With the large-scale integration of the distribution generations (DGs) and the increasing medium-voltage and low-voltage DC power demands, multi-terminal hybrid AC/DC microgrid has drawn great attention from researchers around the world. In order to reduce the number of power conversion stages and meet DC transmission demands under different DC voltage levels, this paper proposes a four-terminal interconnection scheme of the hybrid AC/DC microgrid, connecting one medium-voltage AC (MVAC) terminal, one medium-voltage DC (MVDC) terminal and two low-voltage DC (LVDC) terminals. The proposed interconnection scheme includes a modular multilevel converter (MMC) as the main interlinking converter of the MVAC grid and MVDC microgrid, and a series of dual active bridges (DAB) converters as two isolated LV DC microgrid interfaces. It has more flexibility for power supplies, especially MVDC transmission, and a more robust tolerance for unequal power distribution between the two LVDC Microgrids. To realize the DC capacitor voltage balancing control, an improved energy control method is proposed in this paper. The proposed method keeps DC capacitor voltage balance and AC current zero on the MVDC transmission lines, which contributes to the stability of the MVDC microgrid. In addition, the symmetry of the AC currents is also guaranteed with this control method. Validation results of a four-terminal hybrid AC/DC microgrid verify the effectiveness of the proposed microgrid and control scheme.
Bibliographical noteFunding Information:
This work was funded by China Postdoctoral Science Foundation (Grant No. 2020TQ0222 ), National Key Research and Development Program of China (Grant No. 2017YFB0903300 ), the project of National Natural Science Foundation of China (Grant No. 52061635103 , 51625702 ), and the Joint Fund Project of National Natural Science Foundation-State Grid Corporation (Grant No. U1766210).
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- Arm energy control
- Four-terminal microgrid
- Grid-voltage sags
- Hybrid AC/DC microgrid
- Modular multilevel converter (MMC)
- Voltage balancing control