Patent Number: 047073241
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The pressurized water reactor, PWR, electric power generating system disclosed in FIG. 1 includes a reactor core 1 contained in a reactor vessel 3. A reactor coolant in the form of ordinary water circulates through the reactor core 1 where it absorbs heat generated by controlled fission reactions. The heated reactor coolant is pumped through a hot leg 5 to a steam generator 7 and then back to the reactor core through cold leg 9 by a recirculating pump 11. The steam generator 7 utilizes the thermal energy in the reactor coolant to generate steam which is supplied through a steam header 13 and throttle valve 15 to a turbine 17 which in turn, drives an electric generator 19. Vitiated steam from the turbine is condensed in condenser 21 and the condensate is returned to the steam generator 7 through conduit 23 by pump 25 for regeneration. A typical PWR plant may have two to four steam generators 7 supplying steam to the turbine through separate loops. The throttle valve 15 on the turbine 17 is positioned by a turbine controller 27 in response to a dispatch signal received from a central load dispatcher which allocates load to individual plants in a power grid, and/or from local commands such as operator generated load commands or limits. The temperature of the reactor coolant is controlled by control rods 29 which are inserted into and withdrawn from the reactor core 1 by a rod control system 31. The control rods 29 contain neutron absorbing material which affects the density of thermal neutrons in the core available for sustained fission reactions. The rod control system positions the control rods to maintain a setpoint reactor temperature. The actual reactor coolant temperature is fed back to the rod control system through lead 33. A pressurizer 35 regulates the pressure of the reactor coolant. The typical pressurizer 35 in a PWR includes a heater system which increases coolant pressure by raising the pressure in a head of steam maintained in the pressurizer and a spray system which reduces coolant pressure through condensation of an appropriate proportion of the steam head. Pressurizer pressure is controlled at a setpoint value by a pressurizer control system 37. A protection system 39 monitors the operation of the nuclear steam supply system, which includes the reactor core 1, the steam generator 7, and their associated and interconnecting components such as the pressurizer 35, by gathering data such as various temperatures, pressures, flow rates, the neutron flux density and certain status indications, through numerous inputs represented by leads 41, 43 and 45. The protection system analyzes the data and generates a trip signal on lead 47 which shuts down the reactor by fully inserting the control rods into the reactor core when selected operating limits are exceeded. A load signal derived from the steam pressure in the impulse chamber of the turbine 17, and representative of the load imposed on the reactor by the turbine-generator set, is applied to a rapid power change control 49 through lead 51. This control 49, which forms the core of the present invention, generates a temperature reference setpoint signal which is applied to the rod control system 31 through lead 53. Signals representative of certain operating limits used in the present invention, as more fully discussed below, are sent by the protection system 39 to the rapid power change control 49 as represented by lead 55. The pressurizer control system 37 also sends a setpoint signal to and receives an adjusted setpoint signal from the rapid power change control as represented by lead 57. In addition to rod position control, the temperature of the reactor is also regulated by dissolving controlled amounts of a neutron absorbing material, typically boron, in the reactor coolant. Due to the large amount of reactor coolant, the long loop through which it is circulated and the physical limitations involved in removing boron from the coolant, boron control is used for long term, relatively slow changes in reactor power. The control rods on the other hand, affect reactor power and hence, temperature immediately and hence, are ideal for responding to rapid fluctuations in load. However, continued movement of the control rods results in excessive wear on the rod drive mechanisms and even on the rods themselves as they move relative to guide tubes inside the reactor core. The solution to the problem of controlling a PWR to follow rapid load changes without excessive wear on the control rod positioning and other system components lies in the fact that a decrease in reactor temperature, while maintaining constant control rod position and boron concentration, results in an increase in reactivity. This is the result of two phenomenon. First, the PWR has a negative doppler coefficient which means that a decrease in fuel temperature produces an increase in the rate of fission reactions within the fuel. Secondly, the reactor coolant, in addition to serving as a heat transfer medium, also serves as moderator for slowing down the neutrons released by the fission reactions to the thermal velocities required for sustained reactions. With a negative temperature moderator coefficient, a reduction in the temperature of the coolant increases its density and therefore its moderator effect so that more neutrons in a given neutron generation produce another fission reaction. Thus, when additional load is placed on the nuclear steam supply system, more thermal energy is extracted from the reactor coolant which lowers its temperature and the fuel temperature. This in turn, results in an increase in reactivity to a level which satisfies the new demand. On the other hand, a reduction in the load causes an increase in reactor temperature which results in a decrease in reactivity. Such an increase in temperature, however, could approach various temperature related operating limits which would cause the protection system to trip the reactor. The present invention overcomes these limitations by adjusting the temperature reference setpoint signal for the rod control system 31 as a function of the rapid fluctuations in load so that the setpoint signal matches the variations in the actual temperature resulting from the rapid power changes. Thus, the control rods do not move