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

Because of its low voltage change rate and good harmonic characteristics,multilevel inverter has gradually become a hot spot in the field of power electronics.At present,it is widely used in motor drive,flexible AC transmission,new energy generation and static reactive power compensator.The text takes the modulation strategy and power equalization of cascaded H-bridge multi-level inverter as the research object.The main contents and innovations of the thesis are as follows:In order to solve the issue of power imbalance when traditional PD modulation strategy is applied to cascaded H-bridge nine level inverter,this paper proposes two new refined modulation strategies and corresponding power balancing means.Type I optimal modulation strategy is based on unipolar frequency doubling SPWM modulation technology to optimize the traditional PD modulation strategy,adjusts the waveform of modulation wave and reduces the number of triangular carriers,reduces the triangular carrier frequency to half the original frequency,and reduces the switching loss.On the basis of one optimization,a 1/4 cycle power equalization method is proposed.In this method,the trigger pulse signal of the switch tube of each cell is rotated 1/4 cycle,the switching frequency of all the switch tubes of each cell becomes more uniform,and the power balance of each cell can be realized in one modulation wave period.When the equivalent switching frequency is same,the type I optimized modulation strategy has lower THD than the traditional one and thus has the better waveform.Different from the traditional one,the type II optimized modulated strategy adjusts the negative half period amplitude of the sine modulated wave firstly and restructures the carrier among cells.This strategy completely inherits the superior harmonic performance of the traditional one,outputting the same phase voltage and line voltage frequency spectrum with the number of carrier reducing to half of the traditional one.The optimized modulated strategy has a simple modulated theory and can achieve the power balance among the cascade cells at the full modulation ratio range.A new hybrid modulation strategy is studied for the current reverse problem of asymmetric cascaded H-bridge 13-level inverter.The new modulation strategy combines carrier phase shift and potential comparison modulation,which can effectively avoid the current recharge of inverter.Under the new hybrid modulation strategy,the inverter can automatically realize equalization between low voltage cells and make the inverter output six times frequency,and the inverter has good output waveform quality.