The liberalization of the electricity market has generated significant changes in the planning, operation and management of electric power systems. The open accesses for Independent Power Producers (IPP) results in possible dynamic problems due to the small security margin that characterize competitive markets. For this reason, Evaluation of on-line Transients Stability limits is essential. These limits could be evaluated with modern computed measurement instrumentations that measure in real time and high resolution the fields signal waves such as bus voltages, powers, rotor angles etc, and transmits the measured signals to a data processing center. In this paper we demonstrate by simulation calculations the use of the swing equation in the time domain and in the phase plane, at different configurations to evaluate dynamics constrain that can influence the availability of the transfer capability between two areas. The stability limits have to be added to the thermal and the voltage limits and together they bound the available transfer capability from the source bus to sink bus.