Statistical classification of major event days in distribution system reliability

This paper proposes defining a major event day (MED) in distribution reliability in terms of average frequency of occurrence. This definition is easy to understand for nontechnical people like regulators, seems to be fair to apply to systems of any size, and can be translated into a reliability threshold that can be used to classify individual days. Two possible methods of applying the definition are discussed with an example using real utility data, a bootstrap method, and fitting a probability distribution. Practical issues such as the type of probability distribution to use, how to handle zero-outage days, and calculating normal annual reliability with MEDs removed are discussed and resolved.