in response to the rapid power changes and the change in reactivity required is accommodated by a change in temperature. This change in temperature affects other systems such as the pressurizer pressure. In order to preclude excessive wear on the pressurizer spray and heater components, the setpoint signal for pressurizer pressure control can also be adjusted as a function of the rapid power changes to match the setpoint to the expected change in pressure resulting from the rapid power changes. Similarly, the setpoint signals for the controllers for other system parameters such as pressurizer water level, boron concentration and gray rod insertion (in some of the newer PWRs) can be adjusted alone or in combination with others to minimize control action in response to rapid power changes. The present invention also provides for the use of a widened deadband in the response of the rod control system to rapid load fluctuations over the full power range of the reactor. This is accomplished by varying the width of the deadband as a function of the magnitude of the flucuations in load occurring above a predetermined frequency and by reducing the set point for the rod control loop so that the upper temperature edge of the deadband remains within all temperature limits. A wider deadband can also be used in controlling other system parameters either by itself or with adjustment of the associated controller setpoint. The load signal on lead 51 is passed through a low pass filter 59 to eliminate the rapid power changes and is converted to a load derived temperature reference signal in function generator 61 in a conventional manner. In the typical prior art rod control system, this reference signal is used as the setpoint in a conventional rod control loop 63. In the present invention, the rapidly changing component of the turbine load signal on lead 51 is extracted by a conventional high pass filter 65 which may employ a simple rate/lag transfer function or a more complex function as desired. The particular bandpass frequency of the high pass filter 65 depends upon the specific installation and the characteristics of the load pattern to which the plant is subjected, however, typically fluctuations having a frequency of more than one or two cycles per hour would be extracted from the load signal. Typically, but not necessarily, if the transfer function of the high pass filter 65 is H.sub.2 (S), the transfer function H.sub.1 (S) for the low pass filter 59 is 1-H.sub.2 (S). The high frequency component of the load signal is applied to a filter 67 having a transfer function: ##EQU1## where T(S) is the laplace transform of the reactor coolant temperature, and Q.sub.turb (S) is the laplace transform of the load signal from the turbine. The output of the filter 67 is the expected fluctuation in temperature resulting from the rapid variations in turbine load. This signal is passed through a limiter 69 and is added in summer 71 to the temperature reference signal generated by function generator 61 to produce an adjusted temperature reference signal. The adjusted temperature reference signal is applied to an Auctioneer Low module 73, which as will be seen below, passes the signal along to the rod control loop 63 as T.sub.ref as long as it does not exceed permissible temperature limits. While in theory, the adjusted temperature reference signal should match the variations in reactor coolant temperature induced by the rapid load changes, in practice it is desirable to have a deadband in the response of the rod loop control 63 to assure that the small rapid fluctuations in load do not induce control action. This invention provides a deadband which varies as a function of the magnitude of the rapid power changes. In order to achieve this, the high frequency component of the load signal from filter 65 is also fed to a square law module 75 and then through a unity gain low pass filter 77 which may be a simple first order lag device or a more complex design if desired. The resulting output signal .sigma..sup.2 is a measure of the average squared value (i.e. the variance) of the magnitude of the rapidly changing component of the load. The signal .sigma..sup.2, representing the average squared value of the magnitude of the rapid fluctuations in the load signal, is applied to a gain module 79 which generates a temperature related deadband signal as a function of the magnitude of the variance. The deadband output for a given variance is selected to limit the rod stepping frequency to acceptable limits while also providing good control response. A lower limit on the deadband is required for stability, and an upper limit is required to limit temperature variations for reasons such as component fatique. The intermediate portion may be linear or nonlinear as desired. Since the plant may experience other large transients, such as load rejection, in addition to those normally occuring as a results of economic load regulation, an active high bistable 81 determines when load changes exceed those expected from load regulation and thus, indicates that other large transients are underway which may require more precise control than the control rods 29 can provide with the wider deadband. Such an output by the active high bistable 81 is memorized by a latch 83 whose active (logic 1) output operates a switch 85 to apply a fixed deadband signal to a lead 87. A logic 0 output from latch 83 operates switch 85 to apply the output of gain module 79 to lead 87. The latch 83 can be reset by an operator manual control. The output of switch 85 which appears on lead 87 is the variable deadband signal which is applied to the rod control loop 63. While the signal .sigma..sup.2 can be applied to the bistable 81 as an indication of the magnitude of the fluctuations in load, the output of the square law module 75 may be used instead as illustrated by the dashed line in FIG. 2 and probably is superior for detecting large transients. In order to prevent the reactor temperature from exceeding the license limit and to maintain adequate margins to trip limits, the varible deadband is substracted in summer 89 from the maximum allowable temperature obtained on lead 91 from an auctioneer low module 93. The auctioneer low module 93 selects the lowest of several temperature limiting signals received from the protection system 39 over leads 