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Demonstration of the long-term Demonstration of the long-term reliability of XLPE insulation reliability of XLPE insulation used in MV cables in used in MV cables in

South AfricaSouth Africa

byby

Pravesh HaripersadPravesh Haripersad

Design EngineerDesign Engineer

What is meant by MV cables?

What is the current reliability of such cables?

What are water trees?

How does a cable manufacturer demonstrate long-term reliability of their cables?

Discussion of the 50Hz and 500Hz ageing tests conducted!

What service-life can one expect?

What is the benefit of conducting both ageing tests?

Conclusions

Presentation will address the Presentation will address the following questions or concerns:following questions or concerns:

What is meant by MV cables?

Introduction to MV cablesIntroduction to MV cables

MV cables range from 3.3kV to 33kV

Two primary insulations:

Paper Insulated Lead Covered (PILC)

Cross-linked Polyethylene (XLPE)

SANS 97 SANS 1339

Compulsory

Specifications!

What is the current reliability of such cables?

PILC cables known to have high reliability – some in service well over 50 years!

Similar reliability not reported for the first XLPE cables installed in the 1970’s and 1980’s:- Premature failures occurred mainly due to the presence of

impurities and contaminants within the XLPE insulation

XLPE insulation cleanliness and production/process technologies have evolved considerably

(depending on Manufacturer, ofcourse!!)

The quality of manufactured XLPE cables is so high that it is becoming the preferred insulation even at 500kV!!

- Lower dielectric losses; higher operating temperatures thus higher ampacities; lower environmental impact

Long-term reliability of MV XLPE cablesLong-term reliability of MV XLPE cables

Long-term reliability of MV XLPE cablesLong-term reliability of MV XLPE cables

Service-life expectancy of a good quality, defect free XLPE cable operating under normal conditions is estimated to be in excess of 30 years !!

The following factors reduce this long-term reliability:

- presence of impurities and contaminants- increase in operating voltage (electric stress)- increase in operating frequency

- increase in operating temperature- the presence of water trees

A 10oC increase in operating temperature reduces cable life expectancy by up to 50%!!!

Presence of water trees in XLPE insulation reduces cable life expectancy by up to 43%!!!

What are water trees?

Water trees in XLPE cablesWater trees in XLPE cables

Water has an adverse effect on XLPE insulation as it develops water trees !!

Water trees in XLPE cablesWater trees in XLPE cables

Generally, MV XLPE cables are not water blocked

No definite mechanism whereby water trees arise, but one possibility is by diffusion:

Water enters by diffusion

Add impurities, voltage, temperature

and frequencyBow-tie

tree

Vented tree

Water trees in XLPE cablesWater trees in XLPE cables

Water trees are microscopic and are usually invisible to the naked eye

Insulation is stained and examined using a microscope

A Vented tree

A Bow-tie tree

Water trees in XLPE cablesWater trees in XLPE cables

Water tree development and growth dependant on: - presence of water

- presence of impurities or contaminants - alternating electrical stress - frequency - temperature

Water trees develop into conductive electrical trees which ‘bridge’ the insulation causing failure

Growth of an electrical tree from a needle point inserted into XLPE insulation:

Play

How does a cable manufacturer

demonstrate long-term reliability of their cables?

(50Hz and 500Hz ageing tests)(50Hz and 500Hz ageing tests)

Demonstration of long-termDemonstration of long-termreliability (50Hz or 500Hz ageing)reliability (50Hz or 500Hz ageing)

To demonstrate good quality and long-term reliability, cable manufacturers are compelled to conduct either a:

- 2 year, 50Hz ageing test, or a - 4 month, 500Hz ageing test

The main ageing mechanisms that will be applied to the cable are:

- increased electrical stress (3 x Uo) - frequency (50Hz or 500Hz)

- growth of water trees

Demonstration of long-termDemonstration of long-termreliability (50Hz or 500Hz ageing)reliability (50Hz or 500Hz ageing)

500Hz ageing test is attractive due to shorter duration, 4 months vs 2 years!!

Only two local companies are known to have 500Hz ageing test facilities in the country

CBi electric: african cables 500Hz ageing test lab

Demonstration of long-termDemonstration of long-termreliability (50Hz or 500Hz ageing)reliability (50Hz or 500Hz ageing)

CBi electric: african cables 50Hz ageing test area

Setup as per SANS 6284-5

Demonstration of long-termDemonstration of long-termreliability (50Hz or 500Hz ageing)reliability (50Hz or 500Hz ageing)

At end of ageing, 12 samples, each 10m long are subjected to stepped breakdown test at 1Uo for 5 minutes

Passing criteria is based on breakdown strength, not on water tree count!

