The advances in transmission technologies have allowed project developers to implement new solutions in order to keep up with future network development. Advanced technology trends in conductors are focused on reducing right-of-way (RoW) requirements and increasing the current carrying capacity of transmission lines such as high temperature low sag (HTLS). Another example of advanced transmission technology is gas-insulated lines (GILs), which are suitable for high power transfer if power cables or overhead lines can not be constructed.
A look at some of the technology trends in the transmission space…
HTLS conductors are characterised by high temperature resistance and offer more than double ampacity within the same corridor, without any modification in the tower structure. HTLS conductors have the same or lower sag than existing conductors. Therefore, they require less time to complete the project as compared to constructing a new transmission line. HTLS conductors increase the power delivery on existing RoW without violating the sag criterion. These are ideal for contingencies loading, have lower losses and solve RoW issues.
The use of thermal resistant aluminium alloy (TAL), which is an alloy of aluminium and zirconium, or fully annealed aluminium (1350-O) as conducting material improves thermal stability by enabling operations at high temperatures of up to 230 ºC and 250 ºC respectively. TAL improves the mechanical strength, but decreases conductivity.
Meanwhile, annealing reduces the strength, but improves conductivity and fatigue resistance due to improved softness and ductility. The common reinforcing core materials used for making HTLS conductors include high, extra-high and ultra-high strength steel, INVAR steel (iron and nickel alloy), and carbon fibre composite core (CFCC) and metal matrix composite core, which have an extremely low coefficient of linear thermal expansion as compared to conventional steel wire. The CFCC is of two types – solid single core and stranded core. A coating of aluminium cladding of steel/INVAR steel wire or zinc with 5 per cent aluminium mischmetal alloy improves corrosion protection.
So far, Power Grid Corporation of India Limited (Powergrid) has used HTLS conductors at the 132 kV, 220 kV and 400 kV levels in 34 transmission line reconductoring and new projects (with the existing tower design). The four types of HTLS conductor technologies that have been adopted are INVAR, GAP, aluminium conductor composite core and aluminium conductor steel. A large part of the installation is being undertaken in the eastern region of the country. Powergrid is also using HTLS conductors for forest corridors and other RoW-constrained areas. The main drawback of HTLS conductors is their high capital cost, which is two to three times higher than the cost of aluminium conductors steel reinforced and all alloy aluminium conductors. In addition, there are limitations posed by greater power losses in long transmission lines deploying HTLS conductors.
Superconducting transmission lines
Superconductors are materials that can conduct electric energy below a certain critical temperature without losses. These non-resistive and superconducting transmission lines (SCTLs) can transmit bulk power at low voltage. A cryogenic envelope is needed to keep the superconductor cooled below its critical temperature, which helps in reducing the molecular motion within the material enough so that the flow of current can move unimpeded. SCTLs are compact in size, but they preserve the total capacity. They can help in providing access to remote renewable energy power sources with high capacity transmission. They have minimal environmental impact. There are two types of superconductors – low temperature type superconductors like niobium titanium (TC=9.2 K), cooled by liquid helium, and high temperature superconductors like yttrium-barium-copper oxide (TC=93 K), cooled by liquid nitrogen. A new type of superconductor magnesium diboride was discovered in 2001 with TC=39 K. It can be cooled by either gaseous helium or liquid nitrogen. At present, SCTLs are mainly deployed by global utilities, not on a large scale.
Gas-insulated lines (GILs) use gas with high dielectric strength as the insulating medium instead of air to reduce electrical clearance. They consist of aluminium conductors surrounded by a mix of nitrogen and sulphur hexafluoride inside an enclosure. GILs come with several benefits such as higher transmission capacity and lower transmission losses as compared to overhead lines and other types of underground cables due to the large size of conductors and lower resistance. These lines offer greater reliability with no risk of fire and have electromagnetic fields that are 15-20 times smaller than those of conventional power transmission systems. They offer high operational safety and longer life. GILs are ideally suited for cities that have RoW limitations for overhead lines.
The first GIL installation was in the Black Forest in 1975 with about 4 km length. Although GILs have been deployed by several global transmission and distributions utilities, there are not many examples of GILs in India. They are still not used commercially on a large scale due to their high cost, and are available only in a limited section (maximum 13.5 metre length). The issues related to GILs include limited protection from earthquakes (in the case of underground lines), low reliability, as contamination can lower the dielectric strength, and the risk of breakdown of the insulator.
Cables and towers
Besides conductor technologies, utilities are upgrading cables and towers for efficient transmission. With the growing demand for power in urban areas and industries, underground cable systems are also being deployed by utilities. 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. High surge impedance loading lines, which help to limit voltage drop and thus increase power transfer capability, can also be adopted by utilities.
Cross-linked polyethylene (XLPE) cables are used in places where overhead construction is impractical. XLPE cables have low visual impact due to their underground installation. Land use is minimised, thereby reducing the environmental impact. XLPE cables offer an affordable and justifiable solution as they have a very low maintenance cost. Powergrid is also deploying 320 kV DC XLPE cables for its Pugalur-Trichur HVDC project. The cable will be partially underground and partially overhead. Constraints in the adoption of XLPE cables include high cost and possible damages.
Further, tower upgrade technologies are being deployed by utilities via multicircuit multivoltage solutions, wherein the line is upgraded with more than one voltage circuit on the same tower. Voltage upgrade is a methodology of increasing the line capacity by changing the system voltage from a lower value to a higher value in the same available corridor. It increases the power transfer capacity by over four times. The emergency restoration system (ERS) tower structures are also popular as they are designed to rapidly bypass permanent transmission towers at any voltage in any terrain and are proven to be disaster resilient. ERS towers can be erected in hours and though designed for temporary use, they are kept in continuous service by utilities for over a decade owing to their robust design.
Power grids are expected to operate closer to their maximum capacity. Further, there is an increased need for accurate and better monitoring of the network. Transmission utilities are thus making significant investments in the deployment of wide area monitoring systems and synchrophasor technology. Some of the early real-time PMU applications and visualisations being deployed at the utility level and in ISO control rooms worldwide are angular separation, oscillatory stability monitoring, islanding, and disturbance detection.
Further, various approaches are being adopted to overcome the problem of stability arising due to signal oscillations in an interconnected power system. For instance, STATCOM fine-tunes the grid voltage – generating or absorbing reactive power when the grid voltage changes. It helps stabilise the grid, thus increasing reliability and availability of grid operations. In the event of an immediate voltage drop, a common cause for power outages, STATCOM has a fast response time to inject huge electricity in rush, allowing quick and safe recovery of the network. For instance, Adani Transmission Limited has installed a STATCOM system that can provide up to ±1 MVAr of dynamic reactive power compensation.
Net, net, these technologies have their own learning curves and innovation cycles. Project promoters, regulators and policymakers need to understand each technology and its suitability for project development.