Equipment and structures to distribute electricity

The interface between transmission and distribution lines is the distribution substation, which is commonly found close to residential areas and industrial regions. At the substation, the electricity’s force, or voltage, is reduced from transmission levels (ranging from 115,000 volts to 735,000 volts) to lower levels through substation transformers. This is done because the electricity now only has to travel shorter distances to reach homes, schools and businesses.

The substations also feature:

• circuit switches that direct the flow of electricity. A commonly used switch is a gang-operated, air-break disconnect switch. Using a combination of three separate switches and operating in air (instead of another medium like oil), this switch is used to disconnect the substation from an incoming line, when needed.
  
  
• high voltage breakers that can interrupt the flow of electricity when unexpected surges or faults occur, in order to protect the system from damage.
  
  
• voltage regulators to regulate voltage along the distribution line. (The voltage along a distribution line will change with the amount of electrical load drawn by customers. The regulator automatically raises or lowers the transformer’s voltage to maintain constant voltage levels at different points on the distribution system.)
  
  
• capacitors that improve the quality of the electricity supply to customers, smoothing out voltage depressions caused by increased loads, and “filtering out” voltage distortions.

Once transmission voltages are lowered at the substation, electricity can then be transported to homes and businesses through distribution lines. To do this, substations feed the electricity through primary distribution lines (or “primaries”) at voltages such as 39,000 volts or less to directly supply large customers, like public transit systems, and industrial and commercial customers.

This voltage is still too high for most typical uses, such as homes, business and small industrial users. So another device — the distribution transformer — is used to step the voltage down further. Located at different locations along the distribution system, these operate like the larger transformers in the distribution substation, stepping down the voltage down to the service level required by a particular customer. (A typical residential service uses 120 and 240 volts, whereas most industrial facilities use 480 volts.) Secondary distribution lines (or “secondaries”) then carry power from the distribution transformers to the customer’s meter.