Electricity transmission is the necessary intermediary linking electricity generation and its distribution to the consumers. It is only efficient and effective electricity transmission that ensures high power delivery to target areas. Poor transmission of electricity or drop in voltage across transmission lines has been attributed to a number of factors which comprise those contributed by the nature of the materials used in the development of the conductor and those contributed by the development technique. Electricity transmission loss results in an epileptic power supply or total black out. Research shows that one of the factors contributing to power transmission loss is electrical treeing. This is caused by impurities entrapped in the conductor material and/or mechanical defects imposed on the conductor during the installation, such as abrasion. Electrical treeing manifests as partial discharges or sparks on the conductor when current flows across a portion of the conductor harbouring the entrapment ^[1]. These sparks appear in a tree-like configuration and result in a voltage drop or severe damage to the transmission line. The extreme case of electrical treeing leads to total burning down of the transmission line. The panacea to this defect is the usage of an advanced production technique which produces impurity-free products and the application of advanced hybrid/nanomaterials with high resistance to abrasion and corrosion. Meanwhile, one of the most outstanding conventional transmission conductors is Aluminium Conductor Steel Reinforced (ACSR). Its efficiency in power transmission is affected by the high density of its steel core, high affinity to corrosion of the steel core, and high coefficient of thermal expansion (CTE) of steel materials. These factors limit its current-carrying capacity (ampacity) as well as its cost effectiveness [2,3,4]^[2][3][4]. So, the steel core needs to be replaced with advanced light materials with higher corrosion resistance, higher wear resistance, and lower CTE. It will be recalled that the coefficient of thermal expansion of composite materials is dependent on the thermal conductivity of the constituent elements ^[5] which then determines its sag level when current traverses the transmission line as well as its ampacity. More so, the aluminium conductor composite reinforced (ACCR) is another high-performing transmission conductor in the market. However, Banerjee ^[6] disclosed that the CTE of its metal matrix core (MMC) is relatively high, measuring about 6 × 10⁻⁶ K⁻¹, which makes it susceptible to high sag. This author also revealed that the polymer matrix composite (PMC) used in the production of the aluminium conductor composite core (ACCC), which is another high-performing transmission conductor, can only perform optimally at temperatures below 125 °C, after which degradation ensues. The implication of this is that advanced composites, hybrid or refractory materials need to be used to replace the polymer core that is susceptible to thermal degradation at elevated temperatures.

2. Strengths and Weaknesses of Transmission Conductors

Transmission conductors come in various forms and shapes. Materials used in the production of transmission conductors are numerous. In this section, those materials are discussed, showcasing their strengths and weaknesses. Magnetic properties of materials have negative effect on the electrical conductivity of materials because electrons are repelled from each other for onward transmission/conduction of current. Hence, materials that possess low magnetic properties are required for the development of electrical conductors so that a repulsive force will be of high value. Among all metals, silver has the highest electrical conductivity of 100 on a 0 to 100 scale ranking while copper and gold are ranked 97 and 76, respectively. However, Cu is less expensive and Au is more corrosion-resistant and that is why they are more often applied in electrical conductors than Ag. It is because of the cost effectiveness that Cu became more popular than the other two highly conductive metals. For this reason, Cu also became the international standard conductor to which other conductors were measured. That is why it is called the International Annealed Copper Standard (IACS) from which other electrical conductors are referred. The reference was adopted in 1913 where annealed copper (Cu) was assigned the electrical conductivity of 100% IACS. Till date, Cu cables have been applied in electrical installations of buildings, electrical and electronic gadgets; while Cu-Cu windings are used in power transformers [9,10]^[7][8]. Lloyd and Clement [11]^[9] opined that high density, vulnerability to corrosion, lack of passivation oxides, and its inclination to attacking silicon junctions in electronics have undermined the usage of copper (Cu) cables in electrical applications. Then, the interest shifted to Al conductors as a better replacement for Cu in electrical conductors. Aluminium (Al) has an electrical conductivity range of 21–63% IACS where the conductivity depends on the type of Al and heat treatment it is subjected into.

