Overhead power lines: Answers for Beginners

Overhead power lines: Answers for Beginners

I see overhead power lines everywhere while traveling, As an electrical engineer I can’t get my eyes away from them.

I also know, there are tons of questions about overhead power lines. So in this article, I will answer 6 common questions for beginners. Let’s start.

Are Overhead Power Lines AC Or DC?

All the distribution grid is AC – 3 phase system. I can say that all power lines you come through are almost AC, not DC, however, DC is also used in transmitting high and extra high voltages i.e 500KV and more. Let’s discuss it in detail.

 

Why are AC power lines are better than DC ones? In majority of the overhead transmission systems, AC transmission lines are favored over DC transmission owing to various reasons. This preference could be due to less complexity in design, cost effectiveness and compatibility to the load.

  • Cost Efficiency: It is more complected to step up the DC voltages for long distances transmission lines, this is why DC power lines require very large size conductors due to high values of current compared to AC lines, which in turns increases the cost on overhead system. Even though the transmission efficiency improves a lot, it is still unwanted for short distance transmission systems.

  • Less complexity: Additionally, two converters (AC-DC and DC-AC) will be required at generating and receiving end of the transmission line for stepping down the voltage, as it is very difficult to step down DC voltages. This process decreases the transmission efficiency while putting extra cost on the system. In comparison to that, AC transmission lines are economically viable and has no complexity in design.

High voltage lines wouldn’t require large-size conducting cables and will have no issues in stepping up or stepping down the voltage at both ends.

  • Compatibility: with few exceptions, almost all kinds of loads at user end run on AC voltage. Ultimately, this negates the requirement of using converters, making the system very cost effective with less conversion losses.

Therefore, in overhead lines, it is still preferred to use AC power lines instead of DC power lines.

Why are Overhead Power Lines Not Insulated?

There is really no need to insulate an entire transmission line in overhead systems, simply because of :

  • Its excessive cost
  • Insulation is useless due to the conductor‘s high distance from the ground
  • Increased weight of the conductors
  • Inverse Impact of Di-Electric Strength due to high voltage levels
  • Poor cooling due to insulated conductors

Why Insulation is Useless?

In overhead transmission lines, the distance from the ground is kept very high, whereas two cables on a pole are also kept away from each other to avoid any chances of contact.

This means in normal cases, there is no requirement for any insulation material to provide protection against physical contact.

Instead of spending money on useless insulation, companies spend it on towers, insulation, and accessories for the power lines.

Inverse Impact of Di-Electric Strength:

AC transmission lines carry high values of voltage, which has inverse impact on the di-electric strength (ability to withstand maximum value of voltage before experiencing discharge of current through the insulating material). 

Therefore, instead of spending money on a thing that has little or no return value, it is preferred to rely on natural Air’s di-electric properties to act as an insulator without any cost.

Low Weight and Heat Resistance:

Insulating material can increase the weight of transmission cables, putting extra pressure on towers, which in turns means extra cost in designing towers and conductors carrying accessories.

Insulation will also slow the process of cooling down the conductors, as insulating material won’t allow the free air to pass through it.

Considering all these problems, it is generally preferred to not use any insulation in overhead lines.

What Are Voltage Levels of Overhead Power Lines?

The voltage levels of overhead transmission lines are classified on the basis of the nature of the area and loads.

These are classified as low voltage, Medium Voltage, High Voltage, extra high voltage, and Ultra High Voltage lines.

  • Low Voltage lines (70 to 600 volts): These overhead lines are used in connections involving residential units or small scaled commercial areas. The typical voltage of these lines remains below 1000 volts and mostly remains between 70 to 600 volts depending on the single or three-phase connections.
  • Medium Voltage Lines (750 V to 69 KV): This classification of overhead lines normally remains between the voltage levels of 750 V to 69 KV. However, for economic feasibility, the maximum voltage in these transmission lines remains below the 35KV threshold.

These types of transmission lines are used for feeding power to Low voltage lines and require step-down transformers.

However, in the case of synchronous motors, that typically run above 1KV level, the load can be directly connected to these lines.

  • High Voltage Lines (69KV to 220 KV): For the supply of electric power from generating stations to substations, High voltage transmission lines are used.

These types of overhead lines are economically feasible for long-distance transmission as the transmission losses remain minimal due to the low value of current passing through it.

To avoid any accidental human contact, these transmission lines are installed away from populated areas. The voltage level of these lines typically remains between 69KV to 220 KV.

  • Extra High Voltage Lines (above 300KV and up to 800 KV): For very high-power transmission systems, above 300KV and up to 800 KV, Extra high voltage lines are preferred to reduce the cost and transmission losses.
  • Ultra-High Voltage Lines: For energy resources and generating units far away from load areas, it is preferred to use to Ultra High voltage transmission lines. These lines can carry DC and AC power and have different ratings depending upon the distance and nature of the supply. Economically recommended voltage levels for these lines are at least 800 KV for DC transmission and 1000 KV for AC transmission.

Why Do Overhead Power Lines Have 3 Wires?

Why do power lines have 3 wires

Transmission lines doesn’t typically connect directly to the load, and hence requires no neutral wire to provide added safety factor.

 

Furthermore, in long distance transmission lines, the cost efficiency is an important factor, and therefore instead of transmitting power on a Star connection, companies do prefer Delta connection due to the fact that it has no neutral wire which saves up the cost.

 

The logic behind this phenomenon is the difference in phase angle in a 3-Phase delta system. Every phase angle leads or lags the other by 120 degrees, and if we add these differences together, the net value becomes zero.

It means that for 3-phase high voltage systems, there is no requirement for a neutral wire, and because of this reason, we see only 3 wires in an overhead power line.

