Elucidating the mechanisms behind pre-breakdown phenomena in transformer oil systems

Abstract

The widespread use of dielectric liquids for high voltage insulation and power apparatus cooling is due to their greater electrical breakdown strength and thermal conductivity than gaseous insulators. In addition, their ability to conform to complex geometries and self-heal means that they are often of more practical use than solid insulators. Unfortunately, as with all insulation, the failure of the liquid insulation can cause catastrophic damage. This has led researchers to study the insulating properties of dielectric liquids in an attempt to understand the underlying mechanisms that precede electrical breakdown in order to prevent them. This thesis develops a set of mathematical models which contain the physics to elucidate the pre-breakdown phenomena in transformer oil and other oil-based systems. The models are solved numerically using the finite element software package COMSOL Multiphysics. For transformer oil, the results show that transformer oil stressed by a positively charged needle electrode results in the ionization of oil molecules into positive ions and electrons. The highly mobile electrons are swept back towards the positive electrode leaving a net positive space charge region that propagates towards the negative electrode causing the maximum electric field to move further into the oil bulk. It is the moving electric field and space charge waves that allow ionization to occur further into the oil. This leads to thermal dissipation and creates a low density streamer channel. In comparing the numerical results to experimental data found in the literature, the results indicate that positive streamer propagation velocity regimes or modes are dictated by the onset of different ionization mechanisms (i.e., field ionization, impact ionization, photoionization) that are dependent on the liquid molecular structure and the applied voltage stress. In particular, the field ionization of different families of molecules plays a major role in development of slow and fast mode streamers, especially in liquids that are comprised of many different types of molecules such as transformer oil. The key characteristics of the molecules that affect streamer propagation are their molecular structure (i.e., packing, density, and separation distance) and ionization potential. A direct outcome of this work has been the ability to show that by adding low ionization potential additives to pure dielectric liquids, the voltage at which streamers transition from slow to fast mode can be significantly increased, a result counter-intuitive to conventional wisdom and common practice. For transformer oil with nanoparticle suspensions (nanofluids), the effects of nanoparticle charging on streamer development have been thoroughly investigated. The charging dynamics of a nanoparticle in transformer oil show that electron trapping by conductive nanoparticles is the cause of a decrease in positive streamer velocity. resulting in higher electrical breakdown strength for transformer oil-based nanofluids. Further generalized analysis of the charging of a perfectly conducting sphere from a single charge carrier or two charge carriers of opposite polarity, with different values of volume charge density and mobility and including an ohmic lossy dielectric region surrounding a perfectly conducting sphere or cylinder are also examined. Streamer development in liquid-solid insulation systems, such as oil-pressboard systems, is also investigated. Great effort has been undertaken to model the solid insulation region and a method has been developed to model the oil-solid interface to account for surface charge build up, which is important for streamer dynamics. Various ohmic and migration conduction laws are used for oil and solid insulation to solve for the time and space development of surface charge distributions in closed form for one-dimensional parallel plane and numerically for two-dimensional geometries. The work on streamers in oil-pressboard systems has shown that streamers are attracted to the oil-pressboard interface, due to the larger permittivity of the pressboard. Moreover, the models have shown that the determination of how streamers propagate in the presence of solid insulation is strongly dependent on the extent to which the solid insulation alters the streamer shape and the electric field created by the streamer's space charge. These results obtained from the modeling of streamers in oil-pressboard systems are supported by and help to explain the experimental data in the literature.