An electrical substation contains transformers to change the incoming and outgoing voltage, circuit breakers to protect the equipment from surges, switches and cutoffs to control current flow, and lightning arrestors to protect the lines from storms. Substations may be unmanned, or they may contain a control office if the substation oversees a particularly important section of the power grid. Substations also feature fences and gates for security.
When electricity flows through a power line, some of the current is lost due to resistance. To minimize transmission loss, electrical utilities transmit power through main lines at extremely high voltage. If it flowed directly from these high-voltage lines to a residential area, the voltage would be far too high for use, and it would burn out electrical wires and destroy equipment. Transformers are used to step up and step down the voltage, and substations contain the primary transformers that serve their particular sections of the power grid.
A simple transformer consists of a doughnut-shaped iron core with copper coils wound around opposite sides. When current flows through one set of coils, the magnetic field it creates induces a current in the coils on the opposite side, even though there is no direct path for the current to take. If the side connected to the transmission lines has twice as many coils as the side connected to the residential grid, the voltage is reduced by half. Using a series of transformers, the electric utility transforms the voltage from 110,000 volts or more in the transmission lines to the 110 volts needed for household appliances.