Abstract:
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 system frequency correction is achieved in a load control mode upon separation of the turbine-generator from the utility power grid, by detecting turbine speed deviation beyond predetermined limits and correcting turbine speed to synchronous speed without the need for operator intervention.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to three previously filed patent applications (Ser. Nos. 562,378, now U.S. Pat. No. 4,577,281, 562,507, now U.S. Pat. No. 4,550,380, and 562,508, now abandoned) by the same inventors, all filed Dec. 16, 1983, which are assigned to the same assignee as the present application, and which are all entitled &#34;Microprocessor-Based Extraction Turbine Control&#34;, the disclosures of which are incorporated herein by reference. 
     BACKGROUND OF 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. In any given industrial plant, these requirements vary over time and often they are in competition in the sense that any one provision resource attempting to provide and match these requirements must balance them. 
     Extraction turbines are widely used in industrial environments for cogeneration of process steam and electric power. This is because of their ability to accurately match these competing requirements in a balanced and stable fashion. Industrial utilization of this matching capability 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 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. With rising energy costs and optimization of total system capacity, the extraction turbine becomes a more important factor in the system and especially in the cogeneration sense where power is being sold to the utility and being put onto the utility power grid. Now control of the megawatt output becomes a more important function that it has been in the past. 
     In the prior art of extraction turbine control system design, a speed control loop was incorporated as part of the overall process of bringing a turbine-generator on-line. After the operator brought the turbine to synchronous speed, 3600 rpm, he would examine instrumentation indicating the normal criteria used to synchronize the turbine, such as the phase angle and the generator voltage, and after the main generator breaker was closed the generator would be tied to the local plant power distribution system. If the local plant power distribution system was already tied to the utility power grid, the turbine speed would then be locked to the line frequency of the utility power grid and could not change. 
     While tied into the utility power grid, system frequency correction through speed control was not needed since the turbine speed was locked into the speed determined by the utility system frequency. Even if provided, speed control on the extraction turbine could not affect the system frequency since the extraction turbine represents a small generating capacity. Thus, no speed correction capability was provided, and this meant that the turbine-generator could not operate as a power &#34;island&#34; disconnected from the utility power grid. Without speed correction, a sag in the utility system frequency of sufficient duration or an outright loss of the connection to the utility power grid would cause the plant to shut down since the synchronous motors and pumps that require 60 hertz power at a constant voltage could not be maintained. A set of undervoltage relays on the generator would trip the unit because if the turbine speed was such that power was generated below a certain frequency, this could damage the generator, or the low frequency could damage the synchronous motors in the plant. 
     Thus, if the local plant power distribution system became separated from the large utility power grid and the generator was to continue generating power for use in a local environment, system frequency correction through speed correction was critical to smooth operation. However, prior art attempts at implementation of speed correction upon a loss of the utility power grid were unsuccessful. This was because the connection between the utility power grid and the local plant power distribution system was made through one or more circuit breakers. Typically, these tie-in breakers are owned by the utility, and no provision was made by which the extraction turbine control system could directly sense whether these breakers were or were not maintaining the connection between the local plant power distribution system and the utility power grid. Therefore, the plant operator did not have sufficient time to react to a loss of the utility power grid before taking corrective action by adjusting the turbine speed to maintain the system frequency. 
     The importance of system frequency correction for various system operating conditions is then apparent, but it is so for two separate reasons. If the extraction turbine is to be tied through the local plant power distribution system to a large utility power grid, it is required to enable synchronization of the turbine with the system frequency. If instead the local plant power distribution system becomes separated from the utility power grid and the turbine generator is running localized, it is important the plant equipment not be damaged while the industrial process is maintained. 
     The operator&#39;s control procedure in all of these cases 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. 
