Research on DC voltage balancing control method of star connection cascaded H-bridge static var generator

In-depth study of the DC side voltage control of the star connection cascaded H-bridge multilevel static var generator has been made in this study. Moreover, a three-layer DC side voltage control system has been established. The first layer is overall DC side voltage control. The overall DC side voltage of three phases A, B and C is maintained constant through generating fundamental positive sequence active current. The second layer is balancing control between three phases. Through injecting zero sequence voltage into the voltage command, the authors can redistribute three-phase active power, in order to maintain three-phase DC side voltages balanced; in this method, fast dynamic control can be achieved through feeding forward the power that grid voltage and converter current produce. The third layer is balancing control between modules of each phase. Through adjusting voltage commands of H-bridge modules of each phase along the direction of the converter current, they can redistribute the active power absorbed by each module, in order to ensure that DC side voltages of modules of each phase are equal to each other. Experimental results verify the correctness and reliability of the proposed control system in the end.