Improving of Transient Stability of Power Systems By supplementary Controllers of UPFC Using Different Fault Conditions

Long distance AC transmission is often subject to stability problems , which limits the transmission capability.Large interconnected power systems often suffer from weakly damped swings between synchronous generators and subsystems.This paper presents two methods , based on the use of nonlinear system model , to improve transient stability and damping of power swings using UPFC. A statevariable control strategy has been derived using local available signals of real and reactive power. The results of simulation tests of two control methods, undertaken using a small multi-machine system model, have been presented. The simulations show that the methods make good results in different fault places and clearing time. The results present that controlling series part of UPFC can improve transient stability by state-variable control method, but simultaneous control of shunt and series part of UPFC can damp voltage oscillations by direct Lyapunov method. Key-Words: Transient stability, Local measurement, Flexible ac transmission systems (FACTS) , UPFC , Power oscillations damping.