Patent ID: 8138632

Claim:
A method for controlling a converter having at least two phase modules, each phase module having a first and a second valve branch wherein each of the first and second valve branches has at least three series-connected two-pole subsystems, in the event of a failure of at least one subsystem of a valve branch, the method comprising the steps of: identifying a failed valve branch of a phase module having at least one failed subsystem, said at least one failed subsystem located in a first valve branch of a phase module, controlling at least one subsystem in a corresponding first valve branch of each of the others of the at least two phase modules so as to cause a terminal voltage of the at least one subsystem in the corresponding first valve branch of the other phase modules to be zero, controlling at least one subsystem in each of the second valve branches so as to cause a terminal voltage of the subsystems in each of the second valve branches to be equal to a capacitor voltage of the subsystem sequentially controlling the two-pole subsystems of a valve branch of a phase module with control signals having a mutual time offset so as to change an output voltage of the valve branch.