cables surges

8/9/2019 Cables Surges

1/9

61

1

EEE Transactions on Power Delivery, Vol.

9,

No. 2 A p d

1994

EFFECTS

OF

VOLTAGE SURGES ON EXTRUDED DIELECTRIC CABLE LIFE

PROJECT UPDATE

by

Richard

A.

Hartlein, Member V.

S.

Harper, Member

Georgia Power Company Georgia Power Company Electric Power Research

Harry Ng, Member

AtlanG, Georgia Atlanta, Georgia

KEYWORDS

Cable, Lightning, Water

Tree,

mpulse, Thumper

ABSTRACT

Electric utility engineers have commented

[I], [2]

that

extruded distribution cables frequently fail during or shortly

after a thunder storm. These engineers also comment that

failures often reoccur on cable circuits where previous

failures were located with a thumper. Linemen at Georgia

Power often make similar comments.

To

investigate this observation, crosslinked W P E ) and tree

retardant crosslinked

(TRXLPE)

cable designs were

subjected t accelerated water treeing tests. Samples were

subjected to simulated lightning surges

or

simulated

thumping surges. Crosslinked cables removed after

15 years

of

service operation were also subjected to these surges.

The results show that,

in

some cases, lightning surges do

reduce extruded distribution cable life. Also, high level

thumping surges appear to reduce cable life once cables are

well aged.

INTRODUCTION

The Electric Power Research Institute (EPRI) sponsored the

work

to

investigate the effects of voltage surges on extruded

dielectric cable life under Project

RP2284-01.

Since voltage

surges may affect insulation materials differently, five

commonly used insulation types were initially chosen for this

test program. These include high molecular weight

polyethylene (HMWPE), tree retardant high molecular

weight polyethylene (TRHMWPE) and ethylene propylene

rubber (EPR) aswell as XLPE and TRXLPE.

Preliminary results and a detailed description of the test

93 SM 357-4 PVRD

by the IEEE Insulated Conductors Committee of the IEEE

Power Engineering Society for presentation at the IEEE/

PES 1993 Summer Meeting, Vancouver, B.C., Canada. July

18-22, 1993.

made available for printing April 22, 1993.

PRINTED IN USA

A paper recommended and approved

Manuscript submitted January 4, 1993;

.

Institute

Palo Alto, California

program were published in

[3]

and

[4].

Those results

provide

data

on cables aged in the laboratory and subjected

to 40

kV,

7 0

kV and

120

kV lightning surges. The

data

also

include laboratory aged cables which were subjected

to

a

25

kV thumper surge to simulate surges

use

to locate field

failures.

Aging times of up to 600

days

were reported.

To gather additional information on the effects of surges on

aged cable, a second phase was added to the test program.

In this phase, aging was continued on selected cables beyond

600 days, crosslinked polyethylene cables aged

in

service

(XLPEF) were added to the test program and a 25 kV

lightning surge magnitude was also introduced.

To minimize time and cost, only XLPE, TRXLPE and

XLPEF cables were included

in

the second

phase.

These

compounds were selected because they represent the majority

of cable insulations used by electric utilities.

This paper presents the results gathered to

date

on the cables

tested in phase two.

This

new

data

provides greater insight

into the effects of lightning and thumping

surges

on aged

cables.

A

brief review of the test program is also included.

TEST CABLES

Fifteen kV cables were used because they are very common

and 15 kV cables are easily managed in accelerated

laboratory tests.

Since the ac voltage and surge voltage

stress distribution varies with insulation thickness, cables

with different insulation thicknesses were evaluated.

Table

1

outlines the cables utilized for this program.

The XLPE and TRXLPE cables were made by one

manufacturer specifically for use in this test program.

This

was done to minimize the variations in cable quality that

can

occur between manufacturers.

They were manufactured to

the AEIC CS5-82 specification for crosslinked polyethylene

insulated power cable using a triple extrusion, steam curing

process. The conductor shields were extruded, conventional,

semiconducting XLPE with a nominal thickness of 15 mils.

The nominal insulation thickness was either

175

mils or 220

mils. The insulation shields were extruded, semiconducting

XLPE with a nominal thickness of

30

mils.

Extra smooth

or extra clean shields were not available at the time these

cables were manufactured.

