JEEE Transactions on Dielectrics and Electrical Insulation
Vol. 9, No. 4; August 2002
Experience With Development and Evaluation of
Corona-Suppression Systems for HV Rotating Machines
Hassan El-Kishky
Department of Electrical Engineering
The University of Texas at Tyler
3900 University Blvd.
Tyler, TX 75799, USA
Beant S. Nindra
National Electric Coil
800 King Avenue (43212)
Columbus, OH 43216, USA
Mazen Abdel-Salam
Department of Electrical Engineering
Assiut University
Assiut, 71516, Egypt
and Eugene Williams
California Department of Water Resources
Divisron of Engineering
1416 9th St
Sacramento, CA 95814, USA
This paper presents the results of a project aimed a t the development of reliable
corona-suppression systems for high voltage rotating machines. These systems are
based on both conducting and semi-conducting dry and B-stage tapes a s well as
paints of different resistivity. Three groups of anti-corona systems were developed;
namely, paint (on the slot portion) paint (on the slot exit) system, paint (on the
slot portion) tape (on the slot exit) system, and tape (on the slot portion) tape
(on the slot exit) system. Sample results of standard evaluation and acceptance tests
are presented. The accelerated life-aging test was run on test coil samples at the
National Electric Coil HV testing facility. A numerical 2-dimensional finite difference model for the electric potential and field analysis along the end-turn zone was
developed and the model results are used a s guidelines through the design process
of the grading system. Laboratory test results on randomly selected sample coils
were confirmed by an independent testing facility. The proposed corona-suppression systems were applied on thousands of production coils.
H E failure of a generator or a large motor can cause
considerable financial losses to the utilities and industries due to non-planned outage costs in addition to the
cost of repairing and/or rewinding the machine. The reliability of HV machines depends mainly on the soundness
and integrity of the insulation system [1-141. Exposed to
complex electrical, mechanical and environmental stresses,
the stator winding’s main insulation is the most vulnerable
to deterioration and failure. Long exposure to internal and
surface discharges due to high localized concentration of
electric stress can directly accelerates the deterioration
and ultimately, complete failure of the ground-wall insulation. In particular, intensified electric stress in the overhang portion of the high voltage stator winding of hydroelectric machines is of special concern. Breakdown of the
gas adjacent to the insulation in the immediate vicinity of
the slot exit leads to the development and propagation of
discharges over the end-turns [1,5-7,13-201 which in tum
results in gradual degradation of the ground-wall insulation. Therefore, the application of stress-grading systems
along the end-turn is considered essential for high voltage
It is a common practice that corona-suppression systems are designed such that a high voltage winding passes
a set of standard acceptance tests [1,13,14,21-241 as well
1070-9878/1/$17.00 0 2002 IEEE
El-Kishky et al.: Experience With Development and Evaluation of Corona-Suppression Systems
as meets customer’s special requirements. It is normally
required that a HV winding passes the high potential
blackout test at a level ranging from 100 to 150 percent of
the nominal line-to-ground voltage with no visual corona
discharges [1,14,22-241. Yet, it is not uncommon that special requirements are set up, such as passing a voltage level
that exceeds 200% of the nominal line-to-ground in the
high voltage black-out test without detecting any visual
corona discharge on the stress-grading system. Moreover,
a randomly selected, sample of coils has to pass the standard voltage-endurance test with no repairs on the
corona-suppression system.
Proper design, analysis, and development of corona
suppression systems can significantly reduce the possibility of surface discharge inception and hence, extend the
life of the H V stator winding which in turn, significantly
enhances the reliability of H V machines.
This paper presents the results of a research project
carried out at National Electric Coil and University of
Texas at Tyler towards the development of reliable
corona-suppression systems for HV machines. The main
drive behind the project was the development of coronasuppression systems, which can withstand elevated thermal and electrical stresses. Moreover, the trend of thinning ground-wall insulation for the purpose of machines
up-rating and subsequent stress intensification, i.e. use of
higher design electrical stress has become a major challenge to the research and development of insulation and
corona suppression systems in HV rotating machines. New
insulation and anti-corona systems that can withstand
higher electrical and thermal stresses have to be developed. Moreover, new test requirements and standards
have to be generated for the development, evaluation, and
acceptance of these systems. Within this work, several
anti-corona systems were developed, tested and applied to
thousands of production coils.
