In the process of substituting sulphur hexafluoride in medium voltage load break switches by atmospheric gases, the inferior arc quenching capabilities of possible substitutes have to be compensated. By introducing a polymer nozzle into the switching gap of a load break switch, the interruption capability can be enhanced as the ablated nozzle material changes the composition of the arc plasma. In this contribution the interruption capability of a model load break switch is investigated using different nozzle materials. The results show a good interruption capability when using polypropylene and polyamide 6.6 for high blowing pressures. Polytetrafluorethylene  shows good results across a wide blowing pressure range. Polylactide has the lowest interruption capability among the polymers in this work

Bendig, Marvin

Ermeler, K.

Götte, Nicolas

Kalter, Achim

Krampert, Thomas

Schaak, Martin

Publikationsserver der RWTH Aachen University

Investigations on the effect of the nozzle material on the interruption capability of a medium voltage load break switch

Bendig, M.

Krampert, T.

Götte, N.

Schaak, M.

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

doi:10.14311/ppt.2019.1.15Plasma Physics and Technology 6(1):15–18, 2019 © Department of Physics, FEE CTU in Prague, 2019INVESTIGATIONS ON THE EFFECT OF THE NOZZLE MATERIAL ONTHE INTERRUPTION CAPABILITY OF A MEDIUM VOLTAGE LOADBREAK SWITCHM.Bendiga,∗, T. Kramperta, N. Göttea, A. Kalterb, M. Schaakb,K. Ermelerca Institute for High Voltage Technology, RWTH Aachen University, Schnikelstr. 2, 52056 Aachen, Germanyb Siemens AG, Energy Management Division, Carl-Benz-Str. 22, 60386 Frankfurt am Main, Germanyc Siemens AG, Energy Management Division, Nonnendammallee 104, 13629 Berlin, Germany∗ bendig@ifht.rwth-aachen.deAbstract. In the process of substituting sulphur hexafluoride in medium voltage load break switches byatmospheric gases, the inferior arc quenching capabilities of possible substitutes have to be compensated.By introducing a polymer nozzle into the switching gap of a load break switch, the interruptioncapability can be enhanced as the ablated nozzle material changes the composition of the arc plasma.In this contribution the interruption capability of a model load break switch is investigated usingdifferent nozzle materials. The results show a good interruption capability when using polypropyleneand polyamide 6.6 for high blowing pressures. Polytetrafluorethylene shows good results across a wideblowing pressure range. Polylactide has the lowest interruption capability among the polymers in thiswork.Keywords: load break switch, nozzle material, ablation, alternative gases.1. IntroductionMedium voltage load break switches (LBS) arewidely used switchgear in the secondary distributiongrid. They serve as combined load and disconnectorswitches and are capable of interrupting load currentsin the range of a few hundred amps. Due to thespace limitations in urban areas LBS are mainlybuilt into metal-enclosed switchgear filled with theinsulation gas sulphur hexafluoride (SF6). SF6,with its high dielectric strength and superior arcquenching capabilities, allows for a compact andreliable switchgear design. Nevertheless, SF6 has aglobal warming potential of 23500 carbon dioxide(CO2) mass equivalents, which makes it the mostpotent greenhouse gas known. Possible atmosphericsubstitutes like air or CO2 have been investigated inprevious works. As their arc quenching capabilityis inferior to SF6, additional measures like an axialarc blowing have to be applied [1, 2]. One way ofenhancing the interruption capability of a gas switchis by introducing a gassing polymer nozzle into theswitching gap. If a switching arc in drawn betweenthe contacts, the polymer ablates and the releasedgas is changing the composition of the plasma. Ithas been shown, that polymers that release eitherfluorine or hydrogen are good candidates to be usedas a nozzle in LBS [3, 4].This work analyses the influence of the applied nozzlematerial on the interruption capability of a modelload break switch. The investigated polymers arepolytetrafluorethylene (PTFE), as a polymer releasingfluorine as well as polypropylene (PP), polyamide 6.6(PA6.6) and polylactide (PLA) as polymers releasinghydrogen. All tests were performed using a nitrogen -carbon dioxide mixture as quenching and insulationgas at an elevated filling pressure of p = 1800 hPaabsolute to enhance the electric strength.2. Experimental methodTo evaluate the switching performance the thermalinterruption capability as well as the rate of the dielec-tric recovery are determined individually. By usingthis method, the effects of the used nozzle materialcan be analyzed for the different phases.2.1. Thermal interruption limitA successful thermal interruption is dependent on thecurrent