In consequence of numerous random and uncertain parameters, the operating condition in the power system constantly changes. As a result, sometimes system blackouts take place, which lead to enormous economic, social and environmental losses. The wish to diminish hem is only atural. To ensure this task, the concept of risk, which reflects a combination of the probability of failure states and the level of their consequences, is used. To minimize risk, it is possible to use two main approaches: either on probabilistic basis or on deterministic basis. If the probabilistic approach is used for estimating the risk level, it is necessary to know the loss function as well as the distribution laws of numerous influencing parameters. When estimating the risk level, the Monte-Carlo method has to be used.