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Last Updated: Wednesday, 9 August 2017
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Home   |  Substations

Substations


sub-transmission lines they carry power from local substations to customers

The Utility poles are commonly used to carry two types of electric power lines, distribution lines (or "feeders") and sub-transmission lines they carry power from local substations to customers. They generally carry Voltages “Currents” from 4.6 to 33 Kilovolts (kV) for a distances up to 30 miles (48.2803 km) approximately and the transformers to step the voltage down from the primary voltage to the lower secondary lower voltage is carried to the customers.

subtransmission line are mounted under the higher voltage lines
Mobile Substations-Design and Development of 110KV Mobile Substations

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Subtransmission lines carry higher voltage power from regional substations to local substations. They usually carry 46 kV, 69 kV, or 115 kV for distances up to 60 miles (96.5606 Km). 230kV lines are often supported on H-shaped towers made with two or three poles. Transmission lines carrying voltages of above 230kV are usually not supported by poles, (transmission towers).

The distribution line is often carried on the same poles as a subtransmission line but mounted under the higher voltage lines.

Distribution lines carry relatively low voltage electric power from local substations to customers. Distribution lines may be strung from utility poles or buried under ground at considerable extra expense. Voltages range from 5 to 35 kilovolts (kV’s) for distances up to thirty miles. At a service drop (on or near the customer’s premises) a transformer steps the voltage down to the lower voltage of 240/120 volts used by the customer.

Subtransmission lines carry higher voltages, from 50 to 100 kV, from regional substations to local substations, for distances up to 60 miles.

Transmission lines carry the highest voltages – 200 to 500kv. They are usually not supported by poles but by metal transmission towers.

For economic or practical reasons distribution lines may be strung on the same pole as subtransmission lines but mounted under the higher voltage lines, a practice known as “underbuild.”

The standard utility pole is about 40 ft long and is buried about 6 ft in the ground. Poles can be as high as 100 ft if required to clear structures or terrain. They are usually spaced no more than 125 ft apart. Poles are usually owned by one utility company, which then leases space on it to other service providers.

Most utility poles are made of wood, pressure-treated with some type of preservative for protection against rot, fungi and insects. The most common tree used for telephone poles is the Southern yellow pine, but Douglas or Pacific-fir, Jack pine, and western red cedar are also common. The traditional preservative is creosote, but environmental concerns have led to the use of alternatives such as pentachlorophenol and copper naphthenate.

Wood poles have a 25 to 50 year life span, and require periodic inspection and preservative treatments. Newer poles may be made of steel, concrete, or fiberglass. Wood poles, because of their relatively low cost, remain the most common.

Electric wires strung on poles are usually single conductor, non-insulated aluminum cables. They are supported by insulators commonly mounted on a horizontal crossarm. Power is commonly transmitted using the three-phase system, requiring three wires. Subtransmission lines comprise these 3 wires, plus sometimes an overhead ground wire called a "static line" suspended above them. The overhead ground wire acts like a lightning rod preventing power surges and fires.


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