This paper presents an integrated portable measurement system for thestudy of high speed and high temperature unsteady plasma flows such as thosefound in the vicinity of high current switching arcs. The system permits direct andnon-intrusive measurement of arc light emission images with a capture rate of 1million images per second (1MHz), and 8 bit intensity resolution. Novel softwaretechniques are reported to measure arc trajectories. Results are presented on singlehigh current (2kA) discharge events where the electrode and arc runner surfaces areinvestigated using 3D laser scanning methods; such that the position of the arc rootson the runner can be correlated to the measured trajectories. The results showevidence of the cathode arc root stepping along the arc runners<br/

Cross, K.

Kim, S.Y.

McBride, J.W.

Sharkh, S.M.

English

Southampton (e-Prints Soton)

XVIIIth Symposium on Physics of Switching Arc Ski Hotel - Nové Město na Moravě, Czech Republic  September 7 - 11, 2009  MOTION STUDIES OF HIGH CURRENT ARCS USING AN OPTICAL FIBRE ARRAY BASED IMAGING SYSTEM   J W McBride1, S M Sharkh1, K Cross2, S Y Kim3 1School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom, J.W.McBride@soton.ac.uk, suleiman@soton.ac.uk  2Taicaan Ltd, 2 Venture Road, Southampton Science Park, Southampton, Hampshire, SO16 7NP, United Kingdom, info@taicaan.com 3Hyundai Heavy Industries Co. Ltd, 1, Jeonha-Dong, Dong-Gu, Ulsan, Korea, 682-792, sykim74@hhi.co.kr     Abstract: This paper presents an integrated portable measurement system for the study of high speed and high temperature unsteady plasma flows such as those found in the vicinity of high current switching arcs. The system permits direct and non-intrusive measurement of arc light emission images with a capture rate of 1 million images per second (1MHz), and 8 bit intensity resolution. Novel software techniques are reported to measure arc trajectories. Results are presented on single high current (2kA) discharge events where the electrode and arc runner surfaces are investigated using 3D laser scanning methods; such that the position of the arc roots on the runner can be correlated to the measured trajectories. The results show evidence of the cathode arc root stepping along the arc runners.  Keywords: electric arc, arc imaging, circuit breaker.   Introduction There are many engineering applications in aerospace, combustion and electrical systems where the medium of interest is a gas at high temperature in an unsteady flow regime. Improved understanding of critical flow parameters is required to enable the optimisation of these devices and systems. Using high speed imaging and image-processing techniques, spatially and temporally resolved parameters including object location, size, area, number, velocity and direction of movement can be determined from the recorded images. This work builds upon an established methodology described fully in [1,2], and with an established research programme associated with current limiting devices, for example [3-6]. In this work an integrated system for the measurement of transient displacement and velocity in complex high speed and high temperature unsteady gas flow is described. The optical fibre array imaging system is defined as an Arc Imaging System (AIS). The system presented here has been developed commercially, [7] and is based on the system described in [1, 2]. Table 1 provides an overview of the old [1, 2] and new [7] systems. The key advantages of the new AIS over the old system, is that the temporal data is increased by a factor of 2, the resolution of the system is increased by a factor of 4, and the gain flexibility allows a significant improvement in the control of the system sensitivity. Methods The test system has been fully described in previous research publication, [1, 2]. In brief a capacitive discharge system is connected to a flexible test apparatus, allowing a peak short circuit current of approximately 2 kA over a 10 ms half cycle. The configuration of the arc chamber is shown in Fig1, where the fibre positions are shown.    For the tests conducted here, new materials were selected for each test, as shown in Table 2. In all cases the cathode was consistent with Fig.1, the vent was 15% open, and the moving contact was connected to a high speed solenoid opening device, (>6 m/sec), [5]. The optical fibres were mounted into an optical head such that the fibres are recessed 20 mm from the event, thus reducing the field of view of the fibres. The arc chamber is closed with quartz glass to protect the optical head. 4 experimental conditions are presented with the only parameter the material used on the fixed (cathode) contact. Experiments 1 and 2 used new Ag/C contact tips commonly used in these devices for improved arc root mobility. The experiments are used to determine the repeatability of the system and events. Experiment 3 used the same Ag/C material imbedded within the Cu runner. This has been shown previously to further improve arc mobility, [5]. In the final experiment the Ag/C is replaced with the Cu runner embossed with a hemispherical surface contact region, [5].     