Low-voltage fault ride through of the modular multilevel converter in a battery energy storage system connected directly to the medium voltage grid

Nowadays, even small and medium sized power generators are required to ride through low-voltage faults without disconnecting themselves from the grid. To the modular multilevel converter (MMC) in a battery energy storage system based on split batteries (sBESS), this presents a unique requirement: In case of a low-voltage grid-fault, the arm voltages leave their steady-state trajectory which can lead to undervoltage or overvoltage in the modules, provoking an unacceptable shutdown of the whole converter. This paper presents a new control strategy to support the arm voltages by making use of the split batteries in case of a fault. The converter can ride through consecutive grid-faults while continuing to supply a current to support the grid and can immediately resume operation after the fault is cleared.