Corona Stabilization Research Articles

Partial discharge measurement and analysis at standard oscillating switching and lightning impulses on a GIS with artificial protrusion defects

Partial discharges (PDs) induced by artificial fixed protrusions inside an SF6 gasinsulated switchgear (GIS) test object under oscillating switching and lightning impulse voltages (OIVs) were investigated experimentally. The features of PD current and light pulses (sequences) under impulses were analyzed. The characteristics of PD behaviors under oscillating lightning impulse (OLI), oscillating switching impulse (OSI) and 50 Hz AC voltages were studied according to their statistical distributions. The experimental data were quantitatively analyzed to compare the effectiveness of AC and OIV voltages in exciting the PDs in terms of PD inception and PD activity. Most PDs occurred successively around the peaks of the oscillation period of OIVs and periodical PD sequences were formed. Compared to positive point PDs, negatives occurred more frequently, but with smaller magnitudes. The positive OSI was found to have relative high efficiency in exciting PDs from fixed protrusion comparing to other types of voltage investigated. The occurrence frequency of PD under negative OLI was one or more orders of magnitude greater than that under AC voltage. The average PD magnitudes under positive OIVs were much greater than the others. Both the average PD magnitude and the occurrence frequency were greater at 0.2 MPa, than at 0.4 MPa. This indicates that the AC PD test is not very sensitive to local enhanced field defects, such as fixed protrusions on conductors, because of the limited PD measurement sensitivity and the corona stabilization under AC voltage.

Comparative Dynamics and Sequence Dependence of DNA and RNA Binding to Single Walled Carbon Nanotubes.

Noncovalent polymer-single walled carbon nanotube (SWCNT) conjugates have gained recent interest due to their prevalent use as electrochemical and optical sensors, SWCNT-based therapeutics, and for SWCNT separation. However, little is known about the effects of polymer-SWCNT molecular interactions on functional properties of these conjugates. In this work, we show that SWCNT complexed with related polynucleotide polymers (DNA, RNA) have dramatically different fluorescence stability. Surprisingly, we find a difference of nearly 2500-fold in fluorescence emission between the most fluorescently stable DNA-SWCNT complex, C30 DNA-SWCNT, compared to the least fluorescently stable complex, (AT)7A-(GU)7G DNA-RNA hybrid-SWCNT. We further reveal the existence of three regimes in which SWCNT fluorescence varies nonmonotonically with SWCNT concentration. We utilize molecular dynamics simulations to elucidate the conformation and atomic details of SWCNT-corona phase interactions. Our results show that variations in polynucleotide sequence or sugar backbone can lead to large changes in the conformational stability of the polymer SWCNT corona and the SWCNT optical response. Finally, we demonstrate the effect of the coronae on the response of a recently developed dopamine nanosensor, based on (GT)15 DNA- and (GU)15 RNA-SWCNT complexes. Our results clarify several features of the sequence dependence of corona phases produced by polynucleotides adsorbed to single walled carbon nanotubes, and the implications for molecular recognition in such phases.

The fate of a designed protein corona on nanoparticles in vitro and in vivo.

A variety of monodisperse superparamagnetic iron oxide particles (SPIOs) was designed in which the surface was modified by PEGylation with mono- or bifunctional poly(ethylene oxide)amines (PEG). Using 125I-labeled test proteins (transferrin, albumin), the binding and exchange of corona proteins was studied first in vitro. Incubation with 125I-transferrin showed that with increasing grade of PEGylation the binding was substantially diminished without a difference between simply adsorbed and covalently bound protein. However, after incubation with excess albumin and subsequently whole plasma, transferrin from the preformed transferrin corona was more and more lost from SPIOs in the case of adsorbed proteins. If non-labeled transferrin was used as preformed corona and excess 125I-labeled albumin was added to the reaction mixtures with different SPIOs, a substantial amount of label was bound to the particles with initially adsorbed transferrin but little or even zero with covalently bound transferrin. These in vitro experiments show a clear difference in the stability of a preformed hard corona with adsorbed or covalently bound protein. This difference seems, however, to be of minor importance in vivo when polymer-coated 59Fe-SPIOs with adsorbed or covalently bound 125I-labeled mouse transferrin were injected intravenously in mice. With both protein coronae the 59Fe/125I-labelled particles were cleared from the blood stream within 30 min and appeared in the liver and spleen to a large extent (>90%). In addition, after 2 h already half of the 125I-labeled transferrin from both nanodevices was recycled back into the plasma and into tissue. This study confirms that adsorbed transferrin from a preformed protein corona is efficiently taken up by cells. It is also highlighted that a radiolabelling technique described in this study may be of value to investigate the role of protein corona formation in vivo for the respective nanoparticle uptake.

