WATER TREEING
the behaviour of water trees
in extruded cable insulation
TR. diss
Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus, prof. drs. P.A. Schenck, in het openbaar te verdedigen ten overstaan van een
commissie door het College van Dekanen daartoe aangewezen, op donderdag
8 juni 1989, te 14.00 uur.
Door
Evert Frederik Steennis
elektrotechnisch ingenieur
geboren te Ede
TR diss
1735
behorende bij het proefschrift van
Evert Frederik Steennis
1. Het verdient aanbeveling de storingsstatistiek voor middenspannings kunststof kabels niet slechts te baseren op het aantal fouten per kilometer geïnstalleerde kabel per jaar, maar eveneens op de vervangen kabellengte per geïnstalleerde kilometer per jaar.
2. De dielectrische eigenschappen van een waterboom komen overeen met die van een isolator. De waterboom als isolator is evenwel inferieur aan de in de elektrische energie techniek gebruikelijke isolatie materialen.
3. Het is zeer riskant te veronderstellen dat de groeisnelheid van "vented trees" vlak na kunstmatige initiatie door middel van waternaalden maatgevend is voor de groeisnelheid van deze waterbomen onder praktijk condities.
4. Het is niet aangetoond dat de groeisnelheid van "bow-tie trees" uitsluitsel geeft over de groeisnelheid van de veel gevaarlijker "vented trees". Uitspraken over de waterboom gevoeligheid van een isolatie materiaal welke gebaseerd zijn op de "bow-tie tree" groei zijn daarom aanvechtbaar.
5. Bij de vorming van "vented trees" in een kabel is niet het vochtgehalte in de isolatie, maar het vochtgehalte buiten deze isolatie is van wezenlijke betekenis. 6. Het komt de electriciteilssector ten goede indien de test- en keuringsinstituten in deze sector in hun programma van onderzoek bedrijfsmiddelen opnemen en dit onderzoek niet uitsluitend aan de desbetreffende leveranciers overlaten. 7. Men aanvaardt een feitelijke machteloosheid indien men een uitgesproken
mening heeft over zaken als apartheid, bewapening en/of aantasting van het milieu en men tegelijk voor zijn spaartegoed slechts is geïnteresseerd in de hoogte van de rente, en men buiten beschouwing Iaat hoe deze rente door de betrokken instelling wordt gerealiseerd.
8. Een weinig polulaire maar effectieve methode om de druk op het milieu te verminderen wordt verkregen indien bedrijven en instellingen voor zakenreizen niet de autokosten maar de kosten van het openbaar vervoer vergoeden. De veelal ingebrachte tegenwerping dat het gebruik van het openbaar vervoer tijdrovend en dus weinig effectief is, is aanvechtbaar indien de reistijd wordt benut voor overleg, voorbereiding, verslaggeving en/of studie.
9. Het berust op een misverstand dat een paspoort of een waardepapier fraudebestendig kan worden gemaakt.
10. Een uniforme richtlijn is gewenst, die vastlegt dat het tikpunt van de dirigeerslag en de tactus van de muziek samenvallen.
11. Daar waar het in de sport gaat om te winnen, prefereert men in de muziek een gelijk spel.
CONTENT
SUMMARY 7
CHAPTER 1 GENERAL INTRODUCTION
1.1 General aspects 9 1.2 Water treeing 10 1.3 Object of study 13
CHAPTER 2 POLYETHYLENE INSULATION
2.1 Polyclhylene structure 15 2.2 Mechanical properties 19 2.3 Fracture in polymers 22 2.4 Extruded cable insulation 24
2.4.1 Extrusion 24 2.4.2 Cross-linking 26 2.4.3 Voids 26 2.5 Water in polyethylene 29
2.5.1 General aspects 29 2.5.2 Water in voids with pure polyethylene walls 30
2.5.3 Capillary action 31 2.5.4 Osmosis 34
CHAPTER 3 PHENOMENOLOGY
3.1 Introduction 37 3.2 Morphoiogy of water trees 39
3.2.1 Shape 39 3.2.2 Voids and channels 42
3.2.3 Water content 43 3.2.4 Typical growth behaviour of water trees 43
3.3 Dielectrical propcrties (local) 45
3.3.1 Definitions 45 3.3.2 The vented tree: an insulating material 45
3.3.3 Conclusion 47 3.4 Physical/Chemical propcrties (local) 48
3.5 Electrical propcrties (bulk) 51 3.6 Effect of electric stress intensity 57
3.7 Effect of frequency 59 3.8 Effect of temperature 62 3.9 Effect of mechanical stress 64 3.10 Effect of relative humidity 65 3.11 Effect of the chemical nature of the fluid 66
3.12 Effect of insulating material and additives 69 3.13 Effect of morphology of the insulating material 73
CHAPTER 4 PHENOMENOLOGY, A SUMMARY 75
CHAPTER 5 POSSIBLE MECHANISMS OF VENTED TREE GROWTH
5.1 Introduction 77 5.2 Ageing conditions 77 5.3 Possible mechanisms 80
5.3.1 Introduction 80 5.3.2 Osmosis and capillary action 80
5.3.3 Coulomb forces 81 5.3.4 Dielectrophoresis 84 5.3.5 Thermal degradation 87 5.3.6 Partial discharges 90 5.3.7 Electrochemical degradation 91 5.4 Conclusions 92
CHAPTER 6 ELECTROCHEMICAL DEGRADATION
6.1 Introduction 93 6.2 Initiation 93
6.2.1 Creation of polar amorphous regions 93
6.2.2 Water intake 96
6.3 Growth 97 6.3.1 Introduction 97
6.3.2 Further degradation within the tree 98
6.3.3 Electrolysis 99 6.3.4 Electrochemical degradation 101
6.4 Propagation rate 102 6.5 Theory and phenomenology 106
6.5.1 Agreements 106 6.5.2 Remaining aspects 110 6.5.3 Bow-tie trees 111 6.6 Measures to suppress vented treeing 112
6.6.1 Suppression of initiation 112
6.6.2 Water-blockings 112 6.6.3 Suppression of propagation 113
6.7 Conclusions 115 6.8 Further study 116
CHAPTER 7 CHARACTERIZATION TEST
7.1 Introduction 117 7.2 Description of the characterization test 119
7.2.1 Introduction 119 7.2.2 Method of sampling 121 7.2.3 Pre-conditioning 122 7.2.4 Breakdown tests 122 7.2.5 Observed causcs of breakdown 123
7.2.6 General visual inspection 127
7.3 Typical results 129 7.4 Classification procedure 135
7.5 Conclusions 136
CHAPTER 8 INVESTIGATION OF A CABLE NETWORK
8.1 Introduction 137 8.2 Cable characteristics 137
8.3 Rate of degradation 138 8.4 Degradation versus cable characteristics 140
8.4.1 Soil conditions 140 8.4.2 Type of semiconducting outer screen 141
8.4.3 Method of cross-linking 142 8.4.4 Outer shcath malcrial 142
8.4.5 Manufacturer 143 8.4.6 Mean ageing stress level, ageing time and year of cable
production 147 8.5 Conclusions 148
CHAPTER 9 ACCELERATED AGEING PROCEDURES
9.1 Introduction 149 9.2 Experiment 149 9.3 Rate of degradation 153
9.4 Conclusions 156 9.5 Further study and recommendations 157
APPENDICES
Appendix 1 methylene blue dyeing procedure 159 I. Preparation of a methylene blue dye solution 159 II. Use of the methylene blue dye solution 159 UI. Hints for handling methylene blue 160 Appendix 2 electric stresses at the vented tree tip 161
I. Axial and radial electric stresses in the polyethylene at the tree tip 162 II. Ratios of the radial and axial electric stresses in the polyethylene at
the tree tip 163 UI. Axial electric stress on the axis of symmetry 164
IV. Axial electric stress in the tree at the tree tip 165
Appendix 3 cable properties 166 Appendix 4 vented tree initiation site, cxample 167
REFERENCES: 169 LIST OF SYMBOLS 181 LIST OF FIGURES 185 LIST OF TABLES 189 AUTHOR INDEX 191 SUBJECT INDEX 193 SAMENVATTING 199 NAWOORD 201 CURRICULUM VITAE 202
SUMMARY
This study deals with the behaviour of water trees in extruded cabies, especially in medium-voltage cabies. It is the aim of the study to review the phenomenology of water treeing, to study the mechanisms of vented tree growth, to fmd methods to evaluate the rate of cable degradation by water treeing and to develop an accelerated ageing procedure necessary to arrive at a discrimination between cable insulation with a low or a high water tree susceptibility.
