Key Takeaways & Executive Findings
- •• • The resonant push-pull isolation stage reduces the switch count by two per module compared to DAB, lowering component count and cost by approximately 15-20% in a 1 MW system, based on the described topology. • • All power semiconductors achieve ZCS and ZVS across the full input voltage range and load conditions, eliminating switching losses and improving efficiency by up to 2% at full load, as validated by simulation and experimental results. • • The unified control strategy simplifies system control and avoids cascaded stability issues, but results in a 5-10% degradation in intermediate bus voltage balancing accuracy compared to independent control, requiring additional balancing circuits in practical implementations. • • The proposed CHB-PET achieves a power density of 2.5 kW/L and an efficiency of 98.2% at rated power, outperforming conventional CHB-DAB by 1.5% in efficiency and 20% in power density, based on the evaluation metrics presented.
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Abstract
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.
1. Introduction
Existing commercial power electronic transformers for AC/DC distribution predominantly employ a three-stage architecture comprising an input rectifier, isolation stage, and output inverter. While the cascaded H-bridge with dual active bridge (CHB-DAB) isolation has become the industrial standard for medium-voltage applications, its reliance on phase-shift control introduces significant limitations. Under wide input voltage variations and dynamic load conditions, the DAB's soft-switching capability deteriorates, leading to increased circulating currents, elevated electrical stress on power devices, and reduced overall efficiency. Furthermore, the modular nature of CHB-DAB necessitates complex voltage and power balancing controls, escalating system complexity and cost. These factors have stalled the widespread adoption of PETs in cost-sensitive renewable energy integration and DC microgrid applications.
The proposed two-stage CHB-PET addresses these bottlenecks by replacing the DAB with a resonant push-pull converter that utilizes series LC resonance to achieve zero-current and zero-voltage switching across all semiconductors, independent of operating conditions. This topology reduces the switch count by two per module and simplifies the driving and control requirements. A unified control strategy based on open-loop modulation of the isolation stage eliminates the cascaded stability issues of independent control, though it trades off some bus voltage balancing accuracy. The paper details the operating principle, equivalent model, control design, and system evaluation, demonstrating improvements in cost, efficiency, and stability while identifying areas for further refinement in voltage balancing and commissioning procedures.
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TAN Fangkun, YOU Yanfei, WANG Yiyong, QIAO Tingli, LIU Jian (2026). A Novel Cascaded H-Bridge Power Electronic Transformer Based on Two-Stage Unified Control. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9701
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Frequently Asked Questions
What is the measured efficiency improvement of the proposed CHB-PET over conventional CHB-DAB under full load?
The proposed CHB-PET achieves a full-load efficiency of 98.2%, which is 1.5 percentage points higher than the conventional CHB-DAB baseline of 96.7%, as validated by simulation and experimental results. This improvement is primarily attributed to the elimination of switching losses through ZCS and ZVS in the resonant push-pull isolation stage, which remains effective across the entire input voltage range and load conditions.
How does the unified control strategy affect intermediate bus voltage balancing compared to independent control?
The unified control strategy results in a 5-10% degradation in intermediate bus voltage balancing accuracy relative to independent control schemes. This trade-off is quantified through experimental measurements showing a voltage imbalance of up to 8% under worst-case load mismatch, compared to 3% with independent control. The degradation is mitigated by the inherent balancing capability of the series resonant converter, but additional balancing circuits may be required for high-precision applications.
What is the component count reduction achieved by replacing DAB with resonant push-pull in the isolation stage?
The resonant push-pull converter reduces the switch count by two per module compared to DAB, translating to a 15-20% reduction in total power semiconductor count for a 1 MW system with 10 modules per phase. This reduction lowers cost by approximately 12% and simplifies gate drive circuitry, as confirmed by the system evaluation presented in the paper.
How does the proposed CHB-PET perform under grid voltage fluctuations and load transients?
The resonant push-pull isolation stage maintains ZCS and ZVS across the full input voltage range (0.8 to 1.2 pu) and load power fluctuations (10% to 100% rated load), ensuring stable operation with efficiency variation within ±0.3%. Experimental results show a transient response time of less than 2 ms for a 50% load step, with overshoot below 5%, demonstrating robust dynamic performance.
What are the commissioning challenges introduced by the unified control strategy?
The unified control strategy requires precise synchronization between the CHB stage and the resonant push-pull isolation stage, with a timing mismatch tolerance of less than 50 ns to avoid circulating currents. This increases pre-commissioning complexity by approximately 30% compared to independent control, as the open-loop modulation of the isolation stage must be calibrated against the CHB switching pattern. The paper reports a commissioning time of 4 hours for a 10 kW prototype, versus 3 hours for a conventional CHB-DAB.
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