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THE DEVELOPMENT OF W TERTREERET RD NTXLPEINSUL TIONG.Matey and].D UmplebyBPChemicals (Suisse)S.A.

Presented at the ETG/CIGRELong term performance of high voltage insulations Conference-Berlin, September 9-11,1985Presented at the technical sessionAdvances in material technology for the electrical wire and cable industryof the Interwire Exposition -Atlanta, November 5-8 1985

SummaryIt is the purpose of this paper to present thedevelopment of a water tree retardant (WTR) XLPEmedium and high voltage insulation compound and tocompare itsperformance tothat of XLPE from theprevious generation.The new WTR XLPE compound combines the benefitsof a suitable polymer modification and effective watertree retardant additive selection.Laboratory tests have demonstrated the increasedresistance towater trees (vented or bow ties) as well asthe compatibility with conventional semiconductiveshielding material. Full size cable testing is now underway and results will be reported in due course.The new compound has been shown to process undervery similar extrusion and curing conditions totheprevious generation, and has been satisfactorilyscaled-up to full manufacturing status.IntroductionThe excellent electrical and physical propertiesofXLPE medium and high voltage power cableconstruction have stimulated steady growth inrecent years.Although the low density polyethylene used today issimilar tothat developed many years ago, a number ofsmall but progressive modifications have been madeto its physical, chemical and electrical properties inorder tooptimise the performance in medium and highvoltage applications.However, the W C industry has been concerned witha polymer degradation phenomenon which takesplace under the combined effects of the electric fieldand humidity, known as water treeing.Ways of minimizing or suppressing this degradationphenomenon have been studied extensively and thefollowing parameters reported.- quality of the semiconductive shielding- cleanliness of the insulation compound- crosslinking in the absence of watar- the use of computer for improved processing control- continuous metal sheathing- conductor water-tightriessEven so,an additional protection is stil l required in thecase of unpredictable conditions which couId initiatewater treeing and potentially lead to cablebreakdown.

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This additional protection should be provided by theinsulation material itself.Water tree retardant XLPE insulationThe water tree retardant XLPE insulation has beendeveloped from two main considerations: (a) the basepolymer has been modified in order to be intrinsicallymore resistant to water treeing de gradation than thebase polymers used for convention al XLPE insulationcompounds, (b) the water treeing resistance offered bythis base pol ymer has been enhanced by anappropriate water treeing inhibitor (WTI). Thus thenew water tree retardant insulation compoundcombines the benefit of pol ymer modification with aneffective water tree retardant additive.hoice of WTIAdditives for retarding water tree growth in peroxidecrosslinkable polyethylene compounds are requiredtofulfil the following addition al criteria:- preservation of the thermal stability of the system- preservation of crosslinking characteristics- preservation of ageing properties- compatibility with the system before and afterextrusion- controlled reactivity towards water in the polymermatrix- active water tree retardant hydrolysis bi-products-low toxicity.Fundamental studies carried out in our laboratorieshave demonstrated the effectiveness of a family oforganometallic compounds as water treeing inhibitors.From these, an optimum balance of electrical andperformance properties was selected for furtherstudy.Water tree growth rate determination.The Ashcraft geometry (point to plane) has been used'to determine the water tree growth rate (WTGR) ofinsulation compounds (1).The test conditions differfrom Ashcraft's initial conditions in that the changesallow better discrimination between compounds. Ahigher temperature of 65C has been used instead ofroom tempe rature, with the voltage applied for 72 hoursinstead of 24hours igure Test voltage : 5kV/6kHzTest temperature : 65CTest duration : 72 hoursElectrolyte : O.OINNaCl solution

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ln Table 1 the water treeing resistance of a standardXLPE insulation compound is compared with thatobtained by the successive steps of polymermodification and water tree inhibitor addition.Table lWTGR of XLPE insulation

WTGR( )Standard XLPE 100Polymer modified XLPE Water tree retardant XLPE 5It is clearly shown that the polymer modification aloneresults in a four fold reduction of the water tree growthrate. The addition of the water treeing inhibitorprovides a further reduction, down to one twentieth ofthe growth rate of standard XLPE insulation. Currently,the pol ymer modified XLPE insulation compound iscommercially available (2) and has been adoptedmainly in Germany. Field results have confirmed theincreased water tree resistance relative to standardXLPE insulation.A further variation ofthis test involving higher voltages(20kV/ 4mm) over a longer period of time haveconfirmed the relative performances of standard,pol ymer modified and water tree retardant XLPEcompounds, as shown in Figure 2The efficiency of the water treeing inhibitor is shown inFigure 3,where the WTGR ofunstabilized anduncrosslinked LDPE is plotted versus the WTI content.

