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Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Nonlinear Dielectric Response of Water Treed XLPE Cable Insulation

Sverre Hvidsten

A dissertation submitted to

the Norwegian University of Science and Technology Department of Electrical Power Engineering

in partial fi.dfilment of the requirements for the degree of Doctor Ingeni@r

July 1999 This document has used FrameMaker@ as editor. The font is Times New Roman. Grapher@ from Golden Software has been used for graphical representation.

ISBN 82-471 -0433-4 1999:63 ... PREFACE m

PREFACE

This thesis, submitted for the degree of doctor of engineering (dr.ing.), is restricted to work performed at the Norwegian University of Science and Technology (NTNU) during the years 1994-1999 under the supervision of Professor Erling Ildstad.

I am especially grateful to my supervisor and Dr.Ing. Rolf Hegerberg at SINTEF Energy Research (SEfAS) for their guidance and inspiring discussions. I would also like to thank Oddvar Landr@ at NTNU for his help with the construction of equipment, and to all my friends and colleagues at the Department of Electrical Power Engineering at NTNU and the Materials Science group at SEfAS for their help and assistance.

I am also grateful for having the pleasure of working with Bjorn Holmgren and Peter Werelius at the Royal Institute of Technology (KTH) at Stockholm.

Finally I would like to express my deepest gratitude and love to my wife K&hild and our children for their patience and support during the work.

Trondheim, June 1999

Sverre Hvidsten

ABSTRACT v

ABSTRACT

Condition assessment of XLPE power cables is becoming increasingly important for the utilities, due to a large number of old cables in service with high probability of failure caused by water tree degradation. The commercial available techniques are generally based upon measurements of the dielectric response, either by time @olarisation/depolarisation current or return voltage) or frequency domain measurements (&’,&‘‘). Recent- ly it has been found that a high number of water trees in XLPE insulated cables causes the dielectric response to increase more than linearly with increasing test voltage. This nonlinear feature of water tree degraded XLPE insulation has been suggested to be of a great importance, both for diagnostic purposes, and for fundamental understanding of the water tree phenomenon itself.

The main purpose of this thesis have been to study the nonlinear feature of the dielectric response measured on watertreed XLPE insulation. This has been performed by dielectric response measurements in both time and frequency domain, numerical calculations of losses of simplified water tree models, and fiially water content and water permeation measurements on single water tiees.

The dielectric response measurements were performed on service aged cable samples and laboratory aged Rogowski type objects. The main reason for performing laboratory ageing was to facilitate diagnostic testing as a function of ageing time of samples containing mainly vented water trees. A new method, based upon inserting NaCl particles at the interface between the upper semiconductive screen and the insulation, was found to successfully enhance initiation and growth of vented water trees. AC breakdown strength testing show that it is the vented water trees that reduce the breakdown level of both the laboratory aged test objects and service aged cable samples.

Vented water treeing was found to cause the dielectric response to become nonlinear at a relatively low voltage level. However, the measured frequency domain dielectric response was larger, and found to be more nonlinear than values measured in time domain.

This thesis describes a new mechanism for the nonlinear dielectric response. It is assumed that at low or no applied electric stress the water treed region is characterised by sphericrd micro voids filled with liquid water separated by channels of crazed insulation. vi ABSTRACT

The effect of increasing the test voltage is to cause Maxwell mechanical tensile stresses strong enough to open up the crazing zones and elongate the water droplets into the mechanically weak crazing zones.

Finite Element Method (FEM) calculations show that the effect of the re-opening of crazing zones by an increased test voltage, strongly increases the dielectric loss of the water treed insulation. This is qualitatively in good agreement with the experimental results obtained on water treed insulation, where increasing the test voltage above a certain value caused the losses to increase. The typical frequency independent dielectric response of water treed insulation can, however, not be explained by this model.

Numerical calculations of losses, indicated that the mechanism of voltage assisted ingress of water is more likely in treed regions with rather low contents of water. The mi- I cro-FTTR measurements of single vented water trees indicated that such regions were likely to be present 3-400pm within the tree tip, and close to the insulation screen.

The process of refilling water into water tree structures is likely to be associated with a hysteresis effect. When removing (or reducing) the electric field, mechanical relaxation causes the channel to collapse and to slowly recover its former structure. Dielectric response measurements showed that a hysteresis was typically present when the response was nonlinear. CONTENTS

CONTENTS

... Preface m Abstract v

1. Introduction 1

1.1 Background 1 1.2 Main purpose of work 3 1.3 Description of chapters 3

2. Theory 5

2.1 Introduction 5

2.2 Theory of watertree initiation and growth 5 2.2.1 Typical features of water trees 5 2.2.2 Structure of water trees 8 2.2.3 Electro-mechanical theory of watertreeing 9 2.2.4 Electrochemical theory of watertreeing 10

2.3 Dielectric response and relations between time and frequency domain measurements 12 2.3.1 The dielectric response function 12 2.3.2 Measurements of the dielectric respouse in the time domain 12 2.3.3 Measurements of the dielectric response in the frequency domain14 2.3.4 Relations between time and frequency domain 15

2.4 Mechanism causing a nonlinear dielectric response 18 2.4.1 Effect of application of electric fields 18 2.4.2 Hypothesis for increased dielectric losses 18 CONTENTS

3. Diagnostic equipment and test procedures 21 3.1 Introduction 21

3.2 Equipment for time domain measurements 21

3.3 Equipment for frequency domain measurements 24

3.4 Guarding and preparation of test objects 25 3.4.1 Rogowski test samples 25 3.4.2 Cable samples 27 3.4.3 Thermal treatment 27

3.5. Numerical evaluation of data 28 3.5.1 Methods for estimating of the degree of nonlinearity 28 3.5.2 Fourier transformation of time domain measurements 29

3.6 Test procedures 30 3.6.1 Measurements in time domain 30 3.6.2 Measurements in frequency domain 30

4. Characterisation of the service aged cable samples and laboratory test objects 31

4.1 Introduction 31

4.2 Experimental methods 31 4.2.1 Manufacturing of the rogowski type test object 31 4.2.2 Type of cables 35 4.2.3 Water tree analysis 35 4.2.4 AC breakdown strength testing 36

4.3 Service aged cable samples 38 4.3.1 Results from water tree analysis 38 4.3.2 Results from AC breakdown strength analysis 41

4.4 Laboratory aged test objects 42 4.4.1 Test condition and ageing procedure 42 4.4.2 Results from water tree analysis 42 4.4.3 Results from AC breakdown strength analysis 48

4.5 Discussion 49 coNms

4.6 Conclusions 49

5. Measurements of the time and the frequency domain dielectric response 51

5.1 Introduction 51

5.2 Time domain measurements 51 5.2.1 Time dependence 51 5.2.2 Voltage dependence 54 5.2.3 Sensitivity analysis 56 5.2.4 Water and thermal treatment 56

5.3 Frequency domain measurements 58 5.3.1 Frequency dependence 58 5.3.2 Voltage dependence 58 5.3.3 Hysteresis effect 64

5.4 Relation between time and frequency domain dielectric responses 64 5.4.1 Magnitude of the dielectric response 64 5.4.2 Degree of nonlinearity 66

5.5 Discussion 68

5.6 Conclusions 69

6. Computer simulation of proposed mechanism for nonlinear response 71

6.1 Introduction 71

6.2 Meted of calculation 71 6.2.1 The water tree model 71 6.2.2 Method of electric field and loss calculation 73 6.2.3 Calculation of electrostrictive pressure 75

6.3 Results from numerical calculations 75 6.3.1 Electric field distribution 75 6.3.2 Dielectric loss 78 6.3.3 Sensitivity analysis 81 6.3.4. Increase of temperature 81 6.3.5 Results from analytic calculations of electristrictive pressure 83 CONTENTS

6.4 Discussion 84

6.5 Conclusion 85

7. Content of water within water trees 87

7.1 Introduction 87

7.2 Experimental methods 87 7.2.1 Equipment for measurements of water content 87 7.2.3 Measurements of water permeation 90