     It would therefore 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 have an extraction turbine control system with a system frequency correction capability that provided an automatic turbine speed correction in accordance with system operating conditions indicating separation of the turbine-generator from the utility power grid. This system would allow the operator to attend to other needs such as other adjustments made to the generator megawatt output. It would also be desirable to provide an extraction turbine control system with a system frequency correction capability that is free from drift in calibration of circuit components, thereby reducing periodic maintenance requirements. 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 operator-chosen setpoint signals and turbine operating level signals. As isochronous control operation is disclosed involving automatic system frequency correction upon separation of the turbine-generator from the utility power grid by sensing a predetermined speed deviation from synchronous speed at which time a speed corrector operates to position the extraction turbine inlet steam control valves so as to correct to synchronous speed without the need for operator intervention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an extraction turbine plant operated by a control system arranged in accordance with the principles of the invention; 
     FIG. 2 shows a detail of the operator&#39;s panel portion of the present invention; and 
     FIG. 3 shows a typical configuration of a microprocessor-based extraction turbine control system employed in the system of FIGS. 1 and 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a microprocessor-based extraction turbine-generator control system having an isochronous control operation through automatic speed correction. The isochronous control operation maintains system frequency by monitoring turbine speed for excessive speed deviations from synchronous speed. In accordance with the magnitude of the excessive speed deviations, a correction signal is determined which is then applied to a valve controller to position the extraction turbine inlet steam control valves so as to correct turbine speed without the need for operator intervention. 
     With reference to FIG. 1, an extraction steam turbine-generator 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) end 14 of the extraction turbine 12 through the upper and lower control valves 16. The steam then exits the seventh stage of the HP end 14 to the industrial process steam header 18 and to the low pressure (LP) end 20 of the extraction turbine 12. Maximum process steam flow 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 atmosphere of the LP section 20. An electric power generator 24 is coupled to the turbine shaft for production of megawatts on the plant power distribution system 25 for use in the industrial process, or possibly for sale to the electric utility power grid (not shown). 
     In the initial system operating mode, the extraction turbine 12 is started in a conventional manner and is operating below synchronous speed with the extraction valve 22 wide open, corresponding to no extraction steam demand. A speed control loop maintains the speed selected by the operator via the speed control raise/lower pushbuttons 26 and in/out split-lens pushbuttons 27 on the operator&#39;s panel 28 (see FIG. 2). The reference demand signal 30 representing the selected speed is fed from the operator&#39;s panel 28 to speed controller 32 which generates in PID (PIDVLIM) functional control block 33 a speed setpoint signal 34 as a proportional plus integral plus derivative function of the speed error signal 36. The speed error signal 36 is the difference between the reference demand signal 30 and the speed feedback signal 38 from the speed transducer 40, as determined by the delta (DBDLTA) functional control block 41. The speed setpoint signal 34 is ultimately fed to a valve controller 42, typically an electrohydraulic valve servo and servo driver loop, for positioning the control valves 16 so as to achieve the speed required by the reference demand signal 30. When the operator has brought the extraction turbine 12 up to synchronous speed, 3600 rpm, he then checks all the normal criteria used to synchronize the extraction turbine 12 before closing the main generator breaker 44 tying the generator 24 to the local plant power distribution system 25. 
     Assuming the local plant power distribution system 25 was already tied to the utility power grid prior to closing the main generator breaker 44, the extraction turbine-generator 12 would now be synchronized with the utility power grid such that its speed would be determined by the utility system frequency. At this point, the speed control which allowed for synchronization would no longer be necessary. The present invention initiates a transfer from the speed control loop into the load control loop so that the megawatt output of the generator 24 can be controlled. This is accomplished by the transfer (TRANSF) functional control block 46. This functional control block 46 is typical of the other transfer functional control blocks 48 and 50, each of which has an algorithm for transfer of one of two analog inputs. Based on a logical state of a mode signal, each transfer functional control block 46, 48 and 50 gates out one of its two analog input signals as its analog output signal. When the mode signal is in a &#34;high&#34; logical state, the signal on input 1 is gated out as the output signal. When the mode signal is in a &#34;low&#34;  logical state, the signal on input 2 is gated out as the output signal. 
