Failure Bunching Phenomena in Electric Power Transmission Systems Roy Billinton Gagan Singh Janak Acharya Power System Research Group Electrical Engineering Dept., University of Saskatchewan Saskatoon, Canada Abstract The physical environment in which a component resides can have a significant effect on the resulting reliability of the system. This is particularly true in electric power systems containing overhead transmission lines. Extreme weather conditions can create significant increases in transmission element stress levels leading to sharp increases in the component failure rates. The phenomenon of increased transmission line failures during bad weather is generally referred to as “failure bunching”. This condition should not be misconstrued as a common mode failure. This is an entirely different phenomenon and one that is important for multi-circuit transmission lines on single tower structures. This paper illustrates the inclusion of weather conditions in the reliability analysis of parallel redundant systems. A series of weather models are presented with application to electric transmission lines. The reliability effects of incorporating common mode failures in multi-circuit tower structures and independent events incorporating normal, adverse and major adverse weather considerations in separated parallel line configurations are illustrated and examined. The applications described in this paper are to electric power transmission lines. The concepts of stress related failure bunching and common mode failures are, however, applicable to a wide range of engineering systems. Introduction Transmission circuits are important elements in an electric power system and exist in two basic forms. They can be located underground in the form of cables or above the ground on appropriate tower structures. Cables normally operate in relatively stable environments while overhead circuits are exposed to a wide range of weather conditions. A usual assumption in generation and transmission system reliability studies is that system component failures are independent and therefore the failure of one component is not related to, or influenced by the failure of another component. A blanket assumption of component independence is inherently optimistic and can lead to quite inaccurate assessments of system reliability in conventional transmission systems. This is illustrated in the paper using a simple two transmission line example. Figure 1 shows two different arrangements for the two three-phase transmission lines. In Figure 1(a), the two circuits are on separate tower structures on the same right of way. In Figure 1(b), the two circuits are located on a single tower structure. This is a very popular 1