Patent Publication Number: US-4550380-A

Title: Microprocessor-based extraction turbine control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to two concurrently filed patent applications bearing Ser. Nos. 562,378 and 562,508  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. 
     In the prior art of extraction turbine control system design, emphasis was placed on control of the process steam extraction operation so as to achieve the extraction process steam pressure required by the industrial plant. Extraction process steam pressure is the important control parameter where the extraction process steam is being used to feed heaters in the plant, such as auxiliary heaters, furnace heaters and building heaters, or where the steam is being used to power steam-driven pumps. 
     Other uses of extraction process steam include various quenching processes associated with steel mill operations, such as coke-quenching and quenching of hot metal strip as it exits the rolling mill. In these uses, the important control parameter is mass flow of extraction process steam. 
     For a given extraction steam pipe arrangement, control of either pressure or flow at a specific value necessarily corresponds to a specific value of the other parameter, though uncontrolled. The control scheme for control of either parameter adjusts the extraction valve in accordance with plant requirements. The ability of the control system to switch control modes from a pressure control mode to a flow control mode takes on increasing importance with the expansion in the number of possible ways to utilize the extraction process steam in the industrial process. 
     Prior art extraction turbine control systems required an operator to perform a complex, lengthy and delicate set-up procedure to accomplish this transfer of control modes. A major difficulty of this set-up procedure was presented by the requirement that it was performed so as to avoid a process upset, that is, that it was bumpless. Therefore, in a transfer from a pressure control mode to a flow control mode, the operator had to establish the flow setpoint at the mass flow value already existing while in the pressure control mode. This required visual comparison of various measurement parameters, introducing the possibility of operator error which would create a large swing in the controlled parameter as the new control mode was entered. 
     The operator&#39;s set-up 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. 
     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 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. 
     It can be seen that prior art extraction turbine control systems reflected control strategries 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 industrial 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 process steam requirements during various process steam extraction modes. 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 method of bumpless transfer between mutually exclusive extraction control loops directed to pressure or flow control is disclosed. Two transition setpoint controllers are provided, one for a transition to a pressure control mode and one for a transition to a flow control mode. Depending on which transition is in progress, each transition setpoint controller operates with an extraction transition reference controller which examines the process variable present in the existing level of turbine operation, and the appropriate transition setpoint controller then operates to produce an extraction valve setpoint signal equal to that process variable value, so as to provide bumpless transfer upon transition to the new control mode. Upon a return from the manual mode of turbine operation to the automatic mode, a particular extraction control loop is automatically placed in service without the need for operator intervention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an extraction turbing plant operated by a typical prior art control system; 
     FIG. 2 shows a detail of the operator&#39;s panel portion of the present invention; 
     FIGS. 3, 4 and 5 show an extraction turbine control system arranged in accordance with the principles of the invention, in which: 
     FIG. 3 shows an operator&#39;s panel, an extraction control loop selection controller and a reinsertion logic controller; 
     FIG. 4 shows an extraction valve transition reference selection controller; 
     FIG. 5 shows an extraction valve pressure transition setpoint controller, an extraction valve flow transition setpoint controller, an extraction valve setpoint selection controller, and an extraction turbine arrangement; and 
     FIG. 6 shows the configuration of a microprocessor-based extraction turbine control system employed in the system of FIGS. 2, 3, 4 and 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a typical 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 extraction turbine 12 through a pair of upper and lower control valve 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 or extraction cavity 18 and to the low pressure (LP) section 20 of the extraction turbine 12. 
     Maximum steam flow to the 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 (not shown). 
     The extraction turbine 12 is stated 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 steam demand in an initial system operating mode. When extraction steam demand is present, control of the extraction steam operation is provided by two independent setpoint signal proportional (P) controllers, the extraction valve flow setpoint signal controller 26 and the extraction valve pressure setpoint signal controller 28. Each setpoint signal controller interfaces with the operator&#39;s panel 30 for establishing the level of performance within the processs steam extraction mode of turbine operation, as represented by the two extraction reference signals, the extraction flow reference signal 32 and the extraction pressure reference signal 34. The extraction valve flow setpoint signal 36 and the extraction valve pressure setpoint signal 38 are each fed to a signal summer 40. Depending on which mode of operation is in progress, the extraction valve setpoint signal 42 will be determined by the greater of these two signals, and this signal is then fed to a valve controller 44, typically an electrohydraulic valve servo and servo driver loop for positioning the extraction valve 22. A steam pressure transducer 46 and a steam flow transducer 48 on the industrial process steam header 18 provide feedback signals 52 and 50 to the respective extraction valve setpoint signal controllers 28 and 26 to maintain a stable extraction operation. 
