A device (2) for control of a power flow in a three-phase ac transmission line (L2, La, Lb, Lc) has, for each of its phases (a, b, c), a transformer (12a, 12b, 12c) with a primary winding (121c) and a secondary winding (122c). The secondary winding is serially connected into the respective phase of the transmission line. A voltage dependent on a controllable part of the voltage between the other two phases of the transmission line is applied to the primary winding of the transformer. The device comprises, for each of the phases of the transmission line, a series circuit with a first (T1a, T1b, T1c) and a second (T2a, T2b, T2c) terminal and a connection point (Ja, Jb, Jc). The series circuit comprises a first reactive impedance element (21a, 21b, 21c) with a fixed reactance connected between the first terminal and the connection point, and a second reactive impedance element (22a, 22b, 22c, 23a, 23b, 23c) with a variable reactance connected between the connection point and the second terminal. One of said terminals is coupled to the respective phase of the transmission line and the other terminal is coupled to a terminal at each of the other two series circuits so that, for all the phases, either the first or the second terminal is coupled to the transmission line. The mentioned primary winding is coupled between the connection points at the other two series circuits.