2.1. All Aluminium Transmission Conductors

Presently, there are about four major types of Al conductors, namely: aluminium alloy conductor (AAC), all-aluminium alloy conductor (AAAC), aluminium conductor alloy reinforced (ACAR), aluminium conductor steel reinforced (ACSR) as shown in Figure 1. AAC consists of many strands of hard-drawn 1350-H19 tempered Al alloy with minimum electrical conductivity of 61.2% IACS. This conductor is utilized in municipal electricity distribution that possess limited spacing and closely positioned supports ^[4]. Its corrosion resistance is appreciable and so used in coastal regions where ice and dews are salty. Meanwhile, it has low strength, and because it uses closely positioned supports, it is not cost effective. These challenges have undermined its use as a transmission conductor. Moreover, AAAC, according to Hesterlee, Sanders [13]^[11] consists of 6201-T81 aluminium alloy which enjoys high corrosion resistance of AAC together with high strength of heat-treated Al. It is applied in power transmission/distribution that requires sparsely located supports such as in valleys and rivers crossing. In addition, it is utilized where there is a corrosion challenge. Its conductivity is about 52.5% IACS which is below that of AAC ^[4]. The third type of Al conductor is the aluminium conductor aluminium reinforced (ACAR) which enjoys the high electrical conductivity of 1350 Al alloy and high strength of 6201 Al alloy to produce a stable conductor of high strength and excellent electrical conductivity ^[4]. It is made up of several layers of 1350-H19 aluminium strands wrapped on 6201-T81 aluminium wires, with a central core made of 6201 Al strands. The flexibility of the design is in such a way that both the outer strands and the core can be interswitched so as to satisfy the demand of the area of application. As 1350-H19 serves as the core and 6201-T18 the outer surface in one region, they can be interchanged in another region based on the demand of the area [13]^[11]. The most versatile Al conductor is the aluminium conductor steel reinforced (ACSR). Hesterlee, Sanders [13]^[11] described ACSR as the traditional transmission conductor which came into use since 1900, and consists of stranded galvanised steel central core enclosed by layers of 1350-H19 Al wire. The quantity of steel in the core determines strength of the conductor and the quantity of steel incorporated in a typical ACSR is in the range of 7–40%. Its application is in very long span crossing such as long rivers and hills. The strength of this conductor is the steel core which equips it to withstand more ice and wind loads. Its ampacity is higher than other all-Al conductors but the sag level is greater. Unfortunately too, the steel core has high density which impacts negatively to the cost of using this TC. In addition, the steel core is highly susceptible to corrosion ^[2]; while its maximum operating temperature is between 95–100 °C. It is due to these challenges identified in the all-Al conventional conductors that high-temperature low-sag (HTLS) conductors were developed. Alawar et al. [14]^[12] opined that as the demand for electricity kept surging at a rate of 25% per decade, while electric transmission facilities are upgraded or maintained at a rate of 4% per decade, that it will be imperative to invent more robust transmission conductors with more durability, and higher ampacity.

2.2. HTLS Transmission Conductors

One of the interesting characteristics of HTLS conductors is that it can be mounted in an existing Al or Cu transmission line and give double of its ampacity; thereby reducing cost, time, and power outage which would have occurred during conventional upgrading of transmission lines. So, HTLS conductors were invented to be used in replacing traditional aluminium or copper conductors in the grid without elaborate modifications. They can operate between 100 °C and 250 °C with very minimal sag and low loss of strength. Their benefits include saving time, minimal labour and low cost [15,16,17]^[13][14][15]. Types, material properties and deficiencies of HTLS conductors are shown in Table 1. The essential properties of HTLS conductors cannot be overemphasized. However, there are still some challenges ravaging them which are itemized in Table 1. For example, ACCR is a high-performing transmission conductor but has relatively high CTE which affects its sag level; and its core which is alumina fibre is consolidated with extrusion. This extrusion technique is liable to contaminate the conductor with cracks, piping and impurities [18]^[16] which can result in the treeing defect witnessed in transmission lines. Thus, the functionality of the TC is usually impaired. Moreover, ACCC is another highly rated transmission conductor but is ravaged with low strength; and its operating temperature is relatively low (130 °C) ^[6]. The production technique is pultrusion which is susceptible to contamination, warped shape, and irregular cross-section [19]^[17]. Studies show that all other steel-based HTLS conductors have high density and susceptible to corrosion. Therefore, it has been seen that the materials applied in the existing transmission conductors, both all-Al conventional conductors and HTLS TCs have deficiency in one way or the other. These deficiencies affect their efficiency, ampacity, durability, and cost effectiveness; and so, constitute the bane of the power grid. Hence, there is the need to research on more robust techniques and materials with more robust characteristics.