Controllable series line compensation by series flexible ac transmission system (FACTS) devices can provide significant transient stability improvement and effective power oscillation damping. The work presented in this paper gives an overview of control issues of series FACTS devices contributing to stable power system operation when applying suitable controllers with appropriately selected system measurements. Firstly, this paper describes two internal control schemes, which are based on the automatic power flow control mode and line impedance compensation mode, for a switching converter type based series FACTS device. Secondly, it shows that the external (secondary) control can give a solution for overall system optimization by providing well-established supplementary controls and adaptive reference changes to the internal controller. Case studies are presented with simulation results by applying the proposed controllers to the series FACTS device on a power system.