The power sector is undergoing dynamic transformation with constantly increasing generation and transmission requirements. However, inadequate and obsolete networks are a key challenge. Even though investments in new transmission lines present an ideal solution, these involve high capital costs, long lead times and difficulties in land acquisition due to right-of-way issues. Therefore, re-conductoring of lines, that is, refurbishment and improvement of the existing lines, is being increasingly viewed as a preferred alternative solution. Further, re-conductoring of the existing lines using high temperature low sag (HTLS) conductors is gaining traction. HTLS lines have higher current carrying capacity, low sag-tension property, low voltage losses, and are quick and easy to install with nearly no modification to the existing towers, making them appropriate for re-conductoring. Further, as opposed to the widely used conventional aluminium conductor steel-reinforced (ACSR) conductors, HTLS conductors can ease the tension in transmission lines, and reduce transmission and distribution losses resulting from additional power.
In May 2016, the Central Electricity Authority (CEA) released the draft guidelines for HTLS conductors and their installations. As per the CEA, HTLS conductors can address the issues of congestion and right of way, and can reduce losses leading to increased power flow per unit. A host of HTLS conductors are available in the market. A look at some of these…
Super thermal aluminium conductor invar reinforced (STACIR) conductors use aluminium-zirconium alloy rods with super-thermal resistant alloy wires, forming the outer layers and inner invar (nickel-iron alloys that react negligibly to change in temperature of lines) steel core. This type of conductor is preferred for uprating the existing transmission lines, as it does not require any modification in the existing tower infrastructure. At the same time, it can carry double the current with maximum sag and tension, which are the same as that of the traditional ACSR conductors of the same size.
Thermal alloy conductor steel reinforced (TACSR) conductors use thermal-resistant aluminium alloys for the outer layers while the inner core is made of galvanised steel. The current carrying capacity of these conductors is about 50 per cent more, have the same sag value as that of the existing ACSR conductors and can be seamlessly integrated into the existing transmission infrastructure.
Aluminium conductor steel supported (ACSS) conductors are made of annealed aluminium wires while the inner core is made of zinc-aluminium mischmetal coated steel wires. These conductors are preferred for re-conductoring as well as new line applications as they have about 63 per cent higher conductivity, can be used to transfer more than double the power rating of ACSR conductors and offer low sag even at temperatures as high as 250 ºC.
These conductors are manufactured using super thermal-resistant aluminium alloy wires with high tensile strength steel core. The nomenclature “gap-type” comes from a small annular cavity filled with grease between the steel core and the first layer of aluminium strands. This helps the tension to be restricted only to the steel core, making it a preferred re-conductoring option. Gap-type conductors have the capability to increase the capacity to almost double as compared to the existing lines.
Aluminium conductor composite core (ACCC) conductors comprise carbon, glass fibre and aluminium layers along with a composite core that is about 25 per cent lighter than the traditional steel core. ACCC conductors can reduce line losses by about 40 per cent and offer power sag as compared to conventional conductors with the same specifications, making them favourable for re-conductoring and new line applications.
Power Grid Corporation of India’s Maharani Bagh project was the country’s first HTLS project and since then, over 12,000 km of HTLS lines have been ordered. In addition, 4,600 km of invar cables, 4,000 km of ACCC cables, 500 km of ACCS and 3,300 km of gap-type conductors have been ordered. Given the rapid expansion of the power sector, and the government’s aim to electrify the remote parts of the country, re-conductoring will play a vital role in the necessary refurbishment of transmission networks.