Most power transformers have taps on either primary or secondary windings to vary the number of turns and, thus, the output voltage. The percentage of voltage change, above or below normal, between different tap positions varies in different transformers. In oil-cooled transformers, tap leads are brought to a tap changer, located beneath the oil inside the tank, or brought to an oil-filled tap changer, externally located. Taps on dry-type transformers are brought to insulated terminal boards located inside the metal housing, accessible by removing a panel.
Some transformers taps can be changed under load, while other transformers must be de-energized. When it is necessary to change taps frequently to meet changing conditions, taps that can be changed under load are used. This is accomplished by means of a motor that may be controlled either manually or automatically. Automatic operation is achieved by changing taps to maintain constant voltage as system conditions change. A common range of adjustment is plus or minus 10%. At Reclamation powerplants, de-energized tap changers (DETC) are used and can only be changed with the transformer offline.
A very few load tap changers (LTC) are used at Grand Coulee between the 500-kilovolt (kV) (volts x 1,000) and 220-kV switchyards. A bypass device is sometimes used across tap changers to ensure power flow in case of contact failure. This prevents failure of the transformer in case excessive voltage appears across faulty contacts.