7.3 Experimental results 92 7.3.1 Condition and amount of water in vented water trees 92 7.3.2 Distribution of water close to a water tree tip 93 7.3.3 Effect of eleetric field upon the water permeation rate 96

7.4 Discussion 100

7.5 Conclusion 101

8. Discussion 104

9 Conclusions lW

Appendices 111

References 122 CHAPTER 1

INTRODUCTION

1.1. BACKGROUND

In the Nordic countries, crosslinked polyethylene (XLPE) insulated power cables have been used for more than 30 years. Service experience has revealed that in wet environment the lifetime in of these polymeric cables seem to be shorter than expected, mainly caused by water tree degradation. The number of faults for medium voltage (12 and 24

- 24 kV 30 w 12 kV

I 20 I

o Ld 70 72 74 76 78 80 82 84 86 88

Year of production

PIGURE 1.1. Numberof servicefailuresdueto watertreeingin MV XLPE cablesin Norway reportedin 1998[1]. 2 INTRODUCTION

kV) XLPE cables is particularly high for cables produced before 1980. This is illustrated in Figure 1.1, showing that 8090 the faults reported up to 1998 occurred in cables produced with insulation screens consisting of graphite painting and serniconducting tapes. In addition, laboratory ageing of old cables equipped with strippable insulation screens, show a significant reduction of the breakdown strength due to water trees [2]. Water tree degradation of XLPE cables has therefore become an increasing problem, as a high number of cables with graphite painted and strippable insulation screen is still in opera- tion [1].

The utilities have a need for a safe and reliable transmission of energy to the customers, and to use the high voltage power cables as long as possible. In order to avoid unexpect- ed insulation faults, and to reduce the cost of replacement, there is a need for nondestruc- tive diagnostic methods that can assess the condition of the insulation [3].

Electrical tests such as the detection of partial discharges or the AC withstand test are traditional on-site diagnostic methods. The advantage of such tests is that the weakest spots in the cable can be located and the sections containing such weak spots can be removed and replaced. However, these tests are normally performed at an elevated voltage with a high risk of failure during testing, and there is also no indication of the degree or nature of ageing of the rest of the cable [4, 5]. In order to avoid these problems, diagnostic methods based upon measurements of the dielectric response have been proposed as tools to assess the average ageing state of XLPE cable insulation [6]. This is based upon the assumption that water tree ageing results in measurable changes of the dielectric response in a certain range of frequency, time and voltage.

By performing a single measurement of the dielectric response, only the average degree of degradation can be obtained. In case of water treeing this is not sufficient, as electric breakdown is closer related to the length of the longest water tree than to the density or the degree of generalised ageing. Long water trees maybe located to a short section of the total cable length, or in other cases few but long vented water trees may develop, de- pending on the cable design and its service conditions. In such cases the resulting average increase in permittivity and dielectric loss will be low and critical water tree degradation may not be detected.

During the last 20 years dielectric response measured either in time or frequency domain has been a subject for many studies in order to characterize the degree of water treeing in polymeric insulation [7]. Measurements of depolarisation current, return voltage, capacitance and loss factor are traditional methods for obtaining the dielectric response and assessing non-destructively the quality of the insulation also in the low frequency range (lmHz to 10Hz). Generally, the magnitude of the dielectric response is found to increase with the time of water tree ageing [6].

However, another feature of water tree degraded XLPE insulation have been suggested CHAPTER1 3

to be of a great importance. Recently it has been found that a high number of bow-tie type water trees in XLPE insulated cables causes the time domain dielectric response to increase more than linearly with test voltages above the rated voltage UO[8]. Such a nonlinear increase of the time and frequency domain responses has also been found in XLPE Rogowski type samples containing a high density of bow-tie trees [9]. In case of frequency domain measurements on service aged cables containing a large amount of long vented water trees, nonlinear voltage dependence was detected at voltages far below UO[10]. It is therefore of interest to investigate further whether this nonlinearity can give more information about the state of the cable insulation.

1.2. MAIN PURPOSE OF WORK

The aim of this work has been to test the following three hypotheses.

1. Condition assessment of water tree degraded XLPE cables can be performed by measurements of the dielectric response either in time or frequency domain.

2. Vented watertrees, considered more detrimental to service performance than bow-tie trees, causes both the time and the frequency domain dielectric responses to become nonlinear.

3. The nonlinear feature of the dielectic response is caused by voltage assisted ingress of water into watertreed sections of the cable insulation.

1.3. DESCRIPTION OF CHAPTERS

The proposed mechanism causing a nonlinear dielectric response in watertreed XLPE cable insulation is presented in Chapter 2, where also a review of the theories of water treeing is included. Finally, mathematical relations between time and frequency domain methods for obtaining the dielectric response are presented.

The manufacturing process and the water tree ageing of the Rogowski test samples is presented in Chapter 3. This chapter also includes experimental results from water treeing and breakdown strength analysis of the Rogowski and service aged XLPE test samples.

Chapter 4 describes the diagnostic equipment and test procedures of the dielectric response measurements in time and frequency domain. Results from the response measurements are presented in Chapter 5, where the effect of frequency and magnitude of test voltage is closer examined. 4 INTRODUCTION

In Chapter 6 the results from calculations using the Finite Element Method (FEM) on simplified models of a‘ water tree’ units are presented. Here the effect of voltage assisted ingress of water into initially collapsed channels of crazed insulation is numerically examined.

In Chapter 7, the results from measurements of water content in the water treed regions by using rnicro-FTIR is presented. Also results from the water permeation experiments are included, where the influence of electrical fields upon the diffusion rate of water is investigated.

Finally a discussion of the results obtained is presented in Chapter 8 and concluding re- marks follow in Chapter 9. CHAPTER 2

THEORY

2.1. INTRODUCTION

In the first part of this chapter theories of water treeing in polyethylene and XLPE cable insulation are briefly reviewed, including some of the reported experimental observations. This summary is mainly based upon previous review papers [4, 11, 12, 13, 14, 15]. Many alternative mechanisms such as electro-mechanical, thermodynamic and electrochemical processes have been proposed to explain water treeing, and most authors iden- tify either a mechanical or chemical mechanism to be fundamental [4].

In case of diagnostic testing, initiation and growth of water treeing is not in question, but merely what occur within a water tree degraded insulation when a test voltage, either DC or low frequency AC is applied. Section 2.3 presents the mathematical relations between the time and frequency domain diagnostic methods used to characterize the water tree degraded samples and to study the nonlinear dielectric response.

Up to now, no theoretical explanation of the nonlinear voltage dependence of the dielectric response has been presented. Section 2.4 presents a possible mechanism for this nonlinear voltage dependence, mainly based upon the mechanical approach to water treeing.

2.2. THEORY OF WATERTREE INITIATION ANDGROWTH

2.2.1. ‘&pical features of water trees

Water trees are generally found to initiate and grow in polymeric insulation exposed to an AC electric field and humidity.

As illustrated in Figure 2.1, there exists two types of water trees; bow-tie and vented water trees. Bow tie trees are initiated from impurities and voids within the insulation, and grow in two directions. Vented water trees are initiated at the interface between the sem- 6 THEORY

a) b)

FIGURE 2.1 Examplesof a) ventedandb) bow-tiewatertreesfoundin a XLPE cableremovedfrom service.Thiscablewasequippedwiththeold type of constrictionconsistingof semi- wnductivetapeandgraphitepainting.

iconductive screens and the insulation. The trees grow predominantly along the axis of the applied field. After a rapid initial growth the bow-tie trees seem to saturate in length. Vented water trees generally need a longer initiation time than bow tie trees, and experience with laboratory and service aged cable insulation has shown that vented water trees continue to grow throughout the ageing period and may finally penetrate the insulation wall. Thus vented water trees are considered to cause a more severe ageing than bow-tie trees.

Microscopic examinations have revealed that drying of cable slices containing water trees makes the trees invisible. When again placed in a humid environment water easily re-enters the water tree structure making it visible. This can be carried out repeatedly with the same result, indicating that the physical and chemical changes of the water treed insulation are permanent.

