We investigate the influence of switching arcs on different polymers and their interaction. We describe a set of experiments on a simplified model geometry typical for low voltage switchgear. In a broad range of experimental conditions and parameters such as arc current, polymeric material or contact material, the voltage, the mass loss and the corresponding pressure build-up are examined. From this raw data, we deduce the arc influence on the ablation process as well as the feedback on some arc plasma properties

Bator, M.

Bianchetti, R.

Suetterlin, P.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2017.2.149Plasma Physics and Technology 4(2):149–152, 2017 © Department of Physics, FEE CTU in Prague, 2017POLYMERIC ABLATION INDUCED BY FREE BURNING ARCS IN AIRM. Bator∗, R. Bianchetti, P. SuetterlinABB Corporate Research Center Switzerland, Segelhofstrasse 1K, 5405 Baden-Daettwil, Switzerland∗ matthias.bator@ch.abb.comAbstract. We investigate the influence of switching arcs on different polymers and their interaction.We describe a set of experiments on a simplified model geometry typical for low voltage switchgear. Ina broad range of experimental conditions and parameters such as arc current, polymeric material orcontact material, the voltage, the mass loss and the corresponding pressure build-up are examined.From this raw data, we deduce the arc influence on the ablation process as well as the feedback onsome arc plasma properties.Keywords: free burning arcs, low voltage switchgear, plasma properties, polymeric ablation, thermo-plastics.1. IntroductionLow voltage switchgear and the corresponding devel-opment processes matured over decades and todayare highly optimized. Thus, most development oftenconcentrates on optimization of very specific detailssuch as arc chamber geometries, contact materials andvery often the exact composition of plastic parts. In-terestingly, when testing new polymers, the approachis often to test a wide range of materials and settlewith the best as the understanding of the underlyinginteraction between the arc and ablated polymers isoften lacking.The goal of the investigation presented in this pa-per is to improve exactly this understanding. Twosimplified laboratory setups were used, one with twoparallel polymeric plates with an arc ignited betweentwo cylindrical electrodes, and the other with a poly-meric cylinder in order to constrict the arc axially.We investigated the mass loss, the arc voltage and thepressure build up in dependence on the current as wellas the polymeric and contact material. SpecificallyPA6, PA66, PTFE and PE were investigated in pureforms as well as containing additives. The electrodesconsisted of either copper or steel.2. Experimental SetupThe setup consisted of a capacitor bank with 32mFcharged up to 950V. It was discharged through aninductance and the DUT (device under test) witha pseudo 50Hz pulse. A second variation used anadditional buck converter allowing for an actively con-trolled current discharge of the capacitor through ourDUT (see Figure 1). In these cases, the capacitor bankwas used in a 16mF configuration and was typicallycharged up to 1600V. Two different geometries (seewere used with both setups.In the setup including the buck converter, currentcontrol was achieved by switching an ABB HiPakIGBT via a feedback PI control (see Figure 1). AsFigure 1. a) Schematics of the active current controlsetup. A pre charged capacitance (C) is discharged viaan IGBT and an inductor (L) through the DUT. Thecurrent (A) is used to control the semiconductor. b)Currents (0.6, 0.8, 1.0 and 1.2 kA) set using the setupin a). In one instance, the control loop was stoppedafter 5ms; the others were stopped after 10ms.long as the current was below the set value, the IGBTwas on and the current rose with a time determinedby the used inductance (1.5mH in our case) and thevoltage difference between arc and the capacitor bank.After reaching the set current, the ABB HiPak IGBTis switched off and the current decreases as energyis dissipated in the arc. As a result, a small ripple(determined by amongst others the inductance andthe control frequency) is visible on the current trace,which typically was in the range of ±20A on a 1 kAset current. Either after a set time or when the capac-itor voltage drops below the voltage of the DUT nomodulation of the current occurs and it drops to zero.2.1. Parallel Plate GeometryAs mentioned, two different geometries were used asDUT, both utilizing a thin wire for ignition. In thefirst, two polymeric plates were arranged in paral-lel with one electrode mounted on the right betweenthe two plates (see Figure 2) allowing for optical ac-cess from the side perpendicular to the arc direction.The second electrode