Study on the Effect of Helium on the Dielectric Strength of Medium-Voltage Vacuum Interrupters

energies

Article Study on the Effect of Helium on the Dielectric Strength of Medium-Voltage Vacuum Interrupters

Paweł W˛egierek* , Michał Lech , Damian Kostyła and Czesław Kozak

Faculty of Electrical Engineering and Computer Science, Lublin University of Technology, Nadbystrzycka 38A, 20-618 Lublin, Poland; [email protected] (M.L.); [email protected] (D.K.); [email protected] (C.K.) * Correspondence: [email protected]; Tel.: +48-81-538-4347

Abstract: This paper presents the results of a comparative analysis of the dielectric strength of

disconnecting vacuum interrupters operating on air and helium. The breakdown voltage Ud was measured in the pressure range from 8.0 × 10−4 Pa to 3.0 × 101 Pa for air and from 8.0 × 10−4 Pa to 7.0 × 102 Pa for helium, while varying the interelectrode distance from 1.0 to 5.0 mm. Dedicated laboratory workstations were used to determine the actual pressure values in the vacuum interrupters tested and to precisely measure and record the dielectric strength results of the test object. It was found that the helium-filled vacuum interrupter maintains its full dielectric strength in significantly larger pressure ranges, while the air-filled vacuum interrupter loses its insulating properties. Thus, it is possible to make vacuum interrupters based on the working medium associated with pure helium, with larger working pressure ratings. Under such conditions, it is easier to maintain the tightness of the device and to limit cut-off currents and overvoltages associated with vacuum switchgear.

Keywords: vacuum breakdown; vacuum technology; vacuum interrupter; helium; switchgear

1. Introduction Citation: W˛egierek, P.; Lech, M.; 1.1. Background and Motivation Kostyła, D.; Kozak, C. Study on the Effect of Helium on the Dielectric The continuously growing demand for electricity, as well as the poor technical condi- Strength of Medium-Voltage Vacuum tion of power distribution networks, force the construction of new, more technologically Interrupters. Energies 2021, 14, 3742. advanced power networks with better operating parameters, necessary for the reliable https://doi.org/10.3390/en14133742 transmission and distribution of electricity to the end customers [1,2]. Switching devices in newly designed medium voltage (MV) networks are devices of open or closed construction. Received: 28 May 2021 Sulfur hexafluoride (SF6) and vacuum are mainly used as insulation media [3–6]. Interest in Accepted: 18 June 2021 vacuum technology in recent years is related to its lack of harmfulness to the environment. Published: 22 June 2021 It is a completely neutral medium for the environment, unlike sulfur hexafluoride SF6. When comparing SF6 gas to other working media used in electric power engineering, Publisher’s Note: MDPI stays neutral special attention should be paid to the environmental aspect. SF6 belongs to the so-called with regard to jurisdictional claims in F-gases, i.e., fluorinated greenhouse gases, which, when present in the atmosphere, have published maps and institutional affil- the effect of raising its temperature. According to the International Panel on Climate iations. Change (IPCC) and the U.S. Environmental Protection Agency (EPA), sulfur hexafluoride has been recognized as a compound that contributes to the creation of the greenhouse effect. The Global Warming Potential (GWP), which is an indicator of the greenhouse effect of a substance, for SF6 is over 22,000, an indicator that compares the amount of heat retained Copyright: © 2021 by the authors. by an adequate mass of carbon dioxide, for which the GWP is 1. GWP values for selected Licensee MDPI, Basel, Switzerland. substances are shown in Table1. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Energies 2021, 14, 3742. https://doi.org/10.3390/en14133742 https://www.mdpi.com/journal/energies Energies 2021, 14, 3742 2 of 14

Table 1. Selected chemicals and their GWP100.

Substance GWP100

Carbon dioxide CO2 1 Methane CH4 23 Nitrous oxide N2O 296 Carbon tetraﬂuoride CF4 5700 Freon R-12 (CFC-12) 10,600 Sulfur hexaﬂuoride SF6 22,200

The harmfulness of sulfur hexafluoride use has not been overlooked by many world organizations and institutions. The main documents dealing with this issue are the Mon- treal (1987) and Kyoto (2005) Protocols, which deal with the subject of preventing the formation of the ozone hole. Another document is the European Union Regulation on F-gases, which assumes the reduction of emissions of this type of substances by about 75% by 2050 as compared to 1990 [7–9]. Vacuum insulation, on the other hand, despite the fact that it is not harmful to the environment, has several significant disadvantages that make it difficult to use reliably. These include significant values of cut-off currents and the problem of overvoltages [10], as well as the difficulty of maintaining the integrity of equipment despite its large size. Taking into account the advantages and disadvantages of the used insulating media, scientists are currently working on various types of gas mixtures in order to find a compromise between proper insulating parameters and lack of harmfulness to the environment [11–14]. Such gas mixtures are often enriched with elements of chemical gases, referred to as electronegative [4].

