Positive Streamer Propagation Research Articles

Pressure-dependent propagation of streamers under step voltage in a long point-plane gap in transformer oil

The effect of pressure on positive and negative streamer propagation in a mineral transformer oil is studied in an 80 mm point-plane gap under step voltage. Increased pressure causes reduced stopping length for non-breakdown streamers, higher breakdown voltage, reduced background current, reduced branching in positive streamers and virtually no change in negative streamer shape. The velocities and acceleration voltages are close to being independent of pressure, indicating that the velocity controlling mechanisms take place in the liquid phase. Amplitude and frequency of small current pulses (probably small reilluminations) during negative 1st mode is reduced with increasing pressure, but the effect on reilluminations during 2nd and 3rd mode is small in both polarities.

Numerical simulation of the positive streamer propagation and chemical reactions in SF6/N2 mixtures under non-uniform field

SF 6 /N 2 mixtures is considered as a practical alternative proposal of SF 6 in GIS and GIL. Simulating particle transportations and chemical reactions during the positive streamer propagation in SF 6 /N 2 mixtures assists in the insulation performance optimization of SF 6 /N 2 mixtures. This paper presents a fluid dynamic model coupled with the chemical reaction system for the positive streamer propagation in SF 6 /N 2 mixtures. The Townsend coefficients of the 20%SF 6 /80%N 2 mixture derived from the proposed chemical reaction system are in accordance with the measurement results. Based on this model, the propagation of the positive streamer in a 1 mm needle-plate gap in the 20%SF 6 /80%N 2 mixture is simulated. Simulated results indicate that there exists a negative space charge region in the streamer channel. It can be attributed to the increasing attachment rates of SF 6 yielding SF 5 − and SF 6 −, and the reduction of the ionization rates of N 2 and SF 6 in the streamer channel. Besides, it is noted that the ionization activities of N 2 and SF 6 at the streamer head are equally important to support the continuous propagation of the positive streamer. The ionization of N 2 in the streamer channel serves as an important factor to supply electrons and avoid the accumulation of the negative space charge.

New approach for assessment of positive streamer penetration of long air gaps under impulse voltages

The paper comprises a new systematic approach to determine the critical ambient field needed for positive streamer propagation in nonuniform field gaps. It is shown that such critical streamer penetration field is a function of the basic value previously established for uniform field gaps and the degree of ambient field nonuniformity. The latter quantity is defined by the ratio of the maximum to mean ambient electric fields along the streamer propagation path, which can be determined either analytically or by charge simulation. Such functional dependence of the critical streamer penetration gradient on field nonuniformity has been determined for different rod topologies of rod-plane gap as well as for single and bundle conductor plane gaps. The model is used to determine critical streamer penetration fields and streamer lengths under different impulse voltage levels for different gap configurations and for a tall ground structure exposed to ambient impulse field. The findings of this method are compared to available experimental results and the agreement is found satisfactory.

Indirectly Measured Ambient Electric Fields for Lightning Initiation in Fast Breakdown Regions

AbstractWe report a new approach to measure indirectly the ambient thunderstorm electric fields in fast positive breakdown regions. For a given geometry of the discharged fast breakdown region, we show that there is a minimum ambient electric field required to produce a given charge moment change. We apply this approach to the fast breakdown measurements for two events reported by Rison et al. (2016, https://doi.org/10.1038/ncomms10721) and find that the average ambient electric field in the discharged region is at least V/m at the 9.5 km initiation altitude of these events. This electric field is close to the runaway electron avalanche electric field and to the critical field for positive streamer propagation. These measurements provide a meaningful starting point for more detailed analyses or simulations of what occurs inside the fast breakdown process that is responsible for the initiation of at least some lightning discharges.

Conductivity and capacitance of streamers in avalanche model for streamer propagation in dielectric liquids

Propagation of positive streamers in dielectric liquids, modeled by the electron avalanche mechanism, is simulated in a needle–plane gap. The streamer is modeled as an RC-circuit where the channel is a resistor and the extremities of the streamer have a capacitance towards the plane. The addition of the RC-model introduces a time constant to the propagation model. Increase in capacitance as a streamer branch propagates reduces its potential, while conduction through the streamer channel increases its potential, as a function of the time constant of the RC-system. Streamer branching also increases the capacitance and decreases the potential of the branches. If the electric field within the streamer channel exceeds a threshold, a breakdown occurs in the channel, and the potential of the streamer is equalized with the needle electrode. This is interpreted as a re-illumination. According to this model, a low conductive streamer branch can propagate some distance before its potential is reduced to below the propagation threshold, and then the RC time constant controls the streamer propagation speed. Channel breakdowns, or re-illuminations, are less frequent when the channels are conductive and more frequent for more branched streamers.

