EI-type (including the mountain-shaped) is the more common transformer core. For R-type (also U-type), it is made of two "U" -type iron core. Ring transformers is also known as "0" -type transformer, its iron core is rolled with ribbon-shaped silicon steel sheet.
1.EI core transformer: winding process is simple, low efficiency, low cost (Primarily equipment is cheap).
2.R-type transformer: winding process is simple, high efficiency, but the core production process is more complex, the cost is slightly higher.
(1) Classify according to the structures: there are double-winding transformer, three-winding transformer and auto-transformer.
(2) Classify according to the number of output phase: there are single-phase transformer, three-phase transformer.
(3) Classify according to voltage-regulating mode: there are no-load voltage regulating transformer and on-load voltage regulating transformer.
(4) Classify according to the cooling medium: there are oil-immersed transformer, dry-type transformer and gas-filled transformer.
(5) Classify according to the cooling mode: there are oil-immersed self-cooled transformer, oil-immersed air-cooled transformer, oil-immersed water-cooled transformer, forced oil circulation air-cooled transformer and water-cooled transformer.
(6) Classify according to use: there are power transformer, test transformer, measuring transformer (mutual inductor), mining transformer, voltage regulator, reactor, rectifier transformer, welding transformer, impact transformer etc.
(7) Classify according to the insulation level of neutral point: there are full insulation transformer and semi-insulating transformer.

But what kind of transformers are used in the general plants?
Power plant transformer is dedicated to provide the power plant itself with electricity. From the safety, its short-circuit impedance is larger than that of the general distribution transformer and its technical requirements are consistent with the technology of the general distribution transformer.
The "plant" refers to the power plant, rather than the general factory. This specialized "plant transformer" has its own standard and is dedicated to provide its own power plant with electricity. From the safety, its short-circuit impedance is larger than that of general distribution transformer. In addition, its technical requirements are the same as that of the general distribution transformer.

Basic Composition:
Transformer components include the body (iron core, winding, insulation, lead), transformer oil, fuel tank and cooling units, voltage regulating devices, protection devices (moisture absorber, safe airway, gas relays, oil conservator and temperature measurement devices etc. ) and outgoing line bushing.
1. Iron core: it is the main magnetic circuit part in the transformer. Usually its silicon content is high, thickness is 0.35 mm, 3mm, 27mm, it’s laminated together by the hot-rolled or cold-rolled silicon steel sheets which are coated with insulating paint on the surface. The iron core is divided into two parts: the iron core leg and the horizontal slice. The core leg is wound by the winding and the horizontal slice is used for closing the magnetic circuit. The basic form for the core structure includes two kinds: heart-type and shell-type.
2, Winding: the winding is the circuit part of the transformer, it is wound by the insulated flat wire with double-wire wrapped or enameled round wire.