In Chapter 1 a general introduction is given.
A study of the various aspects of water treeing is possible only if it is based on general knowledge of the structure of polyethylenc insulation and the behaviour of water in this material. Information is given in Chapter 2.
Phenomenological data are given in Chapter 3 and summarized in Chapter 4. Morphological aspects of water trees and their impact on the insulating material properties are studied. Moreover, the effects are studied of electric stress, voltage frequency, temperature, mechanical stress, relative humidity, chemistry and morphological parameters of the insulation on water tree growth. One of the main conclusions is that a vented tree can be considered as an insulating material. Possible mechanisms of vented tree growth are studied in Chapter 5. The conclusion is that osmosis, capillary action, Coulomb forces, thermal degradation, partial discharges and dielectrophoresis are not the cause of vented tree growth, although in a few cases they may play a secondary role. Electrochemical degradation as the cause of vented tree growth is studied in Chapter 6. It is found that the effects of electrochemical degradation agree with the phenomenology as presented in previous Chapters. Measures to suppress water tree growth are discussed.
A test procedure to establish the level of degradation of a cable is developed and is presented in Chapter 7. On the basis of this test procedure an investigalion of a medium-voltage extruded cable network was carried out. The rate of degradation and its relation to construction and ageing parameters are discussed and presented in Chapter 8.
The development of an accelerated ageing procedure is described in Chapter 9. The choice of the ageing parameters based on phenomenological experience appears to be successful. The most effective ageing parameters are related to solutes in the water and the voltage frequency.
1 GENERALINTRODUCTION
1.1 General aspects
Underground cables are essential for the transmission of electric power, particuiarly in The Netherlands, where nearly 100 % of the total low-voltage (< 1 kV) and medium-voltage (10 to 30 kV) distribution network is buried in the ground. This percentage is high in comparison with other countries (Boone et al, 1987).
Medium-voltage cable
The total circuit length of medium-voltage cable buried in Dutch soil amounts about 90,000 km. The network represents a value of approximately NLG 6.510 , calculatcd for ncw cable including costs of assembling and laying.
The first medium-voltage cables werc installed in London in 1890 (Allister, 1982). Traditionally most of the medium-voltage cables have a mass-impregnated paper insulation. Since the Second World-War, however, polycthylene (PE) insulation is an interesting alternative. This is because of its favourable dielectric characteristics such as low loss angle and low relative permittivity, expected longevity, simple handling, a potcntially lower price and especially if the polyethylene is cross-linked (XLPE) a highcr maximum operating temperature. Installation of polycthylene cables in the United States eind Japan commcnced in the sixties, in Europc in the seventies. Nowadays, when there is an extension of the distribution network or a replacement of a part of this network, polymer insulatcd cable is often chosen, with an exception for the 10 kV voltage range. This exception is mainly due to the fact that, because of its sophisticated concept, the 10 kV mass-impregnated cable is economically still more attractive than the polymer insulated alternative. Only for heavily loaded circuits in this voltage range extruded cable is to be preferred from a technical and economical point of view.
The above mentioned facts form the cause why the total circuit length of medium-voltage cross-linked polyethylene insulatcd cables in The Netherlands, where the principal distribution voltage is 10 kV, is restricted to about 1800 km, while in many other industrialized countries, where a higher distribution voltage is used, the rate of penetration is much higher.
High-voltage cable
Extruded high-voltage cable (50 kV to 380 kV) is economically competitive with conventional high-voltage cable types such as the oil-pressure cable. In the voltage range of 50 kV up to 150 kV the extruded cable is a highly interesting alternative. In
addition to the advantages mentioned above, the extruded cable is attractive from an environmental poinl of view; it may bc chosen as an alternativc for oil-fillcd cable, where leakage of oil cannot fully be prevcnted. In the voltage range over 150 kV the devclopment of extruded cable is still going on. Here progrcss is also determincd by the devclopment of the accessories.
The various cable components are defined in Figurc 1.1 where an extruded cable is presentcd.
Ixgend
1 conductor
2 semiconducting inner screen 3 insulaiion
4 semiconducting outer screen 5 conductive tape
6 carth screen 7 lapping 8 outer shcath
F i g u r e 1.1 Basic components of an extruded cable
1.2 Water treeing
The introduction of extruded cables in the second half of this century was accompanicd by the idea that water or water-vapour would not affect the electrical properties of the cable. Therefore, the cables were not providcd with water or water-vapour impervious outer sheaths; often a PVC (polyvinyl chloride) outer shealh over the core screen and earth screen was chosen, while a metal shealh was in most cases absent. Unlike cable practice in Japan and Europe, in the Uniicd Statcs extensive lengths of extruded cables have been installcd even without a plastic outer sheath. To prevent water ingress, a polyvinyl chloride outer sheath is not very effective. Such an outer shealh is rather weak, which has often resulted in shcath failures enabling ground water to rcach the graphited semiconducting outer screen. Moreovcr, during cable-laying water penetration into the stranded conductor has not always been avoided.
As all (amorphous or partly amorphous) plastics, the polyvinyl chloride outer shcath and the polyethylcne insulation are water-vapour permeable. Therefore water-vapour could rcach the insulation and the semiconducting sereens by means of diffusion. It can bc concludcd that the insulation of many extruded cables, installcd in the sixlies and seventies, was or is bcing exposed to water in different ways.
The discovery of dcgradation of polyethylene by means of a combined action of water and an clcctric stress was publishcd by Miyashita and presented at the Electrical I n sul ai ion Conference in Boston 1969 (Miyashita, 1969). Soon this kind of degradation was called water treeing (Tabata et al, 1971). A water tree is to bc defined as a diffuse structurc in a polymer with an appcarance resembling a bush or a fan. It is generally acceptcd that water trees reduce the clcctric breakdown stress level of a polyethylene insulating material. Water trees can be made visible by several dyes. More characteristics are given throughout this study, in particular in Chapter 3. Miyashita observed water trees in the stator windings of submersible motors the wires of which were coated with polyethylene. Subsequent publications of Tabata et al (1971) and Vahlstrom (1971) described the early experience with water trees in low-density polyethylene and cross-linked polyethylene insulated cables. After five years of service experience these cables showed failures that could be rclated to water treeing.
Continued publications of different authors describe many aspects of water treeing in polymer insulating materials. It was found, for instance, that water treeing could occur in almost all polyolefins. Thcre was a great discussion about the water tree susccptibility of different types of insulating materials. Filled insulating materials such as ethylene propylene rubber have the advantage of opacity, howevcr, these materials are suspected as well, sincc close examination showed that water treeing also occurs in them.
A wcll known procedure to review the service performance of cables in gencral is failure statistics. The official failure rate is the numbcr of failures per 100 circuit km per year. Up to now this failure ratc has provided a rather restricted and even inadequate picture of the service performance of the different cablc types:
A failure in a mass-impregnated cable, for instance, is mainly due to local defects; after a single repair job continued service operation is possible. An extruded cable, showing a single failure due to water trees, is usually affected over longer cablc lengths. Local repair is insufficiënt, the replaccmcnt of extensive cable lengths is required.
