Research On Control Method Of Off-grid Multilevel Inverter Based On Cascaded H-bridge

Remote areas are constrained by environmental and hardware conditions,resulting in poor quality and high cost of grid power supply.Establishing a reliable off-grid power supply system is an effective solution to these problems.Cascaded H-bridge(CHB)inverter,as a widely concerned multi-level inverter,has advantages such as modularity and flexible power level expansion,making it suitable for off-grid power supply.Therefore,it is of great significance to study its control methods in off-grid scenarios.This article conducts research on two common off-grid CHB inverter structures(two-stage and parallel),and designs methods based on this.The method designed in this article can meet two power supply requirements:high current tracking quality and low harmonic risk.The specific work is as follows:(1)Analyze the basic structure and principle of CHB inverter,and introduce carrier phase shifted(CPS)modulation strategies.In order to validate methods designed in this article and reduce controller costs,a CHB inverter prototype scheme based on STM32was designed based on the principle and modulation strategy of the CHB inverter,and the feasibility of the design scheme was verified.(2)For the off-grid two-stage CHB inverter,a current tracking method based on sliding mode controller is designed:the PI controller regulates and stabilizes the capacitor voltage of DC/DC part through the Boost circuit;The sliding mode controller calculates the control quantity in real time by sampling the load current,which is used to modulate with the triangular carrier wave to realize the tracking of the load current to the reference current.Then the parameter selection range of sliding mode controller is analyzed from the perspective of nonlinear behavior.The simulation and prototype experimental results show that the load current tracking effect is good,and the total harmonic distortion(THD)is basically less than 5%.(3)For the off-grid CHB parallel inverter,based on its topology and CPS modulation strategy,the relationship between the harmonic content of the integer multiple Nf_c frequency harmonic group of the output voltage and the circulating current between branches,and the branch carrier phase angle is analyzed.Based on this,a CPS method based on branch carrier phase angle switching is designed.This method can compromise the amplitude of the odd times Nf_c frequency harmonic group of the output voltage and the circulating current to about 50%of its maximum value,and achieve a balance between the harmonic content of the output voltage and the circulating current.The effectiveness of the method has been verified through simulation and prototype experiments.(4)To reduce the harm of harmonics to equipment life,a chaotic CPS method for off-grid CHB parallel inverters was designed.This method combines chaotic carrier wave with CPS method based on branch carrier phase angle switching,which reduces the harmonic peak of both output voltage and circulating current while compromising the harmonic content,thereby weakening its adverse impact on the normal operation and lifespan of the equipment.The test results indicate that to avoid an increase in output voltage THD,the maximum additional frequency should be<0.12F_c.The effectiveness of the method has been verified through simulation and prototype experiments.