An aerial substation

A substation is the part of an electricity transmission and distribution system where voltage is transformed from low to high and vice versa using transformers. The range of voltages in a power system varies from 110 V to anything up to 765 kV. Transformation may take place in several stages and at several substations in sequence, starting at the generating plant substation where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household use.

A substation that has a step-up transformer increases the voltage whilst decreasing the current, while a step-down transformer will decrease the voltage while increasing the current for domestic and commercial distribution.

The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station, where there was the generator, and were entirely dependent on it, hence the name.

Substations may be on the surface in fenced enclosures, underground, located in special-purpose buildings, located on power poles (aerial substations) or located in the service areas of the buildings they service. Substations located within the buildings they serve are particularly a feature of high-rise buildings.

It is more efficient to transmit electricity over long distances at very high voltages; the function of the substation is to reduce the voltage from transmission level to values suitable for local distribution.

Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, and metering. Other devices such as capacitors and voltage regulators may also be located at a substation.

Where a substation has a fence, it must be properly grounded (UK:earthed) to protect people from high voltages that may occur during a fault in the transmission system.