1.2. Electronegative Gases as an Insulating Medium Electronegative gas is defined as a gas that shows a tendency to attach electrons, resulting in long-lived negative ions. In order for the reaction to take place, it is necessary that the gas types to be attached have a positive affinity for electrons (>0 eV) [15]. When analyzing electrical discharges in electronegative gases, it is necessary to take into account the so-called capture coefficient η, which reduces the value of the primary (electron) collision ionization coefficient α. This is due to the fact that the intensity of the electron ionization phenomenon decreases as a result of the binding of free electrons, especially by the particles of electrified gases. Therefore, the condition for spontaneous discharge in electronegative gases has been defined as follows [16]: η γ[exp(α − η)d − 1] = 1 − (1) α where: γ—second Townsend coefficient, α—primary (electron) collision ionization coefficient, η—capture coefficient and d—electron avalanche path. Several types of electron capture can be distinguished [17,18]: - capture in two-body processes (resonant and dissociative), - capture in three-body processes. Resonant excitation is described by the following reaction:

e− + M → M− (2)

Dissociative uptake, on the other hand:

e− + M → A− + B (3)

where: M—gas atom or molecule, and A, B—decay fragments. The above reaction is fulfilled by oxygen, sulfur hexafluoride, freon, carbon dioxide and fluorocarbons among others. In these gases, “A−” is a sulfur or carbon atom, while “B” Energies 2021, 14, 3742 3 of 14

is an oxygen atom or one of the halogen atoms or molecules [19,20]. The quantity called capture frequency R for two-body processes has also been described in a mathematical way:

R = vσn = kn (4)

where: v—electron velocity, σ—active cross section, n—density of electronegative molecules, and k—capture frequency constant. In three-body processes, electron binding occurs in a two-step manner. When an electron collides with an electronegative molecule, an excited negative ion is formed, which then undergoes spontaneous decay:

e− + M → M∗− (5)

M∗− → M + e− (6) The disappearance of free electrons initiates the process of creating negatively charged ions, called electron capture or electron binding. The process for both cases is as follows: the liberated free electron binds to an inert atom or molecule of the ground state gas. There are exceptions, however, because the atoms of some elements may tend to bond electrons. Gas dielectrics that are substitutes for sulfur hexafluoride are much more difficult to find than they may appear after initial evaluation. This is because there are many requirements that a potential substitute must meet, and many tests and examinations of the selected gas are required, which are necessary to release it for use. For example, the gas must exhibit a high dielectric strength, which means that the gas must be electronegative. It is worth noting, however, that electrically conductive gases are usually toxic, chemically reactive and harmful to the environment; they also have low vapor pressure, and their decomposition products during gas discharges are extensive and often unknown. On the other hand, it is important to note that non-electronegative gases, which are not environmentally hazardous, usually have low dielectric strength. An example is nitrogen (N), which has a dielectric strength about three times lower than the popular SF6; moreover, it does not have the properties necessary for its application in switchgear. However, very importantly, such gases can prove themselves as admixtures supporting other substances and can find applications in vacuum-based apparatus [21]. The dielectric strength of an insulating system can be increased by incorporating electronegative gases and increasing the gas pressure. These factors must be taken into account when designing the insulation; however, it is very rare to design apparatus where in the gas pressure exceeds 1.0 MPa. One of the gases often used as an admixture to SF6 gas is helium (He). It is a chemical element from the helium group, noble gases, with an atomic number of 2. Helium, being one of the media corresponding to the policy of the world countries defined in the Kyoto Protocol, has been specified as one of the enrichment gases for gas mixtures intended for switching and distribution apparatus responsible for electric arc extinguishing [22]. Anticipated current and future research directions related to the application of gas mixtures (also containing helium) in switchgear are presented in Table2. Energies 2021, 14, 3742 4 of 14

Table 2. Predicted directions of research for gas mixtures used in power engineering [23,24].

Application for Electric Speciﬁcation Application in Insulation Arc Extinguishing

Gases used today 40% SF6 + 60% N2 40% SF6 + 60% N2 and mixtures thereof 50% SF6 + 50% N2 50% SF6 + 50% N2

pure nitrogen (N2) under high pressure, Directions for current ongoing research SF6 + He low concentration of SF6 and pure nitrogen

CO2 SO 2 SF +Ar N O 6 2 SF +CF N +SO 6 4 Forecasted research directions 2 2 SF +C F N +c-C F 6 2 6 2 4 8 SF +N +He SO +SF 6 2 2 6 SF +N +ArHe+electronegative gases SO2+c- 6 2 C4F8SF6+CO2

Mixtures using helium have been the subject of a number of different studies over the years, but they have been for high pressures or other energy industries [25–27]. The design of gas insulating systems with electronegative properties should take into account the electron swarm parameters of the gases, such as the electron attachment and electron- scattering cross-sections. The electron attachment cross-section determines the electron- capture capacity of the gas, which depends mainly on the electron energy. The more efficient the electron capture, the better the insulating properties of the gas. Therefore, it is crucial to select an insulating gas that has a large electron-attachment cross-section value. In contrast, the electron-scattering cross-section determines the ability to reduce the kinetic energy of electrons accelerated by an electric field applied to the system. As electron attachment is more effective than electron ionization from an insulating angle, the use of an electronegative gas is usually sufficient in the design method of insulating systems. Nevertheless, to obtain higher dielectric strength of the insulation, the mentioned electron-swarm parameters cannot be neglected [4,28,29]. From Table2, it can be seen that much of the research on helium has been and is being conducted to form mixtures with sulfur hexafluoride (SF6), which is still present in the insulation despite its reduced content. Studies of the dielectric strength of vacuum interrupters conducted by the authors of this paper are based on the use of helium only, which is a naturally occurring element in the atmosphere, which works in favor of the climate [20,23].