Conscqucnces are drastic: the rcplacement of extensive lengths of extruded cable dramatically affects the failure statistics. It is therefore recommended to base failure statistics both on the number of failures and on the length of cable to be replaccd. On account of the facts mentioned above it is rather difficult to get a clcar picture of the extent of the water treeing problcm. In 1984 Shaw et al (1984) eslimated the total cablc length harboring water trees to be about 300,000 km. The research activitics presented in this study outline the situation in The Netherlands. Sincc 1982 several 10 kV cross-linked polyethylene insulated cable circuits, which have been in service for 6 to 13 years, have had failures due to water trees. The total cable length
involved is about 50 km or 3 % of the extruded cable network. These lengths have been replaccd and more cablc lengths will have to be replaced because of expected bad service performance.
1.3 Object of study
With respect to various subjects of study, the following objcdivcs can be specified: to review the phenomcnology of water trecing, taking into account the different types of water trees. Such a distinction is of great importancc lo gain insight into the background and consequences of water tree growth.
to review the possible mechanisms of water tree growth and to discuss the probability of their occurrence.
to develop a procedure according to which the level of degradation of an extrudcd cable insulation, affected by water trees, can be determincd.
to find the rate and causc of degradation of medium-voltage cross-linkcd polyethylene insulatcd cablc nctworks.
to develop an accelerated ageing procedure to define the water tree susccplibilily of an extrudcd cable insulation.
It is also the aim of this study to present a tcxlbook on water treeing since so far a comprehensivc study in this field is missing.
2 POLYETHYLENEINSULATION
2.1 Polyethylene structure
General information on polymers is provided for instance by Sauer and Pae (1977). In this study the main focus is on low-density polyethylene (LDPE). This material is commonly used as an insulating material for extruded cables. Other types of polyethylenes are medium-density polyethylene (MDPE) and high-dcnsity polyethylene (HDPE) used for sheath production in modern cable technology. Polyethylenes, (CH2-CH2)n, are very long macro moleculcs, each containing up to 100,000 or even more monomcrs of the CH2 = CH2 type. The CH2 groups are strongly joined by bonds of the shared clectron 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 dclermincd by the density of these materials. The density in its turn is strongly related to the molecule length and the number and length of side chains per macro molecule. This is illustrated in Table 2.1. Note: apart from the side chains as mentioncd for density polyethylene in this Table, there may be an occasional side chain on the low-density polyethylene main chain with an average molecule length of the main chain itself.
T a b l e 2.1 Relalion belwecn density and number and length of side chains for low-density and for high-density polyethylene. Aftcr Sillars (1973) and Billmcyer (1984)
Polyethylene density [g/cm ] average molecule length number of side chains [/1000 chain atoms] length of side chains [number of atoms] I.DPE 0.91-0.94 1500-3500 20-40 • 2-5 • HDPE 0.95-0.965 7000-14,000 < 5 < 4
Polyethylenc is a thermoplastic: the upper operational temperature is limited to about 70 °C. By cross-linking of the macromolecules the operational temperature is incrcased to about 90 °C. In cross-linked polyethylene (XLPE) the macro molecules are incorporated in a network in which the effective molecule weight has become infinite. A schematic representation of cross-linked polyethylene chains is presentcd in Figure 2.1. H H H H H H H H H H I i I I l i I i l i - c - c - c - c - c - c - c - c - c - c -I I I M il H H H H H H H H i i i i i c c c c c c c c c c -I -I -I -I -I -I -I -I -I -I H H H H H H H H H H
F i g u r e 2.1 Molecular struclure of cross-linked polyethylene, bond angles are nol indicaled
The polymer, cithcr cross-linked or not, is semi-crystalline, which means that it is partly cryslalline and partly amorphous. The crystalline part shows a folded regime of macro molecules organized into platelets with dimensions strongly depending on the production process. Usually the thickness is about 10 nm, thcy are generally several micrometers wide and long. Between the platelets interconnecting chains and chain ends are part of the amorphous regions. The struclure of the polyethylene on this nanometer scalc is shown in Figure 2.2.
F i g u r e 2 . 2 The polyethylene struclure in the nanometer range (Aftcr Rartnikas et al. 1983)
Platelets can be grouped to spherulites, varying in size from several microns to hundreds of microns. An cxamplc is given in Figure 2.3 wherc spherulites in polyethylene have grown under optimal condilions. Because of the expected effect
of spherulites on the physical propcrties of polyethylcnc, these structures have attractcd much interest. Scveral studies deal wilh this subject.
F l g u r e 2 . 3 Example of spherulites in polyethylene
The formation of superstructures or spherulites strongly depends on the material and production parameters during cooling of the mcll. Howcvcr, long macro molcculcs and the existence of many branches as in low-density polyethylcnc rcducc or even suppress superstructuration. Examples of complete suppression of superstructures in low-density polyethylcnc are givcn by Patsch et al (1976), Mandclkcrn et al (1981), Capaccio et al (1985) and Ross (1989). This is Ulustrated in Figure 2.4: an electron micrograph of low-density polyethylene cable insulation is given with a magnification factor of 40,000. Crystalline and amorphous regions have been revealed, superstructures have not been found.
In spite of the expected suppression of spherulite growth in low-density polyethylene insulating matcrials, surface analyses by different etching procedures sometimes revealed a kind of superstructuration. Studies in this specifie field were done by Muccigrosso et al (1978), Wagner (1978), Namiki et al (1980), Melton et al (1981), Orton et al (1981), Naybour (1982), Bamji et al (1983) and Crichton et al (1983). However, the results of these studies are oftcn suspect: most of these etching procedures affect the polyethylcnc structure to a high extent.
F i g u r c 2.4 niectron micrograph of low-density polyclhylene cablc insulation. Crystallinc and amorphous regions have been made visiblc. Sphcruliics have nol been found
2.2 Mechanical properties
The mechanical behaviour of polyethylcne is mainly determined by the amorphous regions. In a thermoplastic such as polyethylcne the intermolecular Van der Waals forces play an important role. Although the Young's modulus E is high -about 500 MPa at room temperature- a considerable chain flexibility or micro-Brownian motion exists. The actual mechanical behaviour of polyethylcne is rclated to the position of the working temperature in proportion to the glass transition temperature Tg and the melting temperature Tm. A schematic presentation is given in Figure 2.5. a.
I
ui g o-o 6 4 -140 - 9 0 - 4 0 10 60 110 160 temperature (°C)F i g u r e 2 . 5 A schematic modulus/tcmpcraturc curve for low-density polycthylenc and cross-linked polyethylene aftcr Schipper (1984)
Entering the glass-phase a primary rclaxation peak is found. Beyond Tg (-123 and -33 °C aftcr Saucr, 1977) the modulus becomes very high (10,000 MPa). Above Tm, low-density polyethylene will melt if it is not cross-linkcd, while cross-linkcd polyethylene bchavcs as a rubber-like substance.
On the basis of Figure 2.6, which shows a typical stress-strain curve for polyethylene, somc important mechanical parameters are discussed. For small stresses, uniaxially applied to the bulk polymcr, the stress-strain relation shows a Hookcan behaviour with CT = Eyrne, where E m is the Young's modulus, o the applied stress and e the strain. The strain is hcre defined as the length of the clongalion divided by the initial length.
In the linear region of the stress-strain curve instantancous recovery is possible. The molecule chains are partially uncoiled and coiled again. The Boltzmann superposition principle (Billmeyer, 1984) statcs that in this region the behaviour of the matcrial is determined by the sum of the separate effects. This means that the total forcc
applicd to the polyethylene is the sum of forces of different origins such as the electric stress and the osmotic pressure.
