The development of high voltage long distance corridors to deliver electricity to high-demand regions and the introduction of green energy corridors for integrating the increasing share of renewable energy into the grid have spurred the demand for high voltage lines. Obsolete fluid or gas pressure cables are being replaced with new cross-linked polyethylene (XLPE) cables, offering greater tensile strength, elongation and impact resistance. Further, as the government is targeting to provide 24×7 reliable power supply, underground cabling is gaining traction as it is resilient to severe weather conditions, provides ease of network expansion in densely populated areas and protects against theft. It also entails minimum right-of-way (RoW) requirements.
With the growth of smart grid technologies, specialised cables are being designed. For instance, medium voltage smart cables, which integrate optical fibre cable with power cables, are preferred to smart grid applications. Another emerging cable type is electron-beam or e-beam cable, which finds use in high temperature applications in the solar, railway and shipping segments.
Power Line presents an update on cable technologies…
XLPE cables
XLPE cables use cross-linked polyethylene as the main insulating material. Cross-linking inhibits the movement of molecules under the stimulation of heat. This gives these cables greater stability at high temperatures, as compared to thermoplastic materials. XLPE cables can operate at higher temperatures, both under normal loading and short-circuit conditions. These cables have a higher current rating than their polyvinyl chloride counterpart. Extruded XLPE cables are increasingly being deployed by utilities such as Power Grid Corporation of India Limited for setting up of transmission infrastructure in difficult terrain.
The joints for XLPE-insulated cables can be prefabricated at the factory, which reduces the installation time per joint at site. In addition, XLPE cables take less time to manufacture, which means that the same production unit can produce more cable per unit of time. XLPE-insulated cables are also useful in direct current (DC) power transmission. Traditional DC power cables include oil-filled and mass-impregnated non-drain cables that have limitations in long distance power transmission. While the former requires frequent oil refilling, the latter type suffers from low operating temperature.
GILs
Gas-insulated transmission lines (GILs) have nitrogen and sulphur hexafluoride as the insulating medium. GILs comprise aluminium conductors supported by sealed tubes pressurised with gas nitrogen and sulphur hexafluoride in a 80:20 ratio as the main insulation. GILs serve as a viable alternative to overhead lines where RoW is not available for electricity transmission, as GILs can be installed underground as well as in tunnels and trenches. For this reason, these lines are ideally suited for metropolitan areas and cities. Moreover, the resistive losses of GILs are lower than those of overhead lines. GILs offer greater reliability with no risk of fire. They can also be installed in agricultural areas as the ground will still be viable for growing crops. The installation of vertical GILs is popular in hydropower plants as there is no fire hazard associated with them.
Underground cabling
Underground cabling plays a crucial role in providing 24×7 reliable power supply. It offers a host of benefits such as resilience in severe weather conditions, ease of network expansion in densely populated areas and protection against theft. These cables entail minimum RoW and require fewer clearances. While underground power cabling is undoubtedly an expensive choice, demand continues to be driven by the need for reliable supply, safety and aesthetic considerations, and the availability of clearances.
At the distribution and sub-transmission levels, the majority of underground cable projects are being implemented under the Integrated Power Development Scheme. The scheme has sanctioned the implementation of underground cables aggregating 20,130 km across various states, of which 16,766 km has been completed as of June 2020. As of November 2020, works have been completed on 75,000 ckt. km of underground/aerial bunched cables, of which 9,811 ckt. km was commissioned between January and November 2020. Further, underground cabling projects have been proposed under the Smart Cities Mission for reliable power supply and aesthetic value.
As per the Central Electricity Authority, the cost of an underground cabling system is three to four times that of an equivalent overhead system. Another concern with underground cabling is the damage caused to cables from other underground activities. Since these cables are below the ground, they require specialised techniques for fault detection and restoration, and the repair time is much longer. Also, it is difficult to modify underground lines once the cables have been laid.