2. Research Stands, Materials and Methods The research object in conducted research on dielectric strength was a medium-voltage vacuum interrupter of a special design intended to work in overhead switch disconnectors (Figure1). Vacuum interrupters of this type are used to connect current circuits in modern MV switching apparatus of closed construction. They consist mainly of two types of poles: movable and immovable, elastic bellows and a metal condensation screen. The contacts are made of WCu material, an alloy of tungsten (W) and copper (Cu) in a ratio of 70% to 30%. The characteristic technical parameters of the tested vacuum interrupter are shown in Table3. Energies 2021, 14, 3742 5 of 14 Energies 2021, 14, x FOR PEER REVIEW 5 of 14

FigureFigure 1. 1.Research Researchobject—MV object—MV vacuum vacuum interrupter interrupter of of special special design: design: 1—movable 1—movable contact contact adapted adapted to mountingto mounting the interrupterthe interrupter in a laboratory in a laboratory stand, 2—fixedstand, 2— contactfixed withcontact connection with connection for pumping for channel.pumping channel. Table 3. Technical parameters of MV vacuum interrupter used in this study. Table 3. Technical parameters of MV vacuum interrupter used in this study. Type of Parameter Parameter Value Type of Parameter Parameter Value Rated frequency 50 Hz Rated frequency 50 Hz Rated voltage 24 kV Rated impulseRated withstand voltage voltage 12524 kV kV RatedRated impulse continuous withstand current voltage 400125 A kV RatedRated 1s withstandcontinuous current current 16400 kA A RatedRated peak 1s withstandwithstand current current 4016 kA kA Rated short-circuit making current 16 kA Breaking currentRated peak in circuit withstand with low current inductance 40040 A kA RatedMechanical short-circuit durability making current 200016 cycles kA Breaking current in circuit with low inductance 400 A The ﬁrst stageMechanical of work, durability prior to the target dielectric strength2000 tests cycles of the vacuum interrupter, was to determine the actual value of the pressure prevailing in the test facility duringThe the first measurements. stage of work, The prior need to the for target this was dielectric due to strength two reasons. tests of First, the duevacuum to the in- highterrupter voltage, was used to indetermine the tests, the the actual direct value connection of the ofpressure the measuring prevailing head in tothe the test vacuum facility interrupterduring the undermeasure testments. could The have need resulted for this in damage was due to to the two measuring reasons. head,First, due such to as the by high the occurrencevoltage used of anin electricalthe tests, breakdownthe direct connection in the vacuum of the gauge. measuring On the head other to hand, the vacuum measuring in- theterrupter pressure under directly test at could the vacuum have resulted pump setin damage would not to bethe reliable measuring due tohead the, pressuresuch as by drop the acrossoccurren thece pump of an channel: electrical between breakdown the vacuum in the pumpvacuum set gauge and the. On test the object. other Therefore, hand, meas- the author’suring the method pressure of determining directly at the actual vacuum pressure pump value set would in the tested not be vacuum reliable interrupter due to the waspressure developed. drop across the pump channel: between the vacuum pump set and the test object.For Therefore, the application the author of this’s method method, of adetermining prototype vacuum the actual interrupter pressure wasvalue designed in the tested and constructed,vacuum interrupter equipped was with developed. a special ferrule enabling the measuring head to be connected directlyFor to the the application interrupter (atof thethis level method of its, contacts),a prototype as wellvacuum as a connectioninterrupter for was connecting designed theand pumping constructed, channel equipped to it. The with vacuum a special interrupter ferrule is characterized enabling the by meas the sameuring geometrical head to be dimensionsconnected directly as the target to the test interrupter object, so (at the the pressure level of inside its contacts) corresponds, as well to theas a pressure connection in thefor vacuum connecting interrupter the pumping used in channel this research. to it. The The vacuum view of interrupter the completed is characterized prototype vacuum by the interruptersame geometrical and the dimensions complete laboratory as the target station test isobject, shown so in the Figure pressure2. inside corresponds to theThe pressure laboratory in stationthe vacuum consists interrupter of a set of used vacuum in this pumps research. (rotary The and view turbomolecular), of the com- apleted manual prototype dosing valve vacuum and two interrupte vacuumr gauges and the (VG1 complete and VG2). laboratory The system station was is connected shown in toFigure a laboratory 2. computer network, so that through the TPU control unit it is possible to controlThe the laboratory pumps and station to read consists and record of a set measurement of vacuum pumps results (rotary in real-time. and turbomolecu- The helium usedlar), ina manual the study dosing is contained valve and in atwo secured vacuum cylinder gauges with (VG1 avalve and VG2). and a The manometer. system was A schematicconnected of to the a laboratory laboratory computer station is shownnetwork, in Figureso that3. through the TPU control unit it is possible to control the pumps and to read and record measurement results in real-time. The helium used in the study is contained in a secured cylinder with a valve and a manometer. A schematic of the laboratory station is shown in Figure 3.

