“…To meet the increasing power system demands especially in security, the application of unified power flow controller (UPFC) [1,2] becomes an alternative. The UPFC is, without a doubt, the most versatile device of the family of flexible AC transmission (FACTS) devices, since it is able to control the active and the reactive power respectively, as well as the voltage at the connection node, UPFC can be used for power flow control, mitigation of system oscillations and voltage (reactive power) regulation, etc.In the researches of power system security control, some form of transient control due to large perturbations has been hypothesized and its importance has been recognized to deal with instability problem, such as rescheduling of generation (preventive control) [3-5] and arming of load/generation shedding (corrective control) [6][7][8]. Except the instability problem result from the perturbations, the constraint of the power system is also an important issue, such as the frequency deviation constraint and current constraint on the branches.…”