Since the XLPEF cable was obtained from the field, the

0885-8977/94/$04.00

Q 1993 IEEE


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613

applied, the voltage was lowered

to

1X the operating voltage

to ground

(8.6

kV). This was also done to represent field

conditions.

Thumuer Surne Auulication

To apply the thumping surge, a thumper was connected to

each cable rack individually using a dummy cable length

which contained a simulated fault

to

ground. A Biddle

Model

No. 652025

humper with a

12

pf capacitor was used

open end of the test cable is shown in

Figure

2.

r

to apply the thumping surge. A typical waveform at the

Figure 2. Typical waveform as seen at the open (far)

end of the cable subjected to a 25 kV

thumping surge.

Aging of Cables Removed From the Field

The XLPEF cables aged for 15 years in service at 7.6 kV.

An examination of the insulation revealed numerous, large

bowtie and vented water trees. The vented trees were

as

long

as 170 mils

and the bowtie trees were as long

as 60

mils.

A C

breakdown tests were conducted according

to

AEIC CS5-82 starting at 100 V/mil. The resulting

breakdown strength of five, 30 foot long samples was 220,

220,220,220

and

260

V/mil. Since cables with this type

of

watertreeing

and

dielectric strength are considered to be well

aged, the aging voltage used in the laboratory was the same

magnitude as the service voltage (7.6 kV).

Initially, the XLPEF cables were aged in conduits filled with

tap water but with no water in the conductor. The conductor

of this cable was

sealed

immediately after receipt at the

laboratory. This was done to preserve field conditions as

much as possible. However, since no failures occurred after

aging in the laboratory for 200 days, deionized water was

added

to the conductors. Deionized water wasuse to avoid

adding ions which may not have been present in service.

Aging Temuerature

ACCELERATED AGING PROCEDURE

Aeinn of New Cables

The cable aging test was designed

to

simulate and accelerate

field aging and was pattemed after the AEIC CS5-82,

B.5

accelerated water treeing test which

was

in force when this

project was initiated. In accordance with the AEIC

accelerated water treeing test procedure, the XLPE and

TRXLPE cables were thermally preconditioned. This is

done to reduce the high concentration of volatiles contained

in the newly manufactured cable insulations.

Preconditioning was conducted after the cables were placed

in the aging conduits but before they were filled with water.

AEIC specifies a preconditioning conductor temperature of

130 C for

10 days

using current in the conductor.

However, 110

C

was

used

in

this

test to avoid the

possibility of overheating the insulation. All temperature

tolerances were 5

C .

After preconditioning, the

conductors and conduits were filled with tap water.

To

accelerate aging, 3X rated voltage

to

ground

(26

kV,

60

Hz ac) was applied to both the XLPE and TRXLPE

175-mil-wall and 220-mil-wall cables. Thus, the aging stress

was

149

V/mil on the thin wall cables and

118

V/mil on the

thick wall cables.

This

procedure was followed to represent

the different stresses that thick and

thin

wall cables

experience

in

the field. When the lightning surge was

Normally, the AEIC accelerated water treeing test is

performed with sufficient current to achieve a

90°C

conductor temperature in air. However,

in

phase one, the

75°C

rated HMWPE cables were connected

in series

with

the

90°C

rated XLPE, TRXLPE and EPR cables. To

prevent overheating

of

the HMWPE cable and to maintain

the same aging conditions on all cables, a maximum

conductor temperature of 75 C was chosen for all test

cables in phase one. For consistency, this temperature was

also used in the second phase of the test program.

This

temperature was achieved with

260

amperes in the

conductor using a lZhour-on, 12-hour-off load cycle.

Aging With ImDulses

The lightning surges were applied an average of

2.3

times a

week. They were always applied at the end of the

12-hour-on period while the cables were still warm.

Approximately 5 to 10 minutes before the lightning impulse

was applied, the aging voltage was reduced from 26 kV to

8.7

kV.

Thumping surges were applied in a manner that represents

field conditions. Cables used for the thumping test condition

were removed from the aging test and connected to the

thumper. Nine hundred thumping impulses were applied to

each test cable rack individually. The cables were then

placed back in the aging test. Thumping was performed


4/9

614

every

60 days

after the first

120

days of aging.

175 MI1 ConlMl

............................. *...................... *..x

...............

- ?5,M 4??Pu?eA

.....................

