water treeing in polyethylene cables

IEEE Transactions on Electrical Insulation Vol. 25 No. 5, October 1990 989

REVIEW

Water Treeing in Polyethylene Cables

E. F. S t e e n n i s

N. V. KEMA, Arnhem, The Netherlands

F. H. K r e u g e r

Technical University Delft, The Netherlands

ABSTRACT This review discusses water tree growth in polyethylene cable insulations. The characteristics of water trees, the effect of aging parameters on water tree growth and the possible mechanisms of growth are considered, emphasizing vented tree development in polyethylene insulating materials. Moreover, test methods and measures to reduce water treeing are discussed.

1. INTRODUCTION

1.1 HISTORY

H E introduction of extruded cables in the second half T of this century was accompanied by the idea that water or water vapor would not affect the electrical properties of the cable. Therefore, the cables were not provid- ed with water or water-vapor impervious outer sheaths, often a PVC (polyvinyl chloride) outer sheath over the ground screen was chosen, while a metal sheath was in most cases absent. Unlike cable practice in Japan and Europe, in the United States extensive lengths of extruded cables have been installed without any plastic outer sheath.

The discovery of degradation of polyethylene (PE) by the combined action of water and electric stress was published by Miyashita and presented a t the Electrical Insu- lation Conference in Boston 1969 [ lo l l . Soon this kind of degradation was called ‘water treeing’ [152]. A water tree is defined as a diffuse structure in a dielectric insulating material with an appearance resembling a bush or

a fan. It is generally accepted that water trees reduce the electric breakdown stress level of an insulating material.

Miyashita observed water trees in the stator windings of submersible motors of which the wires were coated with PE. Subsequent publications of Tabata et al. [152] and Vahlstrom [162] described the early experience with water trees in low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) insulated cables. After 5 yr of service experience, these cables showed failures that could be related to water treeing.

Continued publications of different authors describe many aspects of water treeing in polymeric insulating materials. Review-type publications are given in Table l. It was found for instance, that water treeing, or more or less similar tree creation, could occur in all polyolefins. There was much discussion about the water tree susceptibility of different types of insulating materials. Filled insulating materials such as ethylene propylene rubber (EPR) have the advantage of opacity, but these materials are suspected as well, since close examination showed water trees also in them.

0018-9307/90/1000-989$1.00 @ 1990 IEEE

990 Steennis et al.: Water Treeing

In 1984 Shaw et al. [136] estimated the total cable LIST OF SYMBOLS length harboring water trees to be about 300000 km.

A area m2

1.2 PE INSULATION

LDPE was often applied as an insulating material for extruded cables. Nowadays, most polyethylene extruded cables have XLPE insulating materials. Other types of PE are medium-density polyethylene (MDPE) and high- density polyethylene (HDPE) used for sheath production in modern cable technology.

Polyethylenes, (CHz-CHa), , are very long macromolecules. The CH2 groups are strongly joined by bonds of the shared electron valence type. The ends of the different chains contain methyl (-CH3) or vinyl (-CH=CH2) groups.

The mechanical properties of the different polyethylenes are mainly determined by the density of these materials. The density in turn is strongly related to the molecular length and the number and length of side chains per macro molecule. This is illustrated in Table 2.

PE is a thermoplastic: the upper operational temperature is limited to % 70°C. By crosslinking of the macromolecules the operational temperature is increased to %

90 'C. In XLPE the macro molecules are incorporated in a network in which the effective molecular weight has become infinite. A schematic representation of XLPE chains is presented in Figure 1.

Table 1. Review-type publications on water treeing.

year author number of references

1977 Wiersma 1978 Eichhorn 1979 Kiss 1980 Nunes et al. 1983 Dissado et al. 1984 Bernstein 1984 Shaw et al. 1989 Steennis 1990 Nicholls et al.

32 22 38 89 43 46 227 147 130

reference number in this paper

c concentration C, specific heat c' osmolarity D dielectric displacement E electric stress E, electric stress in the original unaffected

E, electric stress in the vented tree E.t axial electric stress in the t ip of the vented

tree EPt axial electric stress in the polyethylene

adjacent to the t ip of the vented tree E p d radial electric stress in the t ip of the

vented tree E,, Young's modulus

insulation

F,, force h k' thermal conductivity

parameter of length, length of vented tree

unit-normal vector pressure heat dissipation charge parameter of place, radius gas constant 8.31 time temperature

Tg glass transition temperature T, melting temperature To temperature of the original unaffected

insulation material T, temperature in the vented tree e strain

c p permittivity of the polyethylene

E.

q5 angle n relative permittivity

n. relative permittivity of the vented tree

p density U mechanical stress U, stress in radial direction ut stress in tangential direction ny yield strength U, ultimate strength c, II osmotic pressure

permittivity of the vented tree

path

electrical conductivity of the vented tree

v / m

Pa N m

J /Ksm

N/m2 W/m3

C m

J/Kmole

K K K K

S

k / m 3 Pa Pa Pa P a Pa

(nm)- ' N/m2

The polymer, either crosslinked or not, is semi-crystalline, which means that it is partly crystalline and partly amorphous. The crystalline part shows a folded regime of macromolecules organized into platelets with dimensions strongly depending on the production process. Usually

IEEE Transactions on Electrical Insulation Vol. 25 No. 5, October 1990 QS1

Table 2. Relation between density and number and length of side chains for LDPE and HDPE (After [l?]).

Polvethvlene LDPE HDPE ~ _ _ _

density [g/cm3] 0.91 -0.94 0.95-0.965

average molecule length 1500-3500 7000-1 4000

number of side chains 20-40 < 5 [/lo00 chain atoms] *

Platelets can be grouped to spherulites, varying in size from x 3 to x 700 pm. Indeed, etching procedures sometimes revealed a kind of superstructure [lo, 30,97,105, 110, 112,117,163], although the results are suspect since most of the etching procedures produce artifacts which often are misinterpreted [lo]. Moreover, long macromolecules and the existence of many branches, as in LDPE, reduce or even suppress the superstructure. Examples of complete suppression of superstructures in LDPE are given by Patsch et al. [120], Mandelkern et al. [88], Capaccio et al. [28] and Ross et al. [126]. This is illustrated in Fig- ure 3: an electron micrograph of LDPE cable insulation is given with a magnification factor of 40000. Crystalline

< 4 and amorphous regions have been revealed, superstructures have not been found.

length of side chains 2-5 * [number of atoms1

* Occasionally there is a side chain with an average molecule length

H H H H H

- c - c - c - c - c - H H H H H

H H H H H I I I I I

- c - c - c - c - c - I I I I I H H H H H

I I I I I

I I I I I

10

H

C I

c H

H H H H I I I I

- c - c - c - c - I l l 1 H H H H

H H H H I I I I

- c - c - c - c - I I I I H H H H

Figure 1. Molecular structure of XLPE, bond angles are not indicated.

nrn

Figure 2. The PE structure in the nanometer range (After [12]).

the thickness is x 10 nm, they are generally several p m wide and long. Between the platelets are interconnecting chains and chain ends which form part of the amorphous regions. The structure of the P E on this nm scale is shown in Figure 2.

The mechanical behavior of P E is mainly determined by the amorphous regions. In a thermoplastic such as PE, the intermolecular Van der Waals forces play an important role. Although Young’s modulus E,, is high, x 500 MPa a t room temperature, considerable chain flexibility or micro-Brownian motion exists. The actual mechanical behavior of P E as a function of temperature T depends on the values of the glass transition temperature Tq and the melting temperature T,. A schematic presentation is given in Figure 4. Above T,, LDPE will melt if it is not crosslinked, while XLPE behaves like a rubber-like substance.

On the basis of Figure 5, which shows a typical stress- strain curve for PE, some important mechanical parameters are discussed. For small stresses, uniaxially applied to the bulk polymer, the stress-strain relation shows a Hookean behavior with e = E,,€, where E,, is the Young’s modulus, U the applied stress and E the strain. The strain is here defined as the length of the elongation divided by the initial length.

In the linear region of the stress-strain curve instan- taneous recovery is possible. The molecule chains are partially uncoiled and coiled again. At the yield point slippage of the chains will result in incomplete recovery. The stress at this yield point is called the yield strength e,. Finally, fracture of the bulk polymer will be reached a t the ultimate strength U,,. Some typical data for LDPE and HDPE are given in Table 3.

As all polymers, P E is rather susceptible to effects of time. Important time effects are

1.creep: a t a constant stress the deformation or strain

2.’ relaxation: a t constant strain the required stress slowly slowly increases,

decreases,

go2 Steennis et al.: Water Treeing

Table 3. Typical tensile properties for PE.

polymer LDPE HDPE

density’ [g/cm3] ~0.92 ~ 0 . 9 5

Young’s modulus’ [MPa] 200-400 600-1500

elonaation to fracture2 1%1 400-700 100-600

crystallinity’ [%I = 55 =90

Yield strength’ [MPa] 10-20 25-50 Ultimate strength‘ [MPa] 15-25 25-55

’) according to [17] ’) according to (1291

3. internal friction: a t dynamic loading mechanical energy is converted into heat.

Time effects are of importance a t moderate or high stress levels. At lower stress levels these effects are hardly found and therefore complete recovery is possible a t lower stresses.

Stress cracking or brittle fracture is an important phenomenon that may lead to complete degradation of the polymer. It starts as a localized phenomenon a t a location where a critical stress is present. Stress cracking occurs in the brittle region at low temperatures and/or under alternating stresses during long times intervals.

Fracture is not found under repeated loading conditions as long as the imposed stress remains below a certain endurance limit. This limit is about 1/5 of the static ultimate strength.

A liquid which is capable of dissolving or swelling the Measures to reduce stress polymer promotes cracking.

cracking are e.g.

1. increasing the molecular weight (fewer chain ends and therefore fewer micro-cracks)

2. the use of copolymers (the mechanical properties are improved by the combined effects of the various poly-. mers).

1.2.1 CROSSLINKING

There are several methods of crosslinking. Often a dicumyl peroxide-curing agent is used. Here the dicumyl peroxide, added to the polymer compound, is activated immediately after extrusion in a special tube at high temperature and high pressure. By thermal energy the per- oxyde is split into two chemical free radicals. These free

Figure 3. Electron micrograph of LDPE cable insulation. Crystalline and amorphous regions are visible. Spherulites have not been found.

radicals produce acetophenone and methyl radicals. The methyl radicals in turn give methane and carbon radicals in the polymer chain by removing hydrogen in a limited number of places. It is also possible that the free radicals produce cumyl alcohol and radicals in the polymer chains directly. Finally, the carbon radicals of (different) chains combine to crosslinks [3]. In some production lines crosslinking occurs with the aid of steam a t temperatures between 200 and 220°C and a pressure of 1.6 to 2 MPa [120,157]. This process is called steam curing. Towards the end of the process the cable insulation enters the cooling section where a fairly rapid temperature reduction is achieved.

Today most cables are not cured with steam, but by using hot nitrogen. This suppresses the creation of microvoids considerably, as will be shown. Such a process is called dry-curing. Cooling is performed by using gas or water. Therefore, this process is subdivided into ‘dry- cured dry-cooled’ and ‘dry-cured wet-cooled’. The method of cooling has a minor influence on the creation of microvoids.

Two of the three major chemical residues of the dicumyl peroxide-curing process are cumyl alcohol and acetophenone. These products will eventually diffuse out of the insulation. The rate of diffusion depends on the temperature, the temperature gradient and barriers (e.g. sheaths).

An essentially different method of crosslinking is silane- curing. In this method, curing does not take place directly after extrusion, but in a separate production step. In the one-shot silane-curing process a silane compound is grafted onto the PE chains during the extrusion. Af- ter extrusion the cable is slowly cooled in a water-filled


n 1 0 r a v

k 8 -

Is, -

6 -

cooling trough. Curing takes place afterwards by putting the extruded cable on a reel in a water tank at 85'C. The immersion time for medium-voltage cables ranges between one and five days, depending on the insulation thickness. In this tank the silane groups are coupled chemically under the influence of the water vapor in the insulation. Methanol is also found as a residue. The amount of residual products is much smaller than the amount of these products in the peroxide-curing processes.

4

been measured for typical production processes and are given in Table 4 [3,4,57,68]. In this Table the effect of

I I the method of crosslinking on the presence of microvoids/ , T9

be collected in the amorphous regions of the polymer. If these substances are polar, clustering may take place in existing voids. Sometimes, for instance if supersaturation occurs, cavities will be created. Smaller voids and cavities have dimensions comparable to the dimensions of the interlamellar regions, in most of the XLPE insulating materials the largest voids and cavities have diameters of x 10 pm. In exceptional cases, however, much larger voids are found, with diameters of M 500 pm.

The creation of most of these microvoids is attributed to water vapor or other gases which, during the rapid cooling of the melt, are prevented from diffusing out of the insulation. As a result, supersaturation, in particular of water during steam-curing, is inevitable and microvoids, filled with gases and/or water, will be created. The ap- proximate sizes and densities of the cavities/voids have

. ' 0" ' - . ' the insulation during crosslinking, in combination with the cooling procedure afterwards. A schematic modulus/temperature curve for

LDPE and XLPE (after [1331).

40

HDPE 20

LDPE

Supersaturation and void creation will usually occur in the center region of the insulation. The inner and outer regions will cool down and solidify first, thereby suppressing further diffusion of gases out of the center region of the insulation. Such void concentrations are usually visible as halos in the insulation of steam-cured cables.

'"v oL 012 Oi4 0:s 018 1:O 1(2 1.14 '

strain 6 (m/rn)

Figure 5. Typical stress-strain curve for PE (After [77]).

1.2.2 VOIDS AND CAVITIES

As defined here, voids are filled with gases only, and cavities are filled with liquids and/or solid materials.

I t is generally assumed that during production, water, impurities and residual products from crosslinking will

1.2.3 WATER IN PE

Owing to its non-polarity, PE surfaces are hydrophobic until oxidized. Some water in the PE, however, cannot be excluded. Polar substances in the insulating material are capable of attracting water. I t is known [65,148] that XLPE at a temperature of 20'C absorbs < 100 ppm of water. The amount of water absorbed depends strongly on the temperature, as shown in Figure 6. As was stated in Section 1.2.2, supersaturation temporarily results in high amounts of water, mainly collected in already existing microvoids, or in newly produced microcavities. It WAS found in several cases that most of this water has diffused out of the insulation after several years of service aging [148].

P


Table 4. Concentration, size and volume of voids in PE as a function of the curing process.

L

4 W 0 3

102:

void concentration maximum void size void volume/ curing method PE volume

number/mm3 Pm %

stearn* 1 05-1 o6 30 0.7-7 10~4.1 o4 15 0 .OO 7-0.4

~ 0 . 0 7 silane (one-shot)* = io4 15 uncured** = io5 13 =0.005

*

**

dry*

After [19], as observed in 10 cm3 insulation volume chosen at random, the void volume-PE volume ratio is based on a void diameter of 5 pm. After [57], the void volume-PE volume is based on a void diameter of 1 pm.

e e

e e

e

e e

e

e

1 0 " ' ' ' ' 50 I ' ' ' ' 100 I I I I ' 150

temperature ( "C )

Figure 6. Saturated water content (in ppm) for XLPE as a function of temperature (in "C) (After [65])

2. MORPHOLOGY OF WATER TREES

Figure 7. Vented tree initiated from a void in the semiconducting screen; the void and void content is shown in Figure 0 .

considered as dangerous, since these trees can initiate insulation failures.

Trees can be made permanently visible using different dyes. A well known and generally accepted dyeing procedure is given by Larsen [86] and Shaw and Shaw [136].

2.1 INTRODUCTION 2.2 SHAPE

ATER trees are diffuse structures in polymer insu- W lating materials resembling a bush or a fan, growing in many different kinds of polymers under the action of water (vapor) and an electric field. Water trees weaken the insulation and especially large water trees can be

A distinction must be made between two different types of.water trees [24,148]. These types are the 'bow-tie tree' and the 'vented tree'. This distinction is based on the location where these trees start growing: vented trees

IEEE Transactions on Electrical Insulation Vol. 25 N o . 5 , October 1990 995

Figure 8. Electron micrograph of the void in Figure 7, crystallization of various species.

Figure 9. Typical vented tree grown from the semiconducting inner screen into the insulation of a 10 kV XLPE cable.

are initiated a t the insulation surfaces, bow-tie trees are initiated in the insulation volume. Such a distinction is important since both types show a completely different growth behavior.

This review is focused on vented trees because these trees are more dangerous under service aging conditions than bow-tie trees.

For several reasons the study of vented trees is more difficult than that of bow-tie trees. The concentration of vented trees is often low compared to the concentration of bow-tie trees, and at the beginning of growth,

the propagation rate of vented trees is lower than that of bow-tie trees (in a later stage of growth the opposite is true). Consequently, the study of vented trees is more time consuming than the study of bow-tie trees. For these reasons many publications contain information on bow- tie trees only. In some other cases a distinction between both types of water trees has not even been made: only the description ‘water tree’ has been used. In this review the generic description ‘water tree’ will be used only if bow-tie trees as well as vented trees are under consideration.

Figure 10. Vented tree grown from the graphite outer screen into the insulation of a 10 kV XLPE cable.

2.3 VENTED TREES

The vented tree is defined as growing from the insulating material boundaries to the other side of the insulation, predominantly in the direction of the electric stress.

The origin of vented tree initiation in many cases is difficult to find. However, it is sometimes mechanical damage to the cable insulation. Scratching the insulation for instance may initiate treeing. Another origin of vented tree initiation can be an irregularity in the semiconducting screen where it is in contact with the insulation.

QQ6 Steennis et al.: Water Treeing

In an insulating material which is fairly water tree susceptible, vented trees can reach the other side of the insulation (in 10 kV cables = 4 mm thick) in about 7 yr.

I 1 v fr a

00 pm

Figure 11. ‘ented trees, bent near the insulati ice. These trees were found in the service aged steam-cured 10 kV X

on outer sur- insulation of LPE cable.

