A three-level converter and a method for controlling a three-level converter, wherein the third (S31, S32, S33), the fourth (S41, S42, S43) and the fifth (S51, S52, S53) controllable semiconductor switch of a switching branch having, out of all the switching branches, the most positive voltage in its alternating current pole (AC1, AC2, AC3) is controlled to be non-conductive for the whole period of time when the switching branch in question has the most positive voltage in its alternating current pole, and the first (S11, S12, S13), the second (S21, S22, S23) and the sixth (S61, S62, S63) controllable semiconductor switch of a switching branch having, out of all the switching branches, the most negative voltage in its alternating current pole is controlled to be non-conductive for the whole period of time when the switching branch in question has the most negative voltage in its alternating current pole. (FIG. 2)