Transformer oil is a critical insulating medium in high-voltage equipment, and its discharge characteristics under high electric fields significantly limit overall performance. Current negative streamer classification methods rely primarily on propagation velocity, failing to effectively explain the underlying transition rules and mechanisms. To address this, we developed an impulse discharge experimental platform featuring a shadowgraph optical diagnostic system. By analyzing the morphological evolution, current pulses, and light emission signals of negative streamers across various voltage levels and liquid pressures, this study elucidates their transition mechanisms and mode classifications. The results demonstrate that negative streamer discharge initiates in a primary subsonic mode with a thin-rod channel. It then evolves into either a heavily branched, thick-channeled secondary mode (bush-like) or a faster, less-branched tertiary mode. Under sufficiently high overvoltage, it transitions to a supersonic quaternary mode with filamentary channels. Notably, negative streamer development exhibits distinct gaseous characteristics, and the channel deformation process aligns with the Rayleigh theory of bubble dynamics in liquids. Finally, appropriately increasing liquid pressure compresses the streamer channel, facilitating the transition from the secondary to the tertiary mode.

Severe vibration in power transformers can induce pressure oscillations in thin oil gaps within oil-paper insulation, leading to cavitation bubble formation and potential subsequent partial discharge (PD) initiation. However, the cavitation inception mechanism in vibrating thin oil gaps and its role in PD initiation remain insufficiently understood. In this study, cavitation inception in thin oil gaps bounded by oil-impregnated pressboard was experimentally investigated under different oil temperatures and dissolved gas contents. The observed bubbles were found to originate primarily from local supersaturation and exsolution of dissolved gases during the negative pressure phase of vibration, indicating that gaseous cavitation is the dominant bubble formation mechanism. The vibration acceleration threshold for cavitation bubble inception decreases significantly with increasing oil temperature and dissolved gas content. To interpret this behavior, a dynamic gas separation pressure based on the rectified diffusion model was proposed, showing that enhanced gas diffusion and dissolved gas availability promote the growth of pre-existing nuclei and thereby reduce the cavitation bubble inception threshold. Further PD tests revealed a two-stage decrease in PDIV with increasing vibration acceleration, transitioning from a gap compression dominated slight reduction before stable cavitation to a bubble dominated rapid decrease after cavitation inception. These findings reveal a vibration-induced gaseous cavitation pathway for PD inception in oil-paper insulation and provide support for assessing vibration-related insulation risks in power transformers.

The presence of bubbles severely threatens the insulation safety of power transformers. Understanding bubble accumulation characteristics is essential for predicting potential insulation weaknesses. In this study, we propose a Euler–Lagrange two-phase flow model, combined with an experimental platform, to investigate the accumulation and growth characteristics of bubbles in typical oil channels. The continuous phase flow field is solved within the Eulerian framework, while discrete bubble trajectories are tracked using the Lagrangian approach, simultaneously accounting for drag, buoyancy, and friction forces. The results indicate that bubbles primarily accumulate on the top insulating paper surfaces within horizontal and corner oil channels. In horizontal channels, the accumulation rate correlates positively with flow velocity and bubble concentration; furthermore, paper overlapping gaps intercept sliding bubbles, promoting their coalescence into larger ones. Conversely, at channel corners, the accumulation length and rate decrease as flow velocity increases, with accumulation weakening significantly when the velocity exceeds 0.6 m/s. Additionally, bubble growth is highly sensitive to flow velocity and temperature: higher velocities inhibit bubble coalescence, whereas elevated temperatures accelerate both coalescence and growth. Ultimately, this research reveals the fundamental accumulation laws of bubbles in transformer oil channels, providing a reliable analytical tool and theoretical reference for assessing bubble-induced insulation risks.