What is Electrical Power Cable?
An electrical power cable is a type of cable used for transmitting and distributing electrical energy. It is made up of two or more electrical conductors that are enclosed within an outer protective sheath. The conductors are made of materials such as copper or aluminum and are used to transmit electrical current.
Power cables are often used in situations where overhead lines are not feasible, such as at sea, in airfield crossings, and in other locations. In these cases, the use of underground power cables is preferred, as it provides a safer and more secure transmission of electricity.
However, one major disadvantage of using electrical power cables is that they are more expensive to install and maintain compared to overhead lines. This is due to the higher costs associated with underground installations, including the excavation and trenching required to lay the cables, and the additional materials and labor needed to install the protective sheath.
Construction of Cable:
The parts of the cable are given below.
1. Conductor
2. Insulation
3. Inner Sheath
4. Protective Covering
5. Armoring
6. Over sheath
1. Conductor:
The conductor in an electrical power cable is the component that allows the flow of electrical current. Copper and aluminum are commonly used as the conductor material because of their high electrical conductivity. The conductor can be made of a solid wire or multiple bare wires twisted together.
When multiple wires are used, they are arranged around a central wire in a specific pattern. The number of wires in the first layer is six, the second layer is twelve, and the third layer is eighteen. This pattern continues with more layers having an increasing number of wires. The size of the conductor is represented by a code, such as 7/A, 19/B, or 37/C. The first number represents the number of strands in the conductor, and the second letter represents the diameter of the individual wires, which is given in centimeters or millimeters.
2. Insulation:
Insulation or dielectric material is a crucial component in power cables. It is responsible for providing insulation between the conductors and preventing current leakage. The insulation material must have high electrical resistance, high dielectric strength, good mechanical strength, and the ability to withstand high temperatures.
The most commonly used insulation materials in power cables are impregnated paper, butyl rubber, polyvinyl chloride, polyethylene, and cross-linked polyethylene. Paper insulated cables are popular due to their high current carrying capacity, reliability, and long life.
For cables used in submarine and damp soil, synthetic dielectrics such as polyvinyl chloride and polyethylene are preferred. These materials are lighter than paper and have a nonmigratory dielectric. They also have good dielectric strength, low power loss, and low thermal resistance, making them ideal for these challenging environments.
In addition to the properties mentioned above, the insulation material should also have a low power factor. This means that it should not consume too much power, as this would increase the cost of the system.
3. Inner Sheath:
The inner sheath of a power cable is a layer of material placed around the cable to protect it from moisture. The purpose of the inner sheath is to prevent the ingress of moisture into the cable, which can have a negative impact on the insulation and reduce its effectiveness.
The most common material used for the inner sheath is lead alloy, which is durable and provides excellent protection against moisture. Lead alloy is also a non-magnetic material, which makes it suitable for use in power cables that are required to be non-magnetic.
Another material used for the inner sheath is aluminum, which is cheaper and lighter than lead. Aluminum also has a high mechanical strength, making it suitable for use in power cables that need to withstand internal pressures.
In some cases, corrugated seamless aluminum sheath is used for inner sheath, especially in oil-filled cables and telephone cables. This type of sheath has better-bending properties, reduced thickness, and lighter weight compared to other materials.
Overall, the choice of material for the inner sheath depends on the specific requirements of the cable, such as the level of protection against moisture, weight, and mechanical strength.
4. Protective Covering:
The protective covering, also known as serving or outer sheath, is an important component of the power cable. It is used to protect the cable against corrosion and other external factors like mechanical damage, moisture, and chemicals that could potentially harm the cable.
The protective covering is typically made of materials such as paper, hessian, polyvinyl chloride, or a combination of these. The serving layers are spread with waterproof compounds to ensure that the cable is protected against moisture. The use of serving helps to prevent damage to the cable and extend its service life.
When power cables are laid directly on the ground, they are vulnerable to damage due to corrosion and electrolytes. The protective covering helps to protect against these potential risks by providing a barrier between the cable and the external environment.
In addition to protecting against corrosion, the protective covering also provides mechanical protection to the cable. This is important because the cable may be subjected to physical stress during installation, maintenance, and normal operation. The protective covering helps to absorb these mechanical forces, thereby reducing the risk of damage to the cable.