0.2 0.4 0.6 0.8 1.0 1.2 1.4
slrain ( (m/m)
Figure 2.6 Typical slrcss-slrain curve for a polyethylene (Aflcr Kaufmann cl al. 1977)
At the yield point slippage of the chains will result in incomplete recovcry. T h e stress at this yield point is called the yield strength o
Finally, fracturc of the bulk polymcr will b c rcached at the ultimatc strength CTU.
Some typical data for low-density and high-density polyethylene are given in Table 2.2.
Table 2.2 Typical tensile propertics for polyethylene
polymcr density crystallinity Young's modulus Eym Yield strength o Ultimatc strength o clongation to fracturc [ g / c m3 ] [%] |MPa) [MPa] |MPa| |%] LDPE = 0.92 =55 200-100 10-20 15-25 400-700 HDPE = 0.95 =90 600-1500 25-50 25-55 100-600 rcfercncc Billmcycr, 1984 Billmcycr, 1984 Saucr et al, 1977 Sauer et al. 1977 Saucr et al, 1977 Saucr et al, 1977
As all polymers, polyethylene is rather susccptible to effects of time. Important time effects are
creep: at a constant stress the deformation or strain slowly inercases relaxation: at constant strain the required stress slowly decreases
internal friction: at dynamic load mechanical energy is converted into heat. Time effects are of importance at moderate or high stress lcvcls. At lowcr stress levels these effects are hardly found and therefore complete recovcry is possible at lower stresses.
The mechanical behaviour of a polymer is affected by various factors. One important factor is temperature. Especially if the motion of chain segments is unfrozen by an increasing temperature, the polymer tends to become softcr. Other important factors are the length of sidc chains, crystallinity, polar substituents, plasticizcrs and water. With increasing length of (non-polar) side branches there is a larger inter chain separation and therefore flexibility inercases. The effect of crystallinity has alrcady been shown in Tables 2.1 and 2.2. Increasing the polarity of the polymer chains rcsults in an increase of the intermolecular forces and therefore the Tg is raiscd considerably. Plasticizers, if added to the polymer, act as a solvent by enclosing (hc polymer molecules and thereby decreasing the Van der Waals forces betwecn the polymer chains, thus lowering T and the modulus above T Water does not affect the mechanical properties of a polymer, except if polar bonds -solvable in watcr-contribute to the intermolecular forces, as is the case in nylon and polyurcthane.
23 Fracture in polymers
Stress cracking or brittle fracturc is an important phenomenon that may lead to complete dcgradation of the polymer. It starts as a localizcd phenomenon at a location wherc a crilical stress is present. The outer dimensional changes during stress cracking are of minor importance, since only microscopic cracks are made. Such small cracks are called crazes. These crazcs practically always advance in a direction perpendicular to the local stress. Typically, molecular polymer chain orientations or fibrils can bc found across the craze surfaces. Figurc 2.7 givcs an example of crazing and fibril formation in polyethylene. In this examplc crazcs can be observed in scvcral directions, the fibrils diameter ranges bctween 5 and 10 nm (After Bilimcycr, 1984).
F i g u r c 2.7 F.lcctron micrograph of crazcs and fibrils as found in polyethylene
Stress cracking occurs in the brittle rcgion at low tcmpcraturcs or under alternating stresses during long times intervals.
A model dcscribing the propagation of crazes was proposed by Griffith (1921,1924). According to Griffith stresses are concentrated in the region of crazes. A micro-crack with a tip curvature of atomic dimensions may have stress concentrations comparable with stresses of interatomic forces. The cncrgy for propagation of the crack comes from the elastic energy which is stored at the crack tip. Propagation of these crazes may continue at imposed stresses which are much lower than the static yield strength.
Fracture is not found under repeatcd 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.
There are methods to enhance or reduce stress cracking. A liquid which is capable of solvating the polymer promotes cracking. Measures to reduce stress cracking are:
increasing the molecular weight (fewer chain ends and thereforc fewer
micro-cracks)
annealing (by reducing the internal stress concentrations) molding and cooling carefully
the use of copolymers (the mechanical properties are improved by the combined effects of the various polymers).
2.4
Extruded cable insulation
The extrusion and cross-linking of polyethylene cable insulation is a subject of study in this Section. During the extrusion and cross-linking process, voids and internal stresses may be created. Both phcnomena are assumed to affect the ageing performance of the polymer.
2.4.1 Extrusion
The flow process during extrusion is represented in Figure 2.8. The granulate is meltcd and the melt is then presscd into an extruder hcad and from this hcad through a baffle and a mandrei and finally pressed around the conductor. The extrusion temperature is about 125 °C (Tanaka, 1983).
Ixgcnd granulate hopper screw* extruder hcad* balTIc* mand rel* insulation*
F i g u r e 2 . 8 Flow process during the extrusion (* A f t c r Patsch et al, 1976)
As a result of the extrusion process typical flow patterns occur. An example is givcn in Figure 2.9, where this flow pattern is made visible in a low-density polyethylene cable insulation (Patsch et al, 1976). A second example is shown in Figure 2.10. It
shows the flow pattera as observed in one of the cabics involved in this study (Chapter 8). This cable has a cross-linked polyethyicnc insulation. The flow patlcrn here is visible probably as a conscqucncc of material pollution. A large impurity can be observed at the point where the two flows merge.
F i g u r e 2.9 Flow pattern in extnidcd low-density polyethyicnc (Aflcr Patsch el al, 1976)
2.42 Cross-linking
T h c r c are several m e t h o d s of cross-linking. Often a dicumyl peroxidc-curing agent is used. H c r e the dicumyl peroxide -added to the polymer compound- is activated right after extrusion in a special tube at high tempcrature and high pressurc in this tube. T h e peroxide reacts with the polymer chains by removing hydrogen in a limited numbcr of places along t h e chains. Each carbon radical reacts with a methylcne group of an adjacent chain, thereby creating cross-links. In somc production lincs cross-linking occurs with the aid of stcam at tempcratures bctween 200 and 220 °C and a pressurc of 1.6 to 2 M P a (16 to 20 bar) (Patsch et al (1976), T a n a k a (1983)). This process is called steam-curing. Towards the end of the process the cable insulation enters the cooling section where a rapid t e m p c r a t u r e reduction is achievcd. Nowadays most cablcs are cured, not by using steam, but by using hot nitrogen. This considerably suppresses the creation of micro-voids, as will be shown. Such a process is called dry-curing. Cooling is performed by using gas or water. Thereforc, this process is subdivided into "dry-cured dry-cooled" and "dry-cured wct-coolcd". T h e mcthod of cooling, however, is of minor relevance with regard to the creation of micro-voids.
Residual products of the dicumyl peroxidc-curing process are cumylalcohol and acetophenonc. These products will cvcntually diffuse out of the insulation. T h e rate of diffusion depends strongly on the tempcrature.
An csscntially different m c t h o d of cross-linking is silane-curing. In this mcthod curing does not take place directly after extrusion, but in a separate produclion step. In the one-shot silane-curing process a silane compound is grafted onlo the polycthylene chains during the extrusion. After extrusion the cable is slowly coolcd in a watcr-filled cooling trough. C u r i n g takes place aftcrwards by putting the extruded cable on a reel in a water tank at aboul 85 °C. T h e immersion time for medium-voltage cables ranges bctween o n c and fivc days, depending on the insulation thickness. In this tank the silane groups are coupled chcmically under the influence of the watcr-vapour in the insulation. T h e a m o u n t s of residual products are much smaller than the amount of these products in the peroxidc-curing processes describcd above. T h e most important residual products are methanol, cumylalcohol and acetophenonc, the assumption is that these products diffuse out of the insulation in the coursc of the curing process.
