**1. Introduction**

Uninsulated (bare) cables A50 and AC50/8 (hereinafter also referred to as AC50) are mainly used for transmitting electrical energy in overhead electrical networks on land in all macroclimatic regions with either mild or cold climates. The main difference between AC50 and A50 cables is the presence of a steel core in the AC50 ones. Comparative characteristics of the investigated types of cables according to the manufacturer's certificates [1] are given in Table 1. It is obvious that the presence of a steel core in the AC50 cables of the ACSR (aluminum conductor steel reinforced) type significantly increases the bearing capacity to withstand higher loads than the capabilities of the A50 cable belonging to the AAAC (all aluminum alloy conductor) type. The advantages of the A50 cable include its low weight (since it is completely made of aluminum) and a higher current conductivity (slightly higher than AC50 with a close full diameter of the cable conductor).

**Citation:** Levin, A.A.; Narykova, M.V.; Lihachev, A.I.; Kardashev, B.K.; Kadomtsev, A.G.; Prasolov, N.D.; Panfilov, A.G.; Sokolov, R.V.; Brunkov, P.N.; Sultanov, M.M.; et al. Comparison of Structural, Microstructural, Elastic, and Microplastic Properties of the AAAC (A50) and ACSR (AC50/8) Cables after Various Operation Periods in Power Transmission Lines. *Crystals* **2022**, *12*, 1267. https://doi.org/ 10.3390/cryst12091267

Academic Editors: Ulrich Prahl, Sergey Guk and Faisal Qayyum

Received: 12 August 2022 Accepted: 1 September 2022 Published: 6 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).


**Table 1.** Characteristics of A50 and AC50 overhead power-line cables according to Ref. [1].

a Al/Fe section is of 48.2 mm2/8.04 mm2.

Overhead power lines are mainly constructed of supports, cables, and insulators, with cables (wires in them) being the most weak ("vulnerable") element of this system. The expected service life of the cables listed in Table 1 is at least 45 years [1], but it is not limited to this period and, rather, is determined by the technical condition of the wires. The study of the factors that cause the destruction of wires and the prediction of their serviceability is a vital area of research to develop ways to improve their normal operational properties.

There are several reasons for the premature failure of cables of overhead power lines; therefore, let us briefly analyze some of them. The first reason is wire damage and defects, which can occur at any stage of manufacturing of the cables and processing of the cable wires, including continuous casting, extrusion, wire drawing, or stranding. In the papers [2,3], various defects were investigated and the reasons for their appearance were explained. In particular, one of the most common causes is the presence of oxide particles in aluminum conductors (the starting material for wires) and shrinkage porosity. Such defects (nano and micropores, inclusions such as oxides, carbides, etc.) can indeed be stress concentrators and lead to the formation of microcracks with further destruction [4,5] that reduces their service life. Compliance with the required standards in the manufacture of cables is an important and necessary step to ensure a long service life.

Another reason for the premature failure of cables may be a significant increase in the mechanical load due, for example, to the action of a strong wind or ice formations [6,7]. Furthermore, one of the most dangerous types of loads is high-frequency vibrations due to the laminar wind regime, which induces fatigue fretting (fretting deterioration in between conductor strands) [8]. Fretting deterioration residues oxidize and become harder. These hard particles increase the run-off of conductors [2,3,9,10]. For instance, a study to assess the causes of premature fatigue failure of AAAC cables (Brazil) showed that vibration reduction led to a more than threefold increase in residual service life [11]. Fretting fatigue studies have also been performed in laboratory conditions for AAAC cables (e.g., [12–14]) and for ACSR ones as well (e.g., [15,16]).

