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Review of Features of Fog Chamber at The Ohio State University
for Polymer Insulator Evaluation
Stephen A. Sebo, Edgar P. Casale, JosCR. Cedefio, Wibawa T jokrodiponto and Sheikh A. Akbar
The Ohio State University, Columbus, Ohio 43210-1272, U.S.A .
and
John Sakich and Tiebin Zhao
The Ohio B rass Company, W adsworth, Ohio 44281-0901, U.S.A.
Abstract
Non-ceramic composite) insulators consist of
a fiberglass core rod surrounded by a polymer w eather-
shed housing. The housing is generally nianufacturcd
from an elastomer material. Weathershed materials
may be subject to degradation and aging in service due
to harsh environmental conditions. Performance
evaluation of polymer weathersheds can employ fog
chambers. Design and operational features
of
a new
fog
chamber are reviewed in this paper. Specific charactcr-
istics reviewed are: size and m ain layout of the cham-
ber, insulator arrangement, high voltage source; pro-
tection, water circulation and data acquisition systcms.
Test cycles and techniques are also discussed briefly.
Introduction
A non-ceramic composite) insulator has a
high-strength core surroundcd by a polymer weathcr-
shed housing. Both ends have m etal end fittings.
Non-ceramic m aterials as high voltage insula-
tors were introduced about 30 years ago. These com-
posite insulators have two components. One is a resin-
bonded fiberglass core rod of high tensile strength. The
other is the housing, generally manufactured from an
elastomer m aterial, such as etliylcne propylcne rubber
EPR), which includes ethylene propylcne monomer
EPM) and ethylene propylcne diene nionomcr
EPDM ), silicone rubber, and alloys of EPDW silicone.
Various other polymers, teflon and cycloaliphatic ep-
oxy resins can also be used as housing m atcrials. Fillcr
materials, e.g., alumina trihydrate, are also addcd.
Advantages
of
non-ceramic insulators over
porcelain or glass insulators are well-documcntcd
[
11-
[SI.
The advantages are lighter weight, greater vandal-
ism i.e., impact) resistance, highcr strength-to-weight
ratio, better contamination performance they nccd lcss
cleaning under pollutcd conditions), and improvcd
transmission line aesthetics. There are manufacturing
advantages as well, since they are not fragile and are
easier to handle.
There have been a variety of problems to be
solved, though. Housing m aterial tracking and erosion,
surface changes, water absorption, reduction of con-
tamination flashover strength, bonding failures along
the rod-shed interface were reported
[7],
leading to
changes in materials and m anufacturing processes.
At the same time, the range of tests of non-
ceramic insulators has become broader and more so-
phisticated. The objectives of these tests are to examine
the pcrformance of non-ceramic materials and insula-
tor designs, and to obtain information on the expected
lcngth of service life of a specific non-ceramic insula-
tor.
One of the types of preferred tests is the appli-
cation of fog chambers together with various matcrial
diagnostic techniques. The purpose of this paper is to
review a ncw fog chamber dcsign and utilization proj-
ect prcscntly under way in the High Voltage Labora-
tory at The Ohio State University OSU).
Sections of this paper review the size and
main layout of the fog chamber, insulator arrangement,
high voltage source and protection system, watcr circu-
lation and fog generation system, data acquisition sys-
tcm, and test cycle and procedure considerations.
Re\.icw o Litera ture
Scveral publications describe the d csign of fog
chambcrs, selcction of test conditions, instrunmitation
and da ta acquisition, test proccdurcs and tcst rcsults.
A
1.52111
x
1.521~1 1.37ni high cham bcr with
a pyramidal roof dcvclopcd at the Massacliusctts Insti-
tute of Technology is discusscd i n
[
l].
The first salt-fog chambcr of Dow Corning
Corp. is dcscribcd in dctail in [2]; similar but more
advanced chanibcrs havc bcen dcvelopcd lntcr by
Don
0-7083-3580-5) 1996 IEEE Annua l Repo rt - Conference on Electr ical Insulation and Dielectr ic Phenomena, San Francisco, October
20-23 1996
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The output voltage of the transformer is con-
nected to the high voltage bushing of the
fog
chanibcr.
The bushing is lightweight, custom-made, utilizing a
section of a high vo ltage cable, cable terminators, and
polymer weathershed housings.
There is a circuit breaker that trips at 200A)
in the 240V connection of the high voltage source.
There are 250mA low voltage fuses in the ground con-
nection
of
each insulator.