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(a) (b) (a) (b) Figure 2. Test stand: (a) prototype of a vacuum interrupter designed to determine the actual pres- Figure 2. Test stand:stand: ((aa)) prototypeprototype of of a a vacuum vacuum interrupter interrupter designed designed to to determine determine the the actual actual pressure pressure value in test objects (1—vacuum gauge connection, 2—pump channel connection), (b) view of valuesure value in test in objects test objects (1—vacuum (1—vacuum gauge gauge connection, connection, 2—pump 2—pump channel channel connection), connection), (b) view(b) view of the of the complete test stand. completethe complete test test stand. stand.

Figure 3. Schematic of the stand designed to determine the actual pressure value in the tested Figure 3. 3. Schematic of the stand designed to determine the actual pressure value in in the the tested tested vacuum interrupters (1—prototype vacuum interrupter, 2—vacuum gauge VG2, 3—manual dos- vacuum interrupters (1—prototype vacuum interrupter, 2—vacuum gauge VG2, 3—manual dos- vacuuming valve, interrupters 4—vacuum (1—prototype gauge VG1, 5 vacuum—turbomolecular interrupter, pump, 2—vacuum 6—rotary gauge pre VG2,-pump, 3—manual 7—gases dosing dosed ing valve, 4—vacuum gauge VG1, 5—turbomolecular pump, 6—rotary pre-pump, 7—gases dosed valve,to the 4—vacuuminside of the gauge tested VG1, object). 5—turbomolecular pump, 6—rotary pre-pump, 7—gases dosed to the to the inside of the tested object). inside of the tested object). The idea of determining the actual value of the pressure inside the vacuum inter- The ideaidea ofof determining determining the the actual actual value value of theof t pressurehe pressure inside inside the vacuum the vacuum interrupter interrupter under test was to measure the pressure drop between vacuum gauge VG2, underrupter test under was testto measure was to the measure pressure the drop pressure between drop vacuum between gauge vacuum VG2, mounted gauge at VG the2, mounted at the vacuum interrupter prototype, and vacuum gauge VG1, mounted at the vacuummounted interrupter at the vacuum prototype, interrupter and vacuum prototype gauge, and VG1, vacuum mounted gauge at theVG vacuum1, mounted pump at set.the vacuum pump set. With the results obtained, it was possible to determine the actual Withvacuum the results pump obtained, set. With it the was results possible obtained, to determine it was the possible actual pressure to determine value theinside actual the pressure value inside the target interrupter tested in a later stage. targetpressure interrupter value inside tested the in target a later interrupter stage. tested in a later stage. The target vacuum interrupter dielectric strength tests were performed using a The target target vacuumvacuum interrupter interrupter dielectric dielectric strength strength tests tests were were performed performed using using a dedi- a dedicated, proprietary test-stand in which the test object is mounted. The stand allowed cated,dedicated, proprietary proprietary test-stand test-stand in which in which the testthe objecttest object is mounted. is mounted. The The stand stand allowed allow theed the adjustment of the electrode spacing of the vacuum interrupter, as well as the reliable the adjustment of the electrode spacing of the vacuum interrupter, as well as the reliable

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adjustment of the electrode spacing of the vacuum interrupter, as well as the reliable and safeand connectionsafe connection of the of test the systemtest system that powersthat powers the stand. the stand. The systemThe system consists consists mainly mainly of a high-voltageof a high-voltage transformer, transformer, a capacitive a capacitive measuring measuring divider divider and aand control a cont panelrol panel for the for safe the executionsafe execution of the of measuring the measuring processes. processes. Ensuring Ensuring the appropriate the appropriate value value of pressure of pressure and itsand regulation its regulation was done was usingdone the using vacuum the vacuum set described set described earlier. The earlier. breakdown The breakdown voltages werevoltages measured were in measured accordance in with accordance the ASTM with D2477 the standard.ASTM D2477 The rate standard of voltage. The rise rate was of 2voltage kV/s. rise A schematic was 2 kV/s. diagram A schematic of the diagram laboratory of testthe laboratory stand used test for stand testing used the for dielectric testing strengththe dielectric of the strengthvacuum interrupter of the vacuum is shown interrupter in Figure is4 . shown A detailed in Figure description 4. A of detailed the stand de- alongscription with of its the parameters stand along is givenwith its in parameters publication is [30 given]. in publication [30].

FigureFigure 4.4. SchemeScheme ofof testtest systemsystem forfor testingtesting dielectricdielectric strengthstrength ofof MVMV vacuumvacuum interruptersinterrupters (1—control(1—control panel,panel, 2—power2—power transformer,transformer, 3—capacitance 3—capacitance divider, divider, 4—vacuum 4—vacuum interrupterinterrupter underunder test, test, 5—manual 5—manual dosing dosing valve, valve, 6—vacuum 6—vacuum gauge,gauge, 7—turbomolecular pump, 8—rotary pre-pump, 9—system controller, 10—technical gases dosed to the inside of the tested 7—turbomolecular pump, 8—rotary pre-pump, 9—system controller, 10—technical gases dosed to the inside of the object). tested object).

