International Review on Modelling and Simulations (I.RE.MO.S.), Vol. 4, N. 4 August 2011 Manuscript received and revised July 2011, accepted August 2011 Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved 1690 Improving Transient Stability Using Combined Generator Tripping and Braking Resistor Approach Mostafa Eidiani 1 , Mohammad Ebrahimean Badokhty 2 , Mahdi Ghamat 2 , Hossein Zeynal 3 Abstract – In this paper, improving in transient stability is sought through adevelopment of combined approach. Since, in power systems, the maximum use of existing capacities along with the increased powers transferred through the transition lines make transient stability studies even more important. When the fault occurs, the kinetic energy of system is increased and if the system kinetic energy exceeds a certain amount, system instability will meet. Generator tripping is also one of the most effective methods of improving stability in case of severe faults. In this method, we trip a number of units of a certain unit to stabilize the system. In fact, siftingparticular set ofgenerators, it will decrease the kinetic energy of the system so that stability can be achieved. In generator tripping, for such reasons, the system has to maintain stability where lest number of units can be possibly blown out. Due to its thermal limitations, fixed place of resistor bank and possibility of back swing, however the braking resistor is less efficient than generator tripping. Inour proposed combined method, system stability against severe turbulence is tackledwith minimum tripping of generator units.At this proposal, the intensity of faultwill be valuably lessen by applying braking resistor, and then, for the purpose of improving transient stability, the kinetic energy is reduced by removingcertain unit at the right time.The approach has been tested on 9-bus with 3-generator system to demonstrate promising effects. Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved. Keywords:Transient Stability, Generator Tripping, Braking Resistor, Energy Function Nomenclature V Energy function ܯ ௜ Inertia constant of generator i  ௠௜ Mechanical input power of generators  ௘௜ Electrical power output of generator ߜ ଴ Vertex ߠ ௜ Generator's angle to vertex ߠ ௜௦ The angle at the stable equilibrium point ሺ .ݏ. ݌ሻ ߠ ௜௖ The angle at the instable equilibrium point ሺu. e. pሻ  ௖ Potential energy at instable equilibrium point  ௞ Kinetic energy of the system at fault time  ௄ ′ Kinetic energy after tripping a number of generators of unit i ݐ ଴ Fault time ݐ ௖ Critical time  ௄ ௦௛௘ௗ Total kinetic energy of generator units required to be shed ߙ Percentage of remaining generators of power plant unit j after tripping ߚ Percentage of production, the loss of unit j will be specified ܧ  ௜ Voltage behind the transient reactance of straight shaft of i th generator  ௄ ൌ | ௄ | ௝ఏ  Voltage of kth bus with no generator attached   Matrix of reduced reactance with elimination of all nodes except for the generator's internal nodes  ௦ ሺݐሻ Power absorbed by the resistor | ௄ | Size of voltage of the bus bar on which the brake resistor is installed I. Introduction Dynamic systems are designedto operate in stable conditions. At this point, the system must satisfactorily function and remain stable at all times, especially when a severe disturbance takes place onto the system which can be hedged by drawing sufficient safety margin [1]. Assuming that the system is working within one of its stable modes, if the system eventually returns to its equilibrium condition after a disturbance, the system is said to be stable [2]. If it converges to another equilibrium condition close to the former equilibrium point, it is similarly recognized as a stable working condition for the given dynamic system, and the system is known to be instable if the variables of the system diverge from the equilibrium point over time span. In short, the stability of power systems is, in fact, the tendency of the power system to create recovery forces