The Control Strategy Research Of Cascaded Multilevel Inverter For PV Grid-Connected System

At present,the grid-connected photovoltaic(PV)system always includes DC-DC and DC-AC two-stage power conversion,but the increase in power conversion stages will reduce the efficiency,reliability and simplicity of the overall system,thus designing the new single-stage PV grid-connected inverter to replace the traditional two-stage system is an important research trends of the grid-connected PV topology.Therefore,a single-stage grid-connected PV system is established based on the H-bridge cascaded multi-level inverter in this paper.the strategy of synchronous control method of multiple maximum power point tracking(MPPT)modules is proposed to improve the PV system efficiency,and a H-bridge transmission power match strategy is also proposed.Firstly,the equivalent circuit model and mathematical models of photovoltaic cell under different conditions are established.It was analyzed and simulated that the output characteristics of series and parallel PV array on the non-uniform insolation and uneven temperature distribution conditions.Secondly,as each H-bridge of cascaded multi-level inverter is powered by different photovoltaic arrays,maximum power points(MPP)of each H-bridge unit would be different,so a strategy of using multiple MPPT modules to achieve synchronous control for each H-bridge module is proposed to make the individual MPPT control of each H-bridge module possible.and a new voltage disturbance observation method is used to avoid DC-link voltage collapse phenomenon,which is always caused by step change of insolation.Thirdly,the distribution proportion under different SPWM modulation of the H-bridge module transmission power is analyzed.the amplitude of modulating wave of each H-bridge module under the strategy of multi-carrier phase shift sinusoidal pulse-width(CPS-SPWM)modulation is improved based on voltage errors of the process of H-bridge MPPT control,so the H-bridge module transmission power andmaximum output power of photovoltaic cell can be matched.And the inverter output harmoni of CPS-SPWM modulation is analyzed according to bilateral Fourier transform theory.Finally,a simulation system model is established based on Matlab/Simulink platform,and the strategy of MPPT control and H-bridge transmission power matching are tested and experimented.Experimental results under different test conditions are presented to validate the proposed ideas.