High voltage direct current, HVDC, converter and a method for handling a fault in such converter

A High Voltage Direct Current, HVDC, converter comprising:

• a plurality of converter valves, wherein each converter valve comprises a plurality of cells electrically connected in series, and wherein each cell of the plurality of cells comprises an energy storage and a switch arrangement, wherein the plurality of cells are operable to convert High Voltage Alternating Current, HVAC, to HVDC or HVDC to HVAC, wherein each cell of the plurality of cells further comprises a mechanical bypass switch configured for bypassing the energy storage and the switch arrangement of the cell, and;
• a control system operably connected to the switch arrangements of the plurality of cells and configured to operate each cell of the plurality of cells in an active mode and in an inactive mode, and
• wherein the control system is operably connected to the mechanical bypass switches and configured to operate each cell in a mechanical bypass mode in which the mechanical bypass switch of the cell is arranged to permanently bypass the energy storage and the switch arrangement of the cell;
• wherein the control system is further configured to:
• operate the switch arrangement of each of the cells operating in the active mode so as to contribute to converting High Voltage Alternating Current, HVAC, to HVDC or HVDC to HVAC while the switch arrangement of each of the cells operating in the inactive mode is arranged to bypass the electrical energy storage of the cell,
• determine an error indication for a cell operating in the active mode and switch the cell from operating in the active mode to operate in the inactive mode in response to the error indication,
• determine a recovery indication for the cell operating in the inactive mode,
• switch the cell from operating in the inactive mode to operate in the active mode in response to the recovery indication,
• determine a fault indication for a cell of the plurality of cells and switch the cell to operate in the mechanical bypass mode in response to the fault indication,
• determine a number of cells of a converter valve of the plurality of converter valves which are being operated in the inactive mode and a number of cells which are being operated in the mechanical bypass mode, and
• trip a converter valve of the plurality of converter valves when a predetermined number of cells of the plurality of cells of the converter valve are operated in the inactive mode or the mechanical bypass mode.