The saturation content of water in semiconducting materials is in the range of 1-20% [16]. In XLPE the water content is as low as 0.002- 0.02% at normal operating temperatures [17]. Compared to the XLPE insulation, the serniconductive screens are more per- meable to water, but due to the large absorption capability, they require longer time to become saturated. The relatively long initiation time for vented trees indicate that initiation of vented water trees occur when liquid water is present at the interface between the semiconductor and insulation [14]. The easier access of liquid water and possibly also supply of contaminants from the semiconductive screen to the tips of the vented water trees may explain the higher rate of growth compared to bow-tie trees. CHAPTER2 7

By the use of Transmission and Scanning Electron Microscope (TEM and SEM), it has experimentally been found a higher density of voids inside the treed region than outside [15]. Several experimental studies show that the water tree regions are characterized by rows of voids or ‘string of pearls’ especially when they approach the inner conductor [18]. The diameter of these voids may vary from approximately 0.1 up to 5 pm, and relatively large separations between them may occur [19].

No interpenetrating channels crossing the entire water tree have been observed. Howev- er, short networks of connected channels and rnicrovoids have been located in amorphous regions of the water tree area [18-26]. It has also been found that a void coalescence can appear as channels, particularly at boundaries between crystalline and amorphous sections and other weak paths. The channels may serve as paths where pre- sumably solvated ions can travel [14], and many different elements such as Fe, Cl, N% S, Cu, Ca and K, are found within the tree structure. It is experimentally difficult to ob- serve such small channels, as they could have been cut during the sample preparation (freeze fracture or sectioning) and then appear as isolated cavities in the microscope [25]. In addition, such experiments can only be performed on dried water treed samples, and in this condition the channels are most likely collapsed.

The term channel refers to the situation of an open duct or hollow tube allowing the transport of water. However, results horn fluorescence microscopy of water treed sections stained in rhodamine solution, show microchannels that may not be hollow, but only destructed material where water can diffuse [23]. This has also been experimentally confirmed by TEM studies on water treed regions [15]. Thus it has been proposed that hydrophilic tracks in the nanometer scale may also be present, allowing transport of water by hopping of water molecules from one site to the other as a ‘climbing rope’, thus providing a certain connectivity for water. Hence, it has been suggested that both nanometer tracks and microchannels are present in the water treed structure, forming the elongated structures [15]. 8 THEORY

2.2.2. Electro-mechanical theory of watertreeing

The first mechanical damage theory presented assumed that water treeing is due to mechanical overstressing and formation of rnicrofissures around water droplets within the insulation [27]. Sufficient mechanical stress is assumed to be caused by osmosis, supersaturation and condensation of water and Maxwell stresses due to electric field enhancement at the tip of the water tree channel. This approach gave a useful explanation of initiation and growth of bow tie water trees initiated from voids or impurities within the insulation. On the other hand, as water trees were found to grow at service stresses below what were considered likely to cause mechanical fracture, the theory was modified by including crazing and environmental stress cracking [36]. This meehanism is briefly described below.

From fracture behaviour of polyethylene it is known that crazing develops as a precursor to brittle fracture [28]. Crazing zones are known to be regions of ‘spongy’ plastically de- formed polymer consisting of interconnected systems of microvoids and polymer fibrils. The important feature of craze formation is that porous zones may initiate and grow at stresses far below those neeessary to cause crack propagation [29, 30, 31].

At the interface between a mechanically weak semiconductor and a relatively strong XLPE insulation, vented water tree initiation and further growth may be explained in terms of crazing and environmental stress cracking mechanisms (ESC). It is reasonable

T@of water tree l! F— Vii —F II induced \@#1Gaze tensile \@; stress -‘ . 1

FIGURE 2.2. Tip of a watertreewitha growingcrazingzone [14].The insulationM the tipis cycled betweencompressionandzero in thedirectionof thefield. CHAPTER2 9

to assume that internal mechanical strains frozen in during the production process can eventually lead to micro fissures and crazes at the interface between the insulation and semiconductive screens [32, 33]. This maybe accomplished by local changes in crystal- linity upon heating followed by a rapid cooling [34].

When these fissures are filled with water, Maxwell stresses appear at the boundary between the irregularities and the insulation [27]. This is also the case when the surface of an inclusion is hydrophilic or water soluble and surrounded by a thin layer of liquid water or another conductive liquid [35].

If an ESC active agent is present, the stress needed for initiation and further growth is strongly reduced. Polar liquids such as alcohols, detergents and different types of oils, are known to enhance the initiation, propagation and breakdown of a craze. The exact mechanisms involved is dit%cult to determine, but the liquids must be able to diffuse into the amorphous sections of the polymer or the porous zone of the craze in order to enhance a further formation of these weak zones. In addition, metal ions may also enhance ESC [36]. As a consequence, wetting of the insulation surface by liquid water into micro cracks and crazes is of great importance. Water is naturally expelled from XLPE by surface tension and is kept out by capillary action unless there are polar groups present at the poIymer walls [12]. Consequently, the capillary pressure may assists or impede flow of water into the pores or cracks at the surface [28].

In addition thermodynamic considerations show that it is energetically more favorable for water vapour to condense in areas with high electric field strengths. Thus it is likely that dissolved water in amorphous regions will diffuse towards these condensation sites in the high field areas. In case of supersaturation, the excess water also diffuses towards and eventually condense around condensation nuclei such as micro voids, micro cracks or crazes. The hydrostatic pressure that build up (dependent upon the degree of supersaturation) may be sufficient to cause fracture of the surrounding XLPE. If the voids are interconnected and thin crazing zones are formed ahead of the treed structure, the vented water tree can then grow at low electric field strengths.

2.2.3. Electrochemical theory of watertreeimg

The chemical theory of water treeing is based upon the fact that water enter the amorphous regions of polymers if polar impurities are present. These polar regions can be cre- ated by for example pollution or oxidation of the cable insulation or of the compound. The insulation surface is considered to contain polar groups attached to the end groups of the polymer chains. Water can then enter the amorphous regions containing polar groups. In additiom there will be a diffusion of solvated ions to these sites. However, diffusion of water into the polymer will happen to a certain extent whether such groups exist or not. The effect of the electric field is to enhance the diffusion of water and ions into 10 THEORY

the water tree regions, causing the local permittivity to increase. Consequently, the ion conductivity is enhanced, reactive and/or catalytic species are provided and the local electric field is modified. However, this field is considered too low to cause electrome- chanical degradation [15]. The reaction details of the chemical modification of the surrounding polymer are still unclear, and several subtheories exist. A possible process has been proposed by Fothergill et.al. [4, 12];

Dissociation of water molecules produces radicals and free electrons. These radicals rapidly react with the polymer to produce polymer radicals at existing chain ends or by chain scission, which is most likely in the essential weaker amorphous sections. Such chain scission may lead to rnicrovoid formation. Additionally the free electrons may react with oxygen producing free radicals or oxidizing agents such as H202 forming per- oxide radicals by ion catalysis. This oxidation process results in carbonyl, hydroxyl and carboxylate groups on the end of the chains often generated as the result of the chain scission. As these sites are hydrophilic and the chains themselves hydrophobic, there will be afield driven transport of solvated ions into these sites were they can bind. Sites particularly susceptible to oxidation will allow electrolyte and hydroxyl ions in it to penetrate the polymer and extend the oxidation into the bulk.

End groups may bind ions, and the counterions may also become bounded to these ions

** ●*** cations ‘~ .rystcluim 00.0 anions ~ orncrpkmus ...... I% outside woteftree LVJ ionic endgrcups “ v:-::.:;::. ----

FIGURE 2.3. A modelof a nanometerstructureshowingaproposedhydrophilicnatureof watertrees [15]. CHAPTER2 11

thus causing a stable structure. This process may cause the water in the voids to become de-ionized electrochemically with the metal ions bound to the oxidized surface, which is in agreement with experimental findings [20]. The voids are increasing in volume by chain scission and may finally reach a critical size where it is energetically more favorable for the water to penetrate weak paths than the void to continue to grow [12]. 12 THEORY

2.3. DIELECTRIC RESPONSE AND RELATIONS BETWEEN TIME AND FREQUENCY DOMAINMEASUREMENTS

2.3.1. The dielectric response function

In this study non-destructive diagnostic testing of water tree degraded XLPE cable insulation has been performed by measurements of depolarisation/polarisation current, return voltage and dielectric loss factor. All these methods can be used for obtaining the dielectric response, and their relation will be discussed briefly in this section. The relations are only valid in case of linear insulation systems.