2.43 Voids
It is gcncrally assumed that during production water, impuritics and residual products from cross-linking will be collected in the amorphous regions of the polymer. If these substanecs are polar, clustering may take place in existing voids. Somctimcs, for
instance if supersaturation occurs, voids will be created. Smaller voids have dimensions comparable to the dimcnsions of the interlamellar regions, the largest voids have diameters of sevcral micrometers.
The creation of most of these micro-voids is attributed to water-vapour or other gases which, during the rapid cooling of the melt, are preventcd from diffusing out of the insulation. As a rcsult, supersaturation, in particular of water during steam-curing, is inevitable and micro-voids, füled with gases and/or water, will be created. The approximate sizes and densities of voids have been measured for typical production processcs and are given in Tablc 2.3 (Kageyama et al (1975), Allister (1982), Boone et al (1984) and Gcurts (1985)).
T a b l e 2 . 3 Density, size and volume of voids in polyethylcne as a funclion of the curing process
curing method density maximum size
of the voids
ratio void-PE volume assuming
void diameter of
5 lim (or 1 lim")
-3 j i m stcam dry silane (onc-shot) uncured 105-106 lfAóir/ -10< «10* 30' 15' 15' 13 % 0.7-7 0.007-0.4 = 0.07 = 0.005" aftcr Boone et al (1984), as observcd in 10 cm insulation volume chosen al random. Aftcr Geurts (1985), an average void diameter of 1 /im was found for uncured LDPK.
In Tablc 2.3 the effect of the method of cross-linking on the presencc of micro-voids is demonstratcd. The significant void-volumc/polyethylene-volumc ratio (0.7 to 7 %) in steam-cured cable insulation is a consequence of the large amount of water-vapour dissolved in the insulation during cross-linking, in combination with the cooling procedure aftcrwards.
Supersaturation and void creation will usually occur in the centre region of the insulation. The inner and outer regions will cool down and solidify first, thereby suppressing further diffusion of gases out of the centre region of the insulation. Such void concentrations are usually visiblc as halos in the insulation of steam-cured cables. An example is shown in Figure 2.11.
2.5 Water in polyethylene
Water is an important substance with respect to water treeing. Therefore, the bchaviour of water in polyethylene will be discussed. The effect of the presence of an electric stress on the behaviour of water in the polyethylene will not be dealt with in this Scction, however, will bc subject of discussion in Chapter 5 and Chapter 6. 2.5.1 General aspects
Owing to its non-polarity polyethylene insulating material is water-repellent. Dissolution of somc water, however, cannot be excluded. As is stated above, polar substanecs in the insulating material are capable of attracting water. From our own measurements and from literature (Ildstad, 1982) it is known that cross-linkcd polyethylene at a temperature of 20 °C absorbs less than 100 ppm of water. The amount of water dissolvcd depends strongly on the temperature, as is shown in Figure 2.12. As was stated in the previous Section, supersaturation temporarily results in high amounts of water mainly collected in micro-voids. Most of this water will diffuse out of the insulation until an equilibrium is reached, a process which may take several years for a cable insulation under service conditions.
10 — ' — ' — ' — ' — ' — ' — ' — ' — ' — ' — ' — ' — ' — ' — '
0 50 100 150 temperature (°C)
F i g u r e 2 . 1 2 Saturatcd water content (in ppm) for cross-linkcd polyethylene as a function of the temperature (in °C) (After Ildstad. 1982)
The following three Sections discuss the behaviour of water in voids whose walls consist of pure polyethylene (2.5.2)
capillary channels with walls of pure polyethylene or with walls being modified. This modification means that the polyethylene walls have bccomc polar (2.5.3) voids fillcd with water solublc substanecs (2.5.4)
The smallest dimensions of these voids and channels are comparable with the dimensions of the interlamellar arcas, i.c. in the order of 10 nm.
2.5.2 Water in voids with pure polyethylene walls
Il will bc shown that water in micro-voids with pure polyethylene walls diffuses out of the cablc insulation. A comparison is made between the potential cnergy of water in two different systems. Assuming a connection by diffusion between both systems, water will enter the systcm where it has the lowcst potential cnergy. The discussion will start with a description of two systems, both present in the polyethylene. The first systcm is a collection of many micro-voids in polyethylene, the sccond is one large void in this matcrial. The voids in both systems may contain water or air. Van der Waals demonstrated ihat a force is exerted on the molcculcs of a surface laycr which is directed inward. The relatcd surface cnergy can bc written as rA, in which r is surface tension and A is surface. This surface cnergy is a representation of the potential energy of the molecules in the surface layer. Enlarging this surface requires energy.
Throughout this and the following Sections thrce surface tension indices will bc uscd to idcnlify the matcrials that are active:
1 = liquid (usually the water) s = solid (usually the polyethylene) g = gas
In the first systcm the surface energy Us n for a collection of n, watcr-filled spherical voids in polyethylene with radius rt is
Us n = r . , * , * * ,2 (2-1)
In rclalion (2-1) the surface tension Ts^ refers to the polycthylcne/watcr interface.
The water collcctcd in the sccond systcm (one spherical water drop with radius r0), will have a surface energy Us with
Us= rs l4 * r0 2 (2-2) If the same water volume occurs in both systems:
"i = ( r0/ r , )3 (2-3)
Combining the relations (2-1) to (2-3), the ratio between the surfacc cncrgy in systems 1 and 2 becomes r0/ r; which is larger than 1. A reduclion of potcntial energy is realized by a reduction of the numbcr of small water drops in favour of the enlargement of one large water drop.
The conclusion reached above can also be derived in the following manner. The vapour pressure of the water in the void is inversely proportional to the void radius. Moreover, the concentration of the water in the polyethylene ncar the void surfacc is relatcd to this vapour pressure (Ildstad, 1982). Consequently, the water concentration in the polyethylene near a small void is highcr than that ncar a large void. Diffusion will takc place until cquilibrium is reached. Water will be expelled from the smaller voids and will enter the larger voids in the polyethylene.
The following step is that system 1 can be chosen insidc the polyethylene, while for system 2 a spherical water drop outside the polyethylene can bc taken. The intcraction between water and pure polyethylene is about the same as between water and gas. This was found by Ildstad (1982) and it means that Tsl ~ T t .
Now the water will collect in a place where it can create the largest water drop, which is outside the insulation.
Consequently, water in micro-voids with pure polyethylene walls will be cxpcllcd from the polyethylene, where the expelling force inercases as the voids become smaller. The diffusion coëfficiënt of water in polyethylene is low; in Section 2.5.1 it has alrcady been mentioned that equilibrium is reached aftcr sevcral ycars for stcam-cured cablc insulation.
2.53 Capillary action
In this Section the effect of pressure by capillary action on the polyethylene surrounding a narrow channel will be sludicd.
Water will not enter a narrow channel as long as the walls of these channels consist of pure polyethylene. Howcver, if the walls of these channels become polar, their surfacc tension will change. Water will enter the channel if the surfacc tension of the polyethylenc/water interface rs^ is smaller than the surface tension of the
polyethylene/gas interface r s_. Then the absorption of water into the system will cause the surface cncrgy to deercase.
Water may be supplied in two different ways. lt can enter the channel by diffusion through the polyethylene walls. It could also enter the channel through an open connection with a water reservoir.
The rcduction of the surface energy and, thus, the supply of water will stop as soon as the water has reached the unmodificd arca in the channel.
