measurements by residual gas analysis inside vacuum interrupters

1484 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009

Measurements by Residual Gas AnalysisInside Vacuum Interrupters

Dietmar Gentsch and Thorsten Fugel

Abstract—For low- and medium-voltage applications, thevacuum-interruption principle has been well established over thepast 30 years. Component materials for vacuum interrupters (VIs)have to be designed for very low gas content since this character-istic determines the quality of vacuum inside the VI for at least30 years of service or shelf life. Vacuum integrity in a “sealed-for-life” VI is needed, and, therefore, gas pressure measurementson VIs are conducted during production as a quality-controlprocess. To limit the pressure resolution in the range of 10−7 hPafor a long period, vacuum measuring systems are applied to assurethe vacuum integrity (tightness) of the VI. Those measurementsare performed during production by magnetron (Penning prin-ciple) measurements on each VI. In addition, some selected VIsare analyzed by means of residual gas analysis (RGA—mass spec-troscopy) for further production control and improvement. TheRGA analyzer system consists of a Quadrupole and an Omega-tron analyzer in the ultrahigh-vacuum apparatus, which can beconnected to the VI. This technique is used to detect residual gases,e.g., gas sources, and to investigate the diffusion of gases from thematerial into the vacuum. The measurements of magnetron andRGA are analyzed and compared herein, and a description of howto eliminate gas sources by selection of suitable materials for a VIwill be presented.

Index Terms—Gas analysis, residual gas analysis (RGA),vacuum, vacuum behavior, vacuum integrity, vacuum inter-rupter (VI).

I. INTRODUCTION

NO OTHER kind of interruption device in medium voltageis more reliable than a vacuum interrupter (VI) in terms

of compact size, modern design, reliability, and maintenanceelimination (Fig. 1). Since 1921, VIs have been widely usedfor current interruption in low-, medium-, and high-voltageapplications. VIs are suitable for a wide range of applications,including circuit breakers, contactors, reclosers, and load-breakswitches, as well as vacuum-protecting devices as protectiondevices in a network.

Utilization of vacuum circuit breakers continues to growworldwide and dominates in the medium-voltage sector suchthat more than 60% of today’s switching devices are equippedwith VIs. Today, VIs are available for rated voltages up to 52 kVand 80-kA short-circuit interruption capability.

Manuscript received November 1, 2008; revised March 3, 2009 andApril 5, 2009. First published June 23, 2009; current version publishedAugust 12, 2009.

The authors are with the ABB AG, Calor Emag Medium VoltageProducts, 40472 Ratingen, Germany (e-mail: [email protected];[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2009.2021477

Fig. 1. Dimensions of a compact and modern VG-series VI for ratings of12 kV/31.5 kA (VG4-S). (1) Movable stem. (2) Twist protection. (3) Metalbellows. (4) Interrupter lid. (5) Shield. (6) Ceramic. (7) Main shield. (8),(9) Transverse magnetic field contact system. (10) Fixed stem. (11) Interrupterlid. (12) Vacuum and getter material.

The main advantages of vacuum devices are the following:1) maintenance-free operation over the lifetime of the

device;2) environmentally friendly;3) vacuum integrity by a sealed-for-life system;4) high dielectric strength;5) specific high short-circuit interruption or closing ability.The production of VIs requires a thorough understanding

of the physical interaction processes affecting the vacuumintegrity of the VI [1]. Every gaseous atom is bound to thematerial and penetrates it via diffusion at the surface. Theinternal pressure of the VI increases by degassing of atoms ormolecules. For proper dielectric strength and current interrup-tion in a vacuum device, the vacuum level, as well as its integralresidual gas composition, requires a total pressure level lowerthan 10−3 hPa. However, the target is to reduce the pressure toless than 10−7 hPa to assure a sealed-for-life characteristic formore than 30 years [2].

Ultrahigh vacuum (UHV) requires efficient control of thegaseous content of all materials used in vacuum devices.

The “indirect” gas-pressure measurement takes place afterthe production of the vacuum device by the well-known mag-netron measurement system (Penning principle). With thismethod, the current of residual ionized gas atoms is measuredby generating electrons from a cold cathode [1]. Derived froma calibration curve, the total pressure is assigned by mea-suring the current flow of ionized gas; this determines thetotal pressure of the vacuum device. To be able to compare

0093-3813/$26.00 © 2009 IEEE

GENTSCH AND FUGEL: MEASUREMENTS BY RESIDUAL GAS ANALYSIS INSIDE VACUUM INTERRUPTERS 1485

Fig. 2. Magnetron system: the VI is arranged inside a coil arrangement tosuperpose the applied voltage for cold cathode emission of electrons with anaxial magnetic field. It is enlarged in distance for each electron within thevacuum between both electrodes.

