Overhead Conductors

The wires through which electricity can flow are called electrical conductors. The conductors are mainly made of Copper and Aluminium. The conductors are mainly manufactured by hard-drawn processes. All the hard drawn process reduces the conductivity of the conductor but at the same time, it increases the tensile strength of the conductor.

An electrical conductor may be either a solid type or a stranded type. The solid type conductor has only one solid core or wire. The stranded conductors are made by twisting a number of wires together. Each of the twisted wires is called the strand of the conductor. The standard numbers of strands in a stranded conductor may be either 7, 19, 37, or 61.

stranded conductors

For 7 strands conductor, one strand is at the center of the cross-section of the conductor and six other strands surround that central strand.
For 19 strands conductor, with the just above-mentioned convention, there is an additional layer of 12 strands outside.
For 37 strands conductor, with the just above-mentioned convention, there is an additional layer of 18 stands outside.
For 61 strands conductor, with the just above-mentioned convention, there is an additional layer of 24 strands outside.

Calculation of the number of strands in a stranded conductor

If there are ‘n’ layers of strands in a stranded conductor, 3n(n+1) + 1 will be the number of strands of the conductor.

Calculation of overall diameter of a stranded conductor

If d mm is the diameter of a single strand in the conductor, the overall diameter of the conductor will be (2n +1)d mm.

A stranded conductor is more flexible than an equivalent solid conductor. Also, it has more mechanical strength than an equivalent solid conductor of the same material.

Although the conductivity of copper is much better than that of aluminum, still the use of copper conductors is limited because of its cost. On the other hand, the conductivity of aluminum is poorer than that of copper. But the availability of aluminum is quite more and the cost of aluminum is quite less than that of copper. This is the reason aluminum is a more popular choice as a conductor material than copper.

Calculation of the overall diameter of standard ACSR panther conductor

A standard ACSR panther conductor is represented as 30/7/3.00 mm. It means the conductor has 30 aluminum strands, 7 steel strands and each of the strands for both aluminum and steel has a diameter of 3 mm. Therefore, the total number of strands in the conductor is 30 + 7 = 37.

Let us consider ‘n’ as the number of layers of strands in the conductor. Therefore as per formula, we can write,
3n(n + 1) + 1 = 37 or n = 3.

Now by applying the formula for determining the overall diameter of stranded conductors, we get,
(2n + 1)d = (2X3 + 1)3.00 = 7X3.00 = 21.00 mm.

Copper Conductors

The specific resistance of a hard-drawn copper conductor is 1.72 micro ohm centimeter. The tensile strength of the hard-drawn copper conductor is 4,200 kg per square centimeter. The weight of copper is 8,900 kg per cubic meter. Copper conductors are very rarely used in overhead lines. For electrical distribution purposes, solid copper conductors are used and for electrical transmission purposes, stranded copper conductors are used.

Aluminum Conductors

Previously’ copper conductors were used for transmission purposes. But aluminum is a much cheaper and lighter metal compared to copper. The specific resistance of a hard-drawn aluminum conductor is 2.83 micro ohm centimeter. The tensile strength of the hard-drawn aluminum conductor is 2100 kg per square centimeter. The weight of aluminum is 2700 kg per cubic meter.

The conductivity of an aluminum conductor is 60% of a copper conductor of the same size. The tensile strength of an aluminum conductor is 50% of a copper conductor of the equivalent cross-section. The weight of aluminum is 33% of an equal volume of copper. Therefore for obtaining the same conductivity, the cross-section of an aluminum conductor should be 1.66 times that of a copper conductor. Due to the lightweight of an aluminum conductor, it can be used for a longer span length with comparatively less sag.

An aluminum conductor has a larger diameter than that of an equivalent copper conductor. Because of that, the lines of electric flux originating on the conductor are larger. The concentration of the flux lines is lower surround the conductor surface. Therefore the voltage gradient nearer the conductor surface is smaller compared to the equivalent copper conductor. It reduces the ionization of the surrounding air of the conductor. As a result, the corona effect gets reduced in aluminum conductors.

Aluminum conductors are always made stranded. There are mainly two types of stranded aluminum conductors.

