Nowadays SF6 is commonly used as filling gas for medium voltage load break switches due to its outstanding insulating and arc-quenching properties. However, due to its high global warming potential the interrupting performance of alternative eco-friendly gases are investigated for such devices. This contribution investigates the thermal interruption capability of a medium voltage load break switch using two different oxygen-containing gas mixtures: dry air and a mixture of nitrogen, oxygen and carbon dioxide. The findings indicate an improved performance in comparison to pure nitrogen when considering an admixture of oxygen. In gas mixtures containing 80\% nitrogen, the addition of oxygen results in an enhanced thermal interruption capability compared to an equivalent proportion of carbon dioxide

Ballweber, T.

Castiñeira Suárez, Manuel

Ermeler, K.

Moser, A.

Schaak, M.

English

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

doi:10.14311/ppt.2023.2.107Plasma Physics and Technology 10(2):107–110, 2023 © Department of Physics, FEE CTU in Prague, 2023INFLUENCE OF OXYGEN-CONTAINING FILLING GAS MIXTURES ONTHE INTERRUPTION CAPABILITY OF MV LOAD BREAK SWITCHESM. Castiñeira Suáreza,∗, T. Ballwebera, A. Mosera, M. Schaakb,K. Ermelerca Institute of High Voltage Equipment and Grids, Digitalization and Energy Economics, RWTH AachenUniversity, Schinkelstr. 6, 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∗ m.suarez@iaew.rwth-aachen.deAbstract. Nowadays SF6 is commonly used as filling gas for medium voltage load break switches dueto its outstanding insulating and arc-quenching properties. However, due to its high global warmingpotential the interrupting performance of alternative eco-friendly gases are investigated for such devices.This contribution investigates the thermal interruption capability of a medium voltage load breakswitch using two different oxygen-containing gas mixtures: dry air and a mixture of nitrogen, oxygenand carbon dioxide. The findings indicate an improved performance in comparison to pure nitrogenwhen considering an admixture of oxygen. In gas mixtures containing 80% nitrogen, the addition ofoxygen results in an enhanced thermal interruption capability compared to an equivalent proportion ofcarbon dioxide.Keywords: load break switch, medium voltage, SF6 alternatives, thermal interruption capability.1. IntroductionMedium voltage (MV) load break switches (LBS) arefrequently used in ring main units (RMU) to interruptload currents up to 1.25 kA and serve as disconnectorswitches. To optimize space utilization, LBSs aretypically installed in enclosed compartments as gas-insulated switchgear (GIS). The prevalent choice forfilling gas has been SF6, owing to its outstandinginsulating and arc-quenching capabilities. However,SF6 is characterized by an exceptionally high globalwarming potential (GWP) of 23,500 times that ofcarbon dioxide (CO2) [1]. Atmospheric gases suchas nitrogen (N2), mixtures of N2 and oxygen (O2) orCO2 have a lower dielectric strength compared to thatof SF6 [2]. Consequently, the design of switchgearutilizing these gases must be appropriately adaptedto compensate for the lower insulation performance.This can be accomplished through increased enclosurepressure and/or insulation distance [3].Existing research studies have shown good inter-ruption capability when utilizing mixtures of N2 andCO2 in combination with arc blowing through a poly-mer nozzle [4–6]. However, soot formation could beobserved after conducting the experimental investi-gations. This issue has also been encountered whenmixing perfluoronitriles or perfluoroketones with CO2.In such cases, the admixture of a small percentageof O2 allows minimizing the generation of solid by-products such as carbon deposits [7, 8]. The sameapproach is worth investigating for N2–CO2 mixtures,but it is essential to ensure that the resulting gasprovides a sufficient interruption capability.The present study aims to investigate the thermalinterruption capability of a MV-LBS based on the de-sign proposed by [5], with the incorporation of an O2admixture. Specifically, two gas mixtures are investi-gated, N2–O2 and N2–O2–CO2 with ratios of 80/20and 80/10/10 respectively. In assessing the thermal in-terruption