Draft:Experimental plastic elongation procedure

The experimental-plastic-elongation method describes a physical-mathematical method that is used to calculate the tensile stresses and sagging of overhead lines, especially when using high-temperature conductor cables.

Known methods for calculating tensile stresses and sagging of overhead lines

Methods for calculating tensile stresses and sag are among the most important tools in the planning and design of overhead lines.

Three basic procedures are known in the literature:^[1]

• The method using a pure linear strain model (LE method)
• The method using a linear strain model with simplified integration of creep and settlement behavior (SPE method)
• The method using a non-linear strain model (EPE method)

All methods are based on the calculation of chain lines and strains in different variants and differ significantly in terms of accuracy and calculation effort.

LE method

In the LE (linear elastic model) method, overhead line conductors are modeled as linear springs with a single modulus of elasticity and a single coefficient of thermal expansion. Plastic deformation due to creep or settling processes is not taken into account.

SPE method

In the SPE (Simplified plastic elongation model) method, overhead line conductors are modeled as linear springs, as in the LE method. In addition, a plastic elongation of the conductor is calculated by adding a typical permanent change in length (normally expressed as an equivalent temperature change).

EPE method

In the EPE (Experimental plastic elongation model) method, overhead lines are modeled as non-linear springs that elongate elastically as a function of tensile stress, plastically as a function of tensile stress and time, and thermally as a function of temperature.

While elastic and thermal elongation can be calculated using simple mathematical formulas, the generally non-linear relationship between tensile stress and elongation from a stress-strain diagram is represented by a fourth-degree polynomial in the case of plastic elongation. The coefficients of the polynomial are determined using tensile stress-strain tests in the laboratory. The conductor core and the outer layer are considered separately.

Reasons for using the EPE procedure

In Germany, the SPE method is usually used to calculate tensile stresses and sags. In the course of the initial use of so-called high-temperature conductor cables (HTLS conductors), it was found that programs for calculating sag and distance of overhead line cables using the SPE method do not adequately reflect the physical properties of HTLS conductors. They have weaknesses in determining the mechanical load at which the load is transferred solely to the core of the conductor (transition point; TP). The creep strain behavior of the conductor, which is expressed by an equivalent temperature increase in the SPE method, is also only approximated.

The EPE method can be used to provide a technically sufficient representation with a corresponding accuracy in the calculation of sags and distances. Here, the tensile stress-strain test data determined during type sample tests are used to create a physically more accurate model of the conductor behavior.^[2]

1. Sag-Tension Calculation methods for overhead lines CIGRE-Studie Nr. 324 Task Force B2.12.3 June 2007
2. Dr.-Ing. Udo van Dyk: Zugspannungs- und Durchhangsberechnung nach dem Experimental-plastic-elongation-Verfahren Cuvillier Verlag, accessed on March 13, 2025