55. These signals, which are already available in the protection system, can include: (A) the DNBR (departure from nucleant boiling ratio) limit with an appropriate margin to prevent an unnecessary reactor trip; (B) an exit quality limit which is a measure of the absence of vapor in the reactor coolant leaving the reactor, again with an appropriate margin; (C) the license limit which is the maximum temperature at which the reactor is permitted to operate, and in fact, any other temperature dependent parameter (D) desired, with or without a margin. The maximum allowable temperature adjusted for the deadband in summer 89 is applied to the auctioneer low module 73 along with the adjusted temperature reference signal derived from the load signal in module 61. The lowest of these two signals is selected by the auctioneer low module 73 as the temperature reference signal, T.sub.REF, which becomes the setpoint signal for the rod control loop 63. For moderate thermal loads on the reactor, the temperature reference signal derived from the load signal will be lower in magnitude and will be used as the temperature reference. Near full power, is where it can be expected that conditions would arise where the temperature limiting signals adjusted for the deadband would be likely to be selected as the temperature reference signal, T.sub.REF. With the present invention, however, control of the reactor can be achieved without excessive movement of the control rods right up to full power. The invention can be applied to other control loops in the PWR in addition to, or in place of, the rod control loop 63. For instance, it may be applied to the pressurizer pressure control loop 59 to minimize operation of the pressurizer spray and heater systems. As shown in FIG. 2, the high frequency component of the load signal, which is extracted in high pass filter 65, is applied to a filter 95 having the following transfer characteristic: ##EQU2## where P.sub.PZR (S) is the laplace transform of pressurizer pressure and Q.sub.TURB (S) is as explained in connection with equation 1. The output of filter 95 is the variation in the pressurizer pressure setpoint which matches the expected variation in pressurizer pressure resulting from the rapid changes in load. It is limited in magnitude by module 97, passed through gate 99 and added to the standard (fixed) nominal reference pressure setpoint supplied by the pressurizer control system 37 (see FIG. 1) in summer 101. The adjusted pressurizer pressure setpoint is then returned to the pressurizer control system for use in controlling pressurizer pressure. Since it is very desirable to avoid large pressure deviations which may lead to opening pressurizer power operated relief valves, an interlock which includes gate 99 and an active low bistable with hysteresis 103 is included to allow fluctuations in reference pressure only when pressurizer pressure is below a fixed setpoint. FIG. 3 illustrates schematically the rod control loop 63 to which the temperature reference signal, T.sub.REF from the auctioneer low module 73 in FIG. 2 is applied. The measured reactor temperature on lead 33 is subtracted from T.sub.ref in summer 105 to generate an error signal. Dynamic compensation is applied to the error signal in a compensation circuit 107. The compensated error signal can be summed with other control signals 109 such as, for instance, a power mismatch signal, in summer 111 and the resultant signal is applied to a known control circuit 113 which generates a drive signal for the control rod drives 115 as a function of the applied signal. The control circuit 113 incorporates mean for generating a deadband, D, in the output response. In other words, no drive signal is generated at the output until the applied signal exceeds the magnitude of the deadband. The width of the deadband is controlled by the magnitude of the deadband control signal applied to the control circuit 113 through lead 87. The lockup, L, may be adjusted as a function of the deadband, D, if desired. Activation of the rod drives 115 causes repositioning of the control rods which are indicated collectively components in the plant 117. Similar control of the deadband can be applied to other control systems in the PWR. From the above description, it can be appreciated that the present invention is directed to apparatus which automatically adjusts reference setpoints in the control systems of a PWR in response to rapid fluctuations in turbine load. The system works for all ranges of automatic reactor control up to 100% power. The modules 67 and 95 may use time varying transfer functions to account for normal changes in plant response with core life. Suitable transfer functions can be obtained through well-known system identification techniques, such as those given in Franklin and Powell, "Digital Control of Dynamic Systems", Chapter 8, Addison-Wesley Publishing Co., copyright 1980, second printing June, 1981. The system can be implemented by conventional continuous circuitry or by digital technology. It can also be appreciated that the invention provides for a variable deadband in the response of the rod control system to rapid fluctuations in load on a PWR and that the width of the deadband is a function of the variance of the magnitude of the fluctuations above a predetermined frequency. It can be further appreciated, that the invention permits variable deadband rod control to be used up to full power by limiting the temperature reference signal in the rod control loop such that the high temperature edge of the deadband response is within all reactor temperature limits. The result is reduced wear on rod control components while maintaining full capability to load follow. In an alternate embodiment of the invention, the output of filter 67 or limiter 69 can be used as the input to square law module 75 rather than the output of the high pass filter 65, thus basing the variable deadband width on the anticipated changes in temperature. The setpoints for modules 77, 79, 81 and 83 would have to be changed accordingly. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For instance, while the invention has been described as being implemented by hard wired circuitry, many of the functions can be performed by appropriate software in a programmed digital computer. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.