In the field of power systems, the increasing integration of distributed energy resources (DER) has driven the evolution of microgrid technologies. Researchers at the National Electric Energy Conversion and Control Engineering Technology Research Center, Hunan University, including Lan Zheng and Tu Chunming, have made significant contributions in this area. In their paper published in the 23rd issue of the Journal of Electrical Engineering in 2015, they explored the application of Power Electronic Transformers (PETs) in AC/DC hybrid microgrids.
The study analyzed two key operational modes: grid-connected and off-grid. For the grid-connected mode, the PET input interface was controlled to make the AC/DC hybrid microgrid behave like a "resistive load" or "current source," while the AC and DC output interfaces were maintained as constant voltage sources. This approach ensures stable power exchange with the main grid.
In the off-grid mode, the researchers proposed a hybrid droop control strategy that leverages frequency and voltage information from both the AC and DC sides. This method enables precise power coordination between the AC and DC microgrids, ensuring efficient and stable operation without external grid support.
To validate their findings, a simulation model of the AC/DC microgrid system and PET was developed. The results demonstrated that the PET could accurately and quickly adjust power flow among the main grid, AC microgrid, and DC microgrid, even when there were fluctuations in distributed energy generation. This confirmed the effectiveness of the proposed control strategy.
As DER continues to grow, the need for flexible and reliable power systems becomes more urgent. While DC microgrids offer advantages such as reduced conversion losses and improved controllability, AC microgrids remain dominant due to existing infrastructure. Therefore, the coexistence of AC and DC in hybrid microgrids is expected to be a long-term solution.
The Point of Common Coupling (PCC) plays a critical role in managing energy flows between different parts of the system. A reliable "energy router" is essential to ensure smooth power coordination. The PET, with its ability to perform voltage transformation, isolation, and energy transmission, serves as an ideal candidate for this role.
Current research on PET control often focuses only on basic functions, without addressing the coordinated operation of the entire microgrid. Existing droop control methods also lack comprehensive consideration of both AC and DC side signals. To overcome these limitations, this paper proposes a more integrated control strategy that enhances system reliability and performance.
Overall, the study highlights the importance of PETs in enabling stable and efficient operation of AC/DC hybrid microgrids. Future work should focus on analyzing dynamic responses during DER disconnection and improving the overall stability of the system.
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