High temperature low sag power line conductors

February 2nd, 2015, Published in Articles: Energize, Featured: Energize

 

The increasing need to expand the capacity of transmission networks, and limits on servitudes available, has lead to the development of methods to increase the capacity of existing routes. This has resulted in a new breed of overhead conductor which can remain within physical limits at higher currents and higher operating temperatures.

One of the prime physical limits on overhead transmission lines is conductor sag. Sag is limited to meet the clearance requirements of the line, between the line and ground, and between conductors, under maximum load conditions. If the sag is too large, it may cause a short circuit between the line and objects below it or a short circuit between lines in extremely windy conditions.

Conductor sag in a transmission line span is affected by both temperature and an accumulation of external matter such as ice, which increases the weight on the span. In South Africa, ice on conductors is generally not encountered and conductor temperature is the most influential factor on sag. This limits the amount of current which is allowed to flow in the transmission line.

Ampacity

Ampacity is generally defined as the maximum current that a conductor can carry without suffering immediate or progressive damage. In the case of overhead conductors damage may be interpreted as exceeding thermal expansion limits or annealing temperatures. Ampacity ratings are determined by balancing heat generation and heat dissipation mechanisms, and can thus be affected by such environmental conditions as air temperature, solar radiation, orientation, wind speed and others.

New high temperature low sag (HTLS) overhead conductors have greatly increased ampacity ratings. HTLS conductors can carry much higher currents than conventional overhead conductors with the same physical size.

Temperature effects on aluminium conductors

Standard overhead conductors are made from hard drawn aluminium (HDA), which increases the tensile strength over that of aluminium rod by a factor of almost three. However this limits the temperature, as above 100°C the HDA starts to anneal and lose its tensile strength.

HDA is nearly pure aluminium which is limited to continuous operation below 100°C. Above this temperature aluminium wires lose tensile strength over time and, after extended exposure to high temperature, HDA becomes “fully annealed” wire, which is chemically identical to HDA but all “work-hardening” of the wires inherent in drawing the wires has been removed. Fully annealed wire has a tensile strength less than half that of HDA and breaks at an elongation of 10 to 20% instead of 1%. It is unaffected by further exposure to high temperatures [1].

Temperature also affects the resistivity of the conductor and hence the losses in the conductor. The use of annealed aluminium as a conductor with the tensile strength provided by the core allows higher temperatures. Annealed aluminium wires are attractive for use in HTLS conductors as they can be operated to 350°C without any change in properties [2].

Conductor temperature

The temperature of a conductor depends on a number of factors including:

  • Current flowing in the conductor
  • Ambient temperature
  • Solar radiation level
  • Wind speed.

A high temperature conductor is defined as a conductor which is designed for applications where continuous operation is above 100°C or the conductor is designed to operate in emergency conditions above 150°C [2]. In fact some can be run at over 200°C continuously and over 220°C for short times. Normal operation is limited to a conductor temperature of 50°C.

Sag tension calculations

The allowable clearance between a line and objects underneath the line will depend on the voltage of the line. The sag allowable will depend on the length of the span between towers and the height of conductor attachment point at the tower.

The sag in a conductor span is determined by the tension in the conductor, which is limited by the tensile strength of the conductor. Annealed aluminium conductors have relatively low tensile strength and thus cannot comply with requirements for low sag spans.

Conductor strength and weight directly impact the amount of sag and tension in an overhead conductor. Specifically [2]:

D = w*S²/(8*H)                                                                                                          (1)

where

S =span length

w = weight per unit length

D = sag

H = tension

Thus (H/w) is directly related to specific tensile strength. Generally the higher the H/w ratio, the less the conductor sags. Temperature causes expansion in the conductor which increases the sag and reduces the tension at the same time.

HTLS conductors

HTLS conductors solve the challenges of higher temperature and higher tensile strength by using annealed aluminium or aluminium alloy conductors and a high tensile strength core which carries both the weight and the tension in the conductor span. Cores may be of metallic or composite material construction.

Low sag conductors make use of an inner core with higher tensile strength than the aluminium conductors. This allows higher tension to be applied to the conductor which allows reduced sag. Several materials have been used. Low sag conductors are designed with a centre core of a material that has a higher tensile strength than the aluminium conductor.

Metallic cores

Conventional conductors which consist of an outer layer of HDA conductor with a high tensile steel core have been in use for many years. The tensile strength is a combination of the core strength and the conductor strength. One of the problems is the thermal expansion of the steel core.

More recent developments use a combination of annealed aluminium or aluminium alloy outer conductor with a core consisting of various alloys having higher strength than steel and lower thermal expansion coefficients, as well as lower weight. Corrosion of metallic core material is also a problem and has been countered by various methods such as galvanising, and aluminium coating.

Some typical materials are:

  • High tensile steel – with various corrosion protection systems
  • Invar alloy – has slightly lower strength than steel but much lower thermal expansion

Composite cores

Composite cores consist of high tensile strength non-metallic or combinations of metallic and non-metallic materials, which offer the advantage of reduced weight and allow more conductor material for the same weight per unit length. Some examples are:

  • Fibre glass/carbon fibre – claimed to be 25% stronger and 60% lighter than a traditional steel core
  • Alumina – high purity aluminium reinforced with alumina fibres

Disadvantages

The main disadvantage of HTLS is a higher conductor cost, but this is usually offset by savings of the balance of project plant, and operating costs.

HTLS conductors in common use

ACSR: Aluminium conductor steel reinforced

Fig. 1: ACSR conductor [3].

Fig. 1: ACSR conductor [3].

ACSR is not strictly a high temperature conductor but has improved tensile strength. It consists of single or multi-strand high tensile steel inner core and an aluminium outer core. Steel adds strength but also adds weight, and because the steel core is in contact with the aluminium conductor it can also carry some of the current but has higher resistance and thus higher temperature rise.

J- GAP Gap type thermal resistant aluminium alloy conductor steel reinforced

This consists of a high temperature aluminium alloy outer conductor layer and an inner tube containing ree moving high tensile steel core in grease.

Fig. 2: J-power gap HTLS conductor [4].

Fig. 2: J-power gap HTLS conductor [4].

Aluminium conductor steel support (ACSS)

The outer conductor is annealed aluminium with an inner core of high tensile steel. The HTS core carries the tension in the cable.

Fig. 2: J-power gap HTLS conductor [4].

Fig. 2: J-power gap HTLS conductor [4].

Aluminium conductor composite reinforced (ACCR)

Fig. 4: ACCR conductor [2].

Fig. 4: ACCR conductor [2].

The core is stranded from wires of high purity aluminium reinforced with alumina fibre. The outer, current carrying wires are a hardened aluminium zirconium alloy. The resulting conductor has the same strength as similar size steel core conductors, but is much lighter and sags less [9].

Aluminium conductor composite core (ACCC)

The conductor consists of an outer core of trapezoidal conductors with an inner carbon fibre/fibreglass composite core.

Fig. 5: ACCC conductor [5].

Fig. 5: ACCC conductor [5].

References

[1] EPRI: “High-temperature, low-sag transmission conductors” ,EPRI technical report 1001811, June 2002, www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001001811

[2] B Waering “Types and uses of high temperature conductors”, Cigré AG06 Seminar Bangkok, 28 February 2011.

[3] Aberdare Cables: “ACSR”, www.aberdare.co.za/acsr-british-standard-sizes

[4] Power systems: “Gap type thermal resistant aluminium alloy conductor steel reinforced”, www.jpowers.co.jp/english/product/pdf/gap_c1.pdf

[5] CTC Global: “High capacity transmission conductor product brochure, www.ctcglobal.com/products/accc-conductors

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