     During the turbine synchronization procedure and prior to closing of the main generator breaker 44, the contact sensing logic control signal 52 corresponding to the position of the main generator breaker 44 is in a &#34;low&#34; logical state. Therefore, the mode signal on the transfer functional control block 46 is also in a &#34;low&#34; logical state. As a result, the speed setpoint signal 34 will be gated out of the transfer functional control block 46 to establish the control valve setpoint signal 54 to the valve controller 42. However, once the main generator breaker 44 closes, the contact sensing logic control signal 52 will go to a &#34;high&#34; logical state, such that the transfer functional control block 46 will now gate input 1 as its output, automatically transferring the system from a speed control mode to a load control mode. The speed control loop is no longer operating at this point. The megawatt controller 56, through the megawatt setpoint signal 58, will continue to maintain the electric power output measured by the megawatt transducer 59 in accordance with the level of the reference demand signal 30. 
     The reference demand signal 30 originates in the operator&#39;s panel 28 and is provided as input 2 of the transfer functional control block 48. The mode signal of this transfer functional control block 48 is the &#34;speed in&#34; logic control signal 60 (SPDIN), which is in a &#34;low&#34; logical state because of the operation of the speed flip-flop 62. The speed flip-flop (SRFLOP) functional control block 62 is in the reset state because it has received an indication that the generator breaker 44 has just closed. This indication is provided in the form of a momentarily &#34;high&#34; logic control signal 64 which is provided to the OR functional control block 66 such that a &#34;high&#34; logic control signal 67 momentarily appears on the reset input of the speed flip-flop 62. This logic control signal 67, in its momentarily &#34;high&#34; logical state, resets the state of the flip-flop 62 to provide a &#34;low&#34; SPDIN logic control signal 60. Then the control signal 67 returns to a &#34;low&#34; logical state. The &#34;low&#34; logical state of logic control signal 60 in turn causes the transfer functional control block 48 to gate out the reference demand signal 30 as the reference signal to the megawatt controller 56. 
     The isochronous control operation is now described. As indicated earlier, if the local plant power distribution system 25 becomes separated from the utility power grid while the turbine-generator 12 is in the load control mode, no direct sensing of this separation will occur. The detrimental effect of this separation will be noticed in a disturbance to the speed of the turbine 12 and the system frequency. If, for example, prior to such a separation the extraction turbine 12 were generating power in excess of the local plant requirements, this excess power would be taken up by the utility power grid. Once separated, this excess power capability would translate into an undesired increase in the turbine speed and a disturbance in the system frequency. If unchecked, the local plant power distribution system 25 would disturb the industrial process. Therefore, the present invention provides automatic speed correction to maintain the appropriate system frequency on the local plant power distribution system 25. 
     The present invention provides an operator-selectable speed corrector which can be placed into operation while the utility power grid is still connected, such that system frequency correction can be obtained automatically should the local plant power distribution system 25 become separated from the utility power grid. The operator must choose to have the speed corrector in service prior to the occurrence of such a separation. As mentioned earlier, in the prior art, no provision was made for allowing a speed control loop to be operable while the extraction turbine-generator 12 was tied to the utility power grid. The generation capability of the extraction turbine 12 is considered relatively small in comparison to the utility generation capability, and therefore a speed control loop on the extraction turbine was not provided since it could not affect the overall system frequency while the utility power grid was the determining factor. 
