Comparative Study On Damping Characteristics Of Synchronous Generators Under System Disturbance

Damping torque coefficient is an important characteristic index in the field of transient stability research.It can characterize the ability of power system to maintain stability under large disturbances with complex nonlinear effects.The time-step finite element method is based on the actual structure and magnetic field of the generator to establish a model,which can fully consider the influence of nonlinear factors and accurately account for the changes of the generator operating state and parameters during the dynamic process.The polynomial approximation method uses a quantitative method to determine the damping torque coefficient of the detailed model,and can calculate the time-varying damping torque coefficient.This thesis combines the time-step finite element method and polynomial approximation method to simulate the damping torque coefficient under large disturbances such as fast closing valve,three-phase sudden short circuit and generator demagnetization,and obtained quantitative description of the damping characteristics of different rotor damping structures.The main study contents of this thesis are as follows:(1)Established the field-circuit coupled time-step finite element model,and used the polynomial approximation method to calculate the damping torque coefficient of the three-part damping structure includes rotor slot wedge,iron core and damping bars of synchronous generator under different disturbances.The results show that the damping torque coefficient of synchronous generator is variable during large disturbance;the damping torque coefficient is the largest when the rotor slot wedge acts alone,while the minimum is appeared when the damping bars acts alone;in addition to the case of the slot wedge acts alone,under three-phase sudden short circuit,the damping torque coefficient is the largest when iron core and slot wedge act togetheror,while under two other disturbances,the largest coefficient appears when damping bars and slot wedge act together;the damping torque coefficient is small when three damping structures all work.Based on the above conclusions,this thesis proposed a rotor damping structure that can improve the damping effect of synchronous generators.(2)Compared the calculation results of the polynomial approximation method with the traditional method.The damping torque coefficient calculated by the traditional method is an equivalent constant,since the value of the damping factor is limited,it cannot clearly reflect the difference of the damping effect when different rotor structures act;while the damping torque coefficient obtained by polynomial approximation method has time-varying characteristics with the change of disturbance,which can accurately reflect the variation of generator damping strength in dynamic process.(3)Conducted on rotor damping characteristics when the duration of the fast-closing valve disturbance is different,and compared the damping torque coefficients calculated by the two methods.The results show that the longer the duration of the fast-closing valve is,the larger the damping torque coefficient is,and the more effective the effect on the low-frequency oscillation is;the damping torque coefficient under the traditional method is constant and does not change significantly with the disturbance duration,so it cannot accurately reflect the variation of the damping torque coefficient under different conditions.Therefore,studying the changed damping torque coefficient under large disturbance has important guiding significance for the optimal design of the synchronous generator rotor damping structures.