capability of different gases, the arcing timeconstant τarc serves as an indicator [9]. This param-eter represents the time constant that characterizesthe change in arc voltage resulting from a step cur-rent [10]. As shown in Table 1, O2 exhibits a smallerτarc compared to that of N2 and CO2. This suggeststhat the addition of O2 in the gas mixture may resultin improved performance compared to some of thepreviously studied mixtures.Gas Time constant [µs]N2 210CO2 15O2 1.5Air 20Table 1. Arc time constants for 1 A arc [10, 11]2. MethodThe interruption process in a LBS can be divided intotwo stages, namely the thermal and dielectric phases[10, 12, 13]. The present work focuses on the thermalphase.107M. Castiñeira Suárez, T. Ballweber, A. Moser et al. Plasma Physics and Technology2.1. Thermal interruption capabilityWhen the LBS is opened, an electric arc is ignitedacross its contacts. In case of a successful interruptionthe arc is extinguished at the current zero (CZ) cross-ing, and the recovery voltage (RV) builds up acrossthe open contacts. The thermal phase takes placeimmediately or a few microseconds after CZ [10]. Theresidual conductivity in the switching gap results ina small post-arc current (PAC) as the RV rises. Fora successful interruption, cooling power must exceedthe power input of the PAC. In gas switches, this isaccomplished by blowing cool gases into the switch-ing chamber. If the cooling power is insufficient, theconductivity and PAC increase lead to a thermal re-ignition [13]. In this contribution, the accuracy of thecurrent measurement is not sufficient for examiningthe PAC.The investigation method, presented in detail in [4],is summarized in the following. The current steepnessshortly before CZ (di/dtCZ) is used to quantify thethermal interruption capability of the LBS.Figure 1 shows the test circuit used for the investiga-tion of the thermal interruption capability, consistingof two resonant circuits. Initially, the high-currentcircuit is used to generate a 50 Hz sinusoidal currentrepresenting the load current Ihc. The injection cir-cuit provides a higher frequency current Iinj. Shortlybefore CZ, the thyristor of the injection circuit is trig-gered and Iinj is superimposed to Ihc. This approachallows the investigation of varying di/dtCZ indepen-dently of the load current, by adjusting the chargingvoltage of CV . If the arc is extinguished at the firstCZ of Iinj, the RV builds up as Iinj charges the ca-pacitor Cp through the resistor Rp. Contrarily, if thearc is re-ignited a second oscillation of the Iinj flowsthrough the open contacts of the LBS. To ensure therate of rise of recovery voltage (RRRV) characteris-tic of a 24 kV grid at 50 Hz when setting the currentsteepness of a typical load current of 630 A, a resistorRp = 220 Ω is chosen. Figure 2 shows a comparisonof voltage and current for a current interruption inthe first and second CZ respectively.High Current CircuitLC MS ASRELBSCpRpInjection CircuitLVCVCCFigure 1. Synthetic test circuit for investigation ofthermal interruption capability [5].At the beginning of a test series a low di/dt is setby using a small charging voltage for CV and fivetests are conducted. If all five tests result in a suc-cessful interruption, the charging voltage is increasedby ∆U = 500 V and another series of five tests is per-formed. This procedure is repeated until a chargingIinjIhc + IinjUU, I U, Itime timeFigure 2. Current and voltage in the vicinity of CZduring thermal interruption test.voltage level is reached where at least one test resultsin a failed interruption. The critical current steepnessat CZ crossing (di/dtcrit) is determined as the meanvalue between the lowest di/dtCZ corresponding toa failed interruption and that of the next successfulinterruption below. This value defines the limit forsafe operation of the LBS.2.2. Model switchA simplified cross-sectional view of the model switchis depicted in Figure 3. The switch consists of a pin-tulip contact system made of a tungsten-copper (WCu)alloy with 80/20 weight ratio. The pin contact hasa diameter of dc = 10 mm. Surrounding the contactsystem is a polymeric nozzle made of PTFE, with athroat diameter of dn = 11 mm and a throat lengthof ln = 24 mm. These dimensions have