Behavior of multiple barrier insulation systems under impulse conditions

This paper presents an experimental characterization of the prebreakdown phenomena in liquid/solid interfaces. The characterization is devoted to the 2nd mode positive streamers initiated and propagated along interfaces of mineral-oil and solids with different chemical composition and physical properties. Polymers of low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF) and papers made of kraft paper and a kraft fibril paper (made from cellulosic micro and nano fibrils), lignin-free paper and paper with high lignin content (referred to as k107 kraft paper) are used as the solid to study their influence on the streamer inception and propagation. The streamers are initiated at the interface by applying steps of voltage to a point-plane electrode arrangement with a solid (dielectric barrier) into the gap. The solid is placed diagonal to the oil gap and near to the point electrode. Shadowgraphs, charge and light intensity recordings are obtained during the inception and propagation of the streamers. Thus, estimations of the streamer length, velocity, current and average charge, are also presented. A time delay has been observed before the initiation of the streamer. This delay is probably correlated to the initiation process and formation of the gaseous phase of the streamer near to the interface. The threshold propagation voltage of the 2nd mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. However, the inception voltage is highly influenced by the interface. Additionally, the observed characteristics of streamers propagation (e.g. current, length, velocity, etc) along the tested interfaces cannot be fully explained by a capacitive coupling effect (permittivity mismatch). This open a discussion for the possibility that properties of the solid such as chemical composition, wettability and surface roughness can influence the streamer propagation.

An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm.

Electronegative gas components and gas pressure significantly change residual charge dynamics, which are critical for pulse-periodic streamer discharge behaviors. Evolutions of repetitively pulsed positive streamer discharge and the streamer-to-spark transitions were investigated at high pressures and compared between typical weak (O2) and strong (SF6) electronegative gas mixtures. Pulse-sequence resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. We observe that streamer inception and propagation under subsequent pulses in N2 and N2–O2 mixtures are similar, including the earlier inception of the primary streamer and the accelerated propagation of the secondary streamer. The repetitively pulsed breakdown is extended to the low pulse repetition frequency region with the addition of O2. Discharge evolutions are unexpectedly different in N2–SF6 mixtures. Subsequent discharge channels prefer to propagate around the periphery of the inception cloud region with large radial deviations. Another difference is the precursor channel identified besides multiple streamer channels. Effects of electronegative gas on streamer evolutions under positive repetitive pulses have been qualitatively analyzed. Collisional electron detachment and photo-ionization are crucial in N2–O2 mixtures. With the presence of strong electronegative gas (SF6), the leader formation is probably induced by the earlier corona inception and longer voltage stressing period under following pulses in a pulse train, which are caused by the strong electronegativity of SF6 and the small ion mobility.

Physical mechanisms for streamer initiation and electrical breakdown in water are discussed. The focus is on various processes that could lead to the inception and propagation of streamers in water. Physical details associated with observed disparities in breakdown at the anode versus the cathode, differences in luminosity, and variations in streamer propagation speeds are discussed. Creation of a localized low‐density region near the site of a field enhancement is shown to be essential for streamer initiation. Finally, the use of a trigatron‐type electrically triggered switch with a triple‐point geometry is shown to be a low‐cost, compact alternative to laser (or acoustic) switching. Such controlled breakdown could be used for various applications including water purification and bio‐decontamination.magnified image

A surface streamer is a ubiquitous discharge in electric equipment and surface dielectric barrier discharge. It is expected to suppress this kind of surface discharge in the insulation system, but on the other hand, the surface streamer is also utilized for the generation of active ions and the control of a gas kinetic distribution. As a product of the surface streamer, surface charge influences and characterizes the streamer's behavior. This study observed the two-dimensional transient evolution of the surface charge and the tangential electric field in a single individual surface streamer at the nanosecond level. Using a reflective Pockels effect system and a framing camera system, the surface streamer propagation under a sub-microsecond pulse voltage was measured in real time and in situ. The inception, propagation, and deceleration of a surface streamer are observed. The measurement exhibits an electrode-charge equipotential region that equivalently extends the electrode boundary. Under an anode field with a weak tangential component, the interior of the surface streamer channel exhibits local deviations from the quasi-neutrality, and a reversed tangential electric field is observed. The lateral movement of the positive charge may be responsible for this reverse electric field. The diagnostic technique presented in this study can describe the surface charge transient behavior in pulsed surface discharges, which complements the streamer model.

