Power transmission utilities are increasingly adopting newer technologies for the expansion of the transmission network in order to manage right-of-way (RoW) constraints, ensure efficient power transmission and improve the performance of the existing network. In the transmission conductor space, advanced conductor technologies such as high temperature low sag (HTLS) conductors and high performance conductors (HPCs) are gaining traction. These conductors have higher power transmission capacity than conventional conductors of the same size. On the switchgear front, gas-insulated switchgear (GIS) and hybrid switchgear are being preferred over the regular air- or oil-insulated switchgear due to advantages such as small size and greater safety.
Transmission conductors
The utilities are increasingly deploying new HPCs that carry higher currents and allow higher temperature ratings as compared to the conventional aluminium conductor steel reinforced (ACSR) conductors. HPCs include aluminium conductor steel supported, all-aluminium alloy conductor (AAAC), thermal-resistant aluminium alloy conductor steel reinforced, aluminium conductor alloy reinforced, ACSR, and aluminium conductor carbon fibre reinforced cables.
A more advanced version of the HPC is the HTLS conductor, which can operate at a much higher temperature range than the conventional ACSR conductor, and has low thermal expansion and sag. HTLS conductors comprise aluminium wires helically stranded over a reinforcing core. They are characterised by higher temperature resistance and greater ampacity than conventional conductors. HTLS conductors can withstand temperatures of up to 250 °Celsius, as against conventional conductors steel reinforced and all-aluminium alloy conductors designed to operate continuously at temperatures of 85 °Celsius and 95 °Celsius respectively. In addition to this, HTLS conductors have 30 per cent more ampacity than conventional ones and a low sag feature, which lowers the associated tower requirement.
Broadly, HTLS conductors can be invar type, gap type and synthetic core-based type. Invar-type conductors are 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. Trapezoidal strands of super aluminium alloys are wound around the gap created by the grease to form the conductor. Synthetic core conductors have a synthetic and primarily carbon core surrounded by trapezoidal-sectioned cross-annealed super aluminium alloy strands. Two of the most commonly used HTLS conductors are super thermal alloy conductor invar reinforced and aluminium conductor composite core. The first HTLS project in India was implemented by Power Grid Corporation of India Limited (Powergrid) for the line in line out of one circuit of the 400 kV double-circuit (D/C) (quad) Dadri-Ballabgarh transmission line at Maharani Bagh, wherein it deployed invar type HTLS conductors. A major application of HTLS conductors has been reconductoring of existing lines to increase power transfer capacity. This has been done by several utilities in Kerala, West Bengal, etc., thus enabling them to meet a higher load demand without laying new lines, and achieve higher reliability.
Switchgear
Given the need to economise on space and ensure higher safety, GIS is gaining traction in the transmission segment. GIS is essentially compact and metal encapsulated, consisting of high voltage (HV) equipment such as circuit breakers and disconnectors. In GIS, all the components including busbars, circuit breakers, current transformers, potential transformers and other substation equipment are placed inside modules filled with sulphur hexafluoride or SF6 gas. SF6 is an inert, non-toxic, colourless, odourless, tasteless and non-inflammable gas, about five times as dense as air. It also reduces the distance needed between active and non-active switchgear parts, thereby reducing the size of the equipment.
Another emerging switchgear technology in the transmission segment is hybrid switchgear, which combines conventional air-insulated switchgear (AIS) and modern GIS technology. Hybrid switchgear can be installed both indoors and outdoors, and requires almost 30 per cent less switchyard area and lower foundations per bay. Hybrid substations offer higher reliability, save space, lower civil work cost and entail faster installation. They are mostly used at voltage levels of 72.5-420 kV. They are also useful for setting up transmission infrastructure associated with renewable energy projects. The hybrid switchgear can also be used to enhance the reliability and availability of the existing substations by creating a busbar section. GETCO is one of the pioneers in the installation of hybrid switchgear. A few examples of GETCO’s switchgear are the 220 kV Sartanpur and 220 kV Suva substations.
Transformers
Over the years, a number of technological improvements have been made to transformers to ensure efficient long distance high voltage transmission and for stepping down of voltage for distribution of electricity to consumers. Product-wise, the latest transformer technologies include 1,100 kV high voltage direct current (HVDC) converter transformers, 800 kV HVDC converter transformers and phase-shifting transformers. HVDC technology has gained traction in recent years due to its ability to transmit large amounts of electricity over long distances with lower losses. An HVDC system can reduce transmission losses by around 50 per cent compared to a high voltage alternating current system. HVDC converter transformers form the core of HVDC projects as they transfer power between an AC system and the DC transmission network.
Meanwhile, phase-shifting transformers are special purpose transformers, which are used to control the active power flow in the network by regulating the phase of line voltage. These transformers are used in networks where intensive power wheeling takes place due to deregulation. They help ensure optimum utilisation of transmission lines, thereby enhancing their efficiency. These transformers are site specific and are planned on a case-by-case basis through proper system studies. BHEL commissioned its first indigenously developed phase-shifting transformer at the Kothagudem thermal power station in Telangana in 2014. The transformer is used to control and improve the power flow between 400 kV and 220 kV networks in either direction by shifting the phase as the system requires. Meanwhile, coupling transformers find application in flexible AC transmission systems (FACTS) to enhance the control and stability of the transmission system and increase its power transfer capabilities. These transformers connect the grid with a static synchronous compensator (statcom), which is a FACTS device that ensures the supply of a dynamic, precise and adjustable amount of reactive power to the AC power system the transformers are connected to.
Another key trend in the power transformer segment is the use of smart or digital transformers, which are an integral part of digital substations. They independently regulate voltage and maintain contact with the smart grid in order to allow remote administration and real-time feedback on power supply parameters. These transformers are equipped with intelligent electronic devices, and intelligent monitoring and diagnostic features. Some benefits of smart transformers include real-time monitoring and control facilities, reduced grid losses, increased power supply reliability, and immediate response to fluctuations in the power grid.
Transmission towers
In order to minimise the RoW requirement for setting transmission infrastructure, utilities are increasingly adopting monopoles. These are sleek towers consisting of polygonal tubular sections with a tubular cross-arm arrangement for fixing tension. Powergrid has been installing monopoles since 2008-09 to save space and avoid felling of trees. Powergrid is also using symmetrical monopoles at its 320 kV HVDC Pugalur-Thrissur line. Delhi Transco Limited has also installed monopoles in some areas. Further, the use of narrow-base lattice towers is on the rise. These transformers also help in space optimisation. APTRANSCO has developed narrow-base towers with a base roughly equal to that of a monopole. The transco has deployed these poles in narrow corridors and on traffic dividers in cities. It has developed 132 kV towers with a 1.2 metre base, 220 kV towers with a 2.2 metre base and a 400 kV twin moose with a 7 metre base. Apart from this, multi-circuit towers that are designed to carry three, four or even six circuits to transmit bulk power at an economical rate are extensively being used at higher voltage levels. These are especially useful in forest areas and at substation entries. They significantly lower the space requirement. For instance, the use of quad circuit towers in place of two double-circuit towers can reduce RoW from 106 metres to 46 metres.
Net, net, several technological advancements have been made with regard to transmission conductors, switchgear, transformers, etc., which offer efficient solutions for network expansion and upgradation. However, the deployment of these technologies has been constrained owing to high costs. Going forward, identifying the best-suited technology through a detailed cost-benefit analysis is expected to yield the desired outcome.
Priyanka Kwatra