Water Circulation and
Fog
Generation System
Four air atomizing spray nozzles are em-
ployed, two on each long w all of the fog chamber. It
is a pressurized set-up: the water and air supplied to
each nozzle are under pressure. Flow rates of the w ater
and air of each nozzle can be adjustcd to change tlie
water particle size. Each nozzle fits into an adapter
that is a part of a flange mount, holding the nozzle
securely in place in the polycarbonate wall of the
chamber. Various spray hcads can be employed; the
heads initially selected produce a dcflccted flat spray
pattern. The fog created by this pattern is very uniform;
it fills the chamber gradually, from the base upwards.
Each nozzle has a clean-out needle assembly. The
nozzles are made of stainless steel.
The water flow rate supplicd by each of the
four nozzles can be varied bctwcen 0.1 and 0.47
dm3/min 1.5 to 7.4 gal/hr). The air flow rate at each
nozzle can be varied bctween 45 and 266 dm3/m in 1.6
to 9.4 standard ft3/min), the absolute air pressure range
at the nozzles is 140-500kPa 20 to 70psi). The watcr
and air flow rates are m onitored at each nozzle by ap-
propriate flow meters. Each flow metcr is equippcd
with
a
valve to control the water and air
flows,
rcspcc-
tively.
Eithcr clean fog or salt fog can be applied in
the fog chaiiibcr. In the case of salt fog, a saline solu-
tion is obtaincd by mixing sodium-chloride with
dcionizcd watcr in a 75 litcr plastic tub until the dc-
sired conductivity, e.g., 250 or 2500 pS/cm, is rcachcd.
A
mixed-bed deionizer system is uscd. The dcionizcd
water produced by the unit has a conductivity about 3 5
pS/cni.) The plastic tub is under the ccntcr drainage
point of tlie fog chamber;
it
is on castcrs for easy
handling. A portable nicter is employcd to nicasurc the
conductivity and the tempcrature. The mctcr is
equipped with automatic temperature compensation in
the 0-50 degree C range.
The air is supplied by a comprcssor. The wa-
ter is circulated by a corrosion-resistant pump. The
saline solution is filtered constantly and changcd daily.
Steam fog generation is available for some
flashover tests. It is produced by boiling water in a gal-
vanized steel tub with immersion heaters. It is easy to
adjust the steam production rate grams/hour/m3) as
desired. A different high voltage transformer is uscd
for these experiments by m oving the fog cham ber, that
has casters, to another location.
Data Acquisition System
The data acquisition system nionitors the
Icakage current of each insulator or specimen under
test, via the voltage drop across a series resistor outside
the fog chamber. The leakage currents flowing through
the contaminatcd surface of the insulators under test
are sensed, conditioned, digitized, processed, dis-
played, and finally stored for further analysis.
The
A D
onverter employed by the data ac-
quisition system has 12-bit resolution, 16 single-ended
or 8 diffcrential) input channels, niaximuni acquisi-
tion rate of 100 ksampledsec, and niax. 5 V bipolar
input voltage range if the gain is
1
The gain is switch-
selectable 1, 2, 4 or 8 .
Virtual instrumentation software, DASYLab,
was detcrmined to be the most suitable option for this
application. An icon-based program has bccn devel-
oped
to
classify the leakage currcnt pulse counts using
bins, and to compute cumulative charge, and niaxi-
niuni and minimum current pulse values [111.
A signal conditioning system protects the in-
put terminals of the data acquisition system against
overvoltages in the case of insulator flashover.
The dcsktop computer used is a 16MByte
RAM 586 133MHz type PC. A power line condi-
tioner is uscd to protect the computer from problems
causcd by transicnts and to ensure line-to-load isola-
tion. An unintcrruptible power supply is eniploycd.
Test Cycles and Procedures
The initial step
i n
tcsting polymer insulators
is the cliaractcrization of the wcathcrshcd material
using various diagnostic tcchniqucs, such as Electron
Spcctroscopy for Chemical Analysis ESCA ) and
Fourier Transformed Infrarcd Spectroscopy FTIR).
Other tcchniqucs, for example XP S X-ray Photocmis-
sion Spcctroscopy) and ED S Energy Dispersive Spcc-
troscopy) are also available.
The sccond stcp of the polymcr insulator
evaluation is a scrics of ac flashovcr tests on thc insula-
tors, at diffcrcnt slurry lcvcls charac terized by thc
equivalcnt salt dcposit density [ESDD] figures), at
5
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steam fog conditions. This flashover voltage is one of
the reference points used in evaluating the insulator.
Th e next step is to expose the energized insu-
lators cyclically to salt fog for an extended period of
time e.g., 500 hrs). Then
a
new series
of
ac flashover
tests
in
steam fog is performed
on
the insulators.