The dielectric response function fit) can be defined in terms of the polarisation AP(t) caused by an arbitrarily time-varying signal E(r). In case of a linear system this relation can be expressed by the convolution integral [37, 38];

t (2.1.) M(f) = coJ f(t).E(f– T)dT f(t) = o, (t

The physical interpretation of the convolution integral is that the system retains ‘memory’ of past excitation. If there are no permanent or persistent polarisation in the material, the dielectric response function converges to zero as time reaches infiity.

2.3.2. Measurements in the time domain

In time domain the response function can be obtained by measuring the polarisation current, depolarisation current or return voltage. Figure 2.4 shows schematically return voltage, polarisation and depolarisation currents generated from the application of a DC voltage.

If a step voltage of amplitude UOis applied for a certain time tl, the polarisation current can then be expressed as [37]:

Zp(t) = Uoco &,5(t)+: +f(t) O

The delta function, @t), is due to the instantaneous response of the ‘flee space’ within the volume of the insulation. Normally the current at short times (seconds) is dominated by the polarisation processes represented by the response function, f(t).At longer times the contribution from the conductivity of the material can give rise to a steady current. CHAPTER2 13

As the contribution from the conductivity and the response function cannot be separated by measuring the polarisation current only, it is advantageous to measure the depolarisation current during the short-circuit of the insulation.

The depolarisation current, caused by relaxation of the polarisation processes activated during the application of the step voltage UO, can be expressed as:

zD(t) = Uoco . f(f) t~

Applying a DC voltage for a sufficiently longtime, the conductivity of the insulation can then be estimated by:

0= -&(iP(f)-iD(t)) (2.4.) 00

By using equation 2.4 the DC-conductivity can be calculated from polarisation and depolarisation measurements.

Polarisation Returnvoltage, cmmnt,I~t) u~(t) ~c.c=-nt ------. L~jt

charging I !11 measurements

Time [Seconds]

FIGURE 2.4. Polarisationanddepolarisationcarrentsgeneratedby theapplicationof a DC voltage fromt=Oto t=tl. Alternativelyto depolarisationcarrentmeasurements,theretarnvolt- agecanbe measaredaftera shortgroandingperiod(t2-tl). 14 THEORY

Measurement of the return voltage is the other method for obtaining slow polarisation processes. It is basically a charge measurement, integrating the depolarisation currents when measuring the voltage building up across the capacitance of the sample. However, it may be necessary to take into account the polarisation caused by this voltage.

As can be seen in Figure 2.4, the object is charged with a step voltage UO for a time tI. Then it is short-circuited, and after (t2-tl)seconds the short-circuit is removed, leaving the object in an open circuit condition. This recovery field can be characterized by the following integro-differential equation, where t2

t :uR(t) + Er‘++ uo(f(t)+f(t-tJ) +$J .f(t-~ ) . U(’r)a = o t>tz (2.5.) 0 t2

By solving equation 2.5 numerically, it is possible to determine UR(t) when the dielectric response function, ~(t), the relative permittivity &rand the conductivity ois known. Al- ternatively, it is also possible to calculate the dielectric response function from measurements of the return voltage. By numerically solving Equation 2.5, good correlations have been found between measured and calculated responses [40].

However, when the return voltages are small compared to the charging voltage, and the DC conductivity of the insulation is negligible, the dielectric response function can be determined from these measurements by using the simple relation (see Appendix 1);

dUR(t) f(t-t2)= ;. ~ t>t2 (2.6.) 0 where uR(t’2) = O, and t2 = tl.

2.3.3. Measurements in the frequency domain

When applying a sinusoidal voltage to a dielectric material, the resulting current can be expressed as [37];

r /- \7 1((D)= q &r+x,’ (co)– i(& +X,”(O ] po(~) (2.7.) !_ \-. /-! I where X~‘(@ and X~“(@ is the real and imaginary parts of the dielectric susceptibility, UO the applied voltage, COthe geometric capacitance, @e conductivity, &othe vacuum permittivity and eris the relative perrnittivity at power frequency, typically 2.3 for XLPE I CHAPTER2 15

insulation.

The real and imaginary parts of the dielectric susceptibility are related by the Kramers- Kronig relations [37]. This means that the transformation from the real to the imaginary part (or vice versa) of the dielectric susceptibility should yield the same result. When cal- culating both the real and imaginary parts of the dielectric susceptibility from frequency domain dielectric response measurements, this transformation could be used as a control of the consistency of the measured signals [41].

The relative permittivity is associated to the susceptibility, and the relation can be expressed as shown in Equation 2.8. The last term in this equation is the contribution to the loss from the conductivity. Infrequency domain it is difficult to distinguish between the conductive and dielectric part of the losses.

However, at low frequencies the losses caused by the conductivity may dominate. Thus in such cases it may become possible to distinguish between these losses from measurements performed at several frequencies,

;(0$=e’ (co)–ie’’(crj) = er+X~’ (03)– i X.’’(m)+ ~ (2.8.) ( 0 )

The real and imaginary parts of the complex permittivity, &(o), are linked by the dielectric loss factor, tanii

&“(co) e,” (0) E,” (co) tuna=-= (2.9.) c’ (Cij) E, +x.’ ((0) = s,+ As’ ,(co) where A&r’(@ is the change of the permittivity. The dielectric loss factor can be physi- cally described as the ratio between the energy dissipated per cycle and the energy stored per cycle.

2.3.4. Relations between time and frequency domain

In the frequency domain, the dielectric susceptibility corresponds to the dielectric response functiom and in case of linear systems the time and frequency domain representation are related by the following Fourier transform and its inverse;

XJO$ = X,’ (0$ –X,” (0) = j f(f). e-~wdt (2.10.) o 16 THEORY

Some typical empirical models of dielectric response functions, ~(t), based upon experimental data are shown in Figure 2.5. The Curie von Schweidler model (f(t) - t-n) has experimentally been found to be valid for many dielectric materials [42]. However, the model is only applicable for limited time ranges, as the model does not converge at short times. In addition, when the parameter n is less than zero, the total charge stored in the material diverges.

The low frequency dispersion (LFD) of the response is measured by a decreasing time dependence, and is also widely observed in many dielectric materials [43].

When the dielectric response function can be described by the Curie von Schweidler model, the corresponding complex susceptibility can be analytically calculated as [44];

(2.11.)

\ Curie von Schweidler - t-n, (kn

Time [seconds] (logscale)

FIGURE 2.5. Differenttypesof dielectricresponsefunctionsin timedomainbaseduponexperimen- taldata.The exponentsn andm arecharacterizingthe slope of tbe curves.The single Debye-peakis normallynotfoundin soliddielectricmaterials,butis includedfor comparison. CHAPTER2 17

A n-lr(l_n)co~T(1-n)71 ~’Jo) = p O

%“,(o) = -$. On-lr(l-n)cOs~ 0

It has been shown that the complex part of the dielectric susceptibility can be expressed by the Hamon approximation [44];

f(t) . t x“s(~)= ~n . OJ 0.3

The relation between Hamon frequency and time is given by:

f=y (2.15.)

The expression in Equation 2.14 has an approximation accuracy within some percent for the parameter n in the range of 0.3 en e 1.2. However, if the system display a loss pek the Hamon approximation may not define the position and the magnitude of the peak precisely [46].

