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IEEE
Transactionson Power Delivery
Vol.
6 No. 2, April 1991
ELECTRICAL PERFORMANCE OF POLYMER HOUSED ZINC OXIDE
ARRESTERS UNDER CONTAMINATED CONDITIONS
V. Chaudhry and R. S Gorur
Dept. of Electrical and Computer Engr.
Arizona State University
Tempe, AZ
85287-5706
ABSTRACT
The electrical performance of polymer housed zinc oxide
distribution arresters has been investigated in a fog
chamber. Two types of housing materials, namely,
copolymers of ethylene and propylene (commonly known
as EPM rubber) and terpolymers
of
ethylene propylene and
diene (commonly known as EPDM rubber), both containing
alumina trihydrate (ATH) filler, have been examined. It has
been demonstrated that under relatively low (250 pS/cm)
and moderate
(1000
to 2000 pS/cm) fog conductivity,
degradation in the form of tracking and erosion of the
housing occurs. On some housings, degradation is
dominant in the region of the mold line and is due to non-
uniformity in filler dispersion. The electric field has been
computed and a few electrode modifications to improve the
surface electric field distribution are suggested.
K e y words: polymer housing, EPM, EPDM, dry band
arcing, tracking and erosion.
1. INTRODUCTION
Polymers have been in use for high voltage insulation
applications, such as, line insulators and cable terminators,
for the last
20
years. Their use as zinc oxide arrester
housing for outdoor electr ical systems is very recent [l] The
advantages of choosing polymer insteadof porcelain as the
housing material for arresters are, light weight, shorter
length of arrester possible due to the use of an insulated
mounting bracket, reduced risk of shattering and explosion
of the housing during arrester failure, improved resistance
to moisture ingress due to the close fitting provided by the
polymer, etc. [l] Presently, polymer housed arresters are
available up to
36
kV rating, and there are plans to make
them for higher voltages. Currently available arresters use
either an EPM or an EPDM polymer, and it is expected that
silicone rubber housed arresters will be available shortly
PI.
Considerable amount of service experience and laboratory
tests have demonstrated that there are two types of
polymers, namely silicone rubber and ethylene propylene
rubber (EPR, which is the generic name for EPM and EPDM
polymers), that are well suited for outdoor applications.
Potential problems such as resistance of the material o UV,
chemicals, moisture, etc have been overcome [3]. major
concern which still remains in the use of these materials is
M.
Dyer and R. S hallam
Salt River Project
Phoenix
Arizona,
85072-2025
their behavior under combined high electric stress, moisture
and outdoor contamination. Corona and dry band arcing
are promoted under such conditions
[4].
Unlike with
porcelain, corona on polymers can cause degradation in
the form of radial cracks, and pin holes, and dry band arcing
can cause degradation in the form of tracking and erosion
[3]
racking is defined as the formation of a conducting
layer of carbon deposit formed on the surface due
to
polymer degradation. Erosion is defined as the loss of
material with time.
Tracking and erosion can be minimized with the addition of
inorganic filler, such as alumina trihydrate (ATH). Although
the present materials use sufficient ATH filler (>50% by
weight), tracking and erosion can still occur due
to
non-
uniformity in the filler dispersion[5]. his can happen during
mixing and molding operations. It would be important to
establish limits on the filler dispersion such that premature
housing degradation is prevented.
Zinc oxide arresters for higher voltage ratings are made by
stacking multiple low voltage sections
[l]
For a constant
diameter, the electric field near the high voltage electrode
will increase with the applied voltage, thereby increasing
the risk for corona at operating voltage. It would be valuable
to determine the highest voltage class that polymer housed
arresters could be used without problems from corona.
In polymer housed arresters, the housing fits more snugly
and tightly to the zinc oxide column than is obtained with
porcelain, hence providing the greater resistance to
moisture ingress [l] However, this also increases the
capacitance between the housing and the arrester column.
Consequently, there is a concern by the users that under
contaminated conditions the internal arrester current may
be affected by the external leakage current
[6].
his could
result in heating of the zinc oxide blocks and eventual
failure of the arrester.
If
this is true, an evaluation of the
effect of contamination on the internal current should be
performed.
Another problem with the use of polymers is the lack of a
meaningful laboratory test to predict the performance in
service. Dry band arcing is largely controlled by the surface
electric stress, which varies for different devices. Although
there is data available for polymer outdoor insulators
[7]
and cable terminators
[a],
his may not be applicable for
polymer housed surge arresters. Therefore, it is essential to
have sufficient laboratory data on polymer housed arresters
such that a meaningful accelerated test can be developed
for these devices as well.
5 0 S 295-6 P? i.iRU
A
paper recorninended and approved 2. EXPERIMENTAL
3 y t h e
I G r :
Surge Protective Devices Committee
of
t h e I % P o ue r E n g in e e r in g S o c i e t y f o r ; > r e s e n t a t i o n Fig. 1shows the schematic of the fog chamber used in the
a t t h e I X E / E ; S
1990
Sumer vieeting, Kinneapolis ,
present study. The chamber is made from stainless steel
A.llnnesota, Ju t : 15-19, 1950. ?.:,muscr ipt subm itte d sheets and is 3.6mX3.05mX2.5m high. Four nozzles
August
2 2
1987; made available f o r p r i n t i n g constructed according to IEC specification
[9],
nd located
;lay 15, 1990.
one on each wall of the chamber, are used for fog
generation. Distilled water, to which sodium chloride is
added, is recycled from a 100
I
capacity reservoir. A fresh
.- .