5. Armoring:
Armoring is a method of providing mechanical protection to the cable from external damage. This is typically accomplished by wrapping one or more layers of galvanized steel wires or metal tape around the cable sheath. The purpose of armoring is to prevent physical damage to the cable that could compromise its insulation and electrical performance.
The use of galvanized steel wires is the most common method of armoring, as it provides high longitudinal strength and can withstand significant physical stress. The steel wires are wrapped tightly around the sheath, providing a layer of protection that is resistant to impact, abrasion, and other forms of mechanical damage.
In addition to protecting the cable from physical damage, armoring also serves a secondary purpose. When a fault occurs in the cable, such as a failure of its insulation, the fault current will flow through the armor and be directed to earth. This helps to prevent the fault from causing an electrical hazard and also helps to protect the electrical equipment connected to the cable.
6. Over sheath:
The over sheath of the power cable is the outer layer of the cable and serves as the final layer of protection. It is typically made of a thermosetting or thermoplastic material and is designed to provide mechanical strength to the cable and protect it from external factors like moisture, corrosion, dust, dirt, and other environmental factors. The material used for the over sheath must be durable, long-lasting and have good resistance to external factors like heat, water, and UV radiation.
The over sheath also helps to ensure the electrical and mechanical integrity of the cable by providing a barrier between the inner components and the external environment. It helps to maintain the insulation of the cable, reduce the risk of electrical faults, and increase the overall reliability and lifespan of the power cable.
Types of Electrical Cables:
There are 5 types of electrical cables.
1. Ribbon Electric Cables
2. Twisted Pair Cables
3. Coaxial Cables
4. Shielded Cables
5. Fiber Optic Cables
1. Ribbon Electric Cables:
Ribbon electric cables are composed of multiple insulated wires that run parallel to each other. These wires are usually color-coded and are bundled together using a binding material. The main advantage of using this type of cable is that it allows multiple signals to be transmitted simultaneously, which is useful in a variety of applications, including connecting the central processing unit (CPU) with the motherboard in a computer.
2. Twisted Pair Cables:
Twisted pair cables consist of two insulated copper wires that are twisted together. The twisting of the wires helps to reduce electromagnetic interference (EMI) and crosstalk, which can degrade the quality of the signal being transmitted. These cables are widely used in telecommunications applications, such as telephone lines, and are also used in Ethernet networks. Twisted pair cables can be further classified into shielded twisted pair (STP) and unshielded twisted pair (UTP) cables, depending on whether they have an additional layer of shielding to reduce EMI.
3. Coaxial Cables:
Coaxial cables have a solid copper conductor at their center, surrounded by a layer of insulation and a metallic braid or tape. The braid or tape acts as a shield, reducing electromagnetic interference and crosstalk. The cable is covered with an insulated external jacket for protection. Coaxial cables are commonly used in computer and audio-video networking applications, as they can transmit high-frequency signals over long distances with minimal signal degradation.
4. Shielded Cables:
Shielded cables consist of an insulated wire that is covered by a woven braided shield and/or aluminum Mylar foil. The shield helps to reduce electromagnetic interference and crosstalk, improving the quality of the signal being transmitted. Shielded cables are used in applications where high-voltage electric current is being transmitted and are commonly used in power distribution and control systems.
5. Fiber Optic Cables:
Fiber optic cables are used to transmit optical data signals from a light source, such as a laser or an LED. They consist of a thin, flexible glass or plastic fiber that is surrounded by a cladding material with a lower refractive index. The light signals are transmitted through the fiber by reflecting off the cladding, allowing the signals to be transmitted over long distances with minimal signal degradation. Fiber optic cables are widely used in a variety of applications, including telecommunications, medical equipment, and military systems.
What is the underground cable?
An underground cable is a type of electrical power cable that is used for transmitting and distributing electrical energy. The main advantage of using underground cables is that they are not affected by weather conditions and they don't interfere with the beauty of the environment, making them ideal for use in populated areas. Additionally, underground cables offer a higher level of reliability and safety compared to overhead cables as they are protected from physical damage, accidental contact and environmental hazards.
