Research On Current Circulation Suppression Strategy For Two Parallel Photovoltaic Grid-Connected Inverters With Different Capacities

With the emergence of the energy crisis,the development and utilization of new energy have been concerned widely.Because solar energy is rich in resources and distributed widely,photovoltaic(PV)power generation has broad prospects for development.As part of the photovoltaic power generation system,the performance of grid-connected inverter affects the safety and reliability of the whole system directly.With the constant increase of the PV capacity,the single inverter can not meet the customer’s needs.It needs multiple inverters in parallel.In order to improve the quality of grid-connected current under the condition of weak illumination,a parallel circuit structure of multi inverters with the same capacity and a small capacity inverter is adopted.But the current circulation between the parallel inverters in operation will be produced.The main work of this paper is to study the suppression strategy of the zero sequence current circulation for two parallel inverters with different capacities.Firstly,the topology and working principle of T-type three level inverter are introduced.The mathematical model of T-type three level inverter in synchronous rotating coordinate system is built.The control method of active and reactive currents independent is used for grid-connected control.Secondly,two inverters are connected in parallel by master-slave control.Master inverter is used to track the maximum power point(MPPT)of the simulated PV source by using the method of constant voltage and disturbance observation both combination.The total active current command is obtained,and the total reactive current command is 0.A scheme for obtaining the steady-state current command in proportion and the dynamic current command in slope curve is given.The power loss of the inverter in soft start is analyzed.Finally,the equivalent model of zero sequence current circulation is built,and the loop of zero sequence current circulation is analyzed.On this basis,the zero sequence current circulation suppression strategy is studied deeply.That is current shaping is used to suppress zero sequence current circulation generated by steady-state operation and the dynamic process of two inverters MPPT in the process of PV output power curve variation.The problem of noise and transmission delay in current shaping is analyzed.A synchronous parallel strategy based on synchronous modulation is used to suppress the zero sequence current circulation generated by the input transient dynamic process.Deadbeat zero sequence current circulation controller is used to suppress zero sequence current circulation generated by two inverters tracking MPPT dynamic process in soft start.In order to verify the above control methods,the simulation model is built by PSIM software,and the experimental platform of two T-type three level parallel inverters with different capacities based on control core of STM32F407ZG is built.The feasibility and effectiveness of the control strategies are confirmed by simulation and experimental results.