HTLS CONDUCTORS 101: A COMPREHENSIVE GUIDE

DIFFERENT TYPES OF HTLS CONDUCTORS

As discussed in the last article about the manufacturing of HTLS conductors. A brief overview of HTLS conductors produced on different combinations of core and envelope is emphasized here.

As per our last discussion, it is seen that the high-performance conductors are made by the four different combinations of the core and envelope material.

DIFFERENT COMBINATIONS OF HTLS CONDUCTORS

Steel/ coated steel/ steel alloy core with an envelope of thermal-resistant aluminum alloys

ZTACSR

These htls conductors have the same construction as conventional ACSR conductors, with galvanized steel wires in the core and thermal-resistant aluminum alloy strands (TAL) or thermal-resistant aluminum alloy with zirconium-added strands (ZTAL) as the envelope. TAL and ZTAL aluminum alloys have the same conductivity and tensile strength as conventional electrical grade EC aluminum conductors, and in addition, they can withstand maximum operating temperatures up to 150 and 210 degrees Celsius without the loss in tensile strength over time.

ZTACSR is not a low-sag conductor by design. It has the same thermal elongation as that of conventional ACSR. The main advantage of ZTACSR is that the envelope alloy can withstand the high operating temperature of 150 degrees Celsius for TAL and 210 degrees Celsius for ZTAL materials. These conductors are used to up-rate the existing lines where some additional electrical clearances are available.

ZTACIR / STACIR (Conductor with INVAR core)

These HTLS conductors also have a conventional stranded construction identical to conventional ACSR, with material properties advantage which allows ZTACIR/ STACIR conductors to operate at higher temperatures. ZTACIR conductors have aluminum clad invar alloy steel core and ZTAL strands for the envelope. INVAR is an iron-nickel alloy (Fe-36%Ni) with a very small coefficient of thermal expansion.

Geometrically, these HTLS conductors are identical to conventional ACSR with only a difference in slightly reduced conductivity and much increased maximum allowable temperature as the alloy does not lose mechanical strength at high temperatures.

ZTACIR conductors have a maximum continuous operating temperature of 210 degrees Celsius and can have twice the current carrying capacity as that of conventional ACSR conductors. The coefficient of thermal expansion of INVAR core is approx. one-third of the galvanized steel or aluminum-clad core. However, the tensile strength of the INVAR core is 1080Mpa which is lower than the galvanized steel core. ZTACIR conductor has 8% lower tensile strength than conventional ACSR conductors and equivalent sag tension properties.

The installation methods and accessories for these HTLS conductors are similar to those used for conventional ACSR conductors. A slight lengthening of compression-type accessories is required to satisfy increased current carrying capabilities. Pre-stressing can effectively lower the temperature of the knee-point. Cladding shall be done, if required, to improve the conductivity.

GZTACSR or GAP CONDUCTOR

It is a gap-type HTLS conductors which uses a galvanized steel core surrounded by a thermal-resistant aluminum alloy. The strands of the innermost layer of aluminum are always in a trapezoidal shape and sized such that the inner diameter of the resulting tube is slightly larger than the external diameter of the core so as to maintain a slight gap in between.

This radial gap between the core and the envelope allows independent movement of the two. The gap is filled with heat-resistant grease to reduce friction and prevent water penetration and corrosion. The outer layer can be made trapezoidal to maintain compact stranding minimize the electrical resistance and increase the effective cross-sectional area on aluminum strands.

Gap-type HTLS conductors exhibit the same properties (corrosion & electrical) as a TACSR and its low sag behavior allows it to be operated at much higher temperatures than conventional ACSR. The knee point temperature of the conductor is at erection temperature, which means the sag of the conductor is fully dependent on the sag of the core, allowing maximum use of the low sag properties at very high temperatures. It may be noted that if different sections are erected on different days then the sag temperature behavior will be different for different sections. The expansion coefficient of the conductor above the knee point temperature will be that of the steel core.

The grease used in the gap of the HTLS conductor should have an elevated drop point of at least 300 degrees Celsius. The oil separation point should also be optimal to prevent the migration of grease to the outer surface. Grease must retain its properties over the specified temperature range and under varied environmental conditions.

Installation of these types of HTLS conductors is more complex and labor-intensive. Trapezoidal strands may be used for the outer layer of the conductor for snow-bound areas, as these are less sensitive to snow accretion.

Steel/ coated steel/ steel alloy core with an envelope of annealed aluminum

ACSS and ACSS/TW

Aluminum conductor steel supported or ACSS is among the HTLS conductors which consist of fully annealed strands of aluminum around a stranded steel core. In appearance, the ACSS conductors are essentially identical to conventional ACSR conductors. It is available in round and trapezoidal strand configurations with equal diameter and area to conventional round wires. The steel core used may be of high strength HS, extra high strength EHS, ultra-high strength UHS, mischmetal, or aluminum clad steel core.

As we know, annealed aluminum has higher conductivity than hard-drawn aluminum wires in ACSR, thereby increasing the existing current carrying capacity of the line. However, the tensile strength of annealed aluminum is lower. This can be mitigated by using a high-strength steel core or higher steel core area or both. The tension in the annealed aluminum wires is low, the thermal elongation is thus that of the steel core alone, thereby providing low sag up to 250 degrees Celsius. Also due to low-tension aluminum strands, it does not creep under everyday tension loading.

