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APreventive diagnosis and maintenance of transformers has become more and more useful in recent times in order to improve the reliability of electric power systems. Time domain dielectric testing techniques such as Return Voltage (RV) measurement and Polarisation-Depolarisation Current (PDC) measurement have recently been widely used as potential tools for condition assessment of transformer insulation. In the frequency domain, measurement of the dissipation factor (tanδ) over a frequency range of 0.1 mHz to 1 kHz has been commonly used to assess the condition of transformer insulation. A number of transformers have been tested with these diagnostic techniques. A better understanding and analysis of the dielectric test results is only possible with a clear understanding of the physical behaviour of the insulation system in response to moisture and ageing. A circuit model, which describes the dielectric behaviour of the transformer’s main insulation system, has been modelled in this paper from the frequency domain measurement data and then RV and PDC data have been simulated from the circuit model. This paper then presents discussions of frequency and time domain techniques to find the condition of transformer insulation in terms of moisture and ageing estimation.