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

Jadidian, J.; Zahn, M.; Lavesson, Nils; Widlund, Ola; Borg, Karl

doi:10.1109/tps.2014.2306197

Cited by 16 publications

(24 citation statements)

References 20 publications

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“…Typical velocities for most of the 4th mode propagation in negative streamers is in the range 20-50 km s −1 , however the maximum negative 4th mode velocity is usually 50-80 km s −1 , and may occasionally even exceed 100 km s −1 in some liquids, occurring when the streamer approaches the plane electrode. This is close to twice as fast as the approximate electron limiting velocity of 41 km s −1 suggested in [41] , however it is much slower than the maximum positive 4th mode velocity. If this value is considered as a very rough approximation, this may indicate that the negative 4th mode does not rely on any feed forward mechanisms like e.g.…”

Section: Discussionsupporting

confidence: 52%

“…While the high field conductivity for the 2nd mode streamers are in the range measured experimentally [38,39], the higher conductivities at the higher modes cannot be verified based on existing experimental results. At these voltages the electrons will probably not have mobility high enough in the liquid phase to cause an efficient charge separation on the time scale required (an electron terminal velocity in hydrocarbons of approximately 41 km s −1 is suggested in [41]), or there may not be a sufficient amount of weakly bound electrons in the liquid to warrant the large increase in conductivity required to limit the field at these high speeds. In either case, the conductivity would no longer be able to limit the field in the high field region effectively, and the resulting field would lie somewhere between the Laplacian field and the calculated space charge limited field.…”

Section: Discussionmentioning

confidence: 99%

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Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

et al. 2020

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This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under negative polarity stress. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. Compared to positive polarity, there are less differences in streamer behaviour in the oils examined under negative polarity. Breakdown voltages and acceleration voltages are higher for negative streamers than for positive ones, while their propagation velocities are lower. While propagation modes for positive voltages are quite distinct, the mode changes for negative ones are more gradual. The behaviour of both positive and negative streamers is in line with the hypothesis that the propagation is governed by electron avalanches and quantum chemical properties of liquid components.

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Section: Discussionsupporting

confidence: 52%

Section: Discussionmentioning

confidence: 99%

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

et al. 2020

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“…For this reason, one study multiplies the mobility by 2.5, to make it similar to the gas phase mobility [38], which would increase the streamer propagation speed by the same factor. Conversely, limitations to the maximum speed of electrons have been introduced [75], which would effectively control the maximum speed of a streamer branch. The speed is also proportional to the concentration of seeds (see figure 12), which was calculated from the low-field conductivity of the liquid (see (10)).…”

Section: Discussion Of the Modelmentioning

confidence: 99%

“…FEM models may be better in the end, but for now, such models cannot model a complete breakdown. They are also simplified, for example in the sense that phase changes are not accounted for [75]. Both lattice and FEM models demands much computational power and the mesh size becomes an important parameter, however, this is avoided in the model presented.…”

Section: Discussion Of the Modelmentioning

confidence: 99%

Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

et al. 2018

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A simulation model for second mode positive streamers in dielectric liquids is presented. Initiation and propagation is modeled by an electron-avalanche mechanism and the Townsend-Meek criterion. The electric breakdown is simulated in a point-plane gap, using cyclohexane as a model liquid. Electrons move in a Laplacian electric field arising from the electrodes and streamer structure, and turn into electron avalanches in high-field regions. The Townsend-Meek criterion determines when an avalanche is regarded as a part of the streamer structure. The results show that an avalanche-driven breakdown is possible, however, the inception voltage is relatively high. Parameter variations are included to investigate how the parameter values affect the model. visible light [3], re-illuminations, from one or more of its branches. Above the breakdown voltage, streamers may change between the 2nd, the 3rd, and the 4th mode during propagation. There are usually more reilluminations in the 3rd mode than the 2nd mode. The inception of the 4th mode is associated with a drastic increase in speed and fewer, more luminous, branches [2].There are numerous mechanisms that can be involved in the streamer phenomena, the challenge is identifying their importance during initiation and propagation. Applying a potential to a needle can cause charge injection, giving a space-charge limited current [16] causing Joule heating [16], which in turn can cause bubble nucleation [17]. A breakdown in the gas bubble can then propagate the needle potential, and the process may repeat. This is one way to explain 1st mode propagation. Electric fields can also cause electrohydrodynamic flow, which could cause streamer formation through cavitation [18]. Electrostatic cracking has also been proposed as a cavitation mechanism [19]. A main topic of discussion is whether a lowering of the liquid density is needed before charge generation can occur. Electron avalanches are important in gas discharge, but their importance in liquid breakdown is still disputed. In water, strong scattering could prevent electrons from forming avalanches in the liquid phase [20]. Therefore, discharges in micro-bubbles can be important for charge generation [10,14,20]. The same mechanism was also proposed for non-polar liquids [19], however, the relative permittivity is about 80 in water and about 2 in a typical oil, and this difference can prove important since the field enhancement within a bubble in oil is much lower than in water. Contrary to water, there are indications of electron avalanches in non-polar liquids [16,21,22], furthermore, while the initiation and the propagation length of 2nd mode streamers are dependent on the pressure, their propagation velocity is not pressure dependent [16,23]. This implies that the mechanism responsible for propagation occurs in the liquid phase and that the gaseous channel follows as a consequence. In very high electric fields, field-ionization can occur [24,25], and this mechanism has been proposed for the fast 3rd and 4th propagation mode...

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“…Developing predictive models and simulations is challenging, but many attempts exist [8,9]. Simulations have often focused on one aspect of the problem, such as the electric field [10,11], production of free electrons [12], conductance of the streamer channels [13], inhomogeneities [14], or the plasma within the channels [15].…”

Section: Introductionmentioning

confidence: 99%

Photoionization model for streamer propagation mode change in simulation model for streamers in dielectric liquids

Madshaven

Hestad

Unge

et al. 2020

Plasma Res. Express

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Radiation is important for the propagation of streamers in dielectric liquids. Photoionization is a possibility, but the effect is difficult to differentiate from other contributions. In this work, we model radiation from the streamer head, causing photoionization when absorbed in the liquid. We find that photoionization is local in space (µm-scale). The radiation absorption cross section is modeled considering that the ionization potential (IP) is dependent on the electric field. The result is a steep increase in the ionization rate when the electric field reduces the IP below the energy of the first electronically excited state, which is interpreted as a possible mechanism for changing from slow to fast streamers. By combining a simulation model for slow streamers based on the avalanche mechanism with a change to fast mode based on a photoionization threshold for the electric field, we demonstrate how the conductivity of the streamer channel can be important for switching between slow and fast streamer propagation modes.

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

Cited by 16 publications

References 20 publications

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

Photoionization model for streamer propagation mode change in simulation model for streamers in dielectric liquids

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