Test requirements according to SANS 1339:

Minimum withstand stress

Number of samples that must achieve

this

14 kV/mm 12

18 kV/mm 9

22 kV/mm 5

50Hz and 500Hz ageing tests conducted by CBi electric:african

cables and a discussion of the results!

African Cables have conducted both ageing tests in strict accordance to SANS 1339:

Test results, comparisonsTest results, comparisonsand discussionsand discussions

50Hz ageing test

500Hz ageing test

Started Mar 2003 April 2003

Completed July 2005 Dec 2003

Size 95mm² x 1 Al 240mm² x 1 Cu

Cable voltage 6.35/11kV

XLPE thickness 3.5mm

Length 120m

Applied test voltage 19kV (3Uo)

Test results, comparisonsTest results, comparisonsand discussionsand discussions

50Hz aged sample about to be broken down in 1Uo, 5 minute steps

Test results, comparisonsTest results, comparisonsand discussionsand discussions

Test technician configuring and programming the test set

Test results, comparisonsTest results, comparisonsand discussionsand discussions

The 11kV, 2 year aged sample withstood 80.7kV (13Uo or 30kV/mm) !!!

Test results, comparisonsTest results, comparisonsand discussionsand discussions

Sample cut at breakdown site!!

and discussionsand discussions Test results, comparisonsTest results, comparisons

50Hz ageing test

500Hz ageing test

Withstand 14kV/mm 12 12

Withstand 18kV/mm 12 11

Withstand 22kV/mm 12 11

Type test result SUCCESSFULLY COMPLIED!!

Breakdown assessment of aged samples:

Test results, comparisonsTest results, comparisonsand discussionsand discussions Breakdown data analysed using Weibull Distribution:

63% characteristic breakdown

strength

50Hz 500Hz

37.5 kV/mm

32.5 kV/mm

Unaged cable has a typical breakdown strength of 50 kV/mm!!!

Test results, comparisonsTest results, comparisonsand discussionsand discussions

Water tree analysis:

Test results, comparisonsTest results, comparisonsand discussionsand discussions

Water tree analysis:

50Hz ageing test

500Hz ageing test

Number of vented trees near inner semicon 13 ( 600µm) 0

Number of vented trees near outer semicon 0 0

Number of bow-tie trees 0 2 ( 100µm)

Test results, comparisonsTest results, comparisonsand discussionsand discussions

The bow-tie tree observed in the 500Hz aged sample

What service-life can one expect?

Characteristic breakdown strength vs cable life expectancy of MV XLPE cable samples aged at 50Hz and at 500Hz

0

2

4

6

8

10

12

14

16

18

1 10 100

cable life expectancy (years)

chara

cteri

sti

c bre

akdow

n s

trength

(U

o)

50Hz

500Hz

Cable life-expectancy Cable life-expectancy

n = 9

30 years

What is the benefit of conducting both ageing

tests?

Effectiveness of the two Effectiveness of the two ageing testsageing tests

Both tests met all breakdown strength criteria of SANS 1339

Some differences in results, but not significant differences:

- 63% characteristic breakdown strength of 500Hz is 5kV/mm lower than 50Hz

- slope of 50Hz Weibull plot is steeper than 500Hz plot

- 50Hz produced more water trees than 500Hz

Above indicates that there are numerous ageing mechanisms taking place

Passing both 50Hz and 500Hz test gives greater confidence of good quality and long-term reliability of the cables

ConclusionsConclusions

ConclusionsConclusions

Paper highlights two different approaches taken by African Cables to demonstrate long-term reliability of their MV XLPE cables

Results of ageing show that even after 30 years, cables will have breakdown strengths in excess of 10Uo

Water tree ageing mechanism produced little water trees, confirming high quality and defect free XLPE insulation

Such results enable a customer to easily satisfy himself of the level of quality and long-term reliability of the cable

CBi electric: african cables have demonstrated that they offer only high quality MV XLPE cables with a long-term reliability

Power by Innovation… Innovation Through Partnerships

Thank you and

visit us at:

Thank you and

visit us at:

www.cbi-electric.com

www.cbi-electric.com