Based upon measurements of depolarisation currents (ZD(t) - t-n), the Hamon approximation can be used to calculate the dielectric susceptibility by combining Equation 2.3 and 2.14.

zD(t) o t X“.(ci)) = 0.1. 27W.C0 0.3

In case of measurements of return voltages, a similar expression is obtained by combining the Equations 2.6 and 2.14;

dUR(t) e,. —. dt t #Ja$ = 0.3

2.4. MECHANISM CAUSINGA NONLINEAR DIELECTRIC RESPONSE

2.4.1. Effect of application of electric fields

The dielectric response of water treed polyethylene insulation is found to increase more than linearly at relatively low applied test voltages [10]. It is therefore unlikely that this phenomena is caused by mechanisms such as space charge limited currents or other high field mechanisms [38].

A simplified model of a water tree structure showing ellipsoidicrilly shaped cavities forming a ‘string of pearls’ interconnected by small channels is shown in Figure 2.6. In case of waterfilled voids within the water tree structure, a distortion of their shape or a slight change in volume is likely to happen when they are subjected to electrical fields. This is schematically illustrated in Figure 2.7. At low or no applied electric stress the water treed region is characterised by spherical micro voids fiiled with liquid water and separated by channels of crazed insulation. The water tree will then be observed only as a ‘string of pearls’. Increasing the test voltage will result in Maxwell mechanical tensile stresses strong enough to elongate the water droplets, causing the crazes to open up, and water to penetrate into the mechanically weak crazing zones.

2.4.2. Hypothesis for voltage dependent dielectric losses

A deformation of spherical droplets into prolate spheroids causes the overall permittivity

FIGURE 2.6. Simplifiedmodelof a watertreestmctureshowingellipsoidicallyshapedcavities forminga ‘stringof pearls’interconnectedby smallchannels. CHAPTER2 19

b) Distortionof c)

~ $i&P ‘f ‘i’ Z

stress(F> FJ

‘Mechanically Elongationof Fillingof waterinto weaksections thewaterfiied initiallycollapsed of initiallydried voids channels channels

E = O(or small) E>Et E>Et

FIGURE 2.7. Effectof appliedelectricstressonwaterfiied sphericalvoidswithinaXLPEmatrix. The voids aresubjectedto anincreasingelectricfield, causingthemto be elongated. Thefiial stageis filling of liquidwaterintotheinitiallycollapsedcrazingzone.

to increase with applied voltage [47]. The dielectric response is found to increase with increasing dielectric permittivity, and consequently becoming nonlinear [48]. Such an increase is likely when applying both AC and DC voltages. However, only the transient current flowing in the thin channels causes losses dependent upon permittivity and conductivity of the material inside the channels.

Due to the complex structure and the chemical composition of water tree channels its conductivity may vary within a wide range. In addition, the physical state of water is not clear (liquid, dispersed, or bound). As described in Section 2.2.1, metal ions and oxidation products are frequently found within the tree structure. While ions increase the conductivity of water, oxidation products may bind water molecules causing an equivalent reduction. Subsequently the conductivity of each channel may vary dependent upon the amount and state of these products deposited in the channels [49, 50].

The capacitive current flowing in such channels may contribute to the formation of a nonlinear dielectric response due to increased ohmic losses as more and more channels are filled with water. Strong dipoles bound to the polymer possibly present in the initially collapsed channels, may also contribute to the increased dielectric losses as wetting 20 THEORY

the channels allows these segments to be displaced in an electric field. In case of an AC oscillation this may produce a mechanical local force, resulting in fatiguing effects [4].

In addition higher current magnitudes may also cause local heating which is likely to take place at regions with high field stress or within inhomogenities such as voids or channels. The rate of heating is dependent upon the frequency of applied voltage, local permittivity, dielectric loss factor and the local electric field. This temperature increase causes the mechanical strength of the surrounding XLPE to decrease, enhancing the process of opening collapsed water tree channels.

As a consequence, the nonlinear dielectric response will probably be dependent upon the diagnostic procedure. Dielectric response measured either in frequency or time domain are likely to be different. CHAPTER 3

DIAGNOSTIC EQUIPMENT AND TEST PROCEDURES

3.1. INTRODUCTION

This chapter describes the diagnostic equipment and the test procedures used for testing the water tree degraded XLPE cables and Rogowski test objects.

The dielectric response was obtained by time domain measurements of polarisation and depolarisation currents and return voltages, and measurements of permittivity and dielectric loss factor in frequency domain.

3.2. EQUIPMENT FOR TIME DOMAINMEASUREMENTS

Time domain measurements were performed by using the computerized experimental set up, schematically shown in Figure 3. la).

A high voltage DC source was used to charge the XLPE insulated samples, and measurements of polarisation and depolarisation currents as well as the return voltages were performed using an electrometer (Keithly 617). The voltage source output from the electrometer was used to control the DC source, and the measured signals were transmitted to the computer by optical fibres. A LabView@ program was developed in order to com- municate with the relay controller (IO-tech 488/16) and the electrometer via a GPIB interface.

As the response changes rapidly in the beginning, the sampling of data the 10 first seconds was performed with the fastest sampling rate of the electrometer ( 1s). Later, there- sponse was changing slower, and logarithmically spaced samples were used in order to minimize the number of data. 22 DIAGNOSTICEQUIPMENTANDTESTPROCEDURES

a) HV - relays

— Relay controller R2 RI R3 Sample I HVDCoc Digital / Light trans- voltage Electro- mitterand receiver source s~ meter T T

to the HVDC voltagesource b)

RI = 50MQ R2 = 500kQ R3 = 10IKQ

- to thecompute[ Experimentalarrangement in caseof measurementson Elec@o- theRogowski typesamples. S2 meter B -Jw- to theHVDC voltagesource

FIGURE 3.1. a) The .principle. of time domaindielectricresponsemeasurementsof depolarisation currentsandreturnvoltage.(R1+R2)and(R1+R3)arechmginganddischargingresis- torsrespectively.C representsthesample,andS1andS2arePC-controlledhighvoltage relays(ROSSEng.andKilovac).

b) In caseof measurementson theRogowskiobjects,thehighvoltagerelayS2andthe electrometerwereconnectedto thegroundelectrodeof thesample. CHAPTER3 23

10-8 -@- Depolarisation current i ~ “>, + Polarisation current 10-9 ‘}

i ~ ““1: ~o-lo5 . ‘I,Polarisation curren~ ‘,,pre-chargingfor0.2s 2 ; !

0.5kV, withoutsuppressing theoffsetcurrent

5kV (3.8kV/mm)

q , ~,j 0.5kV (0.38kV/mm) ~o-ls 23510 2351002351000

Time [seconds]

PIGURE 3.2. Polarisationanddepolarisationcurrentsmeasuredon anontreedXLPERogowski sample at0.5 and5 kV.Thefigureshowsthenecessityof suppressingtheoffsetcurrentand pre-chargingtheDC sourcebeforemeasuringthecurrents.

In order to easily measure the polarisation curren~ the HV-relay S2 and the electrometer were connected to the ground electrode of the sample, as indicated in Figure 3. lb).

The high voltage DC source was allowed to stabilize for 200 seconds before charging the sample. This was done in order to avoid possible errors of the polarisation currents due to the charging of the large internal capacitance of the DC source. This is demonstrated in Figure 3.2, where pre-charging the DC source for only 0.2 seconds caused the error to be significant for the first 20 seconds. This effect was eliminated by increasing the period of pr~charging to 200 seconds.

As indicated in Figure 3.2, a permanent offset-current of approximately 10-13 A was found to be present at the termination of the electrometer at no load condition. At low voltage levels this offset-current was in the range of the measured response currents. It was therefore necessary to supress it by detecting the offset-current for 5 minutes prior to each measurement. By proper guarding and shielding, it became possible to measure currents lower than 10-15A, with an instrumentation resolution of 10-16A. 24 DIAGNOSTIC EQUIPMENT AND TEST PROCEDURES

In general, the polarisation current measurements suffered more from noise than the depolarisation current measurements, probably caused by electric noise from the high voltage DC source itself, and from the high voltage relay causing mechanical induced noise.

3.3. EQUIPMENT FOR FREQUENCY DOMAINMEASUREMENTS

The measurements were performed using the system schematically shown in Figure 3.3. It was developed by U.Giifvert (ABB), and commercialized by WabTech/KTH [51, 52]. The instrument can resolve dielectic loss factors below 10-4, at operating voltages up to 14 kVm~, within the frequency range from 10A Hz to 100 Hz.