Residual charges residing at gas volume and trapped at the solid dielectric surface are crucial memory effect agents for evolutions of repetitively pulsed surface streamer discharges. The low-photon-energy laser irradiation can intentionally release and then exploit volume and surface residual charges to regulate discharge characteristics, enabling more flexibilities than conventional voltage waveform modulation methods. However, important parameters and regulation mechanisms of the laser irradiation on evolutions of repetitively pulsed surface streamers are still ambiguous. Evolutions of repetitively pulsed surface streamers in atmospheric pressure air were investigated after low-photon-energy pulsed visible (532 nm) laser irradiations. The effects of the laser irradiation modes and pulse repetition frequency (PRF) dielectric material on streamer inception and propagation features were determined by pulse-sequence and temporally resolved diagnostics. The laser irradiation and increased PRF have similar effects on the evolution of streamer light emission. PRF had little effect on the streamer velocity. When the relative permittivity of the dielectric is smaller, the number of streamer branches increases, which is dominated by the photoionization. Interestingly, the morphology and propagation velocity of the streamer vary greatly depending on the laser irradiation mode. Streamer characteristics exhibit noticeable changes only when the laser perpendicularly irradiates the solid surface rather than the laser beam is tangent to (slightly above) the surface. The effect of laser-induced surface trapped electron desorption on streamer characteristics is analyzed, focusing on the distribution morphology and density of desorption electrons.

This paper presents a study of prebreakdown phenomena (usually called streamers) occurring at high voltage in liquid nitrogen. A special cryostat has been constructed to allow dielectric measurements in liquid nitrogen up to 200 kV, and 0.5 MPa hydrostatic pressure. Streamers are detected and studied with various techniques: high speed visualization, streak camera, and recording of transient currents and emitted light. The propagation of positive and negative streamers is investigated at gap distances up to 80 mm. Streamers are then compared to similar phenomena observed in mineral oil. In liquid nitrogen, streamers are different compared to mineral oil. A transition from slow to fast positive streamers (velocity about 10 km/s) is observed at a threshold voltage below the breakdown voltage. The hydrostatic pressure has almost no influence on this phenomenon. Some experiments carried out with a solid barrier perpendicular to the gap axis are also presented. The practical consequences for the design of HV insulation in superconducting systems are then discussed.

The impact of high velocity air flow on discharge characteristics generated in strongly inhomogeneous electrical fields has been investigated under DC and combined DC + impulse voltages. In the laboratory, a needle plane electrode configuration with a gap distance of 20 cm was exposed to high voltages and a laminar air flow up to 22 m/s. In the first setup, the gap was exposed to a variable DC potential of up to 100 kV in order to create space charges in the vicinity of the electrode. The impact of the air flow on partial discharges and the dynamic behavior of the space charges is evaluated by means of partial discharge measurement and ultraviolet photography. The results show that the air flow increases the frequency of partial discharges in the gap due to an increased rate of space charge removal in the high field area around the tip of the electrode. The partial discharge behavior shows higher dependency on air flow at positive tip polarity as compared to the negative polarity. In the second setup, the standard impulse voltage created by a multistage impulse voltage generator was superimposed to a DC voltage, which continuously created corona and space charges around the tip of the electrode. Breakdown of the gap was triggered by means of the impulse voltage. The DC offset was varied from 0 kV to 100 kV in 20 kV steps and the 50 % flashover voltage was determined for both polarities. The results show that the impact of the air flow is highest at negative DC potential with negative impulse voltage, with an increase in flashover voltage of 40 % when air flow is applied. At positive polarity, the impact of air flow is only observable at 100 kV when the wind extinguishes the stable streamer stem.

SF6 is commonly used as gaseous insulation media in high voltage equipment, but it has high global warming potential and urgently needs to be replaced with eco-friendly alternatives. CO2-based mixtures have shown promising results as SF6 alternatives. Streamers that form the initial phase of electrical breakdown have become important for the reliable design of high-voltage equipment based on gaseous insulation. In this paper, the influence of concentration ratio (90% CO2/10% O2, 80% CO2/20% O2, and 70% CO2/30% O2), applied voltage (±8 kV, ±10 kV, and ±12 kV), pressure (1.5, 2.0, and 2.5 bars), and gap distance (4, 4.5, and 5 mm) on positive and negative streamer formation and propagation is investigated in detail using a 2D axis-symmetric simulation model. Considering the low probability in gas mixtures with higher concentrations of CO2, photoionization is excluded, and background ionization is used for generating free electrons along with the Townsend ionization equation and the Gaussian approximation for the initial electron density distribution. The simulation results show that by increasing the O2 concentration in CO2, the electron density, electric field, and streamer velocity increase under positive and negative polarities. The σ (collision cross section) value of O2 is greater than CO2 at the specific ionization energy, indicating that O2 molecules in a gas mixture have a higher reactivity and a lower molecular stability. The negative streamer has an overall high electron density as compared to the positive streamer. To understand the effect of applied voltage, pressure, and gap distance, only 80% CO2/20% O2 concentration ratio was chosen. By increasing the applied voltage, the electron density, electric field, and streamer velocity increase. Furthermore, a decreasing trend of electron density, electric field, and streamer velocity was observed by increasing the gas pressure and the electrode distance.

