Method and apparatus for controlling the control valve setpoint mode selection for an extraction steam turbine

A microprocessor-based controller for an extraction type steam turbine-generator unit capable of selecting from a variety of predetermined control strategies and implementing corresponding valve position control loops by generating appropriate valve position control signals in accordance with operator-chosen setpoint signals and turbine operating level signals. In a particular control strategy, automatic compensation of megawatt output is achieved during the extraction mode of turbine operation by summing a megawatt setpoint signal from a feedback loop with a feedforward extraction valve setpoint signal and a megawatt reference signal, which sum is then applied to the turbine control valves to enable tight megawatt control during the extraction operation.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present application is related to two concurrently filed patent 
applications bearing Ser. Nos. 562,607, filed Dec. 19, 1983 and 562,508 
filed Dec. 19, 1983 by the same inventors, which are assigned to the same 
assignee as the present application, the disclosures of which are 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
The invention relates to steam turbine control systems, more particularly 
to a control system for an extraction type steam turbine. 
A common aspect of many industrial environments is the required 
simultaneous provision of adequate process steam and electric power. 
Extraction turbines allow a portion of their inlet steam flow to be 
directed to a process steam header by use of an extraction valve. They are 
widely used in industrial environments for cogeneration of process steam 
and electric power requirements because of their ability to accurately 
match these requirements in a balanced and stable fashion. In any given 
industrial plant, these requirements vary over time and an extraction 
turbine control system attempting to provide and match these requirements 
must respond accordingly. 
Industrial utilization of extraction turbines requires appropriate 
adjustment of front-end extraction turbine control valves and the 
extraction valve. These adjustments are made through application of 
well-known valve position control loop technology. 
A control loop is established by a combination of signals, including one 
representing the desired level of turbine operation, and one representing 
the existing level of turbine operation. A prior art analog controller 
functions in the control loop to compare these two signals, and noting any 
discrepancy, it operates to automatically bring the turbine operation to 
that level required to balance these signals. The particular combination 
of signal elements in a control loop reflects the control strategy used by 
the system designer. The combined operation of several control loops 
achieves the overall control philosophy used in the control system design. 
The majority of extraction turbines in service are used in the industrial 
area--steel mills, refineries, paper mills, sewage treatment plants, etc., 
where in the past, generation of electricity by the extraction turbine was 
a byproduct and not really a necessity. The major use of the extraction 
turbine in these cases was for process steam availability. The extraction 
process steam is used to feed heaters in the plant, such as auxiliary 
heaters, furnace heaters and building heaters. It is used to power 
steam-driven pumps and is also used in various quenching processes 
associated with steel mill operations, such as coke-quenching and 
quenching of hot metal strip as it exits the rolling mill. 
Prior art extraction turbine control systems have emphasized process steam 
extraction control at the expense of electric power output or megawatt 
control, that is, they have achieved tight extraction control while 
allowing megawatt output to deviate and float to a level consistent with a 
given process steam extraction requirement. Often, a complex, lengthy and 
delicate valve readjustment procedure was performed by an operator in a 
local control mode to bring megawatt output back to a desired level after 
having deviated due to a previous adjustment in the process steam 
extraction level via the extraction valve controller. A major difficulty 
of this readjustment procedure was presented by the requirement that it 
was performed so as to avoid a process upset, that is, that it was 
bumpless. 
The operator's readjustment procedure was further complicated by the need 
to readjust settings due to the drift introduced by prior art analog 
control system circuitry which depended on discrete electronic components 
such as operational amplifiers, capacitors, diodes and resistors, etc. 
These circuits were prone to drift out of calibration over time and with 
temperature variations. 
With unceasing increases in the costs of energy, personnel and equipment, 
the inadequacies of older extraction turbine control strategies have 
become magnified. The potential for operating cost reductions may be 
available through the application of industrial energy management systems. 
These optimization systems are arranged to provide the front-end plant 
boiler controls with the steam pressure, steam flow, and electrical energy 
requirements for the entire industrial plant. In order for plant 
optimization to occur, the boiler controls must be able to transmit to the 
extraction turbine control system the required level of extraction steam 
pressure and/or flow and/or megawatt output. Use of the boiler control 
system as a remote control system to automatically send into the 
extraction turbine control system all of the various process setpoints 
requires the provision of an extraction turbine control system capable of 
responding to them and moving its operational level in a bumpless fashion, 
without the need for operator intervention. The extraction turbine becomes 
a more important factor in this case especially in the cogeneration sense 
where power is being sold and delivered to the utility power grid. Now, 
tighter control of megawatt output becomes a more important function than 
it has been in the past. 
