For that aim, analysis of future system operation is performed by system operators. Since system operation is stochastic in nature, the assessment of power systems should be based on models and techniques that respond to this behaviour, i.e. probabilistic models and techniques (Billinton and Allan, 1996). Probabilistic assessments attempt to quantify the likelihood, frequency, duration and severity of system inadequacy or security breaches (Billinton and Li, 1994).