An example of a vented tree, initiated a t the boundary area of a void, located in the semiconducting inner screen against the insulation surface, is shown in Figures 7 and 8 [148]. Crystallization of various species (containing Si, S and Ca) did occur in the void. It is assumed that these species were dissolved in the water during aging. The vented tree was grown in a 10 kV XLPE cable insulation which has been aged under service conditions for 8 yr. Muller [106], among others, found that in most cases vented trees at the conductor side in the insulation of full-scale steamcured cables were initiated a t inhomo- geneities in the inner semiconducting layer. For the initiation these examples show that apart from water and electric stress also chemical species in the water (or semiconducting screens) and/or electric stress enhancements may play an important role.

When aged under moderate service conditions (< 4 kV/mm), a vented tree grown from the outside of the insulation is sometimes pencil-like. Trees grown from the inside have branches that spread a little further, although each distinct branch of a large vented trees is also pencil- like. Typical vented trees are presented in Figures 9 and 10. In a few cases, vented trees, bent near the insulation surface, have been observed by the authors. An example is shown in Figure 11.

It has been found that this type of water tree has been responsible for many cable failures. In Figure 12 a vented tree is presented; here also part of a breakdown channel is visible.

Figure 12. Vented tree grown from the semiconducting inner screen into the insulation. A part of the small breakdown channel is visible. marked 1.

2.4 BOW-TIE TREES

The other type of water tree is the bow-tie tree. Bow- tie trees are defined as initiating in the insulation volume. These trees grow in opposite directions, along the electric field lines. Exceptions can be found, in some cases the growth direction has been bent: Karakelle et al. [71] assumed that this is a consequence of frozen-in mechanical stresses. However, Karakelle could also show that this typical growth direction was apparently not affected by the flow pattern that existed in the melt a t the time crosslinking occurred.

The initiation spots are often clearly visible using normal optical microscopy. An example is given in Figure 14. It is generally assumed, and it has been proven in some cases, that these spots contain impurities [137]. Normal- ly the growth of bow-tie trees is strongly reduced after a certain time. The total length is restricted and therefore this kind of water tree is seldom the origin of cable breakdown. There are indications that the length of bow-tie

997 IEEE Transactions on Electrical Insulation Vol. 25 No. 5 , October 1990

Figure 13. Typical bow-tie trees grown in the insulation of a 10 kV XLPE cable.

trees is related to the size of the location containing the impurities.

The transformation of a bow-tie tree, formed close to the insulation surface, into a vented tree was once reported by Naybour [lll]. A collection of typical bow-tie trees in the XLPE insulation of a 10 kV cable is given in Fig- ure 13. A further enlargement of one of these bow-tie trees is presented in Figure 14.

2.5 VOIDS AND CHANNELS IN WATER TREES

Apart from the already existing voids or cavities in the PE, extra void/cavity creation occurs occasionally in a water tree. Collections of such voids are found sometimes in the trunk of vented trees. Near the t ip of the vented tree such microvoids are rarely found [28]. If vented trees have been grown under rather extreme aging conditions, for instance by applying high electric stresses of - 30 kV/mm, some of these microvoids may become in- terconnected by micro channels [28]. In a gas permeation experiment, Cross et al. [32] showed that the diameter of

Figure 14. Bow-tie tree, total length 200 pm. This bow-tie tree was initiated from one of the impurities in the insulation.

the supposed channels, penetrating throughout the insulation, must be < 1 pm. Microscopic investigations of the t ip of a water tree did not even reveal open channels in the nm range [28].

2.6 WATER CONTENT OF WATER TREES

Water trees contain water. If this water is evaporated e.g. by heating, i t is known that the tree will absorb water again if the insulation is exposed to water or water vapor aft erwards.

Meyer [99] measured the amount of water in vented trees grown from water needles. The electric stress near the water needle t ip was very high, a t least 60 kV/mm.


Meyer found that near the tip of the water needles, the vented tree contained 10% water (of the vented tree volume). He assumed that the water had been collected primarily in the microvoids. Such voids are expected to be the result of the high initiation stresses applied. Mey- er also found that a t a certain distance from the water needle, where the electric stress is much more moderate, the amount of water was about 1 t o 2% (of the vented tree volume).

Apart from clustering of water in voids, some water is probably molecularly dispersed in other parts of the tree. Water molecules can be found a t any place where polar groups are attached to P E [125,126,127].

= !? loor * 1

.- 5 80

.- Y 701

f

* 2

0

- -* - 2 10

0 !@ I I I

0 ~~~~

3000 6000 12000 24000 t ime (hours)

= vented tree - = bow-tie tree

Figure 15. Maximum length of vented trees and bow-tie trees for five different cables as a function of the aging time.

2.7 TYPICAL GROWTH BEHAVIOR OF WATER TREES

Many publications show that vented trees and bow-tie trees have a completely different growth behavior. This is illustrated in [147]. Five different medium-voltage cables were aged for 24000 h in a water tank. The cables varied in construction and in method of crosslinking. The electric stress level a t the outside of the insulation was 3.9 kV/mm at power frequency. The tap water in the water tank was kept a t 30°C. The water contained small amounts of NaCl (0.2 kg/m3) and HC1 (pH = 6). The water was admitted under the sheath only. In Figure 15 the largest vented tree and the largest bow-tie tree as observed in the samples are given for each cable as a function

of the aging time. The largest trees observed are presented and not the mean length of the trees, since the largest trees correspond to electrical degradation of the insulation. After 11300 h, breakdowns did occur in cable No. 1 probably as a consequence of the large vented trees. This cable was withdrawn from further aging.

This accelerated aging test shows the typical behavior of water tree growth. After an initial rapid growth within the first 3000 h, the length of bow-tie trees does not seem to increase any more. The length of vented trees increases continuously for most cable insulating materials. After approximately 12000 h of aging, the lengths of both types of trees correspond well to the general picture of tree lengths in the insulation of service-aged cables after 6 to 13 yr.

3. CHARACTERISTICS OF WATER TREES

3.1 T H E VENTED TREE: AN I N S U LATl N G M A T ER I A L

EFORE starting the discussion of dielectrical proper- B ties, definitions should be given of ‘vented tree’ and ‘vented tree path’. The ‘vented tree’ represents the total region within which the tree can be observed (pm to mm scale), using the dyeing procedure described in [86,136].

The ‘vented tree path’ or ‘path’ relates t o the attacked P E only (the nm scale). The ratio of the volume of the vented tree paths to the vented tree certainly is much smaller than 1, but upper and lower limits cannot be estimated in general. There are several indications that both the vented tree and the vented tree paths can be considered as an insulating material.

Koo et al. [81] and Cross et al. [32] studied the dielectric properties of a vented tree in a water needle experiment. The change in capacitance between the water needle and the opposite electrode was measured during the growth of the vented tree a t a frequency of 1500 Hz. The capacitance variations were derived from a voltage change in a resistor placed in series with the electrode. Afterwards, in a model, the vented tree was replaced by metal or by dielectrics with different permittivities. The resulting capacitance variations showed that the dielectric behavior of a real vented tree differs strongly from that of a conductor. Moreover, it was found that comparable voltage variations in this model could be obtained if the water tree was replaced by an actual material with a relative permittivity K. M 6. To explain the measured permittivity, Cross assumed that water was collected in the many

IEEE Transactions on Electrical Insulation Vol. 25 No. 5 , October 1990 909

microvoids distributed over the vented tree volume. Cross pointed out that in such a situation the observed increase in the permittivity of the vented tree volume might be explained by using the multiphase dielectric mixture theory (e.g. Tinga et al. [159]).

Boggs et al. [18] studied the micro-movement of vented trees in a water needle experiment. The trees were not grown in PE but in silicone rubber under the application of an electric stress a t different frequencies. Tree movement has been observed by applying interferomet- ric holography. Boggs found that the movement of the trees was related to E' and therefore probably related to Maxwell stresses. He also concluded that certain out-of- phase movements of the tree in relation to the movement of the needle tip would provide evidence that the vented tree is not conductive.

Recently Ross et al. [126] measured the dielectric properties of a vented tree which had been grown under normal aging conditions in a full-scale XLPE insulated cable. The trees were saturated with water preceding to the measurements. They found that the vented tree has the properties of an insulating material with a permittivity IE = 2.26 and a loss-factor of about ~ O X ~ O - ~ at 50 Hz. There are other observations confirming that a vented tree is an insulating material

"4 axis of symmetry 4

I f y-axis

0 1 - Figure 16.

Electric stresses near the tip of a vented tree path.

3.1.1 B R E A K D O W N CHARACTERISTICS

If a vented tree path were a conductor, then such a path, grown to the other side of the insulation, would initiate thermal breakdown or initiate an electrical tree followed by breakdown. However, it was found that in most cases such long vented trees do not cause breakdown a t stresses of 2 kV/mm or even higher (see Section 4.1 and Figure 17).

In an experiment by Densley [33] electrical trees were I t was initiated in vented trees by inserting a needle.

5

0 0 10 20 30 40 50 60 70 80 90 100

tree size (% of insulation thickness)

- mean value -- 95 % confidence bound

Figure 17 Relation between the breakdown stress level and the water tree size.

found that the vented trees did not provide a more conve- nient path to the electrical trees or the breakdown channels than unaffected PE.

3.1.2 D I R E C T I O N OF V E N T E D T R E E P RO PAG A T I O N

Vented trees and the related paths grow in the direction of the electric field lines. High radial electric stresses a t the tip of the vented tree path would, however, cause the vented tree to fan out. This can be observed a t the tip of a water needle, where the direction of growth near the water electrode is perpendicular to the water needle surface. One of the many examples of this vented tree growth behavior a t the needle tip is given by Filippini et al. [46].

In Steennis [148] various electric stresses near the tree tip are calculated for different vented tree tip radii. The electric stresses were calculated for a range of permittivities and conductivities of the vented tree path. In Fig- ure 16 the assumed electric stresses and the vented tree path configuration are given. High radial electric stresses of 30% of the axial electric stress a t the tip of the vented tree path (Epd/Ept = 0.3) can be expected for vented tree paths with IC, 2 20, or ~ 2 . 5 ~ lo-* (am)-'. In such a situation it is expected that the tree would fan out. It is also shown that for IC, < 10 and cv < 1.3 x lo-' (Gm)-' this ratio Epd/Ept is small. In that case vented tree growth can be expected mainly in the direction of the axial electric stress a t the tree tip. This conforms


with the phenomenology where it is found that the direction of propagation of vented trees is mainly determined by the local electric field lines of the original unaffected PE. For moderate permittivities and conductivities, the electric stresses become independent of the length of the vented tree path: the influence is confined to a local area.

3.2 PHYSICAL/CHEMICAL PROPERTIES (LOCAL)

3.2.1 INTRODUCTION

This Section discusses chemical and physical observations inside a water tree compared to observations outside the tree. Studies are mainly carried out on vented trees from needle tests, from scratched insulation slabs or from full-scale cables.

3.2.2 INFRARED

Infrared (IR) measurements have been carried out by several authors. Bernstein et al. [15] performed measurements on model cables aged for 60 days at 4.6 kHz. After drying the samples, metal ions from the solution were detected a t 1130 cm-'.

Garton et al. [55] tried to find evidence for oxidation in a water tree with FTIR. The insulation of model cable was studied, aged a t a moderate electric stress level a t 1 kHz for 320 h a t 70°C. Garton was not able to find a typical oxidation product such as carbonyl. However, he found absorptions at 600, 1100, 1160, 3550 and 3600 cm-', assigned to ether and alcohol groups.

Abdolall et al. [l] were able to distinguish differences in the IR spectrum between affected (in water tree) and unaffected P E (outside water tree) in a range between 20 and 350 cm-' at a temperature of 4.2 K. Samples were taken from a full-scale XLPE insulated cable. The differences observed were attributed to various possible effects such as hydrogen bonding, strain or inhomogeneous broadening, breakage of polymer chains, or effects due to crosslinking residual products.

Differences in the FTIR spectrum in a range between 500 and 2000 cm-' have been found by Yoshimitsu et al. [173]. These investigators studied XLPE material with an imbedded copper wire aged for up to 10 days a t 1 kHz. They found that in the tree-affected regions the CH2 groups lose their absorbance, but chemical species such as hydroxyl groups and carbonyl groups gain strength compared with the undegraded area.

Bamji et al. [ll] also examined regions with and without water trees using FTIR. Different kinds of test specimen were studied. Absorption was found a t 1160 (and 600) cm-l. Bamji did not attribute these absorptions to ether groups [55] but to sulfate anions. The absorptions a t 1600 cm-' are consistent with the presence of carboxylate anions. Also in this particular experiment the absence 6f carbonyl absorption is noticeable.

In 1987, a publication from Garton et al. [56] described the observation of traces of oxidation. Trees have been taken from service-aged steam-cured XLPE insulated cables. The difference in the carbonyl concentrations a t 1720 cm-' was E 15% of the overall level of oxidation in the insulation. The results have been confirmed by oxidative stability tests where it was found that the tree- affected areas were much less stable than the areas not affected by water trees.

Ross et al. [125] were also able to observe traces of oxidation in the vented trees of an accelerated-aged XLPE cable insulation. The cable was aged for 24000 h, the mean electric stress applied a t 50 HZ during aging was 4 kV/mm. Differences between the material inside and outside vented trees were found a t 1150, 1710 and 1720 cm-' and assigned to hydroxyl groups and carbonyl groups, re- spec tively.

Recently, Ross et al. [127] emphasized that carboxylate groups, which have an absorption a t 1570 cm-', were found in the vented trees of full-scale service-aged XLPE cable insulations. These carboxylate groups can be regarded as end groups of the polymer interacting with water. Such an interaction was also established in the 1000 to 1200 cm-' range, probably as a consequence of ionic sulfur-oxygen groups.

3.2.3 ENERGY DISPERSIVE SPECTROMETRY

Ross [127] observed with energy dispersive spectrometry sulfur and in some cases also silicon in vented trees grown in full-scale service-aged cables.

3.2.4 ELECTRON SPIN RESONANCE

Electron spin resonance was applied by Dorlanne et al. [38] and Crichton et al. [30]. They studied LDPE samples from water needle experiments as well as XLPE samples from full-scale cables. All samples were aged for up to 90 days a t frequency levels of up to 4 kHz. Both investigators detected metal ions from the solution in the water trees, even beyond the visible part of the water tree [30].


3.2.5 DIFFERENTIAL SCANNING CALORIMETRY

trees themselves had been completely dissolved. This experiment shows that the chemical changes assumed above do not result in crosslinking of the tree-affected material.

Differential scanning calorimetry was applied by Bamji et al. [ll]. Trees have been studied in samples taken from service-aged XLPE insulated cable. It was not possible to find a difference in melting endotherms between regions containing water trees and regions without these trees. This indicates that heating during or after water tree growth is of minor importance.

3.2.6 X-RAY ANALYZES

Sletbak et al. [137] detected metal ions and sulfur in the branches of strongly colored bow-tie trees. These ele-

3.2.10 STAINING

Abdolall et al. [l] colored (most probably) vented trees from a full-scale XLPE insulated cable after drying of the insulation for 12 h a t 95'C. For the staining experiment many different solvents were used. The revisibility results show that, with a few exceptions, only solvents with an OH-group a t the end of a molecule made the trees strongly visible again after a few days.

4. CHARACTERISTICS OF THE TREE INFESTED CABLE

DIELECTRIC

ments could also be detected as a fraction of the impurities located a t the initiation site. The bow-tie trees were grown in a XLPE slab for up to 790 h a t a frequency of 50 Hz. Bamji et al. [ll] detected metal ions in a vented tree using X-ray techniques. The vented trees were taken from a full-scale XLPE insulated cable, aged under service conditions.

3.2.7 dc ARC

dc Arc measurements, performed by Bamji et.al. [ll], again show metal ions in vented trees. Moreover, Garton et al. [56] clearly demonstrated the existence of apprecia- ble amounts of sodium, calcium, aluminum and silicon in the tree-affected regions of the insulation of service-aged XLPE insulated cables.

3.2.8 OXYGEN PLASMA ETCHING

4.1 ELECTRICAL PROPERTIES

N this Section the electrical properties of the bulk insu- I lation such as breakdown stress level, loss-factor, resistivity and partial discharges, will be discussed. Studies in this field have mainly been performed on LDPE or XLPE insulating materials which have been aged under service conditions in most cases for a few years. In general, water trees (bow-tie trees as well as vented trees) cause a reduction of the 50 HI, 0.1 Hz, dc or impulse breakdown stress level. This has been observed by many workers [52, 58,60,69,78,79,95,143,144,146,147,155]. In the studies by Franke et al. [52] and Matsuura [95] et al. (1987) indications are given of a relation between the size of the water tree and the 50 Hz breakdown stress level.

A reduced oxidative stability of water trees in relation to the unaffected surrounding P E was found by Ross [127].

In 1986 this relation was demonstrated [146]. More than one hundred medium-voltage cable pieces have been subjected to a 50 He breakdown test. Special techniques

3.2.9 HEAT-TREATMENT

Muller et al. [lo71 carried out some unusual experiments with LDPE slices in which vented trees were present. In an initial experiment slices were heated for 160 h a t 135°C and 20 h a t 190°C. In both cases it was found that the optical structure of the water tree was unchanged. Even after recrystallization of the material by cooling the melt, the structure was not changed. Chemical changes in a water tree are assumed, stabilizing the structure. In a second experiment untreated slices were dissolved in xy- lol. It appeared that not only the P E but also the water

allowed the energy dissipated in the breakdown channels to be reduced. In this way the cause of breakdown could be established for several cases; examples are given in Figures 12, 18, 19. When a vented or bow-tie tree was found, its size was measured. The relation between water tree size and the corresponding breakdown stress level is presented in Figure 17. The breakdown stress level is defined as the arithmetic mean value of the electric stresses during breakdown in the inner and the outer region of the insulation. The main conclusion is that vented trees as well as bow-tie trees weaken the insulation. Moreover, it 'is shown that there is a clear relation between the size of the water trees and the electric breakdown stress level. Finally, it appears that water trees crossing the entire


Figure 18. Vented tree (marked 1) grown from the inside of cable insulation. During the breakdown test not only a breakdown channel (marked 3) was created, but an electrical bow-tie tree (marked 2) was also initiated in the t ip of the water tree.

