Application Expansion of DC Bias Suppression for Hybrid Transformers in Flexible AC Grid Regulation

By combining partially rated converters with electromagnetic transformers, hybrid transformers can manage voltage and power flow in ac grids while enabling a variety of application expansions. For instance, dc bias in ac grids can lead to several detrimental impacts, and the existing suppression methods typically require supplementary equipment. However, hybrid transformers can achieve dc bias suppression by modifying their control strategies without any extra demands. The direct dc output control signal will lead to unbalanced arm power accumulation in hybrid transformers employing modular multilevel converters with half-bridge submodules, eventually causing the converter blocked. This letter proposes a novel asymmetric operation mode of hybrid transformers for dc bias suppression, being able to output a continuous and stable dc component without incurring any additional cost. It also avoids impacting the basic functions of hybrid transformers for voltage management and dc integration. Finally, the proposed operation method has been validated through simulations and experiments. The proposed operation approach is not solely aimed at achieving converter optimization, but rather to serve the specific needs of dc bias in ac grids, expanding the functionality of the hybrid transformers further.