     As noted earlier, this scheme provides pressure control or flow control of an extraction process steam operation. However, the transition from one of these modes to the other requires the operator to perform a complicated procedure to adjust the extraction valve setpoint in the new control mode properly so as to avoid a process upset upon transition. 
     The present invention provides a microprocessor-based extraction turbine control system having a set of mutually exclusive modes of extraction operation through use of individual extraction control loops. Four extraction control loops are provided. These are the local extraction pressure control loop, the local extraction flow control loop, the remote extraction pressure control loop, and the remote extraction flow control loop. 
     While in automatic system control, each of thse control loops operates independently of a provided megawatt load control loop with separate control outputs derived from process feedback. These individual extraction control loops are arranged so as to allow a bumpless transfer between the local extraction pressure control loop and any other extraction control loop, thus avoiding any process upset. Additionally, the present invention is capable of automatic reinsertion of the local extraction pressure control loop upon a return from manual to automatic system control. 
     FIG. 2 shows a detail of the operator&#39;s panel 60 portion of the extraction control system practiced in accordance with the present invention. The panel includes an annunciator display 62 indicating system abnormalities, several digital readout displays, a group 64 indicating desired system operation levels and a group 66 indicating actual system operation levels, valve position panel meters 68, and a series of control pushbuttons 70 for megawatt control, extraction control and manual control. The control pushbuttons 70 allow the operator both to select the system operation mode and to establish the desired level of operation within the selected mode. 
     Operator selection of the extraction control loop under which the extraction operation will proceed is made through pushbutton selection in the extraction control pushbutton group 72 on the operator&#39;s panel 60. Based on this selection, the extraction control loop selection controller 74 shown in FIG. 3, generates logic control signals 75, 76, 77 and 78 representing this selection. The extraction valve transition reference selection controller 80, shown in FIG. 4, responds to this selection and in turn provides an extraction transition reference signal 82 to one of two extraction valve transition setpoint controllers 84 and 86, shown in FIG. 5. The extraction valve setpoint selection controller 88 then selects and feeds the appropriate extraction valve setpoint signal 89 to the valve controller 90, in a bumpless fashion. Thus, the system is not disturbed upon a transition from the local extraction pressure control loop to any other extraction control loop, as further described herein. 
     With reference to FIG. 5, before any extraction mode is entered, the extraction turbine 12 must be in the megawatt load control mode and the flow and pressure transmitters 92 and 94 as well as their respective flow and pressure feedback process variable signals 95 and 96 must not have failed. It is assumed that the extraction valve 22 is wide open at this point, permitting full steam flow through the extraction turbine 12. This is known as the full condensing mode. When the operator closes the generator breaker 98, an extraction limiter (not shown) automatically sets a minimum limit on the extraction valve 22 opening, at 20%, to maintain a minimum flow of cooling steam to LP section 20 of the extraction turbine 12 as noted earlier. Having closed the generator breaker 98, the extraction turbine 12 begins to pick up load on the electric power grid system (not shown). The operator must raise the load on the extraction turbine 12 to a 20% level to enable an extraction operation. The extraction control pushbuttons 72 are ignored below this load level. 
     To begin extracting steam, the operator must select the local extraction pressure control loop as the base mode of extraction operation, via pushbutton 100 on the operator&#39;s panel 60 (see FIG. 2). No other extraction control loop can be selected without the local extraction pressure control loop operating first. Once the local extraction pressure control loop is operating, the operator can select local flow or any of the remote extraction control loops by depressing the appropriate bushbutton in the extraction control pushbutton group 72. 
     Just prior to entering the local extraction pressure control mode, which corresponds to operation of the local extraction pressure control loop, the extraction pressure feedback process variable signal 96 (see FIG. 4) will have a value corresponding to the full condensing mode of operation. As noted earlier, this is the situation in which the extraction valve 22 is 100% open with full steam flow to the LP end 20 of the extraction turbine 12. Upon entering the local extraction pressure control mode, the extraction transition reference signal 82 is set equal to the extraction pressure feedback process variable signal 96, thereby making the transition to the local extraction pressure control loop bumpless. The extraction transition reference signal 82 becomes the extraction pressure reference signal 102 (see FIG. 5) which serves as a reference signal to the extraction pressure PID controller 104. The extraction valve pressure setpoint signal 106 is a PID (proportional plus intetral plus derivative) function of the error signal 108, which error signal 108 is the difference between the extraction pressure process variable signal 96 and the extraction pressure reference signal 102. 