A. A .A. ..A........

.........................................

y ...

y

...

A.A*.A

...A y A n

....... .............

.........

y ......

. nMMkx ......................

175

HII

70kv Pulse

.......................................

.......A...) .

.....................................................

175 MI1

l 2 U V

PUIS0

175 MI1 Thumper

.....................................................

Control samples were exposed to the same aging conditions

as

the surged samples but with no surges applied. All

samples were en er g id , deenergized and filled with water

at the same time. Every effort was taken

to

ensure that the

only difference between the control and surged samples was

the application of the voltage surges.

.....................................................................................

.................................................... ......

.... ...... .x ...........................

Each surge test condition was applied

to

a separate set of

cable samples.

No

cables were subjected

to

both thumping

and lightning impulses.

This

would have been an interest ing

condition to evaluate because it represents the kind of surge

combinations that can occur in service. However, individual

surge effects were considered the most important variables

to

investigate first.

220 MI1 7WV Pula8

_....

.....................

.........

.....

y

.....

.......... .... ....

A u . X

..........................

Redicates

Two, 180-foot-long coils of cable were subjected

to

each test

condition. Each coil was a continuous length which

contained six, 30-foot-long samples. Thus there were 12

samples or replicates for each test condition.

Summarv of Test Variables Investigated

Table 2 shows a matrix of the test conditions examined in

phase

2

A

Cy

indicates that a test covering this condition

was performed.

n (N)

indicates that a test

to

evaluate this

condition was not performed due to time or space

limitations.

Table

2

Matrix showing all of the test conditions

reported in this paper.

175

mil wall

thickness

surge

condition

CNTRL

25kV 4OkV 70kV 12OkV

XLPE

Y N Y Y Y

TRXLPE

Y N N Y Y

XLPEF

Y Y N Y N

220

mil wall

thickness

surge condition

CNTRL 25kV 40kV 70kV 120kV

XLPE

Y N N Y N

TRXLPE

N N N N N

XLPEF

N N N N N

RESULTS

In

phase one, four traditional methods of evaluating the

integrity of an extruded power cable were

used

to determine

the effects of surges on each cable

tested

in the project.

They included ac breakdown tests, impulse breakdown tests,

time

to

failure and visual analysis. After a thorough

investigation, time to failure analysis was the only method

which revealed differences between control and surges

samples. Therefore, only time to failure data is presented

for the cables tested in phase two.

Aging Failures

To simplify the discussion, an abbreviation is

used to

describe the test conditions. The insulation material and

wall thicknesses in mils is followed by the surge or control

test variable. For example, the XLPE, 175-mil-wall cable

subjected to a 70

kV

surge is referred to

as

XLPE, 175/70.

As mentioned earlier, at the start of this project ac

breakdown tests were considered an important diagnostic

tool for evaluating surge effects. Therefore, during the first

360

days of aging in the initial tests, numerous samples were

removed from the test and subjected to

an

ac breakdown

test. By the end of

360

days, almost

no

samples remained.

Thus, the 175/control and 175/70 surged samples of XLPE

and TRXLPE cables were unavailable for further aging

beyond

360

days.

Other conditions examined for these cables

as

well

as

the

XLPEF test cables

started

the test program after the

data

showed that aging time is more important than breakdown

strength. Therefore, data are available for longer than

360.

Also, since unaged TRXLPE cable was still available, the

TRXLPE 175/control condition was repeated.

To compare the time to failure data for all cables, line

graphs are employed which show when each failure

occurred. The data presented in this manner provide a

visual method of observing early failures or clusters of

failures.

n X

at the end

of

a line means either the test

condition was discontinued to perform breakdown tests or

most of the samples failed at the time the X s shown.

No

X at the end of the line means the cable is still under test.

Except for the XLPEF cable, failures during the surge

application are rare. The failures

as

a

function of time are

shown in Figures

3-5.

~

0

100

200 300

400

500

600 700

Oays of Aging

Figure

3.

XLPE failures

as

a function of time in the aging

test. Each failure is represented by a triangle.


5/9

1

175 Mil Cnnlmi

.........................................................................................................................

175 MI1

25 kv Pulse

............................................... f ................2 .............

* ..... ..& ................

. .....................................................

.i .

.. 2

...

.ai.

...............................

75

Mil

70kv Pulse

175 Mil

Thumper

-. ............................................................................... .........A............................