Old AIS   New AIS  Maximum Framing or sample rate 500kHz 1MHz Maximum Number of Optical Fibres 90 1024 Light Intensity Resolution 6bit (0-63)  8 bit (0-255) Memory Allocation 4K 512K Data storage time at Maximum Framing rate 8ms 500ms Sensitivity No Control x1 - x32 Table 1. System Specification           Table 2. Experiment Variable Conditions Contact Exp 1. (51)  Ag/C  Exp 2.  Ag/C  Exp 3.   Ag/C flat Exp 4.  Cu  Contour levels Default values% Values used% 1 12.5  30 2 25  32 3 37.5  34 4 50  36 5 62.5  38 6 75  40 7 87.5  42 Table 3. Default and Defined Contour Values  Results  The main focus of the experiment is on the data in experiment 1, as this provides insights into arc root motion that was not previously reported. Fig 2, shows the arc voltage and current form the experiment: on the top (Fig 2a) the whole waveform, while the left (Fig 2b) is a close up around the centre of Fig 2a. The time base shows the contacts opening after the pre-set time of 2ms. The arc moves rapidly to the arc chamber in approximately 1.2 ms, and then shows a number of pressure wave effects as the arc attempts to fully enter the arc stack [9]. The current peaks at 1.7 kA after 2.2ms. Of particular interest here is the small delay of the arc entry into the stack, which manifests itself as a reduction of the arc voltage near the centre of Fig 2b, at A. Fig 3 shows the light intensity output from selected fibres along the cathode root, while Fig 4 shows the fibre positions superimposed on a jpeg image of the arc chamber. The time base in Fig 3 has been reduced by 2000µsec. Thus the peak intensity for fibre positions U and L corresponds to the time position ~3000µsec, (first peak is at 2963µsec). The second peak in fibre U, suggest the arc is moving backwards, and is only evident in fibre position, U. Fig 5 shows the arc image at 2947µsec. The image shows a scaled and superimposed jpeg image of the arc chamber.  The arc image is created without using a group contouring algorithm used previously in [1, 2], as in this study were are concerned with detailed events during the arcing process that could be lost with group contouring, such as multiple arc paths. Each fibre position is  encoded with calibrated and scaled light intensity values.  In Fig 3 the maximum light intensity values surrounding the events at fibre position U is ~300. To enable a focus on the events surrounding the arc root motion requires a description of the arc plotting methods.   moving contactcopperarcrunner (0,0)copper arcrunnersteelbackingstriparcchambersidewallarc chambervent plateFibre positionFibres used for arc root trajectory plottingarc stackfixed contactpivoting contactmechanismYX  Fig 1. Flexible test apparatus with fibre positions         Fig 2. Arc Voltage and Current, Time base 1ms/division     Fig.3 Scaled Intensities vs Time (time base reduced by 2000μs)   Fig. 6. (a) At time 2.964ms (peak 1 in U), then (b) a stationary arc at 3.021ms V=86.3V, I = 1600A, then (c),the arc moves backwards, re=lighting fibre U (U peak 2) at 3.036ms V= 83.1V,  I=1615A  Fig. 4 Fibre positions along the cathode runner, from left to right BH,AS,AF(highlighted),U,L and E  (a)                                               (b) Fig. 5 Arc images using filters based on maximum value at fibre U At a)2947μs, and b)2959μs current voltage current voltage (a) (b) A cathode  anode (a) (b) (c) U peak 1 at 2963 μs U peak 2 at 3032 μs time (μs) Intensity  Arc Plotting  There are two modes of arc plotting, using dynamic thresholds, or using fixed thresholds. In both cases contours are plotted as a fixed percentage of a maximum light intensity value. With dynamic thresholds the maximum value is the maximum light intensity across all the fibre for a given frame number.  With fixed thresholds the maximum value is the maximum light intensity across all the fibre for all frames. Thus for the former the contours are drawn at different light levels for each frame, and this method is best suited for viewing the arc when the light intensities are low, for example at the start of the arc.  The later is best suited for an overview of the arc process when the arc is in the chamber, as the contours will be fixed, allowing a systematic comparison of arcing events. For the current study as we are interested in arcing events in the chamber and the contour levels are fixed. Table 3, shows the default contour levels, and the levels selected for the images shown in this paper. For the experiment the maximum light level was 850, while the events around fibre U have a maximum of 350. The maximum light level thus corresponds to 41% of the full data set value. Thus the contours are closely bunched around the 42%-30% light values, and fixed for all images.     