TheThe dielectricdielectric strength strength test test methodology methodology is basedis based on makingon making the appropriatethe appropriate electrical elec- connections,trical connections, setting setting the desired the desired electrode electrode spacing spacing andpressure and pressure inside in theside vacuum the vacuum test interrupter,test interrupter conﬁguring, configuring the test the sampletest sample parameters parameters and thenand then running running the measurement. the measure- Throughment. Through a fully automated a fully automated system, the system, voltage the applied voltage to applied the test object to the is test increased object untilis in- ancreased electrical until discharge an electrical occurs discharge between occurs the interrupter between contacts. the interrupter The controller contacts. records The con- the d Utrollerd breakdown records voltagethe U breakdown value, which voltage is used value, for further which testis used analysis. for further test analysis.

3.3. ResultsResults 3.1.3.1. ResultsResults ofof ScalingScaling thethe PressurePressure MeasurementMeasurement SystemSystem AccordingAccording toto thethe manufacturer’smanufacturer’s guidelinesguidelines forfor measuringmeasuring gauges,gauges, inin thethe pressurepressure −1 1 rangerange fromfrom 3.03.0× × 1010−1 PaPa to to 3.0 3.0 ×× 10101 Pa,Pa, the the actual actual pressure pressure value value depends linearly onon thethe readoutreadout value.value. The The vacuum vacuum gauges gauges then then operate operate in in the the thermal thermal head head mode mode (Pirani). (Pirani). Before Be- analyzingfore analyzing the pressure the pressure measurement measurement results obtained results obtained using vacuum using gauges vacuum VG1 gauges and VG2, VG1 itand was VG2, necessary it was tonecessary correct the to correct readings the by readings an appropriate by an appropriate calibration factor.calibration In the factor. case ofIn air,the thiscase was of air, deﬁned this was as: defined as: CAir = 1.00 (7) = 1.00 (7) While for helium: While for helium: CHe = 1.40 (8)

The actual value of the measured pressure was= 1. determined40 by the following relationships(8): The actual value of the measured pressure was determined by the following rela- pe f f = CAir × preading (9) tionships:

= × (9)

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= × (10) pe f f = CHe × preading (10) −1 BelowBelow a a pressure value of 3.00 ×× 1010− 1Pa,Pa, the the pressure pressure reading reading is is equal equal to to the the actual value,value, where where the the heads operate in ionization mode (Penning).(Penning). Above Above a a pressure pressure value 1 equalequal to to 3.0 × 1010 1Pa,Pa, the the actual actual pressure pressure value value should should be be read read from from the characteristics providedprovided by by the the vacuum vacuum gauge gauge manufacturer. manufacturer. However, However, these values were were not subject to measurementmeasurement analysis analysis performed performed for for the purposes of th thisis article. TakingTaking into accountaccount thethe above above relationships, relationships, pressure pressure measurements measurements were were carried carried out outfor airfor andair and helium helium in the in rangethe range 5.0 × 5.010 ×− 104 Pa–1.0−4 Pa–1.0× 10× 101 Pa,1 Pa, reading reading the the values values measured measured by byvacuum vacuum gauges gauges VG1 VG1 and and VG2. VG2 From. From these, these, the characteristicsthe characteristicspVG 1pVG=1 f( =p VGf(p2VG)2 were) were plotted plot- tedand and are are shown shown in Figure in Figure5. 5.

(a) (b)

FigureFigure 5. DependenceDependence of of the the pressure pressure measured measured by by vacuum vacuum gauge gauge VG1 VG1 on on the the pressu pressurere measured by vacuum gauge gauge VG2 takingtaking into into account account correction correction factors for ( a) air and (b) helium.

ForFor the the purpose purpose of of the the correct correct analysis analysis of of the the obtained obtained characteristics characteristics pVG1pVG1 = f(=pVG2 f(p)VG2 for) airfor and air helium, and helium, the auxiliary the auxiliary functions functions y = x werey = xplottedwere plottedon a coordinate on a coordinate system. Thanks system. toThanks this, a tocertain this, adependence certain dependence is visible, is stating visible, that stating above that a pressure above a pressurevalue equal value to about equal 4.0to about× 10−3 4.0Pa for× 10 air− 3andPa forabout air 2.0 and × about10−3 Pa 2.0 for× helium,10−3 Pa the for actual helium, pressure the actual value pressure in the vacuumvalue in interrupter the vacuum is interrupter equal to the isequal readout tothe value readout (pVG1 value= pVG2 ().p VG1Below= p VG2these). Belowvalues, these the actualvalues, pressure the actual values pressure in values the test in interrupter the test interrupter pVG2 arep VG2 greaterare greater than the than pressure the pressure pVG1 measuredpVG1 measured at the at vacuum the vacuum pump. pump. This This phenomenon phenomenon is due is due to the to the greater greater difficulty difﬁculty of maintainingof maintaining a seal a seal on the on thepump pump channel channel at lower at lower pressures, pressures, for both for both air-based air-based and andhe- liumhelium-based-based vacuums. vacuums. Any Any connecting connecting element element where where the the seal seal is is located located is is a a potential potential site site forfor air air to enterenter thethe pumppump channel,channel, thereby thereby increasing increasing the the pressure pressure in in the the system. system. Therefore, There- − − fore,below below a pressure a pressure value value of about of about 4.0 × 4.010 ×3 Pa10−3 for Pa air for and air aboutand about 2.0 × 2.010 ×3 10Pa−3 forPa helium,for helium,the pressure the pressure drop dropbetween between the vacuum the vacuum pump pump set and set the and tested the tested vacuum vacuum interrupter inter- rupterwas determined: was determined: ∆p = pVG2 − pVG1 (11) Δ = − (11) The obtained measurement and calculation results were used to determine the actual pressureThe obtained value in themeasurement tested vacuum and interrupter, calculation inresults the targeted were used dielectric to determine strength the tests. actual pressure value in the tested vacuum interrupter, in the targeted dielectric strength tests.