     If the operator chooses to place the speed corrector in service, operator selection of the speed request pushbutton 27 on the operator&#39;s panel 28 (see FIG. 2) will cause the SPDPBREQUEST logic control signal 70 to momentarily go to a &#34;high&#34; logical state and feed this as an input to the AND functional control block 72. Just prior to placing the speed corrector in service, the SPDIN logic control signal 60 is in a &#34;low&#34; logical state as mentioned earlier. Because of this, the inverter (NOTIN) functional control block 74 will output a &#34;high&#34; logic control signal 76 to the AND functional control block 72. With both inputs in a &#34;high&#34; logical state, the AND functional control block 72 will output a momentarily &#34;high&#34; logic control signal 78 which will be placed on the set input of the speed flip-flop. Since the reset input has a &#34;low&#34; logic control signal 67 as earlier stated, the speed flip-flop 62 will set the SPDIN logic control signal 60 in a &#34;high&#34; logical state, placing the speed corrector in service. 
     Once the speed corrector has been placed in service, regardless of the tie between the utility power grid and the local plant power distribution system 25, the speed corrector will operate to insure that the turbine speed is within certain predetermined limits by examining the actual speed of the turbine and acting to correct it should it exceed or fall below the predetermined limits. The &#34;high&#34; logical state of the SPDIN logic control signal 60 will cause the mode signal of the transfer functional control block 48 to gate out the speed-corrected reference demand signal 80 on input 1. This signal is equal to the sum of the reference demand signal 30 and a speed correction factor 82. The speed correction factor 82 has one of two values, depending upon the system operating conditions. 
     Initially, the speed correct flip-flop 84 operates in the reset state which places a &#34;low&#34; logic control signal 86 on the mode input to the transfer functional control block 50. As a result, the transfer functional control block will gate out input 2 (the null signal 88) as its output, and the first value of the speed correction factor 82 will be zero. The null signal 88 is provided by the analog value generator (AVALGEN) functional control block 89. This means that although the speed corrector is in service, the reference demand signal 30 is not modified by the speed correction factor 82 and maintains its original value. 
     However, the speed corrector monitors the actual turbine speed in the high signal limit and low signal limit (HISIGMTV and LOSIGMTV) functional control blocks 90 and 92. The high signal limit and low signal limit functional control blocks 90 and 92 respectively determine whether the speed feedback signal 38 exceeds or falls below the desired turbine speed of 3600 rpm by comparing the speed feedback signal 38 to preset upper and lower limits. Typically these limits form a deadband of 40 rpm above or below 3600 rpm, for a total deadband of 80 rpm. If either of these signal limit functional control blocks 90 or 92 determines, respectively, the speed feedback signal 38 to be beyond these respective limits, a &#34;high&#34; logic control signal 94 or 96 will be output into the OR functional control block 98, such that a &#34;high&#34; logic control signal 100 will be placed on one input of the AND functional control block 102. With the main generator breaker 44 closed, both inputs to this AND functional control block 102 will be in a &#34;high&#34; logical state, such that the AND functional control block 102 will output a &#34;high&#34; logic control signal 104 to the set input of the speed correct flip-flop 84, thus setting its output 86 &#34;high&#34; and causing the mode signal on the transfer functional control block 50 to gate input 1 as its output. The signal on input 1 of the transfer functional control block is a speed error signal, which is formed by comparing, in the delta functional control block 108, the actual speed feedback 38 with the analog signal 110 representing the desired turbine synchronous speed of 3600 rpm from the analog value generator 111. Thus, the second value of the speed correction factor 82 will be the speed error signal 106, which will be added to the reference demand signal 30 in the summer (SUM2) functional control block 112 to form the new speed-corrected reference demand signal 80 which is then applied to the megawatt controller 56 via the transfer functional control block 48 output signal 113. 
     When the speed-corrected reference demand signal 80 is presented to the megawatt controller 56, an error signal 114 will be generated in the delta functional control block 116. This is because the existing drift in speed away from 3600 rpm is accompanied by a drift in megawatt output, and the existing megawatt level indicated by the megawatt feedback signal 118 will not correspond to megawatt level called for by the speed-corrected reference demand signal 80 now being applied. Therefore the megawatt controller 56 will output a proportional plus integral plus derivative function of the error signal 114, implemented by the PID functional control block 120. This megawatt setpoint signal 58 is applied to input 1 of the transfer functional control block 46, and because the main generator breaker 44 is closed, the &#34;high&#34; logic control signal 52 from the main generator breaker 44 will cause the mode signal to gate out input 1 as the control valve setpoint signal 54. The control valve setpoint signal 54, having thus been adjusted by the speed corrector, will operate with the valve controller 42 to position the control valves 16 in accordance with the necessary correction. 