Fig 5a shows the arc column at 2947 µsec having transferred from the moving contact to the anode arc runner, and conducting to the cathode runner region, outside the fixed contact region. Fig 5b shows the arc 12 µsec later, at 2959 µsec, with the arc column now in the middle of the chamber. This image shows the arc column with a single root on the cathode, but with two conducting paths to the anode surface. This event occurs for the duration of ~10µsec before the arc forms a single column to the anode. This shows the arc root moving rapidly along the anode and actually stepping. This will be discussed in more detail under the arc root trajectory plotting below.     Fig.6 shows the events around the peak in the light intensity values at fibre U also shown in Fig. 3. The first image Fig 6a, is taken 5µsec after image Fig 5b, and corresponds to the first peak in light intensity in fibre U, (see Fig 3). The arc is still in two conducting paths with a single cathode root. Fig 6b is 57 µsec later, and corresponds to the low point in light intensity between the two peaks in fibre U. In this instant the arc voltage is 86.3V, and there is no apparent light in the position of fibre U; a consequence of the close contouring of the image, (Table 3). Fig 6c, is 15µsec later and corresponds to the second peak in fibre U at 3036µsec. In this instant the arc voltage has reduced to 83.1V, and the fibre position U has a signal. The only possibility here is that the arc has moved backwards in the chamber. The observation is consistent with the reduction in the arc voltage apparent in Fig 2b.   Arc Root Plotting  The arc root plotting method was first reported in [8], and is fully described in the reviews [1, 2]. The method is based on an intensity based position averaging technique over selected fibre positions near the surface of the runners (shaded fibres in Fig 1). For the cathode root in experiment 1, the first fibre position B is allocated Y =1mm, and the AZ fibre Y= 40mm (see Fig.4). In the current study the method has been developed further to allow for accurate positioning, since it is essential to define a filter value below which the data is excluded. The averaging method allows for 10 nm resolution in the positioning along the Y axis. The results for the cathode root motion are shown in Fig 7a, and for the anode root motion in Fig7b. Fig 8 shows a close up of the cathode root motion. The results show the arc event starting shortly after 2000µsec, see Fig 2, and the cathode root moving rapidly to the 7 mm position. The cathode root then appears to be stationary for a short time before an irregular stepping motion is exhibited.  These results are consistent with previous observations, on root motion, although these have never been fully explained [10]. The arc roots also exhibit a reversal in the motion around 2964-3036 µsec.   05101520253035401 1001 2001 3001 4001 5001 6001Time (usec)Displacement (mm)0510152025303540451 1001 2001 3001 4001 5001 6001Time (usec)Y Displacement Fig. 7a,  Cathode root motion (Filter= 3);  b Anode root motion (Filter= 3)  0510152025301 101 201 301 401 501 601 701 801 901 1001 1101Time (usec)Y DisplacementFig. 8 Close up of cathode root motion between 2000-3000µsec, (U1 peak at 964 and U2 peak at 1036 Surface Analysis Methods To provide insight into arc root motion, both anode and cathode runners were removed after a single test in experiment 1, and analysed using a 3D con-focal laser profiler, [7]. The anode runner surface exhibits minimal surface damage, however the cathode surface exhibits significant damage, as shown in photos Fig 9.  The 3D surface profile data is shown in Fig 10. The image shows the Ag/C contact surface on the left where there is substantial darkening of the Cu surface. In sections B,C and D, the root appears to have a preference for the upper edge of the runner, which is consistent with the thermal properties of the arc. However around image area C there appears to be a large number of smaller cathode spot regions in the middle of the runner. These spots become more pronounced further along the runner to the right, as there is less darkening of the cathode surface. The spotted areas do not appear in the 3D images, where the colour scale reflects the vertical geometry of the surface. This is because the spotted area although apparent in the photo do not show a significant change in the surface topography, unlike the edges of the runner where there is evidence of melting. A  B  C DF Fig 9. Image of the cathode surface after experiment, with approximate positions of 3D surfaces.   