变频冷水机组的动态特性及多回路PI控制响应(英文)

Dynamic characteristics and multi-loop PI control responses of a variable frequency water chiller

基于质量与能量守恒原理,从控制的角度提出了一套动态冷水机组的集总参数法数学模型,研究了诸如运行频率、热力膨胀阀(TEV)开度等输入变量对系统输出性能的影响.动态响应显示热系统的响应比压力、制冷剂流量的慢许多,表明制冷系统是一个两时间尺度的系统.采用电子膨胀阀(EEV)代替热力膨胀阀,分别采用频率控制器和阀门控制器调节运行频率和阀门开度来实现冷冻水出水温度和压缩机吸气过热度控制.模拟了冷冻水出水温度设定值阶跃变化和阶跃外扰作用下系统的团环响应.结果显示冷冻水出水温度的响应比吸气过热度的响应慢,并且采用TEV的变频制冷系统比采用EEV的变频系统响应速度慢.采用蒸发压力为频率控制器的控制变量时,系统响应比以冷冻水出水温度为控制变量时系统响应快,但是由于实际中很难给出蒸发压力的设定值,因此采用冷冻水温为控制变量的控制更易实现.

A lumped-parameter dynamic model of a water chiller refrigeration system based on mass and energy balance principles is developed. A control-oriented approach to model development is taken and the effect of comtrol inputs, such as compressor operational frequency and TEV （Thermostatic Expansion Valve） opening fraction an the output performance of the system are explored. The transient characteristics show that the thermal system responses are much slower than pressure and mass flow rate responses, revealing a two time-scale property of the system. EEV （Electronic Expansion Valve） was adopted to replace TEV. Two PI controllera are used to control chilled water supply temperature and suction vapor superheat by modulating frequency and EEV position. Closed-loop responses to step ehangea in supply water temperature set point values and disturbances are simulated. It shows that the dynamic responses of variable frequency system with TEV to the change in Twupply-sct were comparatively slower than the system with EEV; the responses of supply water temperature are dower than those of suction vapor superheat. The use of evaporation pressure as controlled variable is also investigated. It indicates that the closed loop responses are faster compared to chilled water supply temperature control. However, from a practical point of view, it is easier to select chilled water supply water temperature set point.