  • All Aluminum Conductor (AAC)
  • All Aluminum Alloy Conductors (AAAC),
  • Aluminum Conductor with Steel Reinforced (ACSR),
  • Aluminum Conductor with Aluminum Alloy Reinforced (ACAR).

All Aluminum Alloy Conductor (AAAC)

In this type of stranded aluminum conductor, all the strands are made of a special type of aluminum alloy. The number of strands may be either 7, 19, 37, and 61, etc.

Some of the electricity utility companies are using AAAC conductors to overcome the problem of conductor theft. Actually, in some areas, AAC (All Aluminium Conductor) and ACSR conductors are often theft due to their pure aluminum metal. The alloy used for AAAC conductors, can not be re-cycled, and therefore the problem of stealing reduces. This is an Aluminium-Magnesium-Silicon alloy with 0.6-0.9% Magnesium and 0.5-0.9% Silicon. Also for avoiding corrosion in the steel core of ACSR conductors, AAAC is used, mainly in the coastal moist areas. But AAAC could not gain much popularity for the purpose of overhead transmission.

Aluminum Conductor with Steel Reinforced (ACSR)

To increase the tensile strength of a stranded conductor, in addition to aluminum strands some steel strands are used at its center. The alternate layers of a stranded conductor are spiraled in the reverse direction.  This arrangement is to prevent unwinding. The stranding provides flexibility to the conductor.

There are three types of ACSR conductors used for EHV transmission purposes.

  • For 132KV systems, ACSR Panther conductors are used. A standard ACSR Panther conductor has steel 7 strands of dia 3.00 mm and 30 aluminum strands of dia 3.00 mm.
  • For 220KV systems, ACSR Zebra conductors are used. A standard ACSR Zebra conductor has steel 7 strands of dia 3.18 mm and 54 aluminum strands of dia 3.18 mm.
  • For 400KV systems, ACSR Moose conductors are used. A standard ACSR Moose conductor has steel 7 strands of dia 3.53 mm and 54 aluminum strands of dia 3.53 mm.

Aluminum used for the strands of the ACSR is above 99.5% pure. Stell used for reinforcement has around 0.65% carbon.

Some properties of ACSR conductors

Diameter of ACSR
  • The overall diameter of ACSR Panther is 21.00 mm.
  • The overall diameter of ACSR Zebra is 28.62 mm.
  • The overall diameter of ACSR Moose is 31.77 mm.
Weight of ACSR
  • The weight of ACSR Panther is 976 kg/km.
  • The weight of ACSR Zebra is 1600 kg/km.
  • The weight of ACSR Moose is 2004 kg/km.

Bundled ACSR Conductors

  • To fulfill extra high current carrying capacity sometimes more than one conductor per phase is used per phase. These conductors are physically parallel to each other with the help of spacers in a specific interval. The individual conductor in a bundle conductor is known as a sub-conductor.
  • In ultra-high voltage systems, i.e. in 400KV or 765KV systems, the voltage gradient in the air adjacent to the conductor surface is quite high. This leads to high corona loss and also a significant interference with communication. When a single conductor is divided into two or more equally spaced sub conductors, the diameter of the flux lines also increases accordingly. As a result, the voltage gradient reduces considerably. Therefore corona and interference effects come down significantly. The standard spacing between sub-conductors is 450 mm in India.
  • It is obvious the current is not divided exactly between the sub – conductors of a bundle conductor. Although the transposition of the sub – conductors within the bundle can solve the issue. But the transposition is not practically done. Because this difference of current is of no practical importance.
  • Bundling conductors per phase reduces the reactance. Actually, bundling increases the Geometric Mean Radius (GMR) of the overall conductor. That is the cause of the reduction of the reactance of the conductor.

Current Carrying Capacity of ACSR Conductors

Heat Balance Equation of Electrical Conductors

There are two reasons for which the temperature of a conductor increases. The first one is the ohmic loss in the conductor due to the flow of current. The second one is sunlight. Again there are two reasons for decreasing the temperature of a conductor. The convectional heat loss and radiation of heat. In the equilibrium condition, the total gain of heat is exactly equal to the loss of heat in a conductor.

