Application Of Improved Generalized Predictive Control In The Combined Control Of Reheat Steam Temperature

In recent years,with the rapid development of the power industry,higher requirements have been placed on the control quality and operation efficiency of thermal power units.The steam temperature at the outlet of the reheater of the thermal power unit is widely controlled by the intermediate reheat system.The temperature adjustment method is that after the new steam passes through the high pressure cylinder to do work,it returns to the reheater of the boiler to absorb heat,and then the heated reheat steam is sent to the intermediate and low pressure cylinder for expansion and work.The original design of reheater steam temperature regulation mode mainly adopts flue gas damper control,and the spray desuperheating control is used as auxiliary(emergency spray)control.However,the temperature regulation mode of flue gas damper generally has adverse factors such as inertia,large delay and serious nonlinearity of the controlled object,resulting in a low automatic input rate and difficulties in control.In actual field applications,most of the units still use the automatic adjustment of water spray as the main control method,and the manual adjustment of the flue gas baffle as the auxiliary control method.According to the thermal efficiency analysis of the unit,it can be seen that the temperature control method mainly based on spray desuperheating significantly reduces the economy of the unit.The temperature control method mainly based on the control of the flue gas baffle,in the case of over-temperature and under-temperature,the control quality is poor,and the safety cannot be guaranteed.Therefore,it is very necessary to combine with the advanced control strategy and realize the joint control of flue gas damper and water spray desuperheating for the reheat steam temperature system,which can ensure both economy and safety.The main research work of this paper is as follows:Firstly,the controlled object of the reheat steam temperature system is modeled based on historical data,and the mathematical model of the controlled object of baffle and desuperheating water is identified by the recursive least square method with forgetting factor.Next,the basic research of generalized predictive control algorithm is carried out.In order to overcome the limitations of generalized predictive control algorithm,an improved stepped generalized predictive control algorithm is designed and verified by simulation.Then,based on the improved generalized predictive control algorithm,three schemes of combined control of reheat steam temperature system with flue gas baffle and desuperheating water are designed and verified by simulation on MATLAB platform.The results show that: The two loops of simple joint control interfere with each other and the tuning is difficult,while the joint control effect of external decoupling and GPC internal feedforward is excellent.Finally,in order to further improve the control quality of the combined control system of damper and water spray desuperheating,a deviation segmented gain scheduling optimization control scheme is proposed.The control principle is: strengthen the control of flue gas damper within the normal temperature deviation range to improve the economy;In the case of over temperature and under temperature,increase the spray desuperheating control to improve the safety.The effectiveness of the control strategy is verified by simulation.