This O-Calc Pro WIKI article provides a brief explanation about how the O-Calc Pro software models conductor creep. Warning: It is strongly recommended that users keep the default values within the O-Calc Pro Master Catalog and do not change the values unless you are confident in your understanding of the engineering, and you have good a reference for the required creep coefficient values.

As is described in the O-Calc Pro “Tension Sag Primer” (see http://o-calcpro.com/wiki/wp-content/uploads/2017/05/OCP-TensionSag.pdf), the O-Calc Pro software uses the hyperbolic formulation for modeling of the various conductor and cable catenary curves. The final tensions and sags of these catenary curves depend on both environmental variables and intrinsic material values. Examples of environmental effects include the temperature, ice buildup, and wind pressure.

The amount of the temperature effect on these catenary curves is dependent on the intrinsic material property as determined by the thermal expansion coefficient. This thermal contraction or elongation, depending on the temperature difference, changes the overall length of the catenary curve. Keep in mind that small changes in the length of the catenary curve can have large effects in other parameters associated with the catenary such as the amount of sag and tension.

Conductor Creep or Plastic Elongation

Conductor catenary curves are also affected by another intrinsic property often referred to as metallurgical creep, or just “creep”. Conductor creep is a plastic elongation of the conductor and is highly dependent on the conductor material. For example, aluminum stranded conductors will have a much larger plastic elongation than say steel stranded conductors, and for conductors of multiple materials, such as an ACSR conductor, the plastic elongation is even more complex.

There are multiple factors that take part in the plastic elongation of a cable or conductor. For example, if the conductor is stranded and first put under tension, the stranding may settle into place. In practice, this strand settling is often accounted for by stringing the conductor and letting it rest for a period, say a few days, before clipping the conductors into the insulators. However, this does not help with the metallurgical creep where the material itself (aluminum or steel) plastically elongates over time as the conductor is continuously under tension.

The long term, metallurgical creep (plastic elongation) is a function of the conductor material, tension over time, and temperature over time. This creep continues over the lifetime of the conductor and does tend to decrease over time. While some literature exists on formulations for conductor creep, it is more often empirically derived using stress-strain curves over time and measuring the amount of plastic elongation. The amount of creep depends on the long term tension on the conductor.

The O-Calc Pro Application of Conductor Creep

When O-Calc Pro applies creep to the conductor, i.e. the Creep Coefficient on the conductor is now zero, the elongation of the conductor is modeled as follows:

               ΔL = εL_oT_av

Where ΔL is the change in the catenary length, ε is referred to as the Creep Coefficient, L_o is the catenary length before creep is applied, and T_av is the average long term maximum tension derived from the minimum temperature catenary curve.

The engineering values for conductor creep coefficients, ε, are not readily available since they are dependent on the long term tension of the conductor. In addition, these coefficients are dependent on the material such as aluminum, aluminum alloys, copper, or steel. Typical values for the conductor creep coefficients, ε, range from 1x10^-5 to 1x10^-8 ((in/in)/lbs). The default value for the conductor creep coefficient within the standard O-Calc Pro Master Catalog is zero, which means that conductor creep will not be modeled.