the pressure values measured by the magnetron system witha reference, a direct measurement is performed by means ofresidual gas analysis (RGA). The standard commercial penningor magnetron gauge is calibrated for a certain gas mixture basedon CO and N2. By applying these gauges, an integral pressurevalue can be measured only, but a very precise value can beobtained by measuring the partial pressure of each individualgas with the RGA system. Only the sum of all partial pressuresrepresents the total pressure of the VI. Therefore, calibrationof the penning or magnetron gauge can be derived from thesemeasurement results. With this analyzer, the vacuum deviceis connected to a sensitive mass spectrometer system. This isan important tool in assuring the quality and the integrity ofvacuum devices and for the optimization of their function.

II. PRESSURE MEASUREMENT: MAGNETRON

AND RGA, EXPERIMENTAL SETUP

A. Magnetron

Fig. 2 shows the general principle of total pressure mea-surement by means of magnetron (Penning) principle using theVI as its own gauge. Applying a high voltage to electrodeslengthwise (Fig. 2) or crosswise, by discharge of cold cathodeelectron emission, leads to ionization of gas molecules. If theVI is arranged within an axial magnetic field, the distance ofthe electrons from the cathode to the anode is enlarged by ahelical orbit-shaped movement.

The helical orbit radius (ionization of gas molecules) iscalculated as

Fz = e · v × B Fz = m · v2/r

−→ r = v · m/B · e (1)

−→ v =(2 · e · U/m)1/2 (2)

whereF force;e electron charge;

B magnetic field;M mass;V velocity;I current;r orbit radius.To generate a homogeneous B field, a Helmholtz coil

arrangement was selected with two coils in series, i.e.,

Ha = n · I · R2/(R2 + a2)3/2 (3)

whereH magnetic field;N coil windings;R radius of coil;A distance between both coils.The current flow is strictly proportional to the number of

ionizing gas molecules within the electrode gap, as shown inFig. 2. This technique is well proven and provides reliable totalpressure measurements; however, the system has to be carefullycalibrated before testing each VI. Therefore, the VI is directlyconnected to an external measurement gauge, and the pressureinside the VI can be controlled. For example, the appliedB-field has a value of 1.06 × 10−2 T, and the orbital radius iscalculated to 1 cm using (1)–(3), on the supposition that theelectron has an energy of 1 keV. This enlarges the distancebetween the electrodes and the ionization yield and, therefore,the gaugeable current of the magnetron. The calibration curvecan be utilized as the basis to apply this principle for qualitycontrol in series production.

B. RGA

To precisely determine the total residual gas pressure, ahigh-resolution mass spectrometer is applied to the VI. Thecomposition of each individual partial pressure of gases has asignificant influence on the performance of the VI. QualitativeRGA in VIs is used to determine not only the compositionbut also the origin and effects of residual gases [3]. The RGAmeasurement setup is shown in Fig. 3 as a basic block diagramof the analyzer equipped with two RGA analyzers based onOmegatron and Quadrupole systems. Both systems are locatedin the analyzer with the same volume to compare the measuredpartial and total pressure results.

The UHV apparatus made of stainless steel works with a bak-ing temperature of up to 300 ◦C and achieves a final pressure of10−11 hPa via permanent pumping. The analyzer consists of aset of pumps connected to the UHV volume (volume 3), a turbo-molecular pump (P1), and a getter-ion pump, which are coupledby UHV valves V4 through V6. To avoid any oil contamination,a pumping system consisting of a dry pump is connected to themolecular pump to achieve the needed prepressure comparedwith the atmosphere. All functional components of volumes1 through 3 are baked out before a VI will be measured. Theconnection point between the VI is suspended from a solderedconnection tube to the metallic lid of the interrupter. In this area,the lid-wall thickness is reduced so that this membrane can bepunctured by a knife linearly moved from the topside by theLN valve. The result is that the residual gas diffuses from the


Fig. 3. Schematic of RGA connected to a VI at the lid of the device. (A) Bake-out oven. (IG) Getter ion pump. (OM) Quadrupole/Omegatron (residualgas analyzer). (M) Turbomolecular pump. (OF 1 to OF 3) Bake-out areas. (Vol.1) Air-lock volume (VL). (Vol.2) Specimen VL. (Vol.3) Analyzing VL.(M1) Mechanical gauge. (Vn) UHV leakage valve. (V1–V6) Valves. (PM) Vacuum gauge > 10−3 hPa. (IIVM) Ion gauge lower than < 10−10 hPa.(KG) Test gas inlet.