Environmental conditions are of grea<sup>t</sup> importance during the operation of cables. As is known, overhead power lines are exposed to atmospheric corrosion [17–19]. The aluminum wires that make up the outer coil of A50 and AC50 cables have high corrosion resistance due to the formation of a resistant amorphous oxide film up to ~10 nm thick on the aluminum surface in ambient air [18–21]. Galvanized steel is used as the central element of the AC50 cable inasmuch as zinc coating is an effective way to protect the steel core from direct contact with the aluminum part of the cable [20]. However, existing operating conditions can lead to the destruction of the zinc layer and the development of galvanic corrosion between steel and aluminum. It has been found that the pH of the aqueous medium (water film, air atmosphere) is of decisive importance for the zinc coating corrosion process since it controls the dissolution of the passive oxyhydroxide surface [22]. Therefore, external conditions are of particular importance here, such as the relative humidity of the air, the amount of precipitation, the presence of industrial enterprises, etc. [19,23]. The results of studies of ACSR cables after operation show different results in assessing the integrity of the zinc coating of the steel core and its effect on the degradation of the structure of the aluminum part of the cables. In fact, some studies have shown that no obvious corrosion is found on a steel core under mild climate conditions [24,25]. On the contrary, a significant violation of the zinc coating and corrosion damage of aluminum wires were observed in some studies, although the mechanical strength of these cables (with steel cores) still

remained at a fairly high level [26–28]. In [29], data on cables with steel cores are adduced, according to which the electrical resistance is significantly increased as a result of galvanic corrosion, so that the local corrosion leads to the failure of power lines. A comparative study showed that the rate of galvanic corrosion in ASCR cables was significantly higher than in AAAC ones [30]. Given the various results of studying the effect of galvanic corrosion on the performance properties of cables, various methods are proposed to improve the solution to this problem, from taking the geographical zoning into account [31] to various methods for assessing its impact [32] (because of the peculiarity of the location of the development of galvanic corrosion and the impossibility of its determination during visual inspection).

Ref. [33] discussed the advantages of using AAAC cables (vs. ASCR) due to the lack of galvanic corrosion, lower resistance, and better nominal current in it. As with ASCR cable corrosion, the main challenge is knowing where to start looking in order to identify incipient damage in time for repair. In [34], AAAC and ACSR conductors were studied in real-life conditions, and it has been concluded that the use of AAAC conductors is more efficient than ACSR, since the latter have a noticeably greater weight, which limits their use (when used on relatively weak structures of wooden supports). However, a comparative study of the fatigue characteristics of AAAC and ACSR cables in laboratory conditions showed that the ASCR type had a fatigue life fivefold greater than the AAAC type, which is an important parameter in evaluating the performance of cables [13,35]. A separate study evaluating the fatigue strength of ASCR cables in a dry environment and in a NaCl corrosive environment showed that wear damage was more severe in a corrosive environment [36]. A similar conclusion about the criticality of corrosion abrasion of cables with a steel core was made in [2].

Analysis of the available scientific data has shown that AAAC and ASCR cables have their advantages and disadvantages depending on the operating conditions. In laboratory studies, it is unlikely to be possible to achieve real operating conditions that take the whole range of factors which provoke early destruction of the conductor into account. The complexity of predicting the behavior of wires during their use is due to the fact that it depends on many factors: the structural state of wire elements, the state of the surface layer, environmental conditions, mechanical stress, friction, the presence/absence of galvanic corrosion, and a number of other conditions. Previously [10,37,38], we studied changes in the structure and microstructure of the individual outer wires of A50 cables (AAAC type) as a function of their service life, correlating with changes in their elastic-plastic properties and surface electrical resistance. In this paper, the methods used to study those cables (namely, the acoustic method for measuring elastoplastic properties and complementary methods of energy-dispersion microanalysis (EDX), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), and densitometry) are applied to similar individual outer wires from AC50 cables (ACSR type). The comparison of two types of cables, A50 and AC50, carried out in this work is of interest because the results obtained can be analyzed, taking into account the identity of their operating conditions, particularly the temperature and environment of use (in the Volgograd region of Russia), scale factor (wire/cable diameter), type of stress state, and service lifetime. The main difference is the presence of a steel core in an AC50 cable, which, as the analysis of literature data shows, has its advantages and disadvantages. In this regard, the identification of the characteristics that change most strongly with time may make it possible to identify the parameters that are precursors of destruction. In this article, complementing [10,37], we contribute to increasing the available data on the analysis and predicting the performance of overhead power line cables by considering wires from AC50 cables compared to wires from A50 cables with comparable service lives of 0 to ~20 years.

#### **2. Materials and Methods**
