IntroductionWater trees are known to be associated with reduc-

tion of dielectric strength and eventual failure of poly-ethylene-insulated cable which does not incorporate aneffective moisture barrier [1,2]. However, past studiesof this phenomenon often correlated poorly with datagathered from field-aged cable [3,4]. In particular,chemical analysis of water treed regions of laboratory-aged polyethylene demonstrated substantial differ-ences from field-aged cable. Such results cast doubt onthe usefulness of typical laboratory studies.

A 1983 survey of solid dielectric cable technologyconducted on behalf of EPRI indicated that several ca-ble manufacturers in Europe and Japan placed greatemphasis on the properties of the semiconductingshields used to manufacture power distribution andtransmission cables. The carbon blacks used in NorthAmerica differed substantially from those used bythese offshore manufacturers. As a result of this find-ing, the Department of Energy initiated a project titled‘‘Interfacial Ageing Phenomena in Power Cable Insula-tion Systems.’’ The findings of this project have beenreported in several technical publications [4-7] whichform the basis for the present summary.

Experimental Test ProgramThe test program was based on a sample configura-

tion which incorporates the interface between the semi-conducting compound and the polyethylene dielectric.The sample cell used in this project (Fig. 1) is a modifi-cation of a sample configuration originated by UnionCarbide. This cell provides a well which allows onesemiconducting layer to be maintained in a controlled,wet environment while the other semiconducting layer

remains dry. This facilitates a direct comparison of thesemicon-dielectric interface when maintained underwet and dry conditions. Ageing was carried out overlong periods of time (typically 3500 and 7000 hours) atmoderate stress (3.4 kV/mm or 85 V/mil and 2.6kV/mm or 65 V/mil). Direct comparison of treesgrown in this cell configuration and at these stresseswith trees taken from field-aged cables indicated verysimilar tree chemical characteristics.

Experiments were carried out using four commercialsemiconducting insulation shield materials (C1 to C4)and four ‘‘model’’ shield materials (M1 to M4) formu-lated to vary such properties as the amount of sulfur inthe shield. The dielectric used in all cases was a com-mercial cross-linkable polyethylene resin in the form ofpellets which had been subjected to 100% optical in-spection.

Raw Material ImpuritiesAnalysis of the cell water showed that in all cases, the

ion concentration of the distilled, deionized water in-creased during ageing, and after as little as 100 hours ofageing, many ion species could be identified whichwere not initially in the water. This effect was inde-

Fig. 1. Test cell design which provides wet and dry semicon-dielec-tric interfaces [4].

Table I [4]

Contaminant Content of Raw Material (weight %)

Sample Surface Bulk Cleaned Surface

C1 .0513 .0746 .0087

C2 .0306 .1038 .0096

C3 .0994 .1072 .0100

C4 .3992 .2970 .0110

C5 .2484 .0286 .0010

M1 .0818 .0355 .0090

M2 .0180 .0860 .0013

M3 .3892 .1430 .0101

M4 .0494 .0581 .0091

Role of Semiconducting Compounds in WaterTreeing of XLPE Cable InsulationS.A. Boggs and M.S. Mashikian, Electrical Insulation Research Center, University of Connecticut

pendent of electrical stress. Various analytical tech-niques were used to evaluate contaminants in and onthe raw material pellets. The results of this analysis areshown in Table I. In addition, the pellets were cleanedthrough ‘‘bathing’’ in hot, distilled, deionized water.

As seen in Table I, the surface contamination gener-ally differs from the bulk contamination. As well, thesurface contamination can be reduced substantiallythrough cleaning in hot, distilled water, which indicatesthat the contaminants are water soluble. The surfacecontamination is believed to come from processingwater used during the pelletizing process. As seen inFig. 2, the surface contamination is substantial and asshown in Fig. 3, it can interfere with the quality of thepellet-to-pellet interface. This effect may be much lesspronounced in extruded cable; however, the ionic con-taminants must still end up dispersed throughout thedielectric.

ResultsFig. 4 shows vented water tree length vs. ageing time

for an ‘‘old technology’’ commercial semiconductiveshielding compound while Fig. 5 shows a similar graphfor a model shielding compound formulated withacetylene black. Note the difference in vertical scales.Obviously the cleaner model compound results in thegrowth of substantially fewer and smaller water trees.Evidently, tree growth is saturated by 7000 hours. Asthe tree growth was thought to be related to the supplyof ions, tree growth might be limited as the supply ofions is depleted. While this could easily occur in the testgeometry used during this project, it is less likely to oc-cur in the field, as installed cables are subjected toground water containing an ample supply of ions andwhich can pick up more ions from the cable conductorwhich is not included in this study. To test this hy-pothesis, water containing ions extracted from ‘‘soil’’

Fig. 2. Contaminants on the surface of Compound C5 (a pellet ofpolyethylene resin).

