A Novel Cascaded H-Bridge Power Electronic Transformer Based on Two-Stage Unified Control
This paper proposes a novel cascaded H-bridge power electronic transformer (CHB-PET) employing a resonant push-pull converter in the DC/DC isolation stage, which reduces the number of switching devices compared to conventional dual active bridge (DAB) solutions. A unified control strategy based on open-loop modulation of the isolation stage is adopted for the two stages, simplifying system control and avoiding cascaded stability issues inherent in independent control schemes. The operating principle, equivalent model, control design, and system evaluation are described and analyzed. Evaluation results indicate that the proposed CHB-PET offers improvements in cost, efficiency, and stability relative to traditional approaches, while exhibiting degradation in intermediate bus voltage balancing and pre-commissioning procedures. Simulation and experimental results validate the effectiveness of the proposed CHB-PET scheme. The resonant push-pull converter utilizes series LC resonance to achieve zero-current switching (ZCS) and zero-voltage switching (ZVS) across all power semiconductors, independent of grid voltage and load fluctuations. The topology reduces the switch count by two per module compared to DAB, lowering component count and cost. The unified control eliminates the need for complex voltage and power balancing among modules, though it introduces challenges in bus voltage equalization. The paper provides a comprehensive analysis of the CHB-PET, including its mathematical model, control parameter design, and performance assessment, demonstrating its potential for medium-voltage AC to low-voltage DC conversion in renewable energy integration and DC distribution systems.