However, the installation of underground cables is much more complicated and expensive than overhead lines. Digging trenches and installing the cable underground is a complex and time-consuming process. Underground cables are also more difficult to maintain and repair compared to overhead cables. In the event of a fault, the entire length of the cable must be excavated to repair the fault.
What is the purpose of Shielded Twisted Pair cable?
Shielded Twisted Pair (STP) cable is a type of cable that has two insulated conductors (normally copper) twisted together and surrounded by a metal shield to prevent electrical interference. The shield acts as a Faraday cage, blocking interference from other electronic devices, power cables or other electrical signals.
The purpose of STP cable is to provide reliable and efficient data transmission in high electromagnetic interference (EMI) environments. It is commonly used in computer networks, Ethernet cables, and other applications where high speed data transmission is required. The twisted pair design helps to reduce crosstalk and signal loss, ensuring consistent and accurate data transmission.
STP cables are also preferred in industrial environments, as they are resistant to environmental factors such as temperature, humidity, and physical impact. This makes them ideal for use in harsh industrial environments, such as manufacturing facilities, mines, and oil rigs.
What is grading of electrical cables?
Grading of an electrical cable involves adjusting the electrical stress distribution across the dielectric material to ensure uniformity and reduce the overall thickness of the cable. The electrical stress on a cable is the result of the electrical current flowing through it, and it is higher near the conductor. If this stress is not evenly distributed across the dielectric material, it can lead to an increase in the thickness of the cable, which can raise the cost of the cable.
Grading aims to equalize the electrical stress across the dielectric material by adjusting its distribution. By doing this, the thickness of the cable can be reduced without compromising its performance. This reduction in thickness can lead to a more cost-effective solution compared to a cable with an uneven stress distribution.
Grading can be achieved through various methods, including using materials with different electrical properties or arranging the layers of insulation in a specific manner. By properly grading the cable, it can have a longer lifespan and perform more efficiently, while also reducing the overall cost.
Types of grading:
1. Capacitance grading
2. Inner Sheath grading
1. Capacitance grading:
In electrical engineering, capacitance grading, also known as dielectric grading, is a process used to equalize the electrical stress in the dielectric of a cable. The electrical stress is a measure of the potential difference between two points, which creates an electric field that can cause the dielectric material to breakdown and cause failure.
The relative permittivity, also known as the dielectric constant, is a measure of the ability of a material to store electrical energy. The permittivity of a material is proportional to the electric field that can be stored in the material, and it depends on the material's composition and temperature.
The electrical stress in a cable is highest at the surface of the conductor and lowest at the outermost sheath of the conductor. To distribute the electrical stress uniformly throughout the dielectric, an infinite number of dielectric materials with different permittivity values are required. However, this is not practical in most applications, so instead, two or more dielectric materials with suitable permittivity values can be used to achieve a uniform distribution of stress.
2. Inner Sheath grading:
In inner sheath grading, the insulator material is separated into several layers with each layer having a different permittivity. This allows for a uniform distribution of electrical stress across the insulator, reducing the overall thickness of the cable.
The inner sheaths are thin cylindrical metal sheaths that are placed between the conductor and the outer sheath. These inner sheaths help to maintain the gradual voltage across the insulator, resulting in a uniform voltage throughout the cable. This method of grading helps to reduce the cost of the cable and increases its overall efficiency. The inner sheath grading method is commonly used in high-voltage cable systems to ensure that the stress is evenly distributed, reducing the risk of cable failure.
Limitations of Grading:
In capacitance grading, the range of permittivity values of insulating materials used in cable insulation is limited. This can lead to changes in the stress distribution and cause insulation breakdown under normal operating conditions if the permittivity of the layers changes.
In inner sheath grading, the inner sheath layers are thin and prone to damage during transportation or installation. The thin inner sheaths are not able to carry the high current charging in long cable lines and hence the current-carrying capacity of the cable is reduced. Additionally, the inner sheath layers can become damaged, which will compromise the overall insulation of the cable. These limitations have led to a shift away from using grading methods in cable design.