The paper describes a study on the operating conditions and small signal stability of flexible AC transmission lines with controlled series and shunt compensation. It is shown that the installation of controllable shunt compensation devices in addition to the existing series allows to increase the transmission power limit by almost two times, provided that the voltage levels are kept within the permissible limits. The minimally possible optimal composition and location of the controlled shunt compensation devices is established to ensure the operation of power transmission in a wide range of power transmission. A set of measures aimed at increasing the static stability margin is justified. The joint coordination of tuning parameters of the series and shunt control devices is implemented.