Overhead Transmission Lines: 8 Answers You Should Know

Overhead Transmission Lines: 8 Answers You Should Know

Overhead transmission lines consists of many accessories, each of these accessories has its own specs. When I was doing a deep internet searching about OHTL I found 8 basic questions every electrician should know its answers.

In this article I will answer these questions in details. Let’s go in details.

What Are OHTL Conductors Made Of?

Not like underground cables, OHTL conductors are not insulated, are not copper and are single conductors.

Which material is used in Overheat Transmission Lines (OHTL) conductors? As a short answer I would say, Aluminum. But keep reading because there are 4 different types of these conductors.

The overhead power lines, mounted on high steel and aluminum towers, are generally used for power transmission. For efficient power transmission, we need good Ohmic conductors i.e., Copper or Aluminum.

Copper is highly efficient with high current density even with low cross-sectional area, yet copper isn’t preferred over aluminum due to its high cost, also because aluminum has some properties that makes it perfect for this purpose.

Mostly, Aluminum is used as the transmission lines conductor was due to its light weight, low cost and negligible difference of conductivity from copper.

You can find detailed answer about Why are OHTL use aluminum not copper? in my article here.

Some types of OHTL Aluminum conductors are:

AAC: All Aluminum Conductor
AAAC: All Aluminum Alloy Conductor
ACSR: Aluminum Conductor, Steel Reinforced
ACAR: Aluminum Conductor, Alloy Reinforced

ACSR Conductor:

Most widely used conductors for transmission lines are Aluminum Conductor Steel Reinforced ACSR.

It has a steel core whose steel content can be varied from (6-40) % depending upon the requirement of strength.

The center wires might be zinc covered (galvanized) steel or aluminum covered (aluminized) steel. The galvanization coatings are thin that are applied to prevent steel from corrosion.

And the purpose of central steel core is to provide additional mechanical strength and so sag is remarkably less than all other aluminum conductors.

The market of ACSR conductors is high because they can be used for all transmission and distribution purposes.

AAC Conductors

AAC (All Aluminum conductor) it is also called as ASC (Aluminum Stranded Conductor).

It has conductivity of almost 61 %. It has a good conductivity as per standards IACS (International Annealed Copper Standard) but due to its relatively poor strength, it has limited transmission and only rural distribution lines.

They are mostly used in urban areas where users need high conductivity rate.

AAAC All Aluminum Alloy Conductor:

They are made up of aluminum alloy 6201 that has high strength aluminum-Magnesium-Silicon Alloy.

It has a good conductivity of almost 52.5% and has a better mechanical strength. AAC’s has lighter weight as compared to ACSR of equal strength and capacity of current.

It can be used for distribution purposes but are not preferred for transmission. They are mostly employed in coastal areas because they have great resistance against corrosion.

ACAR Aluminum Conductor, Alloy Reinforced:

It is formed by wrapping of high purityaluminum (aluminum 1350) on a high strength Aluminum-Magnesium-Silicon alloy (6201 aluminum alloy) core.

It has advantage that it has both mechanical and electrical power equals to ACSR conductors. It can be used in transmission and distribution purposes.

Now let’s move forward to the next question about OHTL insulators.

What Are Power Line Insulators Made Of?

Porcelain, Glass and other composite materials are used to make the insulators. Porcelain itself is a composite material.

In transmission we need an insulator that has low value of di-electric strength.

  • Porcelain Insulators provide the Di-electric strength up to 40-200 kV/inch that is much better than that of glass i.e., 2000-3000 kV/inch. Quartz (SiO2), Clay (Al2O32SiO2·2H2O), Feldspar (KAlSi3O8–NaAlSi3O8–CaAl2Si2O8) and alumina (Al₂O₃) are used to make the porcelain insulators.
  • Glass insulators were used from late 19th century to early 20th century and now have been replaced with porcelain insulators due to their bad Di-electric strength.
  • Silicon Rubber: In highly polluted and temperate regions the Silicon Rubber or EPDM (ethylene-propylene-diene-monomer-rubber) are used because of outstanding hydrophobic properties and excellent thermal resistance.

We all see overhead power lines high towers carrying 6 Aluminum conductors. Did you ask your self, Why 6 conductors instead of 3 (as this is a three phase system)? Well let’s answer this question.