pared into flat rectangular samples, which are usually left
to dry at the ambient temperature in case of paint or
oven-cured in case of tapes. After taking multiple resistivity measurements, a mean value is usually checked against
the required design range. The process may be repeated
with adjustments if deemed necessaly. It is quite normal
that measuring on flat samples may yield different results
from that along the corner of a finished coil. Perhaps this
may be attributed in part to the non-uniform application
of the material in addition to the difficulty of mounting
the measuring probe over the coil surface. Therefore, it is
important to take measurements on finished coils and establish a range of surface resistivity for the corona-suppression system.
Nevertheless, visual inspection of the corona-suppression systems on the slot portion and the end-grading zone
is of considerable importance during the development
stage and before undergoing electrical testing. Surface
roughness, foreign material, and local damage of coatings
and tapes can lead to electric field intensification and SUIface discharge. Based on the extent of the system irregularities, a simple touch up repair or reapplication of the
system maybe performed before proceeding into further
Although, it is evident that the immediate part of the
stress-grading zone next to the overlap area plays the most
decisive part in the grading process, it is insightful to assess the potential distribution along the whole grading
area. Figure 1 shows a schematic of the end-grading zone
of a HV coil. The potential distribution along a paint system is presented in Figure 2. The potential distribution
along the stress-grading system was measured using a small
sphere contact probe tied to a standard 100 kV electrostatic voltmeter [l].The effect of stray capacitance was
significant on the measured potential distribution along
the stress-grading system.
Three groups of corona-suppression systems were developed and investigated; (a) cell (that portion of the system within the slot) conducting paint and stress-grading
paint (paint-paint), (h) cell conducting paint and stressgrading tape (paint-tape) and (c) cell conducting tape and
stress-grading tape (tape-tape). It is worth mentioning that
all tapes and paints used in this project are either silicon
carbide, graphite, or silicon carbide and graphite based.
In the mixing stages, necessaly amounts of graphite are
added to control the paint resistivity.
A set of sample coils of each system are prepared for
both evaluation and acceptance testing phase. Moreover,
each coil with stress-grading system applied on all corners
+tress-grading s y s t e v
Mixing the ingredients to the required viscosity and low
voltage surface resistivity range is the primer of developing corona-suppression systems. Normally, those are pre-
Copper conductor (HV electrode)
Figure 1. Schematic of the end-turn region of a high voltage ma-
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 9, No. 4; August 2002
sional problem. (2) The stress-grading system is approximated by a linear resistive layer of finite thickness. ( 3 ) No
free charge exists at the interface between the gradient
system and the adjacent media.
The electrostatic field distribution is obtained by solving the boundary-value problem formulated in the region
under consideration
Msvurlng pdrd fmm sk4
where, 6 is the electric potential and 2 is the complex
permitivity of the medium, which is given by
Figure 2. Measured potential distribution along the stresr-grading
of a 13.8 kV coil at room temperature Ill.
can he treated as a sample of four test specimens provided that consistency was maintained through the application process and a minimum leads’ clearance does exist.
This phase of testing consists mainly of the high voltage
blackout test and the accelerated aging voltage endurance
test [13,21-301. All tests are performed at room temperature. Systems with reproducible results of the high voltage
blackout test on all samples under similar conditions are
subjected to the accelerated aging voltage endurance test.
The integrity of the anti-corona systems under elevated
thermal and electric stresses can he assessed through the
study of several parameters and characteristics. This includes in particular, the recording of amount of discoloration and powder generation as well as any surface discharge or arcing spotted on the corona suppression system with reference to location and elapsed time under
It is hoped that a following paper that will shed some
light on the interdependence of thermal and electric
stresses and the dynamic nature of degradation process in
the end-turn zone of HV machines will he available to the
reader towards the conclusion of this project.
The model is based on numerical solution of the hounda l y value problem governing the electric.field distribution
along the end-turn region using the finite difference
method (FDM) [31-331. It is worth mentioning that although the model is not meant to provide extensive analysis of the corona-suppression system, it provides some
necessary guidelines throughout the design process. Referring to Figure 1 , x-, y-. and z-coordinates extend along
the axial coil length outside the slot, the coil height, and
the coil width, respectively. To simplify the modeling process, the following assumptions are made: (1) Electric field
is not z-dependent which reduces the model to a 2-dimen-
where p is the volume resistivity of the medium.
