Enhanced Dielectric Performance of Mineral–Synthetic Ester Oil Blend with Varying Morphology of ZnO Nanoparticles for Transformer Liquid Insulation

The growing demand for sustainable transformer insulation drives the development of environmentally friendly alternatives to conventional mineral oil. Although nanoparticle-enhanced ester-based insulating liquids exhibit improved dielectric performance, the influence of zinc oxide nanoparticle (ZnO NPs) morphology on the mineral oil–synthetic ester oil blend remains insufficiently understood. This work investigates a hybrid insulating nanofluid comprising mineral oil, synthetic ester oil, and hydrothermally synthesized ZnO NPs with nanoflower, nanorod, and nanosphere morphologies. Different mineral oil–synthetic ester oil blending ratios were evaluated, and 40% mineral oil–60% synthetic ester oil exhibits the highest alternating-current breakdown voltage (BDV) of 73.9 kV. The synthesized nanostructures were dispersed at different concentrations, and their dielectric characteristics were systematically evaluated. Among the investigated samples, the nanoflower-based nanofluid exhibits the highest BDV of 94.8 kV at 0.001 g/L, followed by the nanorod (87.9 kV) and nanosphere (78.7 kV). A comparative analysis of breakdown voltage, capacitance, and dielectric dissipation factor demonstrates the superior dielectric performance of the nanoflower morphology, which is attributed to its enhanced charge-trapping capability and electron-scavenging behavior that effectively suppress streamer propagation. These findings demonstrate the potential of nanoflower-based hybrid insulating nanofluids for next-generation power transformer insulation.