is mounted to the left of theplates just outside the picture shown in Figure 2. Theplate surface measures 20 x 40mm with an adjustabledistance in this case set to 12mm. This setup resem-149M. Bator, R. Bianchetti, P. Suetterlin Plasma Physics and TechnologyFigure 2. a) Side view of the parallel plate geometry.An electrode is mounted on the right between the twopolymeric plates to be investigated. The second elec-trode, on axis with the first is not shown on the leftside. The plates were separated by 12mm. b) CADmodel of the geometry.bles the typical configuration present in a low voltagebreaker with an arc squeezed in between two polymericwalls.2.2. Nozzle Type GeometryIn the second geometry, a tube replaced the polymericplates. Here, the electrode is mounted gastight insidethe tube on the right (see Figure 3) such that gasflow is only allowed towards the left. The secondelectrode is again mounted outside of the polymericregion (visible on the left side of Figure 3 a)). Thechannel within the tube has a length of 45mm with avariable inner diameter of 5, 7 or 10mm. The closedgeometry allows for an additional pressure measure-ment inside the tube. Differently to the parallel platesetup, optical access is only given at the outlet of thetube and no longer along the whole arc.2.3. Optical MeasurementsA high-speed camera setup is arranged perpendicularto the arc direction in order to visualize the plasmaunder different conditions. Typical exposure timeswere in the range of a few µs. This allowed to eithervisualize a detailed time evolution of the plasma or givea good overview of its distribution. Using bandpassfilters allowed to better distinguish emission fromspecific sources such as the Hα- or specific metalliclines.3. ResultsThe two setups were used to investigate different as-pects of the polymer-arc interaction. The parallelplate setup was used to optically investigate the spa-tial distribution of the arc and how it is influenced bydegassing of the polymers. In the cylindrical setup, weconcentrated on the pressure and arc voltage build-upduring the arcing as well as on the integrated massloss of the polymeric tubes.3.1. Plasma DistributionThe main interest of the investigation was the inter-action of the plasma with the polymeric walls. In afirst step, we investigated how the arc arranges itselfwithin the parallel polymeric plates. This is done bydoing time-resolved optical imaging of the arc usingFigure 3. a) Side view of the nozzle geometry. Anelectrode is mounted on the right inside a hollow poly-meric cylinder. The right side is designed to be airtight, while the left is open towards the ground electrode(also shown). b) CAD model of the geometry. Theelectrode tip has a diameter of 5mm.the high-speed camera. In a first series, neat PA6(neat in this context meaning commercially availablepure compounds) plates were arranged 12mm apart.The applied current was a pseudo 50Hz pulse with3 kA peak value.The ignition wire explodes ca. 0.1ms after currentinitiation filling the volume in front of the contactwith hot metallic plasma and residues of the wire. Inthis early phase, up to ca. 0.5ms (see Figure 4, leftcolumn), the hot plasma is distributed asymmetricallyand touches the polymeric walls (within our resolutionof ca. 0.1mm). The behavior changes quickly as thecurrent increases; the arc becomes brighter and moreconstricted. After ca. 1.5ms, the arc generally stabi-lizes into a symmetric, constricted shape with darkregions between the bright center and the polymericwalls. In all measurements, a sharp edge between thecentral arc plasma and the dark region around it isobserved. A light glow is visible at the polymeric walls(see Figure 4 left column 5.0ms). Optical emissionspectroscopy results (not discussed in this paper in de-tail) show that the spectral distribution in this regionis identical to the center of the arc comprising mostlyionized metal lines. This leads to the conclusion thatthe light is most likely reflected and/or scattered fromFigure 4. Left panel: Time sequence of a single dis-charge with neat PA6 walls and a 50Hz discharge witha 3 kA peak current. Right panel: High-speed frames ofan arc discharge with copper electrodes using a band-with filter at 514.5 nm (3 nm FWHM) exposed for 4µsat peak current.The walls used are neat PTFE.150vol. 4 no. 2/2017 Arc-induced Polymeric Ablationthe central arc and not emission from evaporatingpolymers. The exact shape of the arc is not deter-minable in this setup due to the camera axis beingcentered between the plates and perpendicular to thearc direction. We assume that the initially cylindricalarc is squeezed into a flattened distribution betweenthe plates.With changing current and polymeric material, thegeneral behavior does not