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

The advantages of cascaded H-bridge multilevel inverters lie in its modular design,easy expansion,high efficiency and high-quality grid-connected current.Compared with conventional multilevel inverters,cascaded H-bridge multilevel inverters requires fewer power devices when the number of output voltage levels is the same.In addition,the DC side of each power unit can be independently powered by a photovoltaic panel,avoiding the balancing-control of the DC-side voltage and enabling independent MPPT control.Therefore,Cascaded H-bridge Inverters are well suitable for being applied in photovoltaic power generation.Numerous factors such as nonuniform solar irradiance,unequal ambient temperatures,and inconsistent module degradation will bring about difference in the maximum power point voltage of the photovoltaic panel When the cascaded H-bridge multilevel inverters are applied in photovoltaic power generation.In the meantime,the frequency doubling effect of the traditional carrier phaseshifting modulation strategy fails,the low-frequency harmonic components of the output voltage cannot be eliminated completely.In addition,the difference in maximum power points will also lead to the problem of interphase power imbalance in a three-phase cascade H-bridge photovoltaic grid-connected system.The interphase power imbalance can deteriorate the three-phase output current which eventually lead to the system instability or even grid-connected failure.Therefore,this paper has done some research in accordance with two issues above.(1)The simplified frequency domain analysis of the cascaded system modulating with carrier phase shift modulation strategy is carried out,and the harmonic expression of the output voltage in single phase cascaded system is obtained.Aiming at the shortcoming of existing improved carrier phase-shift modulation strategy(MPSPWM)that it can only be applied in three modules cascade system.,this paper proposes an improved carrier phase-shift modulation strategy that can be applied to multi-module cascade system,it can effectively improve the low frequency harmonic problem caused by the unbalanced operation of the cascade system.(2)For the problem of three-phase grid-connected current asymmetry caused by the interphase power imbalance of the three-phase cascade system,the fundamental frequency zero-sequence injection method is firstly analyzed in detail.Aiming at the shortcoming that it may fails to rebalance three-phase gridconnected current when the interphase power imbalance aggravates furtherly,this paper proposes some improved zero-sequence injection methods by superimposing high-frequency components on the fundamental frequency components,which effectively improve the power balance of the zero-sequence component injection method.(3)A three-phase cascade H-bridge photovoltaic grid-connected inverter experimental prototype has been designed and built,and the software and hardware design principles of the prototype are introduced in detail.Finally,the effectiveness and feasibility of the proposed control strategy are verified by prototype experiments.