Cells in a phase leg of a multilevel converter are controlled through regulating, using tap-changer control, the modulation index of the converter to above 0.80, providing a group of carrier waves for the phase leg, where these carrier waves are identical but displaced in time from each other with a time delay, providing a voltage reference for the phase leg being separate from voltage references provided for other phase legs, stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform and control switching of the cells based on comparisons of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside the selected interval.