TRANSIENT STABILITY EVALUATION OF MULTIMACHINE POWER-SYSTEMS - A PROBABILISTIC APPROACH BASED UPON THE EXTENDED EQUAL-AREA CRITERION

The probabilistic aspects of transient stability evaluation of electri cal power systems are discussed. The stability properties of the syste m after a given fault are characterised by means of the instability pr obability, which is defined in terms of the critical clearing time and the fault clearing time. Particular attention is devoted to the proba bilistic description of the load demand, characterised by a random vec tor, and its effect on the critical clearing time. The instability pro bability is evaluated by using an analytical approach, already proved satisfactory in the single-machine case, as shown in a previous paper by the authors. The method is illustrated with reference to a simple s ystem: the critical clearing time is calculated, for each value of the random inputs, by means of the extended equal area criterion. Its pro bability distribution is then evaluated, by means of a regression tech nique, in terms of the load demand distribution. Such distribution is then used to calculate the instability probability as a function of th e basic statistical parameters of the system. The proposed method allo ws a straight-forward analytical determination of the influence of the system parameters on the instability probability. A peculiar aspect r egarding a multinode system, which is also analysed in the paper, is t he effect of the statistical correlation among the loads on the stabil ity.