Fig. 3. Detail of the interface between two pellets after melting. The‘‘defect’’ in the interface caused by contamination on the surface ofthe pellets is obvious.

Fig. 5. Vented water tree length distribution for several ageingtimes. The semiconducting compound was formulated with acety-lene black, and the cell was aged at 3.4 kV/mm (85 V/mil) [7].

Fig. 4. Vented water tree length distribution for several ageingtimes. The semiconducting compound was an ‘‘old technology’’commercial shielding compound. The cell was aged at 3.4 kV/mm(85 V/mil) [7].

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and from semiconducting compound was added to acell after 7000 hours of ageing. As seen in Fig. 6, thenumber of vented water trees increased at a fairlysteady rate for the samples which had an adequate sup-ply of ions, whereas the number of trees leveled off forthe sample which did not. Likewise, the effect of water-extractable contaminants on tree growth could be dem-onstrated. Fig. 7 shows a comparison of the ventedwater tree length distribution for cells manufacturedwith as-received ‘‘old technology’’ semiconductingshield and cells made with the same semiconducting

material after cleaning to remove water-extractablecontamination. The effect of cleaning is dramatic.

Previous literature indicates that surfactants (deter-gent-like materials which reduce surface tension) in-crease tree initiation and growth. However, the surfac-tants used in previous studies were ionic. Fig. 8 showsa comparison of tree growth for cells in which the semi-con-dielectric surface was treated with an ionic surfac-tant, a nonionic surfactant, and no surfactant. As isclear from the figure, the effect on water treeing is theresult of added ions, not the added surfactant.

During the extensive tree counting required to as-semble the data presented in the above figures, a num-ber of effects were noted. For example, asperities at thesemicon-dielectric interface did not generally result in

Fig. 6. The importance of ions for tree initiation is demonstratedthrough comparison of the number of trees vs. time for cells filledwith deionized, distilled water and cells filled with water contain-ing ions extracted from soil and semiconducting shield material. Inthe case of distilled, deionized water, the ions from the sampleshield become depleted after about 7000 hours and tree initiationstops. Tree initiation continues in the other samples, which have anample supply of ions. Ageing was carried out at 3.4 kV/mm (85V/mil) [7].

Fig. 7. Comparison of vented water tree length distributions forcells fabricated with cleaned and as-received ‘‘old technology’’ com-mercial shielding compound [7].

Fig. 8. Influence of ionic and nonionic surfactants on the numberof vented water trees after 7000 hours of ageing at 3.4 kV/mm (85V/mil). Evidently, water treeing is stimulated by ions rather thansurfactants [7].

Fig. 9. Silicon ion profile as a function of position from the semi-conducting compound into the dielectric. The transition is sharp forthe dry electrode but is smeared out by ion migration from the wetsemicon to the wet dielectric. The sample has been aged for 7000hours at 2.6 kV/mm (65 V/mil) [4].

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the growth of water trees unless they were accompa-nied by some form of contamination. Thus such asperi-ties appear to have much less influence on water treeinitiation than does contamination.

Clearly, the above results suggest that ions are mi-grating from the semiconducting material into the di-electric and water tree. Several studies within the pro-ject demonstrated beyond any doubt that this can occur.For example, Figs. 9 and 10 show the profile of silicon(ion) concentration from within the semicon to withinthe dielectric for the wet and dry semicon-dielectric in-terfaces of the same samples. Clearly the transition isvery abrupt on the dry side but has become smeared asa result of ion mobility on the wet side. Virtually novented water trees grew at the dry semicon-dielectricinterface, while many vented water trees grew on thewet interface.

Conclusions

1. As noted above, detailed comparisons of the chemis-try of trees grown during this project with trees fromfield-aged cable demonstrated excellent correlationin all parameters which could be measured. Thus thetest protocol employed in this study appears tomimic field ageing accurately.

2. As a result of the extensive testing carried out duringthis project, we can conclude that the initiation ofwater trees at a semicon-polyethylene interface re-quires ions and that the required ions are generallypresent in and on the semiconducting material. Theconcentration of water soluble ions on the surface ofsemiconducting material can be reduced throughsimple water extraction, and such reduction has asubstantial effect on the number and length of watertrees.

3. The growth of water trees does not depend solely onions originating from the semiconducting shields.Ground water will generally carry substantial ioniccontamination.

4. The tree length and size distribution differs signifi-cantly for the semiconducting compounds tested,with the newer technology products which incorpo-rate the cleanest carbon black showing a trend to-ward reduced propensity for the growth of ventedwater trees.

5. While the propensity toward treeing appears to de-pend on the degree of surface and volume contami-nation of the shield material pellets, the role of eachimpurity constituent has yet to be determined.

