Grid Supporting Strategy And Stability Analysis Of Grid-connected Converters In Distributed Power Generation System

The large-scale integration of renewable power generation has increased the penetration of power electronic equipments in the power system.Power electronic converters interfaced with distributed generation will gradually replace the synchronous generators and dominate in modern power system.As the most widely used grid-connected interface,the research of voltage source converter promotes the rapid development of distributed power generation system.Among them,the control performance of voltage source converters plays a vital role for the stable operation of the modern power system.Therefore,taking the grid-following control and grid-forming control modes of voltage source converter as the research objects,this dissertation focuses on the current loop control of grid-following converters,the voltage loop control of grid-forming converters,the active power loop control of grid-forming converters and the synchronization stability of grid-forming converters under large disturbance.The main research contents are as follows:1.In the aspect of current loop control of the grid-following converters,an effective decoupling method with ROGI-based decoupled current controller(D-PCI)is introduced to suppress the current coupling between the α-axis andβ-axis in ROGI-based current loop.This method overcomes the degradation of decoupling performance caused by filter parameters and digital control delay in PRX2-based decoupling method,which effectively suppresses the current coupling between α-axis and β-axis,and improves the transient response performance of current loop.Then,D-PCI controller is extended to arbitrary harmonics for tracking the components of unbalanced and harmonics,and the time delay compensation method with the optimal phase angle is discussed,which enhances the stability of current loop.2.For the voltage loop control of grid-forming converters,the converters will lose stability if the resonance frequency of the LC filter fr is designed near fs/6,an active damping control with lead compensator is thus analyzed.This method eliminates the stability boundary at fs/6 and the nearby stability blind-region for the LC filter,which exists in the traditional dual-loop control and active damping control method.And the stability region of the LC filter fr is thus extended from(0,fs/6)to(0,fs/4).Besides,a discrete domain resonant voltage controller applied to grid-forming converters is introduced to avoid the deterioration of control performance caused by the discretization deviation in the digital implementation of traditional s-domain resonant controllers,the design method of discrete domain resonant controller in stationary reference frame is introduced,which improves the dynamic response performance of voltage loop and assures that the output voltage has a fast transient response with small overshoot.3.In term of the active power loop(APL)of grid-forming converters,APL will become a second-order or higher-order system due to the introduction of virtual inertia,an improved VSG control strategy based on power deviation feedforward is proposed to suppress the excessive power oscillation and overshoot in grid-connected(GC)mode.The power deviation feedforward path improves the response speed and effectively suppresses the oscillation and overshoot of grid connected active power.Furthermore,considering the small inertia and damping in the stand-alone(SA)mode,an improved strategy is proposed to disable the power deviation feedforward path,which changes the active power loop control into VSG control based on transient damping,and enhances the inertia and damping characteristics.Therefore,by enabling/disabling the power deviation feedforward path in the GC/SA mode,the proposed control strategy can provide virtual inertia and damping characteristics in SA mode and suppress the active power oscillation and overshoot in GC mode.4.Aiming at the VSG control strategy based on power deviation feedforward,the synchronization stability of grid-forming converters under large voltage drop disturbance is further studied.Compared with traditional VSG control and VSG control based on transient damping,it can be found that the power deviation feedforward path can effectively improve the synchronization stability of GFM converter,and the larger the feedforward coefficient,the stronger the synchronization stability.