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This work aims to study the effect of conducting and semiconducting nanoparticles (NPs) on ac breakdown voltage (ac-BDV) and the electrostatic charging tendency (ECT) of synthetic ester (SE), namely, MIDEL 7131; ac-BDV and ECT are two important properties that must be considered when choosing an insulating oil for use in power transformers. The NPs being tested are iron oxide (Fe3O4, fullerene (C60, and copper oxide (CuO) NPs at 0.1-g/L (0.01 wt%) concentration. An improved nanofluid (NF) preparation procedure and the ECT test cell are presented. This latter consists of a spinning disk covered on both sides by the solid insulator being tested and immersed in the investigated liquid. It is observed that NF with Fe3O4NPs shows the best improvement of ac-BDV; this improvement is about 22.44%. The improvements are about 7.81% and 8.24% with 0.1-g/L CuO and C60NFs, respectively. Regarding ECT, the measurement results show that the liquid volume charge density increases with disk rotation speed over time; a lower ECT is observed with the three NFs. Also, the NFs show a similar ECT at low speed (200 r/min); beyond that speed, the effect of adding CuO and C60NPs on the ECT reduction of MIDEL 7131 becomes more significant than that with Fe3O4NPs.