Mechanisms behind positive streamers and their distinct propagation modes in transformer oil

Comprehensive modeling and analysis is presented for the charge generation, recombination, attachment, and transport of positive ions, negative ions, and electrons in transformer oil between a positive high voltage sharp needle electrode and a large spherical ground electrode. Case studies show that a key mechanism in streamer development is field ionization, which is the direct ionization of molecules due to action of the electric field. Due to the high mobility electrons, which are about 1 × 10⁵ times more mobile than the positive ions, the newly generated electrons quickly exit the high field ionization zone towards the anode leading to the development of a net positive space charge peak which subsequently creates an electric field enhancement in the oil. The process is driven by the applied high voltage creating temporally dynamic space charge and electric field distributions that develop an ionizing wave that drives streamer development. The pre-breakdown modeling and analysis elucidates the development of different streamer modes in transformer oil. In particular, the analysis focuses on mechanisms driving filamentary fast mode streamers discussed in the literature. The results demonstrate that streamer modes arise in transformer oil due to the ionization of different families of hydrocarbon molecules (i.e., aromatic, naphthenic, and paraffinic) at increasing electric field levels (or applied voltages). Ionization of the low concentration aromatic molecules in transformer oil, that generally have lower ionization energies than naphthenic/paraffinic molecules, leads to the propagation of streamers with velocities on the order of 1 km/s. As the applied voltage is increased, the ionization of the main hydrocarbon molecules in transformer oil, high concentration naphthenic/paraffinic molecules, dominates producing high electric field levels and space charge at the streamer tip. This results in the propagation of a very fast streamer with veloci- ies on the order of 10 km/s. Furthermore, these streamers have protrusion spacing in the approximate range of 20-100 μm in ~36 ns like those of electrohydrodynamic instability of charged jets that may be the origin of streamer branching. A preliminary model based on earlier electrohydrodynamic stability analysis is presented that predicts protrusion spacing in the approximate range of 7-30 μm with growth rate 2.5-5 μs.