Energy Optimization based Multi-Objective Circulating Current Control of MMC

The modular multilevel converter (MMC) is one of the prominent topologies for high and medium voltage applications. It is known that any voltage mismatch between its phase-legs causes a second-harmonic-dominant circulating-current (CC) to flow in each of MMC arms. This current, if not regulated appropriately, leads to reduced efficiency, larger energy storage requirement and higher current rating requirement for semiconductor switches. A new CC control strategy is proposed in this paper with the aim to reduce the energy storage requirement in MMC without increasing the current rating requirements of its switches. The proposed control strategy is based on the principle of injecting a second- harmonic circulating-current (SHCC) with predefined amplitude and phase-angle. A comparative study involving the proposed and other major CC control strategies is also presented to establish the main features of the proposed principles. Using the proposed control strategy, the energy storage requirements can be lowered by 30% without increasing the current rating requirements of the semiconductor switches. The proposed control strategy is validated and comparatively evaluated using a 13-level, 3-phase, grid-tied simulation model and also using a 7-level, 3-phase, grid-tied MMC experimental setup.