Conference Record of the 2004 IEEE Intemational Symposium on Electrical Insulation. Indlanapohr. IN USA. 19-22 September 2004 Comparative Evaluation of Glass Conducting Armour Materials for FormWound Stator Coils M. K. W. Stranges, J. E. Hayward, R. Omranipour, J. H. Dymond GE Canada Inc. 107 Park Street North Peterborough, Ontario, CANADA K9J 7B5 meredith [email protected] 1 . Semisondncting armour tape with a particulateloaded hinder is typically used for maintaining the ground potential plane within the slot section of a vacuum-pressure impregnated (I'PI) stator coil. This paper compares the measured resistances as a function of distance from the ground plane for three commercially available semi-conducting glass armour tapes. Results are presented from room temperature resistance tests performed before and after VPI processing, and from an elevated temperature cycle resistance test on the fully processed bars. Similar previous resistance tests and the relative merits of using woven glass and polyester fleece are discussed. The paper also describes results from voltage endurance (VE) tests on different Conducting armour and groundwall configurations, and observations of surface deterioration under the ground plane. Abslrucr: . INTRODUCTION Conducting armour is used to provide a continuous ground plane along the length of form-wound coils within the slot section of medium- to high-voltage stators. The ground plane must extend a distance away from the stator core and toward the end arm radius, usually terminating beneath a layer of semi-conducting grading material. The purpose of the tape is twofold; it suppresses electrical slot discharges, and affords protection to the outer layers of mica tape during the stator winding process. The slot section tapes generally consist of polyester felt, woven glass mat or a combination thereof, with a paint or resin binder medium loaded with conductive particles, usually graphite or carbon black. j Commercially available conducting armour is usually specified by its room-temperature resistance (in units of Rlsq) as it comes off a dry roll. This value can change by orders of magnitude following the introduction of VPI resin. A previous experiment [ I ] has indicated that for vacuum-pressure i m p r e ~ a t e d(WI) systems, epoxy resin impregnants have a strongly negative effect on the conductive properties of the material as a function of distance from the ground plane. A full evaluation of these materials cannot be made unless the room-temperature values for resistance are observed before and after VPI processing. Given that the operating conditions of the conducting armour are rarely at room temperature, it is also good practice to evaluate the resistance characteristics at elevated temperawes in the expected range of operating temperature. 0-7803-8447-4/04/$20.0002004 lEEE 216 The combination of elevated temperature and high voltage can also be expected to produce significant stresses on the conducting armour while in service. Duririghorinal operition; over time, electrical discharges and contamination in the stator winding may affect the bond between the conducting armour and the ground plane. Repetitive discharges in this region can create elevated partial discharge (PD) levels and contribute to rapid deterioration of the conducting armour under impressed stress conditions. Premature failure of the winding may result. Voltage endurance (VE)is a useful tool to studv the effect on the conducting combination ofvol&ge and elevated n i S paper discusses how this method was used to evaluate deterioration in different tvues of armour. ofa This paper is a continuation and further refinement of the experiments developed .in [I]. In the present experiments, three conducting armour materials were measured for their bulk resistance as a function of distance from the ground plane, at room temperahxe and over a range of temperatures representing stator operation. Sample bars taped with one of these materials were also evaluated for dissipation factor (DF) and VE life, followed by observation of the effects of the test upon the conducting armour surface. Two historic systems were similarly compared to the candidate material. EXPERIMENTAL SETUP Coil manufacturers often use armour tapes constructed of polyester fleece and either fully- or semi-processed carbonloaded binder resins. Quite frequently, these materials have been designed for B-stage applications and, may not he particularly suitable for VPI processing. The disadvantage of polyester fleece is its lack of robustness to oxidation from the products of electrical discharges. If certain environmental and electrical conditions are present, eventually the fleece will be eroded by the discharges, leaving a bleached white pattem. In extreme cases the tape will simply vaporize, leaving a gap between the outer layer of insulation and the grounded stator slot. The compromised mechanical and electrical bond will increase the likelihood of vibration andor hrther discharges, thus accelerating the deterioration rate. The authors were interested in establishing a sufficient ground plane while minimizing these mechanical problems. The tapes evaluated in the first part of this experiment bad woven backers constructed of glass fibres. The hulk resistance vs. distance from ground was evaluated for four materials at room temperature and at elevated temperature. VE life was evaluated for bars made using three conducting armour materials in four different configurations. The VE bars were dissected after failure to assess their condition. Resistance Test For the resistance experiment, fifteen