Optical diagnostics can be used to obtain spatially resolved measurements of the density, temperature, conductivity, and electron density of circuit breaker arcs embedded in transonic flows; these can be used to validate the results of simulations, the accuracy of which can currently be assessed in only a limited way. We compare speckle imaging and an interferometric approach. Both use a pulsed nanosecond laser. The speckle imaging setup does not require a reference beam, but only yields information about the gradient of the refractive index. Its accuracy is sensitive to the alignment of the optical components. Interferometry directly yields high resolution images of the index of refraction, from which the density can be calculated using the Gladstone-Dale relation. By using two laser beams, interferometry provides spatially resolved information about the electron density. Such measurements are a significant step towards more accurate CFD models

Carstensen, J.

Doiron, C. B.

Galletti, B.

Sokolov, A.

Stoller, P. C.

English

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

doi:10.14311/ppt.2017.1.44Plasma Physics and Technology 4(1):44–47, 2017 © Department of Physics, FEE CTU in Prague, 2017OPTICAL DIAGNOSTICS OF SWITCHING ARCS NEAR CURRENT-ZERO:SPECKLE IMAGING AND INTERFEROMETRYP. C. Stoller∗, J. Carstensen, B. Galletti, C. B. Doiron, A. SokolovABB Switzerland Ltd., Segelhofstrasse 1K, 5405 Baden-Daettwil, Switzerland∗ patrick.stoller@ch.abb.comAbstract. Optical diagnostics can be used to obtain spatially resolved measurements of the density,temperature, conductivity, and electron density of circuit breaker arcs embedded in transonic flows;these can be used to validate the results of simulations, the accuracy of which can currently be assessedin only a limited way. We compare speckle imaging and an interferometric approach. Both use apulsed nanosecond laser. The speckle imaging setup does not require a reference beam, but only yieldsinformation about the gradient of the refractive index. Its accuracy is sensitive to the alignment of theoptical components. Interferometry directly yields high resolution images of the index of refraction,from which the density can be calculated using the Gladstone-Dale relation. By using two laser beams,interferometry provides spatially resolved information about the electron density. Such measurementsare a significant step towards more accurate CFD models.Keywords: circuit breaker, current-zero, interferometry, imaging, density, temperature.1. IntroductionHigh voltage circuit breakers are used to switch thecurrents that flow under normal conditions in differentsections of the power grid on and off. They must alsobe able to interrupt short circuit currents that arisewhen a fault occurs; these currents can be an orderof magnitude higher than the typical load currentsthat flow during normal operation. The current isinterrupted by drawing an arc between two contacts.Nozzles surrounding these contacts are used to guide atranssonic flow that cools the arc and interrupts it ata current-zero crossing of the alternating current. Thedesign of more effective and efficient circuit breakersdepends on a detailed understanding of the arc physicsrelevant during the roughly 100µs before and afterthe zero-crossing. Information about the arc and thegas flow used to cool it is typically obtained indirectlyvia measurements of the arc voltage, the current, andthe pressure (at specific locations in the arc zone ofthe circuit breaker). These integral measurements donot give sufficient insight into the structure of thearc to allow a detailed understanding of the processesinvolved in interrupting it. Such information can beobtained from computational fluid dynamics (CFD)simulations [1], but such models often lack experi-mental validation (aside from comparison with themeasured current, voltage, and pressure). For thisreason, several spatially resolved optical diagnosticmethods based on well-established techniques thatmeasure the index of refraction (or its gradient) wererecently developed or adapted to image circuit breakerarcs near current zero [2–5]. The advantages and dis-advantages of interferometric techniques compared toother optical methods (such as emission spectroscopy)are discussed in detail in those studies. Here we seek tobriefly outline and compare two of these approaches,speckle imaging and interferometry. Section 2 de-scribes the experimental approaches used. Results ofmeasurements with the two techniques are presentedin Section 3, while Section 4 discusses the differencesbetween the two techniques.2. Experimental techniquesThe speckle and interferometry experimental setupsare described in detail in [3] and [4], respectively.Here we only briefly present the two techniques; aschematic of the speckle imaging setup is provided inFigure 1; the experimental setup for interferometry isgiven in Figure 2. In both cases a 532 nm CrystaLaserCRL-QL532-100-YG pulsed, Q-switched laser with anominal pulse duration of 12 ns was used. To providean additional wavelength for the interferometry mea-surements to measure directly the electron density, a671 nm CNI AO-V-671 pulsed, Q-switched laser witha nominal pulse duration of 16 ns was employed. Thelaser beam(s) were passed through a beam expanderto increase the beam diameter to match the size of theregion imaged. In the case of the speckle setup, thearc zone was imaged onto a glass diffuser using a relaylens. A scientific camera (Phantom V7100 or ProSilicaGT4905) was focused a defined displacement distancebehind the glass diffuser. Shifts in the speckle patternproduced by scattering from the diffuser and observedon the camera (with respect to a reference image inthe absence of an arc and/or gas flow) were used todetermine the line-of-sight integrated gradient in therefractive index at each point. The approach usedto calculate the