Overhead Power Cables Guide
Table of Contents:
- Chapter 1: What is Overhead Power Cable?
- Chapter 2: What is the construction of Overhead Power Cable?
- Chapter 3: What are the differences between Copper and Aluminum conductors?
- Chapter 4: What are the types of Overhead Power Cable?
- Chapter 5: What are the model specifications of Overhead Power Cable?
- Chapter 6: What are the standards of Overhead Power Cable?
- Chapter 7: What are the features of Overhead Power Cable?
- Chapter 8: What are the applications of Overhead Power Cable?
- Chapter 9: What are the precautions of Overhead Power Cable?
- Chapter 10: What is the difference between Overhead and Underground Power cable?
Chapter 1: What is Overhead Power Cable?
Overhead Power cable is the cable present in the overhead electrical power line. The Overhead power cable is for the power distribution. Electrical power transmission and distribution system use Overhead Power cable. To supply energy over vast distances, the cable that works best is Overhead Power cable.
You must have seen in your surrounding the electrical cables. You see them along the streets, behind the trees, backyards, and everywhere.
The name Overhead Power cable is because it is above our heads. The arrangement of the power distribution system is in the air. The suspension is through poles and supporting materials. The Overhead Power cable has specific characteristics that you will get to know as we proceed.
Chapter 2: What is the construction of Overhead Power cable?
Overhead Power cable installation is very witty. To suspend a cable in air, you need to have a lot of things supporting it. You must have observed the poles and many other things on distribution power lines.
Poles: The poles that we use are generally are 30 to 45ft in height. To set them in the land, we dig them 6 to 8ft deep.
Three-phase circuit: Most commonly, you will see a three-phase circuit in power distribution lines. Three-phase circuit means, it uses three wires at a time. The layout is horizontal. 8 to 10ft wooden cross arms are for the support of a three-phase circuit.
Span length: In urban areas, the downrange is 100ft to 150ft. In rural areas, it is 300ft to 400ft.
Neutral: A neutral system is an underbuilt system of distribution circuits. The neutral is present for safety purposes. It acts as a return pathway for any unbalanced loads. It is a safe pathway.
The location of neutral is below the phase conductors. It is present 3ft to 5ft below the phase conductors. In open areas where there is danger of thunderstorms, heavy rains, and lightning, its location is different. Then the neutral places above the phase conductors. In this way, the neutral protects from lightning. It acts as a shield wire.
Secondary circuit: Under the primary circuits are present secondary circuits. They have individual or combined neutral systems.
Pole material: Steel, fibreglass, etc. are the pole materials. Most commonly, wood is the pole material.
The benefits of a wooden pole are that they are easy to climb. Attachments on a wooden pole are simple. A wooden pole acts as an insulator between the ground and high conductor wire.
Conductor: The conductor in Overhead Power cable is aluminium
Insulator: We install insulators along with Overhead Power cable. The presence of an insulator is essential for power distribution lines. If the insulation is not present, the current will flow through the poles to the ground. Therefore, insulators are always present on poles.
Overhead Power cable insulator should have the following properties:
Insulator present on power distribution lines should have high mechanical strength. Its strength will make an insulator able to withstand conductor load. Overhead Power cable suspends in the air. It moves with the wind and strength occurrences. The insulation should have high mechanical strength to face the movements of the conductor.
Leakage currents are a waste. Leakage currents have to be minimum. The insulator should have high electrical resistance to minimize the leakage currents.
The insulator should high relative permittivity. In this way, dielectric strength will be high.
The insulator should have a high ratio of puncture strength to flashover.
Types of Overhead Power line insulator:
Best insulator selection is necessary for safety. If the insulation is not a proper one, the current will go down to the earth. It will make Overhead Power cable useless. Following are the types of Overhead Power line insulators:
- Pin type insulators
- Suspension type insulators
- Strain insulators
- Shackle insulators
Pin type insulators: Distribution systems of 33kV use pin insulators. The location of installation is cross arms of the pole. On the top of a pin insulator is a groove. That’s where we attach the conductor. Conductor wire passes through this groove. Then we bind it with the same cable as of the conductor.
