Incorporation of nanoparticles into transformer oils improves their electrical and insulating properties. However, thermal aging may undesirably decrease the performance of nanomaterials in transformer oils. Herein, pure oil together with TiO2, ZnO, and CNTs incorporated oil (nanofluids) can underwent thermal aging by simulating this phenomenon at 110, 120, and 130 ° C for 30, 30, and 15 days (equivalent of 10, 30, and more than 40 years of normal oil operation, respectively). During the accelerated thermal aging process, the total acid number (TAN), breakdown voltage, and lightning impulse breakdown voltage of all samples were measured periodically. The TAN increased with increasing temperature and time, but never exceeded the allowable level of 1.2 mg KOH/g. As the oil ages, its corrosion rate increases, which is undesirable for the transformer. The results of the breakdown voltage test suggest that the TiO2 was the best candidate, such that the breakdown voltage increased with respect to the pure oil by 17, 27, and 48% at 110, 120, and 130 °C, respectively. The outcome of the lightning breakdown test also indicated that TiO2 still performed better than the other samples. TiO2 was able to improve the lightning voltage at 110, 120, and 130 °C by 33, 8, and 5%, respectively. Therefore, as it was observed, TiO2 has been able to perform the best performance in thermal aging. تفاصيل المقالة