Distribution System Reliability Assessment Incorporating Weather Effects

Electrical distribution systems usually exist in outdoor environments. The weather creates varying degrees of physical stress on system components exposed to fluctuating weather conditions. The failure rate of an element is greatly enhanced in bad weather situations and the likelihood of multiple line failures is much higher in bad weather than in normal weather. The phenomenon of coincident failures of two or more circuits as a result of excessive stresses imposed by weather conditions is designated as failure bunching. Power supply reliability is normally improved by system redundancy and multiple circuit failures severely impact the system reliability. Most customer interruptions are due to problems that arise in distribution systems and a large number of supply outages occur during unfavourable weather situations. Reliability evaluation disregarding weather effects can result in highly inaccurate appraisals. This paper presents an assessment associated with a simple distribution network comprised of two and three line parallel redundant supplies. A series of reliability indices obtained using single state, two state and three state weather models are presented. The results shown illustrate that stress related line failures due to bad weather should be incorporated in practical assessments and that bad weather situations should be divided into at least two categories