The potential solution is unique when Jr satisfies a set
of specified boundary conditions. Referring to Figure 1,
Dirichlet boundary condition must be imposed on the
copper conductor where the voltage is given by the nominal line to ground value, V
Similarly, on the grounded screen, Figure 1, the potential
boundary condition is given by
In the absence of a perfect conductor, no surface current exists at the insulation-stress grading, insulation-air,
or the stress grading-air interface. Using the complex
perimitivity notation given by equation (3), the Neumann
boundary condition applies at all interfacial boundaries.
At the interface between the ith and kthmedium
Extending the modeled region between ( x = 0) which is
located at nearly 4“ deep inside the slot, Figure 1, to the
end of the corona-suppression system ( x = L ) , the horizontal component of the electric field across both ends
( x = 0, x = L ) can be ignored and the boundary conditions on both sides of the insulation may be approximated
Considering the geometrical complexity of the domain
and the type of boundary Conditions, the solution of this
boundary-value problem can only he obtained numerically. In this study FDM with non-uniform grid is used to
calculate the electric potential and field distribution along
the modeled end-turn portion of a high voltage coil, Fig-
El-Kishky et al.: Experience With Development and Evaluation of Corona-Suppression Systems
ure 1 , A versatile FORTRAN.90 program was built for
analysis of the problem under consideration. The program
is capable of simulating both linear and non-linear resistive stress-grading systems, however, only the results of a
simple resistive grading system are presented here. In a
nonlinear resistive grading system, the resistivity can he
expressed empirically as a function of both the electric
field and temperature, p = f ( E , T ) . The resistivity is updated each iteration using the most recent value of the
electric field and temperature. If $PI, E f " , p p l , TP' and
.+I, El""', p. ..l " + ' ) , Tpt" are the corresponding.values
of the electric potential, field, and temperature at node k
after iterations n and n + 1, respectively, this iterative
process can he given by
Table 1. Low voltage resistivity measurement of corona-suppression
coatings at room temperature.
of cell
k Qsq
k n/sq
Surface resistance of
end-grading system
15.0 to
20.0 to
20 to
2.0 t"
3.5 to
3.5 to
Table 2. Experimental test resuits of corona-suppression systems on
13.8 kV Epoxy coils.
endurance test at
ac high voltage
30kY and
black out test
Both electric field and thermal analysis of nonlinear resistive grading systems are yet to he presented in a
manuscript under preparation.
3 four-coils
no visual
corona spotted
at 16kV
3 four-coils
no visual
corona spotted
at 16kV
spotted after
6 four-coils
no visual
corona spotted
at 22kV
no deterioration
spatted after
470 Hrs
2 four-coils
(12kV and
I6 kV level)
no visual
corona spotted
at 12kV and
16kV level
not applied
The project was focused on developing corona suppression systems for both Epoxy and Polyester vacuum pressure impregnated (VPI) coils taking into consideration the
variation in the design and manufacturing process of both
systems such as the average electric stress in the insulation or volts-per-mil (VPM) and the curing cycle requirements.
Figure 2 shows the measured potential distribution
along the stress grading system of a 13.8 kV, Epoxy Coil at
room temperature. The salient non-hearity may be attributed to the electric field intensification in the region
adjacent to the slot exit.
Both conducting and stress grading paints and tapes are
classified according to the supplier and the initial resistivity level. Several samples of each group were manufactured and the low voltage surface resistivity at 500 V and
1000 V (0.5 and 1.0 kV/cm) levels was measured at different locations along the stress-grading zone as well as on
the cell portion. The average low and average high values
are used to determine the range of variation of surface
resistivity. Samples with anti-corona systems' surface resistivity values drastically different from the established range
are corrected or reworked. All samples are subjected to ac
high voltage blackout test, then only passing systems in
which no sign of visual corona during black-out of repetitive samples are subjected to accelerated aging through
voltage endurance test.
