With increasing urbanisation, space is getting constrained in most parts of the country, thereby necessitating the installation of conductors requiring less right of way (RoW) and having high current carrying capacity. A number of new technologies that increase the quantum of power transmitted through conductors and minimiseRoW requirements are available in the market. These technologies also ensure higher efficiency, safety, reliability, ease of design and environmental friendliness.
A look at the technology trends in the cables and conductors market…
The market for cables and conductors has largely been driven by the expansion of the power and infrastructure sectors. As per the Indian Electrical and Electronics Manufacturers’ Association, during 2018-19, the conductor market was estimated to be at Rs 107 billion. During 2019-20 (till December 2019), conductor imports stood at Rs 3.69 billion, much lower than the exports, which were at Rs 17.9 billion during the same period. Meanwhile, the overall size of the cable industry was estimated to be around Rs 558 billion during 2018-19. During 2019-20 (till December 2019), the import of HV and LV cables stood at Rs 9.17 billion, while their export stood at Rs 19.7 billion. The cables segment has shown good growth in exports but domestic demand has been declining, especially for LV and house wiring. However, the cable and conductor segments experienced a rather sluggish growth during 2019-20 (till December 2019). The growth index for the cables industry during 2019-20 (till December) was (-)15.4 per cent while that for conductors was (-)17.6 per cent.
Conductors make up 30-40 per cent of the total cost of overhead EHV transmission lines and thus the type of conductor selected plays an important role. The design of transmission lines equipped with various types of conductors should hence be based on the maximum operating temperature of conductors, RoWoptimisation, losses in the line, cost and reliability considerations.
On the technology front, a number of new alternatives have emerged that enable utilities to augment their capacities while minimisingRoW requirements and increasing reliability. These include overhead line conductors such as all aluminium alloy conductors (AAACs), all-aluminium conductors (AACs), aluminium conductor alloy reinforced (ACAR), aluminium conductor steel reinforced (ACSR); and high performance conductors (HPCs) such as aluminium conductor composite core (ACCC), aluminium conductor steel supported (ACSS), AL59, super thermal alloy conductor invar reinforced (STACIR) and thermal resistant aluminium conductors steel reinforced (TACSR).
ACAR consists of one or more layers of aluminium strands helically wrapped over one or more aluminium alloy wires and provides a transmission conductor with an excellent balance of electrical and mechanical properties. ACSR, meanwhile, comprises a solid or stranded steel core surrounded by one or more layers of strands of aluminium. Conventionally, more steel (26-40 per cent) was used to obtain greater strength in ACSR conductors. Now, with conductors becoming larger, the trend has shifted to less steel content (11-18 per cent).
AAAC has several layers of aluminium alloy stranded in concentric layers designed to achieve a high strength to weight ratio. It is extensively used in bare overhead lines and is utilised in highly polluted industrial areas and coastal regions due to its resistance to corrosion. However, due to the absence of steel, its resistance is lower than ACSR. Nonetheless, AAAC can carry 15-20 per cent extra current and has a longer life span than ACSR. In India, ACSR and AAAC are the most commonly used conductors for the transmission of power through overhead lines.
A more advanced version of HPC is the high temperature low sag (HTLS) conductor that can operate in a much higher temperature range than the conventional ACSR conductor, and has low thermal expansion and sag. HTLS conductors can address issues such as growing congestion in the existing corridor of a power transmission network, enhancement of power flow per unit of RoW and a reduction in losses under normal as well as emergency conditions. HT/ HTLS conductors typically consist of aluminium wires helically stranded over a reinforcing core.
These can be invar type, gap type, or synthetic core-based type. Invar-type conductors are made up of super thermal alloy conductors with aluminium clad invar (an alloy of iron and nickel) core. Gap-type conductors comprise high strength stranded steel core surrounded by grease. Synthetic core conductors, as the name suggests, have a synthetic, primarily carbon, core surrounded by trapezoidal-sectioned cross-annealed super aluminium alloy strands. The stability of the aluminium strands and the physical properties of the core at higher temperatures are the key factors that determine the performance of HTLS conductors. Utilities choose HTLS conductors based on their specific requirements.
A major application of HTLS conductors has been in reconductoring the existing lines to increase power transfer capacity. HPCs should, however, be considered in those corridors where power transfer is constrained due to the thermal loading of conductors. In intra-state transmission systems, the requirement of such conductors is at the 220 kV, 132 kV and 66 kV levels. However, the application of HPCs is not cost effective for the HVDC system and at 765 kV voltage levels.
To avoid RoW issues, the use of extra high voltage (EHV) cross-linked polyethylene (XLPE) cables is being taken up. XLPE cables consist of cross-linked polyethylene as the main insulating material. Cross-linking inhibits the movement of molecules under the stimulation of heat, and this gives these cables greater stability at higher temperatures compared with thermoplastic materials. XLPE cables can operate at higher temperatures, both for normal loading and under short-circuit conditions. XLPE-insulated cables are quite useful in direct current (DC) power transmission. Due to technical limitations, the use of XLPE cables at the EHV level is restricted to a certain length.
Gas-insulated lines (GILs) have gas (nitrogen and sulphur hexafluoride) as the insulating medium, as against physical layers of separation in conventional transmission lines. A GIL comprises aluminium conductors supported by sealed tubes pressurised with gas (nitrogen and sulphur hexafluoride in a 80:20 ratio) as the main insulation. GILs can be installed under the ground as well as in tunnels and trenches. Given that resistive losses of GILs are lower than those of overhead lines and other types of underground cables, these offer greater reliability with no risk of fire. This technology is considered to be a good alternative to EHV XLPE cables, especially where normal current/power flow requirement is high and length is short. However, it is still awaiting widespread adoption in the domestic market.
So far, Powergrid, a few state transmission utilities such as UPPTCL (Uttar Pradesh), MSETCL (Maharashtra), WBSETCL (West Bengal), OPTCL (Odisha), JUSNL (Jharkhand), PTCUL (Uttarakhand), KPTCL (Karnataka), GETCO (Gujarat), RRVPNL (Rajasthan) and some private utilities such as Tata Power, Torrent Power and CESC Limited have already used ACCR and ACCC in transmission line corridors, which are getting overloaded/exceeding the thermal loading limits of the existing conductors. Some more projects of re-conductoring as well as of new lines using HPCs are under way.
Various conductor manufacturers have set up facilities for manufacturing different HPCs. The invar core and polymer composite matrix core of carbon composite core conductors are currently being imported, although the envelope of these conductors is manufactured indigenously. Manufacturers have also started manufacturing accessories for different HPCs.
Overall, with increasing industrialisation and growing population, the need for reliable and efficient power supply has become important. Moreover, increasing renewable energy capacities will necessitate the creation of transmission infrastructure, which would further drive demand for cables and conductors. Going forward, the uptake of HPCs and higher rating cables is expected to see greater traction in the power transmission segment.