Why Do Overhead Power Lines Never Cross Each Other?

In transmission systems, the safety factor is very important and requires necessary ground clearance to avoid any human contact.

Overhead power lines have their potential differences, and in cases, any of the two lines cross each other, it will compromise the recommended level of safety. This is due to the reason of di-electric nature of the air in between two cables.

When two cables cross, having a difference in potential means the air between them acts as a natural capacitor and keeps on charging/ discharging.

This results in heavy transmission losses which are undesirable in transmission systems. Therefore, it is always ensured that overhead power lines never cross each other.

Why Is Aluminum Conductor Steel Reinforced (ACSR)?

The overhead power lines are feeding power over long distances, and in high voltage. In commercial sectors, it is always desired to keep the cost at minimum while providing maximum efficiency.

 

Typically, Copper is preferred in household connections, because of its better conductance. However, in overhead cables, the use of copper could be costly with increased weight and potential safety risk in case of heavy storms and earthquakes.

Why is Aluminum Conductor is  Preferred in OHTL?

In comparison to copper, Aluminum has considerably less conductance (33% less), and may not be an ideal choice for household connection. However, for transmission lines, it is a preferred option due to various reasons.

 

  • Lighter weight: Aluminum conductors have lower density and are lighter in weight than copper, two times lighter, which makes it easier to install at high distances.
  • Compression stress: When we are talking about OHTL we can’t neglect compression stress, Aluminum is less prompt to this stress, which makes it better than copper in OHTL.
  • Economic viability: Aluminum is cheaper and lower in price than copper, and proves an economical solution rather than installing new transmission lines.

Why Using a Steel Core?

Even though Aluminum conductors are bulky in size, their weight is much less than a copper conductor. Now for transmission lines having high span lengths (tower to tower distance), the vulnerability associated with aluminum’s weak strength is compensated by using a steel core in the conductor. As a result, the overall strength of power cable will be increased with high durability and resistance against storms and environmental factors.

What should be the distance between the conductors of OHTL?

There is no particular formula for the calculation of the distance of a conductor from a medium-voltage transmission line. The reason is that so many factors affect this function of distance.

The relation of those factors and their empirical effect in determining the distance makes the formulae really complex. So, normally 3-4 meters distance is kept from the medium voltage transmission lines.

This distance is horizontal if the conductors are to be placed horizontally and vertical if the conductor is to be placed vertically with respect to the position of the transmission line.

So, some general information that can be useful in this topic is as if the transmission lines are carrying a voltage less than 11 kV then a 4-5 meters distance is appreciated.

More than 11kV should be 5-6 meters apart. The height of these transmission lines should be at least 6-6.5 meters.

In southern Asia, this is general practice because the transmission lines are normally overhead transmission lines. In the United States and Europe, transmission lines are normally underground.

The distance of a transmission line with the potential of 66kV from a conductor should be 6.1 meters from the ground and 8 meters over a highway.

The distance of a transmission line with the potential of 132kV from a conductor should be 6.1 meters from the ground and 8.6 meters over a highway.

The distance of a transmission line with the potential of 220kV from a conductor should be 7 meters from the ground and 9.8 meters over a highway.

The distance of a transmission line with the potential of 400kV from a conductor should be 8.8 meters from the ground and 10.8 meters over a highway.

These distances may vary from country to country depending upon their policies.

What is the minimum phase-to-phase clearance of OHTL?

The phase-to-phase clearance of the transmission lines is as:

For 11kV the minimum phase-to-phase clearance must be 787.5 mm.

For 110kV the phase-to-phase clearance must be 990.6 mm.

For 132kV the spacing should be 1219 mm.

For 220kV there should be a distance of 2057.4 mm.

The distance between two transmission lines carrying 765 kV is 45 meters. This is the horizontal distance between these transmission lines.

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

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

Mostly, Aluminum is used as the transmission line conductor due to its lightweight, low cost, and negligible difference in conductivity from copper.

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:

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

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

AAC Conductors

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

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

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

AAAC All Aluminum Alloy Conductor:

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

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

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

ACAR Aluminum Conductor, Alloy Reinforced:

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

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

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 a low value of di-electric strength.

  • Porcelain Insulators provide Di-electric strength up to 40-200 kV/inch which 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 the late 19th century to the 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 Silicon Rubber or EPDM (ethylene-propylene-diene-monomer-rubber) is used because of its outstanding hydrophobic properties and excellent thermal resistance.

We all see overhead power lines and high towers carrying 6 Aluminum conductors. Did you ask yourself, 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 sets 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 has a 3-phase circuit in it. Each wire is 120 ̊ out of phase from the other wire of the same set and a similar design is used for the other set.

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

For more details about short circuits read my article here.

Why Do Overhead Power Lines Glow?

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

The reason for corona discharge is when the high-velocity atoms i.e., the velocity that is more than the dielectric breakdown limits of air, collide with the high Electric field intensity of a power line, and 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 in the area where the air is smooth and slow.

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

When the air density is higher (i.e., high atmospheric pressure) the greater will be the ionization and the 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 an influence on the corona discharge. The larger the spacing the lower the dielectric 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 a 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 conductor in OHTL is the guard wire. This wire is a protection against the strikes of lightning.

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

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?

The balls on the OHTL are made of fiberglass, while the clamp material is usually made of galvanized mild steel.

They are set for airplanes warning about the hazard of the power line close to the flying path.

These balls on transmission lines have a diameter which is greater than 600 mm. Each ball on a 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 contrasted with the background so they can be easily seen.

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

Also, clamps are made of the 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 in a 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.