Table 1 shows the average measured surface resistance
along the cell portion and along the stress-grading zone
spotted in a
2" wide zone
next to the
overlap area after
130 Hrs
no deterioration
for the accepted anti-corona systems according to the criterion mentioned above.
The results of the ac HV blackout and voltage endurance testing on Epoxy coils with different corona suppression systems are displayed in Table 2. Although some
of the systems showed better performance than others,
generally, all the anti-corona systems were able to meet
the basic requirements of application on high voltage rotating machines rated 6.6 kV and above. Despite the fact
that it is economic and easier to apply and repair, the
paint-paint system suffers from application inconsistency
which may he attributed to low-skilled workmanship in
addition to its vulnerability to different levels of visual
damage and its volatility during installation. On the other
hand, anti-corona systems based on B-stage tapes showed
better consistency, durability and discharge resistance.
Yet, the lack of bonding of anti-corona tapes to the
ground-wall insulation could he a major problem, which
may he attributed to the lack of resin contents in the Bstage tape in addition to other factors such as pressure
and other curing cycle parameters. Repair of such systems
IEEE Transactions on Dielectrics and Electrical lnsulotion
Vol. 9, No. 4; August 2002
Figure 3. Stress-grading paint deterioration under elevated electrical and thermal strcsses.
is difficult and may become even impossible without damaging the upper most layers of the ground insulation.
Figure 3 shows one of the coil samples that passed a 16
kV ac HV blackout test undergoing the voltage endurance
test [21]. There is an obvious discoloration across a 5 cm
wide area of the stress-grading system past the overlap
zone. The discoloration may be attributed to relatively high
discharge activity developed in the region after about 130
h in voltage endurance, which demanded intenention to
repair the paint.
Systems based on cell conducting paint along with Bstage stress-grading tapes have shown the best performance through the ac HV blackout test on Epoxy coils,
Table 2. Test voltages exceeding 22 kV line to ground were
applied with no trace of visual corona along the anticorona system. Figures 4a and 4b show the B-stage
stress-grading system applied on a 13.8 kV Epoxy coil before and after the voltage endurance test. No visual corona
or damage of the stress-grading system was detected within
or after the voltage endurance test, Figures 4a and 4b.
Moreover, the surface resistivity of the stress grading
structure remained within the established design range,
Table 1, after exposure to elevated thermal and electrical
stresses for more than 400 hours. Nonetheless, no deterioration was noticed on the end-grading portion of the
anti-corona system after taking the coils out of the voltage
endurance test, Figures 4a and 4b. This may be attributed
in part to the inherent capability of the resin in the B-stage
tape to withstand elevated thermal stresses. Therefore, we
recommend the paint-tape system for application on machines with up rated windings or running under nonfavorable environmental conditions.
According to a contract between National Electric Coil
and one of its customers, the paint-tape system applied on
the customer's 13.8 kV coils has to be tested by an independent laboratoty selected by the customer in addition
to all contract testing performed at National Electric Coil's
Figure 4. (a) Paint-tape corona suppression system before voltage
endurance testing; (b) Paint-tape corona suppression system after the
voltage endurance testing.
HV testing facility. The system has to pass a voltage level
of 16 kV in the high potential blackout test with no visual
corona and also pass the voltage endurance test with no
repairs on the system. Two 10-coils samples of the painttape anti-corona system were sent to a Canadian independent testing laboratoty for evaluation of the new system
along with the assessment of the integrity of the groundwall insulation. Sensitive night vision cameras were used
throughout the high potential blackout test where no visual corona was spotted at a level of 16 kV. The laboratory reported no repairs done on the system through the
voltage endurance test.
The tape-tape corona suppression system applied on
Epoxy coils showed good performance under ac HV
blackout test where two stress-grading system designs were
made to meet a minimum of blackout test requirements of
12 kV and 16 kV, respectively. The main experience was
the difficulty of controlling the surface resistance of the
cell tape as well as adherence of the tape to the groundwall insulation. The latter may depend to some extent on
the porosity of the tape as well as on the VF'I cycle parameters.
