Grid-connected Control Of Synchronverters Under The Grid Voltage Fluctuation

With the dual pressures of energy depletion and the frequent occurrence of traditional large-scale power grid accidents,the disadvantages of centralized power supply in large-scale power grid gradually appears and it is difficult to meet the users'increasing demands for safety and reliability of the power grid.Distributed generation systems came into being,and the synchronverter is one of many distributed generation methods.Because the synchronverter simulates the working principle of the synchronous generator,it has the virtual rotational inertia,which can further realize friendly grid connection and ensure the stability of the power generation system.But when the grid voltage is disturbed and fluctuates,the output power of the synchronverter produces large oscillations and generate negative sequence current,which will seriously affects the stability of the system.Therefore,how to maintain the relative stability of the output power and how to eliminate the negative sequence current under grid voltage fluctuation are still the problems that need to be solved for the synchronverter.In response to the above issues,the specific work of this paper is as follows:(1)The development trend,control strategies and problems that still exist after grid connection of the synchronverter in recent years are analyzed.The basic principle and mathematical derivation of the synchronverter that imitates synchronous generator are made.And the feasibility of the synchronverter's grid-connected operation under different grid parameters is verified by simulation.Aiming at the problem that the synchronverter cannot guarantee the stability of output power and negative sequence current exists under the grid voltage fluctuation,two control strategies are proposed to solve this problem.(2)Grid-connected operation scheme of the synchronverter based on ADC A control strategy.The ADCA(Adaptive Drooping Coefficient Adjustment,ADCA)control strategy is to adjust the active and reactive power droop coefficient autonomously according to the interference fluctuations of the grid voltage of the microgrid,so that the output active power and reactive power of grid-connected synchronverter remain relatively stable when the microgrid voltage is disturbed and fluctuated,and the simulation experiment is used to verify its feasibility and effectiveness.It plays an important role in improving the application of distributed energy in practical projects.(3)Improved negative sequence current optimization scheme under grid side voltage fluctuations.Because the mathematical model of the synchronverter has a negative sequence current under the grid voltage disturbance,and the existence of the negative sequence current is harmful to the whole system,the positive and negative sequences can be decoupled,and then the current loop instruction is designed by taking advantage of the characteristics of the synchronverter in dq coordinate system to realize the suppression of the negative sequence current under unbalanced voltage,which has a significant effect on maintaining the stability of the output active power and reactive power.Finally,using Simulink simulation to verify its feasibility and effectiveness.(4)Simulation GUI design and Experimental platform construction.In order to facilitate the adjustment of the simulation parameters in the next step,a simulation software is designed using the GUI,which makes the design and adjustment of parameters more convenient.Finally,in order to further verify the feasibility and effectiveness of the proposed control strategy,after using Matlab software for simulation verification,the fifth chapter builds a hardware experiment platform,controls the physical inverter with DSP-F28335,and designs a 150W low-power grid-connected system of the synchronverter that is used to verify the above proposed scheme.