A method for operating a power electronic converter device for an electrical power conversion system, the power electronic converter device comprising a converter circuit including an input side with input terminals, an output side with at least one AC output terminal, a first converter with semiconductor devices connected to the input terminals and at least one second converter connected between an AC output of the first converter and the AC output terminal, said second converter comprising at least one floating cell with a DC intermediate circuit and semiconductor devices; the method comprising: switching the semiconductor devices of the floating cell at switching instants determined with optimized pulse patterns or carrier based pulse width modulation; determining a fundamental voltage component for the floating cell, which fundamental voltage component depends on a difference between an actual voltage V C AF of the DC intermediate circuit of the floating cell and a reference value V C AF * for the voltage of the DC intermediate circuit; generating the fundamental voltage component in the actual voltage of the floating cell by modifying the switching instants, such that a voltage V C AF of the DC intermediate circuit is lying in a given reference voltage range; switching the semiconductor devices of the first converter at switching instants determined with optimized pulse patterns or carrier based pulse width modulation; generating a fundamental voltage component in a voltage output by the first converter by modifying the switching instants applied to the first converter, wherein the fundamental voltage component in the actual voltage output by the first converter is determined in dependence of the fundamental voltage component determined for the floating cell; wherein the fundamental voltage component of the voltage at the first converter is modified, such that the fundamental component of the voltage at the output terminals is kept constant; and wherein the fundamental voltage component generated by the floating cell is cancelled out by the fundamental voltage component generated by the first converter. switching the semiconductor devices of the floating cell at switching instants determined with optimized pulse patterns or carrier based pulse width modulation; determining a fundamental voltage component for the floating cell, which fundamental voltage component depends on a difference between an actual voltage V C AF of the DC intermediate circuit of the floating cell and a reference value V C AF * for the voltage of the DC intermediate circuit; generating the fundamental voltage component in the actual voltage of the floating cell by modifying the switching instants, such that a voltage V C AF of the DC intermediate circuit is lying in a given reference voltage range; switching the semiconductor devices of the first converter at switching instants determined with optimized pulse patterns or carrier based pulse width modulation; generating a fundamental voltage component in a voltage output by the first converter by modifying the switching instants applied to the first converter, wherein the fundamental voltage component in the actual voltage output by the first converter is determined in dependence of the fundamental voltage component determined for the floating cell; wherein the fundamental voltage component of the voltage at the first converter is modified, such that the fundamental component of the voltage at the output terminals is kept constant; and wherein the fundamental voltage component generated by the floating cell is cancelled out by the fundamental voltage component generated by the first converter.