In this article, a combined method is used to improving transient stability. 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 occur. Generator tripping is one of the most effective methods for improving stability in case of serious faults. In this method tripped a number of units of a certain power plant unit for stabilizing the system. In fact, by removing the generator decrease the kinetic energy of the system so that stability can be achieved. In generator tripping, for the above-mentioned, it should reach stability by tripping the least generator possible. Due to its thermal limitations, fixed place of resistor bank and possibility of back swing, the braking resistor is less efficient than generator tripping. In combined method, system stability against severe turbulence is reached through minimization tripping of generator units. In this method, we first decrease intensity of fault by applying braking resistor, and then, for the purpose of improving transient stability, it try to reduce kinetic energy by removing the least possible amount of producing the desired units at the right time.Simulations on 9-bus or 3-generator system were conducted, and satisfactory results were obtained.

One of the most important problems in power systems design is increasing the damping of the swing amplitude and also to increase the critical time in transient stability. One of the best techniques is to use resistor braking. However, their performance in power systems, duration of their activity and their place of installation are of great importance. In this paper by using of some of devices in power electronics and by precies modeling of the network and optimal controller is designed which leads as to our final objective. In fact resistor braking are effectively attract additive energy which is a result of disturbances and help us to improve the stability of overall system.

This paper presents a new composite index to analyze power system transient stability. Contingency ranking in power system transient stability is a complicated and time consuming task. To prevail over this difficulty, various indices are used. These indices are based on the concept of coherency, transient energy conversion between kinetic and potential energy and three dot products of the system variables. It is well known that some indices work better than others for a particular power system. This paper along with test results using two practical 230 kV Sistan and 400 kV Khorasan power system in Iran, and 9 bus IEEE test system demonstrates that combination of indices provides better ranking than a single one. In this paper two composite indices (CI) is presented and compared. One composite index is based on Least Mean Square algorithm (LMS) and other based on summing indices by equal weights. Numerical simulations of the developed index, demonstrate that composite index is more effective than other indices.      

Methods for calculating Available Transfer Capability (ATC) of the transmission systems may be grouped under Static and Dynamic methods. This paper presents a fast dynamic method for ATC calculations, which considers both transient stability Limits and Voltage stability Limits as terminating criteria. A variation of Energy Function Method is used to determine the transient stability limit and the determinant of the Jacobian matrix of the system is used as an index to determine the voltage stability limit. A novel method is used to approximately calculate this determinant. Combining these two methods, an algorithm that calculates ATC, based on both voltage and angle dynamic stability is presented The advantage of this algorithm, besides considering both voltage and angle dynamic stability, is its high speed This-speed of calculation makes the algorithm a perfect candidate to be used in screening contingencies and determining those cases that need to be further analyzed. To demonstrate the validity, efficiency, and the speed of the new method, it is used to calculate ATC for numerical examples with 1, 3, 7 (CIGREE), 10,30 (IEEE) and US (Iowa State) buses.

FACTS technology has considerable applications in power systems, such as; improving the steady state performance, damping the power system oscillations, controlling the power flow, and etc. STATCOM is one of the most important FACTS devices used in the parallel compensation, enhancing transient stability and etc. Since three phase fault is widespread in power systems, in this paper STATCOM is used to improve the transient stability of power system when three phase fault occurred. Neural Network has been used for adjusting the gain of the supplementary controller of STATCOM. The simulation performed in MATLAB / Simulink software. Simulation results showed when STATCOM combines with proposed Neural Network based supplementary controller; the transient stability of power system improves.

The Potential Energy Boundary Surface (PEBS) method is a fast and simple method for transient stability assessment of power systems using the transient Energy Function (TEF) concepts and has no convergence problem. However, in some cases its accuracy is questionable and the critical clearing times obtained by using this method may be less or greater than the true values. To overcome this weakness, an iterative PEBS method is proposed in this paper, in which, the conventional PEBS method is firstly employed to compute an initial value for the critical clearing time, then the post-fault system trajectory is used to calculate a new critical clearing time. The simulation results on a 3-machine 9-bus and the 10-machine 39-bus New England test systems show that the iterative PEBS method gives more accurate estimate for the critical clearing time than that of the conventional PEBS method except for in the cases which the so-called "inter-area" mode stability phenomenon occur. The proposed iterative PEBS method is a fast method and can be used for fast transient stability assessment in energy control centers