Computing the value of security

Power systems are operated with some margin of security to ensure that the most likely losses of plant will not cause interruptions to supply. However, this security margin has a cost that may not be justified by the benefit in terms of prevention of lost load. The authors argue that the level of security could be set on the basis of a cost/benefit analysis that balances the cost of operating the system against the expected outage cost. The expected outage cost is determined using a Monte Carlo simulation of the system operation in which random outages are simulated. Such a simulation should not be limited to small but relatively frequent outages but should also include major disturbances that affect a large part of the system. In order to achieve a sufficient degree of accuracy, the simulation models the actions taken to save the system during emergencies, the duration of the restoration process, and possible protection malfunctions. It is shown that sympathetic trippings due to protection malfunction can have a very significant impact on the probability of major incidents and hence on the value of security.