Research On Power Allocation Strategy Of Cascaded Multilevel Grid-connected Pv Inverter

Recently, multilevel inverter has been rapid developed because of reducingthe harmonic and improving the characteristics of voltage and power level.Among them, the cascade multilevel structures are most widely used. In thisdissertation, multi-level cascaded photovoltaic grid inverters are deeplyresearched.Firstly, this thesis describes the cascaded H-bridge topology, analyze theharmonic distortion of multilevel structure, and compare the output voltageharmonic distortion of different voltage level, describe the influencing factors ofvoltage harmonic distortion. Secondly, introduce carrier phase shift PWMmodulation method.Based on cascaded multilevel inverter characteristics, this thesis uses animproved dual-loop control, the inner current, outer voltage control loopcombined with MPPT tracking control method. Implement grid connected,dynamic performance and steady-state performance is good. For the DC-linkvoltage drift problem on modulation stage, using the voltage feed-forwardsystem to change the voltage reference value, to ensure the output voltage of theDC-link.Based on cascaded multilevel inverter, there are two methods of the MPPTcontrol, namely independent MPPT control of multiple photovoltaic arrays andmulti-channel photovoltaic arrays share one MPPT control method. Simulationanalysis and experimental verification are presented. Compare the advantagesand disadvantages of both methods, analyze the difference of the two methods,carry on quantitative analysis and obtain the maximum power point trackingefficiency of two control methods.Finally, aim at the cascaded multilevel three-phase grid-connected inverter,analyze the existing power allocation methods. Based on this method, putforward a new power re-allocation strategy, by injecting zero sequence into theoutput reference vector, so that the three-phase inverter output phase voltages areasymmetrical, to ensure the three-phase inverter output line voltages symmetrical,and the output currents are asymmetrical, meet the grid-connected currentrequirements. Simulation and experimental results verify the correctness of thetheory.