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3.2.3.2. Dielectric Dielectric Strength Strength Test Test Results Results First,First, the the breakdown voltagesvoltages ofof the the disconnecting disconnecting vacuum vacuum interrupter interrupter were were measured meas- −4 uredfor electrode for electrode spacing spacing ranging ranging from 1.0 from mm 1.0 to 5.0mm mm to and5.0 mm pressures and pressures ranging from ranging 8.0 × from10 1 8.0to 3.0× 10×−410 to 3.0Pa by× 10 dosing1 Pa by air dosing into the air interrupter into the interrupter interior. Figure interior.6 shows Figure the 6 dependence shows the dependenceof the measured of the breakdown measured voltages, breakdown both voltages as a function, both of as the a electrode function ofspacing the electrodeUd = f(d) for the selected pressure values and as a function of the pressure inside the tested vacuum spacing Ud = f(d) for the selected pressure values and as a function of the pressure inside interrupter U = f(p) for all electrode spacing settings. the tested vacuumd interrupter Ud = f(p) for all electrode spacing settings.

(a) (b)

Figure 6. The dependence of the breakdown voltages Ud as a function of (a) the electrode spacing d for selected pressures and (b) the Figure 6. The dependence of the breakdown voltages Ud as a function of (a) the electrode spacing d for selected pressures pressureand p (insideb) the pressurethe test vacuup insidem interrupter the test vacuum, for residual interrupter, gases for that residual are pure gases air. that are pure air.

FromFrom the the analysis analysis of of Figure Figure 66a,a, it can bebe seenseen thatthat thethe higherhigher thethe pressurepressure valuesvalues inin-side-side the the tested tested vacuum vacuum interrupter, thethe electrodeelectrode spacingspacing nono longer longer affects affects the the electrical electri- calstrength strength of theof the interrupter.This interrupter.This situation situation occurred occurred for pressuresfor pressures greater greater than than 7.05 7.05× 10 ×0 10Pa.0 Pa.In theseIn these cases, cases, the dielectricthe dielectric strength strength of the of system the system depended depend onlyed ononly the on pressure the pressure inside insidethe test the object. test object. Below Below a pressure a pressure value value of p = of 7.05 p = ×7.0510 0× Pa,100 thePa, the electrical electrical strength strength of theof thevacuum vacuum interrupter interrup waster was affected affected by the by electrodethe electrode spacing spacing in addition in addition to the to pressure the pressure inside. inside.The larger The itslarger value, its value, the value the ofvalue the of breakdown the breakdown voltage voltage was also was larger, also larger, according according to the torelation the relation [16]: [16]: α Ud = C × d (12) = C × d (12) where: Ud—breakdown voltage, d—electrode spacing, C, α—experimentally determined where: Ud—breakdown voltage, d—electrode spacing, C, α—experimentally determined constants. constants.By analyzing Figure6b, two characteristic pressure ranges can be distinguished for eachBy electrode analyzing spacing. Figure In 6b, the two ﬁrst characteristic one, the dielectric pressure strength ranges of can the be vacuum distinguished interrupter for eachremains electrode approximately spacing. In constant. the first one, These the values dielectric are summarizedstrength of the in vacuum Table4. Itinterrupter is worth remainsnoting that, approximately as the electrode constant. spacing These decreases, values are the summarized value of the in pressure Table 4. for Itwhich is worth the notingdielectric that strength, as the electrode of the interrupter spacing decreases, remains constant the value increases. of the pressure In the givenfor which pressure the dielectricranges, for strength the electrode of the interrupter spacing of 1.0remains mm and cons 2.0tant mm, increases. a large In scatter the given of breakdown pressure ranges,voltages for was the observed. electrode spacing of 1.0 mm and 2.0 mm, a large scatter of breakdown voltages was observed.

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Table 4. The measured values of the breakdown voltages U in the compartment with constant Table 4. The measured values of the breakdown voltages Ud ind the compartment with constant dielectricdielectric streng strengthth for for a vacuum a vacuum interrupter interrupter with with residual residual gases gases being being pure pure air. air.

ElectrodeElectrode Spacing Spacing Breakdown Breakdown Voltage Pressure Pressure Range

dd,, mm mm UUdd,, kV kV pp,, Pa 1.01.0 7 7.00.00 8.00 ×× 10−−44––7.707.70 × 101000 −4 0 2.02.0 17. 17.0000 8.00 ×× 10 −4––5.505.50 × 10100 − 3.03.0 31. 31.0000 8.00 ×× 10 −44––4.204.20 × 101000 −4 0 4.04.0 40. 40.0000 8.00 ×× 10 −4––4.004.00 × 10100 5.0 47.00 8.00 × 10−4–3.00 × 100 5.0 47.00 8.00 × 10−4–3.00 × 100

