A model chamber was applied to emulate a switching-off process which is very similar to those in real high-voltage circuit-breakers. The arc between moved W-Cu electrodes through a PTFE nozzle in SF6 was considered. Transparent windows in the chamber wall and a slit in the nozzle enabled an optical in-vestigation of the arc cross section several milliseconds before current zero. The side-on radiance of fluo-rine atom lines has been measured. Considering rotational symmetry of the arc the corresponding radial emission coefficients have been determined. Radial temperature profiles have been obtained with uncer-tainties below 10% considering change of window transmission and optical depth of the line radiation. The experimentally determined temperature profiles are used to validate a CFD simulation of the switch-ing-off process in the model chamber

Franke, St.

Gorchakov, S.

Methling, R.

Petchanka, A.

Reichert, F.

Uhrlandt, D.

English

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

Methling R. et al.: Spectroscopic Study of Arc Temperature Profiles…  Plasma Physics and Technology 2015, 2, 2, 163-166 Spectroscopic Study of Arc Temperature Profiles of a Switching-off Process in a Model Chamber Methling R.1, Franke St.1, Uhrlandt D.1, Gorchakov S.1, Reichert F.2, Petchanka A.2 1Leibniz Institute for Plasma Science and Technology (INP Greifswald), Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany, methling@inp-greifswald.de 2Siemens AG, EM HP CB R&D EN, Nonnendammallee 104, 13629 Berlin, Germany, frank.reichert@siemens.com A model chamber was applied to emulate a switching-off process which is very similar to those in real high-voltage circuit-breakers. The arc between moved W-Cu electrodes through a PTFE nozzle in SF6 was considered. Transparent windows in the chamber wall and a slit in the nozzle enabled an optical in-vestigation of the arc cross section several milliseconds before current zero. The side-on radiance of fluo-rine atom lines has been measured. Considering rotational symmetry of the arc the corresponding radial emission coefficients have been determined. Radial temperature profiles have been obtained with uncer-tainties below 10% considering change of window transmission and optical depth of the line radiation. The experimentally determined temperature profiles are used to validate a CFD simulation of the switch-ing-off process in the model chamber. Keywords: circuit-breaker, arc temperature, spectroscopy1 INTRODUCTION The development of modern switching devic-es, of high-voltage circuit-breakers in particu-lar, suffers from detailed modelling of the pro-cesses in the arc chamber and the forecast of the breaking capability. Despite of the in-creased accuracy of the models and the incom-ing atomic data, validations by experimental measurements are mandatory. Beside the comparison with pressure and voltage meas-urements, the control of the calculated plasma properties of the switching arc is one of the most sensitive methods to validate arc simula-tions including the impact of wall ablation, radiation transport and so on. Optical emission spectroscopy of the arc radiation opens a vari-ety of methods for the spatially resolved de-termination of the plasma temperature and species densities. It has been applied several times to determine the plasma temperature in arc experiments which have been designed to emulate the typical arc behaviour in switching devices like the ablation dominated arc in self-blast circuit-breakers for example [2]. Only few studies deal with experiments in real ge-ometries of circuit-breaker chambers because of the problem of optical access. Optical fibres in the chamber wall can be used to couple out the arc radiation [3]. However, the determina-tion of spatial profiles of plasma temperatures generally requires the radiation analysis over the arc cross section.  In this paper experiments in a SF6 self-blast model circuit-breaker are presented where a thin slit in the nozzle is used for optical emis-sion spectroscopy over the arc cross section. The study is focussed on the high-current phase of the ablation dominated arc in the model breaker. 2 EXPERIMENT The experimental setup consists of a model breaker with an arc chamber very similar to real devices. The electrodes (pin and tulip) are made of W-Cu with pin diameter of 15 mm. The electrodes are separated inside a PTFE nozzle with a narrow part of an inner radius of 9 mm followed by a larger turbulence volume of 25 mm inner radius surrounding the tulip. A slit of 2 mm width perpendicular to the nozzle axis is used for optical investigations. The dis-tance of the pin electrode from the edge of the slit was checked by means of high-speed vid-eo monitoring (switching tests without current or plasma) in order to verify the free spectro-scopic access to the plasma. The setup is con-fined in a closed vessel filled with SF6 at a pressure of 6 bar (absolute). Glass windows at opposite sides of the vessel provide the optical access to the nozzle slit.   