It can be seen that prior art extraction turbine control systems reflected 
control strategies which did not fully exploit the extraction turbine 
capabilities noted earlier. It would therefore be desirable to provide a 
method for selection, from multiple available control loops, a particular 
control loop or combination of control loops reflecting a particular 
control strategy or strategies. It would also be desirable to provide a 
simplified method of extraction turbine control to fully utilize the 
capabilities of the extraction turbine in meeting process steam and 
electrical energy requirements. It would also be desirable to provide an 
extraction turbine control system that makes more efficient use of the 
extraction turbine by achieving tight control of extraction steam 
requirements and tight control of megawatt output through megawatt output 
correction during a process steam extraction mode. It would also be 
desirable to provide an extraction turbine control system with control 
loops that are free from drift in calibration of circuit components, 
thereby reducing periodic maintenance requirements. It would also be 
desirable to provide an extraction turbine control system that is capable 
of accepting remotely generated optimization setpoint signals and 
adjusting its operational level in accordance therewith, without the need 
for operator intervention once the operator has chosen a remote mode. Such 
a control system would enable the realization of front-end boiler fuel 
cost reductions because of the smoother boiler operation associated with 
better and more stable extraction turbine control. 
SUMMARY OF THE INVENTION 
An extraction type steam turbine-generator unit is provided with a 
microprocessor-based controller for selecting predetermined control 
strategies and implementing corresponding valve position control loops by 
generating appropriate valve position control signals in accordance with 
either remotely generated or operator-chosen setpoint signals and turbine 
operating level signals. A particular control strategy is disclosed 
involving automatic compensation of megawatt output during the extraction 
mode of turbine operation by summing a megawatt setpoint signal from a 
feedback loop with a feedforward extraction valve setpoint signal and a 
megawatt reference signal, which sum is then applied to the turbine 
control valves to enable tight megawatt control during the extraction 
operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, a prior art extraction steam turbine control 
system 10 is shown in which an extraction turbine 12 is fed with inlet 
steam at a fixed temperature and pressure from a boiler (not shown) which 
enters at the high pressure (HP) section 14 of the extractor turbine 12 
through a pair of upper and lower control valves 16. The steam drives the 
HP turbine blades and then exits the seventh stage of the HP section 14 to 
the industrial process steam header 18 and to the low pressure (LP) 
section 20 of the extraction turbine 12. 
Maximum process steam flow to a plant process where it is to be used 
corresponds to a minimum opening of the extraction valve 22. However, the 
extraction valve 22 is kept from fully closing to maintain a flow of 
cooling steam to the LP section 20 of the extraction turbine 12, which 
overcomes the heat generated by the friction of the moving LP blades in 
the dense atmosphere of steam. An electric power generator 24 is coupled 
to the turbine shaft for production of electric power for use in the plant 
process, or possibly for sale to the electric utility power grid. 
The extraction turbine 12 is started in a conventional manner, and after 
being loaded, the generator 24 is producing megawatts and the extraction 
valve 22 is wide open, corresponding to no extraction reference signal 34 
in an initial system operating mode. Therefore, the extraction valve error 
signal 29 is zero in this case and does not contribute in summer 25 to the 
control valve setpoint signal 26. A valve controller 27, typically an 
electrohydraulic valve servo and servo driver loop, positions the control 
valves 16 in an open-loop fashion in accordance with the control valve 
setpoint signal 26 which is equal to the megawatt reference signal 28 from 
the operator's panel 30. 
The extraction valve setpoint signal controller 32 interfaces with the 
operator's panel 30 for establishing the level of performance, as 
represented by the extraction reference signal 34, within the process 
steam extraction mode of turbine operation. The extraction valve setpoint 
signal 36 is fed to a valve controller 38 for positioning the extraction 
valve 22. A steam pressure/flow transducer 40 on the industrial process 
steam header 18 provides a feedback signal 42 to the extraction valve 
setpoint signal controller 32 to maintain a stable extraction operation. 
