Voltage balancing in a Modular Multilevel Converter having delta configuration

A Modular Multilevel Converter (MMC) comprising:

• a plurality of phase legs connected in delta configuration, each leg comprising a plurality of series-connected submodules, each submodule comprising an energy storage;
• a circuit breaker for, in a closed position, connecting the MMC to an electrical grid, and for, in an open position, disconnecting the MMC from the electrical grid;
• and
• a control arrangement configured for, in accordance with the circuit breaker being in the open position, for each phase leg:
• setting a voltage reference;
• and
• sequentially selecting submodules in zero state with a sorting algorithm for discharging the energy storages using a circulating current flowing through the plurality of series-connected submodules in each of the phase legs connected in delta configuration, by switching each of the selected submodules to plus or minus state until a voltage deviation from the set voltage reference of the energy storage of each submodule in the each phase leg is within a predefined range;
• wherein the sorting algorithm is configured to perform at least one of following operations in order to balance the submodules such that for the each phase leg, an average direct current (DC) voltage of all submodules in the each phase leg decreases:
• selecting a submodule in zero state with lowest energy storage voltage to be switched to plus or minus state for the energy storage voltage to increase, and
• selecting a submodule in zero state with highest energy storage voltage to be switched to plus or minus state for the energy storage voltage to decrease.