Verification of Finite Duration Uninsulated Conductor Rating Theory

The current a conductor can carry for short durations can be significantly greater than under steady state conditions. In power system applications short duration events are usually associated with faults, typically lasting for 100ms up to a few seconds. Power systems use these differences in current levels to discriminate between normal and abnormal operational conditions. High magnitude short duration currents, such as those which occur during power system faults, can significantly heat conductors, potentially resulting in damage or destruction. Specifying minimum conductor sizes potentially subject to these conditions requires an understanding of the physical effects resulting from the consequential stresses, particularly the heating. This paper details the results of testing undertaken to verify the response of conductors to the established approach to rating these conductors, which uses an equation based on an adiabatic system. This equation, Onderdonk’s equation, is demonstrated to have four identifiable limitations. Novel analytical solutions are proposed for two of the four limitations, being skin effect and cooling resulting in an improved equation for rating conductors for high magnitude short duration currents.