0885-8977/9
/O4OO-0696 0
1 ooO 991
IEEE
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Sample
ID
A
R EGU lMTOR
a GAUGE
Description Housing Leakage ATH
Material Distance
Level
Complete EPDM
395 mm ~ 5 0 %
RE1URN
PUMP
Fig.
1 :
Schematic Diagram of Fog Chamber
saline solution is prepared daily in order to limit the
increase in the water conductivity to less than 10 of the
initial value.
I Arrester
The high voltage is supplied from a 15 kVA, 50kVl120V
transformer bank, located inside the chamber. The output
voltage is controlled by a 0-120 V,
50
A variac, located
outside the chamber. The maximum fluctuation in the
voltage was less than
5 .
Table 1shows the details of the samples evaluated. Some
of the samples, used were complete arresters with zinc
oxide disks and some were polymer housing only. Easy
availability of the housings was the reason for evaluating
the housings only (Sample
B).
Both samples
A
and B had
six skirts on the housing, and the shape of the skirts were
similar. The central cavity diameter was 42mm for both
samples.The complete arresters were tested along with the
insulated mounting bracket. They were fixed to a wooden
post and placed in the center of the chamber. The housings
were attached to stainless steel electrodes and suspended
from a plexiglass ring fixed to the roof of the chamber. The
electrodes for the housing were carefully machined and
Table 1 Details of Samples Evaluated
I
I
B
I Housing Only1 EPM
I
395 mm I >50
inserted in the central cavity using silicone grease in order
to prevent water from entering the cavity. The outside
diameter of the electrodes were such that they were flush
with the housing shank, as was the case with arrester
sample A. At least three samples of each type A and B were
evaluated and the dispersion in the reported data was
within 10 .
The complete arrester was evaluated in the normal mode,
i.
e., the top electrode being the high voltage and the bottom
electrode, the ground. The housings were energized with
the bottom electrode being the high voltage and the top
electrode grounded. This change does not affect the
tracking and erosion performance of the housing, as dry
band arcing is dependent on the direction of the water flow
on the surface. The samples were subjected continuously to
electric stress and fog for 22 hours every day. The test was
stopped for
2
hours in order to facilitate fresh saline solution
preparation and visual examination of the samples.
connected to
channel measuri heavily greased
Ground electrode
connected to channel
measuring internal
arrester current
Fig. 2: Schematic Diagram for Current Measurement
The analog voltage signal which is proportional to the
leakage current, is measured across
a
low value
(100
n)
precision resistor connected in series with the sample on
the grounded end. This signal is fed to a
16
channel,
12
bit
A/D converter (Metra Byte DAS 16-G2), located in an IBM
compatible computer. The computer is programmed to give
the following information on an hourly basis: Peak and
average value of current in the positive and negative parts
of the ac wave, integral of the average current, which is the
cumulative charge, and number of peak current pulses
exceeding several preset current limits. Further details of
the data acquisition system (DAS) can be found in an
earlier paper
[I
01.
The internal current of the complete arrester was also
monitored continuously using the arrangement shown in
Fig. 2. A metal band was inserted above the bottom skirt
and connected to one
of
the channels of the
DAS. The
material between the metal band and the bottom electrode
was heavily greased. In this manner, the channel
connected to the ground electrode measured the internal
current, where as, the channel connected to the metal band
measured the external leakage current.
3. RESULTS
AND DISCUSSION
The arresters and housings evaluated were rated for 9 kV.
The samples were subjected to their maximum continuous
operating voltage (MCOV), which is 7.65 kV for the 9 kV
rated
ANSI)
class arresters. The severity of the test was
varied by changing the water conductivity. The test was
terminated at 500 hours, unless the samples failed earlier.
This time of test has been found to be adequate in order to
distinguish between different materials and designs of
polymer insulating devices [I 1,121. The results reported are
from evaluating at least two samples of the same type.
Table 2 shows the results obtained at various water
conductivities. The conductivity values were chosen such
that it was representative of a wide range of contamination
severity in service [13], and also encompassed the
conductivity used in other accelerated tracking and erosion
tests [14]. For example, conductivity of rain is less than
50
pS/cm, normal tap water is in the range of 200-400 pS/cm;
conductivity used in the tracking wheel test i s 1600 @/Cm
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[I 51, earlier fog chamber ests have used water in the range
of 1000 to 4000 pS/cm [11,12], the German VDE test for
polymer cable accessories uses water in the range of
16000 pS/cm [16]. The following points can be noted from
Table 2
1) No degradation was not iced when the water conductivity
is below 100 pS/cm and at higher values of 16000 pS/cm
and higher.
(2) Degradation in the form of tracking and erosion has
been noticed on the sample
B
at a relatively low water
conductivity of
250
pS/cm.
(3)
Degradation in the form of tracking and slight erosion
has been noticed on both samples A and B at water
conductivities
of 1000
and 2000 kS/cm.
The degradation on both samples A and
B
was init iated at
the bottom electrode, as can be observed from Figs. and
4. This is believed
o
be due to the relative orientation of the
samples with respect
to
the fog nozzles. In the present set-
up, the water film runs down the surface causing dry band
arcing across the tip of the water channel and the bottom
electrode. Thus the difference in the location of the HV and
ground electrodes on the complete arresters and the
housings does
not
produce any difference i n the
degradation pattern.
Table
2:
Fog Chamber Test Observations at Various Water
Conductivities.