Revealing the Binding Structure of the Protein Corona on Gold Nanorods Using Synchrotron Radiation-Based Techniques: Understanding the Reduced Damage in Cell Membranes

Regarding the importance of the biological effects of nanomaterials, there is still limited knowledge about the binding structure and stability of the protein corona on nanomaterials and the subsequent impacts. Here we designed a hard serum albumin protein corona (BSA) on CTAB-coated gold nanorods (AuNRs) and captured the structure of protein adsorption using synchrotron radiation X-ray absorption spectroscopy, microbeam X-ray fluorescent spectroscopy, and circular dichroism in combination with molecular dynamics simulations. The protein adsorption is attributed to at least 12 Au-S bonds and the stable corona reduced the cytotoxicity of CTAB/AuNRs. These combined strategies using physical, chemical, and biological approaches will improve our understanding of the protective effects of protein coronas against the toxicity of nanomaterials. These findings have shed light on a new strategy for studying interactions between proteins and nanomaterials, and this information will help further guide the rational design of nanomaterials for safe and effective biomedical applications.

Surface functionalization affects the zeta potential, coronal stability and membranolytic activity of polymeric nanoparticles

Nano materials are commonly functionalized to boost their physicochemical properties. However, there is little known about the impact of these modifications on cellular systems. Herein, we synthesized eight types of polymeric nanoparticles (NPs) bearing different functional groups, and investigated their effects on interactions with cellular membranes. As models for particle membrane interactions, hemolysis assays using human red blood cells and culture with A549 cells were utilized. Under protein-free conditions, the NPs showed a wide distribution of zeta potentials (ζPs) which showed a good correlation with their hemolytic potential. However, in the presence of serum or lung lining fluid, the ζPs of all NPs coalesced towards a single common negative value and showed neither hemolytic activity nor cytotoxicity to A549 cells. Lipase and protease treatment of the coronated particles did not restore their reactivity. These result simply proves that particle functionalization influences the stability of the particle corona which, if intact, prevents hemolytic activity and membrane disrupture.

The effect of nano‐ZnO on corona aging and photo aging in low‐density polyethylene

AbstractIn this paper, space charge distributions as well as dielectric properties, such as volume resistivity and AC breakdown strength, were studied in low‐density polyethylene (LDPE) and its composite obtained by adding nano‐ZnO nanofiller, before and after photo‐oxidation or corona aging for several days. It was found that a small amount of ZnO nanoparticles could effectively improve both the photostabilization and corona stabilization of the LDPE matrix. Fourier transform infrared (FTIR) spectra of the specimens were investigated to reveal the change of the microstructures. The results showed that a weaker carbonyl absorption peak appeared in LDPE/ZnO nanocomposite than in LDPE. A proper explanation is given according to the photodegradation and corona degradation theory of polyethylene. The results also showed that there was a certain similarity between photo aging and corona aging. IEEJ Trans 2010 DOI: 10.1002/tee.20600

Partial discharges and breakdown at protrusions in uniform background fields in SF6

The breakdown mechanism of compressed SF6 in gas insulation is known to be controlled by stepped leader propagation. This process is reasonably well understood for strongly non-uniform insulation gaps (‘point-to-plane’) and in the absence of pre-breakdown discharge activity (‘corona stabilization’). Open questions still remain for weakly non-uniform insulation gaps with small electrode protrusions (particles, surface roughness), in which pre-breakdown partial discharge (PD) activity is present. This paper presents a first attempt to derive a consistent picture under these conditions, which are characteristic for practical gas insulation systems. Experiments were carried out in a uniform field gap with a short protrusion on one electrode. This configuration was studied at various pressures from 0.1 to 0.5 MPa and both polarities using electrical and optical diagnostics. The results are interpreted using a quantitative model and order-of-magnitude estimates. The emerging picture allows prediction of most of the technically relevant aspects of the discharge processes and their main parameter dependences. It comprises statistical time lags, formative time lags including pre-breakdown PD activity and breakdown fields as a function of gas pressure, protrusion length and polarity.

