The influence of ablation on the du/di behavior of an arc in a model gas circuit breaker was examined. Specifically the transition from a state without ablation in the nozzle towards states with ablation was of interest, since prior work indicated that for high currents the voltage becomes constant or du/di gets even positive if ablation is present. Measurements with different blow pressures and rectangular DC currents of varying amplitude were compared, using PMMA-nozzles and dry air as blowing gas. Ablation was measured by weighing the nozzle, scanning the cross section, and using a coordinate measuring machine. The results agreed well, and confirmed that higher pressure shifts the du/di curve towards more favorable values

Bort, L. S. J.

Franck, C. M.

Freiermuth, V.

English

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

doi:10.14311/ppt.2017.2.145Plasma Physics and Technology 4(2):145–148, 2017 © Department of Physics, FEE CTU in Prague, 2017INFLUENCE OF ABLATION ON DIFFERENTIAL ARC RESISTANCEL. S. J. Bort∗, V. Freiermuth, C. M. FranckHigh Voltage Laboratory, Eidgenössische Technische Hochschule, Zürich, Switzerland∗ lbort@ethz.chAbstract. The influence of ablation on the du/di behavior of an arc in a model gas circuit breakerwas examined. Specifically the transition from a state without ablation in the nozzle towards stateswith ablation was of interest, since prior work indicated that for high currents the voltage becomesconstant or du/di gets even positive if ablation is present. Measurements with different blow pressuresand rectangular DC currents of varying amplitude were compared, using PMMA-nozzles and dry air asblowing gas. Ablation was measured by weighing the nozzle, scanning the cross section, and using acoordinate measuring machine. The results agreed well, and confirmed that higher pressure shifts thedu/di curve towards more favorable values.Keywords: axially blown arc, ablation, passive oscillation, MRTS, HVDC CB.1. IntroductionUnlike in AC, the interruption of DC currents requiresthe creation of an artificial current zero crossing. To-day in DC transfer switches, such as metal returntransfer switches (MRTS), the passive oscillation cir-cuit is used. An LC-circuit is connected in parallel toa gas circuit breaker. If the arc has a negative differen-tial resistance du/di, the damping of the oscillation inthe LC-arc-loop is negative, and eventually will leadto current zero in the breaker [1]. If the arc voltage isnot falling with increasing current, passive oscillationfails and additional circuitry is needed which activelybrings the current to zero[2]. Previous work at HVL[3] and others [4] indicated that a high amount ofablation leads to du/di being positive, but did notanalyze it in detail.2. SetupThe setup used was previously described in [3], [5],[6]. It consists of a model gas circuit breaker, whichuses gas bottles for a freely adjustable blow pressureand two contact pins of which one is movable to drawthe arc. Dry air was used as a blowing gas. The inletpressure is variable, the exhaust pressure is alwaysfixed at 1 bar. Since no gas handling is required toaccess the parts, it is possible to remove and reinstallthe nozzle in a matter of minutes, which made itpossible to determine the ablation after each test inreasonable time.For the measurements presented in this paper, threeidentical nozzles were produced. Each was used for aseries of experiments of one pressure. In the follow-ing, these series are shown as 4.5 bar abs., 8.4 bar abs.and 12.7 bar abs., which refers to the inlet pressureof the nozzle. PMMA was chosen because it is trans-parent, making it possible to observe the arc. Toallow separation along the axis of cylindrical symme-try, each nozzle consisted of two identical parts. Thisaxial position (mm)0 20 40 60 80 100 120 140radialposition(mm)-20-1001020Figure 1. Top: Image of one nozzle half as capturedby flatbed scanner. The contour can be seen, as wellas the grooves for the sealing cord and two of the sixholes for dovel pins. Bottom: Extracted nozzle contour(solid), and position of contacts (dashed). Gasflow wasfrom left to right, cathode was left, anode right.was achieved by milling the contour into two rectan-gular blocks of PMMA, which were bolted together.To ensure tightness, a rubber seal in a groove wasused, and the alignment was guaranteed by dovel pins.Separating the nozzle allowed scanning the contourline in between experiments to track the changes ingeometry, as described in sec. 3.The nozzle geometry is shown in fig. 1, and identicalto the one presented in [5]. The inlet directly attachesto the inlet pressure volume and has a diameter