Streamer and surface charge dynamics in non-uniform air gaps with a dielectric barrier

Streamer behaviour near dielectric surfaces is an important characteristic of air-solid electrical insulation systems. Accurate predictions are important for dielectric design, but dynamic aspects such as surface charging during streamer propagation are not well understood. A drift-diffusion model is used here to simulate positive streamer behaviour in non-uniform fields. The 2D-planar simulation domain includes air gaps between a tip of a HV electrode and a dielectric barrier laying on a grounded plane. The resulting surface charge distributions approach saturation charge conditions, i.e. zero normal electric field on the air side of the boundary. Such charging behaviour was also reported in lightning impulse (LI) experiments. The simulations are also aligned with empirical streamer propagation range estimates. It is demonstrated that saturation charge levels are reachable within a few tens of nanoseconds of exposure to positive streamer channels. Ion drift is shown to be the dominating mechanism of surface charging during positive streamer propagation, although photoemission also plays an important role. Discharge suppression by streamer-deposited surface charge is also demonstrated. Furthermore, the influence of back discharges at the LI tail on the surface charge distribution is shown. Simulating realistic streamer surface charging behaviour with arbitrary electrode and dielectric shapes is an important step toward first principles discharge prediction models.

Slowing Positive Streamer Propagation in Silicon and Ester Transformer Oil Using Multi-nanoparticles Technique

This paper formulates a new multi-nanoparticles technique for reducing positive streamer propagation in silicon and vegetable oils nanofluids; transformer oil nanofluids with multi-nanoparticles suspensions are blended to slow thepropagation of positive streamer more than using individual nanoparticles, hence the insulation qualities are improved andthe breakdown voltage of nanofluid is expanded. This study indicated the influence of nanoparticle radius, nanoparticles’ polarization, the relative permittivity of oil, applied field, and the charge density of nanoparticles interface. It is deduced that transformer oil-based nanofluids with conductive individuals and multi-nanoparticles suspensions have considerably higher impacts on slowing positive streamer than non-conductive nanoparticles. A comparative study has demonstrated the magnitude of deposited charge and charging time as a result of using individual nanoparticles and multi-nanoparticles inside transformer oil.

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.

Investigation on enhancing breakdown voltages of transformer oil nanofluids using multi‐nanoparticles technique

This study presents particularly an innovative technique to enhance charging dynamics in transformer's oil using multi-nanoparticles (NPs). A charging dynamic process in power transformer oils is the main way to slow down the streamer propagation and upping dielectric strength. In addition, transformer oil's based suspension multi-NPs (dielectric, conductive, and semi-conductive) have substantial levels of deposited electrons higher than suspension traditional nanofluids. An experimental work has been done to verify the importance of using multi-NPs for slowing down positive streamer propagation in transformer oils against using individual NP techniques or traditional transformer oils.

A thermo-electrodynamic electric field dependent molecular ionization model to realize positive streamer propagation in a wet-mate DC connector

Complete subsea factory concept, an equivalent of the full topsides processing facility to be operated on the seabed, is envisaged to power longer, deeper and colder subsea oil and gas fields in the future. This concept has been envisioned through a modular stacked subsea DC transmission and distribution system whose subsea umbilical cables and electrical power component on the seabed can be interfaced with each other by wet-mate (WM) DC connectors. Laboratory and theoretical investigations have been carried out to assess various electrical insulation systems and electrode geometries for a WM DC connector which should operate in the steady state as well as switching transients in a corrosive environment for high reliability and minimum maintenance in its lifetime. In this paper, the electrical insulation performance of a needle-sphere electrode geometry defined by IEC 60897 under a positive step voltage is studied. To approach the complicated solid-liquid insulation system envisaged in a WM DC connector after mating, the electrodes are covered by a dielectric solid and oil is enclosed by the dielectric solid as well. A full thermo-electrodynamic electric field dependent molecular ionization Multiphysics model was developed for the simulation of streamer initiation and growth in the oil while dielectric solid is modeled as a perfect insulator. It is shown that stabilization methods, mesh strategies and time step have a great influence on simulation results and guidelines to choose them properly are presented. Based on simulation results, it was found that the higher relative permittivity of the solid insulation the slower streamer propagation in the oil and the less electrical stress on the solid insulation.