bars of glass laminate measuring 9.5 mm x 76.2 mm x 48.8 mm were constructed with all four edges rounded to a 1.5 mm (0.060 in) radius. Holes were drilled in each end of each piece of laminate to allow the assemblies to be hung horizontally during VPI processing. This is a refmement of the sample used in the experiments described in [ I ] and allows a uniform distribution of VPI resin at each end of the sample. Using the prepared glass laminate material as a substrate, 2 half-lapped layers of mica groundwall tape were applied to the sample, ending 1.25 cm inside the edge of the hole at each end. The conducting armour to he evaluated was applied in a half-lapped configuration over the mica tape. This produced a total of five bars per type of conducting armour. samples. At each end of each sample, twelve short lengths of fme copper wire were placed at 1.25 cm intervals, to a maximum of 15 cm from the ground plane. The wire ends were twisted together to form short leads, and the electrical connection to the armour surface was assured by adding a dot of silver conducting paint at the top and bottom of each wire wrap (Figure 1). The resistance as a function of distance from ground was measured at each of the twelve points at each end of each sample; as-taped (i.e., without further processing), after a bake cycle of 2 hours at 160’ C (corresponding to a pre-VF’I bake cycle normally applied to wound stators) and again after a 2-dip VPI process and cure. Three manufacturers supplied the candidate conducting armours for evaluation. All three armours were black tapes made of woven glass hacker with conductive particles suspended in a fully processed binder carrier. The tapes were quite similar in appearance and construction, but each one is a unique product of its manufacturer’s proprietary formulation and process. For the purposes of this paper, the materials are referred to as armours “X, Y, and Z”. The bars were fitted with 2.5 cm wide aluminum plates to act as the ground plane. Fibreglass spacers were used to separate the plates, producing a common interference fit across all the Voltage Endurance and Sample Inspection The second part of the experiment analyzed VE life and the Conducting armour degradation caused by accelerated testing. For VE, three different types of fully processed conducting armour tapes were used in the construction of the samples. The first system employed a polyester fleece conducting armour with carbon-loaded fully processed binder, suitable for use in VF’I coil applications. The second system used a woven glass with a carbon-loaded fully processed hinder, and the third was glass armour overlaid with alkyd-based black conducting paint. The conductors were twenty copper bars measuring 68.5 cm x 3.8 cm x 1.3 cm with an edge radius of 0.8 nnn (1132”). All were gently sanded to obtain smooth clean surfaces, free from sharp points. Using a taping machine, an equal number of half-lapped layers of mica groundwall tape were applied to each bar, followed by a single half-lapped layer of the candidate conducting armour. Each bar then had an overlapping section of semi-conducting grading added to each end. Table 1 shows the layout of the experiment. Table 1 Sample I 1 I 1 1I I Groundwall Tape Outer layer I Inner layers I Polyester Mica in Fleece Painted Glass Mica in Armour Y Mica in Mica in Armour Y (1) FB = film-backed tape (2) N = Number of bars in the set I N (2) I ID Micaout z. FB (1) Mica in Mica out 5 5 ~ L M A key point of the experiment was that each set of IO bars having one type of conducting armour was split into two according to the manner of mica tape application. In the first set, all layers were applied with the mica side facing the conductor, which is the usual process of coil manufacturing. In the second set, the outermost layer of tape was applied with the mica side facing out. The intent was to detenniue if an and aluminum ground plane. 217 ’ I % , improved bond could be achieved by exposing the inner surface of conducting armour to the smooth mica surface of the groundwall tape. Samples with polyester fleece were made with the last layer of groundwall tape applied with mica side out only, because this material had already been evaluated with the last layer of groundwall tape applied with mica side in [I]. For the set with painted glass armour, a layer of mica tape backed on both sides with polyester film was applied under the glass to prevent paint seepage into the groundwall layers while maintaining the smooth surface against the underside of the glass armour. As the table shows, the type of conducting armour and the orientation of last layer of the groundwall tape were the only differences between the four sets of five insulated bars. The taped bars were fitted with aluminum plates set to a measured interference, and then processed through, VPI. Following processing, all of^ the bars received a roomtemperature dissipation factor (DF) test, and one bar was also tested at elevated temperature. Heaters were applied to the plates and all of the bars underwent VE testing at 25 kV applied voltage and 155°C test temperature. . DISCUSSION ' Resistance Tests at Room Temperature Figures 2 4 show the results of the measurement of resistarice as the instrument lead was moved away from the grounded aluminum plate. The data are plotted to compare the three materials on one graph, for each processing step. In each case, the values shown are averages for each distance; there are N = IO values for each distance, for each material. Figure 2 shows a plot of measured resistance against distance from ground for each sample material