Voltage source converter generating a pulse train using two voltage levels

1. A voltage source converter configured to generate a pulse train using two voltage levels, the voltage source converter comprising:

a first converter arm coupled between a junction and a first DC terminal having a first voltage level;

a second converter arm coupled between the junction and a second DC terminal having a second voltage level, wherein at least one of the first and second converter arms comprises a group of cells;

a string of capacitors coupled between the first and second DC terminals;

a control unit configured to control the group of cells used in a transition between the first and second voltage levels for commutating a current running through a corresponding one of the first and second converter arms involved in the transition;

wherein, when controlling the group of cells, the control unit is configured to obtain values of a current running between the string of capacitors and a corresponding one of the first and second converter arms and handle a cell of the group of cells selected to be used last for reaching one of the first and second voltage levels, the handling comprising inserting or bypassing the cell when the current between the string of capacitors and the corresponding one of the first and second converter arms is at a zero level such that resonance in the voltage source converter is limited.

2. The voltage source converter according to claim 1 , wherein capacitors of the group of cells are coupled in parallel with a branch of capacitors during the commutation;

wherein the branch of capacitors comprises at least one of the capacitors of the string of capacitors and the control unit is further configured to determine a difference between the voltage of the branch of capacitors and a sum of voltages provided by the group of cells; and

wherein the cell is handled when the voltage difference is negative.

3. The voltage source converter according to claim 1 , wherein the cell that is to be used last in a specific transition between the first and second voltage levels is selected by the control unit when the transition is ongoing.

4. The voltage source converter according to claim 1 , wherein the control unit is further configured to sort the group of cells according to a cell voltage balancing scheme so that the cell being inserted or bypassed is the last cell of a cell sequence obtained through the sorting.

5. The voltage source converter according to claim 4 , wherein the cell that is to be used last in a specific transition between the first and second voltage levels is selected by the control unit before the transition has been initiated.

6. The voltage source converter according to claim 5 , wherein the current between the string of capacitors and the group of cells is obtained before transition has been initiated and the control unit is further configured to predict when the current running between the string of capacitors and the corresponding one of the first and second converter arms will reach zero based on the obtained values and the cell sequence according to the cell voltage balancing scheme and handle the selected cell at a predicted zero level.

7. The voltage source converter according to claim 1 , wherein the first and second converter arms comprise the group of cells and the last cell being handled comprises a last cell being inserted in the first converter arm.

8. The voltage source converter according to claim 1 , wherein the first and second converter arms comprise the group of cells and the last cell being handled comprises a last cell being bypassed in the second converter arm.

9. The voltage source converter according to claim 1 , wherein the string of capacitors comprises a complete link of capacitors.

10. The voltage source converter according to claim 1 , wherein each cell comprises a first main switch, a first auxiliary switch and a snubber capacitor; and

wherein the first main switches of the group of cells are configured to switch between the first and second voltage levels and the first auxiliary switches of the group of cells are configured to couple snubber capacitors to introduce a positive or a negative slope in the transition between the first and second voltage levels.

11. The voltage source converter according to claim 10 , wherein each of the group of cells further comprises a first damping resistor between the first auxiliary switch and the snubber capacitor.

12. A method of limiting resonance in a voltage source converter that generates a pulse train using two voltage levels, the voltage source converter comprising a first converter arm coupled between a junction and a first DC terminal having a first voltage level, a second converter arm coupled between the junction and a second DC terminal having a second voltage level and a string of capacitors coupled between the first and second DC terminals, wherein at least one of the first and second converter arms comprises a group of cells, the method comprising:

controlling the group of cells used in a transition between the two voltage levels for commutating a current running through a corresponding one of the first and second converter arms involved in the transition, wherein controlling the group of cells comprises:

obtaining values of a current running between the string of capacitors and a corresponding one of the first and second converter arms; and

handling a cell of the group of cells selected to be used last for reaching one of the voltage levels, wherein the handling comprises inserting or bypassing the cell when the current between the string of capacitors and the corresponding one of the first and second converter arms is at a zero level such that resonance in the voltage source converter is limited.

13. The method according to claim 12 , wherein capacitors of the group of cells are coupled in parallel with a branch of capacitors during the commutation;

wherein the branch of capacitors comprises at least one of the capacitors of the string of capacitors; and

wherein the method further comprises determining a difference between the voltage of the branch of capacitors and a sum of voltages provided by the group of cells, wherein the last cell is handled when the voltage difference is negative.

14. The method according to claim 12 , further comprising sorting the group of cells according to a cell voltage balancing scheme so that the last cell being inserted or bypassed is the last cell of a cell sequence obtained through the sorting.

15. The method according to claim 12 , wherein the cell that is to be used last in a specific transition between the two voltage levels is selected when the transition is ongoing.

16. The method according to claim 12 , wherein the cell that is to be used last in a specific transition between the two voltage levels is selected before the transition has been initiated.

17. A voltage source converter comprising:

a first converter arm coupled between a junction and a first DC terminal having a first voltage level;

a second converter arm coupled between the junction and a second DC terminal having a second voltage level, wherein at least one of the first and second converter arms comprises a group of cells;

a string of capacitors coupled between the first and second DC terminals;

a control unit;

a non-transitory memory storing computer program code that, when executed by the control unit causes the control unit to control the group of cells used in a transition between the first and second voltage levels for commutating a current running through a corresponding one of the first and second converter arms involved in the transition by:

obtaining values of a current running between the string of capacitors and a corresponding one of the first and second converter arms; and

handling a cell of the group of cells selected to be used last for reaching one of the first and second voltage levels, wherein the handling comprises inserting or bypassing the cell when the current between the string of capacitors and the corresponding one of the first and second converter arms is at a zero level such that resonance in the voltage source converter is limited.

18. The voltage source converter according to claim 17 , wherein the first and second converter arms comprise the group of cells and the last cell being handled comprises a last cell being inserted in the first converter arm.

19. The voltage source converter according to claim 17 , wherein the first and second converter arms comprise the group of cells and the last cell being handled comprises a last cell being bypassed in the second converter arm.

20. The voltage source converter according to claim 17 , wherein the string of capacitors comprises a complete link of capacitors.