Why Do Transmission Towers Have 6 Conductors?

Generally, the overhead transmission towers, have six wires mounted on each of them.

The reason behind this kind of design is to maximize the power transmission. 2 set of 3-phase circuits are transmitted at the same time saving the infrastructure and increasing the energy transfer from electricity generation plants to the end consumers.

Each set of three wires have a 3-phase circuit in it. Each wire is 120 ̊ out of phase from the other wire of them same set and a similar designing is used for the other set.

Say the potential of first wire is V1 = V<0 ̊. Then the next wire will be at V2 =V<120 ̊.and the last wire will be having a potential of V3 =V<-120 ̊. These sets exclude the guard wires i.e., the ground wires.

Why Does Rain Not Cause Overhead Power Lines Short Circuit?

The rain does not short circuit the transmission lines because:

  • There is a distance of 10 ft to 25 ft between two adjacent lines (varies according to the voltage level). So, it makes it almost impossible to short two wires due to raindrops.
  • Moreover, water isn’t very good conductor because conventionally its conductivity is set zero.

You know what? Cleaning energized overhead power lines using water jets is one of the effective OHTL maintenance. Of course, this risky task is performed under precautions.

In some case when there is acid rain, the conductivity improves and becomes 0.5 μ S/cm. In the case of short circuiting a wire with pylon (the transmission tower) it needs a stream of water to run down from wire to metal of pylon even in that case the precautionary measure i.e., the insulators are already there to handle these kinds of situations.

Why Do Overhead Power Lines Glow?

Lights glow in the power transmission lines due to the corona discharge.

The reason of corona discharge is when the high velocity atoms i.e., velocity that is more than the dielectric breakdown limits of air, collide with the high Electric field intensity of a power line, the Electric field ionizes the air particles.

Thus, these particles release their ionization energies that can be in the form of light and sound, some ozone gas O3 is also emitted in the process.

The higher the transmission voltage the higher will be the corona effect. Also, in the windy areas, the corona effect is much more than the area where air is smooth and slow.

The main reason behind this is the interesting fact that the di-electric breakdown limit is directly proportional to the density of air.

When the air dense is higher (i.e., high atmospheric pressure) the greater will be the ionization and greater will be the corona discharge.

Also, a greater factor for corona discharge is the cross-sectional area of the conductor. The lesser the cross-sectional area the lesser will be the corona discharge as the ionization will be lower than the ionization in the higher cross-sectional area conductors.

The dirt present on the lines increases the corona effect because they reduce the voltage breakdown point.

The spacing between the transmission lines also has influence on the corona discharge. The larger the spacing the lower the di-electric stress on the air, and after a certain limit it may lead to zero.

Why Do OHTL Insulators Have Skirting?

To distribute the high voltage distribution on the cable lug, we use skirting insulators in transmission lines and the pylon as bridge.

Using the Creepage distance i.e., the distance along the surface increases but the total length of the insulator does not increase.

Thus, the high voltage of about 35 kV gets distributed on these coil-like structures and the insulation becomes 100%.

Top Most Conductor in Transmission Line:

The top most conductor in OHTL is the guard wire. This wire is a protection against the strikes of lightning.

At the transformers station from both ends of the power lines, the top wire is connected to an earthing grid.

By this way any lightning strike will find a proper path to discharge through the earthing grid to the ground.

What Are The Balls On OHTL Made Of?

These balls on transmission lines have a diameter which is greater than 600mm. Each ball on transmission line is made of solid color.

Orange balls on power lines must be displayed alternately with white balls or red and white balls on transmission lines. And colored balls should be contrast with background so they can be easily seen.

These round balls on transmission lines are made of 2 hemispheres that are attached on transmission lines with aluminum alloy clamps. And the size of these clamps is dependent on earth wire dimensions.

Also, clamps are made of finest quality and polished on both sides. Moreover, the color is infused with UV-resistant pigment to increase its life.

These balls should be installed with equal spacing along the wire maintaining a gap of 200 feet. If there are multiple transmission lines on certain location then these balls are installed on an adjacent line if the distance of outer lines is greater than 200 feet.

Types of these Balls:

  1. ABS Standard Marker.
  2. EHV 115KV+ Marker.
  3. High temperature Marker.
  4. Modern self-illuminated Marker.
  5. Low-cost Marker.