Electrical Network-Based Time-Dependent Model of Electrical Breakdown in Water

A time-dependent, two-dimensional, percolative approach to model dielectric breakdown based on a network of parallel resistor–capacitor elements having random values, has been developed. The breakdown criteria rely on a threshold electric field and on energy dissipation exceeding the heat of vaporization. By carrying out this time-dependent analysis, the development and propagation of streamers and prebreakdown dynamical evolution have been obtained directly. These model simulations also provide the streamer shape, characteristics such as streamer velocity, the prebreakdown delay time, time-dependent current, and relationship between breakdown times, and applied electric fields for a given geometry. The results agree well with experimental data and reports in literature. The time to breakdown (tbr) for a 100 μm water gap has been shown to be strong function of the applied bias, with a 15–185 ns range. It is also shown that the current is fashioned not only by dynamic changes in local resistance, but that capacitive modifications arising from vaporization and streamer development also affect the transient behavior

Parameters affecting the electrical and thermal properties of transformer oils

Transformer oil is expected to function as an insulating medium and heat transfer agent. In this contribution, contaminants that mostly influence the electrical and thermal properties of transformer oil are addressed. To access the influence of the type of impurities on oil properties, various scenarios were considered. Investigations were therefore performed on new oil, new oil submitted to electrical stress, service-aged oil, reclaimed (by Fuller's Earth) service-aged oil, dehumidified service-aged oil, dehumidified and degassed service-aged oil. The influence of each type of "contaminant" is emphasized and discussed. The results obtained from the measurements show that electrical discharge by-products (charges carriers), mostly affect oil insulating properties, and influence the conduction phenomena which in turn increases the dissipation factor. It is also shown that colloidal suspensions in oil affect the heat transfer capability of this later, as assessed by viscosity