Note: T and
E:
Tracking and Erosion
~~
;ervation on
Sample B
T and E
Fig. 3: Typical Degradation of sample A .
Fig. 4: Typical Degradation of sample B.
3.1
FILLER DISPERSION
The degradation on a few samples B was dominantly in the
region of the mold line as shown in Fig.
5,
and was noticed
in the first
100
hours of salt-fog exposure with fog
conductivities of 250, 1000 and
2000
pS/cm. Such
preferential premature degradation was first observed and
explained in an earlier study with molded cylindrical rods
[5]. he degradation was attr ibuted
o
non-uniformity of filler
dispersion
in the region of the mold line. In order to
determine whether this could be the case with samples B,
small sections were cut around the mold line of the
degraded samples and examined under
a
scanning
electron microscope (Model JEOL JXA-840 SEM) using an
Energy Dispersive X-Ray Analysis (EDX) technique [I 71
Fig. 6 shows the typical variation of the filler dispersion ratio
at various locations (numbered
I
through 10 on the x-axis)
on small sections (about
1
OmmXl OmmX2mm thick)
removed from sample B . The filler dispersion was examined
after the fog chamber test. The various locations represent
different points on the small sections. The filler dispersion
ratio is the ratio of the X-Ray count obtained at the location
to
the maximum X-Ray count obtained on the specimen.
The X-Ray count is an indication of the amount of the ATH
filler present in that location [5]. t is clear that there is a
large filler dispersion on specimens removed from the mold
line of sample B. t is important to note that such highly non-
uniform filler dispersions have occurred even though the
average filler concentration is relatively high (>50 by
weight of the polymer). On specimens removed from other
parts of sample
B,
the filler is dispersed more or less evenly,
indicating adequate mixing of the compound.
These results indicate that molding is responsible for the
non-uniform filler dispersion. This is possible as the
polymer is more mobile than the filler particles, and under
the mold pressure, the more mobile polymer is pushed
outward, leaving the filler behind.
It
would be important
to
determine
if
the filler dispersion along the mold line could
perhaps be improved by the addition of suitable: additives
which would make the polymer and filler mobile to the same
degree.
This study also suggests that a filler dispersion ratio of 0.8
or higher could prevent preferential degradation. It is
to
be
noted that degradation has occurred on housings with a
more uniform filler dispersion, as shown in Figs. 3 and 4
but this is due
to
the repeated dry band arcing caused by
the water film flow pattern.
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5
d
[r
._
m
a
a
.-
._
80-1
00
80-100 0.4 NA
90-1
00
90-1
00 0.4
NA
1 o
0.4
Fig. 5: Degradation in the Mold Region on
SampleB
Mold Region I
lse
where
0 .21
1 , , , .
, .
I
0
2
4
6 8
1 0
Location
Fig. 6. Filler Dispersion on sample
B.
3.2 LEAKAGE CURRENT AND INTERNAL
CURRENT
Table
3
shows the typical leakage current observed during
the test at different water conductivities. The outer surface of
all the samples was hydrophobic initially and the leakage
current was less than
3
mA (peak). However, in less than
20
hours of exposure
to
dry band arcing in the fog chamber the
surfaces became hydrophillic or wettable, and the leakage
current increased
to
the values shown in Table 3. The
following points can be noted from the Table:
(1) Degradation occurs in the fog chamber tests only when
the peak leakage current i s in the range of 10-20 mA.
No
degradation occurs when the leakage current is very small
( 4 0 mA) or very high (>80 mA).
(2) The internal current is not affected by the leakage
current. This suggests that external contamination for the
low voltage arresters tested here has not altered the
internal voltage distribution significantly. However, this may
not be true for higher voltage rated arresters.
The cumulative charge,which is indicative of the duration
and frequency
of
leakage current pulses,was observed to
be about 15% lower in Tests 5 and
6
when compared
to
Tests 3 and 4 . This was despite the fact that in Tests 5 and
6,
a higher magnitude of peak leakage current was
recorded due to the higher water conductivity. This
Table
3:
Typical Peak Leakage Current and Internal
Current at Various Water Conductivities.
Note: NA. Not Applicable.
suggests that leakage current pulses in Tests 5 and
6
occur
in a very random manner. Similar observations have been
made in earlier studies with polymer insulators [18] and
cable terminations [19].
Degradation does not occur at very low values of current
because the arc energy is insufficient
to
break the bonds
in
the polymer. With very high values of leakage current, the
duration of the current pulses is very small and they occur at
random, resulting in an arc energy which is again
insufficient to break the bonds in the polymer. It is only with
intermediate values of current (10
to
20 mA) that the pulse
duration and frequency is sufficient for the arc energy to
cause material degradation. This factor has
to
considered
when analyzing the results from various accelerated
laboratory tests.
3.3
RESISTANCE
TO
MOISTURE INGRESS
A recent publication [20] indicated that zinc oxide arresters
are not affected by moisture ingress because the zinc oxide
material is dense and hence impervious to moisture. Even
so,
other materials inside the arrestor can be affected by
moisture. For example, In the samples A evaluated, the zinc
oxide blocks were wrapped in a fiber glass jacket. Fiber
glass carbonizes under the combined presence of moisture
and electric stress. Therefore, it is very important that the
housing prevent moisture ingress into the zinc oxide
column. A method of detecting moisture ingress
is
the Dissipation Factor or tan 3 measurement. A Phillips
Model PM 6303 meter was used for measuring the
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dissipation factor on the complete arresters. The tan a
(measured at
1
kHz) recorded was between 0.04 to
0.05
on
all the arresters evaluated before and after the 500 hour salt
fog tests. This indicates that there was no ingress of
moisture into the zinc oxide column.
