Mechanism And Characteristic Analysis Of Switching-type Oscillation In Hydro/Thermal Power Generation System

The large number of new energy sources and the application of DC have made the structure of China’s power system more complex,and the probability of some power grids being hit by large disturbances has increased significantly,thus increasing the risk of complex oscillations in traditional hydropower and thermal power units.In recent years,a number of oscillations involving switching links in power generation systems under large disturbances(referred to as "switching-type oscillations" in this paper)have occurred in some power grids.The mechanisms,analysis methods and related characteristics of which are different from those of conventional oscillations under slight disturbances.For hydropower systems,some studies have been conducted,but there is a lack of in-depth and comprehensive analysis;for thermal systems,similar oscillation risks are still rarely reported.In view of this,this paper investigates the mechanism and characteristics of switching-type oscillations in conventional hydro and thermal power systems under disturbances.The main works are as follows:(1)The influence of system parameters on the large-scale dynamical characteristics of hydropower second-order switching systems is analysed in conjunction with the phase diagram method to reveal the dynamical mechanisms of different types of frequency oscillations occurring in the system under different disturbances and damping.Firstly,the equilibrium point characteristics of the hydroelectric second-order switching system are analysed;secondly,the large-scale dynamics of the system under different operating conditions are carved out in conjunction with the phase diagram method,and the existence of the relevant limitcycles is demonstrated;finally,the non-smooth bifurcation characteristics of the system with the variation of load parameters and water hammer effect time constants are analysed.The results show that there is one stable limit-cycle when the equilibrium point is positively damped and the regulation region of the system is negatively damped,which corresponds to frequency oscillations under severe large disturbances;there are two stable limit-cycles when the equilibrium point is negatively damped and the system is limited,which correspond to two types of frequency oscillations with close frequency oscillations under general large disturbances and severe large disturbances respectively.(2)A simplified thermal power non-smooth system model with speed regulation deadband and limit is constructed,its equilibrium point characteristics are analysed,and an approximate analysis method for switching-type frequency oscillations of the simplified thermal power system based on the description function method is proposed.Firstly,a model of thermal power non-smooth system with speed regulation deadband and limit is constructed;secondly,the equilibrium point and damping characteristics of the simplified thermal system are analysed when the conventional parameters of the system are varied;finally,the approximate analysis of oscillations is realised by combining the descriptive function method.The results show that there is a risk of switching-type frequency oscillation with positive damping of the single-unit thermal power system with a speed regulation dead zone and limit participation.(3)A detailed thermal power non-smooth system model is established,and the influence of the excitation system on the damping characteristics of the thermal power system is analysed in combination with the state space method,and the dynamical mechanism of switching-type oscillations occurring in this system under different damping is revealed.Firstly,a thermal power non-smooth system with speed regulation and excitation system is established;secondly,the influence of the excitation system on the damping characteristics of the thermal power system is analysed;furthermore,the risk of switching-type oscillation of the system under different damping is analysed;finally,the dynamics of the system under different disturbances and damping and the non-smooth bifurcation characteristics are summarised.The results show that when the equilibrium point is positively damped or negatively damped,the thermal power system has the risk of switching-type frequency oscillation with the participation of the speed regulation dead zone;when the equilibrium point is negatively damped,there is also the risk of switching-type oscillation with the joint participation of the speed regulation and excitation switching links.