Direct modulation in MMC-based multiterminal hybrid microgrid : a solution for arm capacitor voltage balancing under unbalanced power distribution

Abstract

The rising interest in hybrid alternating current (AC)/ direct current (DC) microgrids among the global research community can be attributed to the widespread adoption of distributed generation systems (DGs). Therefore, this study applies the modular multilevel converter (MMC) in interconnected microgrids to serve as an interlinking converter involving the AC and DC systems. In this topology, the MMC consists of several submodules (SMs) where a low-voltage direct current (LVDC) microgrid is connected to the output of each SM through a dual active bridge (DAB) converter. As a result of using this topology, more LVDC microgrids can be linked, thus enhancing power transition feasibility. However, during unequal power distribution across LVDC microgrids, the arm capacitor voltage balancing becomes a challenging task and if left unsolved, it will result in unbalanced output voltage at the MMC terminal, thereby affecting the overall system. Therefore, this paper proposes the use of the direct modulation method that is capable of naturally producing fundamental and DC components of the circulating current within the MMC. These circulating current components are responsible for uniformly distributing the energy between the arms of the MMC and balancing the arm capacitor voltage. The effectiveness of the proposed method is further assessed through real-time simulation in the OPAL-RT (OP5700) environment. The findings of this study validate that direct modulation can maintain the optimal performance of a multiterminal hybrid microgrid based on MMC under unbalanced power conditions without applying additional controllers, thus simplifying the system design, and improving the overall efficiency.