The need to replace ageing and obsolete transmission and distribution infrastructure, high investments, right-of-way constraints and long gestation periods involved in building new networks have brought to the fore the concept of reconductoring. It refers to the upgradation of the existing infrastructure, enabling utilities to augment the quantum and quality of power transmitted through the existing corridors. It also ensures the timely completion of transmission projects, and enable utilities to significantly scale down losses and prevent instances of power outage.
Several types of conductors are used by the industry for reconductoring. They vary with respect to maximum tension capacity, temperature resistance and current-carrying capacity. Many utilities have been using aluminium conductor steel reinforced (ACSR) conductors in overhead power transmission lines. However, these conductors have the capacity to withstand temperatures in the range of 75-85 °C, which restricts their use in emergency situations. Thus, new technologies are being adopted that can withstand higher temperatures and enable the entities concerned to operate the existing infrastructure with enhanced efficiency. High temperature low sag (HTLS) conductors are aluminium-wired conductors that can be operated at temperatures as high as 250 °C. These conductors are made up of material such as INVAR steel (iron-nickel alloy), aluminium-zirconium alloys, high strength steel and polymer matrix composites. These materials ensure high temperature resistance and increased ampacity, while preventing thermal stress. HTLS conductors can be deployed in new lines or can be utilised for reconductoring existing lines. There are several types of HTLS conductors. Some of these are as follows:
The aluminium conductor composite core (ACCC) comprises carbon, glass fibre and aluminium layers along with a composite core which is significantly lighter than the traditional steel core. ACCC conductors can reduce line losses by up to 40 per cent, compared to conventional conductors of the same weight and diameter. The ACCC is capable of carrying up to 2.5 times the amount of current (1,950 amperes) compared to a conventional ACSR conductor, which has a capacity of 764 amperes and can operate at a temperature of 200 °C in emergency situations.
Another HTLS conductor is the GAP conductor. It consists of aluminium alloy wires that have super thermal resistance and a high tensile strength steel core. GAP-type conductors too come in several varieties. The GAP-type ZT ACSR (GZTACSR) is one such variant. It contains a small annular gap filled with grease between the high strength steel core and the first layer of aluminium alloy strands, which helps restrict the tension to only the steel core. These conductors can operate at temperatures of up to 210°C and carry more than 2.5 times the amount of current (1,979 amperes) compared to a conventional conductor of the same diameter.
Thermal alloy CSR (TACSR) conductors consist of a galvanised steel inner core and thermal-resistant aluminium alloy wires, which form the outer layer of the conductor. Compared to conventional conductors, TACSR conductors can be constantly operated at temperature levels above 150 °C and can carry up to two times more current (1,511 amperes).
Aluminium conductor steel supported (ACSS) conductors consist of one or more layers of hard-drawn and annealed aluminium wires on a central core of steel. The steel core wires are coated with materials such as zinc to prevent corrosion. These conductors are capable of carrying power (1,846 ampere) up to 2.4 times the rating of the replaced conductor with the same diameter and can be operated up to 250 °C, without loss in strength.
Superthermal aluminium clad INVAR reinforced (STACIR) conductors are being increasingly installed for reconductoring purposes. These conductors are low sag and are manufactured with aluminium and zirconium alloy rods with a strong inner INVAR steel core. These conductors increase the current-carrying capacity of the existing line by 2.5 times (1,869 amperes). They enable utilities to draw maximum power since they can function in temperatures as high as 210 °C.
Power Grid Corporation of India Limited (Powergrid) was the first utility in the country to use HTLS conductors for line-in-line-out (LILO) of one circuit of the 400 kV Dadri-Ballabgarh transmission line at Maharani Bagh. It deployed invar-type HTLS conductors for this project. Since then, Powergrid has used several types of HTLS conductors for uprating its existing 400/220 kV lines such as the 400 kV D/C Farakka-Malda transmission line. Moreover, Powergrid has deployed INVAR, GAP and carbon-fibre composite core conductors for a host of new transmission lines in its network.
With the government’s focus on Power for All and last-mile connectivity, reconductoring will play a critical role in the necessary refurbishment of the transmission and distribution networks.