Power by Innovation… Innovation Through Partnerships Demonstration of the long-term reliability of XLPE insulation used in MV cables in South Africa by Pravesh Haripersad Design 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 following questions or concerns:

What is meant by MV cables?

Introduction to MV cables  MV cables range from 3.3kV to 33kV  Two primary insulations: Paper Insulated Lead Covered (PILC) Cross-linked Polyethylene (XLPE) SANS 97SANS 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 cables Long-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 10 o C 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 cables Water trees in XLPE cables  Water has an adverse effect on XLPE insulation as it develops water trees !!

Water trees in XLPE cables Water 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 frequency Bow-tie tree Vented tree

Water trees in XLPE cables Water 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 cables Water 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)

Demonstration of long-term Demonstration of long-term 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-term Demonstration of long-term 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-term Demonstration of long-term reliability (50Hz or 500Hz ageing) CBi electric: african cables 50Hz ageing test area Setup as per SANS

Demonstration of long-term Demonstration of long-term 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 stressNumber of samples that must achieve this 14 kV/mm12 18 kV/mm9 22 kV/mm5

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, comparisons Test results, comparisons and discussions 50Hz ageing test 500Hz ageing test Started Mar 2003April 2003 CompletedJuly 2005Dec 2003 Size95mm² x 1 Al240mm² x 1 Cu Cable voltage6.35/11kV XLPE thickness3.5mm Length120m Applied test voltage19kV (3Uo)

Test results, comparisons Test results, comparisons and discussions 50Hz aged sample about to be broken down in 1Uo, 5 minute steps

Test results, comparisons Test results, comparisons and discussions Test technician configuring and programming the test set

Test results, comparisons Test results, comparisons and discussions The 11kV, 2 year aged sample withstood 80.7kV (13Uo or 30kV/mm) !!!

Test results, comparisons Test results, comparisons and discussions Sample cut at breakdown site!!

and discussions Test results, comparisons Test results, comparisons 50Hz ageing test 500Hz ageing test Withstand  14kV/mm 12 Withstand  18kV/mm 1211 Withstand  22kV/mm 1211 Type test resultSUCCESSFULLY COMPLIED!!  Breakdown assessment of aged samples:

Test results, comparisons Test results, comparisons and discussions  Breakdown data analysed using Weibull Distribution: 63% characteristic breakdown strength 50Hz500Hz 37.5 kV/mm 32.5 kV/mm Unaged cable has a typical breakdown strength of 50 kV/mm!!!

Test results, comparisons Test results, comparisons and discussions  Water tree analysis:

Test results, comparisons Test results, comparisons and 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 semicon00 Number of bow-tie trees0 2 (  100µm)

Test results, comparisons Test results, comparisons and discussions The bow-tie tree observed in the 500Hz aged sample

What service-life can one expect?

Cable life-expectancy n = 9 30 years

What is the benefit of conducting both ageing tests?

Effectiveness of the two ageing 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

Conclusions

Conclusions  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

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