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.

Arc faults within the transformers can generate sudden pressure surges, constituting significant hazards that may precipitate oil tank explosions and severely compromise power system stability. Conventional power−frequency arc discharge experiments encounter limitations in isolating pressure wave characteristics due to persistent gas generation and arc reignition. To circumvent these challenges, an oil-immersed impulse voltage discharge platform was conceived and engineered to investigate pressure wave propagation dynamics. A pressure numerical simulation model and theoretical model of oil−solid interface reflection and refraction were subsequently established to elucidate the pressure propagation mechanism. The experimental and simulation results show that the pressure wave generated by pulsed arc discharge in oil propagates radially in the form of spherical waves. Due to the viscous loss and wave front expansion of transformer oil, the peak pressure decays exponentially with distance, with a decay coefficient β = 1.15. When pressure waves encounter metal obstacles inside transformer oil, there are two propagation paths: direct transmission through and multiple reflections through, and a mode transformation of pressure waves occurs at the oil−solid interface, mainly propagating through obstacles in the form of transverse waves. This work quantitatively delineates the energy pressure wave coupling, propagation dynamics, and attenuation mechanisms, providing critical insights for assessing and mitigating arc fault-induced transformer explosion risks.