Abstract
In order to handle the increased uncertainty in future electric power generation and demand, probabilistic models should be used to model the risk when choosing among the different planning alternatives. If probabilistic data are available for the decision variables (e.g. historic observations), then the uncertainties can be described by suitable probability density functions. If probabilistic data are not available, then the planner can instead use her/his experience and knowledge to construct possible scenarios and their associated probabilities. Network calculations can be performed by specific probabilistic load flow algorithms or a more general Monte Carlo simulation approach. The results of these calculations are the stochastic representation of the nodal voltage and branch current variables, through which the technical constraints can be verified with a relative confidence (acceptable risk of violation).