HIGH-TEMPERATURE LOW SAG HTLS CONDUCTORS
The full form of HTLS conductors is HIGH-TEMPERATURE LOW SAG conductors. Manufacturing of HTLS conductors becomes essential as the conventional conductors used in the power industry are now being saturated to their capacity. Any further increase in the ampacity of the line will result in the rise of temperature of the conductor which will begin to anneal the conductor material. It will start to lose its mechanical strength and eventually fail. The temperature limit for ACSR and AAAC are 85 and 95 degrees Celsius respectively as discussed in the last article of conventional conductors used in the industry.
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Therefore, it is natural to redesign the envelope and the reinforced core of the conductor to make it withstand the high operating temperature without having much effect on its conductivity. Thus these high-performance conductors are engineered and as it also reduces the sag of the transmission lines at high temperatures, it is also called High temperature low sag HTLS conductors. The manufacturing of HTLS conductors basically involves the application of mechanical engineering for the proper designing of the conductor’s envelope and core. The manufacturing process is discussed below.
MANUFACTURING OF HTLS CONDUCTOR’S ENVELOPE
To avoid the thermal limits of conventional conductors, there are two processes by which the thermal limit can be raised in the manufacturing of HTLS conductors.
ANNEALING
In this step of manufacturing HTLS conductor, the conductor material is annealed before purchase and installation, at the factory. Annealing is however a heat-treatment process which involves heating the metal above its recrystallization temperature for a prefixed time before cooling it. This process, however, decreases the tensile strength of the conductor drastically to the range of 48-98 Mpa but it increases the conductor’s conductivity up to 61.8% of IACS. The maximum continuous operating temperature also increases to a 150-degree range after this mechanical heat treatment process.
ALLOYING
It is the other step in the manufacturing of HTLS conductor, in which aluminium is mixed with zirconium in various proportions to form TAL, ZTAL/UTAL, and XTAL designated alloy. It is to be noted that alloying increases the resistivity slightly. Al-Zr alloy conductor has similar if not more tensile strength as of conventional electrical conductor grade (EC) aluminium wire but can operate continuously at higher temperatures of 150-200 degrees Celsius.
TAL(AT1)
It is a thermal-resistant aluminium alloy, it has a tensile strength of 159-169 Mpa. Its conductivity is 60% of IACS. It can reach an emergency operating temperature of 180 degrees Celsius to be operated for less than 10 hours per year with minimal deformation in characteristics.
KTAL (AT2)
It is HS Thermal Resistant aluminium alloy. It has a tensile strength of 225-248 Mpa. Its conductivity is 55% of IACS. The maximum operating temperature of 150 degrees is possible with KTAL material. It can reach an emergency operating temperature of 180 degrees Celsius to be operated for less than 10 hours per year with minimal deformation in characteristics.
ZTAL/UTAL (AT3)
It is Ultra Thermal Resistant aluminium alloy. It has a tensile strength of 159-176 Mpa. Its conductivity is 60% of IACS. The maximum operating temperature of 210 degrees is possible with ZTAL material. It can reach an emergency operating temperature of 240 degrees Celsius to be operated for less than 10 hours per year with minimal deformation in characteristics.
XTAL (AT4)
It is an Extra Thermal Resistant aluminium alloy. It has a tensile strength of 159-169 Mpa. Its conductivity is 58% of IACS. The maximum operating temperature of 230 degrees is possible with XTAL material. It can reach an emergency operating temperature of 260 degrees Celsius to be operated for less than 10 hours per year with minimal deformation in characteristics.
CORE MANUFACTURING OF HTLS CONDUCTORS
The core manufacturing of HTLS conductors requires the selection of material for the core. The material of the core in high-performance conductors or HTLS conductors includes galvanized steel, aluminium cladding steel, mischmetal steel, INVAR steel (Fe-Ni alloy), high-strength steel, metal matrix composites and polymer matrix composites (carbon fibre composites).
As the HTLS conductor with annealed aluminium strands has very low tensile strength therefore the conductor stiffness and breaking strength largely depend on the core of such types of conductors.
For the aluminium alloyed strands type HTLS conductors, the composite conductor strength and stiffness depend on both the reinforcing core and the stranded layers.
LIST OF CORE MATERIALS WITH PROPERTIES
| DESCRIPTION | Modulus of Elasticity (Gpa) | Tensile Strength (Mpa) | Co-efficient of Expansion (x 10-6 / degree C) | Unit weight (mg/mm3) |
| HS Steel | 200 | 1379-1448 | 11.5 | 7.778 |
| EHS Steel | 200 | 1517 | 11.5 | 7.778 |
| EXHS Steel Galfan coated | 200 | 1965 | 11.5 | 7.778 |
| Aluminium Clad 20.3% IACS | 162 | 1103-1345 | 13 | 6.588 |
| Galvanized Invar Alloy | 162 | 1034-1069 | 1.5-3.0 | 7.778 |
| Aluminium Clad Invar | 152 | 932-1080 | 3.7-10.8 | 7.1 |
| Mischmetal (Std, HS) | 200(I) 186(F) | 1379-1448 1517-1620 | 11.5 | 7.778 |
| Metal Matrix | 215 | 1310 | 6.0 | 3.322 |
| Polymer Matrix | 112.3 | 2158 | 1.6 | 1.88 |
COMBINATIONS IN MANUFACTURING OF HTLS CONDUCTORS
The manufacturing of HTLS conductors can be done by any of the following combinations
- Steel/ coated steel/ steel alloy core with an envelope of thermal-resistant aluminium alloy.
- Steel/ coated steel/ steel alloy core with an envelope of annealed aluminium.
- Metal matrix composite core with an envelope of thermal-resistant aluminium alloy.
- Polymer matrix composite core with an envelope of annealed aluminium or thermal-resistant aluminium alloy.
CONCLUSION
The combination of the reinforcing core and the envelope material determines the maximum operating temperature that the HTLS conductor can withstand. Normally there is a compromise to be made while manufacturing of HTLS conductor, between the operating temperature limit and the required tensile strength of the HTLS conductor.
The operating temperature of the HTLS conductor should also take into account the possible deterioration of the connectors and associated hardware. The operating temperature of HTLS conductors is generally less than or equal to the operating temperature limits of all individual components and materials connected.
