Patent ID: 8779730

Claim:
A method of discharging cell capacitors of a cell based voltage source converter having a set of AC terminals and a set of DC terminals, where said AC terminals each have a selectable AC connection to ground, each DC terminal has a corresponding selectable DC connection to ground, the cells are connected in cascade in a set of parallel phase legs between the DC terminals, an AC terminal is provided at a midpoint of a phase leg, thereby dividing the phase leg into two phase arms, and each cell has at least one branch with series connected switching elements in parallel with a capacitor, the method comprising the steps of: blocking all the cells of the converter, connecting the AC terminals to the corresponding AC connections to ground, connecting a first DC terminal to a first DC connection to ground, where either each AC connection to ground or said first DC connection to ground includes a resistor for forming a first capacitor discharging circuit, said first capacitor discharging circuit also comprising the phase arms between the AC terminals and the first DC terminal, thereby placing the cells of these phase arms in parallel with the resistor, wherein the cells of a phase arm in a formed capacitor discharging circuit are grouped into at least one group, simultaneously switching on, in all cells of a group of cells in a phase arm of a first phase leg, the switching elements to configure corresponding cell capacitors to be connected in series between the AC and DC terminals for discharging the cell capacitors in said phase arm, and sequentially repeating the step of switching on for the cells of the other phase arms of the capacitor discharging circuit until all cell capacitors in the formed capacitor discharging circuit have been discharged.