According to the number of phases of a single transformer, it can be divided into three-phase transformers and single-phase transformers. In three-phase power systems, three-phase transformers are generally used. When the capacity is too large and limited by transportation conditions, three single-phase transformers can also be used to form a transformer group in three-phase power systems.  

According to the number of windings, it can be divided into double winding transformers and three winding transformers. Typical transformers are double winding transformers, which have two windings on the iron core, one as the primary winding and one as the secondary winding. A three winding transformer is a transformer with a larger capacity (above 5600 kVA) used to connect three different voltage transmission lines. In special cases, there are also transformers that use more windings.  

According to the structural form, it can be classified into iron core transformers and iron shell transformers. If the winding is wrapped around the core, it is a core type transformer; If the iron core is wrapped around the winding, it is an iron shell transformer. The two are only slightly different in structure, and there is no fundamental difference in principle. Power transformers are all iron core type.  

According to insulation and cooling conditions, it can be divided intooil immersed transformersand dry-type transformers. In order to enhance insulation and cooling conditions, the iron core and winding of the transformer are immersed together in an oil tank filled with transformer oil. In special circumstances, such as in streetlights and mine lighting, dry-type transformers are also used.  

In addition, there are various specialized transformers for specific purposes. For example, high-voltage transformers used in experiments, transformers used in electric furnaces, transformers used in welding, and transformers used in thyristor circuits, as well as voltage transformers and current transformers used for measuring instruments.  

During the operation of oil immersed power transformers, the heat from the windings and iron cores is first transferred to the oil, and then transmitted to the cooling medium through the oil. The cooling methods of oil immersed power transformers can be divided into the following types according to their capacity:

1. Natural oil circulation and natural cooling (oil immersed self cooling type)

2. Natural oil circulation air cooling (oil immersed air cooling)

3. Forced oil circulation water cooling

4. Forced oil circulation air cooling

Special attention should be paid to fire safety measures for oil immersed transformers.         

1. The fire separation distance between oil immersed transformers with an oil content of over 2500kg and oil filled electrical equipment with an oil content of 600kg-2500kg should not be less than 5m.

2. When the fire separation distance between two adjacent oil immersed transformers does not meet the requirements, a fire partition wall or a fire water curtain should be installed at the top of the fire partition wall. Single phase oil immersed transformers can only be equipped with fireproof partition walls or fireproof water curtains.         

3. When the distance between the outer wall of the factory building and the outer edge of the oil immersed transformer outside is less than the specified value in the specification table, a firewall should be used for the outer wall. The distance between the wall and the outer edge of the transformer should not be less than 0.8m.

4. When the distance between the outer wall of the factory building and the outer edge of the oil immersed transformer is within 5m, no doors, windows or holes should be opened below the horizontal line of the total thickness of the transformer plus 3m and within the range of 3m on both sides of the outer edge; The fire resistance limit of doors and fixed windows on the firewall outside its scope should not be less than 0.9 hours.

5. When the single oil volume of oil immersed transformers and other oil filled electrical equipment exceeds 1000Kg, oil storage pits and public oil collection tanks should be set up. 6. The oil immersed transformer shall be equipped with fixed water spray and other fire extinguishing systems according to the current relevant specifications. Oil immersed plant transformers should be installed in separate rooms, and the doors of the rooms should be Class B fire doors that open outward and lead directly to the outside or corridor, and should not open to other rooms.