Types of Cables in Transmission Distribution

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Types of Cables in Transmission Distribution
Power lines used for transmitting electricity have higher voltage capacities than distribution lines. (Thomas Northcut/Photodisc/Getty Images)

The bulk transfer of electricity along power lines is called electric power transmission and distribution. According to the website "Transmission and Distribution," cables used to transmit electricity from the generator site to a substation cables that distribute power from a substation to a home or business share many of the same features.

General Properties for Electric Cables

The book "Electric Power Transmission and Distribution" outlines the basic requirements for cables used to carry an electrical current. These properties apply to both transmission and distribution lines, and include constructing a cable that carries the optimum electrical capacity, is properly insulated and can withstand the above- and/or underground elements faced while carrying electricity from its source to destination.

While copper is the dominant material used for the conduction of electricity along transmission and distribution lines, aluminium is sometimes used as a cheaper and lighter-weight material. Insulating materials can include various types of plastics (including PVC) and rubber (such as ethylene propylene).

Difference Between Transmission and Distribution Cables

The main difference between these two types of power lines is the maximum voltage each is designed to carry. While both are constructed following the basic principles set forth in "Electric Power Transmission and Distribution," transmission lines will have a higher maximum voltage --- as high as 768 kilovolts --- compared to distribution lines, which can carry only one-fifth the voltage.

Low-Voltage Cables

"Transmission and Distribution Electrical Engineering" breaks down transmission and distribution cables into five separate categories. The most basic category is "low voltage." This includes cables used for telephone wires, as well as fire-retarded and resistant cables. These cables have the lowest maximum voltage of the four groups, ranging from 50 to 1,000 volts.

Medium-Voltage Cables

Medium-voltage cables --- which have a maximum capacity ranging from three to 7.2 kilovolts --- are the next class of transmission and distribution lines. These cables are typically used for solid dielectric and MI/MIND purposes.

High-Voltage Cables

The third class of transmission and distribution cables is high voltage. These power lines can carry a maximum voltage capacity of 10 to 150 kilovolts. While these cables can be used for the same purposes as low- and medium-voltage cables, their higher threshold makes them less cost-efficient than lower-grade wires. The main purposes of high-voltage wires include oil-filled cables, as well as gas-pressure or gas-insulated ducts.

Very High-Voltage Cables

Like medium- and high-voltage cables, very high-voltage lines are used for solid dielectric, oil-filled and gas-insulated ducts, but in situations where a higher maximum voltage is required. These cables have a maximum capacity of 150 to 300 kilovolts. Because of their very high-voltage capacity, these are typically transmission lines and not distribution lines.

Extra High-Voltage Cables

Extra high-voltage cables have a maximum capacity of 300 kilovolts or more. These cables serve the same uses as their very high-voltage counterparts, but when even more voltage capacity is necessary. Because the capacity of these cables is higher than the standard maximum of 144 kilovolts for distribution cables, these extra high-voltage cables are typically found along transmission lines.

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