     With reference to FIG. 3, the extraction control loop selection controller 74 employs four set-reset type flip-flop functional control blocks 110, 112, 114 and 116, each corresponding to a transitional operating state into a provided extraction control loop. Selection of a particular control loop is made via logic control signals 117, 118, 119 and 120 which originate in the operator&#39;s panel 60 and which are fed to the respective set inputs (S) on these flip-flop functional control blocks 110, 112, 114 and 116. Each of the reset inputs (R) is used to cancel a selected control loop and these reset inputs are fed by logic control signals 121, 122, 123 and 124 representing undesired system contingencies such as opening of the main generator breaker 98, failure of sensors 92 or 94, or an indication from the operator&#39;s panel 60 to cancel a control loop and its corresponding control mode. 
     The transition into the local extraction pressure control loop, correspondingly to the first transitional operating state, is now described with reference to FIGS. 2, 3, 4 and 5. In FIG. 2, when selection of the local extraction pressure control loop pushbutton 100 is made via the operator&#39;s panel 60, a local extraction pressure loop selection logic control signal 118 is generated in a &#34;high&#34; logical state and, in FIG. 3, is ultimately fed to the set input (S) of the local extraction pressure flip-flop 112 in the extraction control loop selection controller 74. The extraction control loop selection controller 74 operates to generate a corresponding logic control signal, the LOCAL EXTRACTION PRESSURE LOOP IN SERVICE (LEPLIS) logic control signal 76 in a &#34;high&#34; logical state. At the same time, the extraction control loop selection controller 74 generates the other loop selection logic control signals 75, 77 and 78 from the other three flip-flop functional control blocks Remote Extraction Pressure Loop In Servive 110 (REPLIS), Remote Extraction Flow Loop In Service 114 (REFLIS), and Local Extraction Flow Loop In Service 116 (LEFLIS), all in a &#34;low&#34; logical state, since these loops have not been selected. The &#34;high&#34; LEPLIS loop selection logic control signal 76 is fed to the extraction valve pressure transition setpoint controller 86 in FIG. 5, which operates to establish an extraction pressure PID controller 104 as the appropriate controller to achieve a bumpless transfer, as described further herein. 
     The extraction valve transition reference selection controller 80, shown in FIG. 4, employs three transfer functional control blocks 126, 128 and 130. Each transfer functional control block has an algorithm for transfer of one of two analog inputs. Based on the logical state of a mode signal, each transfer functional control block 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 one is gated out as the output signal. When the mode signal is in a &#34;low&#34; logical state, the signal on input two is gated out as the output signal. In this fashion, the extraction valve transition reference selection controller 80 implements the desired control strategy chosen by the operator via the operator&#39;s panel 60, as described further herein. 
     The logic control signal 120 tied to the local extraction flow flip-flop 116 set input (see FIG. 3) originates in the operator&#39;s panel 60 and is also fed to the extraction valve transition reference selection controller 80 (see FIG. 4). Because the operator has not selected the local extraction flow control loop at this time, this logic control signal 120 is in a &#34;low&#34; logical state, so that the AND functional control block 132 of the extraction valve transition reference selection controller 80 will set the mode signal on the first transfer functional control block 126 in a &#34;low&#34; logical state so as to gate out the analog input signal on input two as the output. First intermediate signal 134 takes the value of the extraction pressure process variable signal 96 which has been gated out of the first transfer functional control block 126. 
     The second transfer functional control block 128 gates out the first intermediate signal 134 as its output because the REPLIS logic control signal 75 is in a &#34;low&#34; logical state. This action establishes the second intermediate signal 136 with the same value as that of the first intermediate signal 134, namely, the value of the extraction pressure process variable signal 96. By a similar action, the third transfer functional control block 130 establishes the extraction pressure process variable signal 96 value as the appropriate value of the extraction transition reference signal 82 on a transition into the local extraction pressure control loop. The reason for this is that if the control system is entering into a pressure control mode, to make a bumpless transfer the extraction transition reference signal 82 must be that value of pressure already existing in the extraction cavity 18. That value is represented by the extraction pressure process variable 96 which is used as the extraction transition reference signal 82 in transition. In this fashion, the control system is not being asked to move to a value of extraction pressure different from the value of extraction pressure already existing. 
