Control Strategy Of Microgrid Inverter Based On Virtual Synchronous Generator

With the continuous improvement of distributed energy penetration,in order to give full play to the advantages of distributed energy,the concept of microgrid came into being.However,the inverters in the microgrid generally have low inertia and underdamping,which has a negative impact on the large-scale grid connection of distributed energy.Therefore,virtual synchronous generator(VSG)technology is introduced into the control of microgrid inverter to effectively improve the acceptance ability of traditional power grid to distributed generation.Based on the above background,this thesis systematically studies the control strategy of microgrid inverter based on VSG.Through the simulation of the external characteristics of the synchronous generator,the structural control block diagram of the VSG technology is completely designed,and the appropriate control strategy is selected to realize the operation control of the wind power system,the photovoltaic system and the energy storage system respectively.The establishment of the wind-solar-storage complementary microgrid system based on VSG is completed,and the coordinated control of the microgrid wind-solar-storage is realized in the island and grid-connected modes respectively.This thesis focuses on the operation optimization control of microgrid inverters before and after grid connection.Firstly,the seamless switching of microgrid from island mode to grid-connected mode is realized by pre-synchronization control technology based on virtual power,which avoids large impact current during grid connection.In grid-connected mode,unbalanced grid voltage will adversely affect the output of VSG inverter.In this thesis,a positive and negative sequence separation strategy based on improved second-order generalized integrator(SOGI)is proposed,which effectively reduces the influence of DC component in SOGI input signal on system stability.At the same time,the current inner loop module of VSG is re-improved to control the negative sequence component of the inverter output current to zero,so as to achieve the control target of current balance under unbalanced grid voltage.In addition,the limited capacity of a single VSG is difficult to adapt to today ’s load demand,and it is necessary to operate multiple VSG inverters in parallel.The output power of multi-VSG parallel system is coupled due to the mismatch of line impedance,and the output power cannot be allocated according to the capacity ratio.After deriving the setting principles of the key parameters of the parallel system,this thesis uses SOGI to design an adaptive virtual impedance.By adding an integral link,the virtual impedance can be dynamically adjusted according to the changes of the line conditions.Finally,the virtual impedance is introduced into the VSG double closed-loop control link to achieve the control goal of the output power of the multi-machine parallel system allocated according to the capacity ratio.