The material of the pin insulator is glass, plastic, porcelain. Pin insulators have a covering of petticoats or rain sheds. This covering is significant. We know that the insulator has exposure to rain. Anything moist on the insulation will make it a conductor. The current will flow towards the pole and then to the ground creating a hazardous situation. Thus it is necessary to add rain shed covering on our insulator.
Insulation failure: You should select an insulator according to the line voltage. The insulator should have a property of bearing high electrical stress. Greater the electrical strength of the insulator, the more it can take the line voltage. Flashover or puncture occurs when there is excess electrical stress. It can break the insulation.
Between a line conductor and insulation pin occurs and electric discharge. This is known as flashover. An electrical discharge is in the form of an arc. In the surrounding of An insulator, the air is present. The electrical discharge jumps through the air forming an arch. The safety factor value for pin insulators is 10.
In a puncture breakdown, electrical discharge occurs through the body of the insulator. Electrical discharge occurs from the conductor to the insulation pin. Puncture breakdown permanently damages the insulator. To avoid puncture breakdown, insulation material should be thick.
The safety factor is the ratio of puncture strength to flashover voltage. The safety factor value should be high. A high amount of safety factor shows that flashover occurs before the puncture breakdown.
Suspension Insulators are applicable for voltages above 33 kV. It is because pin insulators get bulky at higher voltages. In a suspension, insulation is present porcelain disks. These disks connect with metal links. Thus it looks like a string. In transmission lines, we prefer suspension insulators.
In dead ends and sharp corners, we use strain insulators. At dead ends of transmission lines is present high tensile load.
In low voltage distribution, we use shackle insulators.
We can fix a shackle insulator directly to a pole or a cross arm.
Shackle insulator installation can be both vertical and horizontal.
Chapter 3: What are the differences between Copper and Aluminium conductors?
The most known electrical conductors are copper and aluminium. They are known for their low resistivity and high conductivity. Both are corrosion resistant. Still, the most conductive metal is silver. But silver is not used as it is very costly. Copper and aluminium have individual features making them specific for various applications.
Conductivity: The International Annealed Copper Standard (IACS) determined copper a standard for electrical conductivity. It is valued 100% with a resistivity of 1.724µΩcm at 20°C. The Alloys of copper decrease its conductivity but enhance other features. Alloys have more temperature resistance, high tensile strength, and high flex endurance.
Aluminium is 61% as conductive as copper of the same size. To match the conductivity of a copper conductor, a much more significant proportion of Aluminum will be required.
|Cost: Copper is expensive in comparison to aluminium. The prices of copper fluctuate, whereas the prices of aluminium are stable.
|Weight: aluminium is three times lighter in weight. That is why overhead Power cable Uses aluminium. It is lightweight and easy to use.
|Insulation: Aluminum has more cross-section areas, thus requires more insulation.
|Tensile strength: Copper has twice the tensile strength.
|Thermal conductivity: Copper has higher thermal conductivity.
Advantages and disadvantages of aluminium:
Aluminium offers the same amount of conductivity as copper because of its large surface area.
Aluminium reduces corona.
Aluminium is cheaper than copper.
Aluminium conductivity is less.
Aluminium is light in weight. Large cross arms are required for the support of aluminium Overhead Power cable as it swings in the air.
Aluminium has less tensile strength.
Aluminium has less specific gravity than copper.
Chapter 4: What are the types of Overhead Power cable?
Overhead Power cable types are according to the conductor it has.
Following are aluminium overhead Power cable:
All Aluminium Conductor (AAC)
- Aluminum Conductor, Aluminum Reinforced (ACAR)
- All Aluminium Alloy Conductor (AAAC)
- Aluminum Conductor Steel Reinforced (ACSR)
- International Annealed Copper Stand (IACS)
Chapter 5: What are the model specifications of Overhead Power cable?
We are going to explain the features of types of Overhead Power cable. You can select for your use accordingly.
