Balancing Control of Cluster Energy in Star-Connected Multilevel Power Conversion System Using Zigzag Transformer

In star-connected cascaded H-bridge converters, it is common to make use of a zero-sequence voltage as a control element for cluster energy balance control. Therefore, during the converter design process, it is crucial to consider and secure the sufficient cluster voltage margin to accommodate this zero-sequence voltage. The proposed method offers a way to reduce the number of submodule cells while maintaining the same output voltage. This reduction is achieved by introducing a zigzag transformer to create a zero-sequence current path. This zero-sequence current can then serve as the control factor for cluster energy balancing, and a dedicated control scheme can be developed. One noteworthy advantage of this approach is that the zero-sequence current can effectively address cluster energy balance even in light load conditions of the star-connected cascaded H-bridge converter, as well as during grid and load imbalances. To validate the feasibility of the proposed method, simulation and experimental studies were conducted. The results confirm the effectiveness and practical applicability of the approach in achieving cluster energy balance under various operating conditions.