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SDIV of OIP material gets altered with thermal ageing temperature of OIP material, supply voltage frequency and the harmonic content in the supply voltage. The UHF signal radiated due to surface discharges formed under AC voltage, high frequency AC voltages and with harmonic voltages, have dominant frequency at about 0.9 GHz. Surface charge accumulation studies indicate that thermal ageing have high impact on amount of charge accumulation. Also number of layers of insulation have high charge trap density, which is ascertained through tdv/dt plot. The amount of charge accumulated is high with pressboard impregnated in titania nanoparticle dispersed transformer oil. The SD activity occurs at the rising portion of AC voltage and at high dv/dt's with harmonic AC voltages. More number of discharges occur with OIPs thermally aged at higher temperature. Physico-chemical changes clearly indicate not much of a change occurs with transformer insulation due to thermal ageing.

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The heat transfer characteristics of nanofluid produced by mixing nano titania with transformer oil, facilitated by addition of surfactants are analyzed. A 2D model is used to analyze the heat transfer and fluid flow characteristics of nano fluid for understanding the formation of hot spots in the chamber filled with nanofluid. Governing equations for conservation of mass, momentum and energy for capturing the above characteristics are described. The temperature along the vertical mid line from the hot spot are measured experimentally and compared with simulation results. Temperature distribution obtained for nanofluid and transformer oil under both steady and transient state has revealed high rate of heat dissipation in nanofluid. Streamlines have shown the presence of press board affects flow in the bulk of the cavity. Nusselt number estimated across the edges of the hot spot has shown higher convective heat transfer in nanofluid.