VI into the calibrated volumes 1 through 3. The residual gasesare measured prior to and after the VI is connected to the RGAanalyzer. The gas is analyzed in volume 3, after valve V4 hasbeen closed, and valve V3 is opened. In case the total pressurevalues are higher than 10−6 hPa at volume 2, the gas influxfrom volume 3 is passing the leak valve to keep the pressure ona required minimum level for the partial pressure measurement.The computer system controls the measured data via an analog-to-digital converter. The current value during the analysis bythe Omegatron or the Quadrupole is measured and depends onthe partial pressure of each gas in volume 3. By varying thefrequency ν of the integrated synthesizer, the masses (partialpressure) can be measured.

III. CHARACTERISTIC MEASUREMENT SPECTRUM

AND RESULTS: COMPARISON WITH

MAGNETRON MEASUREMENTS

To reach a total UHV vacuum pressure level of 10−11 hPawith a continuously pumped UHV analyzer, an extended back-out of the analyzer steel system is needed to enhance thevacuum level. The getter-ion pump remains closed, and thebackground pressure slightly increases from the starting level ofnearly 10−11 hPa to a linear and stable value of up to 10−10 hPaover a period of 2 min. Fig. 4 shows the whole procedureof the measurement from the beginning until the end. Shortlybefore the partial pressure measurement starts, each individualgas is analyzed by the RGA within the analyzer volume to getan indication regarding the background gas composite. Thiscalibration measurement takes place before the vacuum device(VI) is connected to the analyzer. The residual backgroundgases have to be considered when the measurement of the VI

is analyzed. During the connection of the VI, the establishedmembrane at the lid is opened at the stainless-steel materialby a linear movement of the knife within the connecting pipe.At this time, the plastic deformation of the steel creates a gasoutput mainly of hydrogen, as seen in Fig. 4.

In case standard VIs are selected, there is a getter materialwithin each device that leads to a reduction of the total pressureof the analyzer by absorption of gas. A getter material based onZrAl alloy is activated in each individual VI during the standardbrazing process at a temperature of approximately 800 ◦C. Tomaintain the vacuum at a low pressure for more than 30 years,all active gases are absorbed from the getter alloy by chemicalor physical absorption. In normal service or storage conditions,the active gases are being absorbed by the getter, which is madeof a ZrAl material that is activated during brazing of the VIand works during the entire lifetime of the device. The gasdesorption during connection to a standard VI by breakthroughof the device lid will be absorbed by the getter alloy, as wellas some residual gases from the connected RGA analyzer, andthe vacuum pressure decreases nearly after 2 min to perform anRGA measurement.

The pressure development for each individual gas of H2,N2, and Ar is illustrated in Fig. 4 after the breakthrough withthe knife. The partial pressure measurement stops after 2 min.During the whole measurement time of ∼10 min, the totalpressure drops 10−9 to 10−10 hPa. Fig. 5 shows the typical“fingerprint” of a continuously pumped and baked-out RGAanalyzer with temperatures up to 300 ◦C applied for two days.The residual gases for a stainless-steel analyzer are H2, CO2,and CH4 (formed by the chemical reaction of carbon andhydrogen out of the steel component), with a limited outgassingrate from the steel wall into the vacuum atmosphere [4], [5].


Fig. 4. Pressure development during the whole process of RGA measurement of the VI. The pumped and backed-out analyzer has a pressure of 10−11 hPa.After connecting the VI, the partial pressure of the gases H2, N2, and Ar increases. N2 and Ar are diffused in the analyzer vacuum. A stable RGA spectrum canbe obtained at least 1–2 min after connecting with the VI.

Fig. 5. Basic spectrum of continuously pumped and baked-out RGA withtemperatures up to 300 ◦C over two days. Visible is the typical “fingerprint”of an analyzer made of stainless steel with a total pressure of 10−11 hPa andtraces of H2 and CO2.

The figure illustrates a reachable vacuum pressure of 10−11 hPawith traces of these gases.

The background spectrum of the valved (compare Fig. 3; V4)analyzer has to be measured before breakthrough with the knife(LN) through the membrane of the VI lid as illustrated in Fig. 6.The background gas increases after 30 min to a stable valvedvacuum-pressure minimum in the range of 10−8 to 10−9 hPadriven by the increase in traces of the “fingerprint” gases [5].Traces of N+

2 (28 u) can be calculated from the fragments of N+

14 u (with the relative intensity of 5.18%) but are not existingin the spectrum Fig. 6. CO+ is represented in the spectrum at amass of 28 u, and the relative intensity of C+ at 12 u is 4.8%according to the Mass Spectral Data. Hence, the mass 28 urepresents the partial pressure of the CO gas predominantly.

