Profile of water content and degree of polymerisation in the solid insulation of power transformers

A large number of the power transformers in service today have reached an age of 30 years or more. Thus, they might have reached a critical condition. The ageing rate is determined by the thermal load and to a considerable degree also by the water content in the solid insulation material. For an estimation of the total amount of water in a transformer, knowledge about the typical profile of the water content in a transformer as it is created after a certain time of operation is extremely helpful. The decision about drying of the active part of a transformer as a value conserving measure is based finally on the total amount of water in a transformer. In This work results of the investigation of the profiles of water content and degree of polymerisation before and after performing the active part drying procedure will be presented based on data from a 31.5 MVA transformer (110 kV) manufactured in 1955.