Water content

To a small extent, water results from the ageing process of the cellulose insulating material and the oil, but mainly through the diffusion of humidity from the ambient air due to poorly maintained air dehumidifiers and/or a faulty sealing system. Humidity can even arise in the expansion vessel, in spite of an installed air dehumidifier. This fact has been largely ignored in the past. The cause for humidity development lies in the transformer's temperature variations. Moisture in the oil is at an equilibrium with moisture in the solid insulation material, reduces the breakdown voltage level, and accelerates the ageing process. However, this equilibrium is temperature and time-dependent. At present, a precise determination of the humidity in the cellulose is only possible by drawing samples with subsequent Karl Fischer titration. PDC analysis is a modern method for determining the humidity in the insulation.