of50mm. Both the converging and the diverging partare conical, with half angles of 15° and 10° respectively.The throat was 70mm after the inlet and was 20mm indiameter initially. The contacts are copper-tungstenrods with 5mm diameter, which were at the indicatedaxial positions of 40mm and 120mm.145L. S. J. Bort, V. Freiermuth, C. M. Franck Plasma Physics and Technology3. MethodThe ablation of nozzle wall material was determinedusing three different methods. First, the two halveswhere weighed with a scale, with which the mass couldbe determined with ±10mg accuracy, giving ±20mgcertainty for the complete nozzle.Secondly, the two halves were scanned with an offthe shelf flatbed scanner. Assuming cylindrical sym-metry, the change of the contour and the density ofPMMA (1.188mgmm−3) can be used to calculate theablated mass. Additionally, this method shows theaxial location of the ablation. The scanner used hada resolution of 4800 dpi, which gives a theoretical res-olution of 5 µm per pixel. The gray scale images werealigned, cropped and converted to a binary image first,and the contour of the wall was extracted afterwardsusing MATLAB. Figure 1 shows one example imagebefore processing and the result.As a third method, the inner contour of the nozzlewas determined by a coordinate measurement ma-chine (CMM) Mitutoyo KN815, which is usually usedto compare workpieces to the specifications. Thismachine determined the radius every 5mm along theaxial direction, by measuring 8 points along the cir-cumference. Since the machine was not available atthe laboratory, it was only feasible to measure eachnozzle before use, and two of the three after the com-plete series of experiments. The accuracy of thesemeasurements was given by the roundness of the cir-cles, which was about 20–40 µm before and 30–100 µmafter all the experiments.To create the test currents, the FPDCS currentsource as described in [6] was used. As shown infigure 2, each experiment consisted of two phases:During the pre-current phase, only a small currentis flowing while the downstream contact moves fromthe closed position to the open position. This way,an arc is established. Towards the end of this phase,the current is slightly increased and the valves ofthe blowing system are opened. As soon as the gasflow has reached a steady state and the contact hasstopped its movement at the final position, the maincurrent phase is started. During this time, the currentis increased and held at its set value for 10ms. Allresults that are shown in section 4 are calculated onlyfrom the values of the main current phase.After each measurement, the nozzle halves wereweighed and the contour was scanned with the flatbedscanner, to determine the ablation. Two measure-ments per current setting were done to ensure repeata-bility, while keeping the total number of experiments -and therefore ablation - for each nozzle low, to ensurethe geometry is as similar as possible for all tests.For the same reason, the measurements were done inascending order, starting at 300A up to 1400A. Themeasurements with 100A and 200A however weredone afterwards, when it became clear that they aredesired to complete the data set. Since the ablationvalues of 300A were already at the edge of the mea-Time (ms)0 20 40 60 80 100Current(A)0200400600800 CurrentPre-current periodMain Current periodFigure 2. Example current waveform. Start and endof contact movement are indicated with vertical dashedlines, start and stabilization of blowing gas flow withvertical dotted lines. The 85ms of pre-current wereidentical for all experiments, followed by 10ms of maincurrent, 800A in this case.surement range, the 100A and 200A tests were donewith an adapted current shape. The inductance ofthe current source was increased, to reduce rippleand prevent the current to go to zero. Additionallythe duration of the main current phase was increasedfrom 10ms to 40ms, to increase the amount of abla-tion towards measurable values. During the completemeasurement series, the throat diameter increased bya maximum of 10%, which did not change the arcvoltage measurably. This was confirmed by repeatingthe first test at the very end.4. ResultsAccording to the considerations described in [5] and[7], the voltage and current data of the high currentphase was evaluated using the median value insteadof the mean, to be more robust against fluctuations.The error bars in figures 3 and 4 are indicating the15.9 and 84.1 percentiles. Those percentiles give mean-ingful results even if the voltage fluctuations are notgaussian, and equal the standard deviation if they are.Figure 4 shows the du/di behavior is more favorablefor higher