A Gap-type HTLS conductor is a special type HTLS conductor. It exhibits a separate high strength low elongation steel core surrounded by layers of thermal resistant aluminum alloy strands. The typicality of a gap type HTLS conductor is, it maintains a deliberate annular gap between the core and the first layer of aluminum alloy strands. This gap is filled with a heat‐resistant grease. It makes the core free to slide inside the conductor without friction. The aluminum alloy used for outer conductive layers is typically a high‑conductive aluminum‐zirconium alloy. Aluminum-zirconium alloy is slightly less conductive than pure aluminum. However, due to its superior thermal stability, it is a popular choice for manufacturing conductive strands in HTLS conductors. The strands of the inner conductive layer are often trapezoidal “z” shaped wires for compact packing. Here at this inner most conductive layer the compact packing is essential to prevent grease leakage from the gap. The reinforce core normally used in a Gap-type HTLS conductor is an extra‑high‑strength galvanized or aluminum clad steel core. Some designs even use an aluminum-clad INVAR core for ultra low sag. The mechanical strength of INVAR is slightly less than that of steel but due to its low thermal expansion coefficient it is popularly used for the core of HTLS conductors.
In a Gap-type HTLS conductor, the aluminum-zirconium alloy layers carry the current, and the steel or Invar core handles the mechanical tension, especially at high temperatures. This design is what makes Gap-type conductors unique.
Core: Extra-high-strength steel is used for constructing the core of the conductor. These cores are stranded, and the strands are either galvanized or Al-clad to prevent galvanic corrosion. In some Gap-type designs, an Al-clad Invar (Fe–Ni alloy) core is optionally used instead of a steel core.
Gap between Core and Conductor: A small annulus (approximately 1–2 mm) separates the core from the inner layer of aluminum-zirconium alloy strands. This gap is filled with high-temperature grease, allowing the core and the inner conducting layer to slide relative to each other during thermal cycling.
Conducting Strand Layers: Layers of super-thermal Al‑Zr alloy wires surround the gap. The inner most layer is usually of trapezoidal “z” strands, sometimes of round strands.
Installation of Gap-type HTLS Conductors
Gap-type HTLS conductors require a special stringing technique. Typically, only the steel core is clamped and tensioned at the transmission towers during installation, while the aluminum alloy layers are initially left loose. After the core is set, the aluminum alloy layers are compressed into the clamps. This ensures that the core bears the full tension. Above the knee-point temperature, the low-expansion core controls the sag.
Performance Specifications of Gap-type HTLS Conductors
- Temperature Rating: Gap conductors are rated for continuous operation up to 150°C to 210°C without loss of specified strength.
- Current Carrying Capacity: Typical gap conductors carry roughly 1.6 to 2.0 times the current of an equivalent ACSR conductor. For example, the current carrying capacity of GAP – type – HTLS – Panther is about 1050 A at 210°C, and the current carrying capacity of GAP – type – HTLS – Zebra is approximately 1520 A at 210°C. The information are gathered from Sterlite’s catalog.
- Sag Behavior: In GAP – type – HTLS the steel core carries most tension. This limits additional sag. So, the sag remains much lower than ACSR at high temperatures. Below the knee point it behaves like ACSR.
- Mechanical Strength: The tensile strength comes from the steel core. The gap conductors can match or exceed the strength of ACSR conductor. The aluminum‐zirconium alloy layers are hard‑drawn so they retain strength up to operating temperature.
- Creep Resistance: The steel core has negligible creep under normal loading. In contrast, fully annealed aluminum HTLS conductors can creep.
- Electrical Resistance: DC resistance is similar to ACSR of the same cross-section.
Comparison with Other HTLS Types
- ACCC (Aluminium Conductor Composite Core): This is one of the most popular variant of HTLS. It uses a carbon/glass composite core with trapezoidal annealed aluminum strands. It can carry nearly 2 times the current of equivalent size ACSR. The operating temperature is from 180 to 250°C without any significant increase in sag. The cost of ACCC conductor very high, it is about 3 times of an equivalent size of ACSR. Due to the stiffness of the core the overall conductor is less flexible. Although, its core is light weight and strong, but after withstanding conductor’s heat over long periods, the performance can degrade.
- ACCR (Aluminium Conductor Composite Reinforced): This is also a variant of high performance HTLS conductors. It uses an aluminum-matrix composite core. It means the core is constructed using alumina fibers. The outer conductive layers are formed by Al-Zr strands. It achieves 2 times higher ampacity an equivalent ACSR. An ACCR conductor can operate continuously up to the conductor temperature of 210°C. Although, for a short duration at emergency the conductor can be overloaded up to a temperature of 240°C. It is lightweight. It has the mechanical strength similar to that of ACSR. However, it is much costlier. Normally it is 5 times costlier than its equivalent size ACSR. This high cost makes it limited in use in power network system.
- TACSR (Thermal ACSR): This HTLS conductor is nothing but a upgraded version of conventional ACSR. Instead of hard drawn aluminum, it uses aluminum-zirconium alloy for its conducting strands or wires. The strands can be either of trapezoidal or round depending upon design. It uses the standard galvanized stranded steel core. As aluminum-zirconium alloy strands thermally elongate less it offers a moderate ampacity boost over equivalent ACSR. It can tolerate 50 to 60% more current than its equivalent ACSR. A conventional ACSR can maintain its sag within limit up to 75°C to 85°C or sometimes up to 90°C, whereas for TACSR the sag remains within limit up o 150°C. The cost of TACSR is only slightly higher than ACSR. Another advantage of using TACSR is that we can use the same hardware when we replace a conventional ACSR with TACSR. Also the installation procedure of TACSR is similar to ACSR. It is a low-cost HTLS option. It exhibits lower sag at higher temperature compared to its equivalent ACSR. Like conventional ACSR its steel core takes more mechanical load at higher temperature.
- Invar-Core Conductors: These HTLS conductors use an aluminum clad Invar (Ni-Fe alloy) core. The outer conductive layers are formed by thermal Al-Zr alloy. The rated current of Invar core HTLS conductors is roughly 2 times of the rated current of similar ACSR. Above the knee point the Invar core dominates tension, so sag increases very little with temperature. The cost of invar core HTLS conductor is extremely high. It is about 4 times of that of an equivalent size ACSR. These conductors are heavier than equivalent ACCC and ACCR HTLS conductors. The modulus of Invar is nearly 152 GPa compare to 206 GPa for steel. So under heavy ice it can sag more than a steel core conductor. In practice Invar conductors are used only where lowest sag is essential.
A GAP type HTLS conductor offers much higher capacity than ACSR or TACSR but not as high as ACCC and ACCR conductors. It costs roughly twice the cost of an equivalent size ACSR, but that cost is lower than the cost of equivalent composite-core or Invar core HTLS.