Internal Power Balancing of an MMC-Based Large-Scale PV System under Unbalanced Voltage Sags

Modular multilevel converters (MMC)s are capturing the spotlight in large-scale photovoltaic (PV) applications. Studies reported their ability to work under different PV power unbalance conditions; however, no study has covered their operation under voltage sag conditions. This paper proposes a multi-objective internal power balancing control to be applied to a competent PV-MMC configuration to simultaneously deal with both PV power unbalance and voltage sag conditions. The proposed strategy constitutes of two controllers: 1) a circulating current (CC) control which is responsible for balancing the power flow of the MMC under any PV power unbalance condition; 2) arm voltage compensation to deal with the unbalanced voltage sags and ensuring the CCs and submodule (SM) voltages are kept balanced without overmodulation issues. Also, the grid control is designed to keep a balanced current injection to the grid under any condition. The PV-MMC system and the proposed control are designed and tested using Hardware-in-the-Loop (HiL) and experimentally to validate the effectiveness of the control in eliminating any power unbalance under different operation conditions.