A Power Mismatch Elimination Strategy for an MMC-based PV System in Unbalanced Grids

Application of modular multilevel converter (MMC) for grid integration of photovoltaic (PV) generators has gained much attention lately. This paper proposes a modified power mismatch elimination strategy for an MMC-based PV system. Although the existing power mismatch elimination strategy based on internal power flow of the MMC is capable of eliminating power mismatches in a perfectly balanced power system, it will fail to do so if the grid voltage is unbalanced at point of common coupling (PCC). The proposed modified power mismatch elimination strategy, utilizes the internal power flow of the MMC-based PV system to ensure that the converter leg connected to grid phase with higher voltage delivers higher power to the grid and the leg connected to the phase with lower voltage delivers less power to the grid. As a result, the currents delivered to the host grid become balanced and symmetrical in spite of existing power mismatches and unbalanced grid voltage. The effectiveness of the proposed modified power mismatch elimination strategy is demonstrated by time-domain simulations conducted on a model of the PV system in PSCAD/EMTDC software environment.