Methling R. et al.: Spectroscopic Study of Arc Temperature Profiles…   164 The transparency of these windows has been controlled by transmission measurements after every third arc experiment. It turned out that a gas exchange including a complete exhaust of the fume is appropriate for a sufficient trans-mission of the windows and the vessel vol-ume. The discharges were operated with a si-nusoidal current waveform over one half-wave of 50 Hertz frequency and maximum currents of typically about 14 kA (effective current 10 kA) by means of the discharge of a capaci-tor bank.  The radiation from the slit in the PTFE nozzle is imaged through one of the vessel windows over a distance of about 2 m to the entrance slit of a 0.5 m spectrograph as illustrated in Fig. 1. The spectrograph is equipped with an intensified CCD camera (Pimax2, Roper Sci-entific) in order to obtain single 2D spectra typically with a wavelength resolution of 0.06 nm FWHM for a wavelength range of about 20 nm. Spatially, a pixel resolution of 0.14 mm/pixel was obtained.  Fig.1: (a) Experimental setup of optical emission spectroscopy. (b) Details of the model chamber showing electrodes and nozzle with 2 mm slit. Spectroscopic measurements are carried out after the moved pin electrode (right) has passed the slit. The outer chamber of the setup is omitted here. Separate measurements with a tungsten strip lamp were used for absolute intensity calibra-tion of the spectra. Spectra have been meas-ured for several time instants during the fall-ing edge of the current half-wave since the slit is blocked by the pin during the raising edge. The measurements with the CCD camera al-low one record during one shot with a typical exposure time of 200 µs.  3 DETERMINATION OF THE PLASMA TEMPERATURE PROFILE The analysis of the side-on measured radiation intensity of one appropriate spectral line of low optical thickness is used for the determi-nation of the plasma temperature profile of the arc. As in many other works, the plasma com-position and the Boltzmann distribution of ex-cited states according to the local thermody-namic equilibrium is assumed. It is expected that the arc radiation is dominated by atomic and ionic line radiation of the species F, C and S because the chamber is filled with SF6 and the ablation of the PTFE nozzle could be con-siderable. Depending on the ablation the radia-tion contribution of C and S at different times during the current half wave can be hardly forecasted. Therefore, the analysis is focused on fluorine atom lines (F I lines) in contrast to former works [2,4] where carbon ion lines were considered. Finally, the F I line at 641 nm has been chosen because the line is well isolated from other spectral lines and atomic data are available. On the one hand, the relatively low energy of the upper state of the corresponding transition has the advantage that a higher population of the upper state leads to sufficient radiation intensity also in the outer regions of the arc and makes the de-termination of the temperature profile over a larger arc cross section possible. On the other hand, the lower energy of the upper state re-quires a higher accuracy in the determination of the line emission coefficient from the measurements compared to the analysis of a carbon ion line as considered in [2,4]. Another disadvantage is the higher optical thickness of the F I line as the result of the higher popula-tion of the lower state.  The radiation intensity measured in the wave-Methling R. et al.: Spectroscopic Study of Arc Temperature Profiles…   165 length range from 640 to 660 nm and for a range of side-on positions of about 18 mm is, in a first step, calibrated with respect to inten-sity and wavelength. Then, a spectral line pro-file is integrated to determine the radiance of the F I line at 641 nm for every side-on posi-tion. In a next step, Abel inversion is applied to obtain the emission coefficient of the F I line as a function of the radial position in the arc under assumption of low deviations from rotational symmetry of the arc. The radial temperature profile is calculated from the ra-dial course of the emission coefficient accord-ing to the relation to the radiator density and the Boltzmann distribution of the excited fluo-rine atoms. The fluorine atom density is de-termined from plasma composition calcula-tions either for pure PTFE (CF4 vapor) or SF6 vapor dependent on temperature and for pres-sures measured with a pressure probe at the nozzle wall.  