Propagation of positive streamers on the surface of shallow as well as deep tap water in wide and narrow dielectric channels

The results of experiments studying long-living positive streamers propagating on the surface of tap water are presented; the plasma-forming gas is air at atmospheric pressure. Measurement data for the strength of the longitudinal electric field in the surface streamer, the streamer velocity versus time until it stops, and the length of the streamer versus the applied voltage are presented. It is revealed that the depth of the water in a basin influences the streamer length: the deeper the water, the longer the streamer. Besides this, restricting the transverse direction of the area in the water accessible for streamer extension suppresses streamer branching until it fully disappears. It is shown that non-branching streamers propagating in narrow water channels increase their length considerably. Placing a dielectric plate with long, narrow slits of the required configuration on the water enables one to control the trajectory of streamer propagation. Three-dimensional numerical calculations of the spatial structure of the electric field and the current inside and outside the streamer in the water basin with different depths and widths were made. It is shown numerically that the streamer length and its diameter strongly influence both the strength and spatial structure of the electric field in the air around the streamer.

The influence of bremsstrahlung on electric discharge streamers in N2, O2 gas mixtures

Streamers are ionization filaments of electric gas discharges. Negative polarity streamers propagate primarily through electron impact ionization, whereas positive streamers in air develop through ionization of oxygen by UV photons emitted by excited nitrogen; however, experiments show that positive streamers may develop even for low oxygen concentrations. Here we explore if bremsstrahlung ionization facilitates positive streamer propagation. To discriminate between effects of UV and bremsstrahlung ionization, we simulate the formation of a double headed streamer at three different oxygen concentrations: no oxygen, 1 ppm O2 and 20% O2, as in air. At these oxygen levels, UV-relative to bremsstrahlung ionization is zero, small, and large. The simulations are conducted with a particle-in-cell code in a cylindrically symmetric configuration at ambient electric field magnitudes three times the conventional breakdown field. We find that bremsstrahlung induced ionization in air, contrary to expectations, reduces the propagation velocity of both positive and negative streamers by about 15%. At low oxygen levels, positive streamers stall; however, bremsstrahlung creates branching sub-streamers emerging from the streamer front that allow propagation of the streamer. Negative streamers propagate more readily forming branching sub-streamers. These results are in agreement with experiments. At both polarities, ionization patches are created ahead of the streamer front. Electrons with the highest energies are in the sub-streamer tips and the patches.

Modeling of Streamer Dynamics in Atmospheric-Pressure Air: Influence of Rise Time of Applied Voltage Pulse on Streamer Parameters

Results of simulation of positive streamer propagation in atmospheric air, obtained in the framework of a 2-D fluid model, at various rise times of applied voltage pulses are presented. It is shown that the variation of the rise time allows one to control the streamer characteristics: the streamer widths, propagation velocity, densities of charged and excited neutral species, and electric current. At very short (subnanosecond) rise times, it is possible to obtain wide streamers, carrying large currents and producing considerable amounts of reactive species.

The role of free electrons in the guiding of positive streamers

Because positive streamers propagate opposite to the electron drift velocity, their growth not only depends on the local electric field, but also on the electron density ahead of them. We have recently demonstrated the importance of this electron density, by showing that positive streamers could be guided by weak preionization from a laser. Here, we study streamer guiding in nitrogen–oxygen mixtures in more detail, to better understand the propagation of positive streamers. First, the conditions required for guiding are investigated, both experimentally and with three-dimensional numerical simulations. Particular focus is on the role of oxygen and photoionization, which tend to inhibit guiding. Then the position of the guided streamers is investigated, and successfully related to the electron drift.

Microsecond Electrical Discharge in Water in Plate-to-Plate Configuration With Nitrogen Bubble Injection

In this paper, a new method of generation of an electrical discharge in water using plate electrodes is proposed. The electrical discharge in plate-to-plate configuration cannot be easily generated because of a low uniform electrical field. A nitrogen bubble is injected into one of the electrodes to create a favorable condition for electrical breakdown. The generated electrical discharge is studied using a high-speed framing camera. Two interesting effects have been observed. First, the discharge is preferentially initiated on the cathode. Second, the positive streamer reveals nontypical morphology. The physical mechanisms of discharge initiation and positive streamer propagation are discussed.