     Once the plant operator becomes aware of the action of the speed corrector in bringing the turbine back to synchronous speed, he can choose to reset the speed corrector for another operation. This would be the case if the operator is aware that the local plant power distribution system 25 has again become connected to the utility power grid so that the utility system frequency again becomes the determining factor governing turbine speed. To reset the speed corrector, the operator pushes the speed corrector reset pushbutton 121 on the operator&#39;s panel 28 (see FIG. 2), placing a momentarily &#34;high&#34; logic control signal 122 on the reset input to the speed correct flip-flop 84. This will reset the speed correct flip-flop and set the mode signal on the transfer functional control block 50 in a &#34;low&#34; logical state causing the null signal 88 to be gated out as the speed correction factor 82 so that no speed correction is obtained. At this point, assuming that other system operating conditions remain the same, if the speed of the turbine 12 should again go beyond the predetermined limits, the speed corrector will automatically operate once again. 
     As mentioned previously, availability of the speed corrector was made possible by operator selection of this feature at the operator&#39;s panel 28, using the speed request pushbutton 27. This same speed request pushbutton 27 can be used to deactivate the speed corrector. If the speed corrector was already operating, the SPDIN logic control signal 60 would be in a &#34;high&#34; logical state. Therefore, the AND functional control block 124 would have this &#34;high&#34; logic control signal 60 as one of its inputs. If in addition, the main generator breaker 44 was closed, an additional input 52 to this AND functional control block 124 would be in the &#34;high&#34; logical state. The third input to this AND functional control block 124 comes from the speed request pushbutton 27. Under these operating conditions, operator selection of this pushbutton will place a momentarily &#34;high&#34; logic control signal 70 on the AND functional control block 124 and a &#34;high&#34; logic control signal 125 will be output to the OR functional control block 66 which will also output a momentarily &#34;high&#34; logic control signal 67 to the reset input of the speed flip-flop 62. This will result in the SPDIN logic control signal 60 going to a &#34;low&#34; logical state, thus deactivating the speed corrector. 
     Two other system operating conditions will result in deactivation of the speed corrector. The first of these is represented by the speed channel failure logic control signal 126 to the OR functional control block 66 feeding the reset input of the speed flip-flop 62. If the integrity of the speed feedback signal 38 is compromised, the speed channel failure logic control signal 126 will go to a &#34;high&#34; logical state, resetting the speed flip-flop 62 and deactivating the speed corrector. This protects against the situation in which the speed feedback signal 38 has become unreliable, such that the speed correction feature could not operate properly. The other system operating condition in which the speed corrector is deactivated is represented by the logic control signal 64 indicating that the main generator breaker 44 was open and is now closed. This accounts for the situation in which the main generator breaker has just closed and the system has just entered the load control mode. In order to force the plant operator into making a conscious decision regarding the speed corrector, each time the main generator breaker 44 closes, a momentarily &#34;high&#34; logic control signal 64 will be placed on the OR functional control block 66. This will momentarily place a &#34;high&#34; logic control signal 67 on the reset input to the speed flip-flop 62, thus resetting it and deactivating the speed corrector. Therefore, the speed corrector will automatically become unavailable on each such occasion. However, the operator may choose to place the speed corrector in service at this time by using the speed request pushbutton 27 as previously described. 