All Aluminium Conductor (AAC):
AAC has more span length than other types of Overhead Power cable. But it has lesser strength. The cost of AAC and ACSR is equal. AAC has better conductivity at a lower voltage than ACSR.
Aluminium conductor, Aluminum Reinforced (ACAR):
ACAR us the most expansive of all types of Overhead Power conductors. ACAR is cheap as compared to AAAC. ACAR is prone to corrosion.
All Aluminum Alloy Conductor:
It is most favourable for use in distribution lines. In power distribution lines at mountains, riverside, and swamps, AAAC is best to use because it is light in weight. AAAC use over ACSR is because of it’s lightweight.
In longer spans, we use AAAC type of Overhead Power. In AAAC, an alloy is present, which makes it expensive and different than ACSR.
Aluminum Conductor, Steel Reinforced (ACSR):
For longer spans, we use ACSR. Strands of steel and aluminium make ACSR. The number of strands is shown by x/y/z. x indicates the name of aluminium strandsy indicates the number of steel strands
z shows the diameter of each strand.
The amount of steel strands is 7 to 19. Around which aluminium strands are present.
Strands are going to provide flexibility. 7,9, 36, 61, 91, or more could be the number of strands.
Expanded ACSR is the one in which A filler separates aluminium and steel strands. The liner can be a paper. EHV lines use developed ACSR.
Expanded ACSR has a large diameter.
International Annealed Copper Strand (IACS):
It is a standard type of Overhead Power cable. It is a 100% pure conductor, that is why it acts as a standard.
Chapter 6: What are the standards of Overhead Power Cable?
Following are the standards of Overhead Power Cable:
|EN 50182: It is a Conductor for overhead lines. It is a concentric round wire with a lay stranded conductor.
|EN 50183: It is a Conductor for overhead lines. It has aluminium–magnesium–silicon alloy wires.
|BS 183: It has specifications for general-purpose galvanized steel wire strand.
|BS 7884: It has a specification for copper and copper–cadmium conductors that are for overhead systems.
Chapter 7: What are the features of Overhead Power Cable?
Overhead Power cable Is for long-distance power distribution. Electricity supply from power plants to your home is through Overhead Power cable. There are many reasons why we chose the Overhead Power cable.
Following are the features of Overhead Power Cable:
Conductivity: The conductor for Overhead Power cable is aluminium. Its conductivity is less than copper. But the large surface area of aluminium balances its conductivity with copper. Aluminium conductivity is 61% less than copper.
Large surface area: Aluminum has a large surface area. This feature makes Overhead Power cable able to conduct large electric currents.
Insulation: The large surface area of the aluminium conductor requires more insulation material.
Cost: Aluminum is a cheap metal. The cost of Overhead Power Cable depends on the area, type, distance, and load. Overhead Power cable installation is the most costly.
Longer the span of conductor more material it requires for its installation.
Then it will require
The single-phase requires less material for an installation; thus, it costs a little less.
Three phases require more material. Therefore, it costs high. This type of installation is in suburban areas.
Insulator: In the installation of Overhead Power cable, we add an insulator. The purpose of adding insulation is to prevent flashovers and puncture breakdowns. Insulation prevents current from passing through poles to the ground. These events can lead to insulator failure.
Weight: The insulatoThereuponin Overhead Power cable is aluminium
Aluminium is light in weight. Due to its lightweight, we use it in Overhead Power cable. It suspends over vast distances.
Corona: Corona is a phenomenon. In corona, ionization of air due to high voltage. Corona produces violet light around the conductor. This phenomenon also produces a hissing sound.
It is an undesirable situation because it produces ozone gas. The conductor aluminium prevents corona formation. Thus, the corona phenomenon does not occur on Overhead Power cable.
Three-phase wire: They carry a voltage of 120 to 240 volts
If you observe there are three wires,
Two wires carry electricity from the transformer. These two are insulated.
One wire connects to the grounding wire. It is bare neutral wire.