Figure 19. Vented tree (marked 1) grown from the inside of cable insulation. Again, a breakdown channel (marked 3) was created during the test , but also an electrical bow-tie tree (marked 2) was initiated in the t ip of the water tree .

insulation do not cause immediate breakdown under st

vice conditions. Such insulating materials often still ha a breakdown stress level above the service stress level PZ 2 kV/mm.

The breakdown stress of cable insulation can be re- stored by drying the insulation: a breakdown stress of up to a t least 50% of the original level can be obtained [76]. However, water trees do not actually disappear. As soon as water or water vapor is present, this water will be absorbed and consequently the breakdown stress level reduces again:

4.2 LOSS FACTOR (BULK I N S U LATlO N )

Several investigators reported an increase in the loss factor of cable insulation containing water trees: Bahder et al. [7], Tanaka et al. [155], Franke et al. [52], Karner et al. [72] and Fukagawa et al. [54]). The study of Fukagawa showed this increase to be related to insulating materials already having a rather low breakdown stress level; it is possible therefore that these particular insulating materials may contain rather large water trees.

Other investigators also tried to find a relation between a change in the loss factor of the bulk insulation and the presence of water trees. The loss factor has been studied for power frequencies (Kirkland et al. [79], Naybour [112], Srinivas [144]) and frequencies between 0.01 and 10 Hz (Swingler et al. [151]). In these publications it is reported that such a relation was not found. Naybour tried to explain these different findings. He found that poor conductivity of the conducting screens of the cables involved can also be an explanation for an increase of the loss factor.

4.3 RESISTIVITY (BULK I N S U L AT I 0 N )

Water trees do not affect the resistivity of the insulation of a cable as has been reported by Kirkland et al. [79] and Karner et al. [72].

In contrast, a reduction of resistivity was observed by Bahder et al. [7] and Wojtas [168]. In Wojtas’s study the insulation was aged with a dc voltage, making the results suspect. As far as very large water trees are concerned, a reduction of the insulation resistance was found by Taba- ta et al. [153], Tanaka et al. [155], Srinivas et al. [143] and probably also Fukagawa et al. [54]. These results can explain the observed increase of the loss factor described above.

:r- .ve of

IEEE Transactions on Electrical Insulation Vol. 25 No . 5 , October 1900 1003

4.4 PARTIAL DISCHARGES AND ELECTRO-LUMINESCENCE

The growth of water trees is normally not attended by detectable partial discharges. This was found by Bahder et al. [7], Kirkland et al. [ 791, Bamji et al. [9, 111 and Steennis [148] a t a noise level of approximately 0.1 pC.

Optical experiments were performed by Nitta [114] and Bamji et al. [ll]. In water needle experiments both investigators tried to find light emission in front of growing vented trees. Nitta was able to observe the emission of light. However, the light phenomena observed in this needle experiment in all probability originated from mechanisms which are not related to vented tree growth under moderate aging conditions. This is illustrated by Bamji in a similar experiment. The electric stress applied a t the needle tip in the experiment carried out by Bamji was much lower than in Nitta’s experiment. Bamji did not succeed in observing any light emission during tree growth, not even with the aid of a photomultiplier and lens system. Moreover, he was unable to detect partial discharges a t a detection level of 0.05 pC. The experiment performed by Bamji indicates that neither low magnitude partial discharges nor electro-luminescence (emission of light by phosphorescent substances) occurs during the growth of vented trees.

However, under certain circumstances the water tree can initiate an electrical tree, for instance when there are overvoltages. After initiation of electrical treeing, breakdown of the insulation cannot be excluded. Such a mechanism has been described by Tabata et al. [152] and Gronefeld et al. [60].

Steennis [148] showed electrical bow-tie trees grown from the branches of a vented water tree. Such electrical trees were initiated during breakdown voltage tests a t electric stresses much higher than those applied during service conditions. An example is shown in Figures 18 and 19. It is assumed that the cause of initiation of these electrical trees is perhaps the site where a vented tree path crosses a field-disturbing inclusion, such as an impurity or a void. This is based on the following:

1. the electric stress in the vented tree path is of the same order of magnitude as in the unaffected surrounding P E (Section 3.1),

2. there is always a great number of inclusions in the P E and consequently also in the vented tree path. En- hancement of the electric stress can be expected a t many of these inclusions,

Figure 20. Electrical bow-tie tree (marked 3) initiation in the branch of a vented tree (marked 1). The inclusion (marked 2) from which the electrical tree was initiated is indicated.

3. a vented tree path is a ‘poor’ insulating material, the breakdown stress level is lower than that of the surrounding material.

Figure 20 shows an enlargement of the initiation sites of electrical trees, grown from the branches of a vented tree. It is shown that a large amount of impurities is present. The photograph indicates that the electrical trees were initiated a t such inclusions.

5. EFFECT OF OPERATING VA RI A B LES

5.1 EFFECT O F ELECTRIC STRESS IN T ENSlTY

5.1.1 VENTED TREES

The development of vented trees is clearly affected by the electric field strength. An increase of the electric stress intensity leads to an increase of the length of vented’ trees according to Bernstein et al. [15], Yoshimura et al. [174], Srinivas et al. [143], Filippini et al. [45], Hossam


;= 80- E 0 M

70- L m 0 4

~ 6 0 - 0 -0

Eldin et al. [64], Naybour [112] and Muller [106]). An increase of the concentration of vented trees was observed

related to the frequency of the electric stress. To study this subject, water needles were used by Yoshimura et al. [174], Favrid et al. [41], Filippini et al. [44], Pays et al. [123]. Yoshimura found a linear relation between growth and frequency in a range of 200 Hz to 3 kHz. The total test time in this case was 5 h, which is rather short. Longer tests were performed by Favrie et al. [41] and Filippini et al. [44]. These tests show an optimum tree growth between 4 and 8 kHz.

120 days

5.1.2 BOW-TIE TREES

One can measure the time needed for a vented tree to reach a certain length if the electric stress is dou- bled. Most of the studies show that the reduction in time is approximately a factor 2 (Filippini, Hossam El- din, Yoshimura and Muller). These studies all deal with water needle experiments. Bernstein [ 151 and Srinivas [143] tested cables with scratched insulation surfaces a t moderate electric stresses. Here, a reduction factor between 2 and 7 was found. As in these cases only a few

tric stress intensity. Enhancement of the electric stress was found to increase the rate of propagation in experiments carried out by Bulinski et al. [26] and Yoshimura et al. [175]. The opposite effect was observed by Sletbak et al. [137].

5.2 EFFECT OF FREQUENCY

5.2.1 VENTED TREES GROWN UNDER ac STRESS CONDITIONS

From these data it can be concluded that vented tree growth increases a t higher frequencies. However, above 4 to 8 kHz the propagation rate appears t o decrease. As a result of the different growth conditions during these tests it is hardly possible to make an accurate predic- tion of the propagation rate a t a certain frequency level. Figure 22 therefore only gives a rough picture of the relation between the propagation rate of vented trees and frequency.

I I I I 50 85 125 170

applied voltage a t 70 " C (V/mil)

Figure 21. Effect of the electric stress on the vented tree propagation rate (After [15]). Note: 1 mil = 25.4 pxn and 100 V/mil = 3.94 kV/mm 5.2.2 BOW-TIE TREES GROWN UNDER ac

STRESS CONDITIONS

Figure 21 illustrates the relation between the electric stress applied and the time required for a vented tree to grow to a certain length [15].

Bow-tie tree growth in different types of P E was studied by Bahder et a1 [8], Sletbak et al. [137] and Yoshimura


1 10’ i o 2 i o 3 i o 4 i o 5 frequency (Hz)

Figure 22. Graph showing roughly the relation between the vented tree propagation rate and the frequency.

et al. [175]. Bahder used model cables and Sletbak full- scale cables; in the experiment carried out by Yoshimura a blunt needle has been applied. The electric stress in all experiments was moderate. At higher frequencies accel- eration of initiation and growth was found. An optimum frequency was not given.

5.2 .3 WATER TREES GROWN UNDER SIMULTANEOUS APPLICATION OF ac AND

dc STRESS

Pays et al. [123] studied the tree propagation rate in a frequency range of approximately 20 Hz to 10 kHz. In this experiment water treeing was studied under ac stress as well as under simultaneous ac and dc stress conditions. The results of ac plus dc stress were the same as if only ac stress had been applied. The type of water trees found in these experiments has not been described. However, in general these results confirm the results presented by other investigators: in this frequency range the propagation rate increases with increasing frequency. Different types of P E were studied. The aging time of this experiment was not given.

5.2 .4 WATER TREES GROWN UNDER dc STRESS CONDITIONS

If the results described above for moderate electric stresses are extrapolated to very low frequencies, hardly any tree growth is to be expected. This is confirmed in the literature. Water tree growth a t dc voltage has been studied by Franke et al. [51], Yamada et al. [170] and

Pays et al. [123]. The results were obtained from tests on model LDPE insulated cables [51] or full-scale XLPE insulated cables [170] under the application of high electric stresses. Pays et al. [123] studied trees in LDPE plates and vented trees in a needle experiment. The electric stresses varied from 10 to 100 kV/mm for the cable and plate experiments, while in the needle test the electric stresses were much higher.

The shape of water trees grown under dc stress conditions differs from that of common water trees grown under ac stress conditions. It was reported that water trees grown under the application of a dc stress have a remarkably narrow structure, while vented trees as well as bow-tie trees only grow in one direction. Consequently the bow-tie trees have only one plume. Franke observed that during the growth of the water trees under the application of a dc stress the negative copper electrode became black. It is assumed that this is an indication of the presence of an electrochemical process. Pays did not find any tree growth, with one exception: in the plate experiment once a tree-like structure was observed.

There are reasons to suspect results of tests with dc stresses: Very high electric stresses have been applied in order to initiate and grow these trees, and the shape of the trees differs from that of water trees grown under ac conditions. Perhaps there are different mechanisms of (initiation and) growth for trees grown under the application of high dc stress and those trees grown under the application of ac stress.

5.3 EFFECT OF TEMPERATURE

The effect of the temperature on water tree growth can be considered as rather complex. It is thought to depend on the temperature level, temperature gradient, temperature load pattern, water diffusion coefficients and moisture absorption of the insulation as well as the other cable components. In one of his experiments, Muller [lo61 found that as a consequence of a high conductor temperature the relative humidity level of the insulation at the conductor side was low. Moreover, it is well known that cycling the temperature may lead to oversaturation of water in certain regions of the cable insulation.

5.3.1 VENTED TREEING AT CONSTANT TE M PE RATU RE

Several publications discuss vented tree growth at constant temperature levels, without a temperature gradient over the insulation. Most of the tests were carried out

1 006 Steennis et al.: Water Treeing

in water baths on LDPE or XLPE insulation. The test models applied are needle samples by Fourni6 et al. [50], sandwich models with inserted wire by Yoshimitsu et al. [172,173] or more or less full-scale cables by Tabata et al. [153] and Srinivas et al. [143]. From these studies it can be concluded that the concentration of vented trees increases and the length of vented trees decreases if the temperature of the insulation is 2 50°C. Between 20 and 50°C there are conflicting views. Excluding the results of the needle tests [50], the most favorable temperature range for water tree growth to be 30 to 50”c [143, 1531.

Muller was not able to find a statistically justified difference in vented tree growth for steam-cured cables after aging with cycled temperatures, compared to the same cables after aging a t a constant temperature level. With respect to the concentration of these trees, Bulinski noted that temperature cycles reduced it strongly.

5.3.3 BOW-TIE TREES

The development of bow-tie trees has been studied by Naybour [lll], Sletbak et al. [139], Bulinski et al. [27], Fredrich et al. [53] and Marsh et al. [89]. Fredrich showed that bow-tie tree growth is enhanced at higher temperatures. In his experiment the temperature of the water

5.3.2 VENTED TREEING AT CYCLING TEMP E RATU RE

Sletbak et al. [139], Bulinski et al. [27], Marsh et al. [89] and Muller [lo61 studied the effect of cycling the temperature of the conductor and the outer screen. In most cases there was a temperature gradient over the insulation during the experiment. Sletbak, Marsh and Muller used full-scale cable and Bulinski model cables. The duration of the different tests was < 1000 h. The outlines of the different experiments are summarized in Table 5.

Table 5. Cycling temperature and moisture conditions regarding the study of vented treeing.

outside the insulation of the full-scale cable was fixed at 40 or 90°C or cycled from 5 to 9O’C. The conductor contained water, but was unheated. In this particular study the water at high temperatures was oxygen enriched, but it was stated that this could have hardly any influence on the tests; the amount of oxygen in the water a t low temperature would have been even higher. Aging was carried out a t moderate stress levels for < 1000 h.

Naybour observed an increase of the concentration of bow-tie trees if the temperature of the water bath was increased from 35 to 80’C. The other aging parameters were chosen fairly moderate. Tests have been carried out on full-scale dry-cured cables for - 2000 h.

temperature temperature The experiments by Marsh, Bulinski, Sletbak and Miil- ler already have been described in Section 5.3.2. Test re-

(dry or wet) (wet) sults match the findings of Fredrich and Naybour. Slet- bak assumed that the increased bow-tie tree growth dur-

90 55 [I391 ing cycling of the temperature is a result of supersaturation of water in the insulation during the unloaded part

conductor [“C] outer Screen [“cl Reference

55-90 55 20-90 20-55 of the temperature cycles applied. 90 (wet) 55

55-90 (wet) 55 5.4 EFFECT OF MECHANICAL STRESS

90 50 ~ 9 1 20-90 20-50 20 20

20-70 20-70 ~ 7 1 20 20

70-80 20-30 I1 061

In general, one can conclude from these experiments that the growth of vented trees from the outer screen is hardly affected by these variations in test conditions.

A mechanical stress can be imposed on an insulation. It can also be present in the insulation after cooling from the extruded melt. Tanaka et al. [154] studied the relation between internal mechanical stresses and water treeing. Slices were taken from several full-scale (crosslinked) LDPE insulated cables aged in different ways. By count- ing the number of isochromatic lines on a polarization photograph in a cross-section of the insulation, the mech,anical stresses were estimated. It was found by Tana- ka that bow-tie trees and vented trees are concentrated in regions under higher mechanical stresses. Mechanical


stresses between 1 to 8 MPa were observed in the insulation. This is high, but not destructive as described in Section 1.2. Such mechanical stresses may result from the production process or from cable installation. One year later Prigent et al. [124] found water trees in LDPE samples concentrated a t locations with high mechanical stresses. However, Prigent’s evaluation is purely qualita- tive.

More recently, results were published by Tu and Kao [160]. They exposed a P E sample with a water needle to a pressurized atmosphere. The pressure on all sides of the sample was varied over a range between 0.1 and 3 MPa. The test time was short, only 15 h. Tu and Kao found that the initiation was faster, but growth of the vented trees was slower if the pressure was increased.

Recently, Patsch et al. [122] found more evidence for the influence of mechanical stresses. A bent full-scale 20 kV XLPE cable was aged for 5000 h at a voltage level of 24 kV. Patsch found that 90% of all the bow-tie trees were created in the stretched zone (near the conductor where the elongation was 10%) in contrast to 10% of the bow- tie trees found in the compressed zone. This result shows the effect of the mechanical stresses on the initiation rate of bow-tie trees, however, does not inform about the effect of the mechanical stress on vented tree initiation and especially vented tree growth.

5.5 EFFECT OF RELATIVE HUM ID lTY

There are a few studies dealing with the subject of water treeing under different water-vapor conditions. Slet- bak et al. [137,139] and Yoshimitsu et al. [172] showed that the relative humidity of the air surrounding the specimen and inside the specimen is relevant to water treeing. Both found that water treeing becomes rare a t a relative humidity of 65 to 70% or less. Above this level the concentration of bow-tie trees (Sletbak) or vented trees (Yoshimitsu) increases if the relative humidity level is increased. The studies do not give a relation between water tree length and relative humidity. Also, the effect of liquid water around the specimen compared to water vapor with a relative humidity of 100% around the specimen has not been studied.

The experiments were carried out on XLPE samples or cables with electric stresses ranging between 4 and 20 kV/mm.

5.6 EFFECT OF T H E CHEMICAL NATURE OF T H E FLUID

5.6.1 N 0 N - WAT E RY S 0 L U T IO N S

Bahder et al. [8] studied the growth of vented trees and bow-tie trees in normal cables with a LDPE or XLPE insulation. The tests were carried out a t a frequency level of 7.3 kHz and for a duration of 274 days. Flu- ids surrounding the insulation were water and water with CuSO4. Moreover, Hos topap and ethylene glycol, both easily penetrating the polymers, were used. In all cases treeing in the insulation had the same appearance. From this it can be concluded that water treeing is probably a special case in a broader field of insulation degradation. Treeing has also been studied in a liquid paraffin solution, which was chosen because of its extreme small dipole moment. No trees could be observed but it must be mentioned that the test duration was only 12 days. All other studies mentioned below deal with water solutions. The following subjects will be reviewed: type of salts, salt concentration, electrode materials, acidity and solubility.

5.6.2 TYPE OF SALTS

An indication that vented tree length is affected by the type of salts dissolved in the water is given by Bamji et al. [ll]. Bamji used XLPE samples in which water needles were made. A CuSO4 solution produced the greatest tree growth, followed by a NaCl solution and finally by a CaClz solution. No trees were observed in distilled water. The test duration was 90 h, the frequency 1 kHz.

