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 electric fields on the order of 1MV/cm or greater. Streamers transform to surface flashovers when they reach any liquid immersed solid insulation. 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 cannot develop.
In this thesis, we develop an electrohydrodynamic transport model in one, two and three–dimensional geometries to help understand the complicated dynamics of electric charge transport and streamer breakdown in liquid dielectrics. This finite element model clarifies many of the mechanisms behind streamer/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 the 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 reliable tool for designing high voltage components that contain pure liquid dielectrics, nanofluid suspensions and liquid immersed solid dielectrics. This model, for the first time, is able to treat any given electrode shape and gap distance as well as any applied voltage waveform with accurate results, which provides a convenient preliminary way to verify the performance of an insulation system in terms of breakdown voltage, time to breakdown, electric field intensity distribution and ionization level. The model precision is validated through experimental records, analytical solutions and alternative modeling approaches wherever available. Specifically, we verify our one–dimensional numerical results with exact analytical solutions, and our two and three–dimensional modeling results with experimental data found in the literature or provided by ABB Corporate Research, Sweden.
The streamer initiation voltages, number of streamer branches, breakdown voltages and currents are in excellent agreement with the experimental data compared to the prior theoretical research on liquid breakdown physics. Identical results obtained using a finite volume method also confirm the correctness of the finite element approach used in this thesis. The presented model can be employed to search for novel configurations of liquid immersed insulation systems including nanofluids and liquid/solid composite systems.
Prof. Markus Zahn (Thesis Supervisor)
Prof. Jeffrey Lang
Prof. Luca Daniel