AboveAbove the the pressure pressure values values shown shown in Table in Table4, a sharp 4, a decreasesharp decrease in the dielectric in the dielectric strength strengthof the tested of the vacuum tested interrupter vacuum interrupter was observed, was whichobserved, is equivalent which is toequivalent a loss of its to insulating a loss of itsproperties. insulating The properties. breakdown The voltages breakdow forn thesevoltages parts for of these the characteristicsparts of the characteristics are not affected are notby electrodeaffected by spacing electrode but spacing only by but pressure. only by This pressure. corresponds This corresponds to the horizontal to the parts horizontal of the partscharacteristics of the characteristics shown in Figure shown6a. in Figure 6a. AnalogousAnalogous measurements measurements were were performed performed for for the the same same vacuum vacuum inte interrupterrrupter by by inin- creasingcreasing the pressurepressure insideinside by by dosing dosing helium helium into into it. it. Figure Figure7a shows7a shows the relationsthe relationsUd =U f(d d=) U p f(ford) selectedfor selected values values of pressure, of pressure, while while Figure Figure7b shows 7b theshows relation the relationd = f( )U ford = the f(p electrode) for the electrodespacing from spacing 1.0 tofrom 5.0 mm.1.0 to 5.0 mm. V k

, d U

(a) (b)

Figure 7. Dependences of breakdown voltages Ud as a function of (a) electrode spacing d for selected pressure values and Figure 7. Dependences of breakdown voltages Ud as a function of (a) electrode spacing d for selected pressure values and (b) pressure p inside the tested vacuum interrupter,interrupter, forfor residualresidual gasesgases beingbeing helium.helium.

ItIt can can be observedobserved inin Figure Figure7 a7a that, that for, for pressures pressures above above a value a value equal equal to p to= p 4.0 = ×4.010 ×1 10Pa,1 Pa,it is it apparent is apparent that electrode that electrode spacing spacing has no signiﬁcanthas no significant effect on effectthe value on of the the value breakdown of the breakdownvoltages. Below voltages. this pressure,Below this as pressure, electrode as spacing electrode increases, spacing the increases, value of the the value breakdown of the breakdownvoltage increases. voltage increases. InIn Figure Figure 77b,b, analogousanalogous intervalsintervals ofof Ud == f( f(pp)) characteristics characteristics can can be be distinguished distinguished for for eacheach tested electrodeelectrode spacing spacingd .d In. In the the interval interval in which in which the dielectric the dielectric strength strength of the system of the systemremains remains constant, constant, a smaller a scattersmaller of scatter the measured of the measured values of valuesUd voltages of Ud wasvoltages observed was observedcompared compar to the vacuumed to the interrupter vacuum tointerrupter which air to was which dosed. air These was values,dosed. alongThese withvalues, the alongpressure with ranges, the pressure are summarized ranges, in are Table summarized5. Analogously in Table to the 5. case Analogous wherely air to was the dosed case where air was dosed into the vacuum interrupter, there was a sharp drop in the dielectric strength of the system above the pressure values specified in the table below.

Energies 2021, 14, 3742 11 of 14

Energies 2021, 14, x FOR PEER REVIEWinto the vacuum interrupter, there was a sharp drop in the dielectric strength of the system11 of 14

above the pressure values speciﬁed in the table below.

Table 5. Measured values of breakdown voltages U in a compartment with constant dielectric Table 5. Measured values of breakdown voltages Udd in a compartment with constant dielectric strengthstrength for for a a vacuum vacuum interrupter interrupter with with residual residual gases gases being being helium. helium.

ElectrodeElectrode Spacing Spacing Breakdown Breakdown Volta Voltagege Pressure Pressure Range Range

dd, ,mm mm UUdd, ,kV kV pp,, Pa Pa 1.01.0 5.00 5.00 8.00 ×× 10−−44––5.335.33 × 101011 −4 1 2.02.0 13.00 13.00 8.00 ×× 10−4––2.672.67 × 10101 −4 1 3.03.0 29.00 29.00 8.00 ×× 10−4––2.672.67 × 10101 × −4 × 1 4.04.0 36.00 36.00 8.00 × 10−4––2.672.67 × 10101 5.0 46.00 8.00 × 10−4–1.37 × 101 5.0 46.00 8.00 × 10−4–1.37 × 101

ItIt is is crucial from from the the point point of of view view of of the the electrical electrical strength strength of the of vacuum the vacuum interrupter inter- rupterto compare to compare the measured the measured values values of the of breakdownthe breakdown voltages voltagesUd forUd for different different types types of ofresidual residual gases, gases, i.e., i.e. air, air and and helium. helium. The The characteristics characteristics of of the theU Ud breakdownd breakdown voltages voltages as as a afunction function of of pressure pressure (air (air and and helium)helium)p p forfor twotwo electrodeelectrode spacingspacing were selected: 3.0 mm mm andand 5.0 5.0 mm. mm. The The results results are are shown shown in in Figure Figure 88a,a,b.b.

(a) (b)

FigureFigure 8. Comparison of of the the dielectric dielectric strength strength of aof vacuum a vacuum interrupter interrupter ﬁlled filled with airwith and air helium and helium for two for electrode two electrode spacing: spacing(a) 3.0 mm,: (a) (3.0b) mm, 5.0 mm. (b) 5.0 mm.