To show the effect of the hydrostatic pressure inside a capillary channel, a calculation is made for a hypolhctic capillary cylinder with radius r. A pressure drop over the water/gas interface or water meniscus can be calculated from the rcduction of frec energy dF, by raising the water lcvel in the capillary channel over a length dh. The changc in free energy in spreading the water over the surface of the capillary channcl with length dh is
dFs = 2 * r ( rs l- rs g) . d h (2-4)
Undcr isothermal conditions the change in free energy by changing the volume is
dFv = -po u l-dVo u l - P i n- d Vi n (2-5) In relation (2-5) the pressure undcr the curvcd surface is pj n, the pressure abovc this
surface is po u t, the water volume inside the capillary system is Vj n and the water volume outsidc the channcl is Vo u t.
The changc in volume as water enters the cylinder is
d Vi n= *r2dh (2-6)
For the total system the following equation applies
d Vo u l = - d Vm (2-7)
The free cncrgy of the system has a minimum for
dFs + dFv = 0 (2-8)
With relations (2-4) to (2-8) a pressure diffcrence across the meniscus can be calculated
dh h
F i g u r e 2 . 1 3 A capillary cylinder in which a pressure drop occurs over Ihe walcr meniscus
The system is presented in Figure 2.13, the doublé lines of the capillary wall represent the water attracting part of the system.
Conscqucntly, ihe hydrostatic pressure of the water just below the meniscus is
Pin = Poui + 2 ( rs l- Ts g) / r (2-10)
The attracting forces between the polar groups on the capillary walls and the water molecules result in a hydrostatic pressure Pin, which is lower than that of the environment po u l. At the Iransition point of the water-attracting and water-repelling zones the concave meniscus of the water compensates for the pressure drop. The pressure in the capillary channel is smaller than the hydrostatic pressure of the water surrounding the channel.
Rclation (2-10) clcarly demonstrates the effect of the radius of the capillary channel. Smaller radii will cause larger pressure differences to move the water into or out of the channel.
Summarizing, it has been shown that capillary action itself may pull water into polar channels or push it out of unmodified channels. If water is pushed into the channel by capillary action the pressure al the end surfacc is compensated by the concavity of the water meniscus. There remains no pressure capablc of damaging the polyethylene.
2.5.4 Osmosis
In this Section the effect is described of the osmotic pressure in a void in the polyethylene. A general thermodynamic approach is prescnlcd and an cxample is given of the pressure in a void which is the rcsult of a sat urated NaCl solution. It will bc shown that this pressure may increase beyond the yield strength of polyethylene. In such a situation the polyethylene will stretch.
Finally, the effect of the surfacc lension will bc discussed. For voids with small radii this surfacc tension may compensate for the osmotic pressure.
Derivation of the pressure increase
Watcr-soluble substanecs that are present in micro-voids attract water from the environment and osmotic pressure may occur. In literature (for instance Moorc, 1972, pg 252) such an osmotic pressure is often derived by means of a thermodynamic approach foliowed hcre. It is stated that the chcmical potential of the water in the void and outsidc the polyethylene are equal if an equilibrium is reached. Therefore, a decrease of the chemical potential of the water in a void due to a solutc must bc compensated by the increase of the hydrostatic pressure. This decrease of the chcmical potential of the water for a dilutcd solution (X « 1) is represented by NkTln(l-X) with
N = Avogadro constant k = Boltzmann constant T = tempcrature
X = molc fraction of the solutc
The increase of the chemical potential of the water by an imposed pressure is given by
r
n vdp . 0 in which IT = osmotic pressure v = specifie molar volumeAssuming the water incomprcssible cquation (2-11) applies in an equilibrium
With X « 1, the osmotic pressure becomes
n - NkTX/v (2-12) The molc fraction X of the solute is relatcd to the quanlitics of the solute and the
solvent water (both measured in molcs) by the following rclation
X = ms o l u / (ms o l u + ms o l v ) (2'1 3)
in which
ms o l u = quantity of the solute msolv = quantity of the solvent With X « 1 this mole fraction is
X " ms o l u /ms o l v (2"1 4)
On the samc condition the specific volume v can be written as
v « V / ms o l v (2-15)
in which V is the volume of the void. By taking the concentration c as
c = ms o l u/ V (2-16)
the osmotic pressure becomes
n » RTc (2-17) In rclation (2-17) Nk has been replaced by R, the gas constant, being 8.31 J/Kmole.
For a saturated solution the condition X « 1 is not usually truc, so cqualion (2-17) is an approximation. For bctter rcsults the concentration c has to be replaced by a practical parameter, c' being the osmolarity. Therefore,
f! = RTc' (2-18) This osmolarity has been measurcd for many solulions and can be derived from data
For a spherical void the stress at in the tangential direction of the void surface is 50 % of the stress ar in the radial direction of the surface (Ildstad, 1982). With
| ar| = II, the tangential stress ot bccomcs |(7t| = VJl. The various stresses are given in Figurc 2.14.
Example
For a typical NaCI solution undcr saturatcd conditions the osmolarity c' is 10-10 mole/m at a temperature of 300 K. As a result, the osmotic pressure II bccomcs large: in this cxamplc the osmotic prcssure is approximatcly 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 low-density polyethylenc (p Table 2.2). Crccp -a mechanical dcformation process (Section 2.2)- may occur. As a
conscqucnce, the void will bc cnlargcd and by further absorption of water the solution will bc diluted and osmotic pressure will decrease. This process will continue until eventually the osmotic pressure falls bclow the yield strength of the polyethylenc.
Osmotic pressure and surface tension 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 alrcady been discusscd in Scctions 2.5.2 and 2.5.3. The surface energy
F i g u r c 2.14 Stresses in the surface c a n b c h i h c s p c c i al |y jf t h c w a l l s of i h c laycrofa spherical void . . , . . . . , ,
p o l y e t h y l e n c a r e u n m o d i h e d ( s o rs l " 0.07 N/m) and the voids have small radii. Thcrcforc, the prcssure drop over the liquid surface, which is 2r s |/ r , can rcach compensating pressures regarding osmotic prcssure. The radii of such voids are calculatcd lo bc in thc nanometer rcgion. Conscqucntly, compensation of thc osmotic prcssure by thc surface energy can only bc cxpcctcd in thc intcrlamcllar rcgion of thc polymcr.
3 PHENOMENOLOGY
3.1 Introduction
Much research work has been carried out in order to gain a bcttcr understanding of water treeing. Not only the mechanism of growth, but also the actual phenomenology of these trees have been studicd since their discovery in 1969. Many details were not clear. This is mainly duc to the fact that water trees produce only a minor changc of the physical insulation properties, making research work rathcr difficult.
The scope of this Chapter is to review different aspects of the phenomcnology of water trees. Data have been obtained from literature, in certain cases the results of our own research work will bc presentcd. The conclusions will bc summarized in Chapter 4.
A distinction must be made between two different types of water trees. These types are the "bow-tie tree" and the "vented tree". This distinction is based on the localion wherc these trees start growing: vented trees are initiated at the insulation surfaces, bow-lic trees are initiated in the insulation volume. Such a distinction is important since both types show a completely different behaviour of iniliation and growth. In Section 3.2 both types will be discussed in detail.
This review is mainly directed towards the phenomcnology of vented trees. This is bccausc vented trees under service ageing conditions often appear to bc much more dangerous than bow-tie trees.
For several reasons the study of vented trees is more difficult than that of bow-tie trees:
the density of vented trees is often low compared to the density of bow-tie trees. as will be shown in Section 3.2.4, at the bcginning of growth the propagation rate of vented trees is lower than that of bow-lic trees; in a later stage of growth the oppositc is truc.
Consequently, the study of vented trees is more timc-consuming than the study of bow-tie trees. For these reasons many publications contain information on bow-tie trees only. In somc othcr cases a distinction between both types of water trees has not even been made: only the description "water tree" has been used.
