Research On Current-sharing Control Strategy For Three Parallel T-type Three-level Grid-connected Photovoltaic Inverters

With the depletion of traditional fossil fuels,it is imperative to improve the energy structure and develop new energy sources energetically.As an important interface device in photovoltaic power generation system,grid-connected inverters have been developed rapidly.With the increasing capacity of PV installed,the multiple parallel-connected system tends to be adopted to increase the capability,it can improve the reliability and flexibility.When multiple parallel-connected inverters work together,circulating currents will be generated among parallel-connected inverters due to the inconsistency of switching operation and system impedance.The circulating current suppression and current-sharing control strategy of three parallel-connected inverters are studied mainly in this paper.Two of them have the same capacity and one has different capacity to accommodate all kinds of parallel-connected situations.Firstly,a transient mathematical model of single-phase equivalent circuit for the T-type three-level inverter is established in accordance with its working principle.The grid-connected current control based on a division-summation(D-Σ)digital control is mainly studied,which is compared with deadbeat control under two-phase stationary coordinates.Secondly,the circulating currents of two,three and more parallel-connected inverters of the same capacity or different capacities are defined according to the circulation loops of parallel-connected T-type three-level inverters.At the same time,the equivalent mathematical model of multiple parallel-connected T-type three-level inverters is established,the mechanism of the circulating currents is revealed and the ignored problem of controlling zero-sequence circulating currents in a switching period is analyzed.Finally,the current-sharing controller of virtual impedance+voltage compensation is designed to suppress the circulating currents and achieve the sharing currents.The simulation model by PSIM and the experimental platform of three parallel-connected T-type three-level inverters based on STM32F407ZG are built respectively to verify the grid-connected current control of a single T-type three-level inverter and the current-sharing control of two and three parallel-connected inverters under different conditions.The correctness and superiority of the control strategies are confirmed by simulation and experimental results.