Nils Lavesson

Effects of Impulse Voltage Polarity, Peak Amplitude, and Rise Time on Streamers Initiated From a Needle Electrode in Transformer Oil

An electrothermal hydrodynamic model is presented to evaluate effects of the applied lightning impulse voltage parameters such as polarity, magnitude, and rise time on the initiation and propagation of the streamers formed in an IEC defined needle-sphere electrode geometry filled with transformer oil. Instantaneous velocity, column diameter, head curvature, maximum electric field, and the volume charge density have been investigated as the main characteristics of the streamer. Modeling results indicate that greater applied voltage peak amplitudes form streamers with higher velocity, greater head curvatures, and thicker columns. The bushy negative streamers usually initiate at almost twice the applied voltage magnitude and propagate slower than filamentary positive streamers. Results also show that in transformer oil at the same impulse voltage peak amplitude, shorter rise times create thicker positive and negative streamers.

Impulse breakdown delay in liquid dielectrics

Theoretical images of streamers, revealing the mechanisms behind impulse breakdown in liquid dielectrics, are presented. Streamers lead to electrical breakdown by forming paths, capable of carrying large current amplitudes between electrodes. Breakdown delays and terminal currents are calculated for various electrode geometries (40 μm needle and 6.35 mm sphere) and gap distances (up to 10 mm). Modeling results indicate that the breakdown in needle-needle electrodes requires higher impulse voltage amplitudes than in needle-sphere electrodes for the same gap distances. Streamers in needle-sphere geometries are about 50% thicker than streamers propagating in needle-needle geometries under similar impulse voltage amplitudes.

Surface flashover breakdown mechanisms on liquid immersed dielectrics

Flashover formation and expansion mechanisms on the surfaces of different dielectrics immersed in transformer oil have been numerically analyzed. Streamers emanating from a needle electrode tend to transform to surface flashovers if the immersed dielectric permittivity is higher than the liquid permittivity and/or the dielectric interfacial surface cuts the path of the streamer. Perpendicular interface of the immersed dielectric impedes the breakdown by deflecting the streamer and slowing down the surface flashover. The parallel dielectric interface, however, assists the breakdown by regulating the surface flashover velocity to an approximately constant value (∼10 km/s).

Stochastic and deterministic causes of streamer branching in liquid dielectrics

Streamer branching in liquid dielectrics is driven by stochastic and deterministic factors. The presence of stochastic causes of streamer branching such as inhomogeneities inherited from noisy initial states, impurities, or charge carrier density fluctuations is inevitable in any dielectric. A fully three-dimensional streamer model presented in this paper indicates that deterministic origins of branching are intrinsic attributes of streamers, which in some cases make the branching inevitable depending on shape and velocity of the volume charge at the streamer frontier. Specifically, any given inhomogeneous perturbation can result in streamer branching if the volume charge layer at the original streamer head is relatively thin and slow enough. Furthermore, discrete nature of electrons at the leading edge of an ionization front always guarantees the existence of a non-zero inhomogeneous perturbation ahead of the streamer head propagating even in perfectly homogeneous dielectric. Based on the modeling results for streamers propagating in a liquid dielectric, a gauge on the streamer head geometry is introduced that determines whether the branching occurs under particular inhomogeneous circumstances. Estimated number, diameter, and velocity of the born branches agree qualitatively with experimental images of the streamer branching.

Migration-ohmic charge transport in liquid-solid insulation systems

A one-dimensional migration-ohmic analysis of unipolar charge injection and transport phenomena in a series, two-region, liquid-solid composite dielectric system is presented. Steady-state and transient solutions for volume and surface charge densities and charge trajectories in the charge migration liquid region and electric field and voltage drop in liquid and ohmic solid regions are given.

Effects of electrode size and solid barrier orientation on streamer discharge in transformer oil

Geometrical effects of electrodes and solid barriers immersed in transformer oil are investigated on positive streamer initiation, propagation, and transformation to surface flashover using a 2-D axisymmetric model. Electrode radii of curvature in the range of 20 μm to 6.35 mm are selected such that only positive streamers form. Modeling results indicate that the positive electrode size directly determines the streamer initiation voltage, while breakdown voltage and delay are mainly determined by the grounded electrode size. Specifically, sharper positive electrodes require lower voltages to initiate positive streamers and sharper grounded electrodes result in lower delays and higher breakdown voltages. Incorporating perpendicular and parallel orientations of solid barrier interfaces in the electrohydrodynamic model shows that polarization forces from the barrier dielectrics on charge carrying streamers are proportional to the permittivity difference between transformer oil and the solid dielectric. If th...