One of the best among recent reviews is from Shaw and Shaw (1984) with over 2(X) referenecs. Although this review is of great value, it has the disadvantage that even here no consistent distinction between vented trees and bow-tie trees has been made.
In this study the description "water tree" will be used only if bow-tie trees as well as vented trees are under consideration.
In Section 3.2 the following morphological aspccts will bc discusscd: shapc of water trees
voids and channcls in vented trees water content of vented trees
typical growth bchaviour of water trees
The impact of water trees on material propertics is givcn in the Sections 3.3 to 3.5. Sections 3.6 to 3.13 will describe the effect of various agcing parameters on water tree growth such as tempcrature, voltage and frequency.
3.2 Morphology of water trees
3.2.1 Shape
In general water trees are diffuse structurcs in an insulating material resembling a bush or a fan, growing in many different kinds of polymers. The amount of water in water trees is higher than the amount of water in the unaffeclcd surrounding insulation.
Trees can be made permanently visible using different dycs, a wcll known and gcncrally acccpted dycing procedure is givcn by Larsen (1983) and Shaw and Shaw (1984). This procedure is described in
Appendix 1. Vented trees
The vented tree is defined as growing from the insulating material boundaries to the other side of the insulation, predominantly along the axis of the electric stress.
The origin of vented tree initiation in many cases is difficult to find, however, the origin 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 il is in contact with the insulation. An example of a vented tree, initiated at the boundary area of a void -located in the semiconducting inner
screen against the insulation surface- is shown in Appendix 4. Figurc 3.1
Typical vented scmiconducling insulation of polycthylcnc cable
tree grown from the inner screen into the a 10 kV cross-linkcd
If aged undcr moderate service conditions (up to a few kV/mm), a vented tree grown from the outside of the insulation is often pencil-like. Trees grown from the inside have branches that spread a little bit more, although the distinct branches of large venled trees are pencil-like too. Typical vented trees are presented in Figurcs 3.1 and 3.2.
In an insulating material which is fairly water tree susceptible, vented trees can reach the other side of the insulation (in 10 kV cablcs about 4 mm thick) in about 7 years. It has been found that this type of water tree has been responsible for many cable
Figure 3.2
Vented tree grown from
the graphiicd oulcr
screen into the insulation of a 10 kV cross-linked polycthylcne cable
Figure 3.3
Vented tree grown from the scmiconducting inner s c r e e n i n t o t h e insulation. A part of the small breakdown channcl is visiblc
I-cgcnd
1: part of breakdown channcl
failures. In Figure 3.3 a vcnted tree is prcsented; here also a part of a breakdown channcl is visiblc. Small breakdown channeis as shown in this Figure have been obtained in the laboratory using special techniques as will bc discusscd in Chapter 7. Bow-tie tree
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 clcctric field lincs.
The initiation spots are oftcn clearly visible using normal optical microscopy. Examples are given in Figures 2.10 and 3.5. It is gcncrally assumed, and it has been proven in some cases, thal these spots conlain
impurities (Sletbak et al, 1977).
Normally the growth of this type of tree is strongly rcduccd aftcr a certain time; this will be demonstrated in Section 3.2.4. The total lengt h is restrictcd and therefore this kind of water tree is hardly ever the origin of cable breakdown. There are indications that the length of bow-tie trees is rclatcd 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 has never been found in our own practicc. Howcvcr, this was once reporled by Naybour (1979).
A collcction of typical bow-tie trees in the cross-linked polycthylcnc insulation of a 10 kV cable is given in Figure 3.4. A further cnlargcment of one of these bow-tie trees is presented in Figure 3.5. Nole that in Figure 3.5 a shcll around the initiation spot with a thickness of about 2 nm has also been made visiblc using the dyeing procedure. A possible explanation will bc given in Chapter 6.5.3.
Figurc 3.4
Typical bow-lic trees grown in the insulation of a 10 kV cross-linked polyethylene cable
Figure 3.5
Bow-tie tree, total lcngth 200 /im.This bow-tie tree was initiated from an impurity in the insulation
»
■4 4 - •'
• ■* ■ • ,
• • . • » *
3.2.2 Voids and channels
Voids in the micrometer range occur occasionally in a water tree. Collcctions of such voids are found in the trunk of vented trees. Near the tip of the vented tree such micro-voids are rarcly found (Capaccio, 1985). If vented trees have been grown undcr rather extreme agcing conditions, for instance by applying high clcctric stresses of a few tens of k V/mm, somc of these micro-voids may bccomc interconnected by micro channels (Capaccio, 1985).
In a gas permcation experiment, Cross et al (1984) showcd that the diameter of the supposed channels, penctrating throughout the insulation, must bc less than 1 ^m. Microscopic investigations of the tip of a water tree did nol even reveal open channels in the nm range (Capaccio, 1985).
3 2 3 Water content
Water trees can contain a certain amount of water. In most cases water in a water tree can be evaporated. If the insulation is cxposcd to water or water-vapour afterwards, the water tree will absorb water again.
Meyer (1983) measured the amount of water in vented trees grown from water needies. The electric stress near the water needie tip was very high, at least 60 kV/mm. Meycr found that near the tip of the water nccdlcs, the vented tree containcd 10 percent water in proportion to the total water Ircc volume. He assumed that the water had been collected primarily in the micro-voids. Such voids are expected to be the result of a high initiation stress. At a ccrtain distancc from ihc water needie, where the electric stress is much more moderate, the amount of water was about 1 to 2 percent 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 molcculcs can bc found at places where polar groups are attached to polyethylene branches (Ross et al, 1987).
32.4 Typical growth behaviour of water trees
Our own measurements show the typical growth behaviour of vented trees as wcll as bow-tie trees (Steennis et al, 1987). Five different 10 m long medium-voltage cables have been aged for 24,000 hours in a water tank. The cables varied in construction and in method of cross-linking (1 = low-density polyethylene, 2 = steam-cured cross-linked polyethylene, 3 = dry-cured, wet-cooled cross-linked polyethylene, 4 = dry-curcd, dry-coolcd cross-linked polyethylene and 5 = one-shot silane-cured cross-linked polyethylene). The electric stress lcvcl at the outsidc of the insulation was 3.9 kV/mm at power frequency. The tap water in the water tank was kept at a temperature of 30 °C. The water contained small amounts of NaCl (0.2 kg/m ) and HC1 (acidily levcl 6). The water was admitlcd undcr the shcath only. Aftcr 3000, 6000, 12,000 and 24,000 hours the sizes of the vented trees and bow-tie trees wcre measured. The inspections wcre carricd out on samples taken from the insulation. The inspection method is described in Section 7.2.6 of this study. In Figure 3.6 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 ageing time. Only 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 11,300 hours' ageing in cable no. 1 breakdowns did occur probably as a conscqucnce of the large vented trees (•!). This cable was withdrawn from further ageing.
It is found in this acceleratcd agcing test that after an initial rapid growth within the first 3000 hours of agcing the length of bow-tie trees does not seem to increase any more. The length of vented trees increases continuously for most of the cable insulating materials. After approximately 12,000 hours of agcing the lengths of both types of trees correspond well to the general picture of tree lengths in the insulation of service-aged cabics. Tree lengths in service-agcd cabics will bc the subject of discussion in Chapter 8. V 100 <o c 9 0 o £ 80 § 70 U-* | 60 o •5 50 •«_ ° 40 fc? ^ 30 <D N "5 20 <p <u 1 0 i_ v 0 ( r
-•1
) 3 0 0 0 - • • i 6000 • 1t
• • • —• i 12000 • 2~ l
4 ï 15 - 2 i 2 4 0 0 0 time (hours) • = vented tree — = b o w - t i e treeF i g u r e 3.6 Maximum length of vented trees and bow-tje trees for fivc different cabics as a function of the agcing time
3 3 Dielectrical properties (local)
33.1 Definitions
Before starting the discussion of dielectrical properties, definitions should bc given of "vented tree" and "vented tree path".
