Although apparently erroneous, negative effects on transient stability of emergency controls, such as generator tripping, load shedding and rapidly increasing excitation, follow natural laws. These important phenomena are uncovered, and their mechanisms are studied with extended equal-area criterion (EEAC) in this paper. The effects are quantitatively assessed, and their necessary and sufficient conditions are derived as well.

A unified approach has been proposed to overcome the fatal weakness of the AESOPS algorithm. The EEAC method provides a direct way of estimating both real and imaginary parts of the eigenvalue and identifying the dominant cluster of machines related to a swing mode. Moreover, a very simple Newton-Jacobian with an analytical scarlar formula which considers the real and imaginary parts simultaneously is derived in the multimachine state space. Some heuristic rules are proposed to improve the convergence characteristics. Extensive. simulation results on large systems up to 102-machine are presented in this paper to demonstrate the effectiveness and robustness of the improvements

A method is proposed for identifying the critical cluster of machines, i.e. the machines responsible for loss of synchronism in a power system following a large disturbance. it is based on the conjecture that the loss-of-synchronism condition can be recognised by, first, considering 'near-critically cleared trajectories' of the machines and, secondly, observing how they are organised near the system's unstable equilibrium point. The term 'near-critically cleared trajectories' is meant to imply the swing curves of the system in the postfault-phase and is computed for a fault-clearing time that is slightly larger than the actual critical clearing time. To realise the above, the method uses the extended equal-area criterion. This direct criterion makes it possible to assess a convenient clearing time for computing the swing curves; to determine the system's unstable equilibrium point with great ease; and to select the critical machines by observing them at the time corresponding to this unstable equilibrium point. It also makes the speed of critical-cluster identification compatible with real-time requirements. Examples are given using the IEEE test system to illustrate the essential features: reliability in correctly identifying the critical clusters, robustness with respect to its capacity to do so under very stringent conditions, and effectiveness concerning its ability automatically to identify critical clusters of any size, i.e. irrespective of the number of machines that they contain.

A study was conducted on the extra high voltage (EHV) French power system in order to explore the extended equal-area criterion and test its suitability as a fast transient stability indicator. The assumptions underlying the method are reexamined, causes liable to invalidate them are identified, and indices are devised to automatically circumvent them. The selection of candidate critical machines is also reconsidered, and an augmented criterion is proposed. The various improvements were developed and tested on about 1000 stability scenarios, covering the entire 400 kV system. The severity of the scenarios, resulting from the combination of weakened pre and post-fault configurations, subjected the method to particularly stringent conditions. The simulation results are summarized.< >

A unified approach to transient stability assessment of multimachine power systems is presented. It originates from the equal-area criterion applied to a two-machine equivalent of the system, combined with an analytical procedure totally free from any step-by-step numerical integration. The extended equal-area criterion thus devised provides analytical tools for transient stability analysis and sensitivity assessment. In turn, the latter furnishes transient stability limits and paves the way to control. Means to embed these transient stability techniques in a unified security assessment context are developed. Applications to system planning, operation planning and real-time operation are investigated and many practical uses are suggested. The method is scrutinized by a systematic exploration of 10 different power systems and many contingency scenarios, resulting in over 2500 simulations.

A new direct method for transient stability assessment was recently developed on the basis of a dynamic equivalence and a modified Taylor expansion, combined with the equal area criterion. This Extended Equal Area Criterion is a simple, ultrafast method with pure algebraic expressions of both critical clearing time and stability margin. Moreover, analytic sensitivity analysis with respect to generation power was reported. Hence the transient stability domain in the injection space and the transient stability limits of interface flows were formulated.

The extended equal area criterion (EEAC) for online transient stability analysis is considered with the following objectives. The first is to state systematically its main hypotheses and key conditions, justify the former, and suggest means to guarantee the latter. The identification and error analysis of critical machines are among the investigated issues. The second is to scan all possible types of instabilities likely to arise in practice and devise means to treat them. The extension of the EEAC to cases beyond the so-called first-swing stability makes it more robust than all direct methods developed up to now. The third objective is to extract essential information out of a large body of simulations and show that the above improvements and extensions enhance the EEAC accuracy and its capability to work properly even under stringent conditions. Possible EEAC applications are also discussed, and uses of the method as such or as an auxiliary technique for more sophisticated approaches are suggested.< >

A method for online transient stability assessment of large power systems is proposed. It consists of: replacing the multimachine system by a two-machine dynamic equivalent, further amenable to a one-machine-infinite-bus system; reducing the stability problem to a sole algebraic equation, devised from the equal area criterion, or equivalently from the Lyapunov direct criterion; and using this equa...

An analytic sensitivity method is proposed for the transient security assessment of large power systems. It is based on the `extended equal area criterion', and on a related transient stability margin. It is shown that this margin leads to a simple algebraic formulation of sensitivity coefficients. Particular attention is focused on the sensitivity with respect to generation power, and possible applications to power system planning and online operation are suggested. For example, the computation of generation and load supply limits provides transient stability constraints and thence means of online transient security assessment and preventive control. Within the limits of the simplified electromotive force behind the transient reactance model, the method proves to be accurate, demanding very little with respect to computing time and fairly independent of the system size. The simulation results reported corroborate these conclusions