Research On Key Techniques Of Cascaded H-brdige Photovoltaic Inverter

Compared with two-level inverter,multilevel inverter can inject a high quality current with low harmonic distortion to the grid,reach to higher efficiencies with low switching frequency techniques and employ lower volume of the filter inductance.Among the different types of multilevel inverter,cascaded H-bridge is the most attractive topology because of its modularity,simple layout and higher reliability.Additionally,cascaded H-bridge inverter features several dc links,making the maximum power point tracking(MPPT)possible in each solar cell by controlling dc voltage independently if used in photovoltaic(PV)field.Due to partial shading or damaged solar cells,the relevant PV panels will be a serious decline in output power.Since the current flowing through each H-bridge is the same and output power of each H-bridge has great difference,these cells with higher power are likely to over-modulation,which brings about deteriorated grid current even system instability.Moreover,the cascaded H-bridge topology eliminates the bulky transformer between inverter and gird,generating the leakage current in the stray capacitor.Leakage current can reduce the reliability of inverter,threat to the personal safety and result in electromagnetic interference.To address these problems,this paper research on the single-phase cascaded H-bridge photovoltaic inverter and the main work of this thesis is as follows:(1)The main circuit topology of single-phase cascaded H-bridge PV inverter is analyzed in detail and its common modulation strategies and control strategies are presented.(2)A control strategy based on reactive power compensation is proposed,which follows the principle of distributing active power proportionally and reactive power according to requirement and ensure that all power units do not suffer from over-modulation even in serious power unbalance.(3)The mathematical model of leakage current of single-phase cascaded H-bridge PV inverter is researched and the equation of system leakage is deduced.On this basis,a hybrid modulation strategy combined with modified phase disposition pulse width modulation and carrier phase shifting pulse width modulation is proposed,which suppresses system leakage current effectively and is conducive to the system extension and modular design.(4)The simulation model and experimental platform are presented to verify the validity and effectiveness of active power compensation control strategy and hybrid modulation strategy.Finally,the lacks of the research are summarized and the further study is pointed out.