Control strategy for suppressing submodule capacitor voltage fluctuation in cascaded H-bridge solidstate transformer

Nowadays, with the vigorous development of smart grids, cascaded H-bridge solid-state transformers (CHB-SST) used in distribution network systems have gradually become an important research hotspot. The multiport CHB-SST serves as the interfacing device to interconnect different grids, which has the functions of bidirectional, power flow control, reactive power compensation, and voltage regulation. However, the power fluctuation of the power cells should be constrained by the large capacitors which in turn worsens the system power density. The second order current in the DC-link side of the CHB-SST will cause the fluctuation of the submodule (SM) capacitor voltage, affecting the capacitance value selection. Therefore, this paper proposes a method to suppress the SM capacitor voltage fluctuation of CHB-SST through fluctuating power control strategy. The control scheme passes the second order current component of the SM capacitor to the low-voltage side of the CHB-SST to reduce the voltage fluctuation of the SM capacitor, such that the capacitance can be optimized. The proposed method is verified via the MATLAB/Simulink model, proving that the fluctuation of the SM capacitor voltage can be effectively suppressed.