I

1

I75

Mil Cantmi

.............................. ... ........ ..............A........ ..............................

175

Mil

70kv

ulse

.........................................

K.

..........................................................................

175 Mli

IZWv Pulse

...........................................

..A.......

.A..

& ...... A...A.

X

.............

.75 Mi l Thumper

........... .....................................

.......................................................................................................................

0 200

400

600

800

1000

Days of Aging

Figure4. TRXLPE failures as a function of time in the

Each failure is represented by a

ging test.

triangle.

Figure 5 . XLPEF failures as a function of time in the

Each failure is represented by a

ging test.

triangle.

Discussion of Time to Failure Data

Examination of the time to failure

data

begins to provide

interesting insight on surge effects. For the 175 wall XLPE

cables, it is difficult to compare the control condition to the

surge conditions because the control test was terminated

so

early in the test program. Therefore, it is unclear how

strongly the lightning surges influenced cable life. There is

also no clear difference between the 40 kV and 70 kV

lightning surge conditions. However, comparing the 40 kV

and

70

kV lightning surge conditions to the thumper test

condition is useful.

The samples subjected to the lightning surges failed fairly

regularly throughout the test after the first

150

days of

aging. On the other hand, once the samples subjected to the

thumping surge started to experience a few failures, they

failed very rapidly.

In

fact, the thumper failure points on

the graph were artificially spread out to distinguish

individual failures. Thus, the thumping surge did not appear

~

615

to have a significant effect on the cable until approximately

450

days of aging. At that point, the cable failed rapidly

until all the samples were consumed.

For the 220 wall XLPE cables, the only apparent difference

between the control condition and the

70

kV lightning surge

condition is that failures occurred earlier on the samples

subjected to the lightning surge. Interestingly, the

observation made for the 175 wall XLPE cable subjected to

the thumping surge also applies to the 220 wall XLPE cables

subjected to the thumping surge.

The control and

120

kV lightning surge TRXLPE cables

appear to experience similar failure rates. However,

as

before, the samples subjected to the thumping surge fail

rapidly after they have aged for 450 days.

The effect of the 25 kV and 70 kV lightning surges on the

XLPEF cable is very pronounced. Multiple failures

occurred on the samples subjected to lightning surges while

no control samples failed. Once again, there are no clear

differences between the 25 kV and

70

kV surge levels.

Interestingly, the thumper has not had a significant effect on

the cable life.

Unlike the XLPE and TRXLPE cables, the XLPEF cables

often failed when the lightning surge was applied. This

phenomena will be discussed in more detail later.

Statistical Analysis of Time to Failure Data

To provide a more in depth analysis of the time to failure

data, Weibull probability statistics were employed. The

estimated failure rates for the XLPE, TRXLPE and XLPEF

cables using the Weibull model are presented in Figures

6 -

9. The test conditions are noted on the graphs following the

insulation material description. The abbreviations used are

similar to those employed for the aging failure graphs except

that a 0 represents the control condition.

The characteristic life for each test condition is the point at

which 63.2% of the samples have already failed. For

convenience, the 63.2% level is shown with a dotted line on

each graph. If fewer than three failures occurred for a given

test condition, a failure model could not be calculated.

.

--

TIME

DAYS

Figure

6.

Weibull life model for 175 wall XLPE cable.


6/9

616

w

50

1 , , , ' k

1 1

TIME. DAYS

Figure

7.

Weibull life model for

220

wall XLPE cable.

TIME,

DAYS

Figure 8. Weibull life model for TRXLPE cable.

TIME, DAYS

Figure 9. Weibull life model for XLPEF cable.

Confidence Intervals

To gain further insight from the Weibull statistical mdel,

the values for characteristic life (indicated as I* ) ith

corresponding

90%

confidence intervals are shown for each

test condition in Figure

10.

The values for the Weibull,

shape parameter (also indicated as I* )ith corresponding

90% confidence intervals are shown in Figure 11. %e

shape parameter gives an indication of

bow

the probability

distribution function changes with aging time.

Discussion of Statistical Model

F6r the XLPE and TRXLPE cable designs, there are slight

differences in characteristic life between test conditions.

However,

in

all cases, the confidence intervals overlap,

Figure

10.

Weibull life model parameters. 90%

confidence intervals on characteristic life.