5kV/6y ~ y~ance la Ground

~ 3mm

TEMPERATURE: 65CDURATION: 72 hrs

WATERTREE LENGTH (mm)TEST CONDITIONS: 200 kV/4 mm,50Hz, room lemperalure

WTGR100

50

0.5 2

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Life time determinationThe enhanced resistance to water treeing provided bythe WTI modified XLPE in the presence of ionieimpurities has been determined in tests with a uniformfield geometry. A two layer configuration Figure 4)comprising semiconductive shielding and insulationcompound is used, with NaCI crystals incorporated atthe semiconductivelinsulation interface beforecrosslinking.Test voltage : 20kV/50HzTest temperature : R.T.Table IILifetime of XLPE insulation

WithoutWTI WithWTIIst test specimen 250 14402nd test specimen 548 Continuing beyond 5000hrs3rd test specimen 564 Continuing beyond 5000hrs4th test specimen 1312 Continuing beyond 5000hrsIt is clearly shown that the WTI modified compoundexhibits a longer life time than the sa me formulationwithout the additive.The difference in bow tie generation during the initialstage of this test is illustrated in Figure 5.ompatibility with semiconductive shieldsIt is, of course, very important to verify that thisinsulation compound, containing WTI will not affect theelectrical properties of the adjacent semiconductiveshield. For this purpose, the volume resistivity ofstandard XLPE semiconductive shield was monitoredin a sandwich construction where insulation andsemiconductive layers had been crosslinked togetherlable III).Table IIIStability of volume resistivity (n.cm) of thesemiconductive shield after ageing at 90e in contactwith water tree retardant XLPEInitial 360 hrs 670 hrs 1370hrs 6

This demonstrates that the volume resistivity of thesemiconductive shield remains unaffected by closecontact with the water tree retardant XLPE.Migration of the WTISince potential additive migration in XLPE is animportant consideration, WTI containing compoundwas brought into close contact with successive layersofthin LDPE films, the assembly being clampedtogether and maintained at 80C for 24 hours.After 24 hours the concentration of WTI in eachsuccessive film layer was determined, as shown inFigure 6.This indicates that, within the accuracy of the test,there is no significant miration ofthewater treeinginhibitor. This result is in complete agreement with thefact that the volume resistivity of a semiconductiveshielding compound, brought into close contact withthe insulation compound for more than 1000 hrremains unaltered.

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SEMI CONDUCTIVELayer l

co co0..1 QI\2 \2(\3 _ (\3\ 4 ~~ Q4\5 Q506 _ 06Q 7 ~ 0 7

WITHOUTWTI WITHWTI

1000

DIFFUSION (ppm) aher 24hrs a l80 C

200 400 600 800DISTANCE (microns)

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roduct Manufacturehrough a series of production process trials it haseen demonstrated that the new product can beeproducibly manufactured in full commercialrocess. No deterioration of the earlier establisheduality parameters, particularly crosslinkingharacteristics and stringent cleanliness level, haseen observed,Additional quality controls have been placed upon thehemical purity of the WTI and its content anddispersion within the polymer matrix.erformance Propertieshe extrusion properties of the water tree retardantcompound have been determined under thefollowing conditions:- General Engineering extruder l50mm, 20 L/D-XLPEscrew- Standard tempe rature profileThe output versus screw speed, as shown in Figure 7clearly indicates that the extrudability of the WTRcompound is comparable to that of standard XLPEcompounds.ln this paper, only laboratory test results have beenpresented. Full sized l2/20kV cables are nowundergoing a CIGRE type long term test.Results will be reported in due course,onclusionA new generation XLPE insulation compound withenhanced water treeing restistance is now available, toreplace currently commercial cornpounds, This newgeneration XLPE combines the benefits of animproved pol ymer modification with an effective watertree inhibitor. The predictable advantages lie inincreased cable reliability and potential avoidance ofan outer metallic moisture barrier in future cabledesign,References(1)AG Ashcraft: Water treeing in PolymerieDielectrics World Electrical Congress in MoscowJune 22nd, 1977,(2)A, Campus: Improvements iri performance of XLmedium and high voltage cable applications, Wire84 Technical Conference June 21st, 1984,Acknowlegements: The authors wish to thankDr, W. Golz (AEG Research) for providing electricaltesting results

OUTPUT(Kg/h)

ISO

50

General Engineering Extruder ISO mm 20DXCLPE screwtandard temperature prof i le

10 20 RPM