In this syste~ a sinusoidal voltage is applied and the complex permittivity is determined from the amplitude and phase of the resulting current, measured by an electrometer. The balancing arm, consisting of the inverter and the balancing capacitor, cancels the greater capacitive current, thus enabling measurement of small changes in both loss and capacitive current of the test sample [53].

The balancing is only performed at one frequency and at a low voltage. As the change

CH1 Gen CH2 I Frequency response analyser

I-Iv divider

electrometer Balancing arm Liquid nitrogen cooled reference

FIGURE 3.3. The principleof frequencydomaindielectricresponsemeasurements,wherethecom- plex permittivityandloss factorarecalculatedfrom thevoltageandcurrentmeasured withtheelectrometer. CHAPTER3 25

in permittivity, z@.’(o$, was found to be dependent upon the balancing level, the perrnit- tivities at other frequencies and voltages is related to this level [54].

To ensure low losses in the balancing capacitor, the reference polypropylene capacitor is cooled with liquid nitrogen.

3.4. GUARDINGANDPREPARATION OF TEST OBJECTS

Generally, the measurements in both domains were performed with the samples and the measuring circuit kept in a grounded cage in order to increase the sensitivity of thes ys- tem and to reduce electrical noise. All measurements were performed at a room temperature of 20°C.

3.4.1. Rogowski test samples

After being removed from ageing, the objects were short-circuited and kept wet at room temperature for minimum 7 days.

The guarding and shielding of the Rogowski test objects was designed as shown in Fig-

Hv electrode

Water 4 Shield (grounded)

/ Brass ring and copper tape Measuring electrode ‘F

FIGURE 3.4. Guardand shieldarrangementused duringthe dielectricresponsemeasurementsin time andfrequencydomainon theRogowski typetestobjects.The capacitanceof the objectswaslessthan35pF(1.3 mminsulationthickness). 26 DIAGNOSTIC EQUIPMENT AND TEST PROCEDURES

.n.L______Lu 235 235 235 ~o-’ 1o-1 1 10

Frequency Nz] FIGURE 3.5. Results from measurements of dielectric loss factor on a dry and unaged XLPE Rogowski sampleat2 and5kV (1.5 and3.8kV/~ peakvalues). 10-2’r

~o-3 ~ 235 ~235 235 1o-’ 10” 1 10

Frequency @z] FIGURE 3.6. Results from measurements of apparent pennittivity on a dry and unaged XLPE Rogowski sampleat2 and5kV (1.~-and3.8kV/w, p&k values). CHAPTER3 27

ure 3.4. The water-filled object was equipped with a brass ring attached to the outer wall and close to the measuring electrode. In order to avoid couplings between the HV electrode and the measuring electrode a shield was placed above the brass ring and grounded. This was particular impontant when measuring the response in frequency domain. The guard was grounded in order to avoid creepage currents, which could be large in comparison to the measured signals from the test samples, especially at the lower frequencies. Results presented in Figure 3.5, show that without guarding the response was increased by a factor of 10 at 10mHz in magnitude and became nonlinear at lower frequencies. By using the guard arrangement shown in Figure 3.4, the dielectric loss factor becomes linear. Results presented in Figure 3.6, show that the change in apparent permittivity was only slightly dependent upon guarding, and independent of applied voltage.

3.4.2. Cable samples

During the time domain measurements no cable terminations were used, thus preventing contribution from the polar material of the stress cones to the dielectric response. The currents and the return voltages were obtained using the conductor as the measuring electrode.

In case of the frequency domain measurements the cable samples were guarded in accordance with the procedure described in [10] to avoid surface leakage currents and contribution from the stress cones.

To prevent drying the XLPE cable samples were placed in a water-filled plastic tube during the measurements.

3.4.3. Degassing and water treatment

Before thermal treatment all sheets and metallic ground screens were removed from the cable samples.

In order to remove the crosslinking by-products the unaged cable sample D was degassed in a ventilated oven for 8 days at 90°C. One sample of cable B was subjected to drying and water treatment at 90*C. During drying, the cable sample was kept in a ventilated oven at 90°C for 3 days. Wetting was performed by keeping the cable sample soaked in water for 3 days at 90°C. 28 DIAGNOSTIC EQUIPMENT AND TEST PROCEDURES

3.5. NUMERICAL EVALUATION OF DATA

3.5.1. Methods for estimating the degree of nonlinearity

The degree of nonlinearity, is in time domain defined as the ratio between the measured and estimated linear current after 20s at 6kV based upon measurements up to lkV. This is mathematically expressed in Equation 3.1, where the abbreviations are explained in Figure 3.7.

(3.1.)

In frequency domain the degree of nonlinearity is determined as the ratio between the dielectric loss factor or apparent permittivity at 6 and lkV at 0.1 Hz. :

Measured response after20 seconds ~ + 2 % A

Measured (nonlinear response, ZJu, ?) / / .“ ,“ 1 “ Estimatedlinear response, ZJU, t), based upon measurements UP to lkV.

Charging voltage KV]

FIGURE 3.7. A typical nonlinemresponsein timedomain.The responseis linearatlower voltages, and becomes nonlinearat a typically low volt%% Ui.me nofie~ty is ~elY to increasewithincreasingtest voltage. CHAPTER3 29

&“(U, f)lu =fj~v,f=(),l~z (3.2.) ~“=E“(%filu=Mv,f=o.lHz

A&’(M,filU =G~v,f= O.lfLZ (3.3.) k’ = A&’(tJ, f)lU = wv, f= O.lfiZ

In case of nonlinear systems the values are higher than 1.

3.5.2. Fourier transformation of time domain measurements

The numerical calculation of the Fourier transform (Equation 2.10) can be performed by using many different methods. As the sampling of data in time domain is made at loga- rithmic time intervals, the Fast Fourier Transform (FFT) cannot be used since the FFT requires a linear time base.

Inste@ using a method based on an approximation of the experimental data by cubic splines, the time spacing is made linear, and the Fourier - transform of the spline curve can then be calculated directly by using the analytic expressions of the splines [55]. This was facilitated by a computer program (Diel) which determines the dielectric loss factor as well as the change of permittivity from depolarisation current measurements [56]. 30 DIAGNOSTIC EQUIPMENT AND TEST PROCEDURES

3.6. TEST PROCEDURES

3.6.1. Measurements in time domain

The polarisation currents were measured during the charging period of 15 minutes at different DC levels. After grounding for 1s, the depolarisation current or the return voltage

Charging Discharging Measuring Waidng Suppression Numberof Chsr@ng voltages time[rnin] time[s] time[rein] time[rein] time[rein] points WI 15 1 15 30 5 80 0.1-6 (10)

TABLE 3.1. Descriptionof themeasuringparametersduringthetimedomaindielectricresponsemeas- urementsof polarisationanddepolarisationcurrents.

was measured the following 15 minutes.

In order to avoid memory effects caused by the previous DC applications, the samples were short-circuited for 30 minutes before application of the next DC-level. The cable samples and the Rogowski test objects were subjected to charging voltages up to 10 and 6kV respectively. The total measurement time for each object was of a duration of maximum 10 hours.

3.6.2. Measurements in frequency domain

The dielectric response measurements in frequency domain were performed by keeping the applied voltage constant and stepping the frequency from 10 to O.lHz at 3 frequencies per decade. At 2kV the lowest fkquency was 10mHz.

In case of the Rogowski test objects the applied voltages ranged from 0.3 to 6kV. In order to investigate a presence of a hysteresis, a frequency sweep was again performed at 2kV. Approximately the same procedure was followed in case of the cable samples, but the applied voltages ranged from 2 to 9kV. AU the voltages and fields are referred to as peak-values unless otherwise noted. CHAPTER 4

CHARACTERISATION OF THE ROGOWSKI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

4.1. INTRODUCTION

The experiments presented in this thesis are performed on samples taken from service aged XLPE cables and laboratory aged Rogowski type objects.

