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The next compilation of data began in 1976 with 16 and later 21 utilities inNorth America reporting their failure rates on an annual basis for bothpolyethylene and crosslinked polyethylene insulated cables[ 18-31.

Failure data kept by the utilities was rather meager.It was decided to onlyrequest databased on type of insulation, number of failures for each year and thetotal amount of each of those insulation types were in service at the end of theyear. It was also decided to only ask for failures ofknown electrical causes, suchas defective cable, insulation deterioration,lightning etc., and then include allWzknowncauses since treeing analysis was not easily obtainable.

3. PERFORMANCE

Comparable data from EEI for paper insulated, medium voltage power cablesinstalled in the United States is included as Figure 18-1 for the years of 1923through 1966- when the data was no longer collected. Similar data showing theelectrical failures of polyethylene and crosslinked polyethylene for 21 NorthAmerican utilities is shown in Figure 18-2. AEIC then began to collect andreport similar data in 1984 exceptthat data was requested fromall utilities. Amajor future step was to request informationon jackets, ducts, voltage stresslevels, etc. The old 21 company base was not separately recorded, however.They also began to collectdata for tree retardant XLP as well as EPR. SeeFigures 18-3, 18-4, 18-5 and 1 8 6.

AEIC strongly suggests thatthis data be carefidly analyzed and understood.Thisis important since the age of the cables were not known and could skew theresults. For instance, jacketed cable is probably newerth n non-jacketed cableand hence the failure rateof the older cablesmay not be entirely the result of ajacket.

The European community also began to collect data and their results werepublished as UNIPEDE-DISCAB that represents most of the Europeancountries. Their data includesPE XLP, EPR and PVC.

4. ANALYSIS OF DATA

Cable failure rates in theU.S. have historically been calculated on the basis offailures per 100 miles of installed cable. The rest of the world reports failuresper 100 kilometers. ll data is shown as rates per 100 miles for ease ofcomparison.

The most fiequently used form for thedata shows the numberof failures per 100

miles for each year. The disadvantage of such depictionis that older cable is

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looked atin the same light as new cable. This data is more readily available, buta preferred method is to take into accowltthe years in servicefor all cables. Thisis accomplishedby integrating he miles i nst l l ed wi th the years of service. Theexpressionis :

ServiceIndex

where

f X S

ABCDEF

A j i Mi l v j - i ) 18.1)

A = system age in yearj in terms of seMce mile-years,Mi = the number of miles of cable installedin year i5 i = the number of years fromi to j.

Milesof Cable Multiplied by Ape

a2a b3a 2b C4a 3b 2c d5a 4b 3c 2d e6a 5b 4c 3d 2e f etc.

At the endof the yearF the age of the system is:

A F 6a 5b 4c 3d 2e f 18.2)

where a, b, c, etc. represent the number of miles of cableinstalledin year A B, C....

This analysisc n be shownas a summation of failures per 1,000 mile-years, thisis:

Cumulative Milesofear Cableat End of Year Summation

ABCDE

aa ba b ca b c da b c d e

z = 5a 4b 3c 2d e

It is only necessary to add the miles of cable installed each year to thesummationof cable installed in all previous years to obtain updated mile-years

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from this equation.

5.0 PRESENT SITUATION

There is no new data to report regarding cable failure statistics.This s the resultof two factors:

New cable performanceis very good. The few problems withnew cable makes collecting data seem unnecessary.

There are not enough people to do the essentialwork.

The last North American data was collected for 1991 performance byAEIC.The data shows an extremely low failure rate forTR-XLPE and EPR. The XLPErate is not escalating to a troublesome level. he Europeancollectionof data hasalso been discontinued.

This is certainly an indication of the effort thathas been directed towardimproved cables both from the material suppliers standpointas well as thecable manufacturers.

Fipre 18 1PILC Cable Failures in the United States

2.5tiin 1 5

2 1Q 2

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Figure 18 2Electrical Failures of Extruded Dielectric Cables

Figure 18-3AJCIC Cable Failure Data, High and Low Electrical Stress

/ I

- HMWPE* XLPE

EPRTR-XLPE

- E-40vpm-- XLPE40vpm

1983 1985 1987 1989 1991

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Figure 18-4AEIC Cable Failure Data, Duct versus Direct Buried

1983 1984 1985 1QeS 1987 1988 1989 1990 lQ9l

Fallurer par 100 MIIer

Figure 18-5AEIC Cable Failure Data, Jacket versus Non-Jacket Construction

- E-No Jacket-t- XLPE-NO

Jacket-- XLPE-Jacket

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Figure 18 6Cable Failure Data, Seasonal Pattern

4d

= 2

5 150

tn I 0 0e= 50

0

I

L

- PL- alifornia. Palmetto- uget

January June November

6. REFERENCES

[18-1 W. A.Thue. dapted from class notes, Power Cable Engineering Clinic,University of Wisconsin - Madison, October, 1997.[18-21EPRI Report EL-647,1976.

EPRI RP 133: Electrochemical Treeing in Solid Dielectric Cable,

[18-3] W. A. Thue and J Bankoske Operating and Testing Experience onSolid Dielectric Cables, CIGRE, 1980.

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