Large-scale transmission of power is the need of the hour in light of the growing renewable energy capacity, which is targeted to reach 175 GW by 2022. However, the expansion and augmentation of the transmission network is restricted by the lack of right of way (RoW). Utilities are, therefore, focusing on optimising power transfer per unit RoW with higher ampacity conductors. This necessitates the deployment of new generation conductors, which can deliver a large quantity of power without any change or with minor changes in the existing tower/foundation designs. Underground cables, high temperature low sag (HTLS) conductors, extra high voltage (EHV) cables, e-beam cables, covered conductors and gas-insulated lines (GILs) are some of the emerging technologies in the cable and conductor segment. Power Line takes a look at these technologies…
HTLS conductors are characterised by high temperature resistance and greater ampacity than conventional ones. While conventional aluminium conductor steel reinforced (ACSR) and all-aluminium alloy (AAAC) conductors are designed to operate continuously at temperatures of 85 °C and 95 °C respectively, HTLS conductors can withstand temperatures of up to 250 °C. Further, HTLS conductors have 30 per cent more ampacity than conventional ones and the low sag feature results in lesser tower requirement.
HTLS conductors typically consist of aluminium wires helically stranded over a reinforcing core. They can be invar type, gap type, or synthetic core based. The invar type conductors are made up of super thermal alloy conductors with aluminium clad invar (an alloy of iron and nickel) core. The gap-type conductors comprise high strength stranded steel core surrounded by grease. Trapezoidal strands of super aluminium alloys are wound around the gap created by the grease to form the conductor. The core can easily move freely within the conductor in the space created by the 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 comparative performance of HTLS conductors depends on the degree to which the aluminium strand and reinforcing core’s physical properties are stable at high temperature. The conductor choice is made by the utilities based on their requirement, if a particular conductor is selected for a given condition.
HTLS can be deployed in new transmission lines and also used for reconductoring of existing lines to increase power transfer capacity. However, adequate precaution must be taken during the installation process. There is a need to handle the core of the conductor, midspan joints and tension hardware carefully during installation. In the case of interstate transmission system lines, HTLS can be deployed at 66 kV, 220 kV and 132 kV levels. At 400 kV, HTLS can replace quad bundle ACSR and AAAC conductors when the line length is short. The Central Electricity Authority released draft guidelines for HTLS conductors in May 2016.
Some challenges associated with HTLS conductors include extended execution period and the associated cost escalation; in case of reconductoring, outages need to be carried out by utilities; and requirement of specialists for line recovery in case of conductor snaps or tower failures.
Given that transmission losses are reduced at higher voltages, transmission and distribution (T&D) utilities are looking at installing EHV cables at 66 kV and above voltages. EHV cables are made up of aluminium and copper, and various combinations of metal sheaths such as polyaluminium sheath, lead sheath and aluminium sheath in combination with or without copper wire screens. However, most of the demand for EHV cables in the country has been met so far through imports and only a few domestic manufacturers can provide cables up to 400 kV. As per the Indian Electrical and Electronics Manufacturers’ Association (IEEMA), some domestic manufacturers have established world-class technology platforms in EHV cables up to 400 kV, either through technical collaboration or through joint ventures. However, due to inappropriate policies of many utilities, which promote imports rather than investments in the local industry, the country is still importing cables of even 66 kV and above voltage grades. Further, accessories of voltages above 33 kV are also being imported in a big way, in the absence of technology in this segment.
With the growing demand for power in urban areas and industries, underground cable systems are becoming an increasingly indispensable part of T&D systems. Traditionally, the high cost and complexity involved in laying underground cable systems, particularly cable jointing and cable termination, have been the key deterrents to their widespread adoption. However, with growing RoW and safety issues, utilities are increasingly installing underground cables. Underground cables consist of one central core or a number of cores of tinned stranded copper conductors insulated from each other by paper, varnished cambric, vulcanised bitumen, or impregnated paper.
Well-designed and well-installed underground cable systems are expected to have a service life of 30-40 years and are unaffected by failures due to weather phenomena such as lightning, storms, etc. In addition, these cables are out of sight under the ground, and hence offer better aesthetic value to an area or town. These cables often require only a narrow band of land and do not emit any electric field, besides having better power loss characteristics and the ability to absorb emergency power loads.
Utilities are also adopting covered conductors that ensure reliable transmission of power. Covered conductors use an insulating material as protection against accidental contact with other covered conductors or with grounded parts such as tree branches. This covering is sufficient for temporarily withstanding the phase-to-earth voltage. The types of covered conductor systems that are used at various distribution voltage levels include cross-linked polyethylene (XLPE)/ high density polyethylene (HDPE) covered conductors (single or multiple sheathed), aerial cable systems and spacer cables. The main advantages of covered conductors over bare overhead and underground cables are their safety and lower costs.
Other cable technologies
Another emerging cable type are electron-beam or e-beam cables, which find use in high temperature applications in the solar, railways and shipping segments, where cables with cross-linked polymeric insulation with a lower insulation thickness are required. Electron beam irradiation (for cross linking) makes the polymer more stable under the influence of heat, resistant to chemicals, solvents, tough and more abrasion resistant. In conventional cables, cross linking is carried out by thermally induced chemical reaction, affecting the polymers’ quality and life owing to the degradation caused by heat. Meanwhile, e-beam cables are made by electron-beam cross-linking, which is carried out at room temperature through electron beam accelerators. This process of cross-linking polymers is not only fast but also ensures homogeneity in the insulation material. These cables offer superior performance in extreme environments, have high tensile strength, and offer thermal, abrasion and deformation resistance.
GIL is another key emerging cable technology for deployment in areas where overhead lines are not feasible. GILs consist of aluminium conductors supported by insulators contained within sealed tubes pressurised with nitrogen or sulphur hexafluoride gas. The main advantage of GILs over conventional underground cables is higher voltage ratings with systems rated up to 800 kV in operation globally. Further, the terminations at the cable ends are less complex and less prone to failure in GILs. Also, since there are no physical layers of insulation, repair and maintenance of GILs is easy vis-à-vis underground cables. At present, GILs are limited to relatively short distances but the technology is being developed to cover longer distances.
The way forward
Cable design and materials have evolved over the years with dry extruded cables increasingly replacing wet paper insulated ones. XLPE is being preferred over HDPE and low density polyethylene for extruded insulation owing to higher operating temperature and lower cost. XLPE is now available for all transmission applications as its service has been proven for high voltage and medium voltage cables for over 25-30 years, and for EHV cables for over five years.
In the distribution segment, aerial bunched cables (ABCs) are increasingly being deployed by utilities. As compared to conventional bare conductors in overhead distribution systems, ABCs provide more safety, reliability and system economy, reduce power losses, and entail lower installation, and operations and maintenance costs. These are ideal for rural distribution as well as for installations in difficult terrains like hilly areas, forest areas and coastal areas.
Future trends in cable designs are expected to focus on the usage of dry insulation over wet ones, with benefits to the environment. Manufacturers are also focusing on designing cables that offer the advantage of reduced losses, lighter weight, compact size and long shipping lengths (with fewer joints). Further, with increasing digitalisation, smart grids and high voltage direct current projects, the need for advanced cables and conductors is expected to grow in future.