been derivedas part of a parameter study presented in [5].dnlndcPin contactNozzleTulip contactBlowingFigure 3. Section of the model load break switch [4, 6].The contact system and the nozzle are enclosed ina pressurized main vessel with a volume of V = 150 lat 1.8 bar of absolute pressure. An external tankis used to provide the blowing pressure for coolingthe arc. The main vessel and the external tank areconnected via a magnetic valve that is opened shortlybefore the contacts start separating to initiate thegas flow. The pressure inside the blowing tank isadjusted accordingly to investigate different blowingpressure levels. The pressure is measured upstreamthe contact system, behind the magnetic valve. Apneumatic drive with a total stroke of 107 mm is usedto achieve the opening motion of the contact system.The mean opening speed is set to vmean = 5 m/s. Alaser sensor measures the position of the pin contactthroughout the opening motion.Figure 4 displays typical voltage and current val-ues measured during an exemplary interruption test108vol. 10 no. 2/2023 Influence of oxygen-containing filling gas mixtures in MV-LBSusing the provided setup, as well as the travel signalrepresenting the separation of contacts.Figure 4. Typical current, voltage and travel signalsfor an exemplary interruption test (top) and recoveryvoltage (bottom).3. Results and discussionFigure 5 presents the critical current steepness of thetwo gas mixtures investigated in this study, namelyN2–O2 (80/20) and N2–O2–CO2 (80/10/10), as a func-tion of blowing pressure. The investigation has beenperformed with the same test setup and conditions as[6]. In order to compare the results to the gas mix-tures previously studied, the figure includes resultsfrom the mentioned paper. Specifically pure N2 andN2 with admixtures of 20%, 40% and 60% CO2. Linesconnecting the data points are drawn for visualizationand do not correspond to actual data.The two mixtures investigated in this study show alinear increase in di/dtcrit with blowing pressure. Thistendency goes in line with the results obtained forN2–CO2. Similarly, mixtures containing 60% of N2 ormore resulted in a linear behavior. The admixture ofhigher percentages of CO2 contributed to an exponen-tial increase. In the two mixtures containing O2, therequired di/dtCZ of 0.28 A/µs (di/dttarget) for a LBSwith a rated load current of 630 A and a voltage levelof 24 kV at 50 Hz is achieved with a blowing pressureof 660 mbar. Both mixtures exhibit improved thermalinterruption capability compared to pure N2, but areoutperformed by mixtures containing 40% CO2 ormore.Moreover, the N2–O2 (80/20) mixture results in abetter interruption capability than the gas mixturecontaining 10% CO2. The difference in performancecould be attributed to the τarc of O2 and CO2. Asmall τarc indicates a faster change in temperature ofthe arc [10]. The higher concentration of O2, with aτarc ten times shorter compared to CO2, may result in[6][6][6][6]Figure 5. Critical current steepness for various gasmixtures and blowing pressures.a faster reduction of the arc conductance. This couldalso explain why the addition of a 20% CO2 has aminor effect on the interruption capability comparedto pure N2, whereas the admixture of the same per-centage in O2 results in a significant improvement inperformance.Nevertheless, τarc is not sufficient as a quantifica-tion parameter for the interrupting capability of aparticular gas. As indicated in Table 1, both CO2and air exhibit comparable τarc. However, existingresearch shows that pure CO2 leads to a significantlyhigher di/dtcrit compared to the results obtained inthis contribution for air [13]. Furthermore, it must beconsidered that τarc is dependent on the current mag-nitude [10]. The investigations presented in this workare performed using a 50 Hz current with Irms = 630 A,whereas the values presented in Table 1 were obtainedfor 1 A arcs.As shown in Figure 4 (top), a steep voltage increasetakes place shortly before CZ as part of the extinctionpeak. The successful current interruption relies on theformation of a high extinction peak and thus on thecooling before CZ [13]. Therefore a comparison of theextinction peak voltage for both gases investigated isof interest.Figure 6 shows