Research On Modulation Strategy Of Cascaded H-Bridge Multilevel Inverter

The in-depth study of the multi-level inverter topology and its control strategy makes it widely used in high-voltage and high-power systems.In this process,there are not only many new multilevel inverter topologies,but also many novel control strategies.These studies not only improve the overall efficiency of the inverter,but also improve its output power quality.This paper takes the Cascaded H-bridge multilevel inverter as the research object,and conducts a comprehensive and in-depth study on the problems of the modulation strategy of the CHB multilevel inverter.The main research contents are as follows:Aiming at the problems of unbalanced output power of the H-bridge unit at all levels under the In Phase Disposition Pulse Width Modulation strategy of the CHB multilevel inverter and the high total harmonic content of the output line voltage under the Carrier Phase Shift Pulse Width Modulation strategy,two improved multi carrier PWM strategies based on power balance are proposed.One is based on a carrier period segment in the IPD-PWM strategy as the basic unit,and the arrangement of triangular carriers in the vertical direction is changed periodically to achieve the power balance of each H-bridge unit;At the same time,it can ensure that the harmonic spectrum of the output voltage is exactly the same as when the IPD-PWM strategy is adopted;The other is to extend the existing cps-pwm strategy,which is based on controlling the combination of triangular carrier degrees of freedom to enhance the fundamental amplitude of the output voltage,so as to significantly improve the DC voltage utilization of the output voltage;It can also reduce the total distortion rate of the output voltage under the condition of realizing the power balance of the connected units at all levels,and effectively improve the harmonic performance at low modulation degrees.Aiming at the problems of low utilization of DC voltage and unbalanced output power of H-bridge units in the Carrier Disposition modulation strategy of CHB multilevel inverter,an ipd-tpwm strategy based on power balance is proposed,This modulation strategy can not only ensure the quality of the output line voltage waveform under the condition of selecting the appropriate trapezoidal wave triangulation rate δ value,but also greatly increase the fundamental wave amplitude of the CHB inverter output voltage;In addition,by changing the arrangement of triangular carriers,the power balance of each H-bridge unit in one output cycle can be achieved in the full modulation ratio range.Aiming at the problem of local square wave and large low-frequency harmonic content in the output voltage waveform under the conventional hybrid PWM technology of the type III hybrid CHB multilevel inverter,this paper proposes an optimized hybrid PWM technology.Among them,the high-voltage H-bridge unit adopts a combination of modulating wave and triangular carrier,and the low-voltage H-bridge unit adopts a single-pole frequency doubling method to perform a re-logic operation and combination of pulses obtained under hybrid PWM.This method can overcome the local overshoot phenomenon of the low-voltage unit existing under the conventional hybrid PWM,thereby eliminating the 3th,5th,and 7th harmonics in the output voltage waveform.This method makes the output line voltage Total Harmonic Distortion of the technology lower than the PS-PWM technology in the entire modulation degree,which effectively improves the quality of the inverter output voltage waveform.