Comparison of voltage control methods of CHB converters for power routing in smart transformer

For Smart Transformers (ST), reliability is one of the major problems when compared to the traditional low-frequency transformers. Modular ST with advanced control algorithms could increase the reliability compared to the non-modular solutions. This could be achieved by distributing differently the power among the cells depending on their aging. This unequal power transfer is a challenge for Cascaded H-Bridge converters (CHB). ST based on a CHB rectifier with different capacitor voltage balancing schemes are analyzed in the paper. The goal is to evaluate the best scheme for unequal power transfer while maintaining the dc-link voltages constant. The paper focuses on validating the operating power unbalance limits of each cell of the modular system for unbalanced power transfer while maintaining the voltage balance in the medium voltage dc-link stage. The power unbalance limits are established with analytical calculations and verified through simulations and experiments.