Drying of transformer board

The internal insulation in shell type power transformers is usually ensured by the stacking of plates of board impregnated with oil. A board is in equilibrium with the atmosphere at a moisture content varying from seven to eight percent by weight, but, when the insulating function is required, the water content must be lower than 0.5 %. The aim of the present work is the understanding and the modelling of transformer boards drying. The first part of this work presents experimental studies showing that the drying of transformer board follows two regimes: a fast regime in the first period, a slow regime, up to 99 percent of dryness. The effect of heating and the influence of the board thickness on the drying rate are also studied. In the second part of this paper, we present a model capable to represent the observed kinetics of transformer board drying. This model is based on the diffusion of water vapour in the gaseous phase combined with the transfer of water vapour from the fibres to the gaseous phase. The value of the average global transfer coefficient can be deduced from the slope of the curve 1/root 1-x = f(drying time), where x denotes the yield in drying (or moisture loss), and from the values of the physical properties of the board (porosity, tortuosity and specific surface area of the fibres). The proposed model can help a transformer manufacturer to determine the residual water content in the board after drying.