Chaudhry Et Al. - 1991 - Electrical Performance of Polymer Housed Zinc Oxid

7/21/2019 Chaudhry Et Al. - 1991 - Electrical Performance of Polymer Housed Zinc Oxid

1/10

696

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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697

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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698

[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.


4/10

699

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


5/10

700

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


6/10

701

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.

7. REFERENCES

[l ]. D. W. Lenk, F. R. Stockum and D.

E.

Grimes, A New

Approach To Distr ibution Arrester Design , IEEE Trans. on

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[3]. E. A. Cherney and D.

J.

Stonkus, Non-Ceramic

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IEE, IEE

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[5].

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S. Gorur, E. A. Cherney and

R.

Hackam,

performance of Polymeric Insulating Materials in Salt-Fog ,

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[6]. E. C. Saksburg,

J.

S. Kresge, D. A. Mark, G. G. Karady,

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of

Zinc Oxide

Station Arresters , IEEE Trans. on Power Apparatus and

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[7]. M. Cojan et al. Polymeric Transmission Insulators-

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1

42, 1981.

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[8]. L. Johnson and W. Osborne, Contamination Testing of

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11

9-7-PWR, pp383-389,

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[9]. Artificial Pollution Tests on High Voltage Insulators to

be used on AC System , IEC Report 507,1975.


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[ lo]. R. S Gorur, E. A. Cherney and

R.

Hackam, A

Comparative Study of Polymer Insulating Materials Under

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[ l l ] . R. S Gorur, E. A. Cherney and

R.

Hackam, Polymer

Insulator Profiles Evaluated n a Fog Chamber , IEEE Trans

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[12]. R. S Gorur,

E.

A. Cherney and

R.

Hackam, Evaluation

of Polymeric Cable Terminations n a Fog-Chamber , IEEE

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1989.

[13]. C. Gregoire, P. J. Lambeth, M. Sforzini and M. P.

Verma, A Critical Comparison of Artificial Pollution Test

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[14]. G.

R.

Mitchell, Present Status of ASTM Tracking Test

Methods , Journal of Test ing and Evaluation, Vol-2, No.1,

[15].

M.

Kurtz, Comparison of Tracking Test Methods ,

IEEE Trans., Electrical Insulation, Vol. El-6, pp 76-81, 1971.

[16]. VDE Guide for Power Cable Accessories with Rated

Voltage to 30 kV , VDE 0278, October 1982.

175-182, April 1986.

pp 23-31, 1974.

[17]. G. C. Allen and R. K. Wild, Probing the Secrets of

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ppl l-30,

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[18]. R.

S.

Gorur, S Sundhara Rajan and 0. G. Amburgey,

Contamination Performance of Polymers Used for Outdoor

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Aug 1989.

[19]. R. S Gorur, L. A. Johnson and H. Hervig,

Contamination Performance

of

Silicone Rubber

Terminations , Paper No.

90

WM 074-5 PWRD, 1990 IEEE

PES Winter Meeting.

[20]. E. C. Sakshaug, J. Burke and J. Kresge, Metal Oxide

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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 ,

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[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.


8/10

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


9/10

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.


10/10

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 .

Chaudhry Et Al. - 1991 - Electrical Performance of Polymer Housed Zinc Oxid

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Transcript of Chaudhry Et Al. - 1991 - Electrical Performance of Polymer Housed Zinc Oxid