International Electrical Engineering Journal (IEEJ) Vol. 2 (2011) No. 2, pp. 543-549 ISSN 2078-2365 543 Abstract – This paper presents the transient stability assessment of two series FACTS controllers such as static synchronous series compensator (SSSC) and interline power flow controller (IPFC). The optimal controllable parameters in each of these devices are based on the largest reduction in the energy function. The controllable parameters were taken from locally available instantaneous data from the power injection model. The control laws based on certain assumptions were applied and its effectiveness has been viewed for power oscillation damping for small and large signals. The bang-bang control strategy is used for IPFC in which the injected voltage magnitudes were kept constant and only one of voltage angles was varied. The results of the test data have been reported and are optimistic. Keywords: CSD, FACTS, SSSC, IPFC, CCT, injection model, Energy Function. I. INTRODUCTION The transmission networks of the present scenario are subjected to increased stress due to continuous growth in demand. Moreover, maintaining system stability is a big threat in order to prevent blackouts. Flexible AC transmission system (FACTS) device are very efficient in reduction of these stresses and encourages better utilization of existing network. FACTS device improve steady state and dynamic performance of the electric network [1], [2]. The shunt FACTS controller mainly improves the voltage profile and is very effective proven damping system because in some cases, it provides negative damping. The energy function analysis is an effective method of stability assessment [3], [4]. The energy function consists of kinetic energy (K.E.) and potential energy (P.E.). The transient energy is therefore addition of potential and kinetic energy. When a fault occurs in the system, the electrical power output of the machine reduces considerably, but the mechanical input during the period of electromechanical transient remains constant. The machine therefore tend to accelerate on account of P a = P m – P e (damping effect not included) and it gains potential as well as kinetic energy. As soon as the fault is cleared, the conversion between the kinetic and potential energy takes place and this result in rotor swing about its synchronously rotating axis [10-17]. Corresponding Author is Akhilesh A. Nimje, School of Electrical Engineering,KIIT University, Bhubaneswar 751024, India Email: buntynimje@yahoo.co.in However if the magnitude of disturbance is not large enough, the rotor settles down to a new stable operating point thus changing the rotor angle where the transient energy diminishes completely. The transient energy during the fault is dissipated during the energy conversion process (K.E. into P.E. and vice versa) during the post fault period provided the system must have adequate damping. In the absence of damping, the system suffers from sustained oscillations. The faster the energy dissipates, quicker the rotor settles at a new stable operating point (SEP). II. BASIC STRUCTURE OF CONTROLLABLE SERIES DEVICE A static synchronous series compensator is regarded as a CSD. It represents controllable reactive impedance, or a controllable reactive voltage source phase shifted by 90 o with respect to the line current. Energy Function Based Transient Stability Assessment of SSSC and IPFC Akhilesh A. Nimje , Chinmoy Kumar Panigrahi , Ajaya Kumar Mohanty