Drying Process of Insulating Materials of Transformers

The insulating systems of transformers must be dried before impregnation with insulating oil. Some works dealing with the drying process of insulating materials have been published, however, practical methods to analyze the drying process have not. In order to develop a practical method, the drying process of transformer boards was investigated, and results obtained in this investigation were applied to analyze the drying process of transformers. At the beginning, changes in the water vapor pressure in the vacuum container due to the release of water vapor from transformer boards were measured during the drying process. Measured changes were then compared with calculated changes, based on the assumption that water vapor is released from insulating materials due to the difference of water vapor pressure. These results were applied to calculate the drying times of insulating systems in transformers. It was found that if the appropriate permeability of water vapor in insulating materials can be selected, the calculated drying process coincides with the measured drying process.