Effect of gas type on high repetition rate performance of a triggered, corona stabilised switch

This paper reports on the work undertaken to determine the effect of the gas type on the high repetition rate performance of a triggered corona stabilised (TCS) closing switch at a charging voltage of 23 kV. The voltage/pressure (V/p) characteristics as well as the ability of the switch to operate at high repetition rates were measured with SF/sub 6/, air and six different SF/sub 6//air mixtures having an SF/sub 6/ concentration ranging from 75 to 2% by pressure. The high repetition rate tests were conducted with a high-power facility at a pulse repetition frequency (PRF) ranging from 500 Hz up to a maximum of 3 kHz. During the investigation, it was found that for the given nonuniform switch geometry the high repetition rate performance of the device was closely associated with the efficiency of corona stabilisation, as measured from the V/p characteristics. This was clearly manifested by the superior performance of most of the SF/sub 6//air mixtures tested, which displayed a more efficient corona stabilisation compared to pure SF/sub 6/. In contrast, air displayed the worst performance of all the gases tested, due to the weak V/p characteristic and inefficient corona stabilisation.

Insulation properties of a CO2/N2 50% SF6 gas mixture in a nonuniform field

AbstractThis paper describes partial discharge (PD) inception and breakdown voltage characteristics of a CO2/N2/SF6 gas mixture in a nonuniform field. These voltage characteristics were investigated with ac high voltage by changing the mixture rate of each gas of CO2, N2, and SF6 gas and the gas pressure from 0.1 MPa to 0.6 MPa. It was found that adding a small amount of CO2 gas into a N2/SF6 mixture causes a drastic increase in the breakdown voltage. For instance, when the mixture rate of SF6 in N2/SF6 gas mixture is 50%, with the addition of 1% CO2 the maximum breakdown voltage becomes 1.31 and 1.15 times higher than that of a 50% N2/50% SF6 gas mixture and pure SF6 gas, respectively. Moreover, those voltage characteristics of a CO2/N2/SF6 gas mixture were also investigated by changing the electric field utilization factor as well as by applying positive and negative standard lightning impulse voltages in order to discuss the corona stabilization effect, which seems to be one reason for the drastic increase in the breakdown voltage. These results and breakdown mechanism of the CO2/N2/SF6 gas mixture are discussed on the basis of the corona stabilization effect and the dissociation energies of the component gases by observing PD light images, PD light intensities through a blue and red filter, and PD current waveforms. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(3): 34–43, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10019

Space charge behaviour and corona stabilization effect in SF6 gasviewed from sequential generation of a dc partial discharge

We investigated the sequential generation characteristics of partialdischarge (PD) pulses in SF6 gas under the application of a positive dcvoltage, in order to discuss the PD mechanism based on space charge behaviourand corona stabilization effect. Highly time-resolved measurement of PDcurrent pulses enabled us to discuss the relationship between the PD chargeq and the time interval Δt of sequential PD pulses. Experimentalresults revealed that q-Δt characteristics exhibited negative andpositive correlation at small and large Δt regions, respectively.Numerical calculations of ion diffusion and ion drift verified thetime-dependent characteristics of the space charge behaviour and the resultantcorona stabilization effect.

Very fast transient overvoltages in GIS with compressed SF/sub 6/-N/sub 2/ gas mixtures

This paper discusses the characteristics of very fast transient overvoltages (VFTO) in SF/sub 6/-N/sub 2/ gas mixtures at different percentages of SF/sub 6/. A comparison of the VFTO characteristics of pure SF/sub 6/ with those of pure N/sub 2/ is also presented. The investigations are performed using a laboratory model GIS bus duct having a test gap used for simulating a switching event leading to the generation of VFTO. A capacitive voltage sensor is used to measure the VFTO peak magnitude and temporal characteristics. Measurements were carried out at two different gap spacings (0.20 and 0.61 mm) over a pressure range of 100 to 500 kPa. VFTO characteristics for N/sub 2/, SF/sub 6/ and SF/sub 6/-N/sub 2/ mixtures obtained from the experiments show similar trends. The level of surge peak magnitude is <2.0 pu for all cases when the gap was 0.20 mm, but it reaches a maximum of 2.41 pu at 0.61 mm gap. At 0.20 mm gap, in SF/sub 6/-N/sub 2/ mixtures, the difference in peak magnitudes is not significant for 10% and 20% SF/sub 6/ mixtures (between 200 and 400 kPa) and also for pure SF/sub 6/ and 40% SF/sub 6/ (between 200 and 300 kPa). The occurrence of corona stabilization during breakdown of the gap may be the cause for such a behavior. Unlike the above observations at 0.20 mm gap, at 0.61 mm gap, the peak magnitudes strictly increase with pressure for the pure gases and gas mixtures. At 0.20 mm gap, the time to breakdown of the gap is found to be almost constant in all cases. But at 0.61 mm gap, the time to breakdown is seen to be dependent on the mixture, pressure, and breakdown voltage, and this observation is in accordance with Toepler's spark law.

