* Corresponding author: Y. Oubbati, Laboratoire d'Analyse et de Commande des Systèmes d'Energie et Réseaux Electriques (LACoSERE), Université Amar Telidji de Laghouat. Algérie. BP 37G Laghouat 03000, Algeria E-mail: y.oubbati@lagh-univ.dz 1 Laboratoired'Analyseet de Commande des Systèmes d'Energie et Réseaux Electriques (LACoSERE). Université Amar Telidji de Laghouat. Algérie 2 Electrical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Copyright © JES 2016 on-line : journal/esrgroups.org/jes Yousef Oubbati 1, * , Salem Arif 1 , Mohammad Ali Abido 2 J. Electrical Systems 12-4 (2016): 672-686 Regular paper                                                                 !  "                       !   # !              $    $ %&’   !((( )*&                 Keywords: Power System Stability, Transient Stability Constrained Optimal Power Flow (TSCOPF), Improved Particle Swarm Optimizer (IPSO), Optimal Power Flow (OPF), Power System Contingencies. Article history: Received 8 May 2016, Accepted 15 September 2016 1. Introduction Optimal power flow (OPF) is an essential tool for power system operation and planning. The main purpose of an OPF problem is to ensure the economic operation of a power system by setting the control variables such as generator voltages and power outputs, transformer taps, and switchable capacitors. However, the optimal solution obtained from the OPF should ensure the stability of the system under credible contingencies. If the system is transiently unstable under one of the disturbances, the OPF solution must be revised. In the conventional OPF, the solution takes into consideration the static constraints that it is not always powerful in the case of the occurrence of some contingencies. Besides, a large amount of financial loss is due to synchronism in the power system that has been reported in many countries [1]. Recently, due to several blackouts caused by transient instability, dynamic constraints are added into OPF formulation to guarantee the transient stability of the power system against possible contingencies. As a result, OPF formulation presented by adding the transient stability constraints into the conventional OPF problem called transient stability constrained optimal power flow (TSCOPF) [2][1]. It involves the dynamic security and economic operation of power systems, TSCOPF problem which has been extensively investigated and addressed in [3]-[4]. The TSCOPF is considered a nonlinear optimization problem with differential-algebraic equations (DAE) that cannot be solved by employing