Power Transmission Challenges
The main drawback of conventional ACSR (Aluminum Conductor Steel Reinforced) conductors is their limited thermal capacity. These conductors are typically designed to operate at a maximum temperature of 75°C. Although some modern variants can withstand up to 85°C, exceeding these temperature limits by increasing current leads to higher thermal expansion, which in turn increases conductor sag. Excessive sag reduces ground clearance of overhead lines, posing safety and regulatory issues.
To control sag, it is essential to maintain the conductor’s temperature within safe limits. Since electrical current is the primary source of heating in a conductor, the current-carrying capacity must be restricted accordingly. However, in recent decades, the demand for electricity has risen sharply, necessitating an increase in transmission capacity between generation and consumption points.
At the same time, acquiring new land or Right of Way (ROW) for building additional transmission lines has become increasingly difficult due to urbanization, environmental regulations, and public opposition. As a result, constructing new lines parallel to existing ones is often unfeasible. This creates a major challenge in meeting growing power demands by creating additional transmission lines.
In strung ACSR, both steel core and aluminum conductor share the mechanical load. At high current that is at high temperature, due to different coefficient of expansion, steel core is elongated less than the aluminum conductor. Therefore, the aluminum conductor releases its mechanical load to the steel core. Due to higher excessive mechanical load the steel core gets elongated permanently day after day. So, to maintain the sag the current has to be limited in aluminum conductor with steel reinforced to limit the temperature rise.
Introduction to HTLS
The only solution is to increase the current-carrying capacity of the existing transmission lines. That can be done by using conductors that have more current-carrying capacity than conventional ACSR conductors. This is where HTLS (High Temperature Low Sag) conductors come into the picture. Normally, an HTLS conductor can transmit double the current of a conventional ACSR conductor. The range of maximum operating temperature is from 180°C to 250°C. That means the gag of an HTLS conductor remains within limit in this temperature range. The current carrying capacity of a conventional conductor like ACSR, is limited by thermal expansion. Actually, when current increases the temperature of the conductor is increased. Therefore, the steel core as well as aluminum conductor strands are also increased.
Although, the current can be increased in a conductor, provided at the higher temperature the sag is maintained. To maintain the sag HTLS conductors use specialized cores such as invar steel, carbon fiber, or carbon composite materials which have minimum thermal elongation. At the same time to limit the thermal expansion of aluminum strands, annealed aluminum is used instead of hard drawn aluminum.
Basic Construction of HTLS Conductor
Comparison with aluminum conductor with steel reinforced, there are made four major changes found in an HTLS conductor.
1. Core Replacement:
The steel core of an ACSR (Aluminum Conductor Steel Reinforced) conductor is replaced with a carbon composite, carbon fiber, or invar steel core. These materials have a significantly lower coefficient of thermal expansion, so their length remains stable under temperature variations, maintaining constant mechanical tension.
2. Hard Drawn Aluminum Substitution:
The hard drawn aluminum is replaced with annealed aluminum in an HTLS conductor. Annealing reduces the aluminum’s thermal expansion, effectively lowering its temperature coefficient of expansion. As a result, even under high temperatures, the aluminum expands much less than hard drawn aluminum.
3. Mechanical Load Distribution:
In ACSR conductors, the mechanical load is shared between the steel core and aluminum strands. In contrast, HTLS conductors carry the entire mechanical load through the core alone, as annealed aluminum is too soft to bear mechanical stress.
4. Galvanic Protection:
HTLS conductors require a galvanic protection layer, typically a fiberglass layer to prevent galvanic action between the aluminum and composite core. In ACSR conductors, the galvanized steel strands inherently provide this protection via a zinc coating.
5. Effective Cross Section Increment:
Additionally, in some HTLS designs, the aluminum strand geometry is optimized. Instead of circular cross-sections, rectangular or trapezoidal strands are used to minimize air gaps between them, thereby enhancing the effective cross section hence the current-carrying capacity of the conductor.