The purpose of this chapter is to describe the different test objects and to present the results from microscopy water tree and AC breakdown strength analysis. One main objec- tive for performing laboratory ageing of Rogowski type test objects was to facilitate diagnostic testing as a function of ageing time. Secondly a new method was developed to enhance initiation and growth of vented water trees, a technique essentially based upon insertion of salt impurities at the boundary between the upper semiconductor and the insulation.

402. EXPEIUMENTAL METHODS

4.2.1. Manufacturing of the rogowski type test object

The accelerated water treeing ageing test has been performed using a total of 55 Rogowski type test objects; 50 samples with salt (NaCl) inclusions deposited at the interface between the XLPE insulation and the upper semiconductive screen, and 5 reference samples without salt impurities. It was important that the particles were placed at the semicordinsulation interface, as particles embedded within the semiconducting material did not initiate vented water trees at short ageing times [57]. 32 CHARACTERISATIONOFTHER~WSXI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

The objects were manufactured using high quality crosslinkable cable grade materials of low density polyethylene. Both XLPE insulation (LE 4201S) and the serniconducting screen material (LE 0592) was manufactured by Borealis. The serniconductive screens were used as high voltage and ground electrodes as indicated in Figure 4.1 [58].

Basically the manufacturing process described by S.T.Hagen was followed during the production of the objects [59].

1. Homogenisation of granules A laboratory twin-screw kneader was used to extrude insulation tapes in order to secure a homogeneous mixture of polyethylene, dicumulperoxide and antioxidant.

2. Preshupe of insulation and semiconductive screen This tape was then cut into tablets, and pressmoulded in a hydraulic press at a maximum temperature of 115°C, slightly above the melting temperature, to an insulation thickness of 1.3mm. The compression moulding of the semiconductive screens were performed on plaques at 120°C. All semiconductive screens used for the objects had a thickness of o.5mm.

3. Water tree initiation sites Before assembling the test objects, a microliter syringe was used to place 200 droplets of water containing O.lM NaCl solution on the surface of the semiconductive screen. The volume of each droplet was limited to 0.2@ The water was then allowed to evapo- rate in laboratory atmosphere at 20°C and 30!Z0RH for 12 hours. In order to avoid any further contamination of the serniconductive surface, tis drying process was performed in a flow bench with filtered air. The number of particles in the air were measured by a particle counter capable of deteeting particles as small as 0.3prn. The contaminated semiconductive screens were placed close to the filter when no particles were detected. The purpose of this procedure was to create a controlled amount of initiation sites for the growth of vented water trees.

Prior to moulding, typically about 6 to 8 NaCl particles were found at each initiation site after evaporation. These were located in a circle with a typical diameter of 300pm, as indicated in Figure 4.2 a)

4. Assembly In addition, an O.lmm thick steel brushed aluminium backing was placed in contact with the lower semiconductive screen.The aluminium backing serves as an electric contact and a watertight barrier, preventing water from evaporating from the lower semiconductor (the ground electrode) during the water tree ageing as indicated in Figure 4.1.

5. Curing Then the assembled test objects were placed in the mould again, and the temperature was CHAPTER 4 33

raised to 175°C and kept there for 15 minutes.

6. Degas.ring and annealing The objects were annealed in an oven at 115°C for 15 minutes in order to remove internal mechanical stresses. Finally the objects were removed from the mould, and degassed at 70°C for 168h in order to remove crosslinking by-products such as acetophenone and cumylalcohol as these are found to strongly inhibit both the inception and growth rate of water trees [4].

7. Quality assurance In order to investigate the shape of the NaCl inclusions after the curing process, some Rogowski objects were radially cut into slices of 200pm in thickness using a rnicrotome. These slices were then analysed optically by using a lOOx stereo microscope to locate the inclusions for SEM examination.

Some typical SEM micrographs of enclosed NaCl particles at the boundary between the insulation and the upper semiconductor after moukling are shown in Figure 4.2.b) and c). The irregular-shaped particles had a typical thickness of some tens of microns with lengths about 100 microns oriented along the boundary between the XLPE insulation

High voltage electrode (Stainless steel) Cover

0.1 M NaC!l

NaCl particles I

Steel brushed \ k. 0.5mm thick metal backing semiconductive screens

1.3mm thick XLPE insulation

FIGURE 4.1. Schematicrepresentationof thedifferentpartsof theXLPE Rogowski typetestobject. The NaCl - particlesare insertedon the upper serniconductivescreenprior to the assemblyin orderto acceleratethewatertreeageing. 34 CHARACTERISATION OF THE ROGOWSKI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

w NaCl particles ii deposited on the 3 CA upper semiconductor prior to moukiing

Boundary between the upper semiconductor and the XLPE insulation

Semiconductor I XLPE insulation

c) Upper particle in b) enlarged

FIGURE 4.2. a) NaClparticlesdepositedon thesurfaceof theupperserniconductivescreen,b) SEM micrographof typicallyexamplesof NaClinclusionspresentattheboundarybetween theuppersemiconductorandtheinsulation.c) Theupperparticlein b) enlarged. CHAPTER4 35

andthe semiconductor. As indicated in Figure 4.2b), the major part of each particle was generally found to be located within the semiconductor, but clearly in contact with the XLPE insulation.

4.2.2. Type of cables

Some characteristic features of the four XLPE cable samples used are summarized in Ta- ble 4.1.

Cable samples A and B had been in service for 18 and 10 years of service, respectively, and were replaced due to several service failures caused by severe growth of vented water trees from the insulation screen. These two cables were equipped with the old type of insulation screen consisting of graphite painting and semiconducting tape. Cable sample C had strippable insulation screen, and was removed from service after 15 years due to long vented water trees. The unaged cable sample D was used as a reference. It was produced in 1996 and had a modern, fully bonded insulation screens.

Samples Ageing time Insulation Conductor Rated volt- Ageing [years] screen cross section age conditions [rid] &v] A 18 Painted 150 24 service B 10 Painted 400 24 service c 15 Strippable 95 24 service D o Fully bonded 150 24 unaged

TABLE 4.1. The tableshowsa descriptionof theXLPE cablesamplesandtheageingconditions.

42.3. Water tree analysis

Measurements of water tree lengths and density were performed using an optical stereo microscope with a magnification of up to 100 times. The 0.5rnm thick microtomed slices were stained in methylene blue dye solution prior to examination [60].

In case of the service aged cable samples the analysis was performed on a total cable length equivalent to 30mm of the cable insulation (- 25.5cm3) including the breakdown sites. 36 CHARACTSRISATION OF THE ROGOWSXI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

20 slices from each Rogowski type object were microtomed from the centre of the sample including the breakdown channel (- 0.52cm3).

In each slice the following parameters were recorded: i) maximum length and total number of vented trees from the insulation and conductor screen, ii) maximum length and density of bow-tie trees. This water tree investigation is based on the premise that it is the longest water trees that limits the breakdown strength. Therefore only the longest bow-tie and vented water tree (l_) are measured in each slice, and extreme value sta- tistics was used to characterize the distribution of the longest vented water trees. Gum- bels third asymptotic probability function is used, which could be called the Weibull type distribution of the largest value [61],

P(Lmax c lma) = P(lmm) = exp – (4.2.) [ (2%)7 where v is the expected largest value (37% value), d is the insulation thickness and J3is a parameter characterizing the dispersion. The statistical analysis was applied to the data when each slice contained more than 10 water trees.

The asymptotic distribution of 1- can be used to calculate the most probable longest tree in a certain length (L) of the cable, and can be expressed as [62],

L; VL = d–(d–v) ; (4.3.) ()

4.2.4. AC breakdown strength testing

Due to the short cable lengths available, the breakdown testing of the service aged cables was limited to 3 samples of cable A and 5 samples of cable B. These were energized by an AC-ramp voltage increasing until breakdown by a rate of 20kVhninute (7kV/ mmmin). The active test length was 2. lm for samples of cable A, and 5m for samples of cable B using stress-cones as terminations during the test. Cable samples C and D were not subjected to breakdown testing.

In case of the Rogowski samples a similar test procedure was used. They were subjected to an AC-ramp until breakdown increasing at a rate of 20kV/minute (15kV/mmmin). In order to prevent external flash-over, the objects were immersed in silicone oil during the test.

