Growing urbanisation is resulting in space constraints in most countries across the globe, necessitating the installation of more compact transmission lines. Various technological advancements are being made in the transmission conductor industry in an attempt to minimise right-of-way (RoW) issues by increasing the power transmission capacity of conductors. Moreover, new conductors are being deployed for carrying higher currents and allowing higher temperature ratings.

A key growth driver for the adoption of advanced conductor technologies in countries across the globe is the need to upgrade and expand the grid to cater to the growing influx of renewable energy (RE).

Conventional conductor technologies—such as all aluminium conductor (AAC), aluminium conductor steel reinforced (ACSR), all aluminium alloy conductor (AAAC) and aluminium alloy conductor steel reinforced—face several constraints. Conventional conductors cannot be operated at temperatures higher than 100 ºC and it is thereby difficult to acquire RoW for new power lines using these conductors.

Global Transmission takes a look at the recent technological trends in the conductor market.

HTLS conductors

High temperature low sag (HTLS) conductors are characterised by high temperature resistance and greater ampacity than conventional conductors. These can be operated at temperatures above 100°C while exhibiting stable tensile strength and creep-elongation properties. These are used for reconductoring and new lines.

Further, the usage of thermal resistant aluminium alloy or fully annealed aluminium (1350-O) enables high temperature operation without the loss of strength and lower sag. HTLS conductors have 30 per cent more capacity than conventional conductors. The low sag feature in such conductors is of great significance as it reduces the tower requirement.

Various types of HTLS conductors include aluminium conductor steel supported (ACSS), thermal alloy conductor steel reinforced (TACSR), superthermal alloy conductor invar reinforced, ACSS trapezoidal wire, gap-type TACSR, aluminium conductor composite core and aluminium conductor composite reinforced.

The common materials used for making HTLS conductors include INVAR steel [iron and nickel (Fe-Ni) alloy], aluminium-zirconium alloys, annealed aluminium, high strength steel, and metal and polymer matrix composites. HTLS conductors are composed of a combination of aluminium and alloy wires for conductivity and are reinforced by core wires.

However, one of the drawbacks is the high cost associated with the use of HTLS conductors, which is about twice or thrice as much as the cost of ACSR and AAAC conductors. In addition, there are limitations posed by surge impedance loading and greater losses in long transmission lines deploying HTLS conductors. Also, they require non-conventional methods of stringing and a skilled workforce for operations and maintenance. Some utilities have also raised concerns regarding the material behaviour, long-term performance and life expectancy of HTLS conductors.

HTLS conductors are being widely adopted globally.

The first HTLS project in India was implemented by Power Grid Corporation of India Limited (POWERGRID) in 2011, for the line-in line-out of one circuit of the 400 kV double-circuit (D/C) (quad) Dadri–Ballabgarh transmission line at the Maharanibagh substation, where it deployed INVAR-type HTLS conductors.

At POWERGRID, about 34 transmission line reconductoring and new projects (with existing tower design) use HTLS conductors at the 132 kV, 220 kV and 400 kV levels. The utility is also using HTLS conductors for forest corridors and other RoW-constrained areas. The four types of HTLS conductor technologies that have been adopted are INVAR, GAP, aluminium conductor composite core and ACSS.

Paraguay’s state-owned power company, Administración Nacional de Electricidad (ANDE), recently completed the modernisation of the 220 kV Limpio–Villa Hayes line. The scope of work included the replacement of conventional conductors along the 220 kV line with HTLS conductors, which doubled the line’s capacity. The USD4.3 million project was undertaken by Tecnoelectric S.A.

Nepal’s state-owned power company, Nepal Electricity Authority (NEA), under its Asian Development Bank (ADB)-funded Electricity Grid Modernisation Project, has also planned the upgrade of 237 km of 132 kV transmission lines with more efficient HTLS conductors.

Types of HTLS conductors

  • INVAR: INVAR-type conductors are similar to ACSR conductors with respect to their construction, handling and stringing. The core is galvanised and is made up of iron and nickel with a low thermal coefficient of expansion, which is approximately one-third that of steel. At higher temperatures, i.e. beyond the transition temperature, all load is transferred to the core. Hence, the conductor has lower sag as compared to an ACSR conductor. It can be operated at a temperature of up to 200ºC.
  • ACSS conductors: These conductors, which were widely used by American utilities, are similar to ACSR conductors, except that the external strands are aluminium annealed. The aluminium used is pure and hence ductile and soft, which is why the conductor requires more care during handling and stringing. The core is made up of high strength steel and carries most of the load. Hence, the sag is less compared to a conventional ACSR conductor under high temperature. This conductor can also be operated at 200ºC without loss of strength.
  • GAP conductors: These conductors have a core of steel and aluminium strands. A small gap is maintained between the galvanised steel core and the aluminium strands. A GAP conductor is strung by tensioning the steel core and hence has lower sag as compared to an ACSR conductor. It too can be operated at temperatures of up to 200ºC, but it requires special erection techniques while stringing.
  • Carbon fibre composite core conductors: These are fully aluminium annealed and the core is made of a composite material such as glass fibre and carbon. The conductor has less sag due to a low coefficient of thermal expansion and can be operated at temperatures of up to 180ºC. However, it requires special types of dead-end clamps and joints as well as more care while handling and stringing due to the softer aluminium strands. It is able to carry approximately twice as much current as a conventional ACSR cable of the same dimensions. As a result, it is used mostly for retrofitting existing electric power transmission lines without the need for a change in the existing towers and insulators.
  • Metal matrix composite core conductors: These are similar to ACSR conductors, but the core here is made up of a metal matrix comprising aluminium-aluminium oxide fibres. The external aluminium strands are made up of zirconium, a thermal-resistant alloy of aluminium. This conductor can be operated at temperatures of up to 200ºC. However, it is more expensive than the other conductors because of the technology of the composite core metal matrix.

