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 Form-
Wound 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
1..
Abslrucr:
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
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
also describes results from voltage endurance
(VE)
tests on the conducting
ofa
combination
ofvol&ge
and
different Conducting armour and groundwall configurations, and elevated niS
paper
discusses
how
this method
was
used
to evaluate deterioration in different tvues of
observations of surface deterioration under the ground plane.
.
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.
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
j
magnitude following the introduction of
VPI
resin.
A
previous
experiment
[I]
has indicated that for vacuum-pressure
impre~ated
(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.
armour.
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 carbon-
loaded 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
0-7803-8447-4/04/$20.0002004
lEEE
216