what are the types of Transformers

Different types of transformers are categorized based on their purpose, construction, and operating frequencies. Some of the most common types of transformers used in the electrical industry are as follows. The Transformers are largely divided into Power and Distribution Transformers and Step-up or step-down Transformers. Depending upon utility, the following other types of
classification is also in use:
(1) Generator Transformer
(2) Unit Auxiliary Transformer
(3) Station Transformer
(4) Transmission Sub-Station Transformers
(5) Auto Transformers
(6) HVDC Converter Transformer
(7) Arc Furnace Transformers
(8) Traction Transformers
(9) Earthing Transformer or the Zig-Zag
(10) Generator Neutral earthing Transformer
(11) Series and Shunt Reactors (strictly not a transformer)
Types of Windings: -
(1) Distributed:
Used for HV windings of small Distribution Transformers where the current does not exceed 20 amps using a circular cross-section conductor.
(2) Spiral:
Used up to 33 kV for low currents using a strip conductor. Wound closely on Bakelite or pressboard cylinders generally without cooling ducts. However,
multi-layer windings are provided with cooling ducts between layers. No
Transposition is necessary.
(3) Helical:
Used for Low Voltage and high currents. The turns comprising of a
number of conductors are wound axially. Could be single, double or multi-layer winding. Since each conductor is not of the same length, does not embrace the
same flux and of different impedances, and hence circulating currents, the
winding is transposed.
(4) Continuous Disc:
Used for 33kv and 132 KV for medium currents. The coil
comprises of a number of sections axially. Cooling ducts are provided between each section.
(5) Interleaved Disc:
used for voltages above 145 kV. Interleaving enables the
winding to withstand higher impulse voltages.
(6) Shielded Layer:
Used up to 132 kV in star-connected windings with graded insulation. Comprises of a number of concentric spiral coils arranged in layers grading the layers. The longest at the Neutral and the shortest at the Line Terminal. The layers are separated by cooling ducts. This type of construction ensures uniform distributed voltages.