Prebreakdown phenomena in solids and liquids stressed by fast transients: The effect of additives and phase
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- Institutt for kjemi 
This study has been conducted to improve the knowledge of processes responsible for dielectric breakdown of liquid and solid insulation. This is important, as the electric power grid depends on reliable and cost effective insulation materials. Prebreakdown phenomena in liquid and solid neat cyclohexane, neat n-tridecane, n-tridecane with 0.1 M N,N-dimethylaniline (DMA) and ntridecane with 0.1 M trichloroethene (TCE) have been studied in a needleplane geometry with positive and negative needle with impulse voltage. For this purpose an experimental setup for dielectric testing of insulating liquids and frozen liquids in the temperature range from -60°C to +250°C was built. The system uses a sensitive differential charge measurement technique, a computer controlled temperature regulator, a photomultiplier, and a pulse generator with a fast rise time (<40 ns, <60 kV). The electrode gap was 11.5 mm, and most experiments were conducted with a 2 µm tip radius. Experiments were conducted in cyclohexane to make comparison with previous results with similar experimental setups possible, n-tridecane was chosen based on the known similarities in energy bands for polyethylene and alkanes. Thus n-tridecane was used as a model system for polyethylene. The additives were chosen based on their electrochemical properties. DMA has a low ionization potential and TCE is a known electron scavenger. Currents recorded below the inception voltage (Pre-inception currents) have been compared to finite element model calculations to obtain high-field conductivity models for liquid and frozen neat n-tridecane and neat cyclohexane. The conductivity models obtained in this way was then compared to classical conductivity models. In addition field dependent ionization and excitation energies obtained from density functional theory (DFT) simulations have been used to explain the high-field conductivity. The high-field conductivity in neat n-tridecane was found to have similar field dependence as what has been reported for cross linked polyethylene, and high-field conduction in cyclohexane can be explained by the Poole-Frenkel mechanism Conduction currents were observed at lower voltages in n-tridecane than in cyclohexane, corresponding to the low space charge limited field (SCLF) in n-tridecane. This may explain the lower inception voltage in cyclohexane, as the SCLF for cyclohexane is sufficient to make electron avalanches in the liquid phase plausible, while for n-tridecane the SCLF is low enough to limit the field without the creation of a streamer/electrical tree. Thus the formation of electron avalanches in n-tridecane may rely on the formation of a low density region. Adding TCE to n-tridecane increases the pre-inception current and reduces the inception voltage for negative and positive polarity. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Inception voltages, light emission and charge injection were found to be similar for liquid and solid phase in neat n-tridecane and in n-tridecane with additives, which indicates that the same processes are responsible for inception and propagation of electrical trees in solids and streamers in liquids when the material is stressed by a fast transient. The similarity between the measured properties in solid and liquid phase leads to the conclusion that electrical treeing mainly takes place in the amorphous regions of the solid phase. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. In another part of this thesis work, approximations for the electric field and the charge on a spheroid in a uniform background field (or semispheroid on a plane) and of the maximum extent of the SCLF region on axis of the spheroid is presented. This is relevant for electrical tree initiation, as a SCLF region of about 2 µm is needed for inception. The approximations are accurate for step voltages of finite rise time with exponentially increasing conductivity of the form σ0exp(kjEj1=n), where "n" can vary from 1 to 3, which covers the range of functional dependence found in the literature, k and σ0are constants, and E is the electric field.