steepness shortly before current zero (CZ) aswell as on the voltage steepness shortly after CZ. Inthis work the critical current steepness di/dtcrit is usedto quantify the thermal interruption capability. Forthe determination the synthetic test circuit shown inFigure 1 is used. The high current circuit provides onehalf oscillation of a sinusoidal current with a value ofIrms = 630 A and a frequency of f1 ≈ 50 Hz. Shortlybefore the current reaches its natural CZ the thyristorin the high voltage circuit is triggered and a currentwith an amplitude in the range of a few ten amps anda frequency of f2 ≈ 1000 Hz is superimposed over thetest current. Using this superposition method, thecurrent steepness at CZ can be varied independent ofthe test current amplitude. When the current is inter-rupted successfully a recovery voltage rises across the15M. Bendig, T. Krampert, N. Götte et al. Plasma Physics and TechnologyHigh Current Circuit High Voltage CircuitFigure 1. Test circuit to determine the thermal inter-ruption capabilityopen contacts. To set the rate of rise of the recoveryvoltage (RRRV) a parallel resistor with Rp = 220 Ω isused. This resistor is chosen in a way that the RRRVcorresponds to the RRRV of a standardized test forUN = 24 kV when a current steepness according to thesteepness of a 630 A test at a frequency of f = 50 Hzis set. As the parallel resistor is fixed, the RRRVchanges when the current steepness is changed.At the beginning of a test series a low current steep-ness (di/dt) is chosen and five successful interruptiontests are performed. Afterwards the di/dt is increasedand another five interruptions are performed. Thisprocedure is repeated until all five interruptions forone di/dt fail. Afterwards the measured di/dts andRRRVs are plotted into a diagram and a linear regres-sion is performed to make sure that all tests have theright ratio between di/dt and RRRV. In a next stepthe critical RRRV is determined as the mean valuebetween the lowest failed test and the next lowestsuccessfull test. Afterwards, the critical current steep-ness is determined as the product of the regressionslope and the critical RRRV. By using this method,inaccurancies from the low signal to noise ratio of thecurrent signal around CZ are avoided.2.2. Dielectric recoveryThe dielectric recovery of the model switch is deter-mined with the test method and circuit described in[5]. The test object is stressed with a half oscillationof a current with a value of Irms = 630 A. Afterwards,the breakdown voltage at a certain delay after CZ isdetermined by applying a steep impulse voltage tothe test object. This procedure is repeated for severaldelay times to get a time resolved course of the break-down voltage after current zero. For each delay timea minimum of four breakdown tests is performed.3. Test setupA sectional view of the model switch is shown in Fig-ure 2. The contact system consists of a combination ofa tulip and a pin contact made from a tungsten-copper-alloy with a weight ratio of 80/20. The maximumcontact distance is s = 58,5 mm and the contact di-ameter of the pin is dc = 10 mm. The switching arc iscooled by an axial blowing through the tulip contact.The blowing is initiated by a magnetic valve, whichis connected to an external gas volume. The blowingPin contactNozzleTulip contactBlowinglndn dcFigure 2. Section of the model load break switchProperty PTFE PP PA6.6 PLAρ [g/cm3] 2.16 0.91 1.15 1.15TL [◦C] ≈ 330 ≈ 160 260 215Color Gray Gray Natural WhiteVapor F H H -Table 1. Properties of the investigated polymerspressure is measured upstream of the nozzle using aPitot probe and is given as the total pressure dropacross the tulip and nozzle. All investigated pressuresin this work lead to a subsonic flow. The tulip con-tact and part of the contact distance is surroundedby a polymer nozzle with a nozzle throat diameter ofdn = 11 mm and a nozzle throat length of ln = 24 mm.This configuration was identified as a good parame-ter set in previous work [1]. The investigated nozzlematerials in this work are shown in Table 1. Thedensity ρ as well as the melting point TL vary acrossthe different materials. While the used PTFE, PPand PLA are colored, the PA6.6 is used in its naturalivory color. If ablated by the arc, the resulting vapormainly consists of fluorine for PTFE and hydrogenfor PP and PA6.6 [4, 6, 7]. For PLA no data wasfound on the ablation behavior. All nozzles are milledexcept of the PLA nozzle, which is 3D printed. Dueto the manufacturing the internal surface as well asthe structure of the material differs from the othernozzles.The model switch is operated by a pneumatic pistondrive with a stroke of 90 mm and an average open-ing speed of vm = 4,5 m s−1. The stroke is recordedduring the opening process using a laser triangulationmeasurement system.4. ResultsThe