CLASSIFICATION OF TRANSFORMERS
The two types of transformers are air-cooled (dry type) and oil-filled. The transformer rating increases with improved cooling methods. These are the typical ratings for various types of transformers:
Dry Transformers
The two types of dry transformers are self-air-cooled and forced-air-cooled. The heat is removed by natural convection in self-air-cooled transformers. In forced-air-cooled, it is removed by blowers. The rating of dry-type transformers used to be limited to less than 1 MVA. However, modern technology pushed this rating to 20 MVA. This was mainly due to improvements in the quality of insulation and mechanisms of heat removal from the trans- former.
Oil-Immersed Transformers
In this type of transformer, the windings and core are immersed in oil. The main types of oil-immersed transformers are
● Oil-immersed self-cooled (heat is removed by natural convection of the oil through radi- ators)
● Oil-immersed cooled by forced air (heat is removed by blowers blowing air on radiators)
● Oil-immersed cooled by water (the oil is cooled by an oil-water heat exchanger)
The lowest rating of oil-type transformers is around 750 kVA. Since modern dry-type trans- formers are being manufactured up to a rating of 20 MVA, they are replacing oil-type transformers. The main reason is that oil-immersed transformers constitute a fire hazard, and they are very hard to maintain. Dry-type transformers are preferred in most industries.
The rating of oil-immersed cooled-by-water transformers is normally higher than 100 MVA. However, they could be used for transformers having a rating as low as 10 MVA if the transformer is feeding a rectifier. Harmonics (deformation in the sine wave of current and voltage) are normally generated in this application, causing significant heat generation, that necessitate cooling through a heat exchanger.
Most failures in transformers are caused by erosion of the insulating materials. Analysis of a transformer’s oil can provide trends and early warning signs of premature failure. Figure 4.1 illustrates a basic electric power system from the utility to the consumer.