| Hydropower station is a dynamic system with strong nonlinearity and complex hydraulic-mechanical coupling characteristics.Under the effect of nonlinear hydraulicmechanical coupling,the transient process of hydropower system is complicated,especially bifurcation and chaos behaviors.Changes in operating states and conditions induce multidirectional evolution and changes in system performance,which brings huge challenges to unit operation.Moreover,the safe,stable and efficient operation of the hydropower system is restricted.For this reason,the research of characteristics and control of bifurcation and chaos behaviors for hydropower system based on nonlinear hydraulic-mechanical coupling is conducted.For the safe,stable and efficient operation of hydropower station,the mathematical model is established based on the nonlinearity and hydraulic-mechanical coupling characteristics of hydropower system.Aiming at the characteristics and control of the transient process,four aspects of research are carried out: Characteristics of bifurcation behaviors for hydropower system based on nonlinear hydraulic-mechanical coupling.Characteristics of chaos behaviors for hydropower system based on nonlinear hydraulicmechanical coupling.Sliding mode control of hydropower system based on nonlinear hydraulic-mechanical coupling.Stable sectional area of surge tank of hydropower system considering bifurcation and chaos characteristics.Innvative results are achieved as follows:1.Characteristics of bifurcation behaviors for hydropower system based on nonlinear hydraulic-mechanical couplingFirstly,the mathematical model of speed regulating system of hydropower station considering nonlinear turbine characteristics is established.Then,the Hopf bifurcation theory is adopted for the stability analysis of speed regulating system of hydropower station.Both the theoretical analysis and example analysis are carried out to comprehensively explain the rule and essence of stability.Finally,the effect mechanism of nonlinear turbine characteristics on stability of the speed regulating system of hydropower station is revealed by using contrastive analysis.The results indicate that the nonlinear turbine characteristics are composed of nonlinear head characteristics and nonlinear speed characteristics.The emerged Hopf bifurcation of speed regulating system of hydropower station is supercritical.The domain at the lower side of bifurcation curve is the stable domain.The nonlinear speed characteristics of turbine almost have no effect on the stable domain and dynamic response of speed regulating system.The effect of nonlinear turbine characteristics on stability and dynamic response of speed regulating system is mainly realized by the nonlinear head characteristics of turbine.For the stability of the speed regulating system of hydropower station,the nonlinear head characteristics of turbine is favorable under negative load disturbance and unfavorable under positive load disturbance.The rules for the dynamic response of comprehensive characteristics coefficient of turbine reveal the effect mechanism of nonlinear turbine characteristics on stability of speed regulating system of hydropower station.2.Characteristics of chaos behaviors for hydropower system based on nonlinear hydraulic-mechanical couplingFirstly,the nonlinear fourth-order state equation that describes the dynamic behavior of turbine regulating system under effect of nonlinear turbine characteristics is established.Then,the chaotic behaviors of turbine regulating system,i.e.existence of chaotic motion and path to chaotic motion,are studied.The effects of sensitivity parameters on the chaotic behavior of turbine regulating system are analyzed.Finally,the effect mechanism of nonlinear turbine characteristics on chaotic behavior of turbine regulating system is revealed.The results indicate that,with the change of governor parameters,the dynamic behavior of turbine regulating system transforms among damped oscillation,persistent oscillation,frequency division and chaos phenomenon.The chaotic motion only appears under the particular value ranges of governor parameters.The path to chaotic motion of turbine regulating system is period doubling bifurcation.The chaotic motion is more likely to appear when the stability of turbine regulating system is worse.The reasonable selection of sensitivity parameters can eliminate or weaken the chaotic motion.The chaos phenomenon is caused by the nonlinear speed characteristics,not the nonlinear head characteristics.The effect of nonlinear speed characteristics is related to the values of governor parameters.3.Sliding mode control of hydropower system based on nonlinear hydraulicmechanical couplingFirstly,the mathematical model of HTRS considering nonlinear turbine characteristics is established.Then,the sliding mode control strategy(SMCS)of HTRS is designed.The regulation quality of HTRS under SMCS is illuminated by comparing with that of HTRS without SMCS.The effect of gain matrix of SMCS on the dynamic performance of HTRS is revealed.Finally,the robustness of HTRS under SMCS is revealed.The results indicate that the proportional and integral sliding mode surface can satisfy the objective of sliding mode control of HTRS.The SMCS can effectively eliminate the chaos phenomenon and improve the regulation quality of HTRS considering nonlinear turbine characteristics.Under the effect of SMCS,the HTRS enters the stable state and arrives the target state rapidly.The pole assignment has a significant influence on the convergence rate and settling time of the dynamic response processes of state variables.The robustness of the HTRS under SMCS is much better that that of the HTRS without SMCS.After the introduction of SMCS,the anti-interference performance of HTRS considering nonlinear turbine characteristics is improved significantly.4.Stable sectional area of surge tank of hydropower system considering bifurcation and chaos characteristicsFirstly,the nonlinear sixth-order equation of speed regulating system of hydropower station is established.Then the stability of hydropower system is analyzed by Hopf bifurcation.Then the formulae for the critical section area(CSA)of surge tank are derived.The correctness of the formulae for CSA is verified by simulation.The effect on the CSA of differerent nonlinear characteristics,i.e.the nonlinearity of head loss of diversion tunnel,the nonlinearity of throttling orifice head loss,the nonlinearity of turbine,is revealed.Finally,the effect on chaos behavior of area of surge tank is revealed by using contrastive analysis.The results indicate that,under load decrease operation condition,both the nonlinearity of head loss of diversion tunnel and the nonlinearity of throttling orifice head loss can increase the value of CSA of surge tank and are unfavorable for the stability of hydropower station.The nonlinearity of turbine can decrease the value of CSA of surge tank and are favorable for the stability of hydropower station.The superposition of three nonlinear characteristics is unfavorable for the stability of hydropower station.On the contrary,the results are oppsite under load increase operation condition.Moreover,the chaotic motion is more likely to appear when the value of sectional area of surge tank is smaller.It should be noted that,the chaotic motion is generated by the superposition of low frequency and high frequency waves. |
