Amir Hamed Mashhadzadeh - Department of Electrical Engineering, Gorgan Branch, Islamic Azad University, Gorgan, Iran.
Mahmood Ghanbari - Department of Electrical Engineering, Gorgan Branch, Islamic Azad University, Gorgan, Iran.
Amangaldi Koochaki - Department of Electrical Engineering, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
Seyyedmeysam Seyyedbarzegar - Department of Electrical Engineering, Shahrood University of Technology, Shahrood, Iran
Morteza Ghorbanzadeh Ahangari - Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Mazandaran, Babolsar, Iran

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 TiO۲, ZnO, and CNTs incorporated oil (nanofluids) can underwent thermal aging by simulating this phenomenon at ۱۱۰, ۱۲۰, and ۱۳۰ ° C for ۳۰, ۳۰, and ۱۵ days (equivalent of ۱۰, ۳۰, and more than ۴۰ 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 ۱.۲ 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 TiO۲ was the best candidate, such that the breakdown voltage increased with respect to the pure oil by ۱۷, ۲۷, and ۴۸% at ۱۱۰, ۱۲۰, and ۱۳۰ °C, respectively. The outcome of the lightning breakdown test also indicated that TiO۲ still performed better than the other samples. TiO۲ was able to improve the lightning voltage at ۱۱۰, ۱۲۰, and ۱۳۰ °C by ۳۳, ۸, and ۵%, respectively. Therefore, as it was observed, TiO۲ has been able to perform the best performance in thermal aging.

Transformer oil, Thermal aging, Breakdown voltage, Lightning breakdown, Nanoparticle