The calculated characteristic life values are

shown

as

a *'

Figure 11. Weibull life model parameters.

90%

confidence intervals on shape factor. The

calculated shape factor values are shown as a

* U .

making distinctions between the characteristic lives of each

sample statistically inconclusive.

h e XLPEF cable design does show a noticeable difference

in characteristic life between samples subjected to the

25

kV

lightning surge and those subjected to the

70

kV lightning

surge. Although there is a slight overlap in the confidence

intervals of the

two

test conditions, the amount is very

small. Thus the observation that the two test conditions

differ is statistically valid, indicating that lowering the surge


7/9

level from 70 kV to 25 kV increases cable life.

There is a distinct difference in the slope for the 220 wall

XLPE cable between the control and the 70 kV lightning

surge condition.

This

verifies the observation made on the

time

to

failure

data

that the samples subjected

to

the 70 kV

surge failed earlier than the control samples.

Another interesting observation for the XLPE and TRXLPE

Weibull curves is that the slope for the thumper test

condition is consistently higher than the slope for other test

conditions. Although there is

some

overlap in the

confidence intervals, this trend indicates that the samples

subjected to the thumper surge very likely experience a wear

out phase. That is, after a period of occasional failures,

they start to fail rapidly,

as

if the cable reached the end of

its reliable service life.

FAILURE MECHANISMS

Several possible

reasons

for differences between control and

surged sample failure rates

as

well

as

differences in lightning

surge and thumping surge failure rates were discussed in [4].

However, failures of the XLPEF cables during the lightning

surge application is a newly observed phenomena. Almost

all of the other cable designs failed sometime after the surge

was applied. The different failure

mode

for the XLPEF

cable can

be

explained by the degree of water treeing in the

insulation.

The XLPEF cables age at 1X ra td voltage which allows the

dielectric to experience greater deterioration before they fail

than the other cables which were aged at 3X rated voltage.

Thus the ratio of the surging voltage to aging voltage (degree

of deterioration) is larger for the cables operated at 1X than

for the cables operated at 3X rated voltage.

This phenomena

is

demonstrated by the very large bowtie

and vented water trees which formed while the XLPEF cable

was in service. Many of the trees found in the insulation of

this cable extend through

as

much

as

97 9 of the insulation

wall. They continued to grow slowly in the aging test where

the applied voltage magnitude is 1X the operating voltage.

However, they easily become a failure path in the presence

of a large magnitude lightning surge.

The XLPE and TRXLPE cables started the test new, with no

water

trees

present. Since the aging voltage on these cables

was 3X the operating voltage, failures occurred in the aging

test long before the water trees reached the length observed

in the XLPEF cables. Since extremely long water trees are

not present in these cables, they are not nearly

as

susceptible

to failure during voltage surges.

617

CONCLUSIONS

Phase 2

The results of EPRI Project RP2284-01 have provided some

interesting insight into the effects of voltage impulses on

extruded cables aged in the laboratory. Several conclusions

drawn during the first phase of the project are presented in

[3] and [4]. Only new conclusions developed during

phase 2 are

stated

in

this

paper. They apply only to the

cables evaluated in the test program described in this paper.

They may

or

may not

be

applicable to cables operating in

service. Obviously, many questions remain unanswered.

1.

2.

3.

1.

2.

3 .

Lightning Voltage Impulses Can Reduce Cable Life.

This observation was made in phase one and reinforced

in phase 2. The 175 mil and 220

mil

XLPE cables, the

175 mil TRXLPE cables and the 175 mil XLPEF cables

which were surged often failed more than the same

cables that were aged without the voltage impulses.

Thumping Voltage Surges Can Reduce Cable Life.

This is a new observation. After 400 days of aging in

the laboratory, the XLPE and TRXLPE cables subjected

to a thumping surge failed more rapidly than the same

cables subjected the control

or

lightning surge condition.

This implies that once cables have been in service long

enough to develop water tree growth similar to that

obtained after 400 days of aging under the conditions

described in this paper, they may fail prematurely if

subjected to thumping surges.

Lighting Surge Magnitude Does Not Strongly Affect

Cable Failure Rates.

Only the XLPEF cables demonstrated a change in failure

rate

as

a function of surge magnitude. However, the

effect was not observed in the characteristic life, but in

the slope

of

the Weibull model curve.

