Failure Bunching Phenomena in Electric Power Transmission Systems

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.