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]
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.
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.
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
0-7803-8447-4/04/$20.0002004 lEEE
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
n i S paper discusses how this method
was used to evaluate deterioration in different tvues of
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.
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
1 I 1 1I
Groundwall Tape
Outer layer
I Inner layers I
Mica in
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
FB (1)
Mica in
Mica out
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.
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.
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 ,
. ,...- .-
20 0
10 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.
Resistance vs. Distance for Three Armours,
After VPI Processing
Figure 6 - Samples in oven awaiting measurements.
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
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
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.
Elevated Temperature Resistance of
Armour X, vs. Distance from Ground
Distance (em)
Figure 5 -Plot of the ratios of resistance aiier processing to
before processing, for all three materials.
8 3.00
Resistance Tests at Elevated Temperature
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
0i.Uns. (sm)
Figure 7 -Plot of isotherms for armour X.
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
of the samples. For each sample set, the average VE life and
standard deviation are listed. The discolouration of the sample
refers to white bleaching that is generally considered to be the
fust stage of degradation in conducting armour under high
c 4.w
3 3.w
501 2.w
Table 2
Results of Voltage Endurance and Sample Examination
VE Life (h)
ID Average SI,,
Observed First (h) Last (h)
1142.3 2 o f 5
K 926.8
1640.0 3 of5
1 M I 1857.4 I 1068.8 I 1 o f 5
I1640 ' I
(1) S = sample standard deviation
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
Eevated Temperature ksistance of
Armour 2. vs. Lliotance from Ground
c 4.00
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
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
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.
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
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.