Ross [127] studied vented tree growth in similar test objects. Aging lasted for x 1200 h. The upper electrodes consisted of stainless steel, and several 1.71 mmol/l salt solutions were used: NaCl, NazS04, CuC12, and CuSO4. With the CuSO4 solution the longest trees were obtained, whereas with NaCl l o x smaller trees were grown. The other solutions yielded intermediate results.

5.6.3 SALT CONCENTRATION

Many investigators studied the growth of vented trees in relation to the concentration of certain salt ions dissolved in water. The salt concentration can of course be translated into the conductivity of the fluid. In most cases NaCl and CuSO4 solutions were applied. It was found that higher salt quantities enhance the vented tree


propagation rate. Most of these investigators used water needles in order to study vented trees: Ashcraft [4, 51, Yoshimura et al. [174], Filippini et al. [44,45], Hossam Eldin et al. [64]. Test durations < 120 h were used at frequencies < 20 kHz.

between the concentration and pH of the solutions. How- ever, a slight relation between the concentration and the standard entropy of hydrated ions was found.

5.6.6 SOLUBILITY

The effect was confirmed for vented trees in model or full-scale cables (Tabata et al. [153], Katz et a]. [75] and Srinivas et al. [143]). Test durations in the related studies were much longer, < 1 yr, while the frequency level was 50 HZ in [153] and in the kHz-range in [75,153]).

A relation between solubility and initiation of water treeing was found by Kat2 et al. [751. Vented trees have been studied, which start a t the surface of scratched (crosslinked) LDPE cable insulation in a test with a duration of 13 days a t a frequency level of 7.8 kHz.

Fournid et al. [50] found the opposite effect in a 50 HZ LDPE needle test. Rye et al. [128] tried to explain Fournid ' s findings. They proposed that oxygen promotes vented tree growth; moreover, it is known that the solubility level of oxygen is lower in a very strong solution of salts. A combination of these facts could explain the reduced growth of vented trees.

5.7 EFFECT OF INSULATING MATERIAL AND ADDITIVES

5.7.1 VENTED TREEING IN NEEDLE TESTS

The water tree susceptibility of different types of insulating material has often been studied in water needle experiments. Studies with water needles in this specific field have been carried out by Ashcraft [4,5], Isshiki et al.

5.6.4 ELECTRODE MATERIAL

Fournid examined the effect of the electrode material on vented tree growth. The fastest vented tree growth was found if P t or Cu electrodes were used, followed by All Fe and Pb. It is again suggested by Rye that the amount of oxygen in the fluid affects the growth of water trees. The amount of oxygen is lower with Fe or P b electrodes, than when using P t and Cu electrodes.

Filippini [47] carried out similar tests with the same experience. The role of oxygen in water treeing is considered here too. Filippini showed that oxygen formation a t the electrode/liquid interface is not likely. He suggested that the effect on vented tree growth is a consequence of ions in the solution through metal corrosion. Filippini stated that the actual mechanism causing the tree growth is not clear. However, the role of the liquid and the interaction of the liquid/insulation interface is important.

5.6.5 EFFECT OF ELECTROLYTE pH

The effect of pH has been studied by Ashcraft [4,5]. Ashcraft found that the vented tree growth is enhanced a t high pH and decreased a t low pH. Solutions of hy- drochloric acid in 0.01 N NaCl for water-needle experiments in P E were used. The test duration was 24 h a t a frequency of 8.5 kHz. Morita et al. [lo41 examined the concentration of vented trees for a great variety of solutions. Scratched LDPE films were tested for 7 days a t a frequency of 3 kHz. Morita could not find a relation

[66], Kat0 et al. [73], Braun [25], McMahon [96] and Saure et al. [130]. Ashcraft examined the growth rate of trees a t 8.5 kHz for 24 h in polybutene, polystyrene, ethylene propylene diene terpolymer rubber (EPDM), LDPE and in XLPE. Tree growth in polybutene was extreme. The slowest tree growth was found in freshly cured XLPE. This was attributed to crosslink residual products (mainly acetophenone). I t was shown that acetophenone added to P E is able to suppress water treeing in this material.

The retardant effect of crosslink residual products was confirmed by Saure in a similar test, but a t a frequency of 50 Hz. Moreover, Saure found that an antioxidant, used for the stabilization of 'PE, had the opposite effect in this test. Saure also found that silane-cured XLPE was less water tree susceptible than peroxide-cured XLPE, which matches the findings of van de Laar [85] and Kreuger et al. [83]. In 1986 Dissado et al. [36] examined vented tree growth under moderate aging conditions (6 kV/mm, 50 Hz, 30'C) in silane-cured and peroxide-cured (degassed) XLPE slabs for a few thousand hours. In both materials vented trees with a comparable average length could be observed. In the peroxide-cured XLPE, however, the scatter was much higher. These results give the impression that silane-cured P E is less susceptible to vented tree development than peroxide-cured PE. However, none of the investigators tried or could explain these findings.

Braun studied the growth of vented trees in dry-cured XLPE, steam-cured XLPE, polystyrene and epoxy resin a t 60 Hz for 100 days. No treeing was observed in epoxy


resin, the other materials showed about equal vented tree development. It cannot be concluded that epoxy resin is water tree resistant: other investigators also observed water tree development in this material, e.g. Yoshimitsu et al. [171].

Isshiki et al. [66] carried out research work which was mainly intended to find the electric stress level a t which various materials initiate water trees. The rate of propagation was also studied. The total aging time for this experiment was 12 days; the frequency 2 kHz. I t was found that the growth of vented trees is faster in soft materials. The materials PVC, LDPE, XLPE, ethylene vinyl acetate copolymer, HDPE and polypropylene all showed water treeing, while in polystyrene, polycarbonate and nylon some kind of vented treeing was only observed when high initiation stresses were used. In polystyrene, polycarbonate and nylon the trees observed can be considered as suspect since other aging mechanisms may have been introduced as a result of the high initiation electric stresses.

Kato et al. [73] suppressed the initiation of vented trees in very short term needle tests on XLPE samples at 1.2 kHz. He uses a mix of different additives: ferrocene, silox- ane oligomer and 8-hydroxy quinoline. The initiation resistance observed was attributed to the combined action of a migration of these additives to irregularities and a deactivation of electrons and metal ions through'traps.

Another water tree inhibiting additive was presented by McMahon [96]. This additive, dodecanol, has been tested with water needles at different frequencies in P E samples for 28 days. Moreover, the additive was used in full-scale cables, tested for 220 days with a 9O'C conductor temperature. It was found that the dodecanol concentration stabilizes; this indicates that the water tree inhibiting effect observed could be effective over longer periods.

5.7.2 VENTED TREEING IN (MODEL) CABLES

The development of vented trees in cables or model cables has been studied by several investigators: Katz et al. [75], Bahder et al. [8], Srinivas et al. [142,143], Henkel et al. [62], Kalkner et al. [69], Marsh et al. [89] and Fare- mo [40]. Bahder investigated the difference in vented tree growth between LDPE and XLPE insulated cables. He found that LDPE insulated cables were more susceptible to vented trees than XLPE insulated cables. The investigations were carried out on full-scale cables aged for 8 yr under service conditions. The difference observed can be attributed, for instance, to the water tree retardant effect

of crosslink residual products in the XLPE cable insulation. Henkel and Kalkner [62] tested full-scale LDPE and XLPE insulated cables for 250 days at 50 Hz. The cables were pre-conditioned, causing evaporation of these residual products. This evaporation resulted in even more rapid vented tree growth in the XLPE insulated cables than in the LDPE insulated cables. Katz tested pre-conditioned model cables with a scratched inner surface for 15 days a t 7.8 kHz. He found only a small difference in the growth behavior of vented trees in LDPE and XLPE insulated cables.

The effect of crosslink residual products is also confirmed by Srinivas. In his experiments, model LDPE and XLPE insulated cables with a scratched inner insulation surface were tested for 20 to 150 days a t a frequency of - 2 kHz.

Faremo studied vented tree growth in XLPE and some filled ethylene-propylene rubbers. The materials were preconditioned prior t o testing. The experiments were carried out under rather extreme temperature conditions and a t high electric stresses on press molded cups, including plastic semiconducting screens. Faremo concluded that the growth rate of vented trees was about the same in the different materials. These results are all the more important since it has often been suggested that water trees would not exist in ethylene propylene rubber.

5.7.3 BOW-TIE TREES

Bow-tie tree growth has been studied by Katz et al. [74], Yoshimitsu et al. [171], Kalkner et al. [69], Marsh et al. [89] and Faremo [40]. Katz and Faremo did not find any difference in the growth of bow-tie trees between pre- conditioned LDPE and XLPE insulated cables [74] or between XLPE and ethylene propylene rubber press molded cups [40]. The experiments are described above in Section 5.7.2. Marsh [89] observed bow-tie trees in silane-cured and in steam-cured P E insulated cables. The concentration and growth of bow-tie trees in the peroxide-cured cables was shown to be more significant than in silane-cured cables. Marsh tested the full-scale cables for many years under moderate aging conditions. In his experiments different temperature conditions were applied. Yoshimitsu [171] reduced the bow-tie initiation rate successfully in an epoxy by adding a surface active agent that makes the hygroscopic substances (initiating bow-tie trees) hydrophobic. Kalkner [69] found a decrease in bow-tie tree growth if a certain, not further described, additive was used. The experiments by Yoshimitsu were carried out on XLPE or epoxy resin films. A rather high electric stress level of 40 kV/mm at 1 kHz for 63 h was applied. Kalkner tested P E films a t 50 Hz for 130 days.

1010 Steennis et al.: Water neeing

5.7.4 EFFECT OF MORPHOLOGY OF THE I N S U L AT I N G M AT E R I A L

A few publications only discuss the growth of water trees with respect to morphology-related parameters of the insulation.

Morita et al. [lo31 varied the melt index (0.3 to 2 g/10 rnin), density (0.920 to 0.927 g/cm3) and related crystallinity (72 to 76% from density) of LDPE and some other materials. He was not able to find a relation between the growth of the water trees and these morphological parameters. No information is given as to whether bow-tie trees or vented trees were observed. The insulation was tested in a sandwich construction a t a frequency of 50 Hz for 42 days.

Saure and Golz [130] studied vented tree growth in a 50 Hz water needle experiment. The melt index of the LDPE and XLPE samples was varied over a wide range (0.2 to 10 g/10 min). Also no relation could be observed in this study. In the same kind of test, Golz [59] concluded that annealing the insulation resulted in extra vented tree growth. The total effect, however, is small. Anneal- ing was carried out in different ways, often at temperatures > 1OO'C for 15 h. Crosslink residual products had been removed by pre-conditioning from the P E samples. Go18 attributed the effect observed to the increase of crystallinity and therefore of free spaces in the insulation after annealing.

Namiki et al. [110] also concluded that heat-treatment of the material produced extra water tree growth. The author studied the development of bow-tie trees in XLPE sheets aged for 40 h a t a frequency of 50 Hz. Namiki as- cribed this effect to an enhancement of the brittleness of the insulation due to higher crystallization. An explanation for the enhanced water tree growth after heat- treatment [59,110] could be the effect of the (further) evaporation of crosslink residual products. Most results indicate that there is no distinct relation between the morphological parameters of the insulation such as melt index, density or crystallinity and the growth of water trees. Only a few studies are informative in this particular field and unfortunately none of these studies deals with vented trees in full-scale cables.

6. SUMMARY

N summarizing the main characteristics of water trees I and effects of operating variables regarding vented tree growth, the following can be concluded.

Vented trees are more dangerous than bow-tie trees as a result of the difference in growth behavior. Vented trees are diffuse structures, growing in many kinds of polymers. The vented tree probably contains about 1% water, being about 3x the amount of water in freshly steam-cured XLPE insulations and about 100 x the saturation level of water in dry-cured and silane-cured PE. This water is mainly concentrated in micro cavities and channels. Apart from Clustering of water in voids, water is probably molecularly dispersed in the path of the vented tree. Here water molecules can be found a t places with polar groups attached to P E branches. Nevertheless the vented tree can be considered as an insulating material.

A vented tree can be dried and rewetted. To make the tree clearly visible different water-soluble dyes can be used; decolorization of vented trees after dyeing has never been reported. A high concentration of microvoids is mainly found in the trunk of vented trees grown a t high electric stresses. The branches of such vented trees also become more pronounced. A vented tree grows mainly in the direction of the original electric field. The growth rate under service aging conditions ranges from M 20 to M 500 pm per year.

Local measurements on tree-affected regions of the insulation with different chemical and physical detection methods show evidence of oxidation. The presence of different species like sulfate, carboxylate anions and metals in the tree-affected regions has been established.

Bulk measurements on insulating materials with and without vented trees show:

1. There is a relation between the size of vented trees and the breakdown stress level.

2. One cannot exclude an increase of the loss factor and a decrease of the dc resistance of insulating material containing vented trees. However, results from different investigators were not found to be reproducible.

3. At normal electric stresses the growth of vented trees is not accompanied by measurable partial discharges. At higher electric stresses electrical treeing may be initiated from the tip of the vented tree, eventually resulting in a breakdown.

There are no fundamental contradictions between the results of tests where vented trees have been initiated in a water needle experiment and vented trees initiated on the surfaces of scratched or even unscratched insulations. Most of the information justifies the conclusion that the rate of propagation of vented trees is proportional to the electric stress.


There is hardly any vented tree growth under the application of a dc stress. The vented tree propagation rate increases a t increasing frequency; however, above 4 to 8 kHz the tree propagation rate decreases with increasing frequency.

Vented tree growth is not really affected by temperature cycling. The role of a temperature gradient is important and rather complex. At constant temperature level the most favorable temperature region for vented tree growth appears to be 30 to 50°C.

To date, the relation between vented treeing and mechanical stresses is unclear.

At a relative humidity of the air surrounding the insulation of < 70%, vented treeing becomes rare.

The chemical nature of the fluid surrounding the polymer affects vented tree growth. There are several indications that the type of salt is related to vented tree growth; NaCl and CuSO4 solutions are often used to speed up aging processes. Higher salt quantities will usually produce more progressive growth. One test was made with fluids other than water. This also resulted in some kind of treeing, showing that water treeing is probably a special case of treeing in general.

Vented tree growth is not only related to P E bu't can be found in many different materials: polyolefins and epoxy resins. The growth of these trees is clearly affected by additives and crosslink residual products. Additives and residual products are able to evaporate in most cases, resulting in a reduction of this protective effect.

Morphological parameters such as melt index, crystallinity and density probably do not affect the growth of vented trees.

7. POSSIBLE MECHANISMS OF GROWTH

7.1 INTRODUCTION

variety of mechanisms of bow-tie tree and vented tree A growth has been postulated since water trees were discovered. This important matter has been discussed in detail a t a workshop meeting of EPRI in 1985 [24]. The most important mechanisms will be discussed in the course of this Section:

1. capillary action, 2. osmosis,

3. Coulomb forces, 4. dielectrophoresis, 5. thermal degradation, 6. partial discharges, 7. chemical degradation.

The main focus will be on the growth of vented trees, because this type of water tree appears to be the most dangerous one for medium-voltage extruded cable insulations, as was stated before. The discussion of the mechanisms of vented tree growth is preceded by the definition of aging conditions.

Degradation is usually presented under extreme aging conditions. I t is obvious to expect a certain degradation if, for instance, very high electric stresses are applied. Moreover, it is not difficult to show this degradation in experiments with water needles, in which such high stresses are available. These experiments show that high electric stresses should be avoided; however, such experiments are not conclusive on the cause of vented tree propagation a t moderate electric stresses of a few kV/mm. The aging conditions therefore will be emphasized when discussing the various degradation processes.

7.2 CAPILLARY ACTION

Van der Waals demonstrated that a force, directed in- ward is exerted on the water molecules of a surface layer. The related surface energy can be written as a product of the surface tension and the surface area. This surface energy is a representation of the potential energy of the molecules in the surface layer. Enlarging this surface requires energy.

P E insulating materials always contain a number of microvoids. As a consequence of the different surface ten- sions of PE/water, water/air and PE/air interfaces, water inside microvoids with pure P E walls will in principle be expelled from the PE.

Similar considerations describe the behavior of water in a capillary channel. Water will not enter a narrow channel as long as the walls of these channels consist of pure PE. However, if the walls of these channels become polar, their surface tension will change. Water will enter the channel if the surface tension of the PE/water interface is smaller than the surface tension of the PE/ gas interface. Then the absorption of water into the system will cause the surface energy to decrease. In other wdrds, a certain minimum amount of polar material on the walls is required to get an intrusion of water. It could be questioned if capillary action itself is able to damage

r


the polymer system, especially in front of the capillary channel. This is not the case: the reduction of the surface energy and, thus, the supply of water will stop as soon as the water has reached the unmodified area in the channel. The pressure at the end surface is compensated by the concavity of the water meniscus. There remains no pressure capable of damaging the PE [148].

7 . 3 OSMOSIS

tangential stress ct becomes = II/2. The various stresses are given in Figure 23.

7.3.1 EXAMPLE

For a typical NaCl solution under saturated conditions the osmolarity c’ = lo4 mole/m3 at a temperature of 300 K. As a result, the osmotic pressure II becomes large: in this example the osmotic pressure is M 25 MPa. The tangential stress in the surface layer of a spherical void containing this solution is 12 MPa, which is near the yield strength of LDPE (cy in Section 1.2). Creep, a mechan-

the void will be enlarged and by further absorption of water the solution will be diluted and osmotic pressure will decrease. This process will continue until eventually the osmotic pressure falls below the yield strength of the PE.

