Grid-connected Control Of Synchronverters Under Unbalanced Grid

With the increasing penetration of renewable energy,most distributed renewable energy needs DC/AC converter(inverter)to supply power to the power grid,to ensure good dynamic response and stability of the grid voltage balance condition.The synchronverter is used to simulate the original virtual inertia of the synchronous generator,which can realize the reliable connection of the distributed energy sources and ensure the power supply system and the stability.In this paper,the problem of unbalanced three-phase current in the grid connected inverter is studied,which is caused by unbalanced grid voltage,to ensure that the inverter output power while reducing the negative sequence current.The topic of this thesis is of great theoretical significance and practical value.The main works of the thesis are listed as follows:(1)The development of virtual synchronous machines in distributed energy field and the research status of synchronous inverter control are summarized.The synchronization principle and process of synchronverter and the adjustment of power after grid connection are analyzed in detail.Then,based on Simulink platform,according to the basic parameters of the synchronous inverter,the feasibility of the grid connected condition is verified.Put forward the research focus of this paper:Grid-connected Control of Synchronverters under Unbalanced Grid.(2)Control improvement based on Second-Order Generalized Integrator-Phase Locking Loop for extracting component.In order to solve the problem of frequency fluctuation in component extraction control,an improved DSOGI-PLL scheme is adopted to extract the balanced component of unbalanced voltage.Also based on the Simulink tools,the simulation is carried out under the unbalanced voltage amplitude,phase and voltage amplitude / phase,and the feasibility of separating the positive and negative sequence is determined.(3)A positive / negative sequence controller for synchronverters is designed.Because the traditional synchronous inverter mathematical model has many deficiencies in the unbalanced power grid,the positive and negative sequence control algorithm is decoupled,and its independence and adaptability are better.When the unbalance happened in power grid,based on without changing the normal operating mechanism of inverter,it is expounded that the transmission power synchronizationprocess,combined control of inverter in the positive and negative controller under the strategy of system and suppress the negative sequence current.And in the combined action of the two controllers,ensure the inverter grid friendly first,then use the positive sequence and negative sequence controller to keep the inverter to the power transmission power and negative sequence current of system suppression.Based on the Simulink simulation platform,the simulation experiments are carried out under different conditions of unbalanced conditions.(4)Experimental platform is construction and experimental verification of150 W prototype.In order to complete the grid connection under unbalanced grid conditions,the proposed control algorithm is verified by MATLAB.At the same time,the DSP is used to control the inverter,and the 150 W system of the grid connected is designed,the control strategy proposed in this paper has been verified by experiments using this prototype.