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

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…