Water-soluble substances that are present in microvoids attract water from the environment and osmotic pressure

by means of a thermodynamic approach. I t is stated that the chemical potential of the water in the void and outside the P E are equal if an equilibrium is reached. Therefore, a decrease of the chemical potential of the water in a void due to a solute must be compensated by an increase of the hydrostatic pressure. The osmotic pressure derived becomes

may occur. Moore [lo21 derives such an osmotic pressure deformation process, may occur. As a

7.3.2 OSMOTIC PRESSURE AND SURFACE TENSION

II M RTc (1) in which R, the gas constant, 8.31 J/Kmole, T the temperature in K and c the concentration in mole/m3. The basis of this derivation is a diluted solution; however, for a saturated solution Equation (1) is an approximation. For better results the concentration c has to be replaced by a practical parameter, c’ being the osmolarity. Therefore,

II M RTc’ (2)

This osmolarity has been measured for many solutions and can be derived from data given in the Handbook of Chemistry and Physics (D-262 [164]).

t ”

Figure 23. Stresses in the surface layer of a spherical void.

The osmotic pressure may be (partly) compensated by the surface tension of the solid/liquid interface of the void. The effect of the surface tension has already been discussed under ‘capillary action’. The surface energy can be high, especially if the walls of the PE are unmodified so that the voids have small radii. Therefore, the pressure drop over the liquid surface can reach compensating pressures regarding osmotic pressure. The radii of such voids are calculated to be in the nm region. Consequent- ly, compensation of the osmotic pressure by the surface energy can only be expected in the interlamellar region of the polymer. Osmotic pressure itself does not explain vented tree growth. It could act without an electric field, while from the phenomenology it is known that an electric stress is necessary for tree growth.

Sletbak [138], who suggests an osmotic pressure for bow-tie tree growth, indicated that the electrically con- trolled mechanism should be found in a Coulomb action. Another investigator taking osmosis into account is Fe- dors [42]. Kats [74] also indicated the ability of water soluble matter to attract moisture, and included other mechanisms in their overall picture, for example an electrochemical reaction.

7.4 COULOMB FORCES

For a spherical void the stress ct in the tangential direction of the void surface is 50% of the stress cT in the radial direction of the surface [65]. With I F , ( = II, the

Coulomb forces are forces on electric charges, that are caused by an electric stress. Separated stress terms as

IEEE nansact ions on Electrical Insulation Vol. 25 No. 5 , October 1990

the Maxwell stress and electrostriction can be combined into a single relation:

(3)

In which fi is the dielectric displacement vector, E' the- electric stress vector, 5 the unit normal vector, the permittivity of the polymer, and p the density of the polymer.

Figure 24. Pressure at the tip of a vented tree path.

Equation (3) expresses the relation between pressure p and the electric stress E. The first term on the right is related to the Maxwell stress and the second term to electrostriction. The medium here is P E surrounding the vented tree, as is schematically illustrated in Figure 24.

A number of investigators (e.g.Meyer et al. [98]) estimated Coulomb forces as serious. They considered various combinations of- the following: Very high electric stresses in the virgin insulating material, that a water tree is a conducting material, and the vibration of the Coulomb forces in relation to the power frequency.

In certain cases Coulomb forces are assumed to act in combination with another force. With regard to the growth of bow-tie trees, including the effect of osmotic pressure, Sletbak and Ildstad [138,140] should be mentioned here. A high surface tension of the PE/water interface may counteract the effect of Coulomb forces, especially for voids with small radii. A reduction of this surface tension is assumed by Tanaka et al. [156] and Min- nema et al. [loo]. The effect of vibrations is emphasized by Cherney [29], Isshiki et al. [66], Minnema et al. [loo] and Morita et al. [104].

1013

They also found a reduction of the surface tension of the P E surface through the action of an electric stress. On the basis of these two facts it was concluded that the propagation of a vented tree is related to a process which is called 'environmental fatigue failure'.

Cherney [29] starts his calculations with the assumption that the pressure can be derived from the relation F, = QE. In this relation the parameter Fn represents the force on the medium. The electric stress is E and Q represents all the electric charges present in the water of a cavity. This derivation, however, is incorrect because by no means all charges in the water contribute to the force on the medium.

Recently Zeller [176] and Steennis [148] calculated that the Coulomb forces are unable to degrade the polymer. In this case the electric stress E,, has been chosen rather moderate, according to service aging conditions. More- over, Steennis assumed that a vented tree can be represented by an insulating material. There are several results from the phenomenological experience supporting this conclusion:

1. If vented tree growth were related to Coulomb forces, the propagation of the tree would be proportional to E'. However, the results of the phenomenology seem to indicate that its propagation is proportional to E .

2. Cracking of a polymer is normally reduced by the increase of the molecular weight (Section 1.2). Such an effect has not been observed clearly in relation to the propagation of vented trees in P E (Section 5.7).

3. Even electron micrographs did not reveal cracking in the tips of vented tree channels [28]. The voids sometimes observed in the P E a t a certain distance from the channel t ip probably are related to secondary degradation.

7.5 DIELECTROPHORESIS

In an inhomogeneous electric field, water dipoles tend t o move to the point of highest stress. After some time the flow of water in the direction of the electric stress concentration will reach an equilibrium with the diffusion backwards as a result of the water concentration gradient. Several investigators describe this effect (Tanaka et al. [155], Isshiki et al. [66] and Patsch [121]).

The difference in water concentration is found by using a thermodynamic approach. An equilibrium is reached if'the chemical potential of the water in the position of high electric stress is equal to the chemical potential in the Dosition of low (normal) electric stress. From equations

Minnema et al. concluded that the vibrating elastic stresses as a result of the Coulomb forces are essential. , ~ ~ ~ . .


given in the literature it can be calculated that even under the application of a high electric stress enhancement of - 1OOx a t the t ip of a water tree, the increase of the water concentrations a t that specific location is < 3%. The increase of the water concentration becomes negligible if a water tree is considered as an insulating material.

Nevertheless, several authors mention supersaturation of water in PE. This supersaturation is assumed to be caused by dielectrophoresis. Water droplets will be formed, which cause small voids to be filled with water. This process continues until an equilibrium has been reached and may therefore lead to pressure building up and P E being attacked [21,93,94,121,141,167].

It can also be stated that there is no supersaturation. The slightly higher water concentration a t the location with a higher electric stress is in equilibrium with its sur- roundings. Dielectrophoresis is not the cause of water tree growth but may contribute to the transportation of water to locations capable of absorbing water if stress enhancement exists near the surface of these locations. Condensation will occur in these places; in particular water trees can be considered as such dislocations.

7.6 THERMAL DEGRADATION

The thermal behavior of any arbitrary volume is given by the relation

d T a t

pc , - - v . k’VT = qh (4)

With p the density of the material, C,, the specific heat, k’ the thermal conductivity, qh the heat dissipation. The value of qh is given by

qh = % E t (5)

in which q, and E, are the conductivity and the electric stress in the vented tree respectively.

It is possible to assume that a vented tree is a conducting material (water and salts) and that, moreover, the remaining electric stress in the tree is still considerable (M E,). Calculations show that under these conditions the temperature can become unstable (aT/a t # 0). On the basis of these assumptions the investigators Isshiki et al. [66], Tanaka et al. [156], Yoshimura et al. [174] and Meyer et al. [98] state that an increase of temperature may play an essential role. Yoshimura suggested that the water in the tree would even evaporate, with obvious consequences for the polymer.

cable insulation

Figure 25. Cylindrically shaped volume V surrounded by the unaffected PE. The volume V has the electrical properties of a vented tree path.

On the other hand, it is possible to assume that the vented tree can be considered as an insulating material (Section 3.1) and that there is a thermal equilibrium: aT/& = 0 according to [148]. I t can be shown that the temperature in the vented tree hardly differs from the temperature of the surrounding polymer. This is demonstrated for a certain volume V in the PE, cylindrically shaped, as shown in Figure 25, with a length h, radius T and with one of its extreme surfaces A on the outer surface of the insulation. The properties of the vented tree are attributed to the whole cylinder. For the configuration considered, the solution of Equation (4) with aT/at = 0 is according to Wong [169]:

r 2 2h 2k‘ T

Tu = To + qh- In( -)

in which T, is the temperature of the vented tree, T the radius of the cylinder, h the length of the cylinder.

In relation (6) the heat flow through the surface A is neglected. The temperature in the water tree has a maximum for T = 2h/&, thus

c, E,” h2 Tu < T o + 7 ek (7)

Assuming that E, < E,, E, = 2x106 V/m, c, = 1.3x10-’ (am)-’, k’ = 0.25 J/Ksm, h < 1 mm, while e = 2.72.

These assumptions result in

(8) Tu < To + 0.08K

In agreement with these findings microscopic examinations of vented trees do not reveal any thermal degradation. Moreover, Bamji et al. [ll] using Differential scanning calorimetry were not able to detect any difference between the thermal history within PE affected by a tree and the P E free from attack.

7.7 PARTIAL DISCHARGES

Partial discharges may occur in gas-filled cavities under high electric stresses. Under the application of an ac-field


a continuous repetition of the process causes degradation of the surrounding insulating material, so that electrical treeing may occur eventually. These discharges have been studied for many years [82,84,90,91].

In Section 4.4 it was concluded that vented tree growth a t moderate stress levels is in all likelihood not accompanied by such discharges. The experience with partial discharge measurements on medium-voltage cables confirm this, even if these cables contained large vented trees. The noise level during the experiment of Steennis [148] was FZ 0.1 pC, the mean electric stress level in the insulation was < 5 kV/mm.

However, there are two different situations in which partial discharges can be related to vented treeing. In the first situation partial discharges may occur during initiation. If water needles are applied, the very high initiation stress level a t the tip of the water needle may cause partial discharges. This accounts for the light emission from the water needle electrodes in a test by Nitta [114]. Secondly it is possible for electrical treeing to orig- inate from vented trees, for example during overvoltages. These electrical trees cause partial discharges. Examples have already been presented in Figures 18 and 19.

Finally, it is assumed by Dakin [37] that perhaps micro discharges could play a role in the process of water tree growth.

7.8 ELECTROCHEMICAL D EG RA D AT IO N

The concept of chemical action in relation to water tree growth in P E has been mentioned by several authors. Rye et al. [128] showed that in the absence of an electric field, warm solutions of some salts (sodium chloride and various copper salts) attack the interface of P E chemically, possibly oxidizing it. I t is possible that such a process increases the tendency to water tree growth in PE. It has still to be explained why a tree structure arises.

Tabata et al. [153] found tree structures in voltage en-' ergized cable insulation which had been in a solution of H2S and water. The trees had grown from a copper conductor, and contained CuzS and Cu20. This makes the contribution of a chemical action to water treeing proba- ble. Yoshimitsu et al. [171] suppose water tree formation in epoxy resins to be accompanied by chemical reactions.

Henkel et al. [63] supposed an electrochemical process a t the P E interface, in which H 2 0 2 is generated in water, successively attacking the PE. The fact that agents which

are known to either interrupt the electrochemical process or stabilize H 2 0 2 , strongly diminish the formation of water trees, supports this supposition.

Steennis [148] states that water enters the amorphous regions of polymers if polar impurities are present. He gives several reasons for the creation of polar regions (which can be seen as a vented tree initiation process). For instance:

1. pollution or oxidation of the compound or cable insulation during production,

2. diffusion of impurities from a semiconducting screen, which was also estabilished by Crine [31] and Johnson

3. scratching of the insulation surface during or after cable production. This will produce polymer chain ends which may easily be oxidized.

~ 7 1 ,

polar group

Figure 26. Polar groups (.) fixed on the polymer chains at a scratched PE surface.

Figure 7 shows an example of a vented tree initiate( a t such a location. In the cavity were species containing silicon, sulfur and calcium (Figure 8). Growth due to electrochemical action is illustrated in Figure 26 [148]. In a cross section of an insulation surface, polar groups are attached to the polymer chains. In Figure 27 an enlargement is shown. Water enters the amorphous regions as far as these regions contain polar groups, the front of this water intake is represented by line F . In the polar region, up to front F , charge transport by ions can take place, represented by c. Beyond front F the polymer is *pure and acts as a dielectric that transmits capacitive currents, represented by the permittivity cp of the polyethylene in the Figure. At interface F the transport of


F =

t P - -r t F

noter

Figure 27.

b

Water intake at a polarized insulation surface [148] and consequent tree growth by electroly- sis. Up to front F the degraded polar polymer is slightly conductive by ion movement. At F the ions lose their charge and free radicals are formed that attack the virgin polymer. The ionic current through cv is maintained by a capacitive current through c p so that the mechanism works at ac voltage only.

ions is stopped and charge transfer takes place by elec- trolysis as has been described by Kao et al. [70]. Free radicals or oxydizing agents such as H202 are formed. The polymer is attacked, the front shifts further into the polymer and a polar path is created: the polar path containing water forms a vented tree. The vented tree can be considered as a very high-resistive electrolyte: the ionic conductivity is low and a high potential difference remains over the electrolyte. Because the available voltage remains high, the vented tree path may contain several of such fronts in series. The vented tree is thus considered as an insulator, although a poor one. As the ionic current in the water tree in this theory is maintained by capacitive current through the unaffected dielectric, tree growth takes place at ac voltage only which corresponds with the phenomenology described in Section 5.2.

In 1990 Ross [127] introduced an advanced model of the structure of water trees in the nm range, see Fig- ure 28. In this model the hydrophilic nature of water trees is explained by the presence of chemically or phys- ically bonded ionic groups. It is assumed that oxidation occurs during water tree growth as carboxylate groups and ionic sulfur-oxygen groups (observed with FTIR) can be regarded as possible oxidation products. It is suggest-

limiting diameter

I im i tin g void with tropped solt diameter

at . ' . cotions

o O o O anions

crystalline

amorphous

LVJ ionic endgroups PE outside watertree

Figure 28. A model of the structure of water trees (After [127])

ed that acidic sulfur-oxygen groups may be the result of oxidized antoxidants, which derive their functionality from a sulfur atom; in contact with water these groups may be converted to salts. I t shows that antioxidants may play an important role in the process of water tree growth. Ross observed with oxygen plasma exposition that the oxidative stability is locally reduced by water trees, which is another argument that oxidation processes are important. According to Ross there is a further argument. This argument is found in the growth rates of vented trees in needle tests when different salt solutions are applied (for a description of the test results, see Section 5.6). The strongest tree growth is obtained with copper sulfate. According to Ross, sulfates can be considered as oxidizing agents and copper ions are noto- rious catalysts in oxidation processes. Ross further introduced the selective permeability for different entities locally, seeming to bear a close resemblance to ionomer membranes. As a result of the selective permeability (at places with limiting diameter in Figure 28), salts may be trapped once the electric field is removed.

7.9 CONCLUSIONS

Osmosis and capillary action are not related to electric stress, which is the reason why they cannot be considered as the cause of water tree growth. However, both processes may play a secondary role, for instance during the initiation of a water tree or by pushing water into a polarized channel.


Partial discharges have never been detected during the growth of a vented tree under moderate aging conditions and are thus not considered in the first place as the basis of water tree growth. Nevertheless if such discharges are involved, the level is below detectability.

Assuming a water tree represents a conducting material, various basic mechanisms of degradation are con- ceivable such as thermal degradation and degradation by Coulomb forces. About the role of dielectrophoresis there are different opnions. In any case, dielectrophoresis might be of assistance by carrying water vapor to the tip of a vented tree.

On the other hand, assuming a vented tree is not a conducting material but an insulating material, and by selecting aging parameters to comply with the actual use of medium-voltage cables, most of the mechanisms mentioned above become rather improbable. Under these assumptions, however, electrochemical degradation can be considered as a significant mechanism.

8. TEST METHODS

8.1 FIELD TESTING

1. Experience with dc tests on service-aged cables containing water tees (vented trees) shows that many cables, after a first ac breakdown and repair, have an extreme- ly low dc breakdown voltage. Often breakdown voltages smaller than the rated voltage of the cable were reported and further dc breakdowns reveal an ever decreasing breakdown voltage level [49,60,150].

2. To find the effect of the dc field test on the 50 He breakdown voltage level short cable lengths have been stressed with a dc voltage up to several times the rated voltage after which the 50 Ha breakdown voltage level was measured. It was found that in these specific cases there was not a reduction of the 50 Hz breakdown voltage level, although the cables contained many large vented trees [20,60,145].

Gronefeld [60] explained the apparent contradiction between l and 2 as follows: under the application of a dc stress space charges can reduce the stress enhancements in the vicinity of water trees. Then, traveling waves of opposite polarity, occurring after breakdown, will give rise to relatively high stress enhancements a t the tip of the water tree which may result in degradation.

Although it is difficult to prove this assumption, practical experience gives enough reasons to avoid dc stresses

on cables, even for cables with small water trees. Espe- cially dc stresses of several times the rated voltage could lead to cable insulation damage.

Alternative test procedures, such as 0.1 Hz [60] and os- cillating wave [6] are under investigation for instance in Germany and in The Netherlands. It was found recently (to be published, Steennis, Jicable 1991) that 0.1 Hz easily discriminates between cables with different rates of degradation.

A further alternative would be the measurement of the dc current under dc load conditions. According to Hara- sawa et al. [61] this measurement technique has the disadvantage that only heavily deteriorated cables can be discriminated from other cables. A further disadvantage is that still the application of the suspected dc load is required.

A more sophisticated procedure was described earlier by Oonishi et al. [117]. They found that the dc leakage current under negative voltage application is larger than under positive voltage application. As a consequence of this polarity effect, the ac voltage will reveal a dc charg- ing current or a dc leakage current. Their examinations showed a good correlation between the breakdown voltage, the size of vented trees and this leakage current. Moreover, the measurement of the current can be made without interruption of the cable usage. The results indicate that both extensively deteriorated and moderate deteriorated cables can be discriminated from cables having small water trees only. It is not possible to distinguish cables being deteriorated by bow-tie trees since in this case there is no polarity effect. It is the authors’ opinion that this disadvantage is not very important, because bow-tie trees are less detrimental. However, cables having vented trees grown from both sides of the insulation may probably be less sensitive to this procedure.

