an apparatus and method are disclosed for controlling the power flow in an electric transmission system including at least two transmission lines , each transmission line having two ends and carrying alternating current at a selected transmission line voltage and fundamental frequency between the ends . at least two alternating voltages at the fundamental frequency of the alternating currents flowing in the transmission lines , with variable magnitude and controllable phase angle relative to the transmission line current , are generated and coupled in series with the transmission lines . the magnitude and phase of each generated voltage are controlled selectively and individually to adjust the effective reactive and real impedance of each transmission line , to control the real power transmitted between the two ends of the transmission line and , concurrently , to control the amount of real power individually transferred between the transmission lines .