blow pressures. For 4.5 bar, the voltagecurve is falling up to about 400A, and is slightly ris-ing until it becomes flat above. For 8.4 bar the risingpart is less pronounced and for 12.7 bar the risingpart is not observed anymore. The curve graduallytransitions into a flat regime around 800A to 1000A.Additionally, the overall arc voltage and arc power ishigher for higher blow pressures, as shown in fig. 3.Table 1 shows the mass that was ablated during theentire measurement series, for all three pressures. TheCMM measurements agree very well with the weightmeasurement whereas the flatbed scanner results over-estimate the mass loss by about 10%. Figure 7 showsthe nozzle geometry change of the 12.4 bar series, asdetermined by the 29 measured circles of the CMM,and the scan. It can be seen that upstream of thethroat (< 70mm), the scan overestimates the ablation,whereas downstream it shows less in comparison with146vol. 4 no. 2/2017 Influence of Ablation on Differential Arc ResistanceCurrent (A)0 100 200 300 450 550 650 800 1000 1200 1400Power(kW)020040060080010001200ph = 4.5 barph = 8.4 barph = 12.7 barFigure 3. Arc power during the main current phase asfunction of current. Errorbars indicate the 15.9 and84.1 percentiles.Scale Scan CMM4.5 bar 5.43 g 5.56 g n/a8.4 bar 4.05 g 4.41 g 4.14 g12.7 bar 3.38 g 3.72 g 3.35 gTable 1. Sum of the ablated mass of all the experimentsfor each nozzle, determined by weighing, the scans andthe CMM. Scale inaccuracy is at least ±20mg.the CMM. This means the overestimation in the up-stream section has to be higher than 10%. The datafor the individual currents was only recorded with thescale and the scan, and indicates that the scanningmethod has an uncertainty around ±100mg. Sincethe scale had an accuracy of ±20mg, the values fromweight measurements were used for figures 5 and 6.Figure 5 clearly shows that the mass loss due to abla-tion is increasing with current after a certain thresholdaround 350A, and higher blow pressure reduced theamount of ablation for all currents.A more general picture is obtained if the ablatedmass is normalized to the total energy the arc dis-sipated during the high current phase, as shown infigure 6. This especially allows a comparison of testswith low current but long duration with tests of highcurrent and short time. Experiments with 300A for40ms are included, which had a similar specific abla-tion. Figure 6 shows even more clearly, that for thelower blow pressures the ablation increases sharplyabove a threshold current. Qualitatively this is simi-lar to the results of Seeger [8]. Comparing the resultsquantitatively is not possible though, due to the dif-ferent materials and gases.5. DiscussionThe scanning technique produced valuable results,even though the theoretical accuracy was not achieved.It was not possible to detect the edge of the contour onthe scans with single-pixel resolution, since the edgeCurrent (A)0 100 200 300 450 550 650 800 1000 1200 1400Voltage(V)050010001500ph = 4.5 barph = 8.4 barph = 12.7 barFigure 4. Arc voltage as function of current. Themarkers show the median, the errorbars indicate the15.9 and 84.1 percentiles.Current (A)0 100 200 300 450 550 650 800 1000 1200 1400MassLoss(g)00.10.20.30.40.50.60.70.8ph = 4.5 barph = 8.4 barph = 12.7 barFigure 5. Ablated mass as a function of current. Ini-tially all experiments with currents from 300A to1400A used 10ms of main current. Additional ex-periments with 300A and below were performed, usinglonger periods of main current, but ablation was stilltoo low to be measured accurately.is visible as a brightness gradient of roughly 20 px,or 100µm. To improve the edge detection accuracy,painting the surface with different marker pens wasconsidered, but all inks that were tried did not increasethe detection accuracy by much. Other materials orchanges to the background or lighting might be ableto improve this technique. Additional to the edgedetection uncertainty, the contour determined by thescan significantly overestimated the change of theradius around the throat compared with the CMM, asshown in figure 7. This is attributed to the fact thatthe gap where the two nozzle halves are joined is notperfectly gas-tight. The tests with 800A and aboveshowed erosion on the mating surface, up to to thesealing cord groove, which was likely caused by hot gasentering the gap. This erosion most probably erodedthe contour edge as well, which means the assumptionof cylindrical symmetry does not hold perfectly.147L. S. J. Bort, V. Freiermuth, C. M. Franck Plasma Physics and TechnologyCurrent (A)0 200 400 600 800 1000 1200 1400Speci-cAblation(mg/kJ)020406080100ph = 4.5 barph = 8.4 barph = 