The invention concerns a voltage source converter (26) comprising a group of phase legs, at least three connection terminals (AC1, AC2, AC3, DC+, DC−) for connecting the phase legs to power transmission elements, a first group of cells (C1p1, C2p1, C1n1, C2n1, C1p2, C2p2, C1n2, C2n2, C1p3, C2p3, C1n3, C2n3) in each phase leg and a second group of cells (C3p1, C3n1, C3p2, C3n2 C3p3, C3n3). The cells (C1p1, C2p1, C1n1, C2n1, C1p2, C2p2, C1n2, C2n2, C1p3, C2p3, C1n3, C2n3) in the first group are only capable of providing unipolar voltage contributions to the converter and connected for only being capable of such unipolar voltage contributions, while the cells (C3p1, C3n1, C3p2, C3n2 C3p3, C3n3) in the second group are connected to the corresponding cells of the first group and arranged to have bipolar voltage contribution capability.