density and temperature from suchmeasurements is given in [3]. For the interferometrymeasurements, a reference beam was added to theexperimental setup and the probe beam and the refer-ence beam were re-combined at a slight angle using a44vol. 4 no. 1/2017 Optical diagnostics of switching arcsbeam-splitter and recorded by a ProSilica GT4905 orGT2300 camera. The resulting interference patternswere analyzed using Fourier techniques to obtain theline-of-sight integrated refractive index (again makinguse of a reference image). The procedure employed isdetailed in [4].It is important to note that the circuit breaker testdevices used to obtain speckle and interferometric im-ages differed considerably. The test device used in theformer tests, described in detail in [3] is shown in Fig-ure 3. Briefly, this test device consisted of two fixedcontacts (a hollow contact and a solid contact) and anozzle system used to guide the flow of gas (syntheticair in the case of the tests discussed in this work) froma high pressure tank through the arc zone. Opticalmeasurements could be performed in the stagnationregion (through a window in the nozzle) and at thenozzle exit. This test device provided more direct op-tical access to the arc (at the nozzle exit), but had thedisadvantage that the arc was not well-stabilized bythe gas flow (which accelerated out of an intermediatevolume between the arc zone and the high pressuretank). This led to an arc that moved within the arczone and did not always remain parallel to the axis ofthe test device. As discussed in detail in [3] the insta-bility and asymmetry in the arc sometimes preventedreconstruction of the index of refraction profile fromthe measured line-integrated deflection angles (corre-sponding to the line integrated refractive index). Thisis because a three-dimensional reconstruction basedon a single projection is only possible for an objectwith defined symmetry, such as axi-symmetry (tempo-ral and spatial fluctuations can also wash out features,but this effect is negligible for single, nanosecond laserpulses). One approach to overcome this problem (us-ing almost the same circuit breaker test device) isdescribed in [5]. In that work, multiple beams wereused to probe the index of refraction distribution,permitting, in principle, a three-dimensional recon-struction from the line integrated measurements evenin the absence of axi-symmetry. However, it likelythat at least eight beams are required to achieve areasonable reconstruction.The test device used in the interferometry experi-ments is illustrated in Figure 4. It was designed specif-ically to ensure a stable, axi-symmetric arc by allowingthe gas (argon in the case of the tests discussed inthis work) that cools and interrupts the arc to acceler-ate from rest in the arc zone. The three-dimensionalindex of refraction and density distribution in this arcand surrounding gas flow can be re-constructed froma single projection. This test device is described indetail in [4].3. ResultsThe speckle technique was used to image the indexof refraction distribution in the stagnation region ofthe test device described in the previous section. Thedensity was determined from the measured index ofFigure 1. Illustration of the optical setup used forspeckle imaging.Figure 2. Sketch of the optical setup used for interfer-ometry.refraction using the approach presented in [3]. As-suming that the pressure distribution is the same asthe one obtained from measurements in the absenceof an arc (an assumption explained in [3]), the tem-perature can also be calculated. These results arepresented in Figure 5 for illustration; additional re-sults are given in [3]. Figure 5 shows the results forsix different measurements and illustrates the scatter(twice the standard deviation). This scatter does notrepresent the uncertainty in the measurement; instead,it demonstrates that there are fluctuations in the arcfrom test shot to test shot. The largest variation canbe seen in the temperature at the arc boundary; thegradients are very steep, and a large change in tem-perature corresponds to only a small difference in therefractive index.Similar measurements were performed with the in-terferometric technique, although it should be notedthat the test device, the gas, and other aspects ofthe experiment differed. An example of such a mea-surement for a 100A quasi-direct-current arc and astagnation pressure of 1560 kPa are shown in Figure 6.The measurements were also performed in the stagna-tion point, but in the test device that was optimizedto achieve a stable, axi-symmetric arc (Figure 4). Thevariation in the arc density profile observed from shotto shot with the test device shown in 3 cannot beseen in the measurements made on the stable arc; theresults are consistent from test shot to test shot.The interferometric technique was carried out withtwo laser wavelengths, permitting the measurement ofthe electron density in addition to the neutral particledensity. Two laser wavelengths allow the measurementof the electron density because the contribution ofthe electrons to the index of refraction is stronglywavelength dependent, while the contribution of theheavy particles is not. The details of this approachare described in detail in [2, 4]. An example of a45P. C. Stoller, J. Carstensen, B. Galletti et al. Plasma Physics and TechnologyFigure 3. Sketch of the circuit breaker test device usedfor speckle imaging.Figure 4. Sketch of the circuit breaker test device usedfor interferometric imaging.measurement of the electron density in the stagnationpoint of the arc zone is given in Figure 6. Assuminglocal thermodynamic equilibrium, the temperatureof the arc core can be determined from the electrondensity measurement [4].4. Comparison of techniquesThe measurements made with the speckle and theinterferometric experimental setup allow the two ap-proaches to obtaining spatially