Abstrak Studi aliran daya yang optimal (OPF) dilakukan untuk mencapai salah satu tujuan yang diantaranya untuk meminimalisasi biaya atau kerugian melalui perhitungan kemampuan transfer daya yang tersedia (ATC) dalam lingkungan yang terderegulasi. Pembangkitan terdistribusi (DG) jenis non- Kata kunci: aliran daya optimal (OPF), kemampuan transfer daya yang tersedia (ATC), optimasi particle swarm (PSO), pengontrol aliran daya terpadu (UPFC) Abstract Optimal power flow (OPF) study is conducted on a power system to achieve one of the following objectives: cost/loss minimization or available transfer capability (ATC) calculation in a deregulated environment. Distributed generation (DG) is a small source of electric power conversion from nonconventional energy sources and Hybrid DGs which often the most cost-effective and reliable way to produce power. The optimality of control variables and minimum value of objective functions in OPF study would definitely change when DGs are interconnected to the grid. The change would be respect to the location, quantity and combination of power injection by DGs. On the other hand, FACTS controllers are effective in utilizing the existing of transmission network which is very important especially in a deregulated system. Unified power flow controller (UPFC), a second generation FACTS controller, is well known for minimizing the cost of generation/losses with a good voltage profile as well as for ATC improvement. This paper conducts a detailed OPF study on a 9 bus system for the above mentioned three objectives to analyze the effect of DGs with and without UPFC. From the results, it is found that hybrid DGs along with UPFC yields better performance in many aspects. Keywords: available transfer capability (ATC), optimal power flow (OPF), particle swarm optimization (PSO), unified power flow controller (UPFC). IntroductionOPF is carried out to optimize the power flow solution of a large scale power system by minimizing one of the selected objective functions: economic costs or system losses. While maintaining an acceptable system performance in terms of generator capability limits and the output of the compensating devices, optimized control parameters are determined [1]. Capitanecu [2] addresses the main challenges to the security constrained Optimal Power Flow computations. The state of the art computational solution for the problem is reviewed and the challenges and the approaches to face them are identified. Bhaskar [3] proposed a hybrid genetic algorithm for solving OPF problem to minimize the fuel cost.