Research On Stability And Optimal Control Of Hydro-turbine Regulation System Based On Refined Model

In recent years,the development of new energy and power grid have been continuously and powerfully promoted in China.The proportion of new energy sources such as water,wind and light in the power grid has been increasing.A large number of high-voltage and ultra-highvoltage power transmission projects have been completed,and the composition and structure of the power grid has undergone profound changes.On the one hand,with the high penetration of variable renewable energy,the frequency and power regulation of the system will become more difficult,leading to greater challenges in its safety and reliability.On the other hand,under the complex and multi-variable grid structure,hydropower units,as an important regulation energy,will often operate under variable conditions as required,and its operating environment is more severe than in the past.Therefore,it is of great significance to study the stability and optimal control of hydropower units to improve power quality and system stability.The current research on the stability and optimal control of a hydropower unit is mainly based on linearized hydraulic-mechanical or mechanical-electrical models,and focuses on frequency control mode(FCM),lacking systematic consideration on the unit operating characteristics under different control modes and full operating conditions.On the one hand,the key research is to establish the refined model of the prime mover because it represents to a great extent the dynamic characteristics of the system.At the same time,a complete theoretical system has not been formed for the stability analysis of hydropower units under full operating conditions(FOC),lacking a quantitative evaluation method for the stability of nonlinear system.On the other hand,hydropower units are a non-minimum phase system with time-varying parameters,relatively complex operating environment,variable operating conditions and control modes,which causes the traditional PID control hard to meet the requirements of optimal control.As a result,the design of a controller with variable structure and strong robustness is of great significance to improve the control quality and realize the economic operation of the unit.Taking the stability and optimal control of a hydro-turbine regulation system(HTRS)under the background of the profound changes in the structure of the power grid as the starting point,based on measured data,modern control theory and artificial intelligence algorithm,an in-depth and detailed study in this paper is carried out for the key scientific issues and practical application requirements in the aspects of refined modeling,parameter identification,stability and optimal control of the system.Firstly,a refined modeling method of the prime mover and a system parameter estimation strategy are proposed to establish the accurate mathematical model of the HTRS for the study on system stability and optimal control.On this basis,a theoretical framework for the stability analysis of the HTRS under FOC is proposed to realize the quantitative evaluation for the system stability under FOC,analyzing the change law of the stability with operating conditions.To overcome the shortcomings of the traditional PID control and realize the optimal control of hydropower units,an improved sliding mode control strategy under two control modes is studied to lay a good theoretical foundation for the practical application of sliding mode control in this field.The research contents of this paper mainly include:1.For the sake of refined modeling and simulation of a HTRS,the basic mathematical models of the subsystems of the HTRS are first introduced,including the linear and nonlinear model of hydro-turbine,the water hammer model and characteristic line model of water diversion system,the detailed model of servo-system,the five-order model of generator,the simplified first-order model of excitation system and the equivalent model of power grid,to provide the basis for establishing the hydraulic-mechanical-electrical coupling model of the system.Based on the characteristic curves of the hydro-turbine,the neural network nonlinear models are established for the Francis and Kaplan hydro-turbines respectively.According to the operation situation of the hydropower station,the controller model with different modes and the servo-system model with various nonlinear factors are established.2.An input-output correction method for neural network model of a hydro-turbine is studied to increase the accuracy of the neural network model of the hydro-turbine and to make the simulation results maintain a high consistency with the actual system characteristics under different operating conditions.A refined model of the hydro-turbine based on neural network model and correction term is obtained by optimizing the correction function with the measured data.To realize the modular simulation of hydro-turbine and water diversion system and improve the ability of the chracteristic line model to capture shock waves,the unsteady friction and resampling technique are introduced to propose an improved characteristic line model based on the basic one.The effects of the rigid and elastic water hammer,characteristic line and its variant models on the transient process of water head under the conditions of large and small fluctuations are further studied.The differences and applicable scopes of each model are summarized and the effectiveness of the improved characteristic line model is verified.3.In order to realize an adaptive parameter tuning of heuristic optimization algorithm,to avoid the algorithm falling into the local optimum,and to improve the accuracy of the algorithm to solve the parameter identification problem,an adaptive fuzzy particle swarm optimization(AFPSO)is proposed to improve the basic algorithm from three aspects of automatic tuning in parameters,population topologies and mutation characteristics.The excellent performance of AFPSO is proved by benchmark functions and comparative experiments.On this basis,a parameter identification strategy for HTRS is designed based on AFPSO.The parameter identification effect of AFPSO is theoretically verified with the simulated frequency and load disturbance data.The results show that the identification accuracy of AFPSO is significantly better than traditional algorithms.The parameters of a real unit are estimated by using the data from primary frequency regulation and no-load disturbance test,which further proves the effectiveness of the proposed algorithm and strategy.4.In order to study the stability of the HTRS under FOC and to fully master the change law of the unit stability with operating conditions,a neural network differentiation method(NND)is proposed to calculate the transfer coefficients of hydro-turbines,and a piecewise linear model of HTRS is established based on this method.The stability of HTRS under FOC and two control modes is analyzed from the aspects of stability domain analysis and parameter sensitivity analysis,and the corresponding quantitative analysis indices are proposed.On the one hand,the changes of the area and mass center of the stability domain and the dynamics of the system with operating conditions are analyzed in detail.On the other hand,the comprehensive sensitivity of the system state to each parameter is first calculated under the rated operating condition to determine the parameter with high sensitivity.Then the change laws of the comprehensive sensitivity and trajectory sensitivity with operating conditions is studied based on these parameters.5.In order to improve the robustness and the dynamic regulation quality of controllers,a nonlinear sliding mode control(SMC)strategy is proposed for the optimal control of HTRS.Firstly,taking the ideal HTRS as an example,the problem of traditional SMC in the control of hydropower units are studied.In this regard,a new sliding mode surface is designed,an improved SMC and its parameter optimization framework are proposed,and its effectiveness is proved by simulation.On this basis,the improved SMC for hydropower units under power and speed control modes is studied.Under the weak stable condition of the unit,the robustness of the new SMC is verified by the experiments such as parameters perturbation,step response and noise disturbance,which lays a theoretical foundation for the practical application of SMC in hydropower units.