Material characterization tests are conducted
again on the aged polymer insulator component sam-
ples. Also, aged insulator
samples
are
exposed to
ul-
traviolet UV) rays for several months before anoth er
series of material characterization tests is performed.
Analysis of
Tests
Degradation of
the
polymer insulators can
be
evaluated by their electrical, mechanical, chemical an d
physical characteristics. The analysis of tests considers
several features, such as time to failure, flashover volt-
age, leakage current waveshape, peak magnitudes,
pulse count, change vs. time), cumulative charge,
tracking resistance, surlace condition, surface conduc-
tance, etc.
Summary
The widespread use of non-ceramic insulators
has made the evaluation of their long-term perform-
ance essential. Preferred test methods can supply
quantitative information on the agin g characteristics of
polymer housing materials of non-ceramic insulators
and on the length of their expected outdoor service life.
Application of a fog chamber
is
one of the preferred
test methods.
Testing non-ceramic insulators in a new fog
chamber,
a
joint project
of
The Ohio State University
and T he O hio Brass Company, ha s bcen discussed
in
this paper. Various design and operational features,
test cycles and test procedures have also been reviewed.
Acknowlcdgcnicnts
Discussions and visit with Messrs.
E . A .
Rey-
nnert and G.A. Toskey of Dow-Corning Corp. bcfore
the construction of the fog chamber were valuable. Th e
fog chamber was constructed by Messrs. Carl A. Rus-
sell and William C. Thalgott.
References
[ D.C Jolly, A Qualitative Method for Detennining the
Resistance
of
Polymers to Surface Discharges,
IEEE Trans.
o
El
Insulation Vol. EI-17, August 1982, pp.293-299.
[2] E.A. Reynaert, T. Orbeck, J.A. Seifferly, Evaluation of
Polymer Systems for Outdoor
HV
Insulator Application by
Salt Fog Chamber Testing, Proc.
o
the 1982 IEEE ZntI
Synip.
on Electrical Insulation pp. 242-247.
[3]
R.S. Gorur,
E.A. Cherney,
R.
Hackam, A Comparative
Study of Polymer Insulating Materials Under Salt Fog Con-
ditions,
IEEE Trans. on El. Insulation
Vol. EI-21, April
1986,pp. 175-182.
[4]
K.
Isaka, Y. Yokoi,
K.
Naito et al., Development of
Real-Time System for Simultaneous Observation of Visual
Discharges and Leakage Current on Contaminated
D
nsu-
lators, IEEE Trans. on
El
Insulation Vol. 25, December
1990, pp. 1153-1160.
151 A. De La
0 R.S. Gorur, J.
Chang,
AC
Clean
Fog
Tests
on
Non-Ceramic Insulating Materials and a Comparison wt
Porcelain,
IEEE Trans.
o
Power D elivery
Vol. 9, October
1994,
pp.
2000-2008.
[6] H.M. S c l ~ e i de r , .W. Guidi, J.T. Bunlham, R.S.Gorur,
J.F. Ha ll, Accelerated Aging and Flashover Tests on 138 kV
Nonceramic Line Post Insulators,
IEEE Trans. on Power
Delivery
Vol. 8, January 1993, pp.325-336.
[7] J.F. Hall, History and Bibliography of Polymeric Insu-
lators for Outdoor Applications, IEEE Trans. on Power
Delivery Vol. 8, January 1993, pp.376-385.
[8] S.H. Kim, R. Hackam, Effects of Saline-Water Flow
Rate and Air Speed on Leakage Current in RTV Coatings,
IEEE Trans. o Power Delivery Paper No. 95
WM
248-5
PWRD.
[9] Artificial Pollution Tests
on
High Voltage Insulators to
be Used on AC Systems, Inteniational Standard, lEC 507,
Second Edition , 199 1.
[lo ] Composite Insulators for AC Overhead Lines wt
a
Nominal Voltage Greater Than
1000
V -Definitions, Test
Methods and Acceptance Criteria, International Standard,
IEC 1109, First Edition , 1992.
[
1
11 E.P. Casale, S.A. Sebo, Polymer Insulator Fog Cham-
ber Project: Data Acquisition System Development,
Proc. of
the
1996
CEIDP
companion paper).
Corresponding author:
Prof. Stephen
A.
Sebo
Department of Electrical Eng ineering
Th e Ohio State University
Columbus, Ohio 43210-1272,
U.S.A.
Phone: 1-614-292-74 10
E-mail:
FAX: 1-614-292-7596
446
mailto:[email protected]:[email protected]