Study for Analyzing Unevenness of Negative Corona Discharge

We have been studying the basis of the air flow effect which increases the uniformity and stability of negative corona discharge. The possibility that the stability of the negative corona is disturbed by some interruption for the flow of light to the surface of the wire electrode is investigated. Spectra distributions of the corona discharge in air atmosphere and in N2 atmosphere are measured as a first step. Major peaks are seen in the ultraviolet band. Measured peak heights in negative corona discharge are found to be four times bigger, in air, than those in positive corona discharge. Almost all major peaks in air were confirmed to have the same wavelength as was measured under N2. The measured difference in the spectrum distribution curves between positive and negative corona discharges indicates differences in the discharge mechanisms and a hint for analyzing the air flow effect in negative corona discharge.

Space charge behavior in SF/sub 6/ gas and sequential generation of PD pulses

We observed time-resolved partial discharge (PD) characteristics under ac and dc conditions in order to demonstrate the effects of space charge behavior and the corona stabilization effect in SF/sub 6/ gas. From the experimental results, we found that the charge magnitude of the first PD in the positive half cycle of applied ac voltage depended only on the instantaneous voltage. Under dc conditions we found that the magnitude of the PD charge related closely to the time interval between PD pulses. Space charge behavior and the PD generation mechanism are interpreted diagrammatically, and we describe how positive ions reduce the magnitude of PD charge and activate the corona stabilization effect and that the negative ions shorten the time interval between PD pulses.

Design considerations for corona-stabilized repetitive switches

Self-closing corona-stabilized switches have been developed to operate at pulse repetition frequencies (PRFs) which extend well into the kilohertz regime. The performance of these devices illustrates that corona-stabilized plasma closing switches are an effective alternative to conventional uniform or quasi-uniform field switches which utilize gas-flow techniques, high gas pressures or both to achieve high-PRF operation. Corona stabilization takes place in electronegative gases such as air and and requires the presence of a highly divergent electrical field. Under dc, or slowly rising charging voltage conditions, a space charge develops around the highly stressed electrode which prevents premature breakdown and allows full voltage recovery to take place. This means that PRFs extending into the kilohertz regime can readily be achieved. Self-firing corona-stabilized switches have been demonstrated to provide PRFs of up to 5 kHz and triggered corona switches have been shown to be capable of operation at 10 kHz. The lifetime of corona-stabilized switches has been demonstrated to approach about shots. The paper describes the design of a corona-stabilized switch, including detailed electrostatic field analysis of the electrode geometry. This analysis was undertaken to establish the effect of the electrode geometry on the field distribution within the switch. The switch design was flexible and allowed a study of the effect of certain parameters on the switching performance. These parameters include the gap spacing, the gas pressure and the profile of the electrodes. The switch was operated under single-shot conditions to establish the breakdown voltage against pressure (V-p) characteristics and in a continuous repetitive mode to assess the PRF operation. The results show that the best repetitive performance is produced using electrodes which generate a high field over a region sufficient to generate a stabilizing corona.

Factors affecting and methods of improving the pulse repetition frequency of pulse-charged and DC-charged high-pressure gas switches

The results of this paper describe some of the factors which affect the repetitive operation of high-pressure gas switches (spark gaps) for both pulse-charged and DC-charged operation. Also discussed are methods which may be employed to improve the pulse repetition frequency (PRF) of spark gaps operating under such conditions. Under pulse-charged conditions, the voltage recovery process of the spark gap has been shown to be restricted following partial density recovery by the residual ion population. This restriction may be minimized by applying a suitable bias voltage across the gap to remove the ion influence. It is also possible to manipulate the voltage-pressure (V-p) breakdown characteristic of a spark gap in order to improve the rate of rise of recovery voltage by reducing the recovery voltage dependence upon gas pressure. The combination of these effects has been shown to reduce the voltage recovery of pulse-charged spark gaps from several hundred milliseconds to several milliseconds. Under DC-charged conditions, where no dead time is available for voltage recovery, it is possible to employ corona discharge effects, which occur in highly nonuniform fields, to stabilize and control the breakdown process. The use of corona stabilization has enabled the operation of a self-closing spark gap at a PRF of more than 5 kHz, without employing gas flow techniques. A triggered version of a corona-stabilized spark gap has also been developed which has demonstrated single run capabilities of 10/sup 7/ (4 h continuous operation at 700 pps) and a lifetime of /spl sim/10 shots (maintenance free, sealed switch). The triggered corona switch has also demonstrated controlled switching up to a PRF of 1.2 kHz.