Fig. 7 shows the contribution of the gases obtained from theconnected VI. The dominated mass peak represents a trace ofAr, but the partial pressure of all other gases (H2, CO, CH4,N2/CO, and CO2) decreases due to the gas-getter effect on theside of the activated getter material. No other gases with highermasses than 44 u are detectable in this set of measurements.This means that no traces of those elements would be detectableinside a VI as noted in Fig. 7.

Fig. 6. Partial pressure measurement of a valved analyzer after a 30-minwaiting period, illustrated by a total pressure of around 10−8 hPa and tracesof H2, CO, (N2), and CO2.

Fig. 7. Partial pressure measurement of a connected VI, illustrated by a totalpressure of around 10−9 hPa and traces of H2, CO, (N2), Ar, and CO2. Withinthis distribution, Ar is detectable.

The total pressure of all the residual gases is significantlybelow the defined boundary pressure of 10−7 hPa, which is anallowed quality pressure limit for manufactured VIs [6].

For deeper investigation, VIs that present higher total vac-uum pressure during measurement by means of magnetron are


Fig. 8. Comparison between both measurements: 1) magnetron and 2) RGAanalysis of a VI. The maximum allowed pressure after production is 10−7 hPa.After applying the first method, the measured total vacuum pressure dropsdue to the getter-ion effect within the device. In some cases, the secondmeasurement delivers two-orders-of-magnitude-lower values.

being selected for RGA, as seen in Fig. 8. In each case, themeasured total pressure detected by RGA is lower by one to twoorders of magnitude on average, which is caused by the “getter-ion effect” after magnetron measurement [1]. The investigatedand illustrated VIs in Fig. 8, VK I through VK V, exhibit thatno Ar is detectable, whereas in two cases, there were traces ofit found at about 10−9 hPa.

IV. RESULTS AND DISCUSSION

The RGA investigation is carried out on a large number ofVIs taken from standard production, and some are prepared forfurther investigation activities. It turns out that the vacuum levelfrom standard-produced VIs is far below the defined deliverycriterion of 10−7 hPa (after production) and the physical limitof 10−3 hPa [7], hence less than the 10−11 hPa detection limitof the RGA system. For investigation and comparison of themeasurement systems, some VIs are collected and preparedthat showed a marginally higher total pressure at the magnetronmeasurement compared with the acceptance criteria. Comparedwith the RGA investigation results, it is observed that the totalpressure drops down due to the “getter-ion effect.” All VIs arefound to contain H2, CH4, N2/CO, Ar, and CO2.

Hydrogen is generated by the bulk diffusion and desorptionby the stainless-steel components of a VI or RGA design.Diffusion through the stainless steel causing a reaction at thesurface with atmosphere moisture or hydrogen is negligiblecompared with the concentration of the bulk material [8], [9].The activated getter-material vacuum pump rapidly absorbs H2

and CO.CO is generated by the reaction of metal oxides with the car-

bon content in the steel material. Despite the extreme degassingduring brazing, which is a part of the production process, the re-action C(steel) + 2O(at surface) → CO2 is chemically not fullyaccomplished,1 so that, in combination with H2, the compoundCH4 is formed [2].

1At temperatures of less than 300 ◦C, more CO2 than CO is formed(Boudouard equilibrium).

This result is considered to represent the typical “fingerprint”of the stainless-steel VI and/or analyzer. Degassing of thematerial cannot be avoided and occurs over the shelf life ofVIs at ambient temperatures [10]. The aforementioned gasesare considered as typical gases from the established getter ateach device.

V. CONCLUSION

As derived from the investigation, the delivery criterion fora residual gas pressure in a UHV range < 10−7 hPa representsa defined quality with sufficient safety margin for the total lifecycle of the VI [11].

1) The existing technique of magnetron measurement reli-ably works and is indispensable for quality control duringthe current production [12].

2) In case of higher pressures than 10−7 hPa, the VIs arebeing rejected and checked by RGA.

3) In addition, the “state-of-the-art” RGA method makes thepartial pressure information available besides the totalpressure in the VI.

4) RGA plays an important role in the permanent qualityimprovement process.

Thus, the RGA has become indispensable for the optimiza-tion of production and test procedures and for the selectionand qualification of appropriate materials. The RGA partialpressure measurement is being used as a basis to detect gasesthat penetrate from the material into the vacuum and to system-atically eliminate gas sources by selecting vacuum-compatiblematerials. From this investigation, as proven by measurementsand judged by technical experience, it has become apparent thatthe production process leads to VIs that are “sealed for life” ifthe VI is stored for some days in inert gas quarantine betweentwo magnetron measurements [2].