4. COMPARISON WITH OTHER TEST RESULTS
At this time very little data on the tracking and erosion
performance of polymer housed surge arresters is
available. In a previous paper [I] it has been reported that
EPM housed surge arresters showed no degradation even
after several thousands hours of evaluat ion by the Tracking
Wheel and Fog Chamber methods, which use water
conductivity n the range of
1000
to
2000
pS/cm. However,
the present fog chamber work illustrates hat degradationof
the housing occurs with these values of water conductivity.
This contradiction suggests that more work is required
before a laboratory procedure can be recommended to
evaluate the tracking and erosion performance of polymer
housed arresters.
The IEC contamination test [9] on porcelain housed zinc
oxide arresters is usually performed using a salinity of 10
kg/m3 or higher. This translates to a water conductivity of
16,000 pS/cm and higher [9]. The IEEE/ANSI Contamination
tests are performed with water conductivity in the range of
2000-2500
S/cm (400 to
500
ohm.cm)
[21].
he present
study clearly demonstrates that lower values of water
conductivity cause more degradation of the polymer
housing than the higher values recommended in the
standard tests. Therefore, the evaluation of the
contamination performance of polymer housed arresters
should consist of two parts. The first part, performed
according to the above standards, will give information on
the effect of contamination on the overall arrester
contamination performance. The details of the second part,
which will provide information on the tracking and erosion
performance of the housing, needs further work.
5. ELECTRIC FIELD COMPUTATION
Corona on porcelain insulating devices, which is a result of
local high electric stress, is responsible for undesirable
effects such as radio and TV interference, audible noise
and ozone production. With polymer insulating devices, in
addition to the above undesirable effects, corona causes
material degradation n the form of punctures, pin holes and
radial cracks, which can lead to the ultimate failure of the
device with time. The permissible RIV,
TVI
and audible
noise for polymer devices has to be lower than for the
corresponding porcelain insulated devices.
The critical voltage for corona initiation on outdoor high
voltage devices is affected by electric stress, surface
contamination and moisture, and is significantly lower than
under clean and dry conditions. As the surface
contamination changes with time and location, it is difficult
to
estimate the corona inception voltage under
contaminated conditions. A way to prevent degradation
from corona is
to
ensure that under normal operating
conditions, the corona inception voltage is much higher
than the rated voltage. Corona is initiated when the electr ic
field on the insulating surface or in air exceeds 15 kV/cm
The electric field distribution was computed using a Finite
Element method based computer program POISSON ,
developed by
Los
Alamos National Laboratory [23]. The
PI
maximum number of mesh points allowed by the program is
limited to
60,000,
which was adequate for the present study.
To
improve the accuracy of computation, a variable mesh
size was used, the region near the HV electrode having a
greater mesh density than other parts of the arrester.
Fig. 7 shows the electric field distribution of the surge
arrester in the presently available form. The insulated
mounting bracket was omitted from the computation as it
does not affect the electric field distribution under unfaulted
conditions. It can be observed that the equipotential lines
are more crowded near the high voltage electrode, thus
increasing the electric field in this region. Due to rotational
symmetry, only one half of the arrester is modelled.
i
1
Fig. 7: Electric Field Distribut ion with Existing Design. Each
Equipotential Line Corresponds to 5 of the Applied
Voltage.
About 27% of the applied voltage appears in the first
centimeter along the surface of the top most skirt. For the
electric field to be below 15 kV/cm, the maximum voltage
that can be applied is 15/0.27=55 kV. Experimental
determination of the corona inception voltage on housing
B
indicated that the computed value was accurate to within
10 .
This suggests that the 9 kV rated arresters evaluated
here and arresters up to 36 kV rating with the same design
will not experience any serious problem from corona.
For higher voltage ratings, some type of electric field control
device (corona rings) may be required. Corona rings are
usually separate hardware attachments. It is also possible
that suitable modification of the electrode configuration will
produce the desired effect at a lower cost, at least for
intermediate voltage class arresters.
Fig. 8 shows two possible modifications of the HV electrode
and the resulting electric field distribution. In both cases, the
electric
field on the surface of the top skirts, and in the air
region surrounding the HV electrode is reduced when
compared to Fig. 7. For example, the maximum surface
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b
a
Fig. 8: Electric Field Distribution with Modification of the HV Electrode. Each Equipotential Line is 5 of the Applied
voltage
electric stress along any part of the housing is reduced
to
15 and
12
per cm of the applied voltage, in Figs 8a and
8b, respectively. There is also a reduction by about 20 in
the electr ic field in air along the line of maximum stress
XY
in Fig. 8, when compared to Fig. 7. This indicates that with
suitable modification of the HV electrode, the same
diameter arrester can be used for voltage ratings higher
than 55 kV without corona. However, more work is needed
to
optimize the electrode configuration.
6. CONCLUSIONS
1. This study demonstrates that the polymer housings used
presently for zinc oxide arresters can exhibit tracking and
erosion at low and moderate values, but not at very low and
moderate values of fog conductivity. This should be
considered in the preparation of accelerated contamination
tests for polymer housed surge arresters.