REFERENCES

Minutes of the 78 ' Meeting of the Institute of Electrical

and Electronics Engineers Insulated Conductors

Committee Meeting, April 21-23, 1986. pp. V-C.

Minutes of the

80

Meeting of the Institute of Electrical

and Electronics Engineers Insulated Conductors

Committee Meeting, April 27-29, 1987. pp. V-E2.

R.

A. Hartlein, V. S. Harper and H. Ng. Effects of

Voltage Surges on Extruded Dielectric Cable Life,

IEEE Transactions on Power Delivery, April, 1989

Volume 4, Number 2, pp. 829-841.


8/9

618

4.

Electric Power Research Institute, Effects of Voltage

Surges on Solid-Dielectric Cable Life , Report EL-

6902,

Interim Report, September,

1990.

ANOWLEDGEMENTS

The authors would like to thank the Electric Power Research

Institute for funding

this

research project. It addresses an

area that has concerned electric utility engineers for many

years.

The authors would also like to thank the following Georgia

Power Research Center personnel for their hard work and

dedication to accomplish the project goals.

- Mr.

Larry

Coffeen who designed and constructed the

lightning surge test equipment.

- Mr. Boyd Pettitt who processed the large volume of data

generated and developed many of the graphs and

maintained the surge test equipment.

In addition, the authors would like to thank Dr. Russell G.

Heikes of the Georgia Institute of Technology for providing

the statistical data analysis and the project advisor,

Mr.

P.

Pate1 for his suggestions and guidance.

BIOGRAPHIES

Richard A. Hartlein (M 80) was born in Atlanta, Georgia

on March 20, 1952. He received a Bachelor and Master of

Mechanical Engineering degrees in 1976and 1982 from the

Georgia Institute of Technology in Atlanta, GA. He

has

been employed at the Georgia Power Research Center since

1970 where he is currently manager of the mechanical

section. He has conducted test programs on extruded, thin

wall, 230 kV transmission cable, water tree resistance of

extruded cable designs, and power cable ampacity inside

riser shields. He has also managed EPRI projects on the

short-circuit characteristics of cable metallic shields.

He is a member of the IEEE Insulated Conductors

Committee. He is chairman of ICC Task Group

10-27

which writes the IEEE

404

splice testing standard. He is

also a past chairman of the Cable Engineering Section of the

Association of Edison IlluminatingCompanies and chairman

of the task group which writes the S I C CS5 specification

for XLPE insulated distribution cables.

V.

S. Ha mr (M 74) was born

in

Marietta, Georgia on

October 3,

1943.

He received a Bachelor of Electrical

Engineering degree in 1966 from the Georgia Institute

of

Technology in Atlanta, Georgia. He has been employed

with the Georgia Power Company since

1966 and

is

presently a Research Manager. Previous fields of work

include substation control, power line relaying, fault current

calculations and project engineering for customer

substations.

Mr. Harper is a member of the IEEE Power Engineering

Society, Insulated Conductors Committee and has served on

several EPRI task forces and committees.

H a m W.

Hg

(SM

74)

received his B.S. degree in

Electrical Engineering in

1971

from the University of

Arizona, Tucson, Arizona. From 1971

to

1983 he was with

Tucson Electric Power Co. in Tucson, Arizona in various

distribution engineering positions, starting

as

a Distribution

Engineer. In

1976,

he was appointed Supervisor of

Distribution Engineering.

In 1983 he joined the Electric Power Research Institute in

Palo Alto, California where he is presently Manager,

Distribution System Design and Operations, in the Electrical

Systems Division. His work has been

in

a broad area

related to the distribution of power by electric utilities.

Some of his responsibilities include distribution cables,

electrical fault location, wood pole management, distribution

design and analysis software, energy conservation and

amorphous steel core distribution transformers. He has

authored many papers related to power distribution.

Mr. Ng is a member of the IEEE Power Engineering

Society, the IEEE Insulated Conductors Committee, and the

American

Wood

Preservers Association. He serves

on

the

Wood

Pole

Working

Group of the Distribution Committee of

IEEE, the research committee of the American Wood

Preservers Association and

as

advisor to the Forest Products

Laboratory, Mississippi State University. He is also a

Registered Professional Engineer (Electrical) in the states of

Arizona and California.


