A multi voltage source converter (VSC) based flexible alternating current transmission system (FACTS) device, such as generalized unified power flow controller (GUPFC) has more control parameters than its successor unified power flow controller (UPFC). Owing to its multi VSC based configuration, GUPFC can concurrently control real/reactive power flows of two or more transmission lines and AC bus voltage magnitude. In this work, PSCAD/EMTDC model of GTO based GUPFC is developed consisting of two series-VSCs and a shunt VSC. Each VSC is designed in detail which is comprised of eight two-level VSCs, switching at fundamental frequency of 60 Hz. The so-called "2 angle method" is adapted to control both magnitude and phase angle of each VSC independently. Stability and the reasonable dynamic performance of GUPFC are ensured by the optimization of GUPFC's six PI controllers using simplex method, which is a multi-parameter non-gradient optimization routine. The model is verified and tested in WSCC 3-machine 9-bus test system in different operating conditions and when the system is subjected to two types of fault. The results have proven that the tuned controllers are stable in all operation modes and GUPFC shows competence in dynamic control of real/reactive power flows of transmission lines and bus voltage. Copyright (C) 2011 Praise Worthy Prize S.r.l. - All rights reserved.