Creep is a phenomenon which affects most materials subjected to stress. It manifest itself by an inelastic stretch (or permanent elongation) of the materials in the direction of the stress. Certain materials such as aluminum are more susceptible than others. For example, steel suffers only a limited amount of creep. The increase in conductor length resulting from inelastic stretch produces increased sags which must be taken into account in the overhead line design and installation process so as not to infringe clearances.
Some mathematical models have now been evolved to help the engineer assess the effects of creep and examples are given below:
mm/km (Formula 1)
mm/km (Formula 2)
where:
ε = permanent inelastic elongation (creep)
K = constant
σ = average stress in the conductor
β, Φ, μ, δ = creep indices obtained by test
e = natural logarithm base
t = time in hours
θ = temperature in ºC
Since the total inelastic strain can be considered as the result of geometric settlement of strands and of the metallurgical creep thereafter, the derivation of the constants and indices is of prime importance. Typical values for the constants are involved in the equations above are given below:
Source: Bayliss, C.R. and Hardy, B.J., Transmission and Distribution Electrical Engineering Third Edition, 2007