As noted earlier, this scheme achieves tight extraction control but does so 
with negative consequences for megawatt output. The megawatt output 
existing prior to entry into the extraction operation will now tend to 
deviate and float to a level consistent with the process steam extraction 
requirement once this requirement has been established by the extraction 
valve setpoint signal controller 32. In other words, the inlet steam 
energy is converted to electrical energy by the turbine only to the extent 
that the inlet steam is not extracted for other plant use. 
An attempt is made to roughly correct megawatt output for a change in the 
extraction operation. The extraction valve error signal 29 is used as a 
feedforward signal in summer 25 as to adjust the control valve setpoint 
signal 26. The feedforward signal does not entirely achieve this because 
of the lack of megawatt feedback. 
The present invention provides a microprocessor-based control system for 
operating an extraction turbine to satisfy extraction steam and megawatt 
output demand. The present invention provides two selectable megawatt 
control loops, one of which can be placed in service when tighter megawatt 
control is desired as extraction steam demand is met. Each of these two 
megawatt control loops provides a separate type of contribution to the 
control valves. With one of these megawatt control loops in service while 
the extraction turbine is in the full condensing mode, meaning no 
extraction steam is being taken, the control valve setpoint signal is set 
at a level such that a megawatt error signal is zero. If the extraction 
control loop is in service and tighter megawatt control is desired, the 
second megawatt control loop uses a megawatt setpoint signal as a trim 
signal in conjunction with the extraction valve setpoint signal and the 
megawatt reference signal to generate an extraction-corrected control 
valve setpoint signal so as to compensate for an undesired change in 
megawatt output resulting from a process steam extraction operation. 
FIG. 2 shows the detail of the operator's panel 50 portion of the 
extraction turbine control system practiced in accordance with the present 
invention. The panel 50 includes an annunciator display 52 indicating 
system abnormalities, several digital readout displays, a group 54 
indicating desired system operation levels and a group 56 indicating 
actual system operation levels, valve position panel meters 58, and a 
series of control pushbuttons 60 for megawatt control, extraction control 
and manual control. The control pushbuttons 60 allow the operator both to 
select the system operation mode and to establish the desired level of 
operation within the selected mode. 
FIG. 3 shows the preferred embodiment of the extraction turbine control 
system 70 practiced in accordance with the present invention. Two signal 
controllers 72 and 74 are provided to ultimately generate a control valve 
setpoint signal 75 to the valve controller 76 for positioning the 
extraction turbine control valves 16, a control valve setpoint signal 
controller 72 for use when the extraction control loop is out of service 
and the megawatt control loop is in service and a dual mode control valve 
setpoint signal correction controller 74 for use when both the extraction 
control and megawatt control loops are in service. In accordance with each 
of the several predetermined and distinct control strategies provided by 
the present invention, either the closed-loop control valve setpoint 
signal 78 or the correction mode-dependent control valve setpoint signal 
80 is chosen to be used as the control valve setpoint signal 75. This 
choice is dependent upon the operational state of a second controller, the 
control valve setpoint signal selection controller 82. 
The operational state of the control valve setpoint signal selection 
controller 82 is determined by a control mode selector 84. The control 
mode selector 84 generates a logic control signal 86 which determines the 
choice between the two available control valve setpoint signals 78 and 80 
generated respectively by the two signal controllers 72 and 74. The 
control valve setpoint signal selection controller 82 uses this logic 
control signal 86 to determine which setpoint signal 78 or 80 will 
actually drive the valve controller 76 to the exclusion of the other 
available control valve setpoint signal. The signal controller 72 or 74 
which generates the unused setpoint signal 78 or 80 operates in a 
conventional tracking mode, so as to provide bumpless transfer from one 
control strategy to another. 
The control valve setpoint signal selection controller 82 employs a 
transfer functional control block 88. The transfer functional control 
block 88 has an algorithm for transfer of one or two analog inputs. Based 
on the logical state of a mode signal, the transfer functional control 
block 88 gates out one of the two analog input signals as the analog 
output signal. When the mode signal is in a "high" logical state, the 
signal on input one is gated out as the output signal. When the mode 
signal is in a "low" logical state, the signal on input two is gated out 
as the output signal. In this fashion, the control valve setpoint signal 
selection controller 82 implements the desired control strategy chosen by 
the operator via the operator's panel 50 and the control mode selector 84, 
as described further herein. 