thermal interruption capability is tested for noz-zles made from PTFE, PP, PA6.6 and PLA. Thedielectric recovery is only analyzed for PTFE, PP andPA 6.6. Additionally the mass losses due to ablationare determined for all investigated materials.4.1. Thermal interruption capabilityThe critical current steepnesses for the different mate-rials are shown in Figure 3 for different blowing pres-sures. The lines are drawn for better visualization. Forthe maximum blowing pressure and when using PP or16vol. 6 no. 1/2019 Effect of Nozzle Materials in MV Load Break Switches300 400 500 600 700 800Blowing pressure in hPa0.150.20.250.30.350.40.450.5di/dt crit in A/µsPTFEPPPA6.6PLAFigure 3. Thermal interruption capability for differentnozzle materials and blowing pressures0 100 200 300 400 500Time after CZ in µs020406080Breakdown volteage in KV PTFEPPPA6.6Figure 4. Dielectric recovery after current zero fordifferent nozzle materialsPA6.6 no thermal failure occurs with the maximumsteepness of the test circuit di/dtmax = 0,5 Aµs−1.Dashed lines indicate that the critical steepness isabove this level.For all investigated nozzle materials the critical cur-rent steepness rises with increasing blowing pressure.However, there are differences in the pressure depen-dence. When using PTFE or PLA as a nozzle material,the increase in interruption capability rises nearly par-allel for a rising blowing pressure, with PTFE show-ing significantly higher critical current steepnesses.When using either PP or PA6.6 the critical currentsteepnesses are low for low blowing pressures but theincrease with pressure is much higher than for PTFEor PLA.4.2. Dielectric recoveryThe time resolved breakdown voltages after CZ areshown in Figure 4 for PTFE, PP and PA6.6. PLA wasnot investigated due to its low thermal interruptioncapability. The plotted values represent the meanvalues of at least four breakdowns at one specific delaytime. The blowing pressure is set to p = 330 hPA forall tests.For all investigated nozzles a strong increase inbreakdown voltage occurs after current zero. In thefirst 170 µs the difference between the nozzle materialsis low. Afterwards PA6.6 shows a slight decrease inMaterial PTFE PP PA6.6 PLAAblation [mg] 3 5.8 5.2 11.5Table 2. Ablated mass per 630A half oscillation forthe investigated nozzle materialsFigure 5. Photos of the PA 6.6 (top) and the PP(bottom) nozzle after 250 switching operationsdielectric strength followed by a slight decrease of thebreakdown voltage of PP. Local minima are expectedto result from statistical scatter. From roughly 300µson, the highest breakdown voltages are achieved whenusing a PTFE nozzle followed by PP and PA6.6.4.3. Nozzle ablationTo compare the ablation behavior of the different noz-zle materials the nozzles are weighed after productionand after 100 consecutive interruption tests. Thenthe average weight loss per 630 A half oscillation iscalculated. In Table 2 the resulting ablation massesare given. The ablation rates show large deviationsamong the different materials. Due to its high subli-mation temperature PTFE shows the lowest weightloss. The weight losses of PP and PA6.6 are in thesame range, despite of their different melting tem-peratures. However, as the density of PP is lowerthan of PA6.6 the volumetric losses are higher. PLAshows by far the highest mass losses. This might bedue to the 3D printed structure being not as denseas the other nozzles. Furthermore, the significantlyhigher roughness inside the nozzle throat might leadto increased ablation during switching.On all nozzles except from the PTFE nozzle a sootformation can be observed after the tests. Photosof the PP and the PA 6.6 nozzle after 250 switchingoperations are shown in Figure 5. The original geom-etry is higlighted with a blue curve, while the actualgeometry is highlighted by a red curve. The strongervolumetric loss of the PP nozzle is clearly visible. Asthe arc has a preferred footpoint on the top of thetulip, the maximum ablation occurs there.17M. Bendig, T. Krampert, N. Götte et al. Plasma Physics and Technology5. DiscussionAs the different nozzle materials show different effectsin the thermal and the dielectric phase the two phasesare discussed individually.5.1. Thermal phaseThe ablated vapor of PP and PA6.6 mainly consistsof hydrogen. Due to its high thermal conductivityan efficient arc cooling can be achieved. However,the ablated masses are almost twice as high as forPTFE, which leads to a higher pressure rise insidethe nozzle due to ablation. It is assumed, that thishigher pressure rise leads to a nozzle clogging at lowerblowing pressure, resulting in an insufficient flow fieldand therefore a reduced interruption capability. Athigher blowing pressures the pressure rise due to ab-lation is lower than the external pressure leading to asufficient