Surface discharge behavior and simulation analysis of transformer oils with varying hydrocarbon compositions

The rapid development of ultra-high voltage direct current (UHVDC) transmission has sharply increased the demand for high-performance insulating oils. However, the limited supply of premium naphthenic oils such as KI50X necessitates viable alternatives. This study compares the surface discharge behavior of three transformer oils with distinct hydrocarbon compositions, namely, a low-aromatic naphthenic oil (N-LA), a high-aromatic naphthenic oil (N-HA), and a paraffin-based oil (P-LA), in oil–pressboard insulation systems. A synchronized platform was built to monitor partial discharge (PD) activity and white mark formation under AC stress. A digital workflow was developed to extract the geometric and intensity features of white marks. Density functional theory (DFT) was used to analyze the ionization energy and Fukui index distributions of representative hydrocarbons. The proposed mechanisms were validated using a finite element method–cellular automata (FEM–CA) simulation. Two discharge modes were observed: N-LA exhibited stable, low-voltage surface discharges (~19 kV) with sustained white mark growth, and N-HA and P-LA exhibited streamer discharges in bulk oil above 30 kV. Aromatics suppress discharge inception by homogenizing local fields via space charge and scavenging radicals through reactive hydrogen sites. In contrast, paraffins in P-LA enhanced dielectric strength but promoted gas formation once PD began.