     FIG. 2 shows a detail of the operator&#39;s panel 28 portion of the present invention. The panel 28 includes an annunciator display 128 indicating system abnormalities several digital readout displays, a group 130 indicating desired system operation levels and a group 132 indicating actual system operation levels, valve position panel meters 134, and a series of control pushbuttons 136 for megawatt control, extraction control and manual control. The control pushbuttons 136 allow the operator both to select the system operation mode and to establish the desired level of operation within the selected mode. 
     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™ 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™ turbine control system hardware configuration 138 is shown in FIG. 3. The MTCS-20™ turbine control system uses a standard WDPF™ Multi-bus® chassis configuration 139 with six printed circuit cards and with Westinghouse Q-line I/O, all of which is disclosed in a series of patent applications entitled &#34;Houser et al.&#34;, all assigned to the present assignee (Serial Nos. 508,951; 508,795; 508,771; 509,122; 508,769; 509,071; 509,251 and 508,770, all filed June 29, 1983 and 531,821, filed Sept. 13, 1983) and incorporated herein by reference. The pertinent part of these applications is the portion dealing with the &#34;drop overview&#34; as the MTCS-20™ turbine control system is currently sold by Westinghouse as a stand-alone controller not connected to a data highway. ®Multibus is a registered trademark of Intel Corp. MTCS-20™ and WDPF™ 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 140 and 142 give the MTCS-20™ turbine control system its first level of redundancy. The primary processor 140 is responsible for control loop execution while the normal function of the secondary processor 142 is tuning of the controller, listing the control loop, and displaying control parameters. If the primary processor 140 fails, the secondary processor 142 will automatically begin executing the control loop where the primary processor 140 left off. These two boards also contain duplicate sets of the algorithm library, which is described further herein. 
     The ®Multibus-DIOB interface card 144 gives the processors access to the I/O system. The Q-Line I/O bus 146 allows mixing of printed circuit point cards of any style anywhere on the bus 146. These cards are located in the I/O crates 148 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™ turbine control system 138 these cards provide the interface to the field I/O signal group 150, the engineer&#39;s diagnostic panel 152, the operator&#39;s panel 28, and the manual system 154. 
     Two memory components of the MTCS-20™ turbine control system 138 perform separate functions. A shared-memory board 156 is a 128K RAM board providing communication between the two functional processors 140 and 142. A battery-backed RAM board 158 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 ®Multibus chassis 139 is an RS-232C interface board 160 which interfaces a cassette recorder 162 used for permanent storage of the software application program for the control loops, and a keyboard/printer 164 used for entering, changing, and tuning the control loops. 
     The second level of redundancy in the MTCS-20™ turbine control system 138 is an analog system, the manual system 154. 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&#39;s panel 28 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 148 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&#39;s diagnostic panel 152. This panel 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&#39;s diagnostic panel 152 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 use by a two-position keylock switch 166. 
     The field I/O signal group 150 is made up of the I/O signals from the field I/O hardware which includes field instrumentation such as feedback transducers 40 and 59 in FIG. 1, and field actuators that are located on the extraction turbine and the associated steam flow piping. The annunciator output signal grouping 168 indicates system abnormalities and is typically tied to multiple annunciator display panels in the control room or elsewhere. The analog input signal grouping 170 is segregated and tied directly to the manual system 154 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 172 includes the valve servo position loop signals to and from the servo actuators which tie into the valve controller 42 (see FIG. 1). 
     The software application program for the control loop of FIG. 1 is furnished in the MTCS-20™ microprocessor in the form of software application program algorithms based on the use of modular functional control blocks. The functional 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™ 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 (shown in capital letters enclosed by parentheses in the specification and shown in the algorithm library Appendix A) 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. 
     Appendix A contains a preferred algorithm library set for use with the present invention. Appendix B contains the preferred application program listing for use with the present invention. Appendix C contains an address label conversion table for locating the DIOB address of digital and analog input and output labels used in the preferred application program listing. Appendix D contains a set of Q-line card types used for specific algorithms in the preferred algorithm library. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6##