Table 3 shows the results of testing Polyester coils with
tape-tape corona suppression systems. Cell conducting
tapes as well as stress grading tapes from different suppli-
El-Kishky et al.: Experience With Development and Eualuation of Corona-Suppression Systems
Table 3. Experimental tcst results of corona-suppression systems O n
13.8 kV Polyester coils.
ac high voltage
ac high voltage
black out test
black out test
corona(B- stage stress
(dry stress-grading
grading tape)
Supplier A
2 four-coils
Supplier B
2 four-coils
faint glow spotted
at lOkV
not applied
no visual corona
spotted at 16kV
not applied
Supplier C
2 four-coils
sam des
faint daw spotted
at lOkV
no visual corona
spotted at 12kV
ers were experimented including dry VPI and resin-rich
B-stage tapes.
Some contribution is claimed in successfully applying
resin-rich B-stage stress-grading tapes to low cure-temperature Polyester coils. That system is based on the slow
curing of the stress-grading tape at low temperature during the post-bake cycle of the Polyester. The system based
on tapes from supplier B gave the best results that were
easily reproducible on different samples of Polyester coils,
Table 3.
Sample of the FDM model results is presented in Figures 5 to 8. The effect of the gradient system resistivity on
the total electric field distribution along the end-turn of a
modeled 13.8 kV coil is shown in Figures 5 and 6. Considerable local stress intensification is noticed at the slot exit
for a high resistivity stress-grading system. Lowering the
resistivity of the gradient system significantly lowered the
local field intensification, Figure 6.
Figures 7 and 8 show the total electric field distribution
along the modeled end-turn region of a 26 kV class high
voltage coil with different applied voltage stress values,
E,. The intensification is relatively less than that on a 13.8
Distance from slot exit, m
Figure 6. Total electric field distribution along the end-turn of a
S/m and E. = 2.36 kV/mm.
13.8 kV coil with u = 4x
Distance from slot exit
Figure 7. Total electric field distribution along the end-turn of a 26
kV coil with u = 4 x lo-'' S/m and E. = 2.36 kV/mm.
0 1
Distance from slot exit, m
Figure 5. Total electric field distribution along the end-turn of a
13.8 kV coil with u = 4x lo-'" S/m and Em= 2.36 kV/mm.
Distance from slot exit, m
Figure 8. Total electric field distribulion along the end-turn of a 26
kV coil with u = 4x lo-'' S/m and E, = 3.15 kV/mm
ZEEE Transactions on Dielectrics and Electrical Insulation
kV class coil which may be attributed in part to a higher
thickness of the main insulation in the 26 kV coil which
enhance the non-linearity of the y-component of the main
insulation electric field. In other words, it means less electric field at the interface between the main insulation and
the end-grading system. Thinner insulation may lead to a
higher local field intensification and subsequent discharge
along the end-tum zone and within the insulation and
hence, overall shorter insulation life.
This may he of special interest to the utilities and coil
manufacturers in regards to the issue of up rating electrical machines output power. Although thinner insulation
simply means more space for copper and hence higher
output and may improve heat transfer on the one band, it
does lead on the other hand to higher electrical stresses in
the main insulation and along the gradient system, which
may lead to surface discharge and faster degradation of
the insulation system. The model can be used to predict
the optimal up rating level of a HV winding without overstressing the main insulation.
1. Several corona suppression systems based on conducting and stress-grading paints and tapes were developed. The new systems withstand voltage levels ranging
from 110% to well above 275% of the nominal line to
ground voltage on the HV blackout test.
2. The paint-tape anti-corona system with superior performance on both the ac HV blackout test and the voltage
endurance test is recommended for application on high
voltage machines with up rated windings.
3. Stress-grading system based on resin-rich B-stage
tapes was successfully applied on low curing-temperature
Polyester high voltage coils.
4. A 2-dimensional FDM model is introduced to predict the electric stress distribution along the end-turn region of high voltage machines.
5. Local intensification of the electric field is significant1y:affected by the resistivity of the gradient system.
6. The model predicts the resistivity range of the gradient system in order to keep the maximum surface stress
below a threshold value.
7. i h e model can he used to predict the optimal up
rating level of a HV winding w‘ithout overstressing the
main insulation.
The authors would like to acknowledge the support from
National Electric Coil, Inc. and University of Texas at
Tyler throughout this work. Special thanks go to Dan
Bucklew and Fred Dawson of National Electric Coil, Inc.
for their great support of the project.
Vol. 9, No. 4; August 2002
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