Current and Magnetic Flux DC Bias Suppression Strategy for MMC-Based DC Transformers

This letter proposes a current and magnetic flux dc bias suppression strategy for the modular multilevel converter (MMC)-based dc transformers. With this strategy, dc bias can be suppressed within half a switching cycle under different transient conditions, including ac-link voltage amplitude, switching frequency, and phase-shift angle variation. By rearranging the moment and quantity of SMs insertion or bypass, the volt–seconds balance of each arm voltage can be realized, thereby eliminating dc bias caused by amplitude step changes. Besides, dc bias caused by switching frequency and phase-shift angle step changes can be eliminated by adjusting phase-shift angle in the transient period. Experimental results are provided to validate the feasibility and effectiveness of the proposed dynamic control.