## 6 Concluding Remarks  
The previous sections have shown that there are many factors that affect the calculation of the \(r_{DC}\) resistance, such as the following:  
* The exact value of the electrical conductivity of the conductor materials, which has a large influence on the \(r_{DC}\) value.
* The temperature coefficient of resistance, which is an intrinsic property of the conductor material that depends on the specific alloy composition and can vary between similar conductors. Its effect is more pronounced in HTLS conductors, due to the wide range of operating temperatures.
* Knowledge of the lay ratio values.
* Deviation of core and outer diameters from datasheet values.
* Uneven temperature distribution along the conductor.
* Differences in \(r_{DC}\) and \(r_{AC}\) values obtained with different formulas suggested by regulatory and internationally recognized organizations.
* The inability to quantify the effect of the contact resistance between the strands, which affects the effective transverse resistivity and the uneven current distribution of the stranded conductors.  
In addition, the calculation of the \(r_{AC}\) is affected by other factors:  
* The inability to obtain explicit and exact equations to determine the effect of eddy currents in stranded conductors and, even worse, in steel core conductors.
* The transformer effect in steel core conductors, which makes the \(r_{AC}\) value dependent on the current level due to the non-linear magnetization of the steel core and the  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Conductor Parameters** & **Value** \\ \hline Number of trapezoidal Al strands & 36 (8/12/16) \\ Measured conductor diameter [mm] & 32.87 \\ Core diameter [mm] & 9.53 \\ Layer thickness [mm] & 3.89 \\ Aluminum area [mm\({}^{2}\)] & 723.975 \\ Equivalent diameter of strands [mm] & 5.06 \\ Lay ratios EN 50182 [14] & 15/13.5/11.5 \\ Minimum conductivity of Al 1350-0 [31] & 61.8\% IACS \\ Measured temperature coefficient of resistance, \(\alpha\) [\({}^{5}\)C\({}^{-1}\)] & 0.00436 \\ Measured \(r_{DC}\) at 20 \({}^{\circ}\)C [u\(\Omega\)/m] & 39.00 \\ Calculated \(r_{DC}\) at 20 \({}^{\circ}\)C [u\(\Omega\)/m] from (3) & 38.55 \\ Calculated \(r_{AC}/r_{DC}\) ratio from (7) & 1.0539 \\ Calculated \(r_{AC}\) at 20 \({}^{\circ}\)C [u\(\Omega\)/m] from (3) + (7) & 40.63 \\ Measured AC resistance per unit length (20 \({}^{\circ}\)C) [u\(\Omega\)/m] & 39.45 \\ Difference between measured and calculated values of \(r_{AC}\) [\%] & \(-\)2.90\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Main parameters of the HTLS (Dhaka ACCC/TW conductor.  
dependence of the magnetic permeability on the current level, temperature, and tensile stress.
* The methods found in the technical literature to determine the \(r_{AC}/r_{DC}\) ratio usually make important assumptions, such as a homogeneous current distribution among the different layers of the aluminum strands, or a homogeneous distribution of the magnetic field in the steel core, which are typically not met by real conductors.
* There are many stranded conductor topologies and materials used for overhead transmission lines. Such conductors present a wide range of operating currents and temperatures, coupled with the impossibility of obtaining highly accurate explicit formulas to determine the \(r_{AC}\) resistance.  
As a result, the AC resistance of multi-stranded conductors, with or without a magnetic core, is therefore a complex multiphysics problem that depends on many parameters that are difficult to know and control, and some of which are likely to evolve with the aging of the conductor. As a result, it is extremely complex to develop accurate analytical formulations to determine the \(r_{AC}/r_{DC}\) ratio of stranded conductors, so it is important to perform laboratory or on-line measurements of \(r_{AC}\) to develop accurate and reliable DLR approaches and conductor models.