Chapter 8: What are the applications of Overhead Power Cable?
Following are the applications of Overhead Power cable:
|Overhead telephone line
|Cable TV lines
|Overhead power lines
Chapter 9: What are the precautions for Overhead Power Cable?
Broken Overhead Power cables are dangerous when they come in contact with vehicles. Even the intact Overhead Power cable is risky when the touch vehicles. Heightened vehicles such as buses, tractors, trains, cranes are more susceptible.
Place warning signs outside the vehicles. Farmers and drivers should be counselled about what to do when in such a dangerous situation. They should know the meaning of warning danger these steps if you are in danger:
If the risk is reasonable: stay calm, disengage gently, back away if possible, call an emergency number, remain in the cab.
If the risk is high: jump clear and keep others away. Don’t remain down. Jump with your both feet on the ground. Your foot should not touch the vehicle and land at the same time. Do not enter the vehicle unless emergency services declare it clear.
Extreme the anger: never make contact between the machine and the ground.
Distance from Overhead Power cable:
|For 0 to 50kV voltage, observe 10 feet distance
|For Over 50kV to 200kV energy, find 15 feet distance
|For Over 200kV to 350Over voltage, find 20 feet distance
|For Over 350kV to 500Over voltage, observe 25 feet distance
|For Over 500kV to 750kV energy, observe 35 feet distance
For workers: When you are working near Overhead Power Cable, keep the precautionary distance. Do not use steel and metallic ladders. When you are using heightened equipment which may touch the Overhead Power cable, look here and there for its presence. Power lines can’t be often seen in between trees. Even the non-metallic substances can carry a high voltage current. For workers to climb, electric poles should have space.
Conductors must have support so they can carry their weight in cold area where ice accumulates on conductor wire. The pressure of the wind moves the conductor. Giving appropriate support to conductors prevents harmful events.
Trees can conduct an electric current. Children climbing trees should know about the Overhead Power cable and its dangers.
In thunderstorms breaking of Overhead Power cable happens a lot. Every year severe injuries. Coming in contact with an Overhead Power cable is a prevalent accident. It can kill a person. When a person comes near to Overhead Power cable, a flashover occurs.
In agricultural areas, farmers are suffering as they are using heightened vehicles that come in contact with Overhead Power cable.
In construction areas, cranes, lorries may come in contact with Overhead Power cables.
Even without touching an Overhead Power cable, you can get an injury. Because as soon as you come near, a flashover occurs. A voltage lower than 230kV can kill a person.
When insulating materials are wet, they act as conductors. Thus a dirty and moist rubber, plastic, and wood can carry electrical current.
Chapter 10: What is the difference between Overhead and Underground Power cable?
Overhead Power cable is susceptible to extremes of the natural environment. Whenever there is a thunderstorm, sand storm, hurricane, Overhead Power cable face destruction.
They always plan to shift Overhead Power cable to underground. Every area has its consequences.
The underground Power cable system is expensive. It requires high maintenance.
Underground transmission line costs 4 to 14 times more than the Overhead Power line of the same voltage and over the same distance.
Underground power lines require high technology. Wire placement, grounding methods, boring techniques, etc. are the methods for the underground power line.
Underground cables remain safe from extremes of the environment, such as winds and thunderstorms. But underground cables undergo insulation deterioration.
If we use good material, we can slow down the process of loss of insulation. But the loss of insulation does occur in underground cables. It will cause a line fault.
When a line fault occurs, it’s tough to detect it as lines are not visible through the naked eye.
We have to dig at different places to find where the fault is.
It makes the repair of an underground cable very costly.
More money is spent in the digging process than on repairing the cable.
Underground Power cable repair will require eight or nine days because they have additional steps.
First, the detection of the fault takes time. Then digging and trenching is a lengthy procedure.
Overhead Power cable repair takes two or three days because its detection is easy.
Maintaining the cost of the underground power line is exceptionally high. Underground cables can not undergo modification easily.
Tapering phone lines can be done in Overhead Power cables. But this modification is difficult in underground power cables.