Galvanization is prone to degradation in temperatures above 200 degrees Celsius, however, the aluminum-clad, mischmetal (Al-Zinc) alloy-clad cores are more robust against such heat degradation. Mischmetal coating on the steel core can be which can withstand up to 250 degrees Celsius of temperature for continuous operation. The mechanical and physical properties of mischmetal wires are similar to galvanized steel wires. The corrosion resistance of mischmetal is however superior to galvanized steel.

The reduced tensile strength of annealed aluminum wires makes the knee-point of these HTLS conductors relatively low. It can be significantly reduced by pre-stressing the conductor. The pre-stressing results in plastic deformation of the aluminum wires such that an even greater proportion of stress is carried by the steel core. This reduces the vibration fatigue damage in challenging installations such as river crossing.

The installation splicing and termination of these HTLS conductors is almost similar to ACSR. However, the annealed aluminum strands being soft should be handled with care. These conductors should not be dragged in the bare grounds, rocks, or fences. High-temperature tolerant suspension clamps must be used to allow maximum operating temperature in the HTLS conductor.

Metal-matrix composite (MMC) core with an envelope of thermal-resistant aluminum alloys

ACCR

These types of HTLS conductors are made of metal matrix composite (MMC) core. The envelope is made of thermal-resistant aluminum alloys. The core is made of wires of alumina fibers in an aluminum matrix forming the composite material. The core looks similar to the steel core but it is about 8 times stronger than aluminum and has about the same stiffness as the steel core. Each core wire contains thousands of small-diameter ultra-high strength aluminum oxide fibers. These fibers are continuously oriented in the direction of the wire and fully embedded within high-purity aluminum.

The composite core and the outer envelope contribute to the overall conductor strength and conductivity. The composite core has a low coefficient of thermal expansion above its knee point in comparison to the steel core. Therefore it significantly helps in reducing the coefficient of thermal expansion of the conductor as a whole. The core material is significantly lighter than steel, hence less weight. The composite core is stronger and has a higher elastic modulus than the steel core. Conductivity is also better than the steel core.

These HTLS conductors can be operated with a continuous maximum temperature of 210 degrees Celsius and in emergencies up to 240 degrees Celsius with an AT3 alloy envelope. The conductor is essentially all aluminum and hence rejects the galvanic corrosion that is predominant in steel core conductors. It has no undesirable magnetic properties. Unlike the conductors with a ferrous core experiences an increase in resistance due to its magnetic properties. The magnetic effects are eliminated in the MMC core.

The compression-type hardware for the dead-end assembly of these conductors uses a modified two-part approach. One part grips the composite core and then the outer sleeve grips the aluminum strands. This approach prevents the notching of the core wires. This gripping method ensures that the core remains straight to evenly loaded envelope. It also ensures that the envelope strands suffer no lag in loading relative to the core.

The composite cores are anisotropic materials which means it has high tensile strength, lack shear strength, and transverse and torsional strength. Thus these conductors have limited ability to conform to low bend radius. Thus care must be taken in choosing the correct diameter sheaves that is travellers, bull wheel size, pulling tension, and conductor reel size to prevent excessively low bending radius during installation.

Polymer-matrix composite (PMC) core with an envelope of annealed aluminum/ thermal-resistant aluminum alloy (Carbon Composite Core (CCC) Conductor)

ACCC, CFCC, HVCRC, ACFR

In these types of HTLS conductors, the core is usually made of a polymer matrix composite (PMC), usually carbon fibers in a resin or epoxy resin matrix, with an annealed aluminum or thermal-resistant aluminum alloy envelope. The polymer matrix can be made with thermoplastic or thermosetting compounds.

The core is protected against galvanic corrosion by either an annular sleeve made up of glass fibers, all in the same resin matrix, or protected by an aluminum alloy welded tube or other methods while the envelope can be round, trapezoidal, or Z-shaped.

PMC cores have higher tensile strength compared to steel and compensate for the lower strength of fully annealed aluminum wires. While the aluminum strands are fully annealed, offering the highest degree of conductivity for any aluminum available today, the composite core offers a very low coefficient of thermal expansion than the steel core which allows for less sag at high-temperature operation.

Less sag and low weight can be utilized to have increased spans on fewer/shorter structures along with reduced line losses. Generally, the composite core used is a solid, single-piece rod with no interstices. However, the stranded configuration does also exist. As the core has a smooth surface and bears the overall tensile strength of the conductor, the dead-end assembly has been designed to create a stronger crimp compared to that of the ACSR conductor forming a very solid aluminum press that fits around the composite core.

The core resists degradation from vibration, corrosion, ultraviolet radiation, corona, chemical and thermal oxidation, and, most importantly, cyclic load fatigue. However, the core made of multiple strands may be more susceptible to thermal oxidation.

Although CCC has significantly less thermal sag than other High-Performance Conductor designs, its core is quite elastic and sags more than other designs under ice load. For ice-loading conditions, a core with a higher modulus has to be designed.

For very heavy ice-loading regions, an extra high-strength composite core should be used to improve Sag values. This conductor requires special fittings, such as splice and dead-end connections which are patented. The composite materials are highly anisotropic.

CURRENT CARRIYING CAPACITY OF HTLS CONDUCTORS AT VARIOUS TEMPERATURES