Distribution substation

Distribution substations receive high-voltage electricity from the transmission system, transform the voltage to distribution levels, and then direct the current to distribution lines. In this case, a step-down transformer is used, which lowers the voltage from transmission level to distribution level. A typical step-down transformer might reduce voltage from 230 kV or higher on the transmission side to 34.5 kV or lower on the distribution side. 

The electricity then travels over a series of busbars, which are solid or hollow pipes used to conduct electricity. They are used because electric lines in a substation are in close proximity to work areas, and using less stable wires would be dangerous, especially in windy conditions. The busbars connect to circuit breakers. The circuit breaker in the substation has the same function as a circuit breaker in your house. It protects the wire from overloading or from too many amps producing too much heat. 

Busbars connecting to circuit breakers in a substation (photo credit: Stephen Jones)

The substation also may include voltage regulators including capacitor banks, reactors, or static var compensators (SVCs) that allow voltage to be managed as loads fluctuate on feeders served by the substation; additional protective equipment; and various types of communications and control equipment including a SCADA connection to the distribution operations center.

In many instances the electricity then moves underground beyond the edge of the substation and up a pole and onto the distribution lines that wind up and down streets and highways delivering power to end users. 

A more complex distribution substation might have additional transmission lines coming into the substation, additional transformers to deal with potentially different transmission and distribution voltages, and more distribution lines on the secondary side of the substation that would carry the electricity in many different directions. 

Distribution substation
Distribution substation in Longmont, Colorado, U.S. (photo credit: Stephen Jones)