8.2 ACCELERATED AGING

The evaluation of the water tree susceptibility of a cable insulation and in particular the evaluation of water tree retardant insulating materials must be made by means of accelerated aging tests. I t is the object of such a test to obtain an indication of the service behavior of the cable insulation in a short period of time.

The development of such a procedure is rather difficult, as’ history shows. A material test is still under development [131]. With respect to full-scale cable tests, the AIEC test [2] and the ‘Accelerated life test’ [87] (ACLT,


introduced by Lyle and Kirkland) are being used especially in the United States. In Europe, the 'Accelerat- ed water tree aging test' [134] (ACWT, introduced by Schroth, Kalkner and Fredrich) is under consideration. The effectiveness of these three tests can be questioned. It was shown that, probably as a consequence of the temperature regimes applied, the tests lead to an extensive bow-tie tree growth and a reduction of the sizes of the more important vented trees. Therefore, the water tree growth patterns obtained are not according to service experience [134,148]. It was shown also that a bad cable passed the AEIC test [148].

Alternative test procedures are under development, for instance within CIGRE WG 21-11 [113]. As long as a few thousand hour test is not available, a 2 yr test with moderate test conditions (such as 2 . 5 ~ the rated voltage, room temperature, power frequency) can be applied according to [148].

8.3 C H A RACT ER lZATl0 N TESTS

In order to determine the level of degradation of aged cables (and if necessary the reference level of unaged cables) a characterization test is carried out. Such a test comprises the following three basic elements:

1. supply of several cable samples is necessary for breakdown tests and visual inspections in the laboratory,

2. breakdown voltages (in most cases a step voltage test) give information on the electrical strength of the cable insulation,

3. visual inspections give information on the sizes, densities and types of water trees.

Breakdown voltages are found by testing cable samples with a length of a t least about 5 m. The mean breakdown voltage level can be calculated by means of Weibull statistics [14, 1581.

Visual inspections are carried out on dyed slices of the cable insulation. Microscopic evaluations are necessary to find water trees with sizes in the range of 100 p m t o 1 mm. The slices can be taken from randomly chosen places, the vicinity of the places of breakdown and the breakdown site.

AEIC [2], Weck [165] and CIGRE [149] recommend their own characterization test procedures.

After aging in the AEIC qualification test [2] the rate of degradation of the cable is characterized by means of the ac breakdown test. The characterization is based on

2 cable samples, each a t least 5 m long, subjected to a step test until breakdown. A breakdown voltage of 10 kV/mm minimum is required for each of the two cable samples. Moreover visual inspections are carried out on slices taken near the places of breakdown. In the 'Stufen- test' described by Weck [165] three cable samples of 15 m are subjected to a step test. The step test is terminated a t a level of 9 x the rated cable voltage (15 to 20 kV/mm). It is assumed that water treeing is of minor importance in cables having breakdown voltages above this level. Vi- sual inspections are carried out both on randomly chosen insulation volumes and near and a t the breakdown sites.

01 I ' ' ' ' I I I ' I 0 10 20 30 40 50 60 70 80 90 100

largest woter tree observed in insulotion [ % of the insulation thickness]

service operation J cable circuit with breokdown(s) during

Figure 29. The 63% breakdown stress level and the largest water tree found in the insulation plotted for different cables in one graph. In this Figure, the cable circuits which had failures during service are indicated with a lightning mark [149].

An extensive but very informative test procedure is the so called characterization test of CIGRE [149]. The total number of 5 m cable samples subjected to the breakdown test is 12. In this test (apart from visual inspections on randomly chosen small cable samples) visual inspections are carried out a t the breakdown sites. In order to obtain better visual inspection results, it is recommended to

r


adjust the equipment for the breakdown tests in such a way that the energy dissipated during breakdown is min- imized. As a consequence the actual causes of breakdown are often found which is very informative (examples are given in Figures 12, 18 and 19, vented trees were found as the cause of breakdown). Especially for cables having water trees exceeding about 30% of the insulation thickness this inspection is useful. The largest water tree observed in combination with the 63% breakdown stress level represent one dot in a graph (see Figure 29). This graph makes it possible to get an indication of the level of degradation of the involved cable (each dot in this graph represents one cable circuit under investigation) in comparison to other cables. The region of bad cables (indicated with lightning marks) starts with a (63%) breakdown voltage smaller than about 15 kV/mm and with water trees having a length exceeding - 40% of the insulation thickness.

9. MEASURES TO REDUCE WATER TREEING

WO different methods can be applied to obtain a bet- T ter aging performance of cables under wet conditions. On the one hand it is possible to make water-tight cable constructions. On the other hand it is possible to accept water in contact with the insulation and to apply so called water tree retardant insulating materials.

9.1 WATER TIGHT CONSTRUCTIONS

In many countries today the utilities install cables with water-tight constructions having longitudinal water blockings or a combination of longitudinal and radial water blockings.

To avoid any radial water ingress by diffusion utilities often apply cables having radial water barriers. Such a radial water barrier for medium voltage cables is usually a metal foil under the plastic outer sheath. Water vapor is not able to permeate through the metal. Examples of such constructions are given in many publications, for instance by Bourjot [22], Nagabasami [lo81 and Bow [23].

To avoid axial water ingress under the outer sheath after a damage of this sheath, longitudinal water barriers can be applied. These barriers are situated in between the outer sheath and the cable core and mostly consist of swelling tapes or swelling powders [135]. Cable damage can be expected also when liquid water from the soil can reach the conductor, but this conductor can be made water tight for instance by means of a solid conductor or by the application of swelling powders between the strands.

Concerning the swelling material under the outer sheath, these materials have the ability to absorb water vapor diffusing through the plastic outer sheath into the cable. In this way, by absorption, the swelling material is able to keep the relative humidity low for a long period of time for cables having a HDPE outer sheath (with a relatively low water diffusion coefficient). For example, assuming dry swelling materials right after production, it is expected that this period will be a few decades at least. It is the authors’ opinion that it would be of interest to prove that with a MDPE outer sheath (having a much higher diffusion coefficient) this period is long enough to avoid to much water in the cable. Such a situation would be reached when in and near the cable insulation the relative humidity level exceeds a level of x 70% (also see Section 5.5).

9.2 WATER TREE RETARDANTS

Compound manufacturers and cable manufacturers, often in close cooperation, put much effort in the development of water tree retardant insulating materials. Pub- lications show that developments are based on different philosophies concerning the mechanism of water tree growth. As will be shown, the water tree retardant materials presented are modified polymers, polymers in which chemical additives are incorporated or both. In many cases a modification of the polymer is required to prevent that these additives become fugitive. The test results as described in the various publications give the impression that tree retardants can be effective a t least over the limited time period of the tests.

Already in 1980 Soma et al. [141] formulated a bow-tie tree inhibiting material. Soma assumed that bow-tie trees are initiated by condensation of water in a void under the action of an electric field. Further condensation will lead to a pressure build up and creep of the void surface. The mechanically damaged surface contains polar groups and water will enter the polymer. The water tree retardant material (intended to suppress bow-tie tree development) is based on additives having hydrophilic groups absorbing the water in the polymer structure around the voids. The publication describes different additives and presents their effectiveness in a 3000 h test on a full-scale cable.

In 1984 Nagasaki et al. [log] presented a water-tree retardant insulating material containing an ethylene copolymer (EVA) that acts as a barrier against water tree growth. The cause of this retardant effect has not been clarified. Life tests up to 30000 h were carried out on medium- voltage cables showing that the material suppresses the growth of both bow-tie trees and vented trees.


One year later Matey et al. [92] presented a modified base polymer which should intrinsically be more resistant to water tree degradation. Matey assumes that mechanical fatigue plays an important role in the process of water treeing. The modified polymer system should offer an enhanced resistance to crack propagation. In addition the resistance is enhanced by water treeing inhibitor, being an organometallic compound. With respect to this material, it is assumed by Saure et al. [132] that by migration the polar additives have the ability to reduce the electric field stress on places of field enhancement. In this publication Saure emphasized that the basic polymer is a PE copolymer stabilizing the presence of the additives. Short duration tests show that both vented trees and bow-tie trees are reduced in number and length. Long-term aging tests up to 10000 h reveal the good performance of the material both with respect to the breakdown strength and the bow-tie tree development. However, no data are presented concerning the growth behavior of the vented trees.

In 1986 Fisher et al. [49] described the development of a water-tree retardant insulating material called HF- DA 4202. The development program was intended to find a material without a filler and with additives being nonfugitive. From [119] it is known that the additive is an organo-metallic compound. Short-term water needle tests show the retardant characteristics of this material. No more information about test results of long-term tests have been found.

One year later Fischer et al. [48] described a water-- tree retardant material which is based on a modified base polymer with nonfugitive additives. The additives are known to stabilize hydroperoxides by deactivation of the catalytically active metal ions via adsorption and by a deactivation of the initiating sites at which the assumed electrochemical reactions occur. The background of this electrochemical process during tree growth was published two years earlier by Henkel and Muller [63]. Fischer performed aging tests up to 6000 h and observed a reduction of both densities and lengths of bow-tie and vented trees.

Recently, Field et al. [43] described the results of 3000 h tests on full-scale cables. Assuming the water tree growth is related to free water, to ionic contaminants and to oxidation products, different water tree retardant materials were tested among which successfully a homopolymer with an additive and two different modified polymers. In- dication of tree size reduction of both bow-tie trees and vented trees was obtained. Further testing to demon- strate long-term benefits are in progress.

Apart from water-tree retardant insulating materials it is generally assumed that smooth semiconducting layers

and a reduction of the amount of impurities in these layers would be helpful in retarding the water tree growth, especially the vented tree growth. A study of types and amounts of impurities is given by Belhadfa et al. [13]. Im- purities diffusing from the layers into the insulation are described by Crine et al. [31] and Johnson et al. [67]. De- velopment of semiconducting layers with lower contaminants levels have been reported for instance by Nitta [115] and Umpleby [161].

Unfortunately, the above described developments and test results are not informative about the actual degradation mechanism, since modified polymers as well as additives can be considered as polar material. For all mechanisms discussed above, polar groups have watertree retardant consequences since polar groups are able to reduce the electric stress locally (effective for all mechanisms) or to reduce the interfacial energy of polar interfaces which reduces the absorption of water in general (effective especially considering electrochemical degradation). It is concluded that water-tree retardant insulating materials or a combination of these materials with smooth and clean semiconducting layers can be considered as serious candidates for solving the water treeing problem. However, in many cases long duration tests at least up to two years have not yet been performed; such tests are required to come to better evaluations of the characteristics of these materials on the long term. These tests should emphasize material stability and fugitivity of the additives. Moreover, tree growth of especially the vented trees should be taken into account.

REFERENCES

[l] K. Abdolall, H. E. Orton, M. W. Reynolds, B. D. Robert, R. Kennedy and B. P. Clayman, “Some Physicochemical Aspects of Water Trees”, Annual Report Conference on Electrical Insulation and Di- electric Phenomena, pp. 604-614, 1982.

[2] AEIC, “Specifications for Thermoplastic and Crosslinked Polyethylene Insulated Shielded Power Cables Rated 5 through 35 kV”, 9th ed. AEIC CS5- 87. Association of Edison Illuminating Companies (AEIC), Birmingham, Alabama, October, 1987.

[3] D. Mc. Allister (ed), Electric Cables Handbook, Granada; London, Toronto, Sydney, New York, 1982.

[4] A. C. Ashcraft, “Treeing Update Part 111: Water Trees”, Kabelitems 152, Union Carbide Corporation. Based on “Water Treeing in Polymer Dielectrics”, presented a t World Electrotechnical Congress in MOSCOW, June, 1977.


[5] A. C. Ashcraft, (‘Factors Influencing Treeing Identi- fied”, Electrical World, December 1, pp. 3 8 4 0 , 1977.

[6] C. Aucourt, W. Boone, W. Kalkner, W. Naybour and F. Ombello, “Recommendations for a New Af- ter Laying Test method for HV Extruded Cable Sys- tems”, 1990 International Conference on Large HV Electric Systems, CIGRE, Paper 21-105, Paris 1990.

[7] G. Bahder, G. S. Eager and R. G. Lukac, “Influence of Electrochemical Trees on the Electrical Proper- ties of Extruded Polymeric Insulation”, Annual Re- port Conference on Electrical Insulation and Dielec- tric Phenomena, pp. 289-301, 1974.

[8] G. Bahder, C. Katz, J . Lawson and W. Vahlstrom, “Electrical and Electrochemical Treeing Effect in Polyethylene and Crosslinked Polyethylene Cables”, IEEE Trans. PAS, Vol. 93, pp. 977-990, 1974.

[9] S. Bamji, A. Bulinski, J . Densley and N. Shimizu, “Light Emission from Polyethylene Subjected to Highly Divergent Fields”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 592-597, 1982.

[lo] S. Bamji, A. Bulinski, J . Densley and A. Garton, “Etching and the Morphology of Crosslinked Poly- ethylene Cable Insulation”, IEEE Trans. EI, Vol. 19 No. 1, February, pp. 32-41, 1983.

[ll] S. Bamji, A. Bulinski, J . Densley, A. Garton and N. Shimizu, “Water Treeing in Polymeric Insu- lation”, Confkrence Internationale des Grandes Rkseaux Electriques Haute Tension (CIGRE), session September. Paper 15-07, 1984.

[12] R. Bartnikas and R. M. Eichhorn (ed), Engineering Dielectrics Volume IIA. Electrical Properties Solid Insulating Materials: Molecular Structure and Elec- trical Behavior, ASTM Special Technical Publica- tions 783. ASTM Publication Philadelphia, March 1983.

[13] A. Belhadfa, A. J . Houdayer, P. F. Hinrichsen, G. Kajrys, J . St-pierre, G. Kennedy, J.-P. Crine and N. Burns, “Impurities in Semiconductive Compounds used a s HV Cable Shields”, IEEE, to be published; 1989.

[14] G. Bernard, “Application of Weibull Distribution to the Study of Power Cables Insulation”, (On behalf of Working Group 21-09 of CIGRE). Electra, Vol. 127, pp. 77-83, 1989.

[15] B. S. Bernstein, N. Srinivas and P. N. Lee, “Electro- chemical Treeing Studies: Voltage Stress, Tempera- ture, and Solution Penetration Effects under Accel- erated Test Conditions”, Annual Report Conference

on Electrical Insulation and Dielectric Phenomena, pp. 296-302, 1975.

161 B. S. Bernstein, “Recent Progress in Understand- ing Water Treeing Phenomena”, Conference record IEEE International Symposium on Ins. Electr. Mon- treal, June 11-13, pp. 11-21, 1984.

171 F. W. Billmeyer jr , Textbook of Polymer Science, 3th edition. Wiley-Interscience Publication. Wiley & Sons. New York, Chichester, Brisbane, Toronto, Sin- gapore, 1984.

[18] S. A. Boggs, and S. Rizzetto, “The Application of In- terferometric Holography to the Study of Watertree- ing” , Conference Record of 1986 IEEE Internation- al Symposium on Electrical Insulation, Washington DC, June 9-11, pp. 140-143, 1986.

[19] W. Boone, E. F. Steennis, P. A. C. Bentvelzen and A. M. F. J . van de Laar, “Development and Trial of EHV XLPE Cables in The Netherlands”, Confkrence Internationale des Grandes Rkseaux Electriques a Haute Tension (CIGRE), session September 1984. Paper 21-02, 1984.

[20] W. Boone, P. V. M. van Nes and E. F. Steennis, “Water, Onoverwinlijke of Overwinbare Vijand van Kunststof Middenspanningskabel”, Elektrotechniek, Vol. 68, No. 3, March, pp. 219-225, 1990.

[21] 0. Bottger, R. Niederhagen and W. Schuchard, “Hypothese zur Water Tree Entstehung und deren Uberprufung”, Dauerverhalten von Hochspannungs- isolierungen, Electrotechnische Gesellschaft Fach- berichte 16, VDE-Verlag GmbH, Berlin, pp. 109- 113, 1985.

[22] P. Bourjot, P. Gauthier and A. Pinet, “Compor- tement thermomkcanique et ktanchkitk des cibles moyenne tension a isolant synthktique (Themome- chanical behavior and watertightness of medium- voltage insulated cables)”, Jicable 84, International Conference on Polymer Insulated Power Cables, Paris, March, pp. 63-67, 1984.

[23] K. E. Bow, “The Development of Underjacket Mois- ture Barrier Cable as a Counter Measure Against Treeing”, IEEE Trans. PES, Vol. 5, No. 1, January, pp. 47-53, 1989.

[24] E. L. Brancato and B. S. Bernstein (ed), Water Tree- ing Theory, Collection of papers and reports of EPRI Workshop report, Monterey, California, December, 1985.

[25] J. M. Braun, “Comparison of Water Treeing Rates in Steam and Nitrogen Treated Polyethylenes”, IEEE Trans. EI, Vol. 15 No. 2, April, pp. 120-123, 1980.


[26] A. Bulinski and R. J . Densley, “The Voltage Break- down Characteristics of Miniature XLPE Cables Containing Water Trees”, IEEE Trans. ET, Vol. 16, August, pp. 319-326, 1981.

[27] A. T. Bulinski, S. S. Bamji and R. J . Densley, “The Effect of Frequency and Temperature on Water Tree Degradation of Miniature XLPE Cables”, IEEE Trans. EI, Vol. 21 No. 4, August, pp. 645-650, 1986.

[28] G. Capaccio, W. Golz and L. J . Rose, “The In- fluence of Morphology and Polymer Structure on the Watertreeing Resistance of Crosslinked Polyeth- ylene”, Elektrotechnische Gesellschaft Fachberichte

[37] T . W. Dakin, “Rapporteur’s Report, Session D, Treeing in Polyethylene”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 197-206, 1974.

[38] 0. Dorlanne, M. R. Wertheimer, A. Yelon and J . R. Densley, “ESR Study of Cu2+ in Polyethylene Containing Electrical and Water Trees”, Annual Re- port conference on Electrical Insulation and Dielec- tric Phenomena, pp. 136-143, 1980.