Fig.2: Radial temperature profiles for the current 1.7 kA for different parameters of the radiation analysis as given in the text 4 RESULTS AND ERROR ESTIMATION Figure 2 presents results for the radial temper-ature profile obtained from one and the same spectral measurement at the current 1.7 kA (0.4-0.2 ms before CZ). The results belong to different values of pressure p, window trans-mission factor T and transition probability A to demonstrate the influence of variations of these parameters. The black curve corresponds to the measured values of p and T and the tab-ulated value of A (NIST database). For all curves expect the last one plasma composition for SF6 is considered. It becomes obvious that expected uncertainties of pressure and trans-mission measurements lead to errors less than 3 % (blue and red curves). Using A from Ku-rucz data base the temperature is decreased by about 5% (green). Considering PTFE compo-sition yields larger values by about 5 % (ma-genta). All these errors are lower than the ex-pected maximum error which could result from Abel inversion at larger radial positions – about ±700 K – cf. the error bar in Fig. 2. An additional significant source of errors is the optical thickness of the F I line. For an es-timation of the influence on the temperature values the optical depth has been calculated from the integral of the absorption coefficient (considered as the emission coefficient divid-ed by the Planck function for the given tem-perature profile) over the radial position for the center of the F I line. Then, the measured line intensity is corrected by the optical depth, and the approach of the temperature determi-nation is applied again. This procedure is re-peated until sufficient convergence. It results in a temperature profile which is about 7 % larger. Since the optical depth has been calcu-lated for simplicity only for the line center and not for all wavelength values separately, the result gives an upper limit.  Table 1: Parameters of the measurements for three different currents together with the determined maximum temperature Imom  (kA) ptot  (bar) tCZ  (ms) d  (mm) Tmax  (kK) 3.5  6.8  0.7-0.5 1.80-2.50 14.0 3.3  6.8  0.7-0.5 0.68-1,42 14.6 1.7  6.5  0.4-0.2 2.81-3,74 15.3 Results for three different currents are pre-sented in Fig. 3. The corresponding parame-ters at the instant of spectroscopy (measured current Imom, measured pressure ptot, time tCZ before current zero, and electrode distance d) are given in table 1 together with the tempera-ture in the arc center. The positive distance of Methling R. et al.: Spectroscopic Study of Arc Temperature Profiles…   166 the pin electrode from the utmost edge of the slit verifies that free spectroscopic access to the plasma was guaranteed. These results have been obtained using A from NIST data base and SF6 composition without a correction with respect to the optical depth. With decreasing current the temperature profile becomes nar-rower whereas the maximum temperature in-creases.   Fig.3: Radial temperature profiles for three different currents corresponding to the parameters given in Table 1 A further question is the possible influence of eroded electrode material on the plasma prop-erties in the vicinity of the electrode. A certain cooling effect would be expected mainly due to the radiation of copper. From the spectro-scopic measurements used for temperature determination no information can be deduced about this effect since there are no significant copper lines in the wavelength range around the F I line at 641 nm. However, additional measurements reveal that copper line radiation around 510 nm is always present but with a weak contribution to the overall emission. Furthermore, no dependence between the pin electrode distance d and the calculated plasma temperature T can be deduced from the exper-imental data.  5 SUMMARY AND OULOOK Radial temperatures profiles of the switching arc have been obtained in an experiment with a model chamber during the falling edge of the current. With the spectral recording and analy-sis of the radiation of a fluorine atom line at 641 nm the temperature profiles could be de-termined with an accuracy better than 10 % despite of relatively high optical depth of this line. These results are appropriate for the vali-dation of results of a corresponding CFD model of the arc in the model chamber [1]. Next steps will include a more detailed study of the changes of the composition during the current half wave and a more sophisticated consideration of the optical depth in the analy-sis of the F I line radiation.  REFERENCES  [1] Petchanka A, Reichert F, Methling R, Franke St, abstract is submitted to this conference, (2015) [2] Kozakov R, Kettlitz K, Weltmann K-D, Stef-fens A, Franck C M, J. Phys. D: Appl. Phys. 40 (2007) 2499-2506  [3] Tanaka Y, Yokomizu Y, Ishikawa M, Matru-mura T, Kito Y, J. Phys. D: Appl. Phys. 30 (1997) 407-416 [4] Eichhoff D, Kurz A, Kozakov R, Gött G, Uhr-landt D, Schnettler A, J. Phys. D: Appl. Phys. 45 (2012) 305204   

Spectroscopic Study of Arc Temperature Profiles of a Switching-off Process in a Model Chamber

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Spectroscopic Study of Arc Temperature Profiles of a Switching-off Process in a Model Chamber

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