Patent ID: 8848407

Claim:
A method in a voltage source chain-link converter comprising one or more phases, each phase comprising a number of series-connected full bridge converter cell modules comprising four valves, each valve comprising a semi-conductor switch, the four valves being connected in a full H-bridge arrangement with a capacitor unit, each semi-conductor switch having a respective free-wheeling diode connected in parallel thereto, each full H-bridge arrangement comprising a first phase-leg and a second phase-leg, the method comprising the steps of: detecting a failure of a position of one of the converter cell modules, the failed position comprising a failed semi-conductor switch, failed free-wheeling diode or failed gate unit; blocking the first phase-leg comprising the failed position; discharging to zero the capacitor unit of the converter cell module comprising the failed position; and providing a current path for a phase current through the converter cell module with the failed position, the current path comprising a first branch, in turn comprising corresponding positions of the first phase-leg and the second phase-leg in the full H-bridge arrangement, the first branch being connected in parallel to a second branch comprising the failed position, the second branch comprising two remaining positions of the full H-bridge arrangement, wherein the phase current is shared between the first branch and the second branch.