Patent Number: 047073241
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for controlling the response of a pressurized water reactor in a nuclear power plant to rapid fluctuations in load. More particularly, it is directed to such a method and apparatus in which a control system setpoint is varied in response to small rapid changes in plant turbine load in such a manner that the setpoint variations match the expected variations in the controlled parameter. It is also directed to such a method and apparatus in which the width of the deadband in the response of the rod control system to changes in a load signal is varied as a function of the magnitude of rapid fluctuations in the magnitude of the load signal. PRIOR ART Nuclear power plants have typically been operated at constant high power levels due to their low fuel costs and high capital investment. When the total nuclear generating capacity exceeds the utilities minimum load demand, however, it is necessary for some or all of these nuclear units to operate in a "load follow" mode. In other words, they must adjust their power level so that total generation balances total demand. In general, the utility desires to determine which plants will load follow based on the economic consideration of minimizing total generation cost. This function is often performed by an Economic Dispatch Computer which optimizes the distribution of power generation among the various units supplying the grid. Signals are then sent to each plant instructing the control system in each as to how much power the plant should produce. This signal typically consists of a slowly changing component representing normal daily grid load variations plus small, rapid changes representing load changes induced by individual users. Alternately, small rapid changes in plant load may be induced by systems (either remote or local) which act to maintain grid frequency at its reference value as grid load changes. The slow component is accommodated by normal control systems. The fast component can be accommodated by the normal control systems, but the result is excessive wear on the control rod drive mechanisms coupled with increased operating difficulty due to core power distribution fluctuations resulting in reduced availability. This may be due to difficulty in complying with power distribution technical specifications and/or reduced margin to trip resulting in increased probability of a reactor trip. The fast component of load can also cause excessive operation of other systems such as the spray system and heaters used to control pressurizer pressure, the pressurizer water level control and the boron system. A recent design of an Integrated Control System by the assignee of this invention, provided for manual widening of the rod control system deadband which would minimize rod motion by allowing larger temperature variations to occur in the nuclear steam supply system. This avoided the rod wear problem because reactivity changes associated with the power changes were accomplished via feedback of the moderator temperature coefficient. It also avoided the power distribution changes associated with control rod motion. Unfortunately such control was only possible below approximately 85% power since at high power levels the wider deadband would allow the reactor coolant system temperature to exceed the full power (license) value. SUMMARY OF THE INVENTION In the present invention, the setpoint for a control system in a pressurized water reactor is adjusted by an amount corresponding to the expected variation in the value of the controlled parameter resulting from rapid fluctuations in the load imposed on the reactor. In this manner, the control system accommodates to the load change by allowing the value of the parameter to vary with the small rapid load changes rather than taking numerous control actions to maintain the parameter at the programmed setpoint value. This is accomplished by extracting the high frequency component of the load signal and applying to it a transfer function representative of the relationship between the value of the setpoint and the corresponding value of the parameter taking into account system response times to generate an adjustment signal which is added to the setpoint signal. The adjustment signal is limited in magnitude so that the control system responds to large changes in load with normal control action, and if required, the correction is applied to the setpoint signal only if the parameter is below a preset value in order to prevent excursions which might result in a reactor trip. Also in the present invention, the deadband in the rod control system of a pressurized water reactor automatically widens in order to accommodate for the small rapid load fluctuations described above. The amount of widening is a function of the magnitude of the fluctuations of the load signal occurring above a predetermined frequency. In the preferred embodiment of the invention, the width of the deadband is varied as a function of the average squared value (variance) of the magnitude of these flucuations. In a pressurized water reactor, the temperature is usually controlled as a function of power level. Accordingly, the load signal is converted to a temperature reference signal which is used in a feedback control loop to position the control rods to drive the measured reactor temperature toward the value commanded by the temperature reference signal. As another feature of this invention, the temperature reference signal for the rod control system is modified, if required (normally at high power levels only), so that the high-temperature edge of the rod control system deadband does not exceed a maximum value which provides adequate margin to reactor trip and in addition, does not violate the license maximum temperature limit.