Modulation and Active Midpoint Control of a Three-Level Three-Phase Dual-Active Bridge DC-DC Converter under Non-Symmetrical Load

This paper discusses the operation of a three-phase three-level dual-active bridge (3L-DAB3) dc-dc converter in medium-voltage dc (MVDC) grids with a symmetrical monopole configuration. Due to the demand of bipolar mode operation, active control of the midpoint potential is essential to ensure balanced grid voltages even under non-symmetrical loads. The proposed solution is achieved with a novel modulation scheme, which allows a power and midpoint control to enable voltage balancing. Due to the large number of degrees of freedom, look-up tables (LUTs) are generated and used to operate with a maximum converter efficiency. The proposed method is implemented and validated in simulations and on a low-power prototype setup.