9/9

619

surge had

a

very noticeable effect on cable performance. In

Figure

3,

the

175

mil and

220

mil cables clearly had more

failures earlier

in

the test when the

40

kV and

70

kV surges

were applied. In the case of the field aged XLPE cables, the

surged samples failed and the samples which were not

surged experienced no failures.

Diecussion

MARC AUDET, KILBORN INC. TORONTO, CANADA :

The authors have reeented data on impulee

testing of XLPE RND EPR insulation under

laboratory conditione, and thie data seems

to indicate little difference in erformance

between the control samplee a n 8 the test

samples, although th e teet eamples were

eub ected t o impulee waves to simula te

ligitni n surges. It appeare from thi e that

any furtxer teeting of similar cables under

labor atory co nd iti on s with bar e eam 1,s

immersed in water is not liable to resul t in

much usef ul information.

The teat data presented by the authore on1

reinforces reviouely ubliehed data tha z

XLPE E%R ine ulat ea cables and cablee

ineulatnedd with similar compounds fail when

subjected to moieture, generally from water

treeing.

Various pap ers have been presented, which

provide information or teat data concerning

the performance of such cablee of varioue

CO fi U atione and compounds. Some of thie

in?or#afion wae preeented as etatietical

data on cablee installed in th e field. The

data indicates that all similar compounde,

whether TRXLPE or EPR provide good life if

the cablee are sealed against moieture at

the shield. and at the conductor

termination.

It appears that futu re efforte in regard to

cab1 m r v ent s uld be dire ed tow

provfdt n8 %ez?er cakne conf guraelone

w l l g

filled conductors, sealed condinuoue

ehielding tapes or eimilar barriers againet

moisture if long cable life is to be

realized.

Some really useful information could have

been obtained if t he teat eamplee had been

made up in such a manner ae to reflect

actual field conditions. with am ro ve d

terminations etrees -relief debices

rounded shielde, and impervious jackete:

8uch teete would yield really useful data

for cable suppliere, s ecifiere, and

installere. Perhape t he facifitiee available

to th e authore did not permit thi e manner of

testing.

It would have been intereeting to know where

the test cablee failed. whether they f led

at or near th e termina'tione due t o vo?tage

streae, or somewh ere in the middle of the

cables, or randomly. Also, were th e

compou nde teated in any ray after failure,

to eee if any chemical change had occurred?

In

many

cases,

Weibull statistics do show that the

characteristic life of a specific cable design may be similar

for control and surged conditions (Figures

6

-

9).

However,

the slope of the Weibull curves in Figures

6

and

7

is very

different for control samples and surged samples. This is

direct evidence that there is indeed a difference between

control and surged samples.

This

is

demonstrated again in Figure

11

where the

confidence intervals are plotted for the Weibull shape factor.

For example, the there is a significant difference in the

calculated shape factor and the corresponding confidence

interval for the XLPE

220

control samples and the XLPE

220

samples subjected to the

70

kV lightning surge.

Test Cables

Since

the scope of our work was to evaluate the effect

of

lightning surges on

aged

underground cable, we used a very

common procedure of aging cables in an accelerated water

treeing test. They had unfilled, stranded conductors and no

jackets to allow for maximum accelerated aging. Water

impervious jackets and/or blocked conductors would have

significantly increased the time to achieve significant aging.

Other tests are currently underway in

our

laboratory to judge

the effects of jackets and moisture barriers on cable aging.

These results will

be

presented sometime in the future.

It

is important to note that there

is

a considerable amount of

cable in service which is very similar to the cable in our

test. The results obtained in this test program should be

applicable to those cables.

Manuscript

received

August 9,

1993.

Test Procedure

RICHARD A. HARTLEIN,

V. S.

HARPER AND HARRY

NG: The authors would like to thank the discussor for the

opportunity to clarify and provide comments on several

aspects

of the paper.

Suree Effects

Although there were some test conditions where the surge

appeared to have no effect, there were others where the

All test samples were constructed with standard terminations

as used in the field. All cable neutrals were grounded. All

reported test sample failures occurred in the cable within the

water filled tube.

Termination failures were repaired and

not considered cable failures. These procedures are

commonly used throughout the cable industry in accelerated

aging tests on extruded dielectric cables.

Manuscript received October 5, 1993.

cables surges

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