kw rating of the set. In all cases the exhaust pressure is 200 kpa. electricity supply system. The larger steam volume flows require large;' and/or multiple throttle steam valves. Operation in different mills and factories and with various local electricity supply systems present unique problems. Steam turbine manufacturers have gained extensive specialist experience of these problems. They are constantly reviewing the design of turbo-alternator governor systems in an endeavour to improve and simplify the design of various components and the methods of operation. Acknowledgement The authors wish to thank the directors and management of W. H. Allen Sons & Co. Ltd., for permission to publish this paper, colleagues who helped in its preparation, and the British Sugar Corporation for permission to use extracts from Mr. Lanyon's paper. REFERENCE Lanyon W. M. (1973) Steam plant management in the sugar beet processing industry. Inst. Mechanical Engineers, Con. Pub. 12 1973, London. Convention on steam plant operation. Paper C116/73. 79-84.