DC-link voltage modulation for individual capacitor voltage balancing in cascaded H-bridge STATCOM at zero current mode

Individual capacitor voltage balancing of cascaded H-Bridge converters is one of the challenges in the field of multilevel converters, especially when the converter is operating at zero-current mode. This is due to the fact that the balancing algorithm requires information about the power direction for proper insertion of the different cells of the converter. In case of delta configuration of the phase-legs of the converter, a circulating current inside the delta can be used for balancing purposes. However, this approach can not be used for the star configuration. Focusing on the star, this paper first highlights the issue related to the capacitor voltage balancing at zero-current mode and second proposes a novel method to overcome this problem. The proposed method is based on modulation of the DC-link voltages. The method provides a small amount of current flow during the zero-current mode operation. This current enables the sorting algorithm to provide an appropriate individual balancing. The proposed method is applied to a 7-level star connected cascaded H-Bridge STATCOM modeled in PSCAD and simulation results show the effectiveness of the proposed method in balancing the capacitors voltages when no current is exchanged with the grid. Finally the proposed method is experimentally verified through a down-scaled laboratory set-up.