Arrangement for converting between alternating current, AC, and direct current, DC, said arrangement comprising: a number of power transfer modules (24A, 24B, 24C) connected in series between two DC potentials, where one DC potential is the voltage (U d) of a DC power line and each power transfer module (24A, 24B, 24C) comprises a first branch comprising a string of converter cells (CA1, CA2, CB1, CB2, CC1, CC2), wherein a cell provides a voltage contribution of a cell capacitor or zero, said first branch being connected in parallel with a second branch comprising a capacitor (C sa, C sb, C sc) and a secondary winding of a transformer (20A, 20B, 20C), the primary winding of which is connected to a corresponding AC phase (22A, 22B, 22C) of an AC network, the arrangement further comprising at least one control unit (26A, 26B, 26C) comprising an adding unit configured to obtain an AC voltage reference (u vx_ref) and a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term (u d /3) depending on the number of power transfer modules and on the difference between the two DC potentials, and add the DC dependent control contribution to the AC voltage reference for forming a first control expression, and characterised by the at least one control unit further comprising a dividing unit configured to obtain the first control expression, obtain a cell voltage reference sum (u cx_ref) for use in a second control expression, where the cell voltage reference sum is a sum of cell capacitor voltage references provided for a corresponding power transfer module, and divide the first control expression with the second control expression in order to obtain a reference voltage signal (r x) for use in PWM control of the cells of the string of a corresponding power transfer module, where the signal is compared with a triangular carrier wave. a number of power transfer modules (24A, 24B, 24C) connected in series between two DC potentials, where one DC potential is the voltage (U d) of a DC power line and each power transfer module (24A, 24B, 24C) comprises a first branch comprising a string of converter cells (CA1, CA2, CB1, CB2, CC1, CC2), wherein a cell provides a voltage contribution of a cell capacitor or zero, said first branch being connected in parallel with a second branch comprising a capacitor (C sa, C sb, C sc) and a secondary winding of a transformer (20A, 20B, 20C), the primary winding of which is connected to a corresponding AC phase (22A, 22B, 22C) of an AC network, the arrangement further comprising at least one control unit (26A, 26B, 26C) comprising an adding unit configured to obtain an AC voltage reference (u vx_ref) and a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term (u d /3) depending on the number of power transfer modules and on the difference between the two DC potentials, and add the DC dependent control contribution to the AC voltage reference for forming a first control expression, and obtain an AC voltage reference (u vx_ref) and a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term (u d /3) depending on the number of power transfer modules and on the difference between the two DC potentials, and add the DC dependent control contribution to the AC voltage reference for forming a first control expression, and obtain a cell voltage reference sum (u cx_ref) for use in a second control expression, where the cell voltage reference sum is a sum of cell capacitor voltage references provided for a corresponding power transfer module, and divide the first control expression with the second control expression in order to obtain a reference voltage signal (r x) for use in PWM control of the cells of the string of a corresponding power transfer module, where the signal is compared with a triangular carrier wave.