The paper is devoted to studying the possibilities of using Wide Area Measurement Systems (WAMS) data in the control of electric power systems to increase transient and small-signal stability. The authors present a developed North-West – Center – Belarus intertie model with corresponding system connections. It is confirmed that intertie has rather low indices of small-signal oscillatory and transient stability. To improve reliability of the power system, traditional frequency control is supplemented by feedbacks of phasor difference and its second derivative measured in different parts of interconnection. Its operation was tested at several large power plants during short circuits at various points of the system. It is shown that transient stability increases by more than four times, and in addition, small-signal oscillatory stability grows by more than three times. In a number of cases turbine control is replaced by action of battery energy storage systems (BESS) operating in post-emergency situations. Various options for its control are considered. It is shown that transient stability of the intertie with exponential controlled BESS increases up to six times.