Oil-filled distribution transformers are transformers that change the input and output voltages in the desired ratios up to a voltage level of 36 kV and a power of 5000 kVa, where the heat generated during operation is removed by the transformer oil and cooling is provided by the transformer oil so as not to exceed the permitted operating temperature. These types of transformers can be classified into two main categories: hermetic transformers and expansion tank transformers:
Hermetic Oil Distribution Transformers
25 – 5000 kVA power, up to 36 kV high voltage, three-phase or single-phase, oil-cooled, naturally cooled (ONAN), with vacuum tap changer or automatic tap changer under load, can be used for outdoor and indoor applications. Hermetic oil-cooled distribution transformers are manufactured on the principle of balancing oil pressure with atmospheric pressure at the desired temperature, filling the oil under vacuum and preventing the cooling oil from coming into contact with the atmosphere.
In oil-sealed distribution transformers, the internal heat resulting from the change in oil pressure due to thermal changes is controlled by the natural expansion and contraction of the boiler wave walls. The mechanical design and construction are designed to resist these effects.
Hermetic oil-filled distribution transformers are lower than expansion tank transformers. They can therefore be used in smaller areas. Since the oil does not come into contact with the atmosphere in oil-sealed transformers, it is not necessary to change the oil at certain times, as is the case with expansion tank transformers.
Transformer with Expansion Tank
Transformers with expansion tanks have the same core and coil structure as hermetic transformers. The dimensions of the expansion tank of this type of transformer are designed and manufactured following calculations to store the expansion of the oil whose volume increases with temperature.
Since transformers with expansion tanks are open to the atmosphere, the pressure of the oil changes due to heat, and the air is renewed by dehumidifying the air in the transformer tank by means of a clikajel (air dryer).
Due to the exchange of air caused by temperature differences depending on the load of the transformer with the expansion tank put into operation, the clikajel loses its properties over time and causes the oil to become wet and its structure to deteriorate. For this reason, it is necessary to carry out maintenance operations such as replacing the silica gel at certain times and measuring the perforation voltage by taking samples of the oil.