before any further processing. As expected, the resistance is linear with respect to distance. The graph suggests that, when coming straight from the roll, armour Y has the largest range;of resistance with distance from the ground plane. Armour .Xshows the most consistency with distance and lowest overall values, with armour Z falling between the two. Figure 3 shows a plot of measured resistance against distance from ground for each sample material, after pre-baking. The graph suggests that the pre-bake process has not changed the resistance of armours X and Z appreciably. Their performance is very similar under these conditions. The 'resistance of armour Y has dropped. There is presently no explanation for this phenomenon. Figure 4 shows a plot of measured resistance against distance from ground for each sample material after VPI processing. This is the most important condition assessed, because it reflects the condition of the conducting atpour during machine operation. Armours X and Z show similar resistance with respect to distance after VPI. Of the three materials, armour Y demonstrates the lowest overall resistance for every point along the sample. Resistance vs. Distance for Three Armours. Before Processing - Resistance vs. Distanca for Three Armours, Afler PreBake Cycle , . ,...- .- 0.0 00 50 100 150 20 0 5.0 10 0 15.0 20.0 Distance (cm) Distance (cm) Figure 3 -Room temperature resistance plot of average values for pre-baked samples. Figure 2 - Room-temperahue resistance plot of average values fox unprocessed samples. 218 Resistance vs. Distance for Three Armours, After VPI Processing moo 5000 0.0 5.0 10.0 15.0 Figure 6 - Samples in oven awaiting measurements. 20.0 Distance (cm) taken on each of the five samples at only four intervals per end, rather than twelve as in the room-temperature tests. The resulting data from each end were averaged in a manner similar to that used for the room temperature tests. Figure 4 -Room temperature resistanceplot of average values for VPI'd samples. Figure 5 shows a plot of the ratio of the resistance after W I to that of the sample after taping, for all three materials. Armour Y was most able to maintain its conductive properties following WI, and of the three materials its ratio was the most consistent with distance. Ratio of Resistance Altermefore VPI Processing, vs. Distance from Ground 25.0 - Figure 7 shows the plot containing isotherms for armour X. The armour X samples demonstrated the highest resistance at ambient temperature, and showed a drop in resistance with increasing temperature. The resistance maintained its linear 20.0 2 P 2 15.0 - The samples were tested at the oven ambient temperature (approximately 15"C), cycled through 8 0 T , IOOT, and 120°C, then back down to ambient, where they were measured once more. No ageing factor was used in this experiment; the samples were heated to temperature, the measurement taken, then stepped up or allowed to decrease to the next temperature. The resulting resistance is given in w2. z 2 Elevated Temperature Resistance of Armour X, vs. Distance from Ground 10.0 5 6.00 5.0 5.00 0.0 0.0 5.0 10.0 15.0 20.0 Distance (em) 4.00 s Figure 5 -Plot of the ratios of resistance aiier processing to before processing, for all three materials. 8 3.00 5 f 2.00 Resistance Tests at Elevated Temperature 1.00 For each type of armour, the resistance with respect to distance from ground was evaluated over a temperature range comparable to that seen by the material in service. The samples occupied considerable space in the oven, with a large number of connections (Figure 6) exiting to an extemal terminal box. To reduce set up time, the measurements were 219 0.00 0.0 5.0 10.0 0i.Uns. (sm) Figure 7 -Plot of isotherms for armour X. 15.0 20.0 relationship to distance regardless of the temperature, and ranged &om about 1 !& to 5 !& from the location adjacent to ground to the far end of the sample. Voltage Endurance After manufacturing and VPI treating the samples, a dissipation factor @F) test was performed at ambient Figure 8 shows the plot containing isotherms for armour y. temperature on all of the bars, and at 155°C on,one bar from ' The resistance of armour y increased very slightly With each group. The DF values were found to be within a normal temperature. As with armour X, the data were neatly clustered range for the system. and linear over the entire distance. The samples were tested on VE at 25 kV and 155°C. Dnring the test, the area between the grading system and the plates Elevated Temperature Resistance of was periodically inspected for degradation and erosion. Table Armour Y. vs. Distance from Ground 2 summarizes the results of VE testing and visital inspection 6.W of the samples. For each sample set, the average VE life and standard deviation are listed. The discolouration of the sample 5.w refers to white bleaching that is generally considered to be the .'