Research On Adaptive Control Strategy Of Distributed Power Inverter Based On Virtual Synchronous Machine

Wind power,photovoltaic and other non-water renewable energy distributed generation systems are generally connected to the grid through power electronic converters.The output power of the converter based on vector control is decoupled from the system frequency,which does not have the same inertia and damping characteristics as the synchronous generator.Therefore,large-scale grid connected distributed generation will lead to the lack of inertia of the grid system.The inverter control technology based on virtual synchronous generator(VSG)can make the inverter have the same inertia and damping characteristics as the synchronous machine,which is an effective way to solve the above problems.In order to further improve the performance of VSG control,the traditional JD adaptive control strategy adjusts the moment of inertia J and damping coefficient D of VSG according to the system frequency change rate under different working conditions,so as to achieve the purpose of optimizing the dynamic performance.However,this method does not consider the influence of damping coefficient D on the output power of the inverter when the system frequency is lower than the rated frequency;because the change of system damping ratio ζ is not considered,the problem of insufficient stability margin may occur.Based on this,an improved JD adaptive control strategy considering the system damping ratio ζ constraint is proposed in this paper.The adaptive equation of moment of inertia J is established according to the frequency change rate of the system.When the moment of inertia J is determined,the system damping ratio ζ is used to constrain the damping coefficient D,which limits the VSG control to the state of under damping,thus ensuring the stability of VSG in the operation process.Considering the influence of damping parameter D on the output power of inverter when the system frequency is lower than the rated frequency,this paper selects different damping ratio ζ according to the system frequency offset,and adjusts the damping coefficient D through ζ.The inverter can output more active power when the system frequency is lower than the rated frequency,and reduce the output active power when the system frequency is higher than the rated frequency.A VSG pre synchronization control strategy is proposed to solve the problem that the grid connected inverter and the grid voltage are not synchronized,which brings the impact current at the moment of grid connection.By adding an integral link in the power loop of VSG,and changing the VSG frequency reference value and voltage reference value into the frequency value and amplitude value of the grid voltage respectively,the PI controller can realize the characteristics of no difference regulation,and control the output voltage of the inverter to make it consistent with the frequency,amplitude and phase angle of the grid voltage.Finally,the simulation model of the fixed parameter,traditional adaptive control and the control strategy proposed in this paper is built on the MATLAB / Simulink software platform.Through the simulation comparison,the superiority of the adaptive control strategy proposed in this paper is verified.At the same time,the VSG pre synchronization control model is built,and the effectiveness of the VSG pre synchronization control strategy proposed in this paper is verified by comparing with the grid connection method without pre synchronization control.