change. In all measure-ments, the arc becomes more constricted in the centerregion, and most likely distributes wider along theoptical axis of the camera with increasing currentsas shown in Figure 4 (right column). These pictureswere taken with an additional bandpass filter around514.5 nm with a width of 3 nm to enhance the observedradiation coming from atomic emission lines of cop-per. Furthermore, the wall material in this column isPTFE and not PA6 as in the images on the left. Inter-estingly, no emission is observed between the polymerand the bright arc regardless of the current. Evenadditional spectroscopy and absorption measurements(not presented here) do not allow for a classification ofthe "dark zone" between the polymeric walls and thecentral arc. A disadvantage of the parallel plate setupand in particular the open sides along the optical axisis the build-up of free-floating fumes in front of theplasma as can be observed towards the top left ofthe 5.0 kA high speed picture in Figure 4. It coversthe plasma, as well as the dark region and thereforefalsifies the optical impression of the distance of thearc to the polymeric walls. This effect is considerablystronger using PA6 walls.The two take-away observations are that the poly-mer gases are optically not accessible due to mostlikely being too cold to generate radiation. And sec-ondly that a strong spatial constriction of the arcis observed which considerably increases with highercurrents and therefore larger degassing of the polymer.3.2. Mass Loss and Pressure Build-upIn the second set of experiments, using the nozzlesetup, the arc constriction was as apparent as before.At the nozzle opening, the bright plasma region inthe center was surround with a dark region towardsthe walls of the polymeric tube. Still, using this setupthe focus of the investigations shifted from the opticalinvestigation to the pressure build-up and voltage dur-ing the arcing as well as the mass loss of the polymertube.The pressure build-up is an important indicatorfor the interruption process as the ablated polymersinteract in complex ways with the arc. In general,it is considered advantageous to increase the amountof ablated polymer mostly due to its effect of cool-ing the arc and thus leading to higher arc voltages.Furthermore, the flow can be used either to controlthe movement of the arc or to diffuse the arc in spe-cific nozzle configurations. Depending on the exactpolymeric material used, an additional benefit is theFigure 5. Measured total mass loss (red circles, leftaxis) and average pressure build-up (blue squares, rightaxis) for different nozzle materials using a controlled1.2 kA discharge during 10ms and different materialcombinations.reduced conductivity compared to a metallic plasmaand a possibly higher dielectric stability after cur-rent zero. Still, there are also disadvantages relatedto a large amount of hot polymeric gases. Too highpressures might lead to an explosive destruction ofthe device, in particular at higher currents when theoutgassing is scaled up by the current. Furthermore,some polymeric materials, such as e.g. PTFE can leadto higher conductivity in regions with large concen-trations [1] leading to either re-strikes outside of thearc chamber during the interruption or to re-ignitionsafter current zero.Therefore, it is often necessary to take the decisionfor a polymeric wall material on a per-device baseleading to additional work and uncertainties in thedevelopment process. A better understanding of theablation process on a very general base would helpto reduce the necessary testing and ideally enable abetter predictability of polymer choices.In the following we will discuss the findings usingcombinations of copper, steel, and silver electrodeswith neat PA66, "black" PA66 (PA66 with <1% ofcarbon black), neat PTFE, "grey" PTFE (PTFE withmolybdenum disulfide), neat PE and "black" PE (allcommercially sourced). The findings are shown inFigure 5. The top panel shows the mass loss andaverage pressure for different polymers using copperelectrodes, while the bottom panel compares PA66and "black" PA66 using copper and steel electrodes.In both cases, we did not use a 50Hz discharge, but a151M. Bator, R. Bianchetti, P. Suetterlin Plasma Physics and TechnologyFigure 6. Measured average pressures versus averagedarc voltage in all recorded discharges using the nozzletype setup and the 50Hz synthetic circuit.controlled 10ms long current (as shown in Figure 1).The values are averaged between 3 and 8ms wherethe variation is the lowest and the influence of theignition process is no longer present.We observe a clear trend towards higher ablationratios for the "black" and "grey" variants of PA66 andPTFE. It has to be mentioned that the