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Abstract: Transformer oil-based nanofluids (NFs) with TiO2 semiconductive nanoparticles (SNFs) exhibit substantially higher AC breakdown voltage than that of pure transformer oils at variable relative humidity from 20% to 80%. Charge accumulation and decay characteristics of pure oils and SNFs were measured by the pulse electroacoustic technique (PEA). It reveals that SNFs have more uniform internal electric fields and higher charge decay rate compared to pure oils under high relative humidity. It is confirmed by the test results of electrophoresis and thermally stimulated current (TSC) that the nanoparticles adsorb water molecules at the nanoparticle–oil interface, giving rise to the higher shallow trap density and resulting in better charge transport in SNFs. [Copyright &y& Elsevier]