flow. The high hydrogen content results inan increased interruption capability in comparison toPTFE.Due to the lower ablated mass when using PTFE, theeffective flow field is more efficient which results ina good interruption capability. As the ablated gasis very similar to dissociated SF6, an admixture ofablated PTFE to the plasma increases the interrup-tion capability as well [7]. As PLA has a significantlyhigher ablation rate, the flow is insufficient even athigh external blowing pressures.5.2. Dielectric phaseAs the experiments for determining the dielectricrecovery are performed with a blowing pressure ofpb = 330 hPa, similar effects can be assumed to thosein the thermal phase. This leads to PP and PA6.6having an insufficient cooling and therefore also aslower recovery than PTFE. Additionally, the ablatedgas almost completely dissociates in vicinity of thearc. Due to the absence of the highly reactive fluo-rine conductive carbon black might be formed in therecombination process leading to a lower breakdownvoltage and therefore a slower dielectric recovery.To isolate the effect of carbon black formation, exper-iments with a higher blowing pressure should be con-ducted. However, as the recovery voltage at mediumvoltage load interruption is not as demanding as i.e.during fault interruption, the dielectric recovery inthe latter part is sufficient for all investigated cases.Only the first 120µs might be critical.6. ConclusionsThe thermal interruption capability as well as therate of the dielectric recovery were experimentallydetermined for a model load break switch with anaxial arc blowing using different nozzle materials. Thefollowing results were obtained: PTFE shows a good thermal interruption capabilityfor all investigated blowing pressures as well as agood dielectric recovery, due to its low ablationrate. Polymers that release hydrogen in high parts dur-ing arcing, like PP and PA6.6, show an increasedthermal interruption capability for high blowingpressures. However, due to their higher ablationrate, insufficient cooling may result from cloggingat lower blowing pressures. The dielectric recovery may be slower when usingPP or PA6.6 due to clogging and the formation ofcarbon black during the recombination process. 3D printed PLA is not a suitable nozzle material,as the high ablation rate leads to a low interruptioncapability.Summarizing, PTFE is a good candidate for a noz-zle material in load break switches using alternativeinsulation gases. If sufficient blowing pressures areavailable using PP or PA6.6 can result in an evenhigher thermal interruption capability. The disadvan-tage is the higher nozzle wear as well as the formationof soot after switching.AcknowledgementsThis work was supported by the Federal Ministry forEconomic Affairs and Energy on the basis of a decision bythe German Bundestag.The authors would like to thank Tim Ballweber for hisassistance in this work.References[1] M. Bendig, N. Götte, T. Krampert, A. Schnettler,A. Kalter, and M. Schaak. Investigations on the switchingcapability of medium voltage load break switches in analternative quenching gas. In Proc. of the 22nd Int.Conf. on Gas Discharges and their Applications, 2018.[2] N. Sasaki Aanensen, E. Jonsson, and M. Runde.Air-flow investigation for a medium-voltage load breakswitch. IEEE Trans. Power Delivery, 30(1):299–306,2015. doi:10.1109/TPWRD.2014.2334360.[3] D. Gonzales, H. Pursch, and F. Berger. Experimentalinvestigation of the interaction of interrupting arcs andgassing polymer walls. In 57th Holm Conference onElectrical Contacts, 2011.doi:10.1109/HOLM.2011.6034774.[4] H. Taxt, K. Niayesh, and M. Runde. Medium voltageload current interruption in presence of ablating polymermaterial. IEEE Trans. Power Delivery, 33(5):2535–2540,2018. doi:10.1109/TPWRD.2018.2803165.[5] M. Bendig, N. Götte, T. Krampert, A. Schnettler,A. Kalter, and M. Schaak. A method to determine therate of he dielectric recovery in a medium voltage loadbreak switch with a free burning switching arc. In Proc.of the 22nd Int. Conf. on Gas Discharges and theirApplications, 2018.[6] P. Andre. Composition and thermodynamic propertiesof ablated vapours of PMMA, PA6-6, PETP, POM andPE. J. Phys. D: Appl. Phys., 29(7):1963–1972, 1996.doi:10.1088/0022-3727/29/7/033.[7] M. Kriegel. Einfluss des Düsenmaterials auf dasAusschaltverhalten von SF6-Selbstblasschaltern.Dissertation, RWTH Aachen University, 1999.18

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Investigations on the Effect of the Nozzle Material on the Interruption Capability of a Medium Voltage Load Break Switch

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Investigations on the effect of the nozzle material on the interruption capability of a medium voltage load break switch

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