ForFor an an electrode spacingspacing ofof 3.03.0 mm, mm, the the maximum maximumU dUvoltagesd voltages for for both both air air and and helium he- liumreach reach approximately approximately the samethe same values, values, equal equal to about to about 30.0 30.0 kV. WhatkV. What is important is important in this in thiscase case is the is the moment moment when when the the insulating insulating properties properties of theof the interrupter interrupter are are lost. lost. When When air airis dispensedis dispensed into into it, theit, the dielectric dielectric strength strength drops drops rapidly rapidly after after a value a value equal equal to top = p 4.20= 4.20× ×10 100 0Pa. Pa. ForFor anan interrupterinterrupter whereinwherein thethe residualresidual gasesgases consist of helium, a constant value ofof dielectric dielectric strength strength is is maintained up to a pressure levellevel equalequal toto pp == 2.672.67 ×× 101 Pa.Pa. Only Only aboveabove this this value value is is there there a a sharp sharp drop drop in in the the measured measured breakdown voltages. ForFor an an electrode electrode spacing spacing of of 5.0 5.0 mm, an analogous situation occurred. The The constant valuevalue of of the the electric electric strength strength of of the the vacuum vacuum interrupter interrupter,, to to which which air air was dosed dosed,, was was maintainedmaintained up up to to a a pressure pressure value of p = 3.0 × 10100 0PaPa and and amounted amounted to to about about 46.0 46.0 kV. kV. For For the helium-filled interrupter, the constant electrical strength value reached a similar value, but was maintained up to a pressure value equal to p = 1.37 × 101 Pa. In order to illustrate the better insulating properties of the helium-filled vacuum interrupter at higher interior pressures compared to the air-filled interrupter, the characteristics in the electrode spacing range of 1.0 mm to 5.0 mm were compared. A refer-

Energies 2021, 14, 3742 12 of 14

the helium-filled interrupter, the constant electrical strength value reached a similar value, but was maintained up to a pressure value equal to p = 1.37 × 101 Pa. In order to illustrate the better insulating properties of the helium-filled vacuum interrupter at higher interior pressures compared to the air-filled interrupter, the characteristics in the electrode spacing range of 1.0 mm to 5.0 mm were compared. A reference value of 1 pressure inside the interrupter, equal to pref = 2.0 × 10 Pa, was chosen and the Ud breaking voltages for the two types of residual gas were compared. While the insulating properties of the air-filled vacuum interrupter were lost for each of the electrode spacing tested at the reference pressure, they were still maintained at a safe level for the helium-filled interrupter. The measured values are summarized in Table6.

Table 6. Comparison of the breakdown voltages Ud of vacuum interrupters ﬁlled with air and helium, 1 for electrode spacing from 1.0 to 5.0 mm and reference pressure equal to pref = 2.00 × 10 Pa.

Breakdown Voltage Ud, kV

Electrode Spacing Residual Gas Type d, mm Air Helium 1 pref = 2.0 × 10 Pa

1.0 Ud(Air) = 1.20 Ud(He) = 3.50 2.0 Ud(Air) = 0.90 Ud(He) = 8.00 3.0 Ud(Air) = 1.10 Ud(He) = 27.00 4.0 Ud(Air) = 1.00 Ud(He) = 31.00 5.0 Ud(Air) = 1.00 Ud(He) = 40.00

4. Conclusions On the basis of the conducted tests of the dielectric strength of vacuum interrupters ﬁlled with two types of gases: air and helium, it was found that the interrupter ﬁlled with helium maintains full switching capability in a larger pressure range. These results are promising due to the possibility of increasing the operating pressure of the vacuum interrupter. The rated operating pressure of currently manufactured vacuum interrupters is approximately 1.0 × 10−3 Pa, so the insulating medium proposed by the authors of this article makes it possible to increase the pressure inside the interrupter many times. As a result, it will be easier to maintain the tightness of vacuum interrupters used in switching devices, and the values of switching overvoltages occurring in power systems will be reduced. Moreover, the complete elimination of harmful greenhouse gases, such as SF6, is in line with the current climate policy of the European Union. Measurements of the electrical strength of the vacuum interrupters were made for electrode spacing in the range of 1.0 to 5.0 mm, even though the electrode spacing in the interrupters operating in the currently used vacuum switching apparatus is about 9.0 mm. This distance was not measured, however, due to the limitation of the voltage capabilities of the test system to 50 kV. Further research plans by the authors of this paper concern the use of other insulating gases and their mixtures to improve the performance of vacuum interrupters, focusing on both dielectric strength and arc processes. Furthermore, the research will cover an increased range of electrode spacing due to the acquisition of a voltage set with a rated test voltage of 110 kV.

Author Contributions: P.W. proposed to study the inﬂuence of helium on the dielectric strength of medium-voltage vacuum interrupters and guided the work. M.L. and D.K. did the calculations and wrote the paper. P.W. and C.K. revised the results and contributed to the discussions. All authors have read and agreed to the published version of the manuscript. Funding: This research was founded by Lublin University of Technology, grant number FD-20/EE- 2/704. Institutional Review Board Statement: Not applicable. Energies 2021, 14, 3742 13 of 14

Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by The National Centre for Research and Development and co-financed from the European Union funds under the Smart Growth Operational Programme (grant # POIR.04.01.04-00-0130/16). Conflicts of Interest: The authors declare no conflict of interest.

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