2. Fil ler dispersion played an important part in the tracking
and erosion of the polymer housings evaluated. It appears
that this parameter could be used as a quality control
measure by eliminating housings with gross filler
dispersion.
3. The polymer housing seems to adequately protect the
arrester column from moisture ingress.
4. Electric field computation indicates that the present
arrester design will be corona free up to 55 kV. For higher
voltage ratings, suitable modification of the HV electrode
can significantly ncrease the safe operating voltage.
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[17]. G. C. Allen and R. K. Wild, Probing the Secrets of
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S.
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[19]. R. S Gorur, L. A. Johnson and H. Hervig,
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Considerations , Paper 89 WM 071-2 PWRD, IEEE/PES
Winter Meeting, New York, 1989.
[21]. IEEE Standard for Metal Oxide Surge Arresters for AC
Power Circuits , ANSVIEEE Std. C62.11-1987.
[22].
E.
Kuffel and M. Abdullah, High Voltage Engineering ,
Pergamon Press, 1970.
[23]. Poisson/Superfish Group of Codes , Los Alamos
National Laboratory, Report LA-UR-87-126.
Vikrant Chaudhry was born in
Ujjain, India on June 1, 1967. He
graduated with a B S degree in
Electrical Engineering from the
Regional Engineering College in
Bhopal, India in 1988. Since Aug
1988, he is with the Arizona
State University, persuing a MS
degree in Electrical and
Computer Engineering. Mr.
Chaudhry's fields of interest are,
high voltage engineering and
semi-conductors.
Ravi Gorur was born in
Bangalore, India on July 31,
1958. He graduated with a BS
degree f rom Bangalore
University in 1981, MS degree
from the Indian Institute of
Science in 1983, and a Ph. D
degree from the University of
Windsor, Canada in 1986. He
worked as a Post Doctoral
Fellow at the University of
Windsor until Aug 1987. He
joined the Department of
Electrical and Computer Engineering at 'Arizona State
University as an Assistant Professor, n Sept. 1987.
Dr. Gorur is currently involved in research on polymer
materials and insulating systems for power transmission
and distribution and space applications. He is the chairman
of
the IEEE Dielectric and Electrical Insulation Society's
Outdoor Service Environment Committee. He is a member
of the IEEE working groups on Insulator Contamination,
Non-Ceramic Insulators and Insulated Conductors. He has
several publications both in the IEEE Transactions and
Conference Proceedings.
Michael Dyer was born in St.
Louis, Missouri, on June 1, 1954.
He graduated with a
BS
Degree
in Electrical Engineering from the
University of Il linois at Urbana-
Champaign, in 1977. He joined
Illinois Power Co. in 1977, where
he worked in the Danville
Service Area. In 1980, he joined
Salt River Project in Phoenix,
Arizona, where he is responsible
for Distribution and Transmission
equipment applications, for the Electrical Apparatus
Section. Mr. Dyer is a member of the Western Underground
Committee, and is a registered Professional Engineer.
Rao Thallam received the BS
and MS degrees in Electrical
Engineering in India, and the
Ph.D degree from the University
of Waterloo, Canada. He joined
General Electric Company in
1975, where he worked on
HVDC projects Engineering,
Surge Arrestor Engineering, and
Electr ic Ut i l i ty Systems
Engineering Departments. In
1985, he joined Salt River
Project in Phoenix, Arizona, where he is responsible for
conducting application, design and specification studies for
the Electrical Apparatus Section.
Dr. Thallam is active in IEEE, as a member of the Surge
Protective Devices committee, DC Transmission and DC
Converter Stations Subcommittees. He is the utility industry
advisor to EPRl for research projects on HVDC
transmission, insulation coordination, and metal oxide
surge arrestors. He has published several papers in IEEE
and other international journals and conferences, and was
awarded one patent in 1982. He is a registered
professional engineer, senior member of IEEE, and
member of CIGRE.
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7 3
Discussion
JEFFRY P. MACKEVICH, Raychem Corporation, Menlo
Park, CA
This paper is an interesting initial investigation into the
contaminated performance of pol er housed distribution
arresters.
A
task group has been c E g e d by Working Group
3.3.10
of the Surge Protective Devices Committee to address
accelerated aging testing of polymeric housings and this paper
is very timely.
The scope of the aper is limited in terms of the materials
tested, the test metgods used, and the claims which result. The
reader should note that this work should be viewed
as
only a
preliminary investigation. It is agreed with the authors that
there ...is the lack of a meaningful laboratory test to redict the
performance (of polymeric materials) in service. A i s is true,
however, do the authors suggest that the salt fog chamber
testing presented is a more meaningful test than any other test?
This topic has been under discussion in the Insulated
Conductors Committee and various Insulator working groups,
however, a standardized test method is not yet available.
Introduction
The authors claim that Considerable amount of service
experience and laboratory tests have demonstrated that there
are two types of pol ers, namely silicone rubber and ethylene
propylene rubber EPR), that are well suited for outdoor
applications. While it is well known that both silicone rubber
and EPR are used
as
outdoor insulating materials, they are not
the only polymeric materials used, and the reader
is
left to
question the purpose of this statement, given the lack of
references or data to support the claim. There are other
materials known to perform well in outdoor applications. One
such example is semi-crystalline heat-shrink materials, with a
known service life in excess of twenty years, used in medium-
voltage shielded power cable terminations.
