Research On The Control Of Three-level Photovoltaic Inverter Under Unbalanced Grid Voltage Conditions

In recent years,renewable energy such as solar energy and wind energy has been widely developed.Solar energy is paid extensive attention in the world because of cleanliness,abundant resources and wide distribution.Solar photovoltaic(PV)generation is one of the most important ways to achieve efficient use of solar energy.In grid-connected photovoltaic generation system,the topology,modulation strategy and grid connected control scheme of grid-connected inverter are the key factors affect the PV grid-connected generation system.Grid voltage imbalance will cause grid current distortion,even affect the stable operation of the whole grid connected system in severe cases.Therefore,this paper mainly studies the control strategy of grid-connected inverter under the condition of unbalanced grid voltage.This paper focuses on three-phase three-level PV grid-connected inverter as the main research object.First,the inverter working principle is analyzed,and three different modulation strategies are studied in detail from the perspective of system leakage current suppression capability.And the mathematical models of the system are established according to the two conditions of the balanced grid voltage and unbalanced grid voltage,which lays a theoretical foundation for the system grid connected control.Then,the problem of grid synchronization in the case of unbalacned grid voltage is studied in deepth.First of all,the working principles of the traditional phase-locked loop(PLL)and notch method are introduced,and the existing problems under unbalanced grid voltage are analyzed.On this basis,a new digital synchronization method which can quickly detect the synchronization phase information of the grid voltage is proposed,and the new structure principle and parameter design are analyzed in detail.Afterwards,combined with the proposed grid synchronization method,the grid-connected control strategies for different grid-connected current control requirements are presented,and the simulation results are verified by MATLAB/Simulink.Finally,on the basis of theoretical analysis,the main circuit of the system and the digital experimental platform based on TMS320F28335 DSP and XC6SLX9-2TQG144 CFPGA are built.The grid-connected control experimental study under balanced grid voltage and unbalanced conditions are tested and analyzed,which proves the feasibility and effectiveness of the proposed scheme.