Virtual anode effect in the propagation of positive streamers

Experiments made in a parallel-plate arrangement, where one of the plates is the ground plane and the other a positively charged dielectric sheet, show that at low air pressure the dielectric injects streamers onto the surrounding air. The similarities and differences between the streamers thus produced and those emitted from electrodes are discussed. The streamers here obtained were found to have diameters and speeds that are clearly dependent on pressure. Often the streamers were seen to decelerate markedly to the point of coming to a complete stop and on occasions even returning back to the dielectric they came from. The deceleration and the reflection of the streamers are attributed to a virtual anode created by the electrostatic image of the head's charge behind the ground plane. Field calculations performed corroborate this hypothesis. Through the use of an equation of motion of the positive head, it is then possible to obtain an estimate of the magnitude of its charge. The charge thus obtained was found to be in the 2.5–26 nC range.

Effect of Fe3O4 nanoparticles on positive streamer propagation in transformer oil

Fe3O4 nanoparticles with an average diameter of 10 nm were prepared and used to modify streamer characteristic of transformer oil. It was found that positive streamer propagation velocity in transformer oil-based Fe3O4 nanofluid is greatly reduced by 51% in comparison with that in pure oil. The evolution of streamer shape is also dramatically affected by the presence of nanoparticles, changing from a tree-like shape with sharp branches in pure oil to a bush-like structure with thicker and denser branches in nanofluid. The TSC results reveal that the modification of Fe3O4 nanoparticle can greatly increase the density of shallow trap and change space charge distribution in nanofluid by converting fast electrons into slow electrons via trapping and de-trapping process in shallow traps. These negative space charges induced by nanoparticles greatly alleviate the electric field distortion in front of the positive streamer tip and significantly hinder the propagation of positive streamer.

Sub-nanosecond delays of light emitted by streamer in atmospheric pressure air: Analysis of N2(C3Πu) and N2+(B2Σu+) emissions and fundamental streamer structure

Theoretical analysis of ultra-short phenomena occurring during the positive streamer propagation in atmospheric pressure air is presented. Motivated by experimental results obtained with tens-of-picoseconds and tens-of-microns precision, it is shown that when the streamer head passes a spatial coordinate, emission maxima from N2 and N2+ radiative states follow with different delays. These different delays are caused by differences in the dynamics of populating the radiative states, due to different excitation and quenching rates. Associating the position of the streamer head with the maximum value of the self-enhanced electric field, a delay of 160 ps was experimentally found for the peak emission of the first negative system of N2+. A delay dilatation was observed experimentally on early-stage streamers and the general mechanism of this phenomenon is clarified theoretically. In the case of the second positive system of N2, the delay can reach as much as 400 ps. In contrast to the highly nonlinear behavior of streamer events, it is shown theoretically that emission maximum delays linearly depend on the ratio of the streamer radius and its velocity. This is found to be one of the fundamental streamer features and its use in streamer head diagnostics is proposed. Moreover, radially resolved spectra are synthesized for selected subsequent picosecond moments in order to visualize spectrometric fingerprints of radial structures of N2(C3Πu) and N2+(B2Σu+) populations created by streamer-head electrons.

Branching and path-deviation of positive streamers resulting from statistical photon transport

The branching and change in direction of propagation (path-deviation) of positive streamers in molecular gases such as air likely require a statistical process which perturbs the head of the streamer and produces an asymmetry in its space charge density. In this paper, the mechanisms for path-deviation and branching of atmospheric pressure positive streamer discharges in dry air are numerically investigated from the viewpoint of statistical photon transport and photoionization. A statistical photon transport model, based on randomly selected emitting angles and mean-free-path for absorption, was developed and embedded into a fluid-based plasma transport model. The hybrid model was applied to simulations of positive streamer coaxial discharges in dry air at atmospheric pressure. The results show that secondary streamers, often spatially isolated, are triggered by the random photoionization and interact with the thin space charge layer (SCL) of the primary streamer. This interaction may be partly responsible for path-deviation and streamer branching. The general process consists of random remote photo-electron production which initiates a back-traveling electron avalanche, collision of this secondary avalanche with the primary streamer and the subsequent perturbation to its SCL. When the SCL is deformed from a symmetric to an asymmetric shape, the streamer can experience an abrupt change in the direction of propagation. If the SCL is sufficiently perturbed and essentially broken, local maxima in the SCL can develop into new streamers, leading to streamer branching. During the propagation of positive streamers, this mechanism can take place repetitively in time and space, thus producing multi-level branching and more than two branches within one level.

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 the barrier permittivity is greater than the oil permittivity, the solid dielectric applies an attractive force on the volume charge that turns the streamer into a surface flashover expanding on the barrier surface. On the other hand, a low permittivity pressboard interface repels the approaching streamer. Theoretical analysis of electric field propagation and charge distribution along the streamers, surface flashovers, and through the interfacial surfaces is presented. Barrier dielectric relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.