This paper presents the principles and foundations, on which construction and functioning of distributed system destined for optimal control of large power interconnections. Such power interconnections consist of power systems of independent countries and independent markets of electricity. Solution of optimization problems in large power systems according to these principles is carried out as coordinated solution of optimization problems of two levels: optimization problems of subsystems on lower level and optimization problem of upper level – determination of optimal power flows between subsystems. This organization of solution of optimal operation problems forms the basis for creation of efficient distributed control system providing global optimality of large power interconnections and self-determination of independent power systems. Structure and functioning of this control system are also presented in this paper.