Solar cables
Solar cables are designed to suit the specific purpose of evacuating solar energy from photovoltaic (PV) modules. These cables connect individual modules with the string combiner box. Solar DC cables are of two types – module cables or string cables and DC main cables. Module cables/String cables are integrated into PV solar panels and are equipped with suitable connectors to be interconnected. DC main cables are special extension cables that are used to connect positive and negative cables from strings to the generator connection box (or directly to the solar power inverter). They are used outdoors. The DC cables between modules as well as between the generator connection box and the solar power inverter are two-core cables – a current carrying wire, typically a live red wire, and a negative blue wire. Both are surrounded by an insulation layer.
Solar cables have a high thermal rating to withstand extreme temperatures, high resistance to heat pressure, and superior abrasion and notch resistance. For solar tracking panels, the cables need to be flexible as the panels keep moving along with the sun. A key consideration while selecting the solar cable is the right cable size for connecting various components of a solar PV system. Broadly, the size of the wire to be used depends on the generating capacity of the solar panel (the larger the quantity of current generated, the bigger is the size) and the distance of the solar panel system from the loads (for greater distance, a bigger-sized wire is used). Correct sizing of the solar cables minimises overheating and loss of energy.
Cables for data centres
Cabling infrastructure is at the core of every voice, data and multimedia network. Ensuring proper planning, configuration and installation for connectivity in a data centre is essential; otherwise, it can cause inefficiency and excessive heat in the system. The devices in a data centre require physical cabling that is reliable, scalable and manageable.
Cabling within a data centre may be either structured or unstructured. Structured cabling uses predefined standards-based design with predefined connection points and pathways. The cabling used in a structured wiring design is specified by the bandwidth requirements of the system and is tested to ensure proper performance. As compared to an unstructured system, a structured cabling system may take longer to install and has a higher initial cost, but the operational cost is lower and the life cycle of the system is longer.
Also known as “point-to-point” cabling systems, unstructured systems do not use predefined standards, connection points, or pathways. This type of cabling system can lead to cooling issues because the air flow is typically restricted. In addition, managing system growth becomes difficult because of the lack of a plan to change cable locations or run new cabling. An unstructured cabling system is inherently unreliable and may result in extended downtime. It may take less time to install and have a lower initial cost, but the operational cost will be high and the life cycle will be shorter as compared to a structured system.
The key network cable options used in a data centre are AC/DC power, ground, copper and fibre optic cables. The interface on the equipment used in the data centre is the primary means for determining the type of cabling. The network data cabling may also be selected based on the bandwidth requirements of the equipment being used in the data centre.
Other specialised segments
Specialised cables are used for various other segments depending on their needs and requirements. Various cables used in wind power plants are designed to meet particular requirements. Copper cables are used for the transmission of temperature signals to control the operating temperature with the help of resistance thermometers. The data cables used are highly flexible. They are installed from the housing to the base of the wind power plants. Meanwhile, cables used at nuclear power plants are designed to operate at high temperatures, and are required to satisfy various thermal ageing tests, radiation resistance tests, as well as fire performance and environmental performance tests. Nuclear cables are designed to meet a life expectancy of 40-60 years. Apart from this, different kinds of cables such as high voltage cables and elastomer cables are being designed to meet the individual requirements of various segments such as mining, oil and gas, and railways.
Conclusion
Indian power cable manufacturers have attained maturity in terms of technology for HV cables up to 220 kV and have been found competitive in the global scenario. However, due to ineffective policies, the country is still importing cables of 66 kV and above voltage levels. Further, accessories of voltages above 33 kV are being imported in a big way.
Domestic manufacturers must therefore explore the possibility of manufacturing these cables to meet the likely demand. There is a need for the standardisation of specifications, which would lead to improved efficiencies, reliability and replaceability of products. Given the increasing focus on renewables, the demand for cables for solar and wind plants is expected to remain high in the coming years.
Nikita Gupta