Dielectric measurements have been used for long time for assessment of electrical insulation. The insulation system of a power transformer consists of oil and cellulose, whose dielectric properties are strongly influenced by moisture, temperature and ageing. This condition can therefore be evaluated using the response of the dielectric system. In this work we utilize data published to establish a circuit model of power transformer. Analysis techniques are then suggested in order to simplify the testing procedure for moisture in bulk insulation.