Patent ID: 7764523

Claim:
A method of operating a converter circuit, wherein the converter circuit has a converter unit with a multiplicity of controllable power semiconductor switches and the converter unit is connected at the AC voltage end to an electrical AC voltage network, wherein the controllable power semiconductor switches are controlled by means of a control signal, wherein the phase voltages (U Na , U Nb , U Nc ) of the AC voltage network are determined, the negative sequence amplitude of the phase voltages (U Nn,A ) is calculated, the Park-Clarke transformation amplitude of the phase voltages (U Ndq,A ) is calculated, the negative sequence amplitude of the phase voltages (U Nn,A ) is monitored at a settable value, the Park-Clarke transformation amplitude of the phase voltages (U Ndq,A ) is monitored at a settable value, the controllable power semiconductor switches are locked by means of the control signal if the negative sequence amplitude of the phase voltages (U Nn,A ) exceeds the settable value or if the Park-Clarke transformation amplitude of the phase voltages (U Ndq,A ) falls short of the settable value.