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

Inter‐correlations between emitted lights and corresponding currents associated to different energy level spark discharges in mineral oil submitted to high‐voltage direct current stresses

International audienceThe present work is devoted to the analysis of currents and emitted lights associated to electrical spark discharges in mineral oil in a point-plane electrode system under high-voltage direct current stress. A systematic survey has been led on currents and emitted lights that characterise both positive and negative events through large applied field levels. On the other hand, interrelationships have been established between the currents and their corresponding emitted lights for all types of discharges. A particular attention will be accorded to the inter-correlation function between the recorded currents and light emissions and between their corresponding fast Fourier transforms. In spite of the similar shapes, major differences have been found between the spectral behaviours between currents corresponding to low-energy level discharges and those corresponding to high-energy level ones. On the other hand, photonic signals are found to be more accurate and expressive than the recorded current ones to investigate the physicochemical processes involved in the degradation of the insulating properties of the oil. The total energies of all types of discharges are of comparable magnitudes for both polarities. Their derivatives, i.e. powers, dissipated along the propagation are more expressive of the nature of discharge