Simulation Study Of Variable Frequency Control Of Marine Central Cooling Water System And Parameter Optimization

The research on energy-saving technology of ships,which are the major energy consumers and emitters,is in response to the urgent need of building an environmentfriendly society in China.However,the large design margin and the lack of sufficient energy saving measures of the seawater pump of central cooling water system lead to huge energy waste.Since the variable frequency control can be used to continuously adjust the speed of the motor which could achieve on-demand supply of seawater flow and save energy,it is of great significance to study the variable frequency control of the seawater pump of central cooling water system.In this paper,the central cooling water system of an ocean-going container ship is taken as the research object,and the mathematical models of the relevant systems are established by using the principle of modular modeling.Then the simulation and energy saving analysis of variable frequency control of central cooling water system are carried out in MATLAB/Simulink.Finally,in order to optimize the flow control effect of the seawater cooling system,the PID parameters are optimized and its superiority is proven in the simulation model.The main work consists of the following parts:(1)Firstly,the structure and design requirements of the central cooling water system are studied,and the heat transfer processes of the relevant equipments are analyzed according to the heat transfer theory.The simulation model of the main engine cylinder liner and relevant equipments is established,and the correctness of the model is verified under the design condition.Then,the relationship between the main engine working condition and sea water temperature and cooling water temperature and sea water flow under different conditions such as changing the operating condition of main engine and the seawater temperature is obtained.(2)Then the pipeline characteristics of the seawater cooling system are analyzed,and the operating characteristic curve of the seawater pump under the design condition is obtained by fitting the real ship data.On this basis,the operating characteristics of the variable frequency seawater pump are analyzed,and the calculation formulas between the seawater flow rate and the motor speed of the seawater pump in different operating modes are obtained.Then the theoretical basis of direct torque control is studied,and the direct torque control simulation model of asynchronous motor is established.The operation conditions of the motor under different sea water flow are obtained,and the energy-saving analysis is carried out.The results show that the energy-saving effect of the variable frequency controlled motor is obvious.(3)Finally,in view of the difficulty of tuning parameter for PID,the BP neural network are combined with the traditional PID to realize the parameters self-tuning PID for its self-learning characteristics.In order to further optimize the control effect,the genetic algorithm(GA)is used to optimize the initial weight and threshold of the BP neural network.The obtained parameter self-tuning PID is applied to the flow control module of the central cooling water variable frequency control system,and the simulation calculation is performed.The simulation results show that compared with the traditional PID and BP neural network PID which randomly generates initial weight and threshold,the control effect of the GA optimized BP neural network PID has certain advantages.