Control of the extraction valve 22 to regulate extraction steam flow or 
pressure is implemented through an independent controller, the extraction 
valve setpoint signal controller 90, which responds to the extraction 
reference signal 91. An extraction feedback control loop is employed, 
which includes the use of a valve controller 92, a steam pressure or flow 
sensor 94 and an extraction feedback signal 95. A signal element of the 
extraction control loop, the extraction valve setpoint signal 96, is used 
to correct the operation of one of the two megawatt control loops in 
accordance with a particular control strategy described further herein. 
The control mode selector 84 generates two logic control signals 86 and 98 
in response to pushbutton selections made at the operator's panel 50. The 
"megawatt loop in service and extraction loop out of service" (MWINEXTOUT) 
logic control signal 86 determines the operational state of the selection 
controller 82 and the "both loops in service" (BOTH LOOPS) logic control 
signal 98 determines the correction mode of the dual mode control valve 
setpoint signal correction controller 74, all in accordance with the 
predetermined control strategies herein described. 
With reference to FIG. 3, the operation of the extraction turbine control 
system 70 is now described. Assume an extraction turbine operation in 
which little or no extraction steam is being taken from the extraction 
turbine 12, and the generator 24 is generating megawatts. As there is no 
extraction steam being taken, no extraction control loop has been selected 
in pushbutton group 100 on the operator's panel 60. Therefore, the 
"extraction loop in service" EXTIN logic control signal 101 is now in a 
"low" logical state. If the operator chooses to place a megawatt control 
loop in service at this time, selection of pushbutton 102 on the 
operator's panel 60 (see FIG. 2) causes the "the megawatt control loop in 
service" (MWIN) logic control signal 103 to go to a "high" logical state. 
The control mode selector 84 interprets both logic control signals 101 and 
103 so as to cause the MWINEXTOUT logic control signal 86 to go to a 
"high" logical state. This establishes the first operational state. In 
this event, the transfer functional control block 88 of the control valve 
setpoint signal selection controller 82 transfers input one as its output, 
so that the control valve setpoint signal 75 is now equal to the 
closed-loop control valve setpoint signal 78 derived from the control 
valve setpoint signal controller 72 in a megawatt feedback control loop 
utilizing the megawatt output demand as represented by the megawatt 
reference signal 104 from the operator's panel 50, the megawatt feedback 
signal 105 from the megawatt transducer 106, and the valve controller 76. 
As shown, the extraction reference signal 91 and the megawatt reference 
signal 104 are generated by the operator's panel. In the preferred 
embodiment, the extraction reference signal 91 and the megawatt reference 
signal 104 can be generated by a remote control system (not shown) which 
tracks the existing extraction steam level as represented by the 
extraction feedback signal 95 and the existing megawatt output as 
represented by the megawatt feedback signal 105 and generates an 
equivalent extraction reference signal and an equivalent megawatt 
reference signal, respectively, so as to achieve a bumpless transfer on a 
transition to the remote control mode. This same approach would be used to 
accomplish a bumpless transfer from the remote control mode back to the 
local control mode. 
When the operator chooses to place the extraction control loop in service 
in addition to the megawatt control loop already in service, pushbutton 
selection of any of the several extraction control loop "in service" 
pushbuttons 100 on the operator's panel 50 (see FIG. 2) is interpreted by 
the control mode selector 84 so as to cause the logical state of the 
MWINEXTOUT logic control signal 86 to go to a "low" logical state while 
the BOTH LOOPS logic control signal 98 goes to a "high" logical state. 
This combination establishes the second operational state. 
In this event, the BOTH LOOPS logic control signal 98 controls the 
correction mode of the dual mode control valve setpoint signal correction 
controller 74 so as to initiate generation of a correction mode-dependent 
control valve setpoint signal 80. Based on this same operator choice, the 
MWINEXTOUT logic control signal 86 sets the mode signal of the control 
valve setpoint signal selection controller 82 to a "low" logical state. 
