Patent Number: 047770090
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear power and particularly to the control of recirculating steam generators in pressurized water nuclear steam supply systems (NSSS). More specifically, the present invention is directed to automatic water level controls for steam generators of nuclear power systems. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character. 2. Description of the Prior Art The nuclear steam generator of a pressurized water nuclear power plant is typically controlled as a function of three primary operating parameters which are monitored, i.e., water level (L) steam flow (W.sub.S) and feedwater flow W.sub.fw). The signals corresponding to the monitored parameters are processed in proportional/integral and lead/lag circuits to generate a feedwater flow demand signal for controlling the amount of water introduced into the steam generator for the production of steam. The principal concern, and therefore the operating parameter on which the control action is primarily based, is the steam generator water level. In practice, the control of the steam generators of NSSS has proven to be an unusally difficult task. As a result, a significant proportion of major nuclear power plant outages have been caused by reactor trips due to steam generator operation outside the desired range. Many of these outages are due to reactor trips on low or high steam generator water levels. Typically, about 80 percent of steam generator low water level trips occur below 20 percent system rated power, and nearly 90 percent of the high water level trips occur below 20 percent power. The problem of maintaining steam generator water level within proper limits is particularly acute during plant startup, when the operators have had relatively little experience in steam generator water level control. A major complexity incident to steam generator control, particularly at low power levels, resides in the water recirculation characteristics of the system including the steam generator. Thus, during low power operation, the sensitivity of the steam generator water level to changes in feedwater flow increases. Also, at low power there is a seemingly anomolous behavioral characteristic which is manifested by an initial decrease in steam generator water level when there is an increase in the feedwater flow. This behaviour often confuses the operator, and usually causes the operator to further increase the feedwater flow, causing a further decrease in the water level and introducing "positive feedback" into the system which may lead to uncontrolled oscillation of the water level and to a reactor trip. Conventional controllers, even of the above-mentioned three parameter type, are unreliable at low power operation because the steam flow and feedwater flow signals are themselves not reliable under such operating conditions. In most instances, because of this known lack of reliability, the operators elect to manually control the water level. Attempts at manual control have met with only limited success to date. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a control technique for a recirculating steam generator in a nuclear steam supply system and particularly a method which is capable of automatic water level control over the full power operating range of the steam supply system. In accordance with the invention, there is provided a control system and a method of control for a recirculating nuclear steam generator that takes into account the power related variations in the dynamic characteristics of the steam generator. Thus, the present invention automatically controls the feedwater flow rate to a steam generator to maintain satisfactory downcomer water level during steady-state operation and during the following load maneuvers: (a) 10 percent steps in NSSS load between 15 percent and 100 percent NSSS power. PA0 (b) 1 percent/minute ramps in NSSS load between 0 percent and 15 percent NSSS power and 5 percent/minute ramps in NSSS load between 15 percent and 100 percent NSSS power. PA0 (c) Load rejections of any magnitude. PA0 (a) Reactor trip PA0 (b) Loss of a Feedwater Pump during two feedwater pump operation. PA0 (c) High steam generator downcomer water level. The present invention also provides for automatic operation in the event of the following plant conditions: In accomplishing the foregoing, the present invention automatically opens and closes, in a sequential manner, the downcomer and economizer feedwater control valves. Additionally, the invention coordinates the adjustment of the economizer feedwater control valve, the downcomer feedwater control valve and main feedwater pump speed setpoint to automatically regulate the feedwater flow between 0 percent and 100 percent NSSS power to control the steam generator water level. In accordance with a preferred embodiment, the invention regulates the feedwater flow rate to control the steam generator downcomer, i.e., steam generator, water level after a reactor trip by sensing the T.sub.AVG signal from the associated primary coolant loop. This action minimizes the possibility of overcooling the primary coolant loop after a reactor trip. The system automatically returns to the low power level control mode when steam generator water level returns to its setpoint. A control system in accordance with the invention is configured to minimize the necessity for separate adjustments by the operator during manual operation of the feedwater pump speed setpoint and the feedwater control valves. This minimizes operator actions and thus minimizes possible operator error. A particularly unique feature of the invention is its ability to position the feedwater control valves as a function of power speed such that at low flow rates feedwater flow is predominantly regulated by valves while pump speed control is the primary mechanism for feedwater flow adjustment at high flow rates. In the practice of the invention, a signal commensurate with the measured steam generator water level is passed through a lead-lag circuit. The lead improves the control responsiveness and compensates for the delays in the steam generator process, while the lag improves the steady state response and the stability margin. The lead and lag settings are automatically varied with power level to compensate for the dynamic characteristics of the steam generator. The thus processed water level signal is then passed through a proportional-integral controller, where the gain and reset rate are also adjusted as a function of power to further compensate for the steam generator dynamic characteristics.