It is ideal to judge whether the transformer insulation is affected by moisture by measuring the moisture content of insulating parts. There are two methods to measure the moisture content of insulating parts: one is to directly measure the moisture content of insulating parts, and the other is to use the equilibrium water vapor pressure method. The equilibrium water vapor pressure method is usually used for on-site measurement. The equilibrium water vapor pressure method mainly utilizes the principle that the moisture content in insulating parts maintains equilibrium with a certain water vapor pressure inside the oil tank at a certain temperature. That is, the water vapor pressure inside the oil tank is measured, and then the moisture content of the internal insulating parts is calculated based on the relationship curve (PPer line) between the moisture content in insulating parts and the water vapor pressure in the air.