Study On A Fractional Order PID Grading Control Policy For Pressure Difference Between The Chilled Water Supply And Return Pressures Of Primary Pump Variable Flow Of Air-conditioning Chilled Water System

Compared with ACCWS-PPCF,ACCWS-PPVF has been widely used due to its advantages of saving electricity consumption,flexible layout and convenient management and saving electricity consumption,etc.for air-conditioning units and the chilled water pumps.The pressure difference between the chilled water supply and return pressures（PDCWSRP）is very important to maintain hydraulic balance of ACCWS-PPVF and to adapt to the dynamic change of cooling load for air-conditioning users.However,the integer order PID mode is mostly adopted for the present regulation on PDCWSRP,which can lead to the corresponding problem such as the larger steady-state error,the larger overshoot and excessive oscillation,etc.In view of this,combining the air-conditioning technical requirements for ACCWS-PPVF,fractional order PID control technology and advanced intelligent algorithm,the research objective of this paper proposes a fractional order PID control policy for PDCWSRP and designs a new algorithm of tuning parameters of controller in order to promote the control quality of PDCWSRP.Meanwhile,considering the demands of high and low cooling load for air-conditioning users and to ensure that pressure difference between the chilled water supply and return pressures（denoted asΔP）equals to the corresponding setting value(denoted asΔP_set),a grading control method is adopted to regulate variable frequency for primary pump and the flow of bypass water for the actuator installed between water separating and collecting vessels,respectively,and then the corresponding high and low flow of the chilled water is output to the water loops for air-conditioning users.So the demands of the dynamic change of air-conditioning cooling load and hydraulic balance are satisfied.The main contents of this paper are as follows:1.The operation mechanism principle and classification for the air-conditioning chilled water system and the principle of fractional order PID control are introduced.Analyzing the advantages and disadvantages of control modes associated with the pressure difference between the chilled water supply and return pressures,the temperature difference between the chilled water supply and return temperatures and the flow of bypass water etc.for ACCWS-PPVF,a fractional order PID grading control scheme is proposed to improve the control quality of PDCWSRP and the corresponding flow of the chilled water is output to adapt the dynamic change of air-conditioning cooling load.In addition,for the brain of this fractional order PID grading control system,i.e.,fractional order PID controller,an improved Oustaloup filtering algorithm is used to accurately fit it.Through the analysis of frequency domain characteristics of the Bode diagram and the example verification in the existing literature,this fractional order PID controller has better control performance than the integer order PID one.2.Based on the standard biogeography-based optimization algorithm（SBBOA）,the structure model and operation process of a modified biogeography-based optimization algorithm（MBBOA）are constructed by decreasing migration factor linearly and keeping its structure and the other parameters constant.Through the verification of the examples of the classical testing functions,the results show that this MBBOA is superior to SBBOA in terms of convergence and diversity.In order to further test the performance of MBBOA,based on MBBOA,this paper selects min ITAE as its objective function.Through numerical simulation and experimental test on PID control for the level of water tank,this MBBOA is verified to tune parameters of the PID controller and the optimal values of three parameters are obtained.3.On the basis of the mechanism analysis,the input and output characteristics of each component of this fractional order PID grading control system such as the controlled plant associated with PDCWSRP,fractional order PID controller for pressure difference between the chilled water supply and return pressures（PDCWSRP-FOPIDC）,variable frequency pump,measuring transmitter for PDCWSRP and actuator for the flow of bypass water,etc.are derived,respectively,and the corresponding transfer functions are set up.4.Using Matlab/Simulink tool,the simulation model of this system is configured and the corresponding simulation is simulated under the air-conditioning operation condition in summer.MBBOA is synchronously run to tune five parameters of PDCWSRP-FOPIDC to obtain the optimal values of the corresponding parameters,and the dynamic response of PDCWSRP is output.The results indicate that this system is feasible,the control performance such as faster response,less overshoot and stronger anti-jamming for PDCWSRP is achieved and the demand ofΔP=ΔP_setis met.Moreover,for the same controlled plant with PDCWSRP,the PID control strategy,the fractional PID control strategy and the fractional PID control strategy based on SBBOA are simulated,respectively.Comparison of the related results shows that the proposed fractional order PID control policy is obviously prior to the other three control modes.