From the large number of magnetron pressure measurementsof VIs that show a total pressure of less than 10−7 hPa, thepresence of high quality in the industrial VI mass productioncan be concluded.

Comparison of the magnetron total pressure values and thoseobtained from the RGA analyzer has revealed values of 10−9

up to 10−10 hPa being far below the defined limit of 10−7 hPa.In most cases, the VIs have shown a UHV that is lower than thephysical limit of RGA analyzers of 10−11 hPa [13].

Only by collection of VIs from production presenting in-frequently measured pressures of around 10−7 hPa can infor-mation be gathered concerning the dominating partial pressureand, thus, a link to possible gas sources. After applying themagnetron method, the measured total vacuum pressure dropsdue to the getter-ion effect within the device. Thereby, theRGA measurement is more likely to show up to two-orders-of-magnitude-lower values. In some cases, traces of Ar arebeing observed presenting the highest partial pressure. The inertAr gas remains in the vacuum atmosphere, and the absorptioncan take place neither on active surfaces nor on the gettermaterial; hence, partial pressure values of up to 10−8 hPa canbe observed. Therefore, the primary objective is to avoid gassources and to reduce the quantity of outgassing released intothe vacuum atmosphere.


REFERENCES

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[3] D. Gentsch, “Quality control for circuit breakers through residual gasanalysis,” ABB Rev., pp. 1–8, 1995.

[4] R. Calder and G. Lewin, “Reductions of stainless-steel outgassing inultra-high vacuum,” Br. J. Appl. Phys., vol. 18, no. 10, pp. 1459–1472,Oct. 1967.

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[6] W. F. H. Merck, G. C. Damstra, and R. J. B. Gruntjes, “Estima-tion of vacuum circuit breakers,” in Proc. 18th ISDEIV, Eindhoven,The Netherlands, 1998, pp. 506–509.

[7] W. L. Robert, T. Slade, and P. Slade, “Residual gas analysis of vacuuminterrupters,” in Proc. 3rd Int. Conf. Elect. Contacts, Arcs, Apparatus,Appl., 1997, pp. 491–498.

[8] K. Walczak, J. Janiszewski, and H. Moscicka-Grzesiak, “Evaluation ofinternal pressure of vacuum interrupter based on dynamics changes ofelectron field emission current and x-radiation,” in Proc. Int. Symp. HighVoltage Eng., London, U.K., 1999, pp. 192–195.

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[10] P. Frey, K. W. Jäger, N. Klink, and K. E. Saeger, “Investigation on therelease of gas from contact materials,” in Proc. ITK Elect. Cont., Berlin,Germany, 1982, pp. 317–320.

[11] B. M. Cox, O. Lloyd, and A. J. Ralls, “Gas pressure limitation of vacuumswitches,” in Proc. 5th ISDEIV, Poland, 1972, pp. 351–354.

[12] F. R. Fronzek and D. König, “Methods for internal pressure diagnostic ofvacuum interrupter,” in Proc. 18th ISDEIV, Eindhoven, The Netherlands,1998.

[13] P. A. Readhead, J. P. Hobsen, and E. V. Kornelsen, The Physical Basis ofUltrahigh Vacuum. New York: Amer. Inst. Phys., 1993, pp. 329–331.

Dietmar Gentsch received the Dipl.-Ing. degree inmechanical engineering from the Technical Univer-sity of Hannover, Hannover, Germany, in 1992 andthe Dr.-Ing. degree from the Technical UniversityBraunschweig, Braunschweig, Germany, in 2002.

He has worked in the field of interruption per-formance of vacuum interrupters in medium-voltageswitchgear and simulations in test devices. Since1993, he has been with ABB AG, Calor EmagMedium Voltage Products, Ratingen, Germany,working in the development of materials and in the

design of vacuum interrupters. Since 2005, he has been the leader of theresearch and development of vacuum interrupters and, since 2007, of pole partsindoor and outdoor.

Thorsten Fugel received the B.S. degree in elec-trical engineering from the Technical UniversityBraunschweig, Braunschweig, Germany, and thePh.D. degree in the field of vacuum circuit break-ers from Darmstadt University of Technology,Darmstadt, Germany.

He is currently with ABB AG, Calor EmagMedium Voltage Products, Ratingen, Germany,where he has been the Global Product Manager re-sponsible for vacuum interrupters, embedded poles,switches, and fuses since the end of 2006 and has

been the Technology Center Manager since 2007.

measurements by residual gas analysis inside vacuum interrupters

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