Dual-Model Predictive Control for Cascaded H-Bridge Multilevel Active Rectifier with DC Voltage Balancing in a Solid-State Transformer

Solid-state transformers (SSTs) have attracted recent interests due to its potential in providing ancillary services in addition to step-up/step-down of voltages. In SSTs, a common topology used in the first AC-DC stage is the cascaded H-bridge (CHB) multilevel active rectifier. This paper proposes a novel dual-model predictive control method for the CHB multilevel active rectifier that achieves sinusoidal source current, maintains DC voltages at set-points, and balances the DC voltages in each of the H-bridge cells. The primary model of the CHB multilevel active rectifier aims to enable control over source current and output voltage. The secondary model uses conservation of energy to calculate the reference source current, which eliminates the need for a PI-control-based outer loop. A Luenberger-based observer is also implemented to estimate the DC load currents. The effectiveness of the proposed method under steady-state and transient conditions is validated through a laboratory prototype of a single-phase 7-level CHB active rectifier.