In the field of power systems, researchers from the National Electric Energy Conversion and Control Engineering Technology Research Center at Hunan University, including Lan Zheng and Tu Chunming, have made significant contributions to the study of power electronic transformers (PETs) in AC/DC hybrid microgrids. Their work, published in the 23rd issue of the Journal of Electrical Engineering in 2015, explores the operation modes of these transformers—grid-connected and off-grid—and presents corresponding control strategies.
In grid-connected mode, the PET input interface is controlled to make the AC/DC hybrid microgrid behave like a "resistive load" or "current source," while the AC and DC output interfaces are regulated to function as constant voltage sources. For off-grid operation, a hybrid droop control strategy is introduced, which allows for precise power exchange between microgrids based on frequency and voltage information at the interface, combined with the system's drooping characteristics.
A simulation model of the AC/DC microgrid system and the power electronic transformer was developed. The results demonstrated that PET can effectively and rapidly adjust power flow between the main grid, AC microgrid, and DC microgrid under fluctuating distributed energy generation conditions, ensuring stable operation of the hybrid system and validating the proposed control method.
The increasing integration of distributed energy resources (DER) is driving the evolution of power systems. Microgrids offer an effective solution for managing large-scale intermittent DER. Connecting DER in DC form reduces the need for complex conversion stages and eliminates the need for phase and frequency tracking, significantly improving controllability and reliability. While DC microgrids are gaining attention, AC microgrids remain dominant for now, making the coexistence of AC and DC in hybrid microgrids a long-term structure in future power systems.
The bidirectional power flow caused by DER variability makes the Point of Common Coupling (PCC) a critical junction for energy transfer between the distribution network, AC microgrid, and DC microgrid. Effective coordination at the PCC is essential for precise power management, requiring a reliable "energy router."
Power Electronic Transformers (PETs), composed of high-frequency transformers and power electronics circuits, offer voltage transformation, isolation, and energy transmission capabilities. They can act as an "energy router," enabling efficient energy coordination at the PCC.
Current research on PET control primarily focuses on basic functions without addressing microgrid coordination. Traditional droop control methods consider only AC or DC side voltage signals, neglecting interactions between both sides. Some studies propose bidirectional droop control, but they often introduce redundant control links, reducing system reliability.
This paper focuses on the application of PETs in AC/DC hybrid microgrids, analyzing their operation strategies in both grid-connected and off-grid modes. In grid-connected mode, the PET interface is controlled to ensure in-phase current and voltage, while the AC and DC outputs are maintained as constant voltage sources. For off-grid mode, a hybrid droop control method is proposed, enabling accurate and rapid control of bidirectional power flow between microgrids and optimizing power distribution among the main grid, AC, and DC microgrids.
A simulation model was built to validate the performance of the proposed control strategy. Results show that PET can effectively manage power flow during distributed energy fluctuations, ensuring stable operation of the AC/DC hybrid microgrid.
Although this study focused on DER power variations, further analysis of AC microgrid frequency and DC bus voltage dynamics during off-grid operation would enhance the understanding of system stability. This remains an important area for future research.
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