Research On Modulation Method And Control Strategy Of Hybrid Cascaded H-bridge Inverter

In order to respond with the increasingly grave energy and environmental problems,renewable energy power generation technologies such as photovoltaic power have been extensive applicated.As the key equipment of the new energy generation system,the performance of the inverter not only affects the reliability of the system,but also is closely related to the quality of the converted power.Cascaded multilevel inverters have received extensive attention in recent years due to their advantages of modularity,multilevel output and low harmonic content.Hybrid cascaded H-bridge multilevel inverter,as a form of cascaded H-bridge inverter,is the current research focus in multilevel inverter field.This thesis will focus on the hybrid cascaded H-bridge inverter topology,and research the modulation method and control strategy of the inverter.The main work is as follows:(1)An improved hybrid carrier pulse width modulation method based on carrier level-shift is proposed.According to the topology of the hybrid cascaded H-bridge elevenlevel inverter with the DC side voltage ratio of 3:1:1,the current incrush problem caused by the inconsistent output voltage polarity of the high and low voltage cells in the inverter under the traditional modulation strategy and the unbalanced output power problem between the low voltage cells caused by different PWM modulation methods are analyzed.An improved hybrid carrier pulse width modulation strategy based on carrier level-shift is researced.Using carrier freedom and redundant switch states,the working modes of low voltage cells is optimized,so that the high and low voltage cells maintain the same output voltage polarity throughout the output cycle,and the problem of current incrush in the inverter under the traditional modulation method is solved.At the same time,the two low voltage cells in the method work in the high-frequency states,so that the output voltage waveforms of low voltage cells are complementary and symmetrical,and the output power between low voltage cells is selfbalance.For the power balance of low voltage cells,further analyzed the modulation method based on 1/4 or 1/2 cycle pulse.The output voltage corresponding to the inverter under different conditions is analyzed,and the pulse cycle conditions that can achieve power self-balancing are summarized,and the corresponding carrier realization method is given.(2)The grid-connected control of two-stage hybrid cascaded H-bridge inverter is studied.Combined with the structure of two-stage hybrid cascaded H-bridge inverter with boost and H-bridge,focusing on voltage and current double closed-loop control strategy.The front boost circuit of the system completes the maximum power point tracking,the back H-bridge circuit uses PI controller to achieve the stable control of the DC side capacitor voltage,and the PR controller to achieve the tracking of the grid-connected current.The influence of DC voltage feedforward control on the output change of inverter under the condition of unbalanced PV input power is analyzed.In the case of slight and serious imbalance of input power,the irradiance is changed according to the conditions of high voltage cell,low voltage cell,high voltage cell and low voltage cell respectively.The changes of DC side voltage,grid-connected current and output voltage of the inverter with and without power control are analyzed,and the reasons for each situation are analyzed theoretically.(3)The simulation platform and experimental platform of the hybrid cascaded Hbridge eleven-level inverter are built to verify the correctness of the proposed modulation strategy and the theoretical analysis of the control strategy.Figure [68] table [6] reference [62]...