the extinction peak in dependenceof current steepness. The results are grouped intosets of four data points within a 5 mA/µs range. Thecircles represent the mean extinction peak voltage foreach set and the horizontal markers the maximum andminimum values. The extinction peaks are shown forinterruption tests performed with a blowing pressureof pB = 330 mbar (top) and pB = 660 mbar (bottom).For 330 mbar, higher extinction peak voltages canbe observed for N2–O2 in the current steepness rangewhere the di/dtcrit of the worse performing gas isfound (di/dt ≈ 0.14 A/µs). However, since the scat-tering of the voltages measured for both gases overlap,no clear relation between extinction peak and di/dtcritcan be derived for this pressure level. This could ex-plain the small difference in di/dtcrit of both gases for109M. Castiñeira Suárez, T. Ballweber, A. Moser et al. Plasma Physics and TechnologyFigure 6. Extinction peak voltage using a blowingpressure of 330 mbar (top) and 660 mbar (bottom). Redcrosses and plus signs correspond to individual testsresulting in failed interruption for N2–O2 and N2–O2–CO2 respectively.this pB value.For pB = 660 mbar there is a significant differ-ence in the extinction peak voltage measured forboth gases. The mean of the voltages measured forN2–O2 is approximately 100 to 130 V higher untildi/dt ≈ 0.32 A/µs. The lower extinction peak mea-sured for N2–O2–CO2 could be related to a higherarc conductivity shortly before CZ and thus a worsecooling performance, which could explain the lowerdi/dtcrit.The incorporation of O2 into the gas mixture pro-vides beneficial effects in the thermal interruptioncapability. Nevertheless, it is important to considerthat O2 exhibits strong oxidizing properties [8]. Thepotential drawbacks associated with its oxidizing na-ture should be carefully evaluated before implementingsuch mixtures in practical applications.4. ConclusionThis study presents an investigation on the thermalinterruption capability of a model LBS using two gasmixtures containing O2: N2–O2 (80/20) and N2–O2–CO2 (80/10/10). The influence of blowing pressurecan be examined through implementation of an ex-ternal vessel. The results indicate that the additionof O2 improves the interruption capability comparedto pure N2 or N2 with comparable CO2 percentages.The gas mixtures containing 40% CO2 or more resultin superior performance. Substituting CO2 with anequivalent ratio of O2 may potentially enhance theinterruption capability further. However, it is cru-cial to consider the long-term effects of O2 due toits strong oxidizing properties, which could pose chal-lenges in practical applications. Further investigationof the dielectric recovery phase is necessary to evaluatethe performance of these gases during the completecurrent interruption process.References[1] G. Myhre et al. Anthropogenic and natural radiativeforcing. In Climate Change 2013: The Physical ScienceBasis. Contribution of Working Group I to the Fifth ARof the IPCC. Cambridge University Press, 2013. ISBN978-92-9169-138-8.[2] D.-Y. Lim and S. Bae. Study on oxygen/nitrogen gasmixtures for the surface insulation performance in gasinsulated switchgear. IEEE Transactions on Dielectricsand Electrical Insulation, 22(3):1567–1576, 2015.doi:10.1109/TDEI.2015.7116352.[3] L. Niemeyer. A systematic search for insulation gasesand their environmental evaluation. In GaseousDielectrics VIII, pages 459–464. Springer-Verlag, 1998.ISBN 978-0-306-46056-2.[4] M. Bendig et al. Investigations on the effect of thenozzle material on the interruption capability of amedium voltage load break switch. Plasma Physics andTechnology, 6(1):15–18, 2019.doi:10.14311/ppt.2019.1.15.[5] M. Bendig and M. Schaak. Design rules forenvironmentally friendly medium voltage load breakswitches. IEEE Transactions on Power Delivery,36(5):1–9, 2020. doi:10.1109/TPWRD.2020.3024983.[6] T. 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Influence of Oxygen-containing Filling Gas Mixtures on the Interruption Capability of MV Load Break Switches

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Influence of Oxygen-containing Filling Gas Mixtures on the Interruption Capability of MV Load Break Switches

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