The control valve setpoint signal selection controller 82 then operates to 
select the correction mode-dependent control valve setpoint signal 80 from 
the two setpoint signals 78 and 80 available to it, so as to transfer the 
correction mode-dependent control valve setpoint signal 80 as the actual 
control valve setpoint signal 75 which is then fed to the valve controller 
76. This operation of the control valve setpoint signal selection 
controller 82 establishes a second megawatt control loop, replacing the 
control valve setpoint controller 72 with the dual mode control valve 
setpoint signal correction controller 74. The second megawatt control loop 
provides tighter control of megawatt output during an extraction 
operation. 
Generation of the correction mode-dependent control valve setpoint signal 
80 in the dual mode control valve setpoint signal correction controller 74 
is based upon three combinations of three signal inputs. The input signals 
are the extraction valve setpoint signal 96 from the extraction valve 
setpoint controller 90, the megawatt reference signal 104, from the 
operator's panel 50, and the megawatt feedback signal 105 from the 
megawatt transducer 106. 
In operation, an increase in the demand for extraction steam affects the 
extraction turbine 12 by decreasing the steam flow to the low pressure 
portion 20 of the extraction turbine 12. A drop then occurs in the 
megawatt output of the extraction turbine 12, and the dual mode control 
valve setpoint signal correction controller 74 senses this in the first of 
its dual correction modes by comparison of the megawatt reference signal 
104 with the megawatt feedback signal 105 to generate a megawatt error 
signal 108 which is the difference between these two signals and which is 
determined by the delta functional control block 110. 
The megawatt error signal 108 is fed to a PID functional control block 112 
which operates to generate a megawatt setpoint signal 114 based on a 
proportional plus integral plus derivative function of the megawatt error 
signal 108. The megawatt setpoint signal 114 is then fed to input one of 
the transfer functional control block 116 where it is gated out since the 
BOTH LOOPS logic control signal 98 has set the mode signal in a "high" 
logical state. The summer functional control block 118 sums the megawatt 
setpoint signal 114 with both the megawatt reference signal 104 and the 
extraction valve setpoint signal 96 so as to generate the correction 
mode-dependent control valve setpoint signal 80. The summer functional 
control block 118 algorithm produces an analog output which equals the sum 
of its three analog inputs, each of which has a gain term. The gain terms 
are used to weight the inputs differently with respect to each other. 
As noted earlier, this is the particular control strategy in which the 
extraction valve setpoint signal 96 is used to correct the operation of 
one of the two provided megawatt control loops. In the above example, the 
increase in extraction steam demand is represented by an increase in the 
extraction valve setpoint signal 96 which is utilized in a feedforward 
fashion by the summer 118 of the dual correction mode control valve 
setpoint signal controller 74. 
The combination of signals in the summer 118 results in tighter megawatt 
control because the control valve setpoint signal 75 is derived from the 
correction mode-dependent control valve setpoint signal 80 from the dual 
mode control valve setpoint signal correction controller 74. By using the 
extraction valve setpoint signal 96 as a feedforward signal in summer 118, 
the correction mode-dependent control valve setpoint signal 80 is 
extraction-corrected and begins corrective control of the steam turbine 
operation in anticipation of a power generation drop that would otherwise 
occur as a result of the increased extraction steam demand. 
If the operator has chosen to operate the extraction turbine without 
utilizing the megawatt setpoint signal 114 as a trim signal, selection of 
the megawatt control loop "out of service" pushbutton 120 on the 
operator's panel 50 causes the MWIN logic control signal 103 to be in a 
"low" logical state, which will be interpreted by the control mode 
selector 84 so as to cause the MWINEXTOUT logic control signal 86 to be in 
a "low" logical state. The mode signal of the control valve setpoint 
signal selection controller 82 will also be in a "low" logical state, so 
that the correction mode-dependent control valve setpoint signal 80 will 
be selected as the control valve setpoint signal 75. 
In this event, regardless of the status of the extraction control loop 
pushbuttons 100, the control mode selector 84 will also generate a BOTH 
LOOPS logic control signal 98 in a "low" logical state, which will set the 
mode signal on the transfer functional control block 116 so as to gate out 
input two as its output, which is a null input generated by the analog 
value generator functional control block 122. 
