Research On Key Technology Of Improving Dynamic Quality Of Low-pressure Oil Servo Motor System

Steam turbine is the prime mover of steam turbine generator set,which is widely used in thermal power plant,nuclear power plant and geothermal power plant.The electro-hydraulic regulating system of steam turbine must control the opening of regulating valve in real time to make the output power and speed of generator set meet the power demand of users at any time.Therefore,its regulating quality directly affects the economic and safe operation of steam turbine unit and the stability of power system.At present,more and more small and medium steam turbine units,accounting for more than20%of the total capacity of power generation equipment in China,use low-pressure turbine oil electro-hydraulic control system,friendly and pollution-free to environment.The module of oil servo motor,which plays the core function of regulating system,contains multi-stage hydraulic amplification mechanism.The complex structure and non-linear factors make it easy to have problems such as large sluggish rate,small shaking,unstable internal leakage and so on.In order to improve the dynamic quality of the low pressure oil servo motor system,the structure of the system is refined and decomposed,and some key technologies are studied,such as pilot pressure control,flow characteristics of thick orifice,mathematical modeling of complex hydraulic system,internal leakage control of multiple matching structure,etc.The main content in this paper is summarized as follows.（1）A new scheme to realize two-stage pilot control pressure based on multi-stage thick orifice and solenoid directional valve is proposed.The basic matching principle of number and diameter of series orifices is established combing with theoretical analysis,experiment and practical physical system application.It is shown that the scheme not only can realize two-stage pilot control pressure,but also early warn the abnormal state of solenoid valves and improve the reliability of the system.Through CFD simulation,the influence of flow resistance in the integrated structure on the two-stage pilot control pressure has been studied to realize the prediction of the pilot control pressure and the improvement scheme of the integrated structure is proposed.（2）The experimental device is designed to study the flow characteristics of thick orifice.The difference of flow characteristics between two-stage and single thick orifices has been analyzed in the combination of theoretical analysis,CFD simulation and experiment.It is found that the front stage installation hole of two-stage thick orifice can slow down the change trend of flow field parameters,and its discharge coefficient is obviously larger than that of single thick orifice.On this foundation,the influence of cavitation,different length-diameter ratio and machining error on the flow characteristics of two-stage thick orifice has been deeply studied through experiments and CFD simulation.The steady values of the discharge coefficient of the orifice with or without cavitation are obtained with fixed length（L=2 mm）and different diameter（d=1 mm,1.2mm,1.6 mm,2 mm,2.4 mm,3 mm,3.5 mm）.The results support the correct selection of two-stage thick orifice and the setting of discharge coefficient when modeling very well.（3）An experimental device is designed and manufactured to study the control performance of two half bridges,which are the half bridge-B₁ consisting of a two-stage thick orifice and a disc valve in the electro-hydraulic converter and the half bridge-B₂composed of a slender damping hole and a spool valve with circular ports in multi-stage hydraulic amplifying structures of oil servo motor respectively.The mathematical formula transforming the valve displacement to control pressure and the leakage trend of the two half bridges have been obtained,which lays the foundation for the subsequent system modeling and internal leakage research.Basing on AMESim software,simulation model of half bridge-B in different conditions is established to study the influence of system oil source pressure fluctuation,types of fixed and variable hydraulic resistance on the control performance and leakage of half bridge-B.The result provides technical guidance for the optimization design of half bridge-B.（4）The structural characteristics and working mechanism of oil servo motor system has been analyzed.Through the dynamic analysis of all levels of hydraulic amplification mechanism hydraulic power components and mechanical-hydraulic servo feedback mechanism,the transfer function was deduced to complete the mathematical modeling.The simulation model of oil servo motor system has been built using MATLAB/Simulink software,and verified its correctness by experiment.On this basis,the static and dynamic performance of the system and its main influencing factors are analyzed,and the results show that the system has hysteresis phenomenon and slow response speed.Therefore,the characteristic table of the influence of parameter changes on the static and dynamic performance of the system is established,and the key design parameters are improved to enhance the dynamic quality of the system.（5）The internal leakage way of oil servo motor,which has integrated structure with cylinder body and magnifying mechanism is analyzed and experiments are carried out for oil servo motors,different individuals of the same structural form and different structural forms with the same matching requirements.The research results reveal the variation rule of internal leakage.Therefore,the zero-position leakage detection method is proposed to check the consistency of the assembly process of oil servo motor and enhance the stability of the internal leakage quantity.The physical object-oriented method is used to establish a comprehensive leakage simulation model for the regulating system by AMESim software.After the model verification,the influence of radial clearance of different fit positions on the total leakage and system performance has been analyzed to determine key factors affecting the total leakage.On this basis,the radial clearance control range of the multiple matching structure is improved according to the internal leakage control objective.The research shows that it has a certain academic significance and engineering application value for improving the dynamic quality of low-pressure oil servo motor system and enriching the research content in the field of low-pressure electro-hydraulic control system of steam turbine.