Stochastic Dynamic Model And Oscillation Mode Analysis Of A Hydropower Generation System

In recent years,the demand for the construction of large-medium hydropower stations is increasing with the rapid development of hydropower industry.As an important part of hydropower station,hydro-turbine governing system is a complex nonlinear system composed of pressure diversion subsystem,hydraulic generator unit and governor subsystem.There are stability problems with the units running such as such as penstock breaking due to water hammer and self-oscillation of unit,et al.The above problems are directly related to power generation efficiency and safe operation of hydropower stations,and even affect the impact on the power system when the units are connected to the grid.In this paper,firstly,the system parameter?generator speed?is introduced to establish the novel hydro-turbine governing system stochastic model.Based on the Chebyshev polynomial approximation,the influence law of the stochastic parameter on the stability of the hydro-turbine governing system is studied.Then,the first order oscillation modes,composite oscillation modes and resonance modes are defined based on the modal series theory to reveal the dynamic phenomena of the hydro-turbine governing system.Finally,the stability of the shafting system of a hydro-turbine generator unit is studied by modal interaction index analysis and considering the hydraulic unbalance force of the shafting system of a hydro-turbine generator unit.The contents and conclusions are mainly divided into the following three aspects:?1?Considering the stochastic fluctuation of generator speed in the process of load rejection,the stochastic dynamic system is transformed into a deterministic system by introducing stochastic variable into generator speed.And the stochastic dynamic model of load rejection of the hydro-turbine governing system is established by using the Chebyshev polynomial approximation method.The dynamic evolution of the hydro-turbine governing system model with time is analyzed by numerical simulation from two aspects?the relative deviation of generator speed and the relative deviation of rotor angle?.It is found that the hydro-turbine governing system undergoes two processes of unstability?0?t<0.15s?,steady-state?0.15s?t<1.5s?during load rejection.Using Runge-Kutta method,the reliability of the stochastic dynamics model of hydro-turbine governing system established in this chapter is verified by comparing and analyzing the stochastic dynamic model with the existing mathematical model.The results provide a reference for the stochastic dynamic modeling and stability analysis of the hydro-turbine governing system in the process of load rejection.?2?Considering that the hydro-turbine governing system exhibits complex dynamic phenomena when disturbed,such as multi-order oscillation,it is not conducive to stable operation of the system.A nonlinear model of six-dimensional hydro-turbine governing system is established in order to reveal the oscillation characteristics of hydro-turbine governing system under disturbance.The linear method,Runge-Kutta method and modal series method are used to compare and analyze the dynamic trajectory of the system with time to verify the accuracy of the modal series theory.The first order oscillation mode analysis of modal theory shows that the oscillation mode?₁ is most easily affected by the system disturbance.The modal equation of composite oscillation of the hydro-turbine governing system is obtained by modal analysis of composite oscillation.Resonance mode analysis shows that it is less likely for a hydro-turbine governing system with multi-subsystem coupling to have resonance in a multi-oscillation mode.The research results provide theoretical support for the research on the oscillation mechanism of hydro-turbine governing system under disturbance.?3?In order to study the oscillation characteristics of the shafting model of a hydro-turbine generator unit when considering the hydraulic unbalance force,this chapter takes the shafting model of a hydro-turbine generator unit as the research object and establishes the shaft system model of the hydro-turbine generator unit.Due to the complex coupling within the shafting system,the modal series theory is introduced into the shafting model of a hydro-turbine generator unit to reveal the oscillation characteristics of the system.The accuracy of modal analysis is verified by comparing the simulation results of the nonlinear modal series method and Runge-Kutta method.The interaction relationship between state variables and modes is obtained by damping ratio and participation factor.The oscillation characteristics of the shafting system of a hydro-turbine generator unit are studied by quantifying the interaction index of the system mode.Based on the above research results,it is concluded that adjusting the parameter amplitude of the system in the dominant oscillation mode is conducive to the stable operation of the shafting system of a hydro-turbine generator unit.The research results provide theoretical reference for the stable operation of the shafting system of a hydro-turbine generator unit in the transient process considering the hydraulic unbalance force.