spread inidentical measurements could be explained by slightlyleaky arrangement in particular after several shotswhen the inner diameter widened slightly. Carbonblack is typically used to enhance stability versus UVradiation [2]. Due to the high temperatures in thearcing plasma, usually more than 50% of the radiationreaching the polymeric walls has a wavelength below300 nm [3]. A stronger absorption in the top-mostlayers therefore leads to an increase of the ablationprocesses. Similarly, the higher ablation when usingsteel electrodes can be explained. The higher net emis-sion compared to copper [4] would result in a largeramount of radiation reaching the walls and thus leadto larger ablation. Furthermore, more ablated metalin addition can change the interaction with the poly-meric walls. Interestingly, the pressure in PA66 staysmostly the same for neat PA66, black PA66 and PA66Mo, while the mass loss is measurably higher for theblack version. A possible explanation is that this com-parison only includes ablation into the gaseous phase,while the generation of particles and droplets, whichcan strongly vary between different polymers, typi-cally has less influence on the plasma properties butstrongly influences the mass loss. Hence, one couldassume that in the case of the black PA66 the strongerabsorption leads to a slight increase in gaseous abla-tion but a stronger increase in generation of particlesand droplets. Comparing PTFE and black PTFE onthe other hand shows a measurable increase in both,the mass loss and the average pressure. Therefore, inthis case one can conclude that the higher absorptionis a more direct translation into a larger generationof polymeric gas.In order to observe the general correlation betweenthe pressure and arc voltages, all measurements usinga 50 Hz discharge are shown in Figure 6. The valuesare taken at the peak currents, but include all mate-rial combinations as well as different current settings.Even considering this wide variety and the findingsdiscussed before, a general linear trend is observed.Placing an additional floating electrode at the exitof the nozzle we found that ca. 60-70% of the volt-age drop appears in the nozzle. In order to explainthe increase of the arc voltage with the pressure, wehave to consider the findings from the parallel platesetup regarding how the current influences the arcshape and the extent of the dark layer dominated byrelatively cold polymeric gases. The increase of thecurrent leads to higher ablation rates and thereforehigher pressures. At the same time, the hotter arc isbeing constricted in a smaller space. The electricalconductivity is very low at temperatures below 5 kK,grows linearly until ca. 20 kK where it finally flattensout [5]. Therefore, the slightly higher temperaturein the core of the arc is not able to compensate theexponentially lower conductivity in the dark regionsof "cold" polymer gases leading to an increase of thearc voltage in conditions where a high pressure isexpected.4. ConclusionsWe investigated the interaction between polymericablation and an electrical arc. We could clearly showthe influence the ablated polymer has on the shapeof the arc and its flow patterns. Furthermore, wecould show how dominant the dark region of relativelycold polymeric gases is not only on the shape but alsoon the electrical properties of the arc, in particularthe arc voltage. Together with the findings that theablation of polymers can be significantly increasedusing compounds that incorporate e.g. carbon blackthe results can be used to further optimize predictionmodels in switchgear development. The differencesin ablation when using steel electrodes compared tocopper electrodes further show that the influence ofthe splitter plates in a real breaker on the ablationproperties should not be neglected.References[1] Y. Okano et al. Quenching performance of air arcdischarge with polymer insulators. 20th Int. Conf. onGas Disch. and Their Appl., 745(A39):199–202, 2014.[2] R.J. Young and P.A. Lovell. Introduction to Polymers.Third Edition. CRC Press, 2011.[3] ABB. Internal communication/publication.[4] Y. Cressault and A. Gleizes. Thermal plasmaproperties for Ar-Al, Ar-Fe and Ar-Cu mixtures used inwelding plasma processes: I. net emission coefficients atatmospheric pressure. J. Phys. D: Appl. Phys, 46:415206,2013. doi:10.1088/0022-3727/46/41/415206.[5] S. Franke et al. Temperature determination incopper-dominated free-burning arcs. J. Phys. D: Appl.Phys., 47:015202, 2014.doi:10.1088/0022-3727/47/1/015202.152

Polymeric Ablation Induced by Free Burning Arcs in Air

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Polymeric Ablation Induced by Free Burning Arcs in Air

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