Why should large transformers be filled with nitrogen during

Due to the excessive weight of large transformers, in order to reduce the weight during transportation, the oil in the transformer body will be partially or completely discharged, and the space after the oil discharge will be filled with nitrogen, so that the body of the transformer will not touch the air, prevent the inner insulation of the transformer from being damp. After transporting, the transformer should be filled with qualified transformer oil immediately after the nitrogen gas is released.