Transient Characteristics Of Hydro-turbine Governing System Of Hy Dropower Station With Free Surface-pressurized Tailrace System

In-depth research on the performance characteristics of open tailrace channel or sloping ceiling tailrace tunnel can be applied into an underground hydropower station to replace or reduce the sectional area of tailrace surge tank on the premise of meeting the requirements of the guaranteed calculation for regulation and the stable operation. That has obvious positive effect both on scientific technological progress and engineering. The level of hydropower construction and design will be developed by the widespread application of open tailrace channel and sloping ceiling tailrace tunnel. This study is proceeded from the phenomenon of free surface-pressurized flow in the tailrace tunnel, which is the most significant hydraulic characteristics of the underground hydropower station with an open tailrace channel or sloping ceiling tailrace tunnel in the transient process. Aiming at the hydro-turbine governing system of hydropower station with free surface-pressurized tailrace system, the following two points of focus are taken as goals:(1) The stage of hydropower station design:Reveal the transient characteristics (i.e. stability and regulation quality) of the hydro-turbine governing system of hydropower station with free surface-pressurized tailrace system from the perspective of theoretical analysis, and then provide a theoretical foundation and optimization direction for the design of open tailrace channel and sloping ceiling tailrace tunnel as well as their combined layout with tailrace surge tank.(2) The stage of hydropower station debugging and running:Reveal the physical phenomenon of inverse analysis for the debugging tests (i.e. load adjustment test and load rejection test) of hydropower station with free surface-pressurized tailrace system from the perspective of numerical simulation. And to explore, perfect the relevant numerical simulation method and theory of inverse analysis. Then further understand the effects of the hydraulic characteristics of sloping ceiling tailrace tunnel on the operation of hydroelectric generating set. In addition, the rationality of the design and the reliability of the numerical simulation method can be verified through inverse analysis. According to the above two goals, The following three aspects are the main contents in this dissertation:1 Transient characteristics analysis of the hydro-turbine governing system of hydropower station with open tailrace channel.On the basis of the state-space method, a novel linear mathematical model of the unsteady flow for the tailrace system with an open channel is proposed. This novel model is an elastic linearized model of water hammer. The validity of the model is verified by several numerical simulation examples, which are based on a finite difference technique. Then, the complete mathematical model for the hydro-turbine governing system of hydropower station with an open tailrace channel, which is used for simulating the transient process of the hydro-turbine governing system under load disturbance, is established by combining the models of hydro-turbine, generator, governor and open tailrace channel. Finally, according to the complete model, the stability and regulation quality for hydro-turbine governing system with an open tailrace channel under load disturbance are studied, and the effects of open tailrace channel and tailrace surge tank on the stability and regulation quality are analyzed. The results indicate that:(1) The state-space method can be reliably applied into the computation and analysis of the stability and regulation quality of hydro-turbine governing system with an open tailrace channel under load disturbance. (2) For the hydropower station without tailrace surge tank, the influence of the open tailrace channel on the stability and regulating quality of hydro-turbine governing system can be neglected, while for the hydropower station that have tailrace surge tank, the length of the open tailrace channel and the depth of water have a larger effect on the stability of the turbine regulating system, and there exits a length threshold of the open tunnel.in its sides, the effects of channel length for stability domain instead. (3) For the hydropower station without tailrace surge tank, the influence of open tailrace channel on small fluctuation can be ignored. Under load disturbance, the open tailrace channel influences the stability and regulation quality of hydro-turbine governing system by affecting the surge level fluctuation in the surge tank, and its length is the dominant factor. The stability and regulation quality of hydro-turbine governing system can be improved by considering the influence of open channel.2 Transient characteristics analysis of the hydro-turbine governing system of hydropower station with sloping ceiling tailrace tunnel.In order to overcome the problem of nonlinear dynamics of hydro-turbine governing system with sloping ceiling tailrace tunnel, which is caused by the interface movement of the free surface-pressurized flow in the tailrace tunnel, and the difficulty of analyzing the stability of system, the Hopf bifurcation theory is applied to study the stability of hydro-turbine governing system of hydropower station with sloping ceiling tailrace tunnel. Firstly, a novel and rational nonlinear mathematical model of the hydro-turbine governing system is proposed. This model contains the dynamic equation of pipeline system which can accurately describe the motion characteristics of the interface of free surface-pressurized flow in sloping ceiling tailrace tunnel. According to the nonlinear mathematical model, the existence and direction of Hopf bifurcation of the nonlinear dynamic system are analyzed. Furthermore, the algebraic criterion of the occurrence of Hopf bifurcation is derived. Then the stability domain and bifurcation diagram of hydro-turbine governing system are drawn by using the algebraic criterion, and the characteristics of stability under different state parameters are investigated. Finally, the stability working principle of inclined ceiling tailrace in the condition of load adjustment and the influence of step load value, ceiling slope angle and section form of tailrace tunnel and water depth at the interface in tailrace tunnel on stability are analyzed based on stable domain. The results indicate that:The Hopf bifurcation of hydro-turbine governing system with sloping ceiling tailrace tunnel is supercritical. The phase space trajectories of characteristic variables stabilize at the equilibrium points and stable limit cycles when the system state parameter points locate in the stable domain and unstable domain, respectively. Under negative disturbance and positive disturbance, the effect laws of step load value, tailrace tunnel slope angle and water depth in tailrace tunnel are inverse. Tailrace tunnel section form has a significant influence on the stability under negative disturbance, while it has a slight influence on the stability under positive disturbance. With the same step load value, the stable domain of the system under negative disturbance is larger than that under positive disturbance. Among the three section forms, i.e. rectangle, arch and roundness forms, the roundness section is the most favorable for system stability, while the rectangle form is the least favorable. With the increase of water depth in tailrace tunnel, the stable domain of the system becomes smaller under negative disturbance, while it becomes larger under positive disturbance.3 Theories and methods of inverse analysis for the hydropower station with sloping ceiling tailrace tunnel.Aiming at the hydropower station with sloping ceiling tailrace tunnel, the theories and methods of inverse analysis are perfected. Based on a constructed hydropower station with sloping ceiling tailrace tunnel, the mathematical model for inverse analysis is established. Then the inverse calculation and analysis of load adjustment test and load rejection test carried out in the stage of completion and debugging are conducted. The results indicate that: (1) The mathematical model and simulation method used for the inverse calculation in this dissertation can reflect the transient process of real hydropower station with sloping ceiling tailrace tunnel reliably and accurately. (2) According to the verification of the inverse analysis, the requirements of the guaranteed calculation for regulation and the stable operation of the example in the conditions of load adjustment and load rejection can be satisfied, which indicates that the design based on the theoretical analysis, numerical simulation and model test for this hydropower station is correct and rational. (3) The procedures of inverse analysis proposed in this dissertation can be used as a standard process and applied into the inverse analysis of other hydropower station with sloping ceiling tailrace tunnel. (4) The theories and methods of inverse analysis, theoretical analysis, numerical simulation and model test together constitute the four basic methods for the study of the key technologies of sloping ceiling tailrace tunnel.