An Analytical Model for the High Temperature Low Sag Conductor Knee Point Determination

Abstract

The modern high voltage power overhead lines operate with high temperature low sag (HTLS) conductors due to possibility of the current capacity increase. HTLS conductors are material and technological advanced solutions. The main advantage of HTLS conductors is a special designed operation conditions which cause the transformation of tensile stresses from the external aluminium base layers to the core. The conditions of this transformation are called “knee point” because a rapid change of the conductor sag - temperature relationship is observed. Prediction of conditions of the “knee point” temperature (KPT) is a key problem during overhead line design. The KPT is a function of different factors like conductor materials properties, conductor design, span parameters, sagging procedures and overhead line exploitation conditions. The paper presents an original theoretical model for HTLS conductors KPT calculations and shows some examples and comparisons of the different conductor designs and parameters.

Sag‐tension evaluation of high‐temperature gap‐type conductor in operation

“…In [24], the authors discuss traditional models used to determine the mechanical behaviour of ACSR conductors, and the authors refer to the KPT as a temperature range instead of an exact temperature. Figure 12 shows that the change in CTE corresponding to an exact KPT is not abrupt but a gradual one.…”

“…The hypothesis considered a residual tension during conductor manufacturing and established this tension as the reason for the increase in the KPT [23]. In [24], it is determined analytically and after several laboratory tests that the difference between the temperature during manufacture and the temperature during the installation of the conductor produces an increase in the KPT as well as a difference between the expected KPT and the actual one [24]. The difference between the actual and expected KPTs is justified with the theory of the compression tensions in the outer layers of aluminium.…”

Investigation of sag behaviour for aluminium conductor steel reinforced considering tensile stress distribution

“…For any wire n in figure 2, the relationship between the strains along the wire axis and the conductor axis is as follows [15]:εwn=εcnsin2⁡βn,where ε wn and ε cn are the strains along the wire axis and the conductor axis of wire n , respectively, and β n is the helix angle of wire n . It is noted that all the wires have the same ε cn , referred to as ε c in the following [6,13]. β n can be calculated by [15]βn=arctan(ln2πRn),

Figure 2Elongations of ACSR and wire.

…”

“…Moreover, there still exists a free axial elongation difference between aluminium wires due to the variant temperature rise of each aluminium wire. However, constrained by the collaborative deformation of the whole conductor, all the wires show an equal axial elongation [ 6 , 13 ]. Consequently, the interaction forces between the wires appear.…”

“…While, for the case of conductor temperature being higher than KPT, all the tension is only borne by the steel core. Thus, the sag behaviour of ACSR is bilinear [ 6 , 7 ]. As a result, the accurate calculation of KPT is required to obtain sag-temperature behaviour.…”