How are power systems put together?
Understanding the components of the electric power system, and the overhead construction of its distribution circuits
To understand how a utility company neglected to maintain and repair its power lines and poles and liked caused an electrocution accident, you first need to have a basic idea of how power systems are put together.
Components of the electric power system
Power generation: The sources of electric energy are from turbine-generators, which are powered by coal (fossil fuels), fission of nuclear fuel, water, natural gas, and more recently, geothermal, wind, solar, and biomass sources, produce electricity.
Delivery: Via overhead wires, underground cables, and submarine cables form the bones of the system that delivers electric energy from the generation sources to the customers of the world. The U.S. power grid electrically operates as a three phase AC system, with two major components, transmission and distribution.
Transmission: Highest voltages are used to transmit power from the generation plant to the substations, often at 138,000 volts. This is stepped down to 69,000 volts at the switching stations, for its trip to the substations.
Distribution: At the substations, incoming power is stepped down and lowered in voltage for distribution over local areas, first by using primary feeders of one to five miles in length, often operating at 4,800 volts, 8,300 volts or 13,200 volts. In rural areas, these “primaries” may be as long as 10-12 miles. These circuits are usually carried on poles, but in more modern and dense systems, underground conduits may convey the cables, or they may be buried directly in the ground. Secondary distribution is accomplished by step-down transformers that deliver power to businesses and homes at 120 to 240 volts, from “service drops” at nearby poles.
Overhead construction of distribution circuits
Power conductors need supports to get from one place to another, such as towers, poles or other structures made of steel, concrete or wood. Usually, steel poles and towers are used for transmission lines, while wood or concrete poles are used for distribution.
In the U.S., overhead distribution construction more often consists of wood poles with the conductor wires attached to insulators supported on wood crossarms. Advantages of wood are economic, and the natural insulating property of wood make it a safe alternative.
Wood poles and structures decay, and are affected by birds, insects, climbing line workers, weather, chemical exposure and thermal factors. So poles are treated with preservatives and must be inspected periodically. Required pole strength is determined by the weight of crossarms, insulators, wires, transformers, and all other equipment it must carry as well as by ice and wind loadings.