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
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FORM-FIRST CONFERENCE
1974
Fwn c o n T w n
OIL runr
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,
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.
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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.
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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 influencingthe 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 t o variation of speeder gear setting o r 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 P l l O l VALVE
COIITROL VALVE
OIL FROM
CONTROL
011 PUMP
CONTROL VALYt
I OF2
Fig. 4-A mechanical speed governor system operating t w o throttle valves through an
actuator, which amplifies the governor movement, and a fulcrum lever system.
130
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FORTY-FIRST CONFERENCE
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
RELAY TAW
VIA OIL COOLERS
FROM 1 0 0 0 0 k P o
OIL PUMP
-----
CONTROL OIL I O W 0 kP'
TRIP SYSTEM OIL 4 2 0 k ~ a
MODULATED OIL 0 - 1 7 0 0 kPa
FROM 4 2 0 LP.
OIL PUMP
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
m u l t i ~ l ethrottle valves.
1974
FORTY-FIRST CONFERENCE
RETURN SPRING
Fig. 6-Sectional
WWERPISTON
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
Set output
kW
Inlet steam
pressure
k Pa
Inlet steam
temp
5000
8000
10,000
4300
3300
4300
405
420
410
OC
1974
Steam
rate
kg/kWh
Approx.
date
of installation
8.4
8.2
8.0
1970
to
1973
Table showing the reduction in typical steam rate of back pressure turbines, as supplied
t o the British Sugar Corporation, achieved by increasing the turbine inlet steam conditions and k W 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.