[ l ]
Since both a
shielded power cable termination and a surge arrester have
stress-grading characteristics, it would appear that a
termination more closely models the arrester than a n insulator,
and experience with terminations is a plicable. Mak et a1 [2]
and Gorur et
al [3]
have each shown tgat semi-crystalline heat-
shrink termination designs had superior performance in
contaminated environments, similar to the conditions described
in this paper. Gubanski [4] eported
on
work performed with
merry-go-round contaminated environment testing that silicone
rubber, epoxy resin and EPDM had comparable results. Can
the authors explain why they neglected to mention other known
polymer materials besides silicone rubber and EPR?
The broad sweeping statement regarding silicone rubber and
EPR not only omits existing olymeric materials but it also
implies that the base p o k e r
alone
determines the
performance. This statement reduces all material formulations,
product design and manufacturing processes to the simplest
denominator, the base polymer. Such a simplification is
completely incorrect.
s
any materials person or product
designer knows, product performance is a combination
of
the
compound formulation (base polymer(s) and additives to
enhance performance), product design (wall thickness, shed
geomet and orientation, Cree age distance, sealing system,
etc.) anymanufacturing rocess fcumulative heat history during
com ounding and manuracturing). This statement also ignores
pro& design. A poor design with an excellent polymer
material may not provide the desired service performance. Can
the authors elaborate
on
their implicit assumptions?
The authors also indicate that the capacitance between the
arrester column and the polymer housing is increased, relative
to a porcelain unit.
No
data or calculations are provided to
support this statement. Can the authors provide data to
support the statement?
ExDerimental
The salt fog chamber testing conducted
on
arrester housings
has been previously run
on
polymeric insulators and polymenc
cable terminations. Given the statement by the authors at the
end of the Introduction, ...it is essential to have sufficient
laboratory data
on
polymer housed arresters such that a
meaningful accelerated test can be developed for these devices
as well , it is unclear as to the authors' motivation for
conducting this study. There does not appear to be a known
correlat ion between the salt fog chamber testing conducted and
a given field service environment. Without a known correlation
to service conditions, was the sole purpose of the study to
obtain data for one set of salt fo4 chamber conditions? What
additional da ta or further testing is suggested by the authors in
order to develop meaningful tests? Can the authors clarify
what one is to infer from the data presented, with respect to
predicted field performance, and, is it reasonable to make such
an extrapolation?
Although the authors note that silicon rubber and EPR are
resistant to
UV
chemicals, moisture, etc., they present no data
to
support the resistance of the specific test samples to
degradation from these other environmental factors. A
meaningful accelerated test should include simulation of
multiple stress aging factors and not rely
on
separate test data
developed from different sample formulations.
Results and Discussion
The authors tested two different housings in different
configurations. Sample set
B
was tested with an insulating core
and without zinc oxlde which provides stress grading. Can the
authors comment
on
the equivalency
of
the testing samples
A
and
B
with the differences
in
sample construction and electric
field distribution?
One explanation given for the possible cause of degradation
about the bottom electrode was the positioning of the samples
relative to the fo nozzles. Do
the authors mean to imply that
the same sampfes located in a different position and/or
orientation relative to the nozzles would have resulted in
different performance? How would the results differ if there
had been
no
fog nozzles, i.e. service in a high fog are a? Why
would the nozzles
not
represent actual service conditions?
The authors did not define failure criteria for this test. Gorur
[SI presented these data in an earlier publication and reported
failures in less than
100
hours for tests 2 and
3.
Can the authors
explain the inconsistency between
no
reported failures in this
paper and the reporting of these data with failures in reference
S
Other previous apers, which described testing in a salt fog
chamber, claimed tl at a 5 hour test time was sufficient to
discriminate the differences in performance among the samples
evaluated. While that time may be valid for a comparative test,
how should this data be used by others to test other materials
and compare the results, if the testing was not taken to failure?
Since previous papers (using the test chamber described in this
paper) energized the polymer samples at voltages higher than
rated line-ground voltage, why did the authors not run
supplemental tests
on
Sam le
B
housings at elevated voltages to
see what effect stress h a l o n performance? How severe of a
test do the authors feel this was by only energizing the samples
at operating voltage without accelerated stress aging? With
tracking occurring at relatively moderate levels of conductivity,
can the authors comment
on
the suitability of the two materials
tested for use as arreste r housings for applications in polluted
service environments?
Was
the variation in filler dis ersion reported obtained from
one sample set only or did all tge samples tested have the same
filler distribution at the mold line?
Does Figure
6
represent
one sample only or is it an average of a larger sample set?
One aspect, not discussed in the paper, is what the possible
effects of a contaminated test or semce environment might be
7/21/2019 Chaudhry Et Al. - 1991 - Electrical Performance of Polymer Housed Zinc Oxid
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704
on the insulating mounting bracket. Arres ter designs vary, with
some units having a ground-end metal sealing cap, while othe rs
do not. For the latter ar rq em en t, all surface leakage currents
must flow over the housing surface, across the insulating
bracket and disconnector, collecting on the ground lead
mounting stud. Can the authors speculate as to what might
happen to the housing and bracket if there is no concentric
leakage current collector (end cap) between the housing and
bracket?
Resistance To Moisture Ineress
The au thors are correct in identifying the requirement
to
have a
watertight interface between housing and internal components.
Dissipation factor is a valid method for determinin the
integrity of an interface, yet it would seem that 509 %ours
exposure in a salt fog chamber may not be a ve ngoro us
moisture seal test.
Can
he authors comment as to #e severity
of this exposure
as
a moisture seal test. Would it not be more
appropriate to conduct a thermal cycle freeze/thaw test as
discussed by Bradwell[6] for polymeric insu lators?