Fig 10. 3D surface profiles of the cathode surfaces, 5mm x 5mm (5micron spacing) 1001x1001 pixels  Further inspection of surface area D is shown in Fig 11 and 12, with a CCD microscope camera view of the spot feature F, in Fig 11. The microscope image clearly shows the darkened area around the cathode spot. The green line and data value correspond to the vertical height in microns of the laser spot also shown as a cross-hair in the centre of the image. A re-scaling of the colour scale in D produces the image in Fig.11, where now 4 cathode spots become apparent. These are labelled F, G,H,I. The first three regions are apparent in the photographic image in Fig 9, but not the small feature I. Fig 12, further enhances the image of the cathode spots with a full 3D view of the surface features.  The image shows that feature F exhibiting a crater effect at the leading edge, while the trailing edge shows a build up of material. This is also consistent with other observations of erosion of cathode spots [11].  A B  C  D   F G H I Fig. 11, Detailed TaiCaan XYRIS profiler view of region D, with an inset CCD camera image of spot F, (CCD visual area 1mmx1mm).    F H IG Fig. 12, Detailed TaiCaan XYRIS profiler view of region D, with colure scale to enhance the 3D features   Conclusions High speed optical imaging of electric arcs and 3D surface profiles in a circuit breaker reveal evidence of the cathode arc root stepping along the arc runners.   References [1] P.M. Weaver, J.W. McBride, and C.C. Siew, “Integrated measurement system for high speed unsteady plasma flow and its application to electric arcs”, IEE Proceedings - Science, Measurement and Technology, Vol.150, No. 4, pp. 153-160, 2003.  [2] J.W.  McBride and P.M. Weaver, “Review of arcing phenomena in low voltage current limiting circuit breakers”, IEE Proceedings - Science, Measurement and Technology, Vol. 148, No. 1, pp. 1-7, 2001. [3] P.M.Weaver, K. Pechrach and J.W. McBride, “Arc root mobility on piezoelectrically actuated contacts in miniature circuit breakers”, IEEE Transactions on Components and Packaging Technologies, Vol. 28, No. 4, pp. 734-740, 2005.  [4] P.M. Weaver, K. Pechrach and J.W. McBride, “The energetics of gas flow and contact erosion during short circuit arcing”, IEEE Transactions on Components and Packaging Technologies, Vol. 27, No. 1, pp. 51-56, 2004.   [5] J.W. McBride, K. Pechrach and P.M. Weaver, “Arc motion and gas flow in current limiting circuit breakers operating with a low contact switching velocity”, IEEE Transactions on Components and Packaging Technologies, Vol. 25, No. 3, pp. 427-433, 2002.  [6] J.W. McBride, K. Pechrach, and P.M. Weaver, “Arc root commutation from moving contacts in low voltage devices,” IEEE Transactions on Components and Packaging Technologies, Vol. 24, No. 3, pp. 331-336, 2001. [7] www.taicaan.com. [8] J.W.McBride, P.M. Weaver, and P. Jeffery, “Arc root mobility during contact opening at high current”, Proceedings of IEEE Holm conference on Electrical contacts, Chicago, pp.27-34, September 1996. [9] H.Weichert, “Shockwaves in arc chambers of miniature circuit breakers”, Proceedings of the 36th IEEE Holm Conference and the 15th International Conference on Electric Contacts, pp. 580-585,1990. [10] P. R. Zeller, and W. F. Rieder, “Arc structure, arc Motion, and gas pressure between laterally enclosed arc runners”, IEEE Trans. Components and Packaging Technology, Vol. 24, No. 3 , pp. 337-341, 200. [11] J. W. McBride,  J. K. Cross and S. M. Sharkh, “The evaluation of arc erosion on electrical contacts using three dimensional surface profiles”, IEEE Transactions on Components, Packaging and Manufacturing Technology, Vol. 19, No. 1, 1996, pp. 87-97. 

Arc motion and gas flow in current limiting circuit breakers operating with a low contact switching velocity”,

Arc root commutation from moving contacts in low voltage devices,”

Arc root mobility during contact opening at high current”,

Arc root mobility on piezoelectrically actuated contacts in miniature circuit breakers”,

Arc structure, arc Motion, and gas pressure between laterally enclosed arc runners”,

Integrated measurement system for high speed unsteady plasma flow and its application to electric arcs”,

Review of arcing phenomena in low voltage current limiting circuit breakers”,

Shockwaves in arc chambers of miniature circuit breakers”,

The energetics of gas flow and contact erosion during short circuit arcing”,

The evaluation of arc erosion on electrical contacts using three dimensional surface profiles”,

Motion studies of high current arcs using an optical fibre array imaging system

Motion studies of high current arcs using an optical fibre array imaging system

Abstract

Similar works

Full text

Available Versions

Southampton (e-Prints Soton)