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techniques are discussed later int h i s Chapter.

Water treeing s influenced by the following:

Moisture

VoidsContaminantsIonic impuritiesTemperatureTemperature gradtentAgingtimeVoltage stressPH

5. L BOR TORY ESTING

Treeing was considered tobe a laboratory trick until the1970s. Some of theearliest work was done by Simplex Wire Cable. Kitchens, Pratt, Ware,Crowdes, and others reported on work done with one needle embeddedin smallslabs of polyethylene beginning in 1956. From t h i s work, they developed thefirst commercialtree retardantHMWPE insulation. They reported in 1958 thatmoisture wasan inhibitor to tree growth. What was not known at that time wasthat they were lookingonly at electrical trees. They confidently predicted in1958 that W E ay last more than 40 years in water at operating stress up

to 45 volts per mil. They were not awareof the existence of watertrees as wenow understand them nordid they repeat that statement made in that first paperthat ...at the end of40 years, half the lengths of cable will have failed.

Other researchers in that same timeperiod began using two embedded needles.They came up wt similar conclusions. McMahon and Perkinsreported in 1960that corona life of a specimen of HMWE in air is a strong function ofhumidity. A relative humidity of 95 to 100 gives approximately 15 timeslonger life than dry air. They were also only looking at electricaltrees.

After the reported findingsof Lawson and Vahlstrom and the Japanesereports n1972 of sulfide trees in cables removedfrom the field, laboratory work movedtowards wet testing of insulating materials such as the pie plate test ofMcMahon, and Perkins. By 1975, AEIC had developed an accelerated watertreeing test on actual full sized cable samples placedin water filled pipes.

6 TECHNICALDISCUSSION OF TREEING

Treeinghas been demonstrated as one of the most important factors involved inloss of life for medium voltage cables.Electrical t r e e s are considered to be

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associated with thefinal cable failure and do not exist for a long periodof time.Water trees are the slower growing variety. They can extend fiom one electrodeto the other without a service failure. Once they have formed, watertrees seemto be converted to electrical trees forpart or all of their length by dc, surges, andimpulses. Conclusions in recent research work show that treed cables that aresubjected to dc, surges, or impulses have shorter life in service after thatapplicationthan cables not subjectedto those stresses.

There are several possible explanations forthis conversion of a watertree toan electrical tree, but the more commonly accepted explanation is that chargesare trapped in the insulation wall.When hese trapped charges are disturbed byheat or mechanical motion, they can literally bore a hole through the insulationwall. A llkely scenariois that the trapped charges bore a tunnelfirom one void orcontaminant to the next one. The insulation between these voids may bein adeteriorated condition, thus speeding up the damagedfrom the trapped charges.This continues until the wall has been virtually destroyedand the cable canthold even line voltage.

Inception of water trees is likely to be the resultof voltage enhancements atvoids, contaminants, or other imperfectionsin the cable. Another significantfactor is the presenceof ionic impurities have shown to be especially deleteriousto cables. At one time it was thought that the sourceof these ions was fromground water or the like. Itis now established that the frequentsource of theseimpurities is the materials in the cable basically contaminants inthe older

semiconducting shield materials. Microscopically small chunkso sand makethe insulatiodshield interface another source of voltage enhancement.Growthorpropagation of the water treeis apparently quite slow several yearsin a wellmade cable. Bow tiet r e e s may stop propagatingas they grow large enough todecrease he voltage stress at their exbemities.

We know that voltage stress and temperature acceleratethis propagation ofwater trees. Crosslinked and thermoplastic polyethylene are adversely effectedby temperatures above about75 OC as demonstrated by laboratory agingstudies 116-lo].

It is well established that moisture penetrates polymers.What has only beendemonstrated in thepast 20 years or so is that ac brings moisture toward thepoint of higher electrical stress.This is known as &electrophoresis.Tanaka in1974 presented this important concept that helps explain the growth of watertrees.