It is widely believed that moisture ingress is due to pumping
action of the rmal excursions such as might occur as a result of
a lightning discharge. The testing conducted was without any
energy input and the samples remained at ambient temperatur e
over the duration of the test. Can the authors comment on the
adequacy of the sealing systems for those units tested with
regar d to possible pumping ?
Groeger [7] has presented data suggesting that grease along a
separable EPDM elbow connector interface migrates
as
a
function
of
time. Since arrester rubber housings rely on an
inter feren ce fit and residual compression, rm@t not this
henomenon also apply to arresters? Was this investigated?
b id the au thors consider seal degradation and what impact this
would have on long term performance? Can the authors
comment on the need for sample a 'ng rior to seal testing to
test expected performance over the B e o rt he device?
This section contains an erroneous statement that should be
corrected: Fiber glass carbonizes under the combined presence
of moisture and electric stress. Glass fibers do not carbonize.
The authors may have been trying to describe the effects of
moisture and &h electrical stress on the epoxy resin binder in
a fiber glass/epoxy matrix, or on a resinous coating of the glass
fibers.
Conclusiory
1 The first sentence in conclusion number 1 appears to be
missing a word: ... inc oxide arresters can exhibit tracking and
erosion at low and moderate values,... The question is, at low
and mod erate values of what? The reader is left dangling.
2. In the second sentence of conclusion number two, the use of
the word gross is inaccurat e and misleading.
It appears that
the authors means to say non-uniform or grossly uneven
filler dispersion .
3. Conclusion number three doe5 not appear to have a
necessary basis. No long-term humidi cycling, therm al aging
or temperature cycling was con duc ter in the re orted tests.
Any of these factors could disrupt the integn o?the housing
environmental seals, and the potential e z c t s should be
investigated or, at least, considered, before the conclusion was
made that the arrester column is adequately protected from
moisture ingress.
References
[ l ]
R.
Penneck and D. Nyberg, Im rovements in non-trackin
materials, presented at the 7th &EE PES Conference an8
Exposition on Transmission and Distribution, Atlanta, GA,
April 14,1979.
[2]S.Mak and G. Lusk, Contaminated Environment Testing of
Cable Terminations, presented at the 7th IEEE PES
Conference and Exposition on Transmission and Distribution,
[3] R. Gorur, E. Cherney and R. Hackam, Evaluation of
Polymeric Cable Terminations in a Fog-Chamber , IEEE
Transactions on Power Delivery, Vol. 4, No. 2, pp. 842-849,
April 1989.
[4]
S.
Gubanski , Experience with the Me o-round Test ,
IEEE Transactions on electrical Insulation,vi&25, No. 2, pp.
[5]
R.
Gorur, Polymers for Outdoor Insulation , presented at
the 6th BEAMA International Electrical Insulation
Conference, Brighton , May 22-24, 1990.
[6]
k
Bradwell, Importance of preventing moisture ingress to
polymeric insulators , IE E Proceedings, Vol.
131,
Pt. B,
No.
6,
November 1984, pp. 245-251.
[7]
J.
Groeger, presentation made at the 86th Meeting of the
Insulated Conductors Committee, Sub-committee 10 Meeting,
Dearborn, MI, May 1,1990.
Atlanta, GA, April 1-6,1979.
331-340, April 1990.
Manuscript received J u l y 30, 1 9 9 0 .
H.
S.
Brewer and D
W. Lenk
(The Ohio Brass Company, Wadsworth,
Ohio): The authors are to be commended on their initial efforts to develop
a test procedure to evaluate the contamination performance of polymer
housed metal oxide arresters. This is obviously an area that deserves
continuing examination.
We have several questions regarding the Salt Fog Test Procedure and
would appreciate clarifications. Salt fog
tests
were performed with salt-fog
nozzles. What is the orientation of the test specimen relative to direction
of the salt fog spray from the nozzles? Did the authors find this orientation
critical to the rate
of
surface activity and possible degradation?
ANSI C29.11-1989, Test Standard for Composite Suspension Insula-
tors, specifies a 1OOOhour continuous salt fog test procedure utilizing a
turbo sprayer or room humidifier to generate fog. Would you expect test
results to vary significantly as a result of replacing spray nozzles with
either of the above methods?
The authors state that for the small distribution arresters the flow of
internal current is not critical in evaluating the surface degradation charac-
teristic. What are the authors' thoughts on performing tests with internal
MOV disk elements replaced with insulating components? With MOV disk
elements removed, the severity of the fog test is accelerated by increasing
the applied voltage. On larger diameter housings, which would tend to be
slower in developing dry band arcing activity, this might be helpful in
generating surface arcing.
The authors state that the salt fog chamber was shut off for two hours
each day to allow examination of the test samples.
Do
they feel that the
forced drying/wetting duty associated with reenergization was important
to the surface degradation? Would they expect to be able to duplicate test
results utilizing continuous fog energization?
We do not agree with the authors conclusion that it is valid to perform
tests on samples energized with high voltage applied at the bottom end of
the arrester housing. They state . .dry band arcing is dependent on the
direction of the water flow on the surface . Do they have a technical
reference for this statement? If so, will the authors provide this reference?
The authors conclude that poor filler dispersion may have contributed to
the surface degradation protilem. When were the filler dispersion measure-
ments taken? Were they taken on the sample units after completion of the
salt fog exposure tests? If so, the disparity in dispersion ratio measure-
ments could have been the result of intense surface activity, rather than the
cause of the degradation.
