A distribution system originates at a distribution
substation and includes the lines, poles, transformers and other equipment
needed to deliver electric power to the customer at the required voltages. Customers are classed as:
Figure 1. Energy flow through a typical substation
The following are examples of distribution systems components. Collectively
they constitute a typical distribution system. These typically deliver voltages as high as 34,000 volts (34 kV)
and as low as 120 volts.
Most industries need 2,400 to 4,160 volts to run heavy machinery and usually their own substation or substations to reduce
the voltage from the transmission line to the desired level for distribution throughout the plant area. They usually require 3-phase lines to
power 3-phase motors.
Figure 8. Industrial facility distribution transformer
The Lineman's and Cableman's Handbook, Shoemaker, T.
M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
Commercial customers are usually served at
distribution voltages, ranging from 14.4 kV to 7.2 kV through a service
drop line which leads from a transformer on or near the distribution pole to
the customer's end use structure. They may require 3-phase lines to power 3-phase
Distribution transformer to 3-phase service - commercial facility
The distribution electricity is reduced to the end use
voltage (120/240 volts single phase) via a pole mounted or pad-mounted transformer. Power is delivered to the
residential customer through a service drop line which
leads from the distribution pole transformer to the customer's structure, for overhead lines, or underground.
Residential distribution transformer and service drop
Pad-mounted residential distribution transformer
Currently the only electric transportation systems are light rail and subway
systems. A small distribution substation reduces the local distribution
voltage to the transportation system requirements. The overhead lines supply
electric power to the transportation system motors and the return current
lines are connected to the train tracks.
Public transit train powered by overhead electric lines
Substation where electricity is conditioned for powering commuter trains
Figure 15. Power runs from the substation underground to the poles
where power is delivered to the power lines. The circuit
is completed through the train tracks, with lines returning
to the substation.
Electric cables carry electricity to power the train's motors