[39] R. M. Eichhorn, “Treeing in Solid Extruded Electri- cal Insulation”, Electronics & Power, February, pp. 125-131, 1978.

ence Upon the Breakdown Strength of EPR Materi- als”, Jicable 87, International Conference on Poly- mer Insulated Power Cables, Paris, pp. 193-198, 1987.

[29] E. A. Cherney, “Water Trees in Electrical Insula- tion” , Ontario Hydro Research Quarterly, 3d quar- ter, pp. 7-12. 1973.

[30] B. H. Crichton, M. J . Given, 0. Farish and H. M. Bamford, “Measurement of Water Tree Growth by Instrumental Neutron Activation Analysis”, Journal of Physics, Part D: Applied Physics, Vol. 16, pp. L223-L226, 1983.

[31] W . P. Crine, S. Pelissou, H. St-Onge, J . St-Piere, G. Kennedy, A. Houdayer and P. Hinrichsen, “El- emental and Ionic Impurities in Cable Insulation and Shields” , Jicable 87, International Conference on Polymer Insulated Power Cables, Paris, pp. 206- 213, 1987.

[32] J . D. Cross and J. Y. Koo, “Some Observations on the Structure of Water Trees”, IEEE Trans. EI, Vol. 19 NO. 4, August, pp. 303-306, 1984.

[33] J . Densley, “Water Treeing in HV Cable Insulation”, IEEE Canadian Communications and Power Confer- ence, pp. 269-270, 1974.

[41] E. Favrie and H. Auclair, “Effect of Water on Elec- trical Properties of Extruded Synthetical Insulations Application on Cables”, IEEE Trans. PAS, Vol. 99, No. 3 May/June, pp. 1225-1234, 1980.

[42] R. F. Fedors, “Water-treeing as an Osmotic Phe- nomenon”, Polymer, Vol. 21, August, pp. 863-865, 1980.

[43] A. W. Field and V. A. A. Banks, “Development of Water Tree Retardant Crosslinked Polyethylene Ca- ble”, The 6th BEAMA International Electrical In- sulation Conference, Session 7, Brighton, May, pp.

[44] J . C. Filippini and C. T . Meyer, “Effect of Frequen- cy on the Growth of Water Trees in Polyethylene”, IEEE Trans. EI, Vol. 17 No. 6, December, pp. 554- 559, 1982.

[45] J . C. Filippini, C. T. Meyer and M. El-Kahel, “Some

254-258, 1990.

Mechanical Aspects of the Propagation of Water Trees in Polyethylene” , Annual Report Conference on Electrical Insulation and Dielectric Phenomena,

[34] J . Densley, A. Bulinski and T . Sudarshan, “The Ef- fects of Water Immersion, Voltage and Frequency on the Electric Strength of Miniature XLPE Cables”, Annual Report Conference on Electrical Insulation pp. 629-637, 1982.

and Dielectric Phenomena, pp. 469-479, 1979

[35] L. A. Dissado, S. W . Wolfe and J . C. Fothergill, “A Study of the Factors Influencing Water Tree Growth”, IEEE Trans. EI, Vol. 18 No. 6, December, pp. 565-585, 1983.

[36] L. A. Dissado, S. V. Wolfe, J . C. Fothergill, T. Jones and S. M. Rowland, “The Distribution in Length of Vented Water Trees”, IEEE Trans. EI, Vol. 21 No. 4, August, pp. 651-655, 1986.

[46] J . C. Filippini, J . Y. Koo, Y. Poggi, C. Laurent, C. Mayoux and S. Noel, “Aborescences d’Eau et Aborescences Electriques (Watertreeing and electrical treeing)”, Jicable 84, International Conference on Polymer Insulated Power Cables, Paris, March, pp. 87-92, 1984.

[47] J . C. Filippini, J . Y. Koo and J . L. Chen, “Electrode Influence on the Properties of Water Trees in Poly- ethylene”, IEEE Trans. EI, Vol. 12 No. 1, February, pp. 75-82, 1989.


P. Fischer, E. F . Peschke, R. G. Schroth and A. A. Farkas, “Development and Realization of Watertree Retardant XLPE Compounds and Cables”, CIGRE Symposium Vienna, 1987.

E. J . Fisher and B. MC. Clung, “Long-life Insula- tion for Industrial and Utility Cables”, IEEE Trans. IA, Vol. 22, No. 5, September/October, pp. 946-951, 1986.

R. Fourni, J. Perret, P. Recoup and Y. Le Gall, “Wa- ter Treeing in Polyethylene for HV Cables”, Confer- ence Record of 1978 IEEE International Symposium on Electrical Insulation, pp. 110-115, 1978.

E. A. Franke, J . R. Stauffer and E. Czekaj, “Wa- ter Tree Growth in Polyethylene under dc Voltage Stress”, IEEE Trans. EI, Vol. 1 2 No. 3, June, pp. 218-223, 1977.

H. Franke, H. Heumann and D. Kaubisch, “Testing Possibilities and Results Regarding Water Aging of PE/XLPE Insulated Medium Voltage Cables”, Jica- ble 84, International Conference on Polymer Insulat- ed Power Cables, Paris, March, pp. 113-118, 1984.

D. Fredrich and W. Kalkner, “On the Examination of the Watertreeing Behavior of XLPE-insulated Ca- bles”, Conference report of the Fifth International Symposium on HV Engineering. Paper no. 21-04. Braunschweig, Federal Republic of Germany, August 24-28, 1987.

H. Fukagawa, T. Okamoto, N. Hozumi and T. Shi- bata, “Developments of Methods to Estimate the Residual Life of XLPE Cables Deteriorated by Wa- tertrees”, Jicable 87, International Conference on Polymer Insulated Power Cables, Paris, pp. 457-462, 1987.

A. Garton, R. J . Densley and A. Bulinski, “Initial Results of a Study of Water Trees in XLPE by In- frared Spectroscopy”, IEEE Trans. EI, Vol. 15 No. 6, December, pp. 500-501, 1980.

A. Garton, S. Bamji, A. Bulinski and J . Densley, “Oxidation and Water Tree Formation in Service- aged XLPE Cable Insulation”, IEEE Trans. EI, Vol. 22 NO. 4, August, pp. 405-412, 1987.

W. S. M. Geurts, “Veroudering van Kabels met Iso- latie van Lage-dichtheids Polytheen” , Research report. Ref. I11 7722/85, N. v. KEMA, Arnhem, The Netherlands, (in Dutch) 1985.

T . Gloger and R. von Olshausen, “Die Auswirkungen von Water Trees auf die Gleich- und Stossspannungs- festigkeit von Polyethylen- Modelprufkorpern”, Electrotechnische Gesellschaft Fachberichte 16, VDE-Verlag GmbH, Berlin, pp. 135-139, 1985.

[59] W. Golz, “Water-tree Growth in Low-density Pol- yethylene”, Colloid and Polymer Sci, Vol. 263, pp. 286-292, 1985.

[60] P. Gronefeld, R. von Olshausen and F. Selle, “Fehler- erkennung und Isolationsgefahrdung bei der Prufung water tree-haltiger VPE-Kabel mit Spannungen unterschiedlicher Form”, Elektrizititswirtschaft, Jg. 84, Vol. 13, pp. 501-506, 1985.

[61] K. Harasawa, Y. Shimamura, H. Fugagawa, K. Soma and M. Marumo, “Influence of dc Voltage Applica- tion on Dielectric Performances of XLPE Cables”, Jicable 87, International Conference on Polymer In- sulated Power Cables, Paris, pp. 125-132, 1987.

[62] H. J . Henkel, W. Kalkner and N. Muller, “Elec- trochemical Treeing - Strukturen in Modelkabeliso- lierungen aus Thermoplastischem oder Vernetztem Polyet hylen” , Siemens Forschungs und Entwicklungs Bericht, Vol. 10, No. 4, Springer Verlag, pp. 205-214, 1981.

[63] H. J . Henkel and N. Muller, “Additive zur Inhibie- rung von Wasserbaumchen in Polyolefinen fur Kabel- isolierungen”, Electrotechnische Gesellschaft Fach- berichte 16, VDE-Verlag GmbH, Berlin, pp. 132- 134, 1985.

[64] A. A. Hossam Eldin and Z. El Shazli, “A Study of Water Tree Frequency Dependence and Growth Rate in Polymer”, Annual Report Conference on Electri- cal Insulation and Dielectric Phenomena, pp. 638- 643, 1982.

[65] E. Ildstad, Water Migration and Watertreeing in crosslinked Polyethylene Cables, Dissertation. Uni- versity of Trondheim, Norway, 1982.

[66] S. Isshiki, M. Yamamoto, S. Chabata, T. Mizoguchi and M. Ono, “Water Tree in Crosslinked Polyethyl- ene Power Cables”, IEEE Trans. PAS, Vol. 93, pp. 1419-1429, 1974.

[67] J . F. Johnson, J . H. Groeger, M. S. Mashikian, B. S. Bernstein and A. R. Cooper, “Sensitive Analyt- ical Methods for Early Diagnosis of Aging in Solid Insulation under Voltage Stress”, Confkrence Inter- nationale des Grandes R6seaux Electriques A Haute Tension (CIGRE), session August/September, Paper 15-01, 1988.

[68] S. Kageyama, M. Ono and S. Chabata, “Microvoids in Crosslinked Polyethylene Insulated Cables”, IEEE Trans. PAS, Vol. 94, No. 4, July/August, pp. 1258- 1263, 1975.


[69] W. Kalkner, U. Muller, E. Peschke, H. J . Henkel and R. von Olshausen, “Watertreeing in PE and XLPE Insulated HV Cables” , International Conference on Large HV Electric Systems (CIGRE). Paper No. 21- 07, 1982.

[70] K. C. Kao and W. Hwang, Electrical Transport in

[79] J. W. Kirkland, R. S. Thiede and R. A. Reita, “Eval- uating the Service Degradation of Insulated Pow- er Cables”, Conference paper (81 TD645-1) of the IEEE PES, Transmission & Distribution Conference and Exposition, Minneapolis, Minnesota, Sept. 20- 25, pp. 2128-2136, 1981.

Solids, (with particular reference to organic semi- conductors) International Series in the Science of the Solid State, Vol. 14, Pergamon Press, 1981.

[8O] K. D. Kiss, H. C. Doepken and N. Srinivas, “Ag- ing of Polyolefin Electrical Insulation”, In Durability of Macromolecular Materials, ed.: R. K. Eby, ACS

[71] M. Karakelle and P. J . Philips, “The Influence of Structure on Water Treeing in Crosslinked Polyeth- ylene - Tree Morphology and the Influence of Organ- ic Contaminants”, IEEE Trans. EI, Vol. 24, No. 6, Dec., pp. 1101-1108, 1989.

Symp. Ser. 95, pp. 433-466, 1979.

[81] J. Y. KOO, J . D. Cross, M. El-Kahel, C. T . Meyer and J . C. Filippini, “Electrical Behavior and Struc- ture of Watertrees in Relation to their Propagation”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 301-306, 1983. [72] H. Karner, U. Stietzel, M. Saure and W. Golz, “De-

termination of Small Water Contents in Solid Or- ganic Insulating and the Influence Of [82] F. H. Kreuger, Detection and Location ofDjscharges:

in particular in Plastic-insulated HV Cables, Thesis Delft University, 1960.

ture on the Dielectric Properties”, Conference Inter- nationale des Grandes Reseaux Electriques a Haute Tension, CIGRE, session September, Paper 15-02, 1984.

[73] H. Kato, N. Maekawa, S. Inoue and H. Fujita, “Ef- fect and Mechanism of Some New Voltage Stabiliz- ers for Crosslinked Polyethylene Insulation”, Annual Report Conference on Electrical Insulation and Di- electric Phenomena, pp. 229-238, 1974.

[74] C. Katz and B. S. Bernstein, “Electrochemical Tree- ing a t Contaminants in Polyethylene and Crosslinked Polyethylene Insulation” , Annual Report Conference on Electrical Insulation and Dielectric Phenomena,

[75] C. Katz, N. Srinivas and B. S. Bernstein, “Growth Rate of Electrochemical Trees in PE and XLPE In- sulation Systems: Effect of Water Nature”, Annual Report Conference on Electrical Insulation and Di- electric Phenomena, pp. 279-288, 1974.

pp. 307-316, 1973.

[76] C. Katz, G. S. Eager J r , E. R. Leber and F. E. Fis- cher, “Influence of Water on Dielectric Strength and Rejuvenation of In-service Aged URD Cables” , Jica- ble 84, International Conference on Polymer Insulat- ed Power Cables, Paris, March, pp. 127-134, 1984.

[77] H. S. Kaufmann and H. J . Falcetta (ed), Introduc- tion to Polymer Science and Technology, An SPE Textbook. LC 76-16838. Society of Plastics Engi- neering Monographs. Wiley, New York, 1977.

[78] K. Kawahara, T. Yoshimitsu, Y. Ishikawa, H. Aka- hori and H. Yoshida, “The Effect of Temperature on the Electrical Breakdown Voltage of XLPE in the Presence of Water Trees”, Conference record of 1984 IEEE International Symposium on Electrical Insula- tion, Montreal, June 11-13, pp. 33-36, 1984.

[83] F. H. Kreuger and A. M. F. J van de Laar, “Silane Crosslinked Medium Voltage Cable”, Cired 85, Inter- national conference on electricity distribution, Part I. IEE Conference Publication Nr. 250. IEE, London, pp. 231-233, 1985.

[84] F. H. Kreuger, Partial Discharge Detection in HV Equipment, Temple Press, London, 1964 & Butter- worths, London, 1989.

[85] A. M. F. J . van de Laar, “Silane Crosslinked Power Cable: An Investigation of Cable Properties”, Inter- national Conference on Large HV Electric Systems, CIGRE. Paper no. 21-2, 1982.

[86] P. B. Larsen, “Methodes de Coloration d’utilisdes des Arborescences d’Eau dans les Isolation Extrudees de C5bles”, Electra, Vol. 86, pp. 53-59, 1983.

[87] R. Lyle and J . W. Kirkland, “An Accelerated Life Test for Evaluating Power Cable Insulation”, IEEE Trans. PAS, Vol. 100, No. 8, August, pp. 3764-3771, 1981.

[88] L. Mandelkern, M. Glotin and R. A. Benson, “Su- permolecular Structure and Thermodynamic Prop- erties of Linear and Branched Polyethylenes under Rapid Crystallization Conditions”, Macromolecules, Vol. 14, pp. 22-33, 1981.

[89] T. G. Marsh, A. P. Smith and J . Drysale, “Long Term Aging of Water Immersed XLPE Insulations”, Jicable 87, International Conference on Polymer In- sulated Power Cables, Paris, pp. 181-186, 1987.


[go] J . H. Mason, “The Deterioration and Breakdown of Dielectrics Resulting from Internal Discharges”, Proc. Instn. Eng. Electr. Part I, Vol. 98, pp. 44-59, 1951.

[91] J. H. Mason, “Breakdown of Insulation by Dis- charges”, Proc. Instn. Eng. Electr. Part IIa, Vol. 100, Nr. 3, pp. 149, 1953.

[92] G. Matey, and J . D. Umpleby, “The Development of a Water Tree Retardant XLPE Insulation”, Dauer- verhalten von Hochspann ungsisolier ungen, Elec tro- technische Gesellschaft Fachberichte 16, VDE-Verlag GmbH, Berlin, pp. 170-173, 1985.

[93] H. Matsuba and E. Kawai, “Water Tree Mechanism in Electrical Insulation”, IEEE Trans. PAS, Vol. 95, No. 2, March/April, pp. 660-670, 1976.

[94] H. Matsuba, E. Kawai and K. Sato, “Water Tree Mechanism and its Suppression”, Conference Records (CH 1496-9/80) International Symposium on Electrical Insulation, pp. 224-227, 1976.

[95] K. Matsuura, H. Ohno, A. Ishibashi, K. Yatsuka, K. Hirotsu, I. Kajiki and J . Shinagawa, “Investiga- tion on Deterioration of 22 to 66 kV XLPE Cables Removed from Actual Service Lines”, Jicable 87, In- ternational Conference on Polymer Insulated Power Cables, Paris, pp. 471-476, 1987.

[96] E. J . McMahon, “A Tree Growth Inhibiting Insula- tion for Power Cable”, IEEE Trans. EI, Vol. 16, No. 4, August, pp. 304-318, 1981.

[97] C. W. Melton, D. Mangaraj and M. Epstein, “Mor- phology of Thermoplastic and crosslinked Polyeth- ylene Cable Insulation”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 299-305, 1981.

[98] C. T. Meyer, J . C. Filippini and N. Felici, “Water Tree Propagation in Relation to Mechanical Proper- ties of Polyethylene”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 374-381, 1978.

[99] C. T. Meyer, “Water Absorption During Water Tree- ing”, IEEE Trans. EI, Vol. 18, No. 1, February, pp. 28-31, 1983.

[loo] L. Minnema, H. A. Barneveld and P. D. Rinkel, “An Investigation into the Mechanism of Water Treeing in Polyethylene HV Cables”, IEEE Trans. EI, Vol. 15, No. 6, December, pp. 461-471, 1980.

[ 1011 T . Miyashita, “Deterioration of Water-immersed Polyethylene Coated Wire by Treeing”, Proceedings 1969 IEEE-NEMA Electrical Insulation Conference, Boston, September, pp. 131-135, 1969.

[lo21 W. J . Moore, Physical Chemistry, Fourth Edition, Inc. Prentice-Hall, Englewood Cliffs, New Jersey, 1972.

[lo31 M. Morita, M. Hanai and H. Shimanuki, “Some Preventive Methods for Water Treeing in PE and XLP Insulation”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 303-312, 1973.

[lo41 M. Morita, M. Hanai, H. Shimanuki, F. Aida and T. Shiono, “Effects of Hydrated Ions on Watertrees in Polyethylene” , Annual Report Conference on Elec- trical Insulation and Dielectric Phenomena, pp. 195- 202, 1980.