As briefly mentioned previously, thereis only a small distinction between waterand electrochemical trees that results from a naturalstainingof the interior or

the voids. Re-1970 HMWPE insulation was formulated with a staining

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antioxidant. These cable didnot require any dying to see the trees. The changeto non-staining antioxidant around1970 resulted in water trees that could not beseen unless the wafers were putin a dye solution.In the transitionperiod, t wasthought that possibly the staining antioxidant was what had caused the treesThe dying procedure is given at the end ofthis section.

rees also exist andare visible in EPR insulated cable but they can onlybe seenat the d a c e of the cut similar dying procedure is used for EPR but thestaining time must be increased considerably. There are also proprietarymethods for staining EPR cable samples.Tree counts in EPR are lower than forthe non-opaquetypes because of not being able to see down into the material,but they also may be lower because they simply don't tree the same as X P Ecables.

Trees positively initiate at defects within the cable such as at discontinuitiesbetween the interfacesof the insulation and thetwo shields, and at voids andcontaminants- metal particles, threads, oxidized bits of insulationambers) andeven t chunksof undispersed antioxidant.

Trees hat have oneof their points of origin at the insulation/ shield interfaceare called 'tented" trees. They always show up as the dangerous trees ascompared to ones thatstay completely within thewall of insulation the non-vented tree. The probable explanation here is that pressure can build up withinthe non-vented tree andthis suppresses thepartial discharge.

7. METIIYLENE BLUE DYING PROCEDURE

In a 500 ml beaker with watch glass cover, place:

A. 250 ml distilled waterB. 0 50 m methylene blueC. 8 ml concentrated aqueous ammonia

Heat to boiling with continuous stirring Use a fume hood or other adequate

Ventilation.

Place he specimens tobe stained in the solution using a wirefor installation andremoval.

Remove specimens from hot solution from time to time tobe certain that thestaining is neither too lightnor too dark.

When the specimensare adequately stained, remove from the hot solution,rinsein hot water, andwipe dry

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A thin film of oil on the surface of the sample makes observation with amicroscope much less confused by scratches.

8 . OBSERVATION IN SILICONE OIL

An excellent method of observing several inches of insulation at one time is toplace a one foot sample on the insulated cable (the semiconducting insulationshield must be removed ) in a glass beaker with silicone oil that h s been heatedto about 130 C. At about this temperature, all of the crystallinity is gone and theinsulation becomes quite clear. The surface of the conductor shield can beobserved for smoothness. Voids or contaminants in the insulation wall can bereadily seen. Note: voids can be created during the test by moisture in theinsulation resulting fiom service conditions.

9. REFERENCES

[16-1]. Treeing Update Kabefitems Parts I 11 111, Vols. 150, 151 and 152,Union Carbide, 1977. (There are 162 references in these three volumes.)

116-21.January, 1978.

Electrochemical Treeing in Cable, EPRI EL-647, Project 133,

[16-31. R. J. Densley, An Investigation Into Growth of Electrical Trees in

XLPE Cable Insulation, IEEE Vol. EI-14, No 3, June, 1979.

116-41. J. Sletbak, A Theory of Water Tree Initiation and Growth, IEEE Vol.PAS-98, 4 Aug., 1979.

[16-51. R. Lyle, W. A. Thue, The Origin Effect of Small Discontinuities inPolyethylene Insulated URD Cables. IEEE 83 WM 002-3, 1983.

[16-6]. S. L. Nunes and M. T. Shaw, Water Treeing in Polyethylene AReview of Mechanisms, EEE Vol. EL15 #6, December 1980.

116-71. R. Lyle and J. W. Kirkland, A nAccelerated Life Test for EvaluatingPower Cable Insulation, IEEE 8 1 W M 15-5.

[16-81, J. Sletbak and E. Ildstad, The Effect of Service and Test Conditions onWater Tree Growth, IEEE 83 W M 03-1.

l16-91. R. Lyle, Effect of Testing Parameters on the Outcome of theAccelerated Cable Life Test, IEEE 86 T&D 577-1, 1986.

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[16-lo]. M. D. Wdton J. T. Smith, B. S. Bemstein and W. A. ThueAccelerated Aging of Extruded Dielectric Power Cables Parts I 11 111, IEEETEUIS.PD Vol. 7 April 1992 and 93 SM 559-5 1992and 1993.

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