Abstract: In order to investigate the performance and compositional changes of transformer oil under long-term, low-intensity partial discharge conditions, ionization tests were conducted on mineral insulating oil under high-voltage AC fields in hydrogen/nitrogen atmospheres. The insulation properties, physicochemical characteristics, and hydrocarbon composition of the oil samples were systematically analyzed after different durations of ionization. The results show that, with increasing ionization time, the hydrocarbon group composition of aromatic-free insulating oil remains essentially stable and is less affected by the ambient atmosphere. In contrast, in aromatic-containing insulating oil, the content of monocyclic aromatics increases while that of bicyclic aromatics decreases, and these changes are significantly influenced by nitrogen and hydrogen atmospheres. A high concentration of hydrogen promotes the conversion of bicyclic aromatics to monocyclic aromatics and further to naphthenes, while inhibiting the dehydrogenation of naphthenes to form aromatics. After ionization, all insulating oil samples exhibit increased average relative molecular mass, density, kinematic viscosity, and dielectric loss factor, along with degraded insulation performance. Moreover, the gas absorption capacity of high-aromatic oil decreases after ionization, while the gas evolution property of low-aromatic oil improves. However, throughout the entire test process, the gas evolution behavior of high-aromatic oil remains superior to that of low-aromatic oil.

Key words: insulating oil, high voltage electric field, ionization, hydrocarbon composition