     In FIG. 5, the extraction transition reference signal 82 is used in the extraction value pressure transition setpoint controller 86. Because a transition to the pressure control mode is now in progress, the transition-to-pressure logic control signal 136 will be in a &#34;high&#34; logical state. This will set the mode signal on the first transfer functional control block 138 so as to gate out the extraction transition reference signal 82 as the extraction pressure reference signal 102. The delta functional control block 140 operates to compare the extraction pressure reference signal 102 with the extraction pressure process variable signal 96. Because these are the same, as mentioned previously, a zero error signal 108 is fed to the PID functional control block 141. The value of the output of the PID functional control block 142 after transition will be the value of the tracking signal 144 existing just prior to the transition entry into the local extraction pressure control loop. 
     The tracking signal 144 is derived from the output of the second transfer functional control block 146 in the extraction valve pressure transition setpoint controller 86. Prior to the transition to the local extraction pressure control mode, the transfer functional control block 146 has its mode signal set in a &#34;log&#34; logical state. This is because both the REPLIS and the LEPLIS logic control signals 75 and 76 are in a &#34;low&#34; logical state. Therefore, the transfer functional control block 146 gates out the existing extraction valve setpoint signal 89 as its output signal, so that the tracking signal 144 is equal to the existing extraction valve setpoint signal 89. Upon a transition into the local extraction pressure control loop, the initial value out of the PID functional control block 142 is the value of the tracking signal 144 just prior to the transition, which value was that of the existing extraction valve setpoint signal 89. When the transition occurs, the second transfer functional control block 146 will gate out input one as its output because of the presence of the LEPLIS logic control 76 signal in a &#34;high&#34; logical state. This output signal is the extraction valve pressure setpoint signal 148, and its value is exactly the same as the value of the existing extraction valve setpoint signal 89 prior to the transition. 
     The extraction valve setpoint selection controller 88 now operates to take the extraction valve pressure setpoint signal 148 on the second input of the transfer functional control block 150, and because both the REFLIS and LEFLIS logic control signals 77 and 78 are in a &#34;low&#34; logical state, this transfer functional control block 150 will gate out the extraction valve pressure setpoint signal 148 as its output so that the extraction valve setpoint signal 89 (EVSP) is now established and fed to the valve controller 90. 
     Once the transition has passed, the extraction pressure transition setpoint controller 86 will have the first transfer functional control block 138 gate out the extraction pressure reference signal 102 on input two as its output because of the &#34;low&#34; logical state of the transition-to-pressure logic control signal 136. 
     The extraction pressure reference summer functional control block 152 will allow extraction pressure adjustment by incrementing or decrementing the extraction pressure reference signal 102 in accordance with the incremental extraction pressure reference signal 154 coming from the operator&#39;s panel 60 or the remote control system 156 (see FIG. 4) depending on whether a local or a remote control mode is operating. This incremental extraction pressure reference signal 154 is generated by a smoothing function applied to the difference between the desired and actual extraction pressure reference signals. 
     A transition into the local extraction flow control loop, corresponding to the third transitional operating state, from the base mode of operation in the local extraction pressure control loop, is accomplished in a similar fashion, and the extraction valve flow transition setpoint controller 84 utilizes and generates flow-related signals having their pressure-related counterparts in the extraction valve pressure transition setpoint controller 86. 
     In FIG. 4, upon a transition into the remote extraction pressure control loop, corresponding to the second transitional operating state, the extraction transition reference signal 82 is established by the remote control system 156 equivalent to remote control pressure reference signal 158. Likewise, upon a transition into the remote extraction flow contol loop, corresponding to the fourth transitional operating state, the extraction transition reference signal 82 is established by the remote control system 156 equivalent to the remote control flow reference signal 160. Since the remote control system 156 has tracked the extraction pressure process variable signal 96 and the extraction flow process variable signal 95, these remote reference signals 158 and 160 are equivalent to the respective process variable signals 96 and 95 upon transition. Otherwise, the transition to a remote control mode is made in a fashion similar to that which has been described. 
     The local extraction pressure control loop is used as the intermediate mode during a transition between any two other extraction control loops. That is, the local extraction pressure control loop is selected as the first transition in control loops. Once in the local extraction pressure control loop, the transition to any other extraction control loop is accomplished in a similar manner to that described above. 