. fust stage of degradation in conducting armour under high c 4.w Q stress. - t 3 3.w 501 2.w 0 K 1.03 0.W 00 5.0 10.0 15.0 20.0 Table 2 Results of Voltage Endurance and Sample Examination VE Life (h) Discolouration ID Average SI,, Observed First (h) Last (h) J 2339.2 1142.3 2 o f 5 1200 2100 K 926.8 254.3 5of5 700 700 L 1640.0 3 of5 900 1390 2561.7 1 M I 1857.4 I 1068.8 I 1 o f 5 I1640 ' I (1) S = sample standard deviation I Distance (cm) Figure 8 - Plot of isotderms for armom Y. 5 kR over the distance. between the grading tape and the plates. It should be noted that the rest of group M samples failed in less than 2000 h. Following the initial condition assessment, the plates were removed fiom the bars to examine the conducting armonr under the plates. The conducting armour of all samples in group J, and the resin between the outer layer of groundwall tape and the conducting armour were severely eroded; however, this group demonstrated longer life than the group that was prepared with the same conducting ani~ourand with the last layer of mica applied with mica side 'in [I]. When samples of these two groups with close failye time were compared, the samples with mica side out showed less erosion than the samples with mica side in. It should be noted that samples of group J with mica side out had a significantly longer VE life than their sister samples with mica side in, but comparison may be l i i t e d because these two groups were prepared and processed as two different batches at different time. Eevated Temperature ksistance of Armour 2. vs. Lliotance from Ground 6.00 -2 . 5.00 c 4.00 *Q +-2! +Ambient2 3.00 200 00 2 100 0.00 0.0 5.0 a.0 6.0 20.0 Llistance (cm) Figure 9 - Plot of isotherms for armour Z. The group K samples with failure time less than 1000 h showed minor erosion, while severe erosion was detected in samples with failure time greater than 1000 h. It should be 220 noted that only the conducting paint was eroded; the glass backer was still intact. The conducting armour of two group L samples that lasted more than 2300 h had severe erosion; the rest of the samples in this group that lasted less than 2200 h only had minor erosion. A sample from group M lasting more than 3000 h had severe erosion, while another with a life of 1100 to 1700 h had minor erosion. For groups L and M, similar to group K, only the resin containing conducting particles was eroded; the glass material was not. For this reason, the resin beneath the conducting armour could not be examined without cutting and removing the conducting armour layer. At the time of writing, similarly constructed sample bars of armours X and Z were also being tested on VE. The Conducting armour of the samples will not he cut until the VE testing is complete and all the materials are available for comparison. The results of parametric distribution analysis are shown in Table 3. The results suggest that samples with painted glass amour (group K) have significantly shorter life than the other groups. As mentioned earlier, for these samples the last layer of mica tape was different than the rest of the layers. This may have had some impact on the life of the samples. demonstrate a notable increase or decrease in resistance as the temperature increased. During temperature cycling, armour Y demonstrated the lowest resistance at ambient temperature. The principal difference between the samples was the consistency of the results; the distribution for armour Y was not only tight across the isotherms, hut also showed the least increase with distance. In general, as expected, samples with longer VE hours had a higher degree of erosion than those with shorter VE life. Regardless of conducting armour type, the electrical activity (and consequently erosion) began at interfacial surfaces such as wrinkles, and the edges of conducting armour and groundwall tape. Erosion was worse for polyester fleece; this was attributed to the fact that polyester is less resistant to localized heating and ozone generated by partial discharges than glass. For each of the materials employing woven glass as a backer, only the conducting paint or resin was eroded. The glass material remained intact. The orientation of the last layer of mica appears to have an effect on the degradation of polyester-based conducting armours. Table 3 Results of Parametric Analysis Samples with painted glass conducting armour did not last as long on test as the others evaluated in this study. This may be because of the PET-film backed mica tape used in construction of those samples, whose behaviour under corona conditions may differ from those insulated using non-PET mica tapes. For each of the materials, an accelerated degradation may occur at some critical point along the relationship between time to failure and visible degradation. Further work is required to establish this. M Scale Shape Scale 2906.8 1774.1 4762.7 2.0997 2113.5 1.1179 1354.2 3.9439 3298.4 ACKNOWLEDGEMENT CONCLUSIONS Armour Y showed the most consistent performance in room temperature resistance with respect to distance. Although its resistance before processing was the highest of the three candidate materials, after VPI it showed the lowest resistance and the least change relative to distance. The ratio of increase in resistance after VPI was the least for armour Y, and the most consistent. This suggests that VPI processing had the least effect on the room temperature resistance of armour Y. No material showed a significant deviation in the linear relationship of resistance to distance when cycled through the machine operating temperature range, nor did any 22 1 The authors wish to thank the suppliers for sample materials. The authors gratefully express their appreciation to Tim Hargreaves, Cec Mumford and the GE Peterborough manufacturing team for their hard work in preparing the bar samples. Thanks also to laboratoly technicians Maurice Coderre and Dan Leeper for their help with the resistance tests, to Fermin Pascual Espino Cortes of the University of Waterloo Electrical Engineering Department for his work on VE testing, and to Adam Nace for his assistance with the dissections. REFERENCES I. Stranges, M.K.W., I.E. Hayward, R. Omranipour, and J. H. Dymond, “A Comparative Evaluation of Various Conducting Slot Armour Materials”, Proceedings of the EICEMCW, Sept. 23-25,2003, Indianapolis IN.