Finite Element Based Comparative Analysis of Positive Streamers in Multi Dispersed Nanoparticle Based Transformer Oil

The dispersion of dissimilar nanoparticles (NPs) in transformer oil (TO) has a major impact on fast propagating positive streamers. This work investigates the positive streamer dynamics in TO modified by dispersing both Fe3O4 and Al2O3 NPs at a homogenous concentration. The hydrodynamic drift diffusion model of positive streamer evolution and propagation are solved using the commercial software package COMSOL Multiphysics. The impact of multiple NPs (MNPs) has been analysed for streamer propagation, electric field intensity, electron density, and space charge density of modified TO. MNPs successfully reduce streamer propagation velocity by 50%, 17%, and 37.5% comparing to pure oil, Fe3O4 based nanodielectric fluids (NDFs), and Al2O3 based NDFs, respectively. The spatial distribution of electron density reveals the loss of electrons from the ionization region until the saturation of NPs. A comparative study demonstrates that MNPs significantly alter the streamer dynamics and augment the dielectric strength of TO compared to individual NPs.

Progress in modeling streamer and leader discharges­­­

<p>We present recent progress in pulsed discharge modeling in Amsterdam that is motivated by high voltage and plasma engineering and by lightning.</p><p>We perform streamer simulations with adaptive mesh refinement in 2D and 3D using PIC particle models and fluid models, where we now can include complex electrode shapes and dielectric boundaries. For the longer time evolution, we also have added Ohmic heating, gas expansion, and the relevant ­­­plasma chemistry for air and methane-air mixtures.</p><p>Results relevant for lightning physics include</p><ul><li>Validation and verification of streamer propagation models (with S. Dijcks and S. Nijdam for the experimental counterpart)</li> <li>Simulations of streamer branching and comparison with experiments</li> <li>Parameter studies for long non-branching streamers that can accelerate or decelerate, and vary largely in velocity, radius and inner electron density, depending on the electric field</li> <li>Different stagnation behavior of positive and negative streamers in low electric fields</li> <li>Positive streamers in air that can continue to propagate as isolated patches of positive charge, without a conducting channel behind the streamer head</li> <li>Repetitive discharges, heating, and plasma-chemistry</li> </ul>

Elongation and branching of stem channels produced by positive streamers in long air gaps

The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head.