DC voltage control of cascaded inverter-based dynamic voltage restorer

For the cascaded multilevel DVR(Dynamic Voltage Restorer) based on transformerless topology,its steady state characteristics are analyzed quantitatively.Analyses show that DC capacitor voltage can be stabilized by adjusting the injected voltage phase angle.A control strategy is presented,which compensates rapidly the system voltage sags while keeps the DC voltage stable by adjusting the injected voltage phase angle.PI controller is introduced into the injected voltage generation based on the minimal energy compensation model.The DVR absorbs or delivers active power from or to grid to maintain the DC capacitor voltage at its reference.In order to minimize the DC voltage deviation,the PI closed-loop control is used to balance the power among H-bridges of cascaded inverters.The correctness of theoretical analysis and the feasibility of the proposed design scheme are verified by simulation results.