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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student-submitted PDF version of thesis., Includes bibliographical references (p. 223-230)., Liquid dielectrics provide superior electrical breakdown strength and heat transfer capability, especially when used in combination with liquid-immersed solid dielectrics. Over the past half-century, there has been extensive research characterizing "streamers" in order to prevent them, as they are the main origins of electrical breakdown in liquid dielectrics. Streamers are conductive structures that form in regions of liquid dielectrics that are over-stressed by intense electric fields. Streamers can transform to surface flashovers when they reach any liquid-immersed solid insulation. Surface flashovers usually propagate faster and further than streamers in similar electric field intensity. Charge generation and transport is crucially important in liquid dielectric breakdown, since without the presence of the electric charge and its ability to migrate in the liquid dielectric volume and on the interface of liquid/solid dielectrics, streamers and surface flashovers are unable to develop. In this thesis, we develop a finite element method transport model in one, two and threedimensional geometries to help understand the complicated dynamics of electric charge transport and streamer breakdown in liquid dielectrics. This electrohydrodynamic model clarifies many of the mechanisms behind streamer/surface flashover formation, propagation and branching in typical liquid/solid dielectric composite systems. Several key mechanisms have been identified and added to the transport model of streamers, such as effects of electric field intensity on the ionization potential of liquid dielectric molecules and electron velocity saturation, which make the modeling results more realistic. In addition to improving the understanding of electrical breakdown physics in liquid-based insulation systems, a significant effort is made throughout this thesis research to enhance the stability, convergence, speed and accuracy of the model, making it a convenient and reliable tool for designing hig, by Jouya Jadidian., Ph.D.

Experimental and computational images of a 90-μm thick copper wire explosion in atmospheric pressure air are presented. A Marx generator is used to produce a pulsed current density into the wire with a maximum rate of rise of ~ 10[superscript 18] Am[superscript -2] s[superscript -1]. A multiphase numerical model includes mass and momentum conservation equations, thermofield ionization, Maxwell's equations, and heat transfer between the phases. Visual records of the experiment agree with the simulation results.

An electrothermal hydrodynamic model is presented to evaluate effects of the applied lightning impulse voltage parameters such as polarity, magnitude, and rise time on the initiation and propagation of the streamers formed in an IEC defined needle-sphere electrode geometry filled with transformer oil. Instantaneous velocity, column diameter, head curvature, maximum electric field, and the volume charge density have been investigated as the main characteristics of the streamer. Modeling results indicate that greater applied voltage peak amplitudes form streamers with higher velocity, greater head curvatures, and thicker columns. The bushy negative streamers usually initiate at almost twice the applied voltage magnitude and propagate slower than filamentary positive streamers. Results also show that in transformer oil at the same impulse voltage peak amplitude, shorter rise times create thicker positive and negative streamers., ABB Corporate Research Center (Vasteras, Sweden)

Flashover formation and expansion mechanisms on the surfaces of different dielectrics immersed in transformer oil have been numerically analyzed. Streamers emanating from a needle electrode tend to transform to surface flashovers if the immersed dielectric permittivity is higher than the liquid permittivity and/or the dielectric interfacial surface cuts the path of the streamer. Perpendicular interface of the immersed dielectric impedes the breakdown by deflecting the streamer and slowing down the surface flashover. The parallel dielectric interface, however, assists the breakdown by regulating the surface flashover velocity to an approximately constant value (∼10 km/s)., IEEE Dielectrics and Electrical Insulation Society

Streamer branching in liquid dielectrics is driven by stochastic and deterministic factors. The presence of stochastic causes of streamer branching such as inhomogeneities inherited from noisy initial states, impurities, or charge carrier density fluctuations is inevitable in any dielectric. A fully three-dimensional streamer model presented in this paper indicates that deterministic origins of branching are intrinsic attributes of streamers, which in some cases make the branching inevitable depending on shape and velocity of the volume charge at the streamer frontier. Specifically, any given inhomogeneous perturbation can result in streamer branching if the volume charge layer at the original streamer head is relatively thin and slow enough. Furthermore, discrete nature of electrons at the leading edge of an ionization front always guarantees the existence of a non-zero inhomogeneous perturbation ahead of the streamer head propagating even in perfectly homogeneous dielectric. Based on the modeling results for streamers propagating in a liquid dielectric, a gauge on the streamer head geometry is introduced that determines whether the branching occurs under particular inhomogeneous circumstances. Estimated number, diameter, and velocity of the born branches agree qualitatively with experimental images of the streamer branching., IEEE Dielectrics and Electrical Insulation Society, IEEE Nuclear and Plasma Sciences Society