XLPE cables

XLPE cables use cross-linked polyethylene as the main insulating material. Cross-linking inhibits the movement of molecules under the stimulation of heat, which gives these cables greater stability at high temperatures, as compared to thermoplastic materials. XLPE cables can operate at higher temperatures, both under normal loading and short-circuit conditions. These cables have a higher current rating than an equivalent polyvinyl chloride cable.

XLPE-insulated cables are useful in direct current (DC) power transmission. Traditional DC power cables include oil-filled or mass impregnated non-drain cables, which have limitations for long distance power transmission. While the former requires frequent oil refilling, the latter type suffers from low operating temperature. Extruded XLPE cables are increasingly being deployed in new underground transmission throughout the world. Extruded XLPE cables have a high transmission capacity, which is not limited by the route length.

India’s POWERGRID is also deploying 320 kV DC XLPE cables for its Pugalur–Trichur high voltage direct current (HVDC) project. The cable will be partially underground and partially overhead. HVDC extruded XLPE cables can be easily laid in deep oceans as well as on rough terrain over long distances.

The state-owned Egyptian Electricity Transmission Company (EETC) recently contracted China’s TBEA Shandong Luneng Taishan Cable Company Limited as a successful bidder for the 220 kV Zahraa Nasr city interconnection, which entails interconnecting the 220 kV Zahraa Nasr City substation with Cairo East substation and Koraimat substation, through a 5-km-long, 220 kV double-circuit, XLPE underground cable (UGC) each.

Recently, the Danish supplier of power cable solutions, NKT Cables, completed the replacement of four high voltage lines with XLPE cables connecting the power grids of Sweden and Denmark.

High temperature superconductors

High temperature superconductors (HTS) consist of several strands of superconducting wire wrapped around a copper core and a cryogenic cooling system to maintain proper operating conditions. These conductors can carry five to ten times the current carried by conventional conductors. They can transfer more power using the same towers and line corridors. Further, HTS-based devices including cables, fault current limiters, transformers and energy storage solutions can limit over-currents and protect the grid from damage.

They are compact in size and have a lower RoW requirement. Therefore, HTS can be installed in dense urban areas with a high load requirement within the existing RoW. They can also be used to connect two existing substations to create redundancy when transformer addition is not feasible. In addition, the transmission losses associated with HTS are one-fourth of conventional copper/ aluminium conductor losses. Also, electromagnetic radiation is suppressed by the HTS shield. These conductors can operate at a wide voltage range of 5 kV to 765 kV and have lower impedance than conventional conductors.

At present, HTS are largely deployed by global utilities, and not on a large scale. In 2014, RWE Deutschland, Nexans and KIT installed HTS as part of the AmpaCity project in Germany, which aimed to deploy the world’s longest superconductor system to replace inner-city high voltage cables. In India, POWERGRID is considering a demonstration project on the 220 kV HTS cable system in the grid to assess feasibility and operational issues. A successful HTS cable system will facilitate higher power transmission without erecting towers.

Gas-insulated lines

Gas-insulated lines (GILs) have nitrogen and sulphur hexafluoride as the insulating medium as against physical layers for separation in conventional transmission lines. A GIL comprises aluminium conductors supported by sealed tubes pressurised with gas (nitrogen and sulphur hexafluoride in an 80:20 proportion) as the main insulation.

GILs are ideally suited  for metropolitan areas and cities where there is limited RoW for overhead lines. They can be installed under the ground as well as in tunnels or trenches. Underground GILs can also be installed in agricultural areas, leaving the ground viable for growing crops. The installation of vertical GILs is popular in hydropower plants as there is no fire hazard associated with them.

GILs come with several benefits. First, the resistive losses of GILs are lower than of overhead lines and other types of underground cables due to the larger size of conductors and lower resistance. Second, these lines offer greater reliability with no risk of fire and have electromagnetic fields that are 15 to 20 times smaller than those of conventional power transmission systems. Last, GILs are unaffected by high temperatures, high solar radiation and pollution.

Issues related to GILs include deterioration in insulation properties owing to particle contamination and limited protection from seismic activities/earthquakes (in the case of underground lines) and limited maturity of the technology.

GILs have been deployed by a few global transmission and distributions utilities so far. In June 2020, the Swedish-Swiss power and automation technology company ABB, in partnership with SGCC, commissioned the world’s first 1,000 kV ultra high voltage GIL, which was laid in the Sutong tunnel under the Yangtze River located in China’s Jiangsu province. In India, the North Chennai thermal power plant has a 1.5-km GIL, which was supplied by the erstwhile Areva T&D in 2010.

Figure 1: Types of conductors


Conductor technologies are gaining ground particularly in areas with space constraints. Various countries are adopting such technologies to help ensure power reliability. Going forward, the adoption of advanced conductor technologies is expected to be driven by several factors such as growth in power demand, scarcity of land for the construction of new transmission lines, increase in public opposition to RoW, adoption of renewable power sources in far-flung areas requiring higher capacity transmission lines, and the need for more environment-friendly technologies.