An interconnected power system is subjected to load disturbances with rapid change, system frequency may be severely affected and becomes oscillatory. To compensate for such load disturbances and to stabilize the frequency oscillations, the dynamic power flow control of the FACTS devices located in series with the tie line between interconnected power systems, are employed. In this paper an attempt has been made to understand the dynamic performance of Automatic Generation Control (AGC) of a two area multi-unit hydro-thermal power system with Static Synchronous Series Compensator (SSSC) and Thyristor Controlled Phase Shifter (TCPS). The optimal values of the AGC loop integral gain settings are obtained using integral squared error (ISE) technique following a step load disturbance in either of the areas by minimizing a quadratic performance index. Time domain simulations were carried out to study the performance of the power system with SSSC and TCPS and results are discussed. © 2016, UK Simulation Society. All rights reseved.