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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,

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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,

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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-

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[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,

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[

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:

sebo@ee.eng.ohio-state.edu

FAX: 1-614-292-7596

446

mailto:sebo@ee.eng.ohio-state.edumailto:sebo@ee.eng.ohio-state.edu