6. Surface contamination of the insulation pellets re-mains segregated at the interpellet interfaces andstimulates the initiation of bowtie trees.

Technology TransferVarious aspects of the technology developed during

this Department of Energy-supported work have beentransferred to Cablec Polymers, Inc., Dow Corning, Inc.,Union Carbide, Elastimold Corporation, Cabot Corpo-ration, Uniroyal, AT&T, Exxon, and BP Polymers. Thisresearch has resulted in substantial changes to semicon-ducting shield technology and compounds which haveimproved the reliability of solid dielectric cable.

Theoretical Context (Speculation)A full theory for water treeing is still lacking. How-

ever, the outlines of a theory are available. H.R. Zellerhas published two seminal papers in the field [8,9]. Onepaper outlines the basis for condensation of waterwithin a polymer. Using basic thermodynamics, Zellerpoints out that water dispersed in a hydrophobic mate-rial which contains hydrophilic regions will condense

Fig. 10. Silicon ion profile as a function of position from a secondsemiconducting compound into the dielectric. The transition issharp for the dry electrode but is smeared out by ion migration fromthe wet semicon to the wet dielectric. The sample has been aged for7000 hours at 2.6 kV/mm (65 V/mil) [4].

Fig. 11. Change in chemical potential with change in ion concentra-tion as calculated by Zeller for two water-filled cavities connectedby a thin channel [9]. A change in chemical potential over about 1eV is sufficient to ‘‘drive’’ an electrochemical process such as watertreeing.

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into those hydrophilic regions [8]. In the second paper,Zeller provides a credible basis for the ‘‘driving func-tion’’ of water treeing [9]. He points out that the changein chemical potential with ion concentration is suffi-cient to drive water treeing. Fig. 11 shows the changein chemical potential (in electron volts) which accompa-nies the change in ion concentration from zero to thestated value for two small cavities connected by a smallchannel. Anything over 1 eV can drive a chemical reac-tion. Thus this mechanism has the potential to drivewater treeing over a wide range of concentration.However, the driving function drops at sufficientlyhigh and low concentrations. These computed data canbe taken as qualitative and suggestive; they are not de-finitive computations of the ‘‘driving force’’ for a realis-tic geometry. Recently published experimental data in-dicate that water trees consist of cavities connected by‘‘channels’’ of altered material [10].

Taken together, these results suggest that water tree-ing results from a localized chemical reaction whichchanges the polymer from hydrophobic to hydrophilic.Water and ions can travel along and condense intothese hydrophilic paths from cavity to cavity. The com-bination of water, ions, and (possibly) cavities results ina chemical reaction at the channel tip which convertsmaterial from hydrophobic to hydrophilic and propa-gates the water tree as a myriad of such channels. In-itially, the channels are probably not tunnels but ratherpaths of polymer in which the material has been reactedto change from hydrophobic to hydrophilic. Unfortu-nately, the channels tend to be from factions of a mi-crometre to a micrometre in diameter, so analysis of thematerial in a channel to determine the precise chemicalreactions therein poses a substantial challenge whichhas yet to be overcome.

AcknowledgementsThe experimental research described in this article

was carried out by Matthew Mashikian, JosephGroeger, and others at the Electrical Insulation Re-search Center (EIRC), University of Connecticut. Thearticle was written by Steven Boggs, who recentlyjoined EIRC.

Steven Boggs was graduated from Reed College andreceived his Ph.D. and M.B.A. degrees from the Univer-sity of Toronto. He conducted research for 12 yearswith Ontario Hydro, primarily in the areas of SF6 gas-insulated substations, solid dielectrics, and partial dis-charge detection. Prior to joining the Electrical Insula-tion Research Center as Associate Director, he was Di-rector of Engineering and Research at UndergroundSystems, Inc. (Armonk, NY). Steve was elected a Fel-low of the IEEE for his contributions to gas-insulatedsubstation technology.

Matthew Mashikian was graduated from the Ameri-can University of Beirut and received his Dr. of Engi-neering degree from the University of Detroit. Mattworked for ASEA from 1958 to 1962 as an applicationengineer for lightning arresters. From 1963 to 1979, hewas in the Engineering Research Department of DetroitEdison where he rose to the position of Supervisor ofElectrical Equipment and instrumentation. After earlyretirement from this position in 1979, he started his ownconsulting company, Mashikian & Associates, Inc.Since 1983, he has been Director of the Electrical Insula-tion Research Center at the University of Connecticut.Matt was elected a Fellow of the IEEE for his contribu-tions to the technology of solid dielectric power cables.Matt is presently Secretary of the PES Insulated Con-ductors Committee and Chairperson of the DEIS Edu-cation Committee.

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