1、 中文 2175 字 Research on Fuzzy Control for Steam Generator Water Level I. INTRODUCTION The steam generator is one of the main devices in PWR nuclear power plant, in order to ensure the safety of nuclear power plant during operation； the steam generators water level must be controlled in a certain rang
2、e. When the nuclear power plant is running, as the steam flow or the water flow changing, the amount of boiling bubbles in the steam generator will change due to local pressure or temperature change, the instantaneous water level showed “false water level” phenomenon . The existence of “false water
3、level” made it difficult to control the water level. The introduction of feed-forward control to the traditional single-loop PID control can, in a certain extent, overcome the false water level phenomenon. But the conventional PID control method in the process of steam generator water level control
4、has some shortcomings. To the steam generator that has highly complex, large time-delay and nonlinear time-varying characteristics, the PID parameters tuning is a tedious job and the control effect is very poor. Furthermore, to achieve good control performance still as conditions changing, it often
5、needs to change the PID controller parameters. But the analog PID controller parameters are difficult to regulate online. Fuzzy control is a kind of nonlinear control strategy based on fuzzy reasoning, which express operating experience of skilled manipulation men and common sense rules of inference
6、 through vague language. Fuzzy control do not need to know precise mathematical model of controlled object, is not sensitive to the change of process parameters, is highly robust and can overcome non-linear factors, so, fuzzy control has faster response and smaller ultra- tone, can get better contro
7、l effect. Based on understanding above, this paper design a steam generator water level fuzzy controller, the simulation shows that the controller has good control performance and practical value. II. DYNAMIC CHARACTERISTICS OF STEAM GENERATOR The transfer function of PWR steam generators mathematic
8、al model of the general form shows below: y(s)=GW(s)QW(s)+GS(s)QS(s) （ 1） where y is the steam generator water level； QW for the water flow； QS for the steam flow； GW (s) for the impact of the water flow to the steam generator water level； GS (s) for the effect of the steam flow (load) to the steam
9、generator water level. The balance of the steam generator water level is maintained through the match between the water flow and steam flow. The process that water level changes with the steam flow or water flow changing can be regarded as a simple integration process, but impact of the water flow a
10、nd steam flow s change on water level is different. A. Dynamics Characteristics under Water Flow Disturbance Suppose steam flow GS remains unchanged, and water flow GW step increases, on the one hand because the temperature of feed water is much lower than the temperature of saturated water in the s
11、team generator, so that , when feed water entering, it will absorb a lot of extra heat, the vapor phase bubble contents will reduce, resulting in water level decreasing； on the other hand, the increase in water flow GW made it greater than steam load, and cause water level increases linearly. Compre
12、hensive two factors, after the step increase of the water flow, the water level rise has a time delay process, showing a down then up. B. Dynamic Characteristics under Steam Load Disturbance Suppose feed water flow GW remains unchanged, and steam load GS step increases, on the one hand the water lev
13、el will flow down because the steam flow rate is greater than the water flow rate. On the other hand, as the steam load increased, vapor pressure is reduced； the bubble volume on the liquid surface increases, causing the water level increased. Comprehensive two factors, after the step increase of th
14、e steam flow rate, the water level down has a time delay process, showing a up then down. The impact on the water level of water flow or steam flow stepping decreased has similar principle as above. As analysis can be seen as above, when the water flow or steam load change, the water level did not f
15、ollow the change immediately, but there is an opposite process at first. This phenomenon is called false water level phenomenon. III. DESIGN OF WATER LEVEL FUZZY CONTROLLER The conventional PID controller has a poor control performance to the steam generator that exist “false water level” characteri
16、stics, showing a greater overshoot in the tracking time. But a well-designed fuzzy controller is able to overcome the false water level phenomenon, and has good control performance. A. Sstructure of Fuzzy Controller The structure showed in Figure 1. Figure 1. Structure of steam generator water level
17、 fuzzy controller Choose the water level error (e) and change rate of error (ec) as input of the fuzzy controller, the output of the fuzzy controller is the added value of the valve opening signal u. Meanwhile, use the steam flow feed-forward to overcome the false water level phenomenon, use water f
18、low feedback to overcome fluctuations in water supply side . k1, k2 were water flow and steam flow transmitter conversion factor. To ensure the water flow to match the steam flow, k1 and k2 values should be equal to. B. Fuzzy theory, fuzzy subset and Membership Function The fuzzy Analects of e, ec a
19、nd u are -6, 6, both with seven fuzzy sets NB (negative big), NM (negative middle), NS (negative small), ZO (zero), PS (positive small), PM (positive middle) and PB (positive big) to describe. e, ec and, u are all using triangular membership function (see Figure 2). Figure 2. Input and output variab
20、le membership function C. Fuzzy control rule table The establishment principle of fuzzy control rules are: when the error is large, the output control volume should give priority to eliminate error as soon as possible； when the error is small, the Fuzzy controller K1 valve GW(s) dx/dt K2 GS(s) expected water lever Stream flow Water lever water flow e ec + - + + - + + +