12.7 barFigure 6. Ablated mass normalized to the energy dis-sipated by the arc during the main current phase, asfunction of current. Below 350A, the ablation rate isabout as low as the measuring inaccuracy, above thatit rises with current.Axial Position (mm)0 20 40 60 80 100 120 140Radiuschange(mm)00.20.40.60.811.2ScanCMM PointsInterpolationThroat positionFigure 7. Total change of geometry (see fig 1) dueto ablation. Determined by the flatbed scans and thecoordinate measurement machine, after the entire mea-surement series for the 12.4 bar nozzle.The results of the scans and the CMM (see fig. 7)both show the ablation as a function of axial posi-tion. Ablation starts at the position of the upstreamcontact (40mm), increases towards the throat posi-tion (70mm) and sharply decreases afterwards. Inthe diverging part of the nozzle, from 80mm up tothe downstream contact at 120mm the amount ofablation is much smaller than in the upstream sec-tion. One possible reason for this could be higher arcpower in the upstream section. Alternatively, the per-centage of the dissipated energy arc that contributesto ablation compared with other cooling mechanismsis higher upstream. Higher blow pressures leads tohigher arc power and lower ablation rate at the sametime. This means, with increasing blow pressure theoverall share of the ablation as cooling mechanism isreduced. However the present data does not allow fora quantification or explanation of this effect, since toomany factors are still unknown. One of them is thecombustion of PMMA with the oxygen of blowing air.According to [9], combustion of 1 g of PMMA yieldsa similar energy than what is required to vaporize it.This means, it is not possible to determine the energybalance of the ablation properly as long as air is used.6. ConclusionsThe methods described in section 3 worked with anaccuracy as expected. Higher weighing accuracy ishard to achieve since it involves including effects likechanges in PMMA density due to humidity, or changesin buoyancy due to changes in air pressure. The scan-ning method could probably be improved by enhanc-ing the scan quality and the edge detection. Theresults shown in this paper agree well with previousfindings [3], which indicate that increasing blow pres-sure is shifting the u(i) curve towards more favourableconditions for passive oscillation breakers. Addition-ally it can be seen qualitatively that the thresholdcurrent above which ablation starts (300A to 500Aaccording to figure 5) coincides with the point wherethe u(i) changes to unfavorable (flat/rising) condi-tions (see fig. 4). If this is combined with the fact thatthe share of ablative cooling is increasing in the sameregion, the onset of significant ablation is directlycorrelated with the u(i) curve getting flat.AcknowledgementsThe authors would like to thank ABB Switzerland forfinancial support, as well as E. Panousis of ABB CRCSwitzerland for the frequent discussions.References[1] B. Bachmann et al. Development of a 500kV AirblastHVDC Circuit Breaker. IEEE Trans. on PowerApparatus and Systems, PAS-104(9):2460–2466, 1985.doi:10.1109/TPAS.1985.318991.[2] T. Schultz et al. Circuit Breakers for Fault CurrentInterruption in HVDC Grids. In VDE-FachtagungHochspannungstechnik, 2016.doi:10.3929/ethz-a-010795297.[3] M.M. Walter. Switching Arcs in Passive ResonanceHVDC Circuit Breakers. PhD thesis, ETH Zürich, 2013.doi:10.3929/ethz-a-010112102.[4] L. Muller. Modelling of an ablation controlled arc. JPhys D: Appl Phys, 26(8):1253–1259, 1993.doi:10.1088/0022-3727/26/8/015.[5] L.S.J. Bort et al. Effects of nozzle and contactgeometry on arc voltage in gas circuit-breakers. In 2016IEEE Intern. Conf. on High Voltage Eng. and Appl.,2016. doi:10.1109/ICHVE.2016.7800700.[6] A. Ritter et al. Five years of pulsed current testing forHVDC switchgear. In 2016 IEEE Intern. Conf. on HighVoltage Eng. and Appl., 2016.doi:10.1109/ICHVE.2016.7800658.[7] H. Schmid et al. Measuring a Small Number of Samples,and the 3v Fallacy: Shedding Light on Confidence andError Intervals. IEEE Solid-State Circuits Magazine,6(2):52–58, 2014. doi:10.1109/MSSC.2014.2313714.[8] M. Seeger et al. Experimental study on PTFEablation in high voltage circuit-breakers. J Phys D:Appl Phys, 39(23):5016–5024, 2006.doi:10.1088/0022-3727/39/23/018.[9] B.T. Rhodes et al. Burning rate and flame heat fluxfor PMMA in a cone calorimeter. Fire Safety Journal,26(3):221–240, 1996.doi:10.1016/S0379-7112(96)00025-2.148

Influence of Ablation on Differential Arc Resistance

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Influence of Ablation on Differential Arc Resistance

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