Diffusion of moisture through power-transformer insulation

Moisture in power-transformer insulation has long been recognised as a major cause for the deterioration of the dielectric and for shortening the `service-lives¿ of units. Engineers have, therefore, striven to find a method that will accurately predict the degradation of the insulation in service conditions. This must involve a knowledge of how the moisture concentration varies in the insulation with time and load. Until recently, there has been no really satisfactory method of doing this, but now, with the advent of fast, large-storage computers, it is practicable to determine these changes in concentration numerically. Accurate computation depends on realistic mathematical modelling of the diffusion processes in both the liquid and solid insulants. These calculations are complex because the rate of moisture transfer depends on the materials, moisture distribution, temperature and oil velocity. The computations involve the simultaneous solution of the fundamental diffusion and heat-transfer equations. A program has been developed to execute these calculations and it should prove to be a powerful tool for both transformer designers and electricity-supply engineers. The program produces sufficient information for engineers to decide whether the insulation of a transformer is in a satisfactory condition for the unit to be used. The reliability of the computations depends on the provision of accurate data, including a value for the diffusion coefficient of moisture through transformer oil. The magnitude of this parameter was unknown and the paper describes the establishment of a technique for measuring the quantity.