Keer electro-optic measurements in liquid dielectrics

Kerr electro-optic technique has been used to measure the electric field distribution in high voltage stressed dielectric liquids, where the difference between refractive indices for light polarized parallel and perpendicular to the local electric field is a function of the electric field intensity. For transformer oil, the most widely-used insulating liquids in power apparatus and high voltage technology, Kerr effect is very weak due to its low Kerr constant. Previous Kerr measurements have been using ac modulation technique, which is only applicable to dc steadystate electric field mapping while various instabilities develop in liquid under long-term high voltage application. The use of the high-sensitivity CCD camera as optical detector makes it possible to capture the weak Kerr effect in high voltage stressed transformer oil. The first part of this thesis is to demonstrate the reliability and evaluate the sensitivity of the measurements for various cases with identical electrodes under pulsed excitation with insignificant flow effects. After the validation and optimization of the experimental setup, measurements are taken to record the time evolution of electric field distributions in transformer oil stressed by high voltage pulses, from which the dynamics of space charge development can be obtained. Correlation between space charge distribution pattern and impulse breakdown voltage is examined. Hypothetically, bipolar homo-charge injection with reduced electric field at both electrodes may allow higher voltage operation without insulation failure, since electrical breakdown usually initiates at the electrode-dielectric interfaces. It is shown that the hypothesis is testable and correct only under specific circumstances. Besides, fractal-like kinetics for electrode charge injection is identified from the measurement data, which enriches the knowledge on ionic conduction in liquids by offering an experimentally-determined boundary condition to the numerical model. Physical mechanisms based on formative steps of adsorption-