| In this thesis,hydroelectric power systems(conventional hydropower station and pumped storage power station)were taken as the research object,and its transient process dynamics models were established and stability analysis wae carried out.Conventional hydropower stations and pumped storage power stations are complex systems of hydro-mechanicalelectrical coupling.Typical state variables have different dynamic responses over time,so both can be described as complex nonlinear hydroelectric power systems.In the transient process,the hydroelectric power system has a drastic change in operating parameters and a complex internal coupling relationship,so its stability during transients is particularly acute.This paper under the founding of National Natural Science Foundation project "Stability and Control of Hydropower Station System" divided the hydropower system into multiple subsystems for independent modeling of blocks from the perspective of dynamics.Considering the joint effects of hydraulic,mechanical and electromagnetic factors,the coupling mechanism between the subsystems of the hydropower system was deduced during transient process,and the transient dynamics modeling of the hydroelectric power system and its stability mechanism analysis were carried out.Some relatively complete and innovative theoretical results have been achieved.The main research contents and results of this thesis are as follows:(1)The hydro-turbine governing system consists of three subsystems: hydraulic,mechanical and electrical subsystem.The response time of each subsystem has a scale difference.Therefore,the precise model of the hydro-turbine governing system in the transient process has multi-scale coupling effect.In order to study the transient dynamic behavior and stability mechanism of the hydro-turbine governing system at multiple time scales,first the mechanical subsystem,effecting by inertia and clearance,was considered as the slow subsystem of the hydro-turbine governing system.The hydro-turbine governing system was rescaled by introducing the scale factor,and a multi-time scale effect hydro-turbine governing system was established.The numerical simulation was used to analyze the dynamic behavior evolution of the system under different time-scales.It was found that there were significant fast and slow effects in the system(high-frequency small amplitude vibration and lowfrequency large vibration alternately appear)and can be effectively weakened or avoided by increasing the scale factor when it is greater than 0 and less than 1.Furthermore,in order to explore the evolution mechanism of transient characteristics of the hydro-turbine governing system at multiple frequency scales,considering the transfer coefficient of the system changes with the operating conditions,a multi-frequency scale dynamic model of the hydro-turbine governing system was established by introducing the transfer coefficients in form of cyclic excitation.Through numerical simulation,it was found that the multi-frequency scale hydroturbine governing system had typical fast and slow dynamic behavior(period clustering)and the instability mechanism of the system with the increase of the amplitude and frequency of the excitation was revealed.The research results provide a theoretical reference for the multiscale coupled dynamic modeling and stability analysis of the hydro-turbine governing system during transient processes.(2)The torque and flow characteristics of the hydro-turbine governing system change rapidly during transients,which is the key factor determining the applicability of the transient dynamics model.In order to describe the dynamic characteristics of the hydro-turbine governing system in transient process more accurately,the transient torque and flow expressions of the hydro-turbine governing system were obtained by improvement.A dynamic model that can reflect the transient characteristics of the hydro-turbine governing system was established for the load-rejection transient process.The influence of the straight line closing law and the polygonal line closing law of the guide vane on the transient characteristics of the hydro-turbine governing system was analyzed by numerical simulation.The mechanism of the turning point setting in the closing rule of the guide vane on the transient head,speed and flow of the hydro-turbine governing system was revealed.Furthermore,in order to deeply analyze the transient dynamic response and force characteristics of the hydropower plant shafting system,based on the coupling relationship between the hydro-turbine governing system and the shafting system,the transient coupling dynamic model of the hydro-turbine governing system and the shafting system was established.During the start-up transient process,the effects of the straight-line opening and the polygonal-line opening of the guide vane on the transient dynamics of the hydro-turbine governing system and the shafting system were analyzed.The interaction mechanism of the two systems during the start-up process and the transient response and stress on the shafting were revealed.The research results enrich the coupled dynamics modeling theory of hydraulic turbine governing system and shafting system,and lay a theoretical foundation for exploring its transient stability mechanism.(3)There is a phenomenon of mixed flow in the hydropower station system with inclined ceiling tailrace.Compared with conventional hydropower stations,the transient stability of the system is more complicated because its transient influence factors change with working conditions.In order to study the transient energy flow characteristics and stability factors of the hydropower station system with inclined ceiling tailrace,the dynamic modeling and transient energy flow analysis of the hydropower station system with inclined ceiling tailrace under the Hamiltonian theory framework were carried out.Firstly,based on the dynamic model of the hydropower station system with inclined ceiling tailrace,it was transformed into the corresponding Hamiltonian system by orthogonal decomposition method.The factors affecting the energy generation and energy dissipation of the system were obtained by decomposing the structural matrix of the Hamiltonian system.The dynamic response of the hydropower station system with inclined ceiling tailrace under step load disturbance and random load disturbance were obtained by numerical simulation.In the small fluctuation process of unit load regulation,the stability of the hydropower station system under three kinds of tail water structure(with pressurized tail water,pressurized tail water with transient water flow and inclined ceiling tailrace)was studied from the perspective of dynamics.The influence law of the slope of the inclined ceiling tailrace on the transient stability of the hydropower station system was revealed.The research results provide theoretical support for transient energy flow analysis and stability control of the hydropower station system with inclined ceiling tailrace.(4)The pump-turbine is affected by a variety of random factors during operation,making its transient characteristics and its stability mechanism more complicated.In order to study the transient response and stability conditions of the pump-turbine system under random factors,the dynamic model of the pump-turbine system under power generation conditions was established.The numerical simulation was used to analyze the influences of PI control parameters on the transient charactertistics of the pump-turbine system under random load disturbance.Furthermore,considering the random variation of the long-pressure water diversion pipeline,the Chebyshev polynomial approximation method was used to establish the stochastic dynamic model of the pump-turbine system during the load-rejection transient process,and the influence of the random variation of the water-flow inertia on the transient characteristics of the system was analyzed.The influence of the inverse S-zone characteristic curve on the transient stability of the system was given.The effects of characteristic curve slope,friction loss,water flow inertia and moment of inertia on the stability of the system at the runaway point were analyzed by numerical simulation.The research results provide a theoretical reference for the stochastic dynamics modeling theory and stability mechanism of the pump-turbine system. |
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