Research On Power Balance Modulation Strategy Of Hybrid Cascaded H-Bridge Multilevel Inverter

In recent years,with the large-scale application of high-voltage and high-power equipment,multi-level inverters have become a research hotspot due to their high output voltage waveform quality and low electromagnetic interference.Many new multi-level inverter topologies and new modulation strategies continue to appear.On the premise of outputting the same number of levels,the hybrid multilevel inverter needs fewer switching devices and DC voltage sources,and the topology is simpler.This paper takes the hybrid cascaded H-bridge multilevel inverter as the research object,its topology structure,modulation strategy and power equalization method have been deeply studied.The main research contents are as follows:Aiming at the problems that the traditional hybrid cascaded H-bridge inverter cannot take into account the aspects of ensuring the quality of the output voltage waveform,making the high-voltage unit work in a low frequency state,simplifying the modulation strategy,and eliminating the current inversion problem,based on the optimized inverter topology of the traditional hybrid cascaded H-bridge inverter,two improved hybrid cascaded H-bridge inverter topologies are proposed,which are D1-D2 hybrid cascaded H-bridge inverter and T1-3 hybrid cascaded H-bridge inverter.The corresponding modulation strategy and power equalization method are designed for the two topologies.Two hybrid modulation strategies are designed for the D1-D2 hybrid cascaded H-bridge inverter.They are the hybrid carrier stacking modulation strategy that combines staircase wave modulation and carrier stack modulation,and the step wave modulation is used in the high-voltage unit,and the low-voltage unit adopts Hybrid frequency doubling modulation strategy combining carrier phase shift and carrier amplitude shift.The former solves the problem of current inversion and makes the high-voltage unit work in the fundamental frequency state,while the latter makes the inverter have double frequency characteristics and realizes the self-balancing of the output power of the low-voltage unit while retaining the above advantages.Aiming at the problem of power imbalance between units of the same voltage level of the inverter,a power balance method based on logic operation is proposed,which realizes the power balance between units of the same voltage level.Aiming at the problem of unbalanced output power between high and low voltage units of T1-3 hybrid cascaded H-bridge inverters under the hybrid frequency doubling modulation strategy,a power equalization modulation strategy based on dynamic adjustment of conduction angle is proposed.Theoretical analysis establishes the relationship between the fundamental amplitude of the output voltage of the high-voltage unit and the modulation degree.On the basis of this relationship,the power balance constraint equation based on the conduction angle of the high-voltage unit is deduced,and then under different modulation degrees,by dynamically adjusting the conduction angle of the high-voltage unit,the output power balance of each unit under the full modulation degree can be achieved on the premise of maintaining the quality of the output voltage waveform.Finally,the simulation and experimental platforms of the D1-D2 hybrid cascaded H-bridge inverter and the T1-3 hybrid cascaded H-bridge inverter are built,the stability of the proposed topology and the correctness and effectiveness of the modulation strategy and power equalization method are verified.