Influence of the Chemical Composition of Dielectric Fluid on Partial Discharge Characteristics for Diagnostic Purposes

This thesis provides an understanding of the correlation in change of partial discharge (PD) characteristics with the chemical nature of impurities present within dielectric fluids when measured in a needle-plane geometry. Such knowledge could prove vital to support development of a field-deployable diagnostic for dielectric fluids. A reproducible method for applying a protective coating upon the plane electrode to maintain consistent measurements has been developed and demonstrated to be imperative for obtaining PD signatures. Experiments were conducted with hexadecane, which is a relatively pure base fluid. Chemicals commonly associated with ageing processes that occur in dielectric fluids, which include oxidation of fluid, thermal degradation of cellulose, and deposition of copper sulfide from thermally degenerated sulfur-containing compounds were added to hexadecane. Partial discharge inception voltage (PDIV), PD burst characteristics, and phase resolved pattern were correlated to the additive’s experimentally known or computationally calculated electrochemical properties. In general, the presence of low ionization potential compound increases ionization and eases the formation of avalanches relative to pure hexadecane. PD pulse rate is suppressed for negative streamers with compounds exhibiting positive electron affinities (EA), as was evident for paper degradation compounds, polycyclic aromatic compounds, and corrosive sulfur compounds. Deviations from these results were seen with carboxylic acids which reduced the number of negative streamers even though they exhibit negative EA, and could be attributed to a large electron capture cross section from hydrogen bonding. Copper sulfide particulates cause large changes in PD characteristics as a result of its semi-conductive nature. Degradation impurities provide distinct PD pattern signatures that can be easily seen in an illustrative phase resolved PD pattern. Overall, acquiring the number of positive and negative streamers within a certain timeframe, along with the examination of the distinct PD pattern, gives more insight into the altering chemistry of the insulating fluid as compared to the statistically insignificant PDIV or dielectric breakdown test