Heat Transfer Analysis Of Direct Air Cooling System And Optimization Of Back Pressure Control Strategy

At the cold end of the direct air-cooled turbine,the cold air is sent to the bottom of the air-cooled island by an axial flow fan,and the heat is transferred to the steam turbine with higher temperature in the fin tube,finally condensed into water.Compared with other cooling methods,air is used as cooling medium in direct air cooling system,which can save more than 80% of water and occupy a smaller area.Therefore,it has been widely used in many areas of China where ‘rich in coal and little water’.In the direct air cooling system,the back pressure is one of the important monitoring and control quantities in the direct air cooling system,and its control quality is directly related to the safe and economic operation of the whole unit.However,because the direct air cooling system is exposed directly in the natural environment always,its heat transfer process will also be affected by many environmental factors besides the operating conditions of the unit,and the heat transfer performance is directly related to the change of the air cooling back pressure.At present,the conventional PID are often used for the back-pressure control of direct air cooling units.However,the two methods depend heavily on the controlled model,when the system conditions change or the system is unstable due to the change of other factors,the parameters can not be automatically adjusted online according to the change,so the control effect is general.Taking a direct air-cooling system of 300 MW unit of Shanxi province as the research object,defined the heat transfer characteristic of air-cooling system and main influencing factors of back pressure and the specific action of each analyzed-factor on the basis of introducing the principle of direct air-cooling system and operation regulation measures.Under the condition that the environment was basically constant,the particle swarm optimization algorithm was adopted to build the target model of back-pressure control system.According to the characteristics of the system,fuzzy PID control wasselected.The back pressure control system was simulated and compared with the traditional PID control system.The results showed that the fuzzy PID control has a good adaptability to the controlled model,and has a smaller overshoot and a faster adjustment time.But in order to further improve the anti-jamming ability of the system,fuzzy internal model PID control was proposed to optimize the system.The results showed that the proposed method is superior to the first two in both dynamic and static performance,and has stronger stability and anti-jamming ability.It can provide a theoretical basis for the improvement of the control strategy of the back pressure control system of the direct air cooling unit,and has certain reference significance.