Drying of power-transformer insulation

To ensure that large high-voltage power transformers will operate reliably for years without requiring unscheduled maintenance, manufacturers thoroughly dry the insulation after manufacture and before commissioning. Using conventional monitoring procedures, it is difficult to establish the residual moisture concentration throughout the insulation. The paper introduces a computational technique which has the potential of calculating accurately the moisture levels in the windings after various periods drying. The method takes into account the evaporative transfer of moisture from the winding surface to the drying medium and may be applied to all drying cycles which incorporate periods of air, vapour-phase or oil-spray heating and vacuum. To demonstrate the accuracy of the method the drying of a model layer winding was predicted and the results were compared with measurements. The excellent correlation of the results is displayed in the paper.