Numerical Simulation Of A Cascade Control System Of Supply Water Temperature PI~?D~?and Steam Flow PI~?for A Vapor-water Type Heat Exchanger

As one of the important equipments of a thermal station,the vapor-water type heat exchanger?VWTHE?plays some key roles in regulating,converting and distributing the heating medium transported in the central heating system,which has some dynamic characteristics of non-linearity,time-varying parameters,and time-delay,etc.At present,the control methods of PID single loop and PID-PI cascade are the main automatic control ways for VWTHE in practice.However,these conventional control modes can lead to the problems of large error in the supply water temperature,large overshoot and long adjustment time,etc,so it is difficult to achieve the desired control effects.In view of the above,the research aim of this paper is to further improve control modes for VWTHE and to obtain more satisfactory control effect.Firstly,based on analyzing heating technological process for VWTHE and PI^?D^?control technology,this paper proposes a cascade control system of supply water temperature PI^?D^?-steam flow PI^?for VWTHE.Secondly,for the key problem of PI^?D^?and PI^?controller,i.e.is parameter tuning,the modified particle swarm optimization algorithm?MPSOA?is constructed to tune the optimal values of the corresponding parameters.Finally,using MATLAB/Simulink tool,the model of this cascade control system is configured and numerically simulated.The feasibility of the proposed fractional order PID cascade control strategy and MPSOA are verified by the simulation results.The main research tasks and contents are as follows.1.Based on the standard particle swarm optimization algorithm?SPSOA?,the structure model and operating process of MPSOA are constructed by appropriately changing inertia weight w and acceleration coefficient c₁ and c₂.By verifying the examples of classical functions in the existing literature,the results show that convergence and diversity of MPSOA are obviously improved than SPSOA.2.The PI^?D^?controller,the core of this cascade control system,is accurately fitted by using the improved Oustaloup filtering algorithm.Through the analysis of frequency domain characteristics of the bode diagram and verification of the examples in the existing literature,it is shown that the PI^?D^?controller has better control performance than the PID one.In addition,the feasibility of the MPSOA to tune the controller's parameters is verified by the experiment of tank level control.3.The related requirements of central heating technological process and automatic control are analyzed synthetically,and the temperature plant of VWTHE,the measuring and transmitting units of supply water temperature and steam flow,and the steam flow actuator are modeled,respectively.A cascade control strategy of supply water temperature fractional order PID controller?SWT-FOPIDC?as the main controller and steam flow fractional order PI controller?SF-FOPIC?as the secondary controller is determined.Considering min ITAE as the performance evaluation index of fitness function J,the optimal values of eight parameters of SWT-FOPIDC and SF-FOPIC are found by running MPSOA.4.Under the heating condition in winter,this cascade control system has been configured and numerically simulated based on the MATLAB/Simulink tool.The results show that this cascade control system has faster response,less overshoot and better ability of anti-jamming,which is suitable for those plants or systems that have large inertia or overshoot.In addition,the supply water temperature and steam flow can meet the requirements of the central heating technology.5.For the same VWTHE and heating conditions,the numerical simulations of control system modes of supply water temperature PID-steam flow PI cascade are carried out.Based on analyzing the performance indexes of steady state error,overshoot,adjusting time,etc,this cascade control strategy for supply water temperature PI^?D^?and steam flow PI^?controllers whose parameters are tuned by MPSOA is obviously superior to PID-PI cascade controllers whose parameters are tuned by classical tuning method.