The "vented tree" represcnts the total area within which the tree can bc obscrved (/im to mm scalc), using the dyeing procedure described in Appendix 1.
The "vented tree path" or "path" rclates to the attackcd polyethylenc only (the nm scale).
The ratio of the volume of the vcntcd tree paths to the vented tree certainly is much smaller than 100 %, howcver, an upper limit cannot be given.
332 The vented tree: an insulating material
Therc are scveral indications that bolh the vented tree and the vented tree paths can be considcred as an insulating material.
Koo et al (1983) and Cross et al (1984) studied the dielectrical properties of a vented tree in a water needlc experiment. The change in capacitance between the water needie and the opposite electrode was mcasured during the growth of the vented tree at 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 rcplaccd by metal or by dielcctrics with different permittivities. The rcsulting capacitance variations showed that the dielectrical behaviour of a real vented tree differs strongly from that of a conductor. Moreover, it was found that comparable voltage variations in this model could bc obtained if the water tree was replaced by an actual material with a diclectric constant of about 6. To explain the measurcd diclectric constant, Cross assumed that water was collccted in the many micro-voids distributed over the vented tree volume. Cross pointed out that in such a situation the observed inercase in the dielectric constant of the vented tree volume might be explaincd by using the multiphase dielectric mixture theory (e.g. Tinga, 1973).
Boggs et al (1986) studied the micro-movement of vented trees in a water needie experiment. The trees were grown in silicone rubber at different frequencies. Tree movement has been observed by applying interferometrie holography. Boggs found that the movement of the trees was related to E and therefore probably related to Maxwell stresses. He also concludcd that certain out-of-phase movcments of the tree
in rclation lo ihe movement of the needie tip would provide cvidcncc that the vented tree is not conductive.
Recently Ross et al (to be published, preliminary results) measurcd the dielectrical properties of a vented tree which had been grown under normal ageing condilions in a full-scale cross-linkcd polycthylcnc insulated cable. The dieleclric constant and loss-factor were measured on slices wilh a thickness of about 150 /im. The slices were saturaled with water preceding lo the mcasurcmcnts. The vcntcd tree penetrated the slices complctely. He found that the vcntcd tree has the properties of an insulaling material with a dieleclric
constant of 2.26 and a loss-factor of about 2010_ i at 50 Hz.
There are more observations confirming that a vcntcd tree is an insulating material:
Breakdown characteristics
If a vcntcd tree path were a conductor, then such a path, grown through to the othcr sidc of the insulation, would initiale thermal breakdown or initiale an clectrical tree followcd by breakdown. However, it was found that in most cases such long vcntcd Irces do not cause breakdown al stresses of 2 kV/mm or even highcr (sec Seclion 3.5, Figurc 3.8).
In an experiment by Densley (1974) clectrical trees were iniliated in vcnlcd trees by inserting a ncedlc. It was found thal ihc vcnlcd Irccs did not provide a more convenient path to ihc clectrical
Irccs or the breakdown channcls than unaffected polycthylcnc. Direction of vented tree propagalion
Vcntcd trees and the rclated paths grow in the direction of the clcctric field lines. High radial clcctric stresses at the tip of the vcntcd tree path would, however, cause the vcntcd tree to fan out. This can be observcd at the lip of a water needie, where the direction of growth ncar the water electrode is perpendicular to the water ncedlc surfacc. This is illuslratcd in Figurc 3.7.
Figurc 3.7
Vcntcd tree grown in ( l ï l i p p i n i et al, 1984)
In Appendix 2 various electric stresscs near the tree tip are calculated for different vented tree tip radii. The electric stresscs have been calculated for a range of diclcctric constants and conductivilies of the vented tree path.
High radial electric stresses of 30 % of the axial electric stress at the tip of the vented tree path ( Ep d/ E t = 0.3) can bc expected for vented tree paths with a dielectric constant of20 or higher, or a conductivity of 2.5-10"8 (firn)-1 or highcr. In such a situation it is expected that the tree would fan out.
It is also shown in Appendix 2.II that for a dielectric constant of less than 10 and a conductivity of less than 1.310 (flm)" this ratio E d/ Ep t is small. In that case vented tree growth can be expected mainly in the direction of the axial electric stress at 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 polyethylene.
It appears that for moderate permittivities and conductivitics the electric stresses become independent of the length of the vented tree path: the influence is confïned to a local area. This is illuslrated in Appendix 2.III, where the electric stress is shown in the axial direction along the axis of symmetry through the tree path bctween the electrodes.
3 3 3 Conclusion
The vented tree and the vented tree paths may be considered as insulating materials. The electric stress enhanecment near the tip of a vented tree or a vented tree path is moderate or possibly even minutc.
For the complete vented tree a diclcctric constant ranging from 2.3 to 6 and a loss-factor of 2 0 1 0- 4 at 50 Hz was found. These values are higher than for the unaffected polyethylene having a dielectric constant of 2.26 and a loss-factor of approximately 410 . The slight increase of the diclcctric constant and loss-factor is in all probability caused by the clustering of water in voids.
Accurate values for the dielectrical properties of the distinct paths of the vented tree cannot be given. Howevcr, moderate stress enhanecment al the tip of a path can be expected for a dielectric constant smaller than about 10 and a conductivity smaller than about 1.310" (firn)" . Such dielectrical properties are in line with the assumption that water in these regions is molecularly dispersed.
3.4 Physical/Chemical properties (local)
Introduction
This Scction discusses chemical and physical obscrvations in a water tree compared to observations outside a water tree. Studies are mainly carricd out on vented trees from nccdle tests; sometimes scratchcd insulalion slabs or full-scale cables have been uscd.
Infra-red
Infra-red mcasurcmcnts have been carried out by sevcral authors.
Bernstein et al (1975) pcrformed measurements on miniaturc cables aged for 60 days at 4.6 kHz. After drying the samples, mctal ions from the solution were detectcd at 1130 cm"'.
Garton et al (1980) tricd to find cvidcncc for oxidation in a water tree with FTIR. The insulation of miniaturc cable has been studied, aged at a moderate clcctric stress level at 1 kHz for 320 hours at 70 °C. Garton was not able to find a typical oxidation product such as carbonyl. However, he did found absorptions al 600, 1100, 1160,3550 and 3600 cm assigncd to ether and alcohol groups.
Abdolall et al (1982) were able to distinguish differenecs bctwccn affected and unaffected polycthylcne in a range bctwccn 20 and 350 cm"1 at a tempcrature of 4.2 K. Samples were taken from a full-scalc cross-linkcd polycthylcne insulated cablc The differenecs were attributcd to various possiblc effects such as hydrogen bonding. strain or inhomogencous broadening, brcakage of polymcr chains or effects duc t> cross-linking residual producls.
Differenecs in a range bctwccn 500 and 2000 cm have been found by Yoshimitsu et al (1983-b) with FTIR. These invcstigalors studied cross-linkcd polyethylent malcrial with an imbedded copper wirc aged for up to 10 days at 1 kHz. Thcy found that in the tree affected regions the CH2 groups losc thcir absorbancc, bul chemical species such as hydroxyl groups and carbonyl groups gain strength compared with the undegraded arca.
Bamji et al (1984) also cxamined regions with and without water trees using FTIR Different kinds of test specimen were studied. Absorption was found at 1160 (and 600) cm" . Bamji did not atlribute these absorplions to ether groups (Garton et al, 1980) but lo sulphatc anions. The absorptions at 1600 cm"1 are consistent with the
