Control Strategy For Switching Between Grid Connection And Off Grid Modes Of Bidirectional Energy Storage Inverter

The bidirectional energy storage inverter in the micro-grid is the interface between the distributed generation unit and the main power grid.Therefore,the control of the micro-grid can actually be transformed into the control of the bidirectional energy storage inverter.The switching between grid connected and off grid modes of bi-directional energy storage inverter will cause impact effect,which will affect the power safety of important loads.Therefore,in the switching process,it is necessary to adopt the necessary control strategy for the bidirectional energy storage inverter to avoid the impact effect.This paper takes the three-phase three wire bidirectional energy storage inverter as the research object.In order to realize the seamless and smooth switching of the operation mode of the bidirectional energy storage inverter,this paper mainly does the following work.Firstly,the bi-directional energy storage inverter in micro-grid is modeled and analyzed,and then the basic principle of droop control when the line impedance is inductive is analyzed in detail,and its power measurement module,droop control module and voltage and current double loop control module are designed.Then,the control mode and parameter requirements to be adopted when switching between grid connected and off grid are explained.The traditional island detection method has a long detection cycle in the process of switching from grid connection to off grid.An active frequency offset island detection method with positive feedback is proposed to improve the efficiency of island detection and reduce the total harmonic distortion rate of inverter output current.By introducing positive feedback coefficient and piecewise function to optimize the chopper coefficient,the island detection efficiency is improved,and the frequency disturbance direction can be adjusted according to the load impedance.When the bi-directional energy storage inverter is switched from off grid mode to grid mode,aiming at the problem of low accuracy of traditional pre-synchronization control before grid connection,a composite pre-synchronization control strategy based on droop control is proposed,which includes three sub modules: phase-locked loop module,phase and angular frequency synchronization tracking module and amplitude synchronization tracking module.The synchronous tracking principle of the output voltage phase,angular frequency and amplitude of the bidirectional energy storage inverter in the pre-synchronization control is analyzed.At the same time,the flow chart of the pre-synchronization control is given to realize the accurate tracking and synchronization of the output voltage of the bidirectional energy storage inverter to the grid voltage.Finally,by building a model on the simulation software,the micro-grid system is simulated and verified by using the active frequency offset island detection method with positive feedback and the composite pre-synchronization control strategy.The results show that the smooth switching strategy proposed in this paper has better switching efficiency and stability,and its performance is significantly improved compared with the traditional control strategy.