Research On Cascaded H-bridge Photovoltaic Inverter And Its Stable Operation Control Technology

Compared with the traditional two-level inverter,the cascaded H-bridge inverter has the advantages of high efficiency,low harmonic content of output current,small size,easy modularization and so on.And its total number of power devices used is the least when the output voltage level is the same with other multilevel inverters.In addition,the unique advantages of module level MPPT and module level shutdown make cascaded Hbridge grid-connected inverter very suitable for photovoltaic application scenarios.However,the premise of photovoltaic power generation is the stable operation of the grid-connected system.On the one hand,affected by the light intensity and ambient temperature,the output power of PV modules on the DC side of H-bridge units may be different.In order to ensure the power generation and the quality of output current,the corresponding power balance control strategy must be adopted to maintain the stable operation of the grid-connected inverter itself.On the other hand,due to the long-distance transmission and large-scale power electronic devices connected to the grid,weak grid characteristics would be represented from point of common couplings.Therefore,its necessary to ensure the stable operation of the grid-connected system formed by inverter and the weak grid.Hence,the following research work is done for the cascade H-bridge photovoltaic grid-connected inverter in this paper:(1)The basic principle of the cascade H-bridge grid-connected inverter is introduced,and the output voltage characteristics of inverter under different modulation strategies are compared and two system-level control strategies are present.(2)Based on the fundamental wave modulation can be realized of the maximum power H-bridge unit,the existing power balance control strategies are comprehensively summarized and analyzed.An optimized clamped sinusoidal modulation is proposed by combining quasi-square modulation with clamped sinusoidal modulation,which ensures that the maximum fundamental modulation is 4/π and reduces the injecting harmonic content in the modulation signal.(3)The output impedance of the cascaded H-bridge grid-connected inverter is modeled,and the equivalent time delay of the system under hybrid PWM and carrier phase-shift PWM strategies is deduced detailly.Based on this,the stability of the gridconnected system under weak grid is analyzed.A phase angle compensation strategy is adopted to enhance the stability of the grid-connected inverter with harmonic current suppression strategy under weak grid.(4)The effectiveness and feasibility of the optimized power balance control method and phase angle compensation control method are verified by MATLAB/Simulink and experimental platform.