Polymer enclosed distribution class surge arresters have been installed
since 1986. To date, more than 1 OOO OOO polymer units have been
shipped, with no reported failures from surface tracking or erosion. Are
the authors able to correlate the Salt Fog Test Procedure to actual service
conditions?
Manuscript
received
August
3 1990.
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705
R. S . GORUR (Arizona State University, Tempe, AZ): On behalf of
all the authors,
I
would like to thank the discussers for their
interesting comments which add to the value of the paper.
To
Mr. H.
S. Brewer and
D
W.
1 . d
The test samples were
located at about 0.5m below the horizontal plane of the fog nozzles.
As long as the samples were located below the fog nozzles, the
degradation rate was not too different. The main wetting mechanism
is by droplet impingement as the type of fog used is the cold fog.
This orientation was chosen as in the field, rain falls from above on
to the devices.
There is data published to show that the flashover voltage is
dependent, to a certain extent, on the droplet size and temperature of
the fog and test object
[I].
With polymeric cable terminations, the
same
type of termination has performed differently in several test
methods, all using cold fog [Ref. [191 of paper]. The method of fog
generation
is
an important parameter which could influence the test
results, and is an area which needs further investigation.
We have performed numerous tests where the de-energization time
has been varied from zero to
24
hours. The end results were
essentially identical for de-energization times of zero (continuous
fog generation) and two hours.
References 3 and
4
of the paper, provides the information on the
effect of the water flow pattem on the degradation rate.
The filler dispersion measurements were performed after the salt fog
tests. We do not believe that the non-unifonh filler dispersion is the
result of intense
dry
band activity,
as
the filler dispersion was fairly
uniform in locations other than the mold lide, where tracking was
visible. If the discussers statement were to be true, then a non-
uniform filler dispersion should be expected at every location that
showed tracking or erosion due to intense surface activity.
We have not correlated the life in the salt-fog test to that in service.
The fog chamber test used here and other laboratory tests are merely
tools which
are
useful in identifying problems in the design and
materials. It is extremely difficult to associate life in service with
hours of exposure in a laboratory test as the wetting and
contamination severities are not the
same
for any two locations.
Based on experience with polymer insulators, any problems in the
design and material are not visible in the first few years, but appear
later.
TOMr. J. P. Mackevi ch. We have made no claim that silicone and
EPDM rubber are &e onlv tWO tvD of materials that are suited for
outdoor insulation applications. The purpose of the introduction is to
provide a brief background information on polymer housing used
for surge arresters, and not mentioning other materials does not
mean that they are unsuitable for outdoor applications.
Valuable information regarding the details of the material
composition and product design are trade secrets, which
are
not
divulged
to
the user or to universities. Without the exact knowledge
of these aspects, more mistakes in the sample description are
possible and this is not desirable. One way of characterizing
materials is by the public domain information, which is what has
been done in this paper.
Regarding the capacitance, Reference [6] of the paper provides the
supporting data.
The purpose of the present work was to provide information which
can be used for the development of accelerated aging tests on
polymer housed arresters, and not to develop THE STANDARD
itself. There are many more aspects, some of which are mentioned
by the discusser, which require further research.
Regarding the resistance of silicone rubber and EPDM materials to
UV, chemicals and moisture, the data is already available in many
books and also in manufacturers' catalogs, and therefore was
presumed to be well known information which need not be
referenced.
Although the electric field distribution is different for samples A and
B
in the
dry
conditidh, when the surface is covered by a water layer,
the conducting surface wohld be the dominating factor and the
difference in the electric field dismbution is expected to be similar
for both samples.
The type of fog, temperature, droplet size method of wetting are
a l l
known to
affect the contaminriiionperformance of outdoor insulation
[
11.
The study of fog parameters was not performed in this work,
therefore, it is not possible
io
address the discusser's question.
The data reported in Ref [5] of the discussion and in this paper is
consistent, contrary tb the discusser's observation. The
500
hour
test duration
is
only a beiichmark for comparison, each test
procedure has a ceitain duration below which no failures can be
expected. Comparison of results from various test methods is
difficult and this
has
been well illustrated in Ref
[191
of the paper for
polymeric cable terminatibns.
As samples B showed deadation at the normal operating stress, it
would not be surprising to see more degradation at higher than
normal stresses, and ndt much information would be gained by
doing the proposed tests.
All the samples of type B which showed erosion along the mold
release showed non-uniformities in the fil ler dispersion and fig. 6is
the typical plot and not from just one sample.
Without test data, it would ,not be possible to speculate the
performanceof the device mentioned in the discussion.
The
paper mentions that based on the present results, the housing
material showed good resistance to moisture ingress. The aspects
mentioned by tht discusser should be considered
for the
development of a test shdard, and this was not the objective of the
present paper.
Regarding carbonization of glass fibers, this was an oversight, and
it is
true
that it is not the glass fibers which carbonize but the organic
resin which binds the fibers.
The points mentioned in the first two points of the conclusion are editorial.
The conclusions are based on the test results of the present work and
conclusion number three is therefore valid.
Reference
[I]. G . Karady "The Effect of
FOE
Parameters on the Testine of
Artificially Contaminated InsulaGrs in a Fog Chamber , IEEE
Trans. PAS 1975, pp 378-387.
Manuscript r e c e i v e d
October
5, 1 9 9 0 .