In this second correction mode of the dual mode control valve setpoint 
signal correction controller 74, the megawatt setpoint signal 114 will not 
contribute to the summer functional control block 118, and one of two 
signal combinations are used in the summer functional control block 118, 
each combination corresponding to one of two correction submodes for 
generating the correction mode-dependent control valve setpoint signal 80 
as its output. If the extraction valve setpoint signal controller 90 is 
operating because an extraction control loop has been selected, the 
megawatt reference signal 104 and the extraction valve setpoint signal 96 
are used to generate the correction mode-dependent control valve setpoint 
signal 80 in an open-loop corrected fashion. Otherwise, with no 
extraction, only the megawatt reference signal 104 is used. In the latter 
case, the correction mode-dependent control valve setpoint signal 80 is 
really just an open-loop control valve setpoint signal and has no 
correction for the extraction operation, as there is none. Either of these 
cases provides open-loop control over megawatt output the distinction 
between them being whether there is or is not a feedforward contribution 
from the extraction valve setpoint signal 96, which depends on the 
operation of an extraction control loop. The accuracy of the megawatt 
output in either case will depend upon the calibration of the control 
valve cams, mechanical linkages, and position servo loop printed circuit 
cards. This calibration attempts to translate the value of the control 
valve setpoint signal 75 into the actual position of the control valves 16 
without the benefit of a megawatt feedback error signal which would 
compensate for any inaccuracies in the calibration. However, if there is 
an extraction operation, the feedforward contribution of the extraction 
valve setpoint signal 96 still provides the control valves 16 with rough 
compensation for the extraction operation. 
In the preferred embodiment, the turbine control system incorporates use of 
a single-board sixteen-bit microprocessor and an input and output 
interface having analog and digital conversion capability suitable for use 
in process environments, such as the MTCS--20.TM. turbine control system, 
sold by the Westinghouse Electric Corporation. This microprocessor-based 
turbine control system has the inherent advantage of freedom from drift in 
calibration of components, along with ease of start-up and reduced 
maintenance requirements. 
A typical MTCS--20.TM. turbine control system hardware configuration 200 is 
shown in FIG. 4. The MTCS--20.TM. turbine control system uses a standard 
WDPF.TM. Multi-bus.RTM. chassis configuration 202 with six printed circuit 
cards and with Westinghouse Q-line I/O, all of which is disclosed in a 
patent application bearing Ser. No. 508,951, filed June 29, 1983, assigned 
to the present assignee and incorporated herein by reference. The 
pertinent part of this application is the portion dealing with the "drop 
overview" as the MTCS--20.TM. turbine control system is currently sold by 
Westinghouse as a stand-alone controller not connected to a data highway. 
.RTM.Multibus is a registered trademark of Intel Corp. MTCS--20.TM. and 
WDPF.TM. are trademarks of Westinghouse Electric /Corporation and Q-line 
is a series of printed circuit cards sold by Westinghouse Electric 
Corporation. 
The dual functional processors 204 and 206 give the MTCS--20.TM. turbine 
contorl system its first level of redundancy. The primary processor 204 is 
responsible for control loop execution while the normal function of the 
secondary processor 206 is tuning of the controller, listing the control 
loop, and displaying control parameters. If the primary processor 204 
fails, the secondary processor 206 will automatically begin executing the 
control loop where the primary processor 204 left off. These two boards 
also contain duplicate sets of the algorithm library, which is described 
further herein. 
The .RTM.Multibus-DIOB interface card 207 gives the processors access to 
the I/O system. The Q-Line I/O bus 208 allows mixing of printed circuit 
point cards of any style anywhere on the bus 208. These cards are located 
in the I/O crates 210 and can be analog or digital, input or output, in 
any combination, and can accommodate a large variety of signal types. In 
the MTCS--20.TM. turbine control system 200 these cards provide the 
interface to the field I/O signal group 212, the engineer's diagnostic 
panel 214, the operator's panel 50, and the manual system 215. 
Two memory components of the MTCS--20.TM. turbine control system 200 
perform separate functions. A shared-memory board 216 is a 128K RAM board 
providing communication between the two functional processors 204 and 206. 
A battery-backed RAM board 218 is a 16K memory board on which the software 
application program for the control loops is stored. It retains its 
contents for up to 3 hours following a loss of power. 
The last card in the .RTM.Multibus chassis 202 is an RS--232C interface 
board 220 which interfaces a cassette recorder 222 used for permanent 
storage of the software application program for the control loops, and a 
keyboard/printer 224 used for entering, changing, and tuning the control 
loops. 
