Transforming uncertainty into security margins

Summary form only given. Power systems are often operated with deterministic security margins determined by experience and by running simulations of selected contingencies. One way to introduce uncertainty into this framework is to model the sources of uncertainty, translate the sources of uncertainty into uncertainty in the margin and then set a margin so that there is a high probability that the system will remain secure. This high probability of security may be chosen. Choosing a higher probability of security will of course determine a larger margin. Consider a transfer capability margin in a situation in which the transfer capability margin limits some profit-making bulk power transaction. Then changes in the transfer capability margin can be translated into changes in profit made on the transfer. If the costs of the security violation can be estimated, this allows the tradeoff of security versus profit to be quantified. The translation of uncertainty into margins outlined above can be solved in a practical way for some security violations and for some sources of uncertainty. We first discuss some solved cases and then consider an unsolved case. We present NERC data showing power tails in the blackout size probability and discuss how this might relate to cascading failure.