A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic
induction. Electromagnetic induction produces an electromotive
force across a conductor which is exposed to time varying magnetic
fields. Commonly, transformers are used to increase or decrease the
voltages of alternating current in electric power applications.

1. Auto-transformer
An auto-transformer has only a single winding with two end terminals,
plus a third of an intermediate tap point. The primary voltage is
applied across two of the terminals, and the secondary voltage taken
from one of these and the third terminal. The primary and secondary
circuits therefore have a number of windings turns in common.
An adjustable auto-transformer is made by the secondary connection
through a sliding brush, giving a variable turns ratio.

Polyphase transformers
For three-phase power, three separate single-phase transformers
can be used, or all three phases can be connected to a single
polyphase transformer. In this case, the magnetic circuits are
connected together, the core, thus containing a three-phase flow of
flux. The three primary windings are connected together and the three
secondary windings are connected together. The most common connections
are Y-∆, ∆-Y, ∆-∆ and Y-Y. If a winding is connected to earth
(grounded), the earth connection point is usually at the center point
of a Y winding.

current transformer is a measurement device designed to provide
a current in its secondary coil proportional to the current flowing
in its primary. Current transformers are commonly used in metering
and protective relaying, where they facilitate the safe measurement
of large currents. The current transformer isolates measurement and
control circuitry from the high voltages typically present on the
circuit being measured. Voltage transformers (VTs)--also referred
to as potential transformers (PTs)--are used for metering and
protection in high-voltage circuits. They are designed to present
negligible load to the supply being measured and to have a precise
voltage ratio to accurately step down high voltages so that metering
and protective relay equipment can be operated at a lower potential.