Transformer tank with conservator; They are produced in the power range of 25-2500 kVA, in the high voltage level range of 2 kV to 40.5 kV, three or single phase, with oil, natural cooling (ONAN), with idle tap changer or automatic tap changer under load, for external and internal use. The dimensions of expansion tanks of transformers are designed as a result of calculations to store the expansion of oil that increases with temperature. Since the transformer with expansion tank is open to the atmosphere, the oil pressure changing due to thermal effects is dehumidified and air exchange is ensured by the slicagel (air dryer) in the transformer tank. The transformer with expansion tank (transformer with expansion tank) put into operation loses its slicagel feature over time as a result of air exchange caused by temperature differences depending on the load and causes the oil to become moist and its structure to deteriorate. For this reason, maintenance should be carried out such as changing the slicagel at certain periods and measuring the breakdown voltage by taking samples from the oil.