Transmission conductors are a vital component of the power transmission infrastructure. To set up a strong transmission network, it is required that the conductors allow long distance transmission of power at minimum loss level and right-of-way (RoW) requirements. The transmission conductor industry has witnessed a number of technological advancements, which aim to enhance the quantum of power transmitted through conductors in a reliable manner. Some of the emerging conductor technologies are high temperature low sag (HTLS) conductors, high temperature superconductors (HTS) and covered conductors. These technologies aim to create a robust transmission network by tackling the challenges faced in the transmission of power.
HTLS conductors and its types
HTLS conductors can operate at a temperature of 150-250 degrees Celsius without any loss in design conductivity and tensile strength. As compared to conventional conductors, these allow higher power transfer per unit of RoW, ensure lower technical and commercial losses, and reduce the carbon footprint and land requirement, among other things. HTLS conductors address the issue of growing congestion in the transmission network by increasing the transmission capacity.
HTLS conductors comprise a combination of aluminium or aluminium alloy wires, reinforced by core wires. These can carry almost double the current carried by aluminium conductor steel reinforced (ACSR) of the same size, while the maximum sag and tension remain the same for both. HTLS conductors are about two to three times costlier than conventional conductors.
In terms of design, there are several kinds of HTLS conductors, which differ based on the material of the outer layer, material of the inner core, configuration of the conductor, etc. One popular type of HTLS conductor is superthermal aluminium conductor INVAR reinforced. These conductors use aluminium-zirconium alloy rods with super thermal-resistant alloy wires forming the outer layers. Meanwhile, the inner core comprises INVAR, which is a nickel-iron alloy that reacts negligibly to temperature change. Another type of HTLS conductor is aluminium conductor composite core (ACCC), which comprises carbon, glass fibre and aluminium layers along with a composite core. The composite core is about 25 per cent lighter than the traditional steel core. Several 220 kV lines set up by Delhi Transco Limited and a 220 kV (8 km) line by Tata Power are some examples of the use of composite conductors in the country.
A GAP conductor is also a type of HTLS conductor. It comprises super thermal-resistant aluminium alloy wires with a high tensile strength steel core. These conductors have a small annular cavity filled with grease between the steel core and the first layer of aluminium strand, which helps restrict the tension only to the steel core. An example of the use of GAP conductors in the country is some of the 220 kV double circuit lines set up by CESC Limited.
Some of the other HTLS conductor types are thermal alloy conductor steel reinforced, which uses thermal-resistant aluminium alloys for the outer layers and galvanised steel for the inner core, and aluminium conductor steel supported (ACSS), which uses Galfan-coated steel wire inner core with concentrically arranged annealed aluminium strands forming the outer layers.
Deployment of HTLS technology in India
HTLS technology has witnessed wide uptake in the country. The first HTLS transmission line project was executed by Power Grid Corporation of India Limited (Powergrid) at Maharani Bagh, in Delhi. Currently, Powergrid has more than 5,000 km of HTLS (INVAR, carbon composite core, GAP) conductors in operation. As per industry estimates, since the first HTLS line, equipment orders for more than 12,000 km of HTLS transmission lines have been placed by Powergrid, state transmission utilities, state-owned discoms, private utilities, etc. Broadly, the order book for HTLS technology comprises INVAR (4,600 km), ACCC (4,000 km), ACSS (500 km) and GAP (3,300 km).
An emerging avenue for the deployment of HTLS technology in the country is the reconductoring of existing lines. Reconductoring helps augment the quantum of power transmitted through the existing lines, and is useful in corridors where power transfer is constrained due to thermal loading considerations. While undertaking reconductoring, it is essential to ensure that the terminal substations are capable of handling enhanced power transfer. Some of the examples of reconductoring with HTLS technology are reconductoring of three 132 kV ACSR panther conductor lines aggregating 37 km with ACCC lines by Odisha Power Transmission Corporation Limited, reconductoring of two existing 132 kV ACSR lines with ACCC lines by the West Bengal State Electricity Transmission Company, and reconductoring of a 132 kV line with ACCC and INVAR core conductors by Uttar Pradesh Power Transmission Corporation Limited to tackle the clearance requirement and congestion issues in the National Capital Region.
HTS cables comprise HTS tapes composed of BSCCO (bismuth, calcium and copper oxide) and YBCO (yttrium, barium and copper oxide). The tapes serve as the current carrying conductor, surrounded by an electric insulating layer to prevent voltage breakdown. Further, in order to ensure protection during faults, HTS cables use stabilisers, which provide an alternative path for the current when the total current in the conductor exceeds the critical level. Thin strips of helically wound copper are a popular choice of the stabiliser material. Besides, shielding layers are provided to contain magnetic fields generated by the cable.
One of the unique features of HTS cables is that they offer zero resistance in power transmission and a high quantum of power transmission. The current carrying capacity of HTS cables is two-to-tenfold of an ordinary cable. Other benefits of HTS cables include prevention of electromagnetic wave leakages, low levels of transmission losses and reduced congestion in transmission lines. These are suitable for urban areas that have limited space and large power requirements.
In order to understand the scope of HTS in India, Powergrid is setting up a 220 kV HTS cable system demonstration project for assessing the feasibility of the technology and operational challenges involved in the execution.
Another emerging conductor technology is covered conductors. These conductors use an insulating material as protection against accidental contact with other covered conductors or with trees, branches, etc. These conductors cost about 25 per cent more than bare conductors.
Covered conductors vary depending on the insulating material, conductor configuration, etc. The most widely used covered conductors include cross-linked polyethylene (XLPE)/high density polyethylene (HDPE) covered conductors (single or multiple sheathed), aerial bunched cables (ABCs) and spacer cables. XLPE and HDPE are the most commonly used sheath materials for covered conductors. The conductor material can be high conductivity copper, aluminium or ACSR. These can have one, two or three sheath layers at medium voltage (6.6-33 kV), while at the 66-132 kV level, the conductor may have up to five layers. On the other hand, spacer cable systems consist of three covered conductor phases in a polymeric support cradle supported by a messenger cable. The benefits of these conductors include a lower failure rate, improved safety, easy installation and operation, and fewer ROW issues. However, the use of spacer cable systems is not recommended in heavily polluted areas to avoid accelerated ageing of the electrical equipment and accessories. Meanwhile, ABCs are fully insulated three-core cables with an earth screen used for overhead applications. These cables are less susceptible to lightning, but also the most expensive. These are also more reliable as compared to bare overhead distribution lines, since power and neutral conductors are insulated with a better dielectric medium.
In India, Karnataka Power Transmission Company Limited was the first transco to use covered conductors. The transco undertook a pilot project involving the construction of a medium-voltage transmission line in Yelahanka, a suburb of Bengaluru. The 66 kV line was laid alongside a 220 kV multi-circuit line to transfer power from Hoody to the 220 kV station in Yelahanka to improve the quality of power supply to the suburb, which is close to the international airport.
There is a host of advanced conductor technologies available to suit the varied requirements of utilities. The kind of conductors to be used needs to be considered on a case-to-case basis depending upon techno-economic analysis. Conductor technology needs to be adopted taking into account the expected thermal loading of the line, to minimise the cost while obtaining the desired benefits. Besides, it is essential to ascertain whether the technology is compatible with other components in the associated power system.