## 1 Introduction  
Bare aluminum stranded conductors have been used in high-voltage transmission lines for over 100 years. They consist of multiple layers of aluminum or aluminum alloy strands twisted in opposite directions. In the case of steel-reinforced conductors, the core of the conductor, which is used to increase mechanical strength, consists of several galvanized steel wires [1]. As the world increasingly turns to renewable energy and electrification to meet de-carbonization targets, ensuring the safe, efficient, and reliable operation of power lines is essential [2, 3]. To meet this growing demand, the capacity of existing power lines must be maximized [4], so new approaches are being developed. The replacement of conventional conductors with high-temperature low-sag (HTLS) conductors and dynamic line rating (DLR) strategies based on real-time weather dependent thermal ratings [5, 6] are two methods to maximize the capacity of existing overhead transmission lines. DLR solves the conductor heat balance equation based on an on-line measurement of the current flowing through the conductor and its temperature using appropriate sensors, as well as the local weather conditions, including ambient temperature, solar radiation, and wind direction and speed [7, 8]. Since ohmic losses are the most significant source of heat in conductors, they have a major impact on the thermal behavior of the conductor. For this reason, it is very important to determine the AC resistance of the conductor with a sufficient degree of accuracy. However, several factors affect the electrical resistance of such conductors, such as the conductivity of the conductor material, the cross-sectional area, the lay length of the different aluminum layers, and the presence of a steel core used to reinforce the mechanical strength of the conductor [1]. Among these, the electrical conductivity of the aluminum strands plays an important role. Since conductor standards typically specify minimum electrical conductivity values, changes in this parameter can cause AC resistance to vary between conductors of the same type. There are other unquantifiable effects such as the contact resistance between adjacent strands, which depends on the condition of the contact surface layer. It is determined by the crystalline structure, the oxidation state, or the presence of grease [9]. All of the above factors will affect the uneven current distribution among the different strands, thus influencing the electrical resistance and the thermal behavior of the conductor. In addition, the DC and AC resistances per unit length of stranded conductors, \(r_{\text{DC}}\left[\Omega/\text{m}\right]\) and \(r_{AC}\left[\Omega/\text{m}\right]\), respectively, differ due to the effect of the eddy currents, i.e., the skin and proximity effects, although when the distance between the conductors is much greater than their diameter, the proximity effect becomes much smaller than the skin effect, as commonly occurs in overhead conductors [1]. For steel-reinforced conductors, there are other effects, such as the transformer effect due to the magnetization of the steel core, which make the \(r_{AC}\) dependent on the current. The operating temperature also has an important effect on the electrical resistance [3]. However, the operating temperature of overhead transmission line conductors depends on a thermal balance between heating and cooling effects occurring in the conductor. This paper deals with the determination of the \(r_{AC}\) of stranded conductors for overhead transmission lines in order to obtain an accurate prediction of ohmic losses in such conductors. These results can be very useful for the development of DLR approaches and models of stranded conductors. For this purpose, three types of conductors with different geometry and composition, and thus different thermal behavior, are analyzed. They are a conductor with all strands made of aluminum alloy, an aluminum conductor with a steel core, and an HTLS conductor.