Velocity Of Positive Streamers Research Articles

Alternating streamer propagation in mineral oil under bipolar oscillating impulse voltage

This study aimed to clarify the basic process of streamer propagation in mineral oil at bipolar oscillating impulse voltage. Shadow images and light signals of streamers showed that under bipolar oscillating impulse, positive and negative streamers propagated in an alternating manner: after polarity reversal, new streamers with opposite polarity were initiated and propagated first through the gaseous channels left behind by former streamers and then toward the ground electrode. The velocity of positive streamers was found nearly an order of magnitude higher than that of negative ones; thus, positive streamers are primarily responsible for the insulation failure of mineral oil. Negative streamers played the role of maintaining gaseous channels and facilitating positive streamers initiation due to their strong heat effect. High oscillation frequency and large damping factor decreased the durations and amplitudes of positive peaks, which restrained positive streamer propagation and further resulted in the increase in the breakdown voltage. Experiments on dielectric behavior of mineral oil were conducted to verify above inferences.

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.

Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the charging characteristics of the nanoparticles as a function of time and dielectric permittivity and conductivity of nanoparticles and the surrounding transformer oil. This charged nanoparticle model is used with a comprehensive electrodynamic analysis for the charge generation, recombination, and transport of positive and negative ions, electrons, and charged nanoparticles between a positive high voltage sharp needle electrode and a large spherical ground electrode. Case studies show that transformer oil molecular ionization without nanoparticles cause an electric field and space charge wave to propagate between electrodes, generating heat that can cause transformer oil to vaporize, creating the positive streamer. With nanoparticles as electron scavengers, the speed of the streamer is reduced, offering improved high voltage equipment performance and reliability.

Positive and negative streamers in ambient air: modelling evolution and velocities

We simulate short positive and negative streamers in air at standard temperature and pressure. First, double-headed streamers in homogeneous electric fields of 50 kV cm−1 are briefly studied, and then we analyse streamers that emerge from needle electrodes with voltages of 10–20 kV in more detail. The streamer velocity at a given streamer length depends only weakly on the initial ionization seed, except in the case of negative streamers in homogeneous fields. We characterize the streamer evolution by length, velocity, head radius, head charge and maximal field enhancement. We show that the velocity of positive streamers is determined mainly by their radius and in quantitative agreement with recent experimental results both for radius and velocity. The velocity of negative streamers is dominated by electron drift in the enhanced field; in the low local fields of the present simulations, it is little influenced by photo-ionization. Initially it is puzzling that negative streamers can be slower than positive ones under similar conditions, both in experiment and in simulation, as negative streamer fronts always move at least with the electron drift velocity in the local field. We argue that this drift motion broadens the streamer head, decreases the field enhancement and ultimately leads to slower propagation or even extinction of the negative streamer.

Determination of the streamers characteristics propagating in liquids using the electrical network computation

This work is devoted to the modeling of branching streamers propagating in transformer oil using an equivalent electrical network and the electrical network computation. The proposed model enables one to determine the different characteristics of the streamer (i.e., the associated current and the electrical charge, the power and the energy injected in the liquid, the local electric field at the streamer head, the streamer shape and its velocity, the mobility of the charge carriers within the streamer channels, the local viscosity and temperature). It's shown through the simulated values of the mobility of charge carriers, the local viscosity and temperature that both electronic and gaseous mechanisms are implicated in the streamer development. The gaseous nature of streamers and the role of the local electric field are evident. The influence of the conductivity and additives as well as the electrode gap on the propagation velocity of positive streamers is analyzed

The velocity of streamer tips in impulse point-to-plane corona in air, using Lichtenberg figure techniques

Positive and negative streamer lengths were measured in point-to-plane impulse corona in air at atmospheric pressure using Lichtenberg figure technique. Applying short time pulses from 9 to 40 nsec duration time-distance plots were obtained, which allowed one to determine the streamer tip velocity. For a 4 cm gap, a 1 mm diameter point, and a 30 kV pulse an average tip velocity of 1.8×108 cm/sec was found. Comparing these data withHudson's photomultiplier measurement, we may identify the Lichtenberg figure with the “primary” streamer ofHudson. The negative streamer tip velocity in the cathode part of the gap was approximately an order of magnitude smaller than the positive streamer velocity.