Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

power transformer
a transformer that is used to transmit power from one voltage level to another. Power transformers can be of either single phase or three phase design, and include either two or three windings.
shell-type transformer
a power transformer in which the magnetic circuit surrounds and normally encloses a greater portion of the electrical winding.
star-star transformer

See wye-wye transformer
submersible transformer
a transformer, used in underground distribution work, which is capable of operation while submerged in water.
subway transformer
another name for a submersible transformer.
teaser transformer
one of two single-phase transformers used to make up a Scott or T-connection transformer. The teaser transformer is connected between one line of the three-phase voltage system and the center-tap of the main transformer in the T-connection. Also called teaser coil.
See T-connection
three-winding transformer
a transformer with three windings, typically primary, secondary, and tertiary. Common three phase, three winding transformers employ wye connected primary and secondary windings and a delta connected tertiary winding.

In some cases, an autotransformer is used to form the primary-secondary combination.
a device that has two or more coils wound on an iron core. Transformers provide an efficient means of changing voltage and current levels, and make the bulk power transmission system practical. The transformer primary is the winding that accepts power, and the transformer secondary is the winding that delivers power. The primary to secondary voltages are related by the turns ratio of the coils. The corresponding currents are related inversely by the same ratio.
transformer differential relay
a differential relay specifically designed to protect transformers. In particular, transformer differential relays must deal with current transformer turns ratio error and transformer inrush and excitation current.
transformer fuse
a fuse employed to isolate a transformer from the power system in the event of a transformer fault or heavy overload.
transformer vault
a fireproof enclosure in which power transformers containing oil must be mounted if used underground or indoors.
up-down transformer

See buck-boost transformer
V-V transformer

See open-delta transformer
voltage transformer
an instrument transformer specially designed and optimized for voltage measurement and power metering applications. The primary winding is rated to match the system voltage and the secondary is typically rated at a standard value to match common meters and display units.
See potential transformer
wye-delta transformer
a connection of a three-phase transformer with one primary and one secondary which can be considered as three similar single-phase transformers. The primary is connected in wye, that is one terminal from each phase is connected to neutral and one to a line voltage. The secondary is connected in delta, with each phase connected between two line voltages.
wye-wye transformer
a three-phase transformer with both the primary and secondary coils connected in wye. This connection is considered undesirable, due to the triple harmonics in the exciting current. To maintain balance load voltages under varying loads, it is necessary to solidly connect the primary and secondary neutrals to ground. This may allow some secondary current to flow on the primary neutral and may also cause interference with parallel communication lines.
wye-wye-delta transformer
a three-phase transformer in which the primary and secondary coils are connected in wye. In order to overcome the problems with the wyewye connection, a set of tertiary coils are connected in delta to provide a path for the triple harmonic components of the exciting current to circulate.