Our research proposed a methodology for detecting transient instability failures in dynamic simulations for Peruvian Center-South Transmission System. Our findings are disconnection time up to 190ms for 500 kV in overhead lines, an interrupted power greater than 32 MW and the type of line according to its voltage (500 kV) [1]. The case study is the disconnection of line 5011 (Chilca Nueva-Fenix), which had a replacement time of 4h 14 min and an interrupted power of 529.6 MW; obtaining a critical angle of 1.26 rad and a time saving of 4.75 s.