Streamer propagation physics is investigated in 2D and 3D pin-to-plane geometries at atmospheric pressures using a PIC-DSMC code with an air model comprised of standard Townsend breakdown (electron-neutral elastic, excitation, ionization, attachment, and detachment collision chemistry and secondary electron emission) and streamer mechanisms (photoionization and ion-neutral collisions)1 via tracking excited state neutrals which can then either quench through collisions or spontaneously emit based on transition-specific Einstein-Acoefficients2,3. Here we model a 100 $\mu \mathrm{m}$ hemispherical pin electrode (at 10 kV) 1.5 mm from a planar cathode at ground. The gap is filled with air at 760 Torr and a 0.5 eV, 1020m-3 plasma is used to seed the simulation domain. Prior studies have shown that the reduced electric field (E/n) can have a significant impact on streamer evolution4,5 and here we investigate how the approximate inverse (2D) and inverse-square (3D) falloff in field effects streamer evolution. To reduce computational costs, the 3D computational domains are wedges. We sweep the wedge angle from 10 to 45 degrees to examine the influence of wedge angle on streamer branching and propagation. Other parameters of interest include streamer velocity and inception cloud formation.

The formation and propagation of pseudo-filamentary dielectric barrier discharge in atmospheric air are investigated through a 2D fluid model. The discharge development can be divided into three stages: the volume streamer stage, the surface streamer stage, and the reverse discharge stage. The simulations show that the streamer head becomes wider and the electron density of the volume streamer head increases six times when the volume streamer interacts with the dielectric, and the volume streamer transforms into the surface streamer after the interaction. Compared with volume streamers, surface streamers have a smaller radius, a higher electric field, and a higher electron density. Furthermore, the parameters that may influence the discharge characteristics are also studied. It is found that a larger dielectric permittivity, a thinner dielectric, or a shorter voltage rise time leads to earlier inception of volume streamers, faster propagation of surface streamers, and higher current density. It is observed that the velocity of the surface streamer increases first, and then, decreases with the accumulated charges on the surface.

This paper presents an experimental study on the breakdown characteristics of a mineral transformer oil in moderately-uniform fields under standard lightning impulse. To achieve the moderately-uniform fields with the field factors from 5.5 to 31.3, the rod to plane electrode geometry was applied with the rod tip radius of 0.5 mm and gap distances from 3 mm to 25 mm. The negative breakdown voltages raise up steeply against the gap distance when the gap distance is lower than 10 mm and for higher gap distance the increasing rate of the breakdown voltage is reduced. Based on the simulated maximum electric field under the breakdown voltage, the negative transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown is deduced at 9.6 which experimentally verifies the previous extrapolation based estimation. The positive breakdown voltages were also measured under the same conditions, which indicated that all the breakdowns are dominated by the streamer initiation in such moderately-uniform fields. This also evidences that the transition field factor boundary is polarity dependent and negative lightning impulse shows a lower transition field factor boundary value compared to the positive one.

The experimental study was performed to clarify the basic behaviour of streamers traveling over the surface of oil-immersed polyethylene insulated wire under a standard lightning impulse voltage application. The effect of the grounded side electrode on the characteristics of creepage discharges, such as discharge inception voltage, discharge extension length, discharge current and charge, discharge figure has been studied. There is a remarkable difference in these characteristics between positive and negative polarities of the streamer. Though the approach of the grounded side electrode to the wire restrains the generation and progression of streamers, it leads to the reduction of a punch-through breakdown voltage of the wire insulator. The authors also measured the electric potential along the streamer channel and the streamer propagation velocity by means of a capacitive probe. It is indicated that the streamer continues to progress in a constant velocity and the streamer channel is not equipotential. The propagation of streamers appears to be stopped by a finite potential drop along the streamer channel.

The paper deals with basic investigation of AC breakdown behaviour of gaseous insulation with synthetic air at different level of field homogeneity and gas pressure. The measurement was performed at different gas pressure ranging from 0.1 to 0.25 MPa and gap distances from 1 to 9 cm, using an electrode test-set “tip-bowl”. The test parameters were chosen in such a way that different field types could be realized. At small distances the electrical field is weakly inhomogeneous and the breakdown values correspond to the discharge inception level. The measured values show a reasonable agreement to those values based on the calculation of the streamer criterion by using a relevant approximation formula for the effective ionisation coefficient. For larger distances the electrical field becomes more inhomogeneous and the calculated streamer inception voltage corresponds to the partial discharge (PD) inception voltage only. The calculated inception voltage values fit the measured values over the applied gap distance and gas pressure quite sufficiently. The best fit to the measured results was reached by slightly changing the parameters of the approximation formula depending on the gas pressure value. Additional PD measurement shows different PD activity dependent on gas pressure and gap distance.

It is generally known that power system outages caused by lightning flashover present a serious engineering problem. Most of the industrial company take serious action in order to protect their equipment from damage by natural causes such as lightning. It will be the necessary precautions to keep the lightning channel far away from the immediate neighbourhood of flammable and explosive materials. Protection of electric equipment against this power of nature is one objective of high voltage engineering and testing. The aim of this study is to investigate flashover in gap between industrial pipeline and any conducting material around the pipe. Impulse voltage test are used to simulate the lightning and tested on prototype of pipeline. The influence of the spark gap discharge on the statistics of the air breakdown processes has been investigated in a wide range of gap lengths from 1.0cm to 3.0cm under impulse conditions. The results were discussed base on its breakdown voltage affected by the length of the gap.