The transmission segment is grappling with the twin challenges of setting up new transmission lines and enhancing the national grid capacity. High capacity corridors, renewable integration and larger transformer capacity have increased the demand for advanced conductor technologies, which allow high temperature ratings and have low right-of-way (RoW) requirements. This is more so as conductors are the main live elements that carry current and contribute to 30-40 per cent of the transmission line costs. This necessitates the selection of the right type of conductor. Over the past few years, various solutions and advancements have emerged to ensure higher ampacity at higher temperatures, low sag, and greater safety of the towers and foundations of existing lines. A look at the recent advancements in conductors…
HTLS conductors
High temperature low sag (HTLS) conductors are characterised by high temperature resistance and greater ampacity than conventional conductors. They are capable of continuous operation above 150 ºC and do not sag at higher temperatures. Even at temperatures that are above the “knee point temperature”, all the stress of the conductor is borne by the core and hence the sag remains constant. Further, the usage of thermal resistant aluminium alloy or fully annealed aluminium (1350-0) enables high temperature operation without the loss of strength and lower sag. HTLS conductors have 30 per cent more capacity than that of conventional conductors and the low sag feature results in a smaller tower requirement.
The common materials used for making HTLS conductors include INVAR steel (iron and nickel [Fe-Ni] alloy), aluminium-zirconium alloys, annealed aluminium, high strength steel, and metal and polymer matrix composites. HTLS conductors are stranded with a combination of aluminium and alloy wires for conductivity and reinforced by core wires.
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. Powergrid is currently using HTLS conductors of various types (INVAR type, gap type, ACCC type) for uprating the existing 400/220 kV lines. It is also constructing new lines in forest and other RoW-constrained areas.
Types of HTLS conductors
INVAR
INVAR-type conductors are similar to aluminium conductor steel reinforced (ACSR) conductors with respect to their construction, handling and stringing. The aluminium strands can be made thermal resistant by either having fully annealed aluminium or by adding zirconium to obtain a thermal resistant aluminium alloy. The core is galvanised and is made up of iron and nickel (Fe-Ni) with a low thermal coefficient of expansion, which is approximately one-third that of steel. At higher temperatures beyond the transition temperature, all load is transferred to the core and hence, it has a lower sag as compared to that of an ACSR conductor. It can be operated at a temperature of up to 200 ºC.
ACSS conductors
Aluminium conductor steel supported (ACSS) conductors, which were widely used by American utilities, are similar to ACSR conductors, except that the external strands are aluminium annealed. The pure aluminium used is ductile and soft, which is why the conductor requires more care during handling and stringing. The core is made up of high strength steel and carries most of the load and hence the sag is less compared to a conventional ACSR conductor under high temperature. This conductor can also be operated at 200 ºC without the loss of strength.
GAP conductors
This is another type of HTLS conductor with a core of steel and aluminium strands. A small gap is maintained between the galvanised steel core and the aluminium strands. It is strung by tensioning the steel core and hence has a lower sag as compared to an ACSR conductor. This too can be operated at temperatures of up to 200 ºC, but it requires special erection techniques while stringing.
Carbon fibre composite core conductors
This kind of HTLS conductor is fully aluminium annealed and the core is made of composite material like glass fibre and carbon. The conductor has less sag due to a low coefficient of thermal expansion and can be operated at temperatures of up to 180 ºC. However, it requires special types of dead-end clamps and joints as well as more care while handling and stringing due to the softer aluminium strands. It is able to carry approximately twice as much current as a conventional ACSR cable of the same dimensions. As a result, it is used mostly for retrofitting of the existing electric power transmission line without the need for a change in the existing towers and insulators.
Metal matrix composite core conductors
Metal matrix composite core conductors are similar to ACSR Conductors, but the core here is made up of a metal matrix constituting aluminium-aluminium oxide fibres. The external aluminium strands are made up of zirconium – a thermal resistant alloy of aluminium. This conductor can be operated at temperatures of up to 200 ºC. However, this conductor is comparatively more expensive than the other conductors because of the technology of composite core metal matrix.
Salient benefits of HTLS conductors
The use of HTLS conductors is an attractive method of increasing transmission line thermal rating. One of the benefits of HTLS conductors is that their current carrying capacity is higher because of their high temperature operation, as compared to an ACSR conductor. For instance, at a high temperature range of 180-200 ºC the thermal rating or the current carrying capacity offered by HTLS conductors is 2-2.5 times that of an ACSR conductor of the same size, without any appreciable increase in the sag.
HTLS conductors also facilitate thermal uprating of the existing lines that use quadruple-bundled ACSR conductors with higher diameters. Twin bundle HTLS conductors are increasingly being used for uprating high capacity 400 kV extra high voltage (EHV) lines. This helps in increasing the capacity of power flow by 100 per cent. It also avoids capital expenditure, precious RoW, resources, time and effort expended in the construction of additional lines. These special HTLS conductors can be installed and operated without the need for extensive modification of the existing structures and foundations.
Drawbacks of HTLS
The main downside of HTLS conductors is that they have a high cost. In almost all cases, HTLS conductors require special stringing procedures/ requirements, fittings and spare parts, which add to their installation cost, making them two to three times more expensive than ACSR and all aluminium alloy conductors (AAAC). Further, despite their advantage of a high current carrying capacity, limitations are posed by surge impedance loading, thereby causing greater losses in long transmission lines deploying HTLS conductors. This may restrict their full utilisation. Further, these conductors require non-conventional methods of stringing and their operations and maintenance require a skilled workforce. Utilities have also raised concerns regarding the material behaviour, long-term performance and life expectancy of HTLS conductors.
Conclusion
In India, ACSR and AAAC are the commonly used conductors on overhead lines for transmission and distribution systems. However, the need for HTLS-based transmission lines is being felt and the usage of these conductors has increased power flow and reduced congestion. HTLS conductor technology is focused on solving the growing structural and capacity problems of electricity networks. With the National Smart Grid Mission in place and the ambitious target of achieving 175 GW of renewable energy by 2022, specialised conductors will be at the forefront of bringing about a transition in the transmission segment.
Further, with the rising power demand, there is a need to ensure flexibility in the electricity system. In this regard too, HTLS conductors offer an attractive option for high capacity EHV transmission lines and a viable solution for uprating/upgrading of the existing lines. However, choosing the right conductor technology is an important decision. An optimum selection requires a case-by-case assessment based on the techno-economic analysis over the lifecycle of the transmission line.