Modeling streamers in transformer oil: The transitional fast 3rd mode streamer

This paper presents an electro-thermal hydrodynamic model that explains the development of different streamer modes in transformer oil. The focus is on the difference between the slow 2^nd and fast 3^rd mode streamers discussed in the literature. Through the use of numerical methods the model demonstrates that streamer modes arise in transformer oil due to the electric field dependent molecular 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 number density aromatic molecules, that generally have lower ionization energies than naphthenic/paraffinic molecules, leads to the propagation of 2^nd mode 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 number density naphthenic/paraffinic molecules, dominates producing high electric field levels and space charge at the streamer tip. This ultimately leads to the propagation of the 3^rd mode streamer with velocities on the order of 10 km/s.