Impedance Modeling Of Virtual Synchronous Generator And Adaptive Optimization Of Weak Grid

The ’double carbon ’ target brings about an increase in the proportion of distributed power sources in the power grid.With the extensive application of power electronic equipment,there is a lack of inertia and insufficient damping in modern power grids.Because the regulation effect of distributed power supply on frequency and voltage cannot be comparable to the performance of traditional synchronous generator,the virtual synchronous generator(VSG)technology is proposed by introducing the rotor mechanical motion equation of synchronous generator in the control process to improve the operation characteristics of distributed power supply and help to improve the acceptance ability of power grid to new energy generation.Although VSG technology can provide inertial support capability for the system,it also increases the difficulty of control,and there is a problem of grid-connected system stability.At the same time,in order to better play the role of virtual inertia,it is also necessary to optimize the control parameters to improve its adaptability to the power grid and improve the transient process.The virtual synchronous generator control includes active and reactive power control and voltage and current double closed-loop control.When analyzing the stability and transient performance of the grid-connected system,most of them start from the establishment of transfer function or spatial state equation,and analyze the stability through the root locus.Based on this,the control system parameters are designed.This has insufficient consideration of the adaptability of different power grids when selecting parameters,especially when optimizing transient performance.Aiming at the adaptability of VSG control to different grid strength,based on the modeling of VSG output impedance and the stability analysis of grid-connected system,this paper proposes a parameter design method based on the phase angle margin of VSG output impedance and grid impedance.Firstly,the mathematical model of positive and negative sequence output impedance of VSG considering power loop is established by harmonic linearization method.On this basis,the positive and negative sequence output impedance model of voltage and current double closed loop is considered.The influence of inertia,damping and proportional resonance coefficient of voltage and current loop on the frequency characteristics of positive and negative sequence output impedance is analyzed by Bode diagram.Secondly,the Nyquist curve of VSG and grid impedance ratio is used to further analyze the interaction stability between VSG and grid,and the range of VSG control parameters satisfying the stability of grid-connected system is obtained.Based on the above analysis,an optimal design method of VSG control parameters based on the impedance ratio phase margin of the interactive system is proposed.This method improves the adaptability of the power grid strength,and can improve the transient performance of the grid-connected system while taking into account the VSG primary frequency modulation characteristics.The design method and steps are given,and the rationality of the parameter design is verified by MATLAB/Simulink model simulation.At the same time,in order to compare with the existing parameter design method based on the transfer function of the control loop,the parameter range considering the influence of the power grid is further analyzed based on the transfer function,and different control parameters are selected according to the control performance requirements.Finally,in order to optimize the transient process of virtual synchronous generator,the influence of virtual inertia and damping coefficient on transient performance is analyzed,and the adaptive control strategy of virtual inertia and damping coefficient is studied.On the basis of adaptive control,through MATLAB/Simulink model simulation and RT-LAB real-time simulation system,the transient effects of the parameter design proposed in this paper and the parameter range determined by the transfer function parameter design method are compared and analyzed.The optimization effect of the proposed method on the grid-connected transient performance under different grid strength is verified.