The Optimization Research On Modulation Strategy For Cascaded H Bridge Photovoltaic Grid-Connected Inverter

H-Bridge Cascaded Multilevel Inverters Because of their modular design,the DC side of each module can be connected to a photovoltaic panel and achieve independent MPPT control.Therefore,as a new generation of PV grid-connected inverter has become the current research hot,compared with the traditional diode-clamped inverters,cascaded H-bridge inverter has a small output voltage harmonic content,the output filter volume Small,easy to modularized and many other advantages,so cascaded H-bridge inverter has a good application prospect in photovoltaic grid-connected inverter.When the H-bridge cascaded inverters are used in photovoltaic power generation,when the DC-side photovoltaic panels of each module operate at the same MPP,the traditional carrier phase shift modulation strategy can make the equivalent output voltage of the inverter switch Frequency doubling,thus eliminating low-frequency sideband harmonics.However,when the HVDC module of each H-bridge module due to environmental factors,such as aging caused by the imbalance of the module power,the frequency doubling effect of this modulation method will fail.In the meantime,in order to save the cost,the isolation transformer on the DC side of the H-bridge cascaded inverter is canceled,which leads to the leakage current generated by the connection of the photovoltaic panel to the parasitic capacitor and the power grid,thus endangering the system and life safety.The traditional H4 bridge cascade Type topology.When the power is completely balanced,the leakage current modulation method has better suppression effect,However,when the DC power is unbalanced,the suppression effect will be reduced or even invalidated.This paper focuses on the above two issues do the following research:(1)Harmonic expression of output voltage in phase-shifted H-bridge inverter with carrier phase-shifting is obtained through simplified harmonic analysis method,and harmonic voltage characteristics of output voltage under the condition of unbalanced power of photovoltaic module are analyzed Impact.In order to eliminate this kind of influence,an improved carrier phase shift modulation strategy(MPSPWM)is proposed in this paper,which can reduce the low-frequency harmonic distortion(Carrier frequency and its multiple)of the output voltage caused by the power imbalance of the module by adjusting the carrier phase shift angle in real time,and improve inverter output voltage power quality.(2)Aiming at the problem of leakage current suppression under DC power unbalance,a cascade topology of H6 bridge is proposed and a mathematical modeling of leakage current is presented.The mathematical expression of leakage current,And based on the unbalanced DC power,the improved Carrier Phase Shift Modulation(MPSPWM)is applied to H6 bridge cascaded topology.Compared with traditional H4 bridge cascaded topology application carrier cascade to solve the leakage current problem,The proposed method can well solve the problem of leakage current suppression when the DC power is unbalanced.(3)The single-phase H-bridge cascade simulation model and experimental prototype are designed,the parameters of the simulation model and the software and hardware design scheme of the prototype are introduced in detail.Finally,the simulation and experimental results show the effectiveness of the improved modulation strategy.