A Model for the Initiation and Propagation of Positive Streamers in Transformer Oil

An electro-thermal model is presented of streamer initiation and transport that leads to electrical breakdown. The dominant charge initiation mechanism treated is electric field dependent molecular ionization. Analysis using COMSOL Multiphysics shows that molecular ionization results in fast electrons that cause a propagating electric field wave that is the dominant mechanism in the initiation of electric breakdown streamers. To model these dynamics a complete three carrier (positive ions, negative ions, and electrons) liquid-phase electric field dependent molecular ionization based streamer model was developed and solved for a positive needle voltage. The Heaviside function based on a critical energy for transformer oil boiling was used as the criterion to define regions of liquid or gas. In the gas-phase charge generation is described by Townsend ionization.