The second level of redundancy in the MTCS--20.TM. turbine control system 
200 is an analog system, the manual system 215. It protects against 
failure of the digital system, in which case it would be automatically 
switched into operation to take control of the turbine. It also permits 
the plant operator to maintain control, while an engineer changes a 
digital control loop, by allowing the operator to manually position the 
turbine control and extraction valves 16 and 22 from the same operator's 
panel 50 used when the digital system is in control. It also constantly 
monitors the turbine speed and, in case of an overspeed condition, closes 
the turbine valves regardless of which system is in control. 
The two I/O crates 210 can each hold up to 12 Westinghouse Q-Line I/O point 
cards. These cards are periodically polled by the software and all process 
information is retained in registers on the individual point cards. These 
registers appear as memory locations to the digital system which obtains 
data through memory accesses and outputs data by memory store commands 
(memory-mapped I/O). Thus the latest process information is always 
available to the system and the time response is not degraded by 
intermediate data handling or buffering. 
Three point cards are dedicated to the engineer's diagnostic panel 214. 
This panel 214 consists of three modules that allow the engineer to 
monitor the status of the diagnostic alarms, control the mode of the 
digital system, and display the output of any two system signals. The mode 
control module in the engineer's diagnostic panel 214 permits an engineer 
to load a control program, tune algorithms in the loop, or display 
parameters on the display module. The mode control module provides 
security from unauthorized used by a two-position keylock switch 226. 
The field I/O signal group 212 is made up of the I/O signals from the field 
I/O hardware which includes field instrumentation such as sensors or 
transducers 94 and 106 in FIG. 3, and field actuators that are located on 
the extraction turbine and the associated steam flow piping. The 
annunciator output signal grouping 228 indicates system abnormalities and 
is typically tied to multiple annunciator display panels in the control 
room or elsewhere. The analog input signal grouping 228 is segregated and 
tied directly to the manual system 215 so that in the event of a loss of 
the digital control system, essential signals for manual control are 
available. The control valve signal grouping 232 includes the valve servo 
position loop signals to and from the servo actuators which tie into the 
valve controllers 76 and 92 (see FIG. 3). 
The software application programs for the control loops of FIG. 3 are 
furnished in the MTCS--20.TM. microprocessor in the form of software 
application program algorithms based on the use of modular functional 
control blocks. The functional control blocks are designed to replace 
tasks which a typical analog or digital control loop needs to perform. The 
set of available functional control blocks forms the algorithm library and 
includes arithmetic blocks, limit blocks, control blocks, I/O blocks, 
auto/manual blocks, (for manual setpoint entry and control), and 
miscellaneous blocks. The miscellaneous category includes functions for 
generating analog and digital values, generating polynomial functions, 
gating one of two analog signals based on the logic state of a mode 
signal, time delays, etc. 
The MTCS--20.TM. turbine control system is designed for interactive entry 
of functional control blocks on a line-by-line basis, to form the 
application program. Each line of the application program consists of the 
functional control block number, the algorithm name (from the algorithm 
library) corresponding to that functional control block, and each of the 
parameter locations forming the arguments or inputs to that algorithm. 
Each functional control block chosen by the operator and listed on a line 
of the application program is task-specific, with only one output, which 
provides a high degree of flexibility and ease of changing. A translator 
handles the functional control blocks in the order in which they were 
entered by the operator. It translates the algorithm name of the 
functional control block, which the operator understands, into a series of 
data blocks in the pre-specified operator-chosen order so that each data 
block has a block number, algorithm number, parameter location, parameter 
location, parameter location, etc. for as many parameters as that 
particular algorithm requires. The translator also checks the syntax of 
the operator-entered data, and thereby preprocesses the application 
program for block-sequential, run-time interpretation by an interpreter. 
The interpreter executes the application program in the functional 
processor and works on the series of data blocks which the translator has 
created. The interpreter calls the algorithms in the order in which they 
were entered, corresponding to the lines of the application program. The 
interpreter also routes the answers generated by each algorithm to the 
correct location in memory for use by later blocks in the application 
program. The use of a run-time interpreter eliminates compiling, thereby 
saving time and increasing the flexibility and ease of programming. The 
completion cycle time of the control loop is user-selectable.