The creation of the market for electric power causes the problems, connected with an increase in the effectiveness in the use of the existing intersystem connections and guarantee of their static and dynamic stability. With one of the methods of solution of these problems to eat use of the controlled or flexible systems, whose regime can be established independent of operating conditions of the combinable systems and is controlled according to the assigned law. Flexible systems ensure control of the flows of active and reactive power by regulating the parameters of power transmission and have international name FACTS. In the article is given the analysis of control of the 330kV intersystem transfer Estonian Power Plant – Substation Valmiera 330kV in Latvia with the FACTS device on the intermediate substation Tsirgulina 330 kV (Estonia), are determined the parametric domains of FACTS regulation for effective control of the transfer regimes