International Journal of Scientific & Engineering Research Volume 4, Issue 1, January-2013 1 ISSN 2229-5518 IJSER © 2013 http://www.ijser.org FACTS (A Revolution for Improvement of Power System Stability) Saurabh Kumar Bajpai Saurabhbajpai897@gmail.com Vira College of Engineer, Bijnor, uttar Pradesh, India Abstract- In this paper we discuss about the facts role for improvement of power system stability. A power system is a combination of generation, transmission and distribution and covers a wide area with number of connections. In such large area no of interruptions occurs which effect the efficiency as well as stability of electrical energy. Key words- Steady state stability, transient stability, Facts (series and shunt controlled), IPC, SSSC, TCSC, SVC, SATCOM Introduction- The main criterion for stability is that the synchronous machines maintain synchronism at the end of the transient period. So we can say that if the oscillatory response of a power system during the transient period following a disturbance is damped and the system settles in a finite time to a new steady operating condition, we say the system is stable. If the system is not stable, it is considered unstable. This primitive definition of stability requires that the system oscillations be damped. This condition is sometimes called asymptotic stability and means that the system contains inherent forces that tend to reduce oscillations. This is a desirable feature in many systems and is considered necessary for power systems. The definition also excludes continuous oscillation from the family of stable systems, although oscillators are stable in a mathematical sense. The reason is practical since a continually oscillating system would be undesirable for both the supplier and the user of electric power. Hence the definition describes a practical specification for an acceptable operating condition. The stability problem is concerned with the behavior of the synchronous machines after a disturbance. For convenience of analysis, stability problems are generally divided into two major categories-steady state stability and transient state stability. Steady State Stability- Small signal stability is the ability of power system to maintain synchronous operation under small disturbances. In large power system, small signal stability problems may be either local or global in nature. Local modes are associated with the oscillations of generating units at a particular station with respect to the rest of system; these oscillations are localized in a small part of power system. Global modes are associated with the oscillations of many machines in one part of the system against machines in the other parts; these oscillations are also called inter-area mode oscillation. In an interconnected power system, the rotors of each synchronous machine in the system rotate at the same average electrical speed. The power delivered by the generator to the power system is equal to the mechanical power applied by the prime mover, neglecting losses. During steady state operation, the electrical power out balances the mechanical power in. The mechanical power input to the shaft from the prime mover is the product of torque and speed, PM= TMω. The mechanical torque is in the direction of rotation. An electrical torque is applied to the shaft by the generator and is in a direction opposite of rotation. When the system is disturbed due to a fault or the load is changed quickly, the electrical power out of the machine changes. The electrical power out of the machine can change rapidly, but the mechanical power into the machine is relatively slow to change. Because of this difference in speed of response, there