The Analysis Of Time-frequency Domain Characteristic Information Of Hydraulic Turbine Governing System Considering Tail Water Excitation

In order to adapt to energy reform and development,hydropower units are often operating under design conditions,and operating conditions are frequently changed.The hydraulic excitation in the unit,especially the tail water excitation,fluctuates sharply,threatening the safe and stable operation of the hydropower unit.At present,there are abundant researches on the time-domain and frequency-domain analysis of tailwater excitation,but the research on the time-varying amplitude-frequency characteristics of tailwater excitation in both time and frequency domains is not sufficient from the perspective of time-frequency domain.Therefore,it is very important for in-depth study of the stable operation of hydropower units from the energy to establish a more accurate nonlinear mathematical model of the turbine governing system considering the influence of tail water excitation,and to explore the time-frequency domain characteristic information of tail water excitation under design conditions.In summary,the main research content and conclusions of this thesis can be summarized into the following three aspects:(1)Refined non-linear mathematical model of hydro-turbine regulating system.In this paper,the characteristic line method,equivalent circuit method and thin plate spline interpolation method are used to establish the nonlinear mathematical model of the hydraulic turbine governing system with draft tube,and the problem of parameter transmission at the turbine-draft tube interface is solved by the spatial discretization method.With reference to the experimental data provided by the Francis-99 seminar of NTNU-Norwegian University of Science and Technology,this paper uses the MATLAB numerical simulation platform to simulate the steady-state and transient condition respectively,and verify the model from the perspective of the time domain.Besides,the relative error of the numerical simulation of various hydraulic excitation information such as turbine head,flow and tail water excitation in transient condition is basically within 5%.This work provides a reliable theoretical model basis for exploring the operating stability of hydropower units under design conditions.(2)Realize accurate extraction of tailwater excitation characteristic information from the perspective of time-frequency domain.In this paper,genetic algorithm and sample entropy are used to improve the variational modal decomposition method to avoid inaccurate decomposition caused by preset parameter errors.Set the tail water excitation mathematical simulation signal,and carry out method demonstration and data analysis on the improved algorithm.The results show that compared with the conventional method(empirical mode decomposition method)and the original algorithm(variational mode decomposition method),the improved variational mode decomposition algorithm is more accurate in extracting the characteristics of the mathematical simulation information of tailwater excitation.The relative error of the value is within 4.55%,and the relative error of the frequency is within 4.3%.This work provides technical support for studying the time-frequency domain characteristic information of tail water excitation.(3)Quantitatively analyze the frequency characteristics and energy characteristics of tail water excitation from the perspective of time-frequency domains.In this paper,the improved variational modal decomposition algorithm is used to conduct time-frequency domain analysis on the measured tailwater excitation data under steady-state and transient conditions.By analyzing the information components and sources of each frequency band,the numerical simulation frequency range of the model tailwater excitation in this paper is explained(low Frequency band).Furthermore,numerical simulation is used to comprehensively explore the influence of guide vane closing time on tailwater excitation from the perspective of time domain and time-frequency domain,and determine a reasonable guide vane closing time.The results show that the measured tail water excitation mainly includes high-frequency signal(30.27fn),low-frequency signal((0.17～0.50)fn),standing wave signal(2.87 fn,7.60fn)and system excitation source.Among them,the model in this paper can simulate the low-frequency tail water excitation caused by the tail water vortex zone;(2)The longer the guide vane is closed,the more stable the tail water excitation development process,and the smaller the vacuum degree of the draft tube inlet;(3)Excessive extension of the closing time of the guide vane will have little effect on the weakening of the tail water excitation;(4)For this simulation condition,the closing time of the guide vane should be selected at 2.3s～3.3s.At this time,the vacuum degree of the draft tube inlet is small,and the tail water is excited.Frequency and energy fluctuations are relatively stable.The relevant research results can provide theoretical guidance for ensuring the stable operation of the unit.