Surface flashover development on transformer oil-pressboard interface

Highly insulating pressboard immersed in transformer oil applies a polarization force on charge carrying streamers, proportional to the permittivity difference between the oil and the pressboard. If the pressboard permittivity is greater than the oil permittivity, the attractive force turns the streamer into a surface flashover. On the other hand, a low permittivity pressboard interface repels the approaching streamer and reduces the streamer velocity. Theoretical analysis of electric field propagation through the interfacial surfaces, oriented in either parallel with or perpendicular to the streamer propagation direction, is presented. Pressboard relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.

Abrupt Changes in Streamer Propagation Velocity Driven by Electron Velocity Saturation and Microscopic Inhomogeneities

Causes of streamer acceleration caused by electron velocity saturation at intense electric fields are investigated in liquid dielectrics. Findings of this paper explain regimes of higher positive streamer velocity using a nonlinear electric-field-dependent electron velocity model that describes the streamer velocity more accurately when compared with the experimental results in the literature. Streamer branching in inhomogeneous transformer oil is also studied as an alternative cause of sudden changes in streamer velocity magnitude and direction, because after branching, the velocities of the born branches are significantly higher than the main streamer column velocity. In this paper, a 3-D model is established to incorporate nonsymmetrical variations in the streamer shape. The modeling results show that the streamer branching has both stochastic and deterministic origins. The stochastic parameters are determined by the content of inhomogeneity in the liquid and the deterministic origins of branching are defined by the structure of the volume charge layer at the streamer head. The results indicate that the sudden changes in streamer velocities are due to the electric-field-dependent electron mobility, electric-field-dependent ionization potential of dielectric molecules, and the branching capability of streamers even in extremely homogeneous liquid bulk.

Modeling of streamers in transformer oil using OpenFOAM

Purpose – A model for streamers based on charge transport has been developed by MIT and ABB. The purpose of this paper is to investigate the consequences of changing numerical method from the finite element method (FEM) to the finite volume method (FVM) for simulations using the streamer model. The new solver is also used to extend the simulations to 3D. Design/methodology/approach – The equations from the MIT-ABB streamer model are implemented in OpenFOAM which uses the FVM. Checks of the results are performed including verification of convergence. The solver is then applied to some of the key simulations from the FEM model and results presented. Findings – The results for second mode streamers are confirmed, whereas the results for third mode streamers differ significantly leading to questioning of one hypothesis proposed based on the FEM results. The 3D simulations give consistent results and show a way forward for future simulations. Originality/value – The FVM has not been applied to the model before and led to more confidence in second mode result and revising of third mode results. In addition the new simulation method makes it possible to extend the results to 3D.

Streamer dynamics in transformer oil: Influence of applied voltage rise time

Summary form only given. Mineral transformer oil is the most often used dielectric for electrical insulation and cooling in high power electrical equipment. As a result, transformer oil is widely researched and tested to better understand its electrical, chemical and thermal behavior in power transmission equipment. A large portion of this research is focused on understanding the mechanisms and steps that lead to electrical breakdown, as failure often has disastrous consequences. Previous studies have developed an electro-thermal hydrodynamic model which describes the formation and propagation of streamers leading to electrical breakdown in a needle-sphere electrode geometry. The focus of this paper is to evaluate the effect of applied lightning impulse voltage parameters, such as polarity, magnitude, duration, and rise and fall times on the streamers column diameter, head curvature and head velocity. Modeling results indicate that by increasing the applied voltage magnitude, the streamer velocity slowly increases until a critical voltage is reached, where an order of magnitude velocity increase is observed and the streamer transitions to a higher mode. Moreover, the results indicate that negative streamers initiate at almost twice the applied voltage magnitude and propagate slower than positive streamers at the same applied voltage magnitude and rise-time. These results are in agreement with experiments. In addition, this paper systematically investigates the influence of the applied voltage rise-time on streamer dynamics in transformer oil. From numerical studies, we found that in transformer oil at the same peak voltage, short rise-times create large diameter slow streamers, while a longer rise-time creates thinner streamers propagating faster.