Novel modular multilevel converter-based five-terminal MV/LV hybrid AC/DC microgrids with improved operation capability under unbalanced power distribution

Conventionally, the multilevel converter-based multi-terminal hybrid microgrids require a large number of power switches and have a limited operation capability under unbalanced power distribution in medium and low voltage (MV/LV) AC/DC microgrids. To solve this issue, this paper proposes the novel modular multilevel converter (MMC)-based five-terminal MV/LV hybrid AC/DC microgrids. The proposed hybrid microgrids realize the interconnection between the medium-voltage AC (MVAC), MVDC, low voltage AC (LVAC), and two LVDC terminals. In addition, the MVAC grid is connected to the AC terminal of MMC, and the MVDC microgrid is connected to the DC terminal of MMC through a dual active bridge (DAB) converter. Based on MMC, the compact interlinking converters are established, providing three LVDC terminals, which are connected to two LVDC microgrids and one LVAC microgrid through a DC/AC converter. Compared with the conventional MMC-based hybrid microgrids, the proposed topology can significantly reduce the number of power switches. Moreover, to overcome the control challenge of arm energy balancing in MMC and meet the requirement of different operation modes in microgrids, a hierarchical energy control method is proposed, where the low circulating currents are injected to balance the arm energy. Therefore, the system operation capability can be improved under unbalanced power distribution. Validation results in different conditions (power step, power reversal, and unbalanced MVAC voltages) indicate that by the proposed method, the arm energy and capacitor voltages in MMC are well balanced, and the proposed hybrid AC/DC microgrids can operate normally at different modes.