[lo51 J . Muccigrosso and P. J . Phillips, “The Morphology of crosslinked Polyethylene Insulation”, IEEE Trans. EI, Vol. 13, No. 3, June, pp. 172-178, 1978.

[lo61 K-B. Muller, “Zum Langzeitverhalten neues VPE-Kabel unter Feuchteeinwirkung”, Elektriz- itatswirtschaft, Jg. 88, Vol. 26, pp. 1861-1871, 1989.

[lo71 N. Muller and H. J. Henkel, “Phanomenologische Aspekte der Wasserbaumchem in Polyolefinen fur Kabelisolierungen”, Electrotechnische Gesellschaft Fachberichte 16, VDE-Verlag GmbH, Berlin, pp. 119-122, 1985.

[lo81 T . Nagabasami, K. Sasaki, K. Sugiyama and M. Ishitobi, “Development of XLPE Cables with New Laminated Waterproof Layer”, Conference Record of 1984 IEEE Symposium on Electrical Insulation, Montreal, June 11- 13, pp. 4 1 4 5 , 1984.

[lo91 S. Nagasaki, H. Matsubara, S. Yamanouchi, M. Ya- mada, T. Matsuike and S. Fukanaga, “Development of Water-tree-retardant XLPE Cables”, IEEE Trans. PAS, Vol. 103, No. 3, March, pp. 536-544, 1984.

1101 Y. Namiki, H. Shimanuki, F. Aida and M. Mori- ta, “A Study on Microvoids and Their Filling in Crosslinked Polyethylene Insulated Cables”, IEEE Trans. EI, Vol. 15 No. 6, December, pp. 473-480, 1980.

1111 R. D. Naybour, “The Growth of Water Trees in Cross Linked Polyethylene at Operating Stresses and Their Influence on Cable Life”, IEE Conference Pub- lication nr. 177, pp. 238-241, 1979.

[112] R. D. Naybour, “The Growth of Water Trees in Dry and Steam Cured Polyethylene a t 1.9 MV/m, 50 Hz and their Influence on Cable Life in the Stress Range 15 to 4 MV/m”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 620-628, 1982.


[I131 A. W. Nicholls and E. F. Steennis, “Water Tree- ing, State of the Art - Progress Report of CIGRE WG 21-11 on Water Treeing in Extruded Cable In- sulations”, 1990 International Conference on Large HV Electric Systems, Joint session paper 15/21-02, CIGRE, Paris, 1990.

[114] Y. Nitta, “A Possible Mechanism for Propaga- tion on Water Trees from Water Electrodes”, IEEE Trans. EI, Vol. 9, No. 3, September, pp. 109-112. 1974.

[115] Y. Nitta and Y. Kawasaki, “Evaluation of a New Material for Semiconducting Layer in XLPE Power Cables”, IEEE Trans. EI, Vol. 15, No. 2, April, pp. 140-143, 1980.

[116] S. L. Nunes and M. T . Shaw, “Water Treeing in Pol- yethylene - A Review of Mechanisms”, IEEE Trans. EI, Vol. 15, No. 6, December, pp. 437-450, 1980.

[117] H. Oonishi, F. Urano, K. Soma, K. Kotani and K. Kamio, “Development of New Diagnostic Method for Hot-line XLPE Cables with Water Trees”, Confer- ence Record 86 SM 397-4 of the IEEE PES 1986 Summer meeting, Mexico City, July, 1986.

[118] H. Orton and K. Abdolall “An Investigation into the Morphology of XLPE Cable Insulation”, Annual Report Conference on Electrical Insulation and Di- electric Phenomena, pp. 306-313, 1981.

[119] M. J . KEOGH, “Patent no. 0057 286 A2, dated De- cember 21, 1981”, Application number: 81110660.8. Applicant Union Carbide Corporation, 1981.

[120] R. Patsch, H. Wagner and H. Heumann, “Imhomo- geneities and their Significance in Single-layer Ex- truded Polyolefine Insulations for Cables” , Interna- tional Conference on Large HV Electric Systems, CI- GRE. Paper 15-11, 1976.

[121] R. Patsch, “Effects of Moisture and Electrical Field Strength on Polymer Insulations”, Colloid Polym. Sci. Vol. 259, pp. 885-893, 1981.

[122] R. Patsch and A. Paximadakis, “Water Trees in Cables - Experimental Findings and Theoretical Ex- planations”, IEEE Conference Record 89 TD 350- 0 PWRD. Presented a t the IEEE PES 1989 Trans- mission and Distribution Conference, New Orleans, Louisiana, April, 1989.

[123] M. Pays, M. Louis, J . Perret, C. Alquie and J. Lewiner, “Behavior of Extruded HVDC Pow- er Transmission Cables: Tests on Materials and Cables”, International Conference on Large HV Electric Systems, CIGRE. Paper 21-07: 1988.

[124] M. Prigent, J . Bobo and I. Eyraud, “Aging of Pol- yethylene in Water under Electrical Stress”, Confer- ence Record on Dielectric Materials, Measurements and Applications. July 21-25, Cambridge, pp. 32-35, 1975.

[125] R. Ross, W. S. M. Geurts and J . J . Smit, “FTIR Microspectroscopy and Dielectric Analysis of Wa- tertrees in XLPE”, IEE Conference publication number 289, Dielectric Materials, Measurements and Ap- plications, pp. 313-317, 1988.

[126] R. Ross, W. S. M. Geurts and J . J . Smit, J . H. van der Maas and E. T. G. Lutz, “Vented Water Trees - Oxidation and Hydrophilic Behavior”, Kema Scientific Technical Reports, Vol. 7, No. 6, pp 357- 364, 1989.

[127] R. Ross, W. S. M. Geurts, J . J . Smit, J . H. van der Maas and E. T . G. Lutz, “The Hydrophilic Na- ture of Watertrees”, Conference record of the 1990 IEEE International Symposium on Electrical Insula- tion, Toronto, Canada, June 3-6, 1990.

[128] J. Rye, P. M. Brown, G. L. Le Poidevin and W. T . Eeles, “Oxidation of Polyethylene as a Possible Prelude to Water Treeing”, IEE Conference Record on Dielectric Materials, Measurements and Applica- tions. July 21-25, Cambridge, pp. 36-39, 1975.

[129] J . A. Sauer and K. D. Pae, “Mechanical Proper- ties of High Polymers”, In Introduction to polymeric science and technology, H. S. Kaufmann, and J . J . Falcetta, SPE Textbook. LC 76-16838. Society of Plastic Engineering Monographs. Wiley, NewYork, NY 1977.

[130] M. Saure and W. Golz, “Uber den Einfluss von Mischungskomponenten auf das Water-Tree Wachs- tum in Polyolefin-Materialen und Prufverfahren zur Bewertung des Water-Tree Wachstums”, Electro- technische Gesellschaft Fachberichte 16 , VDE-Ver lag GmbH, Berlin, pp. 127-131, 1985.

[131] M. Saure and W. Kalkner, “On Water Tree Testing of Materials - Status Report of CIGRE T F 15-06- 05”, CIGRE Symposium 05-87, Vienna, Section 6.2, Number 620-10, 1987.

[I321 M. Saure, W-D. Schuppe, H. Franke and D. Kaubisch, “Die Entwicklung des Mittelspan- nungskabels Rheytard mit watertreeretardierender VPE-Isolierung”, Elektrizitatswirtschaft, Jg. 88,

[I331 B. A. Schipper, “Dynamic Mechanical Thermal Analysis van Kunststoffen”, Research report. Ref. I11 9168/84, N. V. KEMA, Arnhem, The Nether- lands, (in Dutch), 1984.

Vol. 7, pp. 376-381, 1989.


[134] R. G. Schroth, W. Kalkner and D. Fredrich, “Test Methods for Evaluating the Water Tree Aging Be- havior of Extruded Cable Insulations”, 1990 Inter- national Conference on Large HV Electric Systems, Joint session paper 15/21-01, CIGRE, Paris, 1990.

[I351 W. D. Schuppe, “Progress with XLPE Insulated Medium Voltage Cables in the Federal Republic of Germany”, Jicable 84, International Conference on Polymer Insulated Power Cables, Paris, March, pp. 35-39, 1984.

[136] M. T. Shaw and S. H. Shaw, “Water Treeing in Solid Dielectrics”, IEEE Trans. EI, Vol. 19, No. 5, October, pp. 419-452, 1984.

[137] J . Sletbak and A. Botne, “A Study of Inception and Growth of Water Trees and Electrochemical Trees in Polyethylene and Crosslinked Polyethylene Insu- lations”, IEEE Trans. EI, Vol. 12, No. 6, December, pp. 383-388, 1977.

[138] J . Sletbak, “A Theory of Water Tree Initiation and Growth”, IEEE Trans. PAS, Vol. 98, No. 4, Ju- ly/August, pp. 1358-1365, 1979.

[145] N. N. Srinivas, B. S. Bernstein and R. A. Deck- er, Conference Record 89 T D 365-8 PWRD of the IEEE PES Transmission and Distribution Confer- ence, New Orleans, Louisiana, April, 1989.

[146] E. F. Steennis and W. Boone, “Watertreeing in Service Aged and Accelerated Aged XLPE Cables”, Institute of Electrical Engineers (IEE). Conference Power Cables and Accessories 10 to 180 kV, Lon- don, pp. 162-166, 1986.

[147] E. F. Steennis, C. C. van den Heuvel and W. Boone, “Accelerated Aging to Predict Watertree Behavior in Extruded Cables”, Jicable 87, International Confer- ence on Polymer Insulated Power Cables, Paris, pp.

[148] E. F. Steennis, Water Treeing, the Behavior of Wa- ter Trees in Extruded Cable Insulation, Thesis Uni- versity Delft. ISBN 90-353-1022-5; KEMA, Arn- hem, 1989.

[149] E. F. Steennis and A. M. F. J. van de Laar, “Char- acterization Test and Classification Procedure for Water Tree Aged Medium Voltage Cables”, (On behalf of Working Group 21-11 of CIGRE). Electra, Vol. 125, July, pp. 89- 101, 1989.

161-167, 1987.

[I391 J . Sletbak and E. Ildstad, ‘IThe Effect Of Service [150] E. F. Steennis, w . B~~~~ and A. ~ ~ ~ t f ~ ~ ~ t , uwa- ter Treeing in Service Aged Cables, Experience and Evaluation Procedure”, IEEE Trans. PES, Vol. 5, No. 1, January, pp. 40-46, 1990.

and Test Conditions on Water Tree Growth in XLPE Cables”, IEEE Trans. PAS, Vol. 102, No. 7, July, pp. 2069-2076, 1983.

[140] J . Sletbak and E. Ildstad, “The Validity of the Me- chanical Damage Theory of Water Treeing Tested Against Experimental Results”, Conference Record of the 1984 IEEE International Symposium on Elec- trical Insulation, Montreal, June 11-13, pp. 29-32, 1984.

[141] K. Soma and S. Kuma, “Development of Bow-tie Tree Inhibitors”, Conference Record of the 1980 IEEE International Symposium on Electrical Insu- lation, pp. 212-215, 1980.

[142] N. N. Srinivas, S. M. Allam and H. C. Doepken Jr , “The Effect of crosslinking and crosslinking Agent by Products on Tree Growth in Polyethylene”, An- nual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 380-385, 1976.

[143] N. N. Srinivas, H. C. Doepken, A. L. McKean and M. C. Biskeborn, “Electrochemical Treeing in Ca- ble”, EPRI Final Report EL-647, 1978.

[144] N. N. Srinivas, “Estimation of Life Expectancy of Polyethylene-insulated Cables”, EPRI Final Report EL-3154, 1984.

[151] S. G. Swingler, R. J . Jackson and J . Drysdale, “The Dielectric Response of Polyethylene Cable Contain- ing Water Trees”, Conference University of Lancast- er, 10-13 September, pp. 183-186, 1984.

11521 T . Tabata, T. Fukuda and Z. Iwata, “Investigations of Water Effects on Degradation of Crosslinked Pol- yethylene Insulated Conductors”, Paper 71 TP545- PWR, IEEE Summer Meeting and International Symposium on High Power Testing, Ore. Portland,

[153] T. Tabata, H. Nagai, T. Fukuda and Z. Iwata, “Sul- fide Attack and Treeing of Polyethylene Insulated Cables - Cause and Prevention”, IEEE Trans. PAS, Vol. 91, Nr. 4, pp. 1354-1360, 1972.

[154] T . Tanaka and T. Fukuda, “Residual Strain and Water Trees in XLPE and PE Cables”, Annual Re- port Conference on Electrical Insulation and Dielec- tric Phenomena, pp. 239-249, 1974.

[155] T . Tanaka, T. Fukuda, S. Suzuki, Y. Nitta, H. Go- to and K. Kubota, “Water Trees in crosslinked Poly- ethylene Power Cables”, Paper T73497-5. IEEE PES Summer Meeting & EHV/UHV Conference, Vancou- ver, BC Canada, July 15-20, pp. 693-702, 1974.

July 18-23, pp. 1361-1370, 1971.


[156] T. Tanaka, T . Fukuda and S. Suzuki, “Water Tree Formation and Lifetime Estimation in 3.3 kV and 6.6 kV XLPE and PE Power Cables”, IEEE Trans. PAS, Vol. 95, No. 6, Nov/Dec, pp. 1892-1900, 1976.

[157] T. Tanaka and A. Greenwood, Advanced Power Ca- ble Technology, Volume 11, Present and Future, CRC Press, Inc. Boca Raton, Florida, 1983.

[158] D. R. Thoman, L. J . Bain and C. E. Antle, “In- ferences on the Parameters of the Weibull Distribu- tion”, Technometrics, Vol. 11, No. 3, August, pp. 445-460, 1969.

11591 W. R. Tinga, W. A. G. Voss and D. F. Blossey, “Generalized Approach to Multiphase Dielectric Mixture Theory”, Journal of Applied Physics, Vol. 44, No. 9, September, pp. 3897-3902, 1973.

[160] D. M. Tu and K. C. Kao, “Effects of Hydrostatic Pressure on Water Treeing Properties of Polyethyl- ene”, Annual Report Conference on Electrical Insu- lation and Dielectric Phenomena, pp. 307-31 1, 1983.

[161] J . D. Umpleby, E. P. Marsden, R. J . Turbett and A. Mendelsohn, “Approaches to WTGR Crosslinked Polyethylene”, IEE, Science, Education and Tech- nology Division. Synopses of the opening remarks. Discussion meeting on “The Mechanisms of Water Treeing in Polymeric Insulations”, March 14, 1988.

[162] W. Vahlstrom Jr , “Investigation of Insulation De- terioration in 15 kV and 22 kV Polyethylene Cables Removed from Service”, Paper 71 C42-PWR, IEEE PES Underground Distribution Conference, Mich. Detroit, Sept 27- 30, pp. 1023-1035, 1971.

[163] H. Wagner, “Pseudo-spherulite Structures in crosslinked Low-density Polyethylene” , IEEE Trans. EI, Vol. 13 No. 2, April, pp. 81-86, 1978.

[164] R. C. Weast (ed), Handbook of Chemistry and Physics, 61st edition 1980-1981, CRC Press. Inc. Bo- ca Raton, Florida 1980.

[165] K-H. Weck, “Stufentest eur Ermittlung des Isolationszustands betrieblich vorbeanspruchter PE- und VPE-Mittelspannungskabel”, Elektrie- itatswirtschaft, Jg. 88, Vol. 8, pp. 470-473, 1989.

[166] J . A. Wiersma, “Watertreeing in Cables with Ex- truded Insulation”, Electra, Vol. 55, pp. 25-38, 1977.

[167] W. D. Wilkens, “Thermal Gradient Effects in Pow- er Cable Insulation in Wet Environments”, Annual Report Conference on Electrical Insulation and Di- electric Phenomena, pp. 260-269, 1974.

[168] S. Wojtas, “Investigations of Polyethylene Insula- tion Resistivity of Power Cables”, Jicable 87, Inter- national Conference on Polymer Insulated Power Ca- bles, Paris, pp. 436-440, 1987.

[169] H. Y. Wong, Heat Transfer for Engineers, Longman Group Limited, Inc. Longman New York, 1977.

[170] Y. Yamada, S. Yamanouchi and S. Miyamoto, “Treeing Phenomena in XLPE Insulation under dc Voltage” , Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 500-510, 1979.

[171] T. Yoshimitsu and T. Nakakita, “New Findings on Water Tree in High Polymer Insulating Materials”, Conference Record, IEEE International Symposium on Electrical Insulation, pp. 116-121, 1978.

[172] T. Yoshimitsu, H. Mitsui, K. Hishida and H. Yoshida, “Water Treeing Phenomena in Humid Air”, IEEE Trans. EI, Vol. 18, No. 4, August, pp. 396-401, 1983.

[173] T . Yoshimitsu, H. Mitsui, S. Kenjo and T . Nakaki- ta, “Some Considerations on ac Watertrees in Crosslinked Polyethylene”, IEEE Trans. EI, Vol. 18, No. 1, February, pp. 23-27, 1983.

[174] N. Yoshimura, F. Noto and K. Kikuchi, “Growth of Watertrees in Polyethylene and Silicone Rubber by Water Electrodes”, IEEE Trans. EI, Vol. 12, No. 6, December, pp. 411-416, 1977.

1751 N. Yoshimura and F. Noto, “Voltage and Frequency Dependence of Bow-tie Trees in Crosslinked Polyeth- ylene”, IEEE Trans. EI, Vol. 17, No. 4, August, pp. 363-367, 1982.

1761 R. H. Zeller, “Thermodynamics of Watertreeing”, IEEE Trans. EI, Vol. 22, No. 6, December, pp. 677- 681, 1987.

Manuscript was received on 27 Jun 1990, in revised form 1 9 Sep 1990.

water treeing in polyethylene cables

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