Studies On Synchronization Stability Of Grid-connected System With Grid-following Converter

"Double high" power system,namely high proportion of renewable energy and high proportion of power electronic equipment,has become an important trend in the development of power system.Grid-following converter(GFL)is an important power electronic equipment for "double-high" power system.In the case of a weak grid,the voltage of the grid-connected point of power electronic equipment is sensitive and variable,which greatly increases the probability of GFL synchronization instability and renewable energy off-grid.It is urgent to deeply analyze the synchronization stability and synchronization stability control method of the GFL grid-connected system to realize the safe and stable operation of the new-type power system.In this thesis,the synchronization instability mechanism,quantitative evaluation of transient synchronization stability and the synchronization stability control of GFL have been studied,which provides theoretical support for the synchronization stability of GFL grid-connected system.The main research work and achievements are as follows:(1)The mechanism of GFL synchronization instability is analyzed.The mathematical model of the GFL integrated into the infinite bus system is established,which includes the AC circuit and control system of GFL.Using the dynamic analysis of phase locked loop(PLL),it is found that there are two types of instability of GFL,namely static synchronization instability and transient synchronization instability.The two types of instability mechanism are analyzed respectively,and the key parameters and dominant factors leading to synchronization instability of GFL grid-connected systems are revealed.(2)A method for quantitative evaluation of transient synchronization stability of GFL grid-connected systems based on impedance identification is proposed.A method for identifying the Thevenin equivalent impedance of the GFL grid-connected system is studied.This method can quickly and accurately identify the Thevenin equivalent impedance of the system after a short-circuit fault occurs,and can be used in the evaluation method of GFL transient synchronization stability.Lyapunov function is constructed,and the transient synchronization stability of GFL grid-connected system is quantitatively evaluated by direct method,and the stability region of the system is estimated.(3)The synchronization stability control method of GFL is studied.The self-adaptive current control method is improved.This method adjusts the GFL active and reactive current command values according to the Thevenin equivalent impedance from the fault point to the grid-connected point of GFL after faults to solve the problem of static synchronization instability.The transient instability problem is solved by integrator limiter and variable variable structure PLL.The integrator limiter method can theoretically ensure that the transient synchronization instability is effectively stabilized.To a certain extent,the variable structure PLL control method solves the static error problems of PLL freezing method.