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.
Wang et al. (2026) studied this question.