Research On Power Balance Strategy Of Cascaded Multilevel Inverter Based On Output Voltage Cycling Adjustment

As power demand continues to increase in industrial production,multilevel converters have become an attractive alternative to traditional two-level converters.This paper takes cascaded h-bridge(CHB)inverter as the research object,which has the characteristics of stronger high voltage and high power processing capability,lower total harmonic distortion,smaller electromagnetic interference,easier to realize modular expansion and no transformer required.The traditional CHB inverter consists of multiple H-bridges that are separately powered by the isolated power supply.In this case,it is necessary to control the average output power of cascaded cells to be equal to achieve even power distribution.When the battery pack is used for power supply,the uneven power distribution will cause the discharge of each battery pack to be inconsistent,which will cause the battery pack voltage to deviate from the normal value,so the quality of the inverter output waveform will deteriorate.When the uncontrolled rectifier is used for power supply,the uneven power distribution will bring difficulties to the design of transformer,which is not conducive to harmonic elimination of the input current of transformer.These harmonics can be important due to the amount of power involved in high power applications,making it more difficult to meet standards.In view of the above problems,two power balance strategies of CHB inverter based on output voltage cycling adjustment is proposed.Based on the traditional phase disposition modulation strategy,the concept of basic voltage and basic power is proposed.It is revealed that only the cascaded cell outputs all independent basic powers in a certain order,so the even power distribution can be achieved in the shortest time.In addition,the optimization and comparison of the two proposed strategies are implemented.The number of carriers and carrier adjustments in quadratic optimization strategies are less,which not only can achieve better harmonic control of inverter output voltage,but also can realize power coordinated control between the cascaded cells.Therefore,the adverse effects of poor harmonic characteristics and power uneven distribution are eliminated.Moreover,the proposed modulation strategies have better expansibility.The n-cell CHB type inverter can achieve even power distribution in n/2 output cycles when the carrier distribution limitation condition is satisfied.In this paper,the four-cell CHB inverter is taken as an example for detailed derivation and analysis.Finally,the simulation platform and experimental prototype of CHB inverter are built.The correctness,feasibility and expandability of the proposed new power balance strategies are verified by simulation and experimental results.