Control of MMC during fault

1. A method of 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 a plurality of phase legs, wherein each phase leg comprises a plurality of series connected converter cells, the method comprising:

determining 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, reducing 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;

determining that the fault has been cleared, whereby the reduced voltage in the power grid is returned to the nominal voltage in the power grid; and

for each of the phase legs of the MMC, increasing the reduced DC voltage back to the nominal DC voltage.

2. The method of claim 1 , wherein the predetermined amount is within a range of 5-20% of the nominal DC voltage.

3. The method of claim 1 , wherein the reducing includes updating a voltage reference for each phase leg.

4. The method of claim 1 , wherein the MMC is in double-wye configuration.

5. The method of claim 1 , wherein the MMC is configured for a railway intertie.

6. A computer program product embodied on a non-transitory computer readable medium and comprising computer-executable components for causing a control arrangement of the MMC to perform the method of claim 1 when the computer-executable components are run on processing circuitry comprised in the control arrangement.

7. The method of claim 1 , wherein the MMC is in delta configuration.

8. 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.

9. The control arrangement of claim 8 , wherein the predetermined amount is within a range of 5-20% of the nominal DC voltage.

10. The control arrangement of claim 8 , wherein the control arrangement is operative to reduce the DC voltage of the phase leg by the predetermined amount by updating a voltage reference for each phase leg.

11. The control arrangement of claim 8 , wherein the MMC is in double-wye configuration.

12. The control arrangement of claim 8 , wherein the MMC is in delta configuration.

13. The control arrangement of claim 8 , wherein the MMC is configured for a railway intertie.

14. 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.

15. The method of claim 14 , wherein the predetermined amount is within a range of 5-20% of the nominal DC voltage.

16. The method of claim 14 , wherein the reducing comprises updating a voltage reference for each phase leg.

17. The method of claim 14 , wherein the MMC is in double-wye or delta configuration.

18. The method of claim 14 , wherein the MMC is configured for a railway intertie.

19. A computer program product embodied on a non-transitory computer readable medium and comprising computer-executable components for causing a control arrangement of the MMC to perform method steps of the method of claim 14 when the computer-executable components are run on processing circuitry comprised in the control arrangement, the method steps including recognizing the fault, reducing the DC voltage of the phase leg, recognizing that the fault has been, and increasing the reduced DC voltage back to the nominal DC voltage.

20. A control arrangement for controlling the MMC of claim 14 , the control arrangement including processing circuitry and data storage storing instructions executable by the processing circuitry to cause the processing circuit to perform method steps of the method when the instructions are executed by the processing circuitry, the method steps including recognizing the fault, reducing the DC voltage of the phase leg, recognizing that the fault has been, and increasing the reduced DC voltage back to the nominal DC voltage.