     Another method of entry into the local extraction pressure control loop is that method associated with reinsertion of the local extraction pressure control loop upon return from the manual to the automatic sysem. 
     As noted earlier, the manual system 162 (see FIG. 6) may be in control because of a problem in the automatic system. In the manual control mode, the control loops are operating open-loop and the operator controls the turbine using an analog control system to position the control and extraction valves in accordance with visual process instrumentation readings. During the repair or modification of the automatic system control, the operator may have been implementing an extraction operation in the manual mode. Upon return to the automatic control system, the level of the extraction operation achieved in the manual mode must be preserved in order to avoid a process upset. 
     With reference to FIG. 3, the present invention provides a reinsertion logic controller 164 to accomplish the reinsertion of the local extraction pressure control loop upon a return from the manual to the automatic control mode. The reinsertion logic controller 164 examines the system operation prior to the return to the automatic mode to determine if an extraction operation was in progress during manual control. The reinsertion logic controller 164 employs logic functional control blocks 166, 168 and 170 to make this determination. In the presence of the appropriate system conditions, the reinsertion logic controller 164 internally generates a reinsertion logic control signal 172 signifying the determination that the local extraction pressure control loop should be reinserted. The reinsertion logic control signal 172 representing this determination is then ultimately fed to the extraction control loop selection controller 74 so as to accomplish a transition to the local extraction pressure control loop as previously described. 
     The operation of the reinsertion logic controller 164 is now described. Four system operating conditions represented by logic control signals 174, 175, 176 and 177 are fed to the reinsertion logic controller 164 as part of the examination process. These are: 
     1. &#34;Control was in turbine manual&#34; logic controls signal 174 (WAS MANUAL). 
     2. &#34;Control is in auto&#34; logic control signal 175 (IS AUTO). 
     3. &#34;Main generator breaker is closed&#34; logic control signal 176 (BREAKER CLOSED). 
     4. &#34;Extraction valve position above 99%&#34; logic control signal 177 (EXTRACTION). 
     When the first two of these logic control signals 174 and 175 are in a &#34;high&#34; logical state, a return to the automatic control system operating mode from the manual mode has just been accomplished. When the BREAKER CLOSED logic control signal 176 is in a &#34;high&#34; logical state, the main generator breaker 98 is closed which, as noted earlier, is a precondition for transition into the local extraction pressure control loop. The last system operating condition necessary for reinsertion of the local extraction pressure control loop is represented by the EXTRACTION logic control signal 177. When in a &#34;low&#34; logical state, this signal 177 indicates that the position of the extraction valve 22 as sensed by the position sensor 178 (see FIG. 5) is below 99% which means an extraction operation is currently in progress. 
     When the AND logic functional control block 168 in the reinsertion logic controller 164 determines that all of the above necessary system operating conditions are present, reinsertion of the local extraction pressure control loop is called for because an extraction operation was proceeding in the manual mode prior to returning to the automatic mode. The AND logic functional control block 168 then generates a reinsertion logic control signal 172 in a &#34;high&#34; logical state for ultimate use by the local extraction pressure flip-flop 112 in the extraction control loop selection controller 74. A transition into the local extraction pressure control loop then commences as previously described. 
     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 MTSC-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 200 is shown in FIG. 6. The MTCS-20™ turbine control system uses a standard WDPF™ Multi-bus® chassis configuration 202 with six printed circuit cards and with Westinghous 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 (Ser. Nos. 508,769; 508,770; 508,771; 508,795, 508,951; 509,122; 509,251; and 569,071) 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 204 and 206 give the MTCS-20™ turbine control 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 ®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™ turbine control system 200 these cards provide the interface to the field I/O signal group 212, the engineer&#39;s diagnostic panel 214, the operator&#39;s panel 60, and the manual system 162. 
     Two memory components of the MTCS-20™ turbine control system 200 perform separate functions. A shared-memory board 216 is a 128K AM 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 ®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™ turbine control system 200 is an analog system, the manual system 162. 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 60 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&#39;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&#39;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 use 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 feedback transducers 92 and 94 in FIG. 5, and field actuators such as position sensor 178 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 230 is segregated and tied directly to the manual system 162 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 controller 90 (see FIG. 5). 
     The software application programs for the control loops of FIGS. 3, 4 and 5 are 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 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 miscillaneous 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 (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, paratmeter location, paramater 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. 
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