Research On Seamless On/off-grid Switching Control Strategy Of Microgrid Inverter Based On VS

In the process of in-depth exploration of the global energy crisis and environmental protection,the development and utilization of new energy has become more and more important in society.The concept of microgrid is proposed and applied to realize the absorption of large amount of distributed energy in power system.Virtual synchronous generator(VSG)technology has similar external characteristics and operation characteristics of synchronous generator,which makes up for the lack of inertia and damping in microgrid,and is widely used in microgrid.The microgrid inverter has two operating modes:grid-connected and off-grid.In order to avoid voltage shock and current shock when the inverter switches between operating modes,a seamless switching control strategy is used to achieve seamless switching between operating modes.Based on the microgrid VSG inverter,this paper studies the seamless switching technology of on-grid and off-grid operation modes.Firstly,this paper expounds the background and research status of microgrid and virtual synchronous generator,discusses the research significance of the subject,introduces the pre-synchronization technology,and studies the necessity of seamless switching technology in different operating modes of microgrid and the related research status..Secondly,the control strategy of micro-grid inverter is introduced.By comparing the characteristics of VSG and traditional synchronous generator,the damping and inertia characteristics of synchronous generator are simulated,and the VSG algorithm model with power frequency controller and virtual excitation controller as the control structure is constructed.The photovoltaic power generation system was set up as the DC source,and the converter was established to boost the output voltage of the photovoltaic system.The conductance increment algorithm was designed as the control principle to ensure that the output power of the DC source was at the maximum power point.Based on the analysis and modeling of the battery energy storage system,the bidirectional converter was established.According to the charged state of the energy storage unit and the operating state of the photovoltaic power generation system,the operating state was divided into fixed power state and tracking photovoltaic state,and the VSG power under different operating states was simulated and analyzed.The influence of two key parameters,virtual inertia and damping coefficient,on the output active power and frequency is discussed.In the oscillation process,the overshoot of the frequency and the adjusting time are taken as the key indexes to judge the frequency stability of the system.The effects of virtual inertia and damping coefficient in the control are simulated and analyzed.In addition,the off-grid switching process of the inverter is analyzed.In order to solve the voltage shock and current shock caused by the inverter in mode switching,the amplitude and phase information of the grid voltage is collected through the phaselocked loop.A pre-synchronous control technique based on frequency disturbance is proposed to adjust the voltage amplitude and phase of the micro-grid,so that the voltage of the micro-grid can meet the condition of grid-connected operation.VSG control and PQ control are used to share the current inner loop,and a numerical slow starter is set to realize the smooth transition of the reference current of the outer loop,to ensure the synchronization of control strategy switching time and switching action time,to ensure smooth switching between the two operating modes,and finally realize the seamless switching between the off-grid operating mode.Finally,on the software simulation platform,VSG’s stable operation in parallel/network mode and seamless transition between different operating modes are verified by simulation.The improved control strategy proposed in this paper is compared with the traditional method.