Application of unique measuring techniques to positive impulse corona stabilisation in SF6

Results of the application of unique measuring tools for corona stabilisation under positive impulse conditions in SF6 are presented. The present postulate that corona stabilisation appears and disappears about a distinct threshold of impulse rise-time of approximately 10 µs is tested. For slow-rising impulses (&gt; 10 µs), breakdown is found to be able to occur both with and without corona stabilisation. The measurements are explained by the availability of free electrons during the rise-time of the impulse. The unique measuring tools applied rely on statistical analysis of records of the final spark path and correlation of this data with the time-to-breakdown data. The results of the measuring tools are initially verified through tests under conditions of known corona stabilisation.

Modification of ion flow field by insulator surface with rod-plane geometry in SF/sub 6/

The effect of an insulator inserted along the axis of a rod-to-plane gap on an ion flow field in SF/sub 6/ gas is investigated experimentally in the pressure region where a preceding corona discharge exists. Without the insulator, the calculated electric field on the plane electrode agrees fairly well with the measured one. The high density space charge near the rod electrode produces the corona stabilization effect. With the insulator, the accumulated charges on the insulator affect the current and electric field distribution on the plane electrode which are variable with pressure, while the normalized charge density near the plane electrode is less dependent on pressure. These characteristics may be attributed to the change in the electric field distribution by the accumulated charge, the increase in the ratio of the dielectric strength in gas gap to that along the insulator surface with the gas pressure, and the thermal diffusion of ions near the insulator. On the basis of measured results, the ion flow field in the presence of the insulator is discussed. >

Influence of surface shape on DC surface breakdown characteristics under metallic-particle contaminated conditions in SF6 gas

The influence of a metallic particle attached to a supporting insulator is remarkable in decreasing the dielectric strength in an SF6 insulated apparatus. To solve this problem, a series of studies has been conducted on a method of improving the particle-initiated breakdown performance. In this paper, dc breakdown voltage characteristics and the breakdown mechanism are investigated under metallic-particle contaminated conditions in SF6 gas by varying the particle condition and the surface shape of the acrylic plate with a rod-plane electrode. The main results show that a rib increases the breakdown voltage by a corona stabilization effect and the elongation of discharge path, but a groove decreases its effect drastically at lower gas pressures. The breakdown voltage at pressures higher than 4 atm may be estimated on the basis of the discharge-maintaining field and the discharge length which depend on the gas pressure and the surface shape.

SF&lt;sub&gt;6&lt;/sub&gt;ガス中の金属粒子汚損直流沿面フラッシオーバに対する沿面形状の影響

The influence of a metallic particle attached to a supporting insulator is remarkable in decreasing the dielectric strength in an SF6 insulated apparatus. To solve this problem, a series of studies has been conducted on a method of improving the particle-initiated breakdown performance. In this paper, dc breakdown voltage characteristics and the breakdown mechanism are investigated under metallic-particle contaminated conditions in SF6 gas by varying the particle condition and the surface shape of the acrylic plate with a rod-plane electrode. The main results show that a rib increases the breakdown voltage by a corona stabilization effect and the elongation of discharge path, but a groove decreases its effect drastically at lower gas pressures. The breakdown voltage at pressures higher than 4 atm may be estimated on the basis of the discharge-maintaining field and the discharge length which depend on the gas pressure and the surface shape.

Breakdown characteristics of cryogenic gaseous nitrogen and estimation of its electrical insulation properties

The electrical properties of liquid nitrogen and its vaporized gas have become of great interest since the discovery of high-temperature superconducting materials. The authors present the breakdown characteristics of gaseous nitrogen from room temperature down to 74 K. With uniform field gap geometry, the validity of Paschen's law is confirmed even under the saturated gas condition at 74 K. In the case of a nonuniform field gap, there is a remarkable a difference in the effects of positive and negative polarities on corona stabilization. A wider gap length is necessary for stable negative corona formation than for positive corona at temperatures <100 K. Breakdown criteria are proposed, on the basis of experimental results, to give a practical and reliable procedure for the design of cryogenic insulating systems which use liquid nitrogen and the vaporized gas as the coolant.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>