Where, I2R = Heat generated in the conductor due to flow of current ‘ I ‘ Amperes, R is the resistance of the conductor per meter.
Qs = Heat gain in watts per meter of the conductor due to sunlight,
Qc = Convection heat loss in watts per meter of conductor,
Qr = Radiated heat loss in watts per meter of conductor.
From the above equation, current carrying capacity ‘I’ can be determined as,
The effect of heat gain due to magnetic heat and corona heating and heat loss due to evaporation is usually negligible, hence not considered.

  • The current carrying capacity of ACSR Panther at 75°C is 340 A.
  • The current carrying capacity of ACSR Zebra at 75°C is 480 A.
  • The current carrying capacity of ACSR Moose at 75°C is 540 A.

 

Steel Conductors

The conductivity of a steel conductor is about 10% of a copper conductor of the equivalent cross-section. But the tensile strength of a steel conductor is much more than that of an equivalent-sized copper conductor. It is about 10,000 kg per square centimeter. Also, the cost of a steel conductor is much less than that of an electrically equivalent copper conductor. Because of these reasons, steel conductors are used where cost optimization is an important criterion for constructing overhead lines. For example, in short, in rural distribution networks sometimes steel conductors are used. Rust is a major problem of steel conductors. Therefore steel conductors have to be replaced in a regular interval. This makes the expenditure of maintenance on overhead lines with steel conductors much higher. Sometimes by using a layer of aluminum on the conductor the overall conductivity of the conductor is increased to some extent and also it saves the conductor from rusting. The steel conductor with an aluminum layer is called the alumoweld steel conductor. Although presently the use of steel conductors even for a short distribution rural line has become obsolete.

 

High Temperature Low Sag (HTLS) Conductors

There are mainly three types of HTLS conductors used for overhead transmission lines.

  • Aluminum Conductor with Steel Support (ACSS)
  • Aluminum Conductor with Composite Reinforce (ACCR)
  • Aluminum Conductor with Composite Core (ACCC)

Aluminum Conductor with Steel Support (ACSS)

The Aluminum Conductor with Steel Support (ACSS) conductor was first developed in the year of 1970. Here trapezoidal-shaped aluminum strands are often used to utilize the empty space between the circular strands of traditional ACSR. It increases the effective cross-section of the conductive aluminum portion by 20 to 25%. The ACSS conductor looks like a traditional ACSR. Because like an ACSR, ACSS is made of steel cores and aluminum outer strands. But instead of hard-drawn aluminum used in ACSR, annealed, or “O” tempered soft aluminum is used for ACSS. This is the basic difference between ACSR and ACSS conductors. The conductivity of hard-drawn aluminum in ACSR is 61.2 percent of that of copper whereas, the conductivity of annealed aluminum in ACSS is 63 percent of that of copper. This increases the current carrying capacity of an ACSS conductor. Although the tensile strength of soft aluminum is one-third of that of hard-drawn aluminum. For ACSS nearly the entire tension force acts on the steel support or reinforce.

Above 93°C the aluminum starts annealing. This uneven annealing weakens the ACSR conductors. Therefore breaking of conductor, during wind, and the force during fault may occur mainly in the case of old ACSR conductors. This is why ACSR is strictly restricted for using below 75°C. Since ACSS HTLS conductors use pre-annealed soft aluminum, annealing during service conditions can not further affect or weaken the conductor. Therefore ACSS and other similar types of HTLS can be used at much higher temperatures than 75°C. Theoretically, an ACSS has a temperature limit of 250°C but practically it is kept under 200°C.

Aluminum Conductor with Composite Reinforce (ACCR)

For the last three years, ACCR conductors have gained popularity.  ACCR is a type of HTLS conductor. This HTLS uses wires composed of alumina oxide fibers embedded in an aluminum matrix as the core of the conductor. This arrangement is just like replacing the traditional steel wires in the center of the traditional conductor. This reduces the weight and sag of the conductor.

Aluminum Conductor with Composite Core (ACCC)

It was developed in 2005. It uses a carbon-fiber core as reinforcement of the conductor. The actual material used for the purpose is Carbon Fiber Thermoset Polymer Matrix Composite Core. To avoid the galvanic corrosion between aluminum and carbon fiber, a thin protective layer of glass fiber is provided on the carbon – fiber core.   There is always a high risk of galvanic corrosion with carbon fiber cores because of the substantial difference in electrode potential between aluminum and carbon or graphite.

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