Control of MMC during fault

A control arrangement for controlling a Modular Multilevel Converter, MMC, during a fault in a power grid to which the MMC is connected, the MMC comprising a plurality of phases, each phase comprising at least one phase leg, wherein each phase leg comprises a plurality of series connected converter cells, the control arrangement comprising:

• processing circuitry;
• and
• data storage storing instructions executable by the processing circuitry, whereby said control arrangement is operative to:
• determine that the fault has occurred in the power grid, the fault including a reduced voltage in the power grid that is reduced from a nominal voltage in the power grid;
• for each phase leg of the MMC, reduce a DC voltage of the phase leg by a predetermined amount from a nominal DC voltage to a reduced DC voltage, the DC voltage of the phase leg being a sum of all respective voltages over an energy storage of each cell of the phase leg;
• determine that the fault has been cleared, whereby the reduced voltage in the power grid is returned to the nominal voltage of the power grid;
• and
• for each of the phase legs of the MMC, increase the reduced DC voltage back to the nominal DC voltage.

A method comprising:

• operating a power grid at a nominal power grid voltage;
• recognizing a fault by determining that the power grid is being operated at a reduced power grid voltage that is lower than the nominal power grid voltage;
• in response to the fault, for each phase leg of a modular multilevel converter (MMC) that includes a plurality of phase legs, reducing a DC voltage of the phase leg by a predetermined amount from a nominal DC voltage to a reduced DC voltage, wherein each phase leg comprises a plurality of series connected converter cells and, for each phase leg, the DC voltage is a sum of all respective voltages over an energy storage of each cell of the phase leg;
• after recognizing the fault, operating the power grid at the nominal power grid voltage;
• recognizing that the fault has been cleared by determining that the power grid is being operated at the nominal power grid voltage;
• and
• in response to the fault being cleared, for each of the phase legs of the MMC, increasing the reduced DC voltage back to the nominal DC voltage.