resolved informationabout a circuit breaker arc near current-zero to becompared. It is important to note that both the opti-cal techniques and the test devices used differ. Themain difference that stems from the test device isFigure 5. Speckle measurements of the temperatureand density in the stagnation zone of the test deviceshown in Figure 3 illustrating the variation in themeasurement. The stagnation pressure was 750 kPaand the current was 12A. The density is given in unitsof kg/m3.Figure 6. Interferometry measurement of the densityand the electron density distribution in the stagnationzone of the test device shown in Figure 4. The stagna-tion pressure was 1560 kPa and the current was 100A.Note that noise in the density measurement amplifiedby Abel inversion results in unphysical dips below zeroin the arc core. These unphysical dips are removed byperforming a fit to an analytical function [4].the much more stable arc observed when the gas flowin the arc zone accelerates from rest. In principle,speckle measurements made on such an arc wouldalso show less shot to shot scatter. In other words,this difference is due to the test device and not themeasurement technique.46vol. 4 no. 1/2017 Optical diagnostics of switching arcsAlthough the speckle and interferometry techniquesare both based on measuring the change in indexof refraction between conditions with and withoutan axially blown arc, there are key differences. Theformer technique requires a less complex experimentalsetup. Only a single beam path is required in that case,while the latter approach requires the use of two beampaths, one of which bypasses the test device. Theadvantage of using a reference beam is that the line-integrated difference in refractive index is measureddirectly. The speckle technique (similar to schlierenimaging and Hartmann-Schack inteferometry) onlyallows measurement of the gradient of the refractiveindex. On the other hand, unlike in the case of speckleimaging, interferometry requires that the coherencelength of the laser be sufficient for the reference andprobe beams to interfere.Correct measurement of the distance the diffuser(or speckle plate) is placed out of the focal plane ofthe camera lens is crucial to accurate speckle mea-surements and can in some cases limit their accuracy.In the case of interferometry, only the magnificationof the imaging optics must be determined accurately,but this is generally straightforward.The inteferometric technique can also makes bet-ter use of the available resolution of the camera, asdiscussed in [4]. To summarize, the somewhat morecomplex optical setup required to perform interferom-etry is justified by its ability to directly determine theindex of refraction and by the improved resolution itcan provide.5. Conclusion and outlookBoth speckle and interferometric imaging can be usedto obtain information about the density distribution inthe arc zone of a circuit breaker during the importantcurrent interruption phase. The speckle techniquerequires a somewhat simpler experimental setup—areference beam is not needed and the coherence lengthof the laser does not need to be considered. In addition,the sensitivity of the speckle technique can be tunedby simply shifting the location of the diffuser plateused to generate the speckle pattern. On the otherhand, using only a few additional optical elements, theinterferometric technique permits direct measurementof the index of refraction (rather than of its gradient)and does not require determination of the diffuserplate defocus distance (the accuracy of which canlimit the accuracy of the overall measurement). Thus,while both experimental approaches can be used tostudy the arc in a circuit breaker near current zero, theinterferometric approach is generally to be preferred.In general, index of refraction based optical diagnos-tic techniques provide spatially resolved informationabout the arc and the surrounding gas flow relatedto the density, arc radius, temperature, conductivity,and electron density that cannot be obtained fromconventional measurements of the arc voltage and cur-rent. Thus, these methods provide new data that canbe used to validate and further develop arc modelsused in the simulation and design of circuit breakers.References[1] Frank Reichert, Jean-Jacques Gonzalez, and PierreFreton. Modelling and simulation of radiative energytransfer in high-voltage circuit breakers. Journal ofPhysics D: Applied Physics, 45(37):375201, 2012.doi:10.1088/0022-3727/45/37/375201.[2] Yuki Inada, Shigeyasu Matsuoka, Akiko Kumada,Hisatoshi Ikeda, and Kunihiko Hidaka. Shack-hartmanntype laser wavefront sensor for measuringtwo-dimensional electron density distribution overextinguishing arc discharge. Journal of Physics D:Applied Physics, 45(43):435202, 2012.doi:10.1088/0022-3727/45/43/435202.[3] P C Stoller, E Panousis, J Carstensen, C B Doiron,and R Färber. Speckle measurements of density andtemperature profiles in a model gas circuit breaker.Journal of Physics D: Applied Physics, 48(1):015501,2015. doi:10.1088/0022-3727/48/1/015501.[4] J Carstensen, P Stoller, B Galletti, C B Doiron, andA Sokolov. Measurement of electron and heavy particledensity profiles in an axially blown, stable arc. PhysicalReview Applied, Submitted.[5] M. Stoffels, S. Simon, P. G. Nikolic, P. Stoller, andJ. Carstensen. Development of a multiperspectiveoptical measuring system for investigating decayingswitching arcs at the nozzle exit of circuit breakers.Appl. Opt., 56(7):2007–2019, Mar 2017.doi:10.1364/AO.56.002007.47

Optical Diagnostics of Switching Arcs Near Current-zero: Speckle Imaging and Interferometry

https://ojs.cvut.cz/ojs/index.php/PPT/article/download/5914/5220

Optical Diagnostics of Switching Arcs Near Current-zero: Speckle Imaging and Interferometry

Abstract

Similar works

Full text

Available Versions

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