The observed breakdown values were fitted to the Weibull distribution of the smallest CHAPTER 4 37

value [63]:

P(E c Emin)= p(Emin) = 1 – exp – (4.4.) [ (%’--$)7

Here ECis the expected smallest value (63!% value), EO is the limitvalue for the lowest possible breakdown strength and ~ the shape parameter. As it is difficult experimentally to determine Eo, it is common to choose this value equal to zero.

The probability of breakdown was estimated using equation 4.5 [64].

P(i, n) = * (4.5.) where i is the number of breakdowns at a certain voltage or electric field strength Emin and n is the number of total samples in the test. It has been shown that this estimate is considerably more accurate than other commonly used estimates.

All water tree and breakdown test results have been treated by using the statistical computer program WeibullSmith from Fulton Findings. The parameters of the distributions were estimated by the use of the maximum likelihood method.

Prior to breakdown testing liquid water was kept in contact with the insulation screens to avoid drying of the insulation. 38 CHARACTERISATION OF THE ROGOWSKI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

4.3. SERVICE AGED CABLE SAMPLES

4.3.1. Results from water tree analysis

The results from water tree analysis of the service aged XLPE cables are summarized in Table 4.2 and some typical examples of water trees are shown in Figure 4.3. The differences in vented tree growth of the examined cables are clearly demonstrated.

Cable B was found to have the highest concentration of vented trees from the insulation screen. These vented trees were rather thin and needle shaped. This is different from that observed in cable A and C, where the trees were larger in width.

Figure 4.4 shows the distributions of observed maximum tree lengths (i-) in each examined cable section. As can be seen the distributions fit well to the experimental data and it was found that all the longest trees were vented water trees growing from the insulation screen. The longest observed vented water tree was found in cable A, penetrating 92?Z0of the insulation thickness.

The bow-tie water trees were few and very short compared to the vented water trees. However, the longest bow-tie tree was found in cable B, and had a length of 950prn.

. . a) b) c)

Cable A, Cable B, Cable C, 18 years of service 10 years of service 15 years of service

FIGURE4.3. a),b) andc) Photographsshowingtypicalventedwatertreeingattheterminationof the ageing.The insulationthicknessfor theserviceagedcablesamplesare5.5mm. CHAPTER4 39

a) I 99 Vented Bow-tie

● g 90 1 ii

Cable A

2~ 1 235 10ZX5 100 Non-treed insulation (d-l_)/d [%]

Cable B

I p=21 $=59 2~ 1 235 102X5 100 Non-treed insulation (d-l_ )/d [%]

FIGURE 4.4. Resultsfrom watertree analysis.Lengthdistributionof the shortestremainingnon- treedinsulationusing Gumbels’ thirdasymptoticprobabilityfunctionof the largest valueperslice,a) ServieeagedcableA, b) ServieeagedcableB. 40 CHARACTSRISATION OF THE ROGOWSKI TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMFLES I

Water tree analysis Bow. e Vented kee.s Cable Insulationscreen Conductor screen Ssmple Ageirrg LMx,37% Llax, 37% (b) n kix, 37% w:% [95%c.i.] (bin) n (lm) [cm-z] [~m] [mm-s] [95%c.i.] [95% c.i.] [c~-21 ~m] @m]

230 4060 .-. A 18 (560) 0.01 (5040) 14 (110) 1.8 [160, 290] [3720, 4350] + --- 350 3640 --- B 10 (950) 0.2 (3880) 141 (508) 1.7 [290, 440] [3580, 3690] -.. El=c 15 No waterztree amdysis performed I I

TABLE 4.2. Resultswatertreeanalysis.Lengthof thelongesttreeobservedin theexaminedcable lengthis givenin brackets.The dottedlinesindicatethattherearetoo few observationsto cak%datctheexpectedktrgestwatertree(s’7~0 -value). I CHAPTER4 41

4.3.2. Results from AC breakdown strength analysis

Results from measurements of AC breakdown strength are presented in Table 4.3 and Figure 4.3 b). It is clearly demonstrated that the service aged cables had very low residual breakdown strengths, with especially low values in case of cable sample A. (The dotted line in Figure 4.5 for cable A, indicate the extrapolated breakdown strength level of a 5m long cable sample).

AC breakdowntests

Average 95% Shape breakdown confidence factor, Sample Ageing strength interval ‘ime (63%value) B [yed ~v/m] rkvhmn] m“) (W) 13.5 [13,14] A 18 (38) (36.5,39) 42 L 20.5 [20,21] 10 (57) (46.5,53) 45 c 15 No breakdown strengthanalysisper- I I formed

TABLE 4.3. Resultsfrom theAC breakdowntestson theserviceagedcablesmnplesA andB.

1 2 3 5 10 Breakdown voltage UAJO [pu]

FIGURE 4.5. Distributionof breakdownvoltage (UO=12kV)of serviceaged cable sampleA and B usingWeibullsprobabilityfunction. 42 CHARACTERISATION OF TRE ROGOWSRI TYPE TEST OBJECTS ANO THE SERVICE AGED CABLE SAMPLES

4.4. LABORATORYAGED TEST OBJECTS

4.41. Test condition and ageing procedure

Before water tree ageing the objects were filled with tap water, and kept in an oven at 50°C for 24 hours. This was done in order to increase the content of water and thus enhance the initiation and growth of vented water trees.

The water tree ageing was performed at room temperature and the objects were energized at 50 Hz AC electric field of 4kVhnm, a value slightly above typical service stress for medium voltage XLPE cables (2-4 kV/mm). During ageing the cup-shaped objects were sealed with a polyethylene cover to avoid evaporation of water. Stainless steel electrodes were immersed in water to make electric contact of the high voltage to the semiconductor.

10 objects were removed after O, 8, 12, 16 and 20 weeks of ageing for measurements of dielectric response, breakdown strength and degree of water treeing. The 5 reference samples with no impurity deposits on the upper semiconductive screen were examined after 20 weeks of aging.

Samples to be removed for examination were selected statistically to reduce the influence of unintended factors such as location in the hydraulic press, the succession of introducing salt particles on the serniconductive screen, operator fitness and so on. The randomization was performed by drawing numbered patches from a bag without laying the patches back.

4.4.2. Results from water tree analysis

After ageing both vented and bow-tie water trees were found in the Rogowski samples. In case of vented water trees growing from the upper semiconductive screen, all the longest trees were found to grow from the salt inclusions. The length and the number of the trees were increasing with increasing ageing time. After 20 weeks of ageing 85% of the NaCl inclusions initiated vented water trees. A summary of the obtained data is presented in Table 4.4.

It was observed that some of the 0.5mm thick slices contained only 1 or 2 trees. The extreme value statistic analysis was therefore based upon a total of 10 slices. CHAPTER4 43

Photographs of typical vented water trees growing from the upper semi conductive screen after 8 and 20 weeks of ageing are presented in Figure 4.6. Results presented in Table 4.5, show that after 8 weeks of ageing, the longest observed water tree was 480pm in length, increasing to 880pm after 20 weeks of ageing, thus penetrating 6890 of the insulation wall. The graphs presented in Figure 4.7, show that the longest observed vented water trees were found to fit well to the extreme value distribution.

Vented water trees were also found to grow from the lower semiconductive screen. However, these trees were found to be short, and the longest tree observed was found after 20 weeks of ageing and had a length of 200pm, equivalent to 15% of the insulation thickness. After 20 weeks of ageing the density was as low as 2.6cm-2 , i.e. more than five times lower than that of the upper screen.

The results presented in Figure 4.6 and 4.7 show that the maximum length of the bow- tie trees only slightly increased after 8 weeks of ageing. Typically the bow-tie trees were found to be shorter than 120pm. The longest observed bow-tie tree was, however, found after 20 weeks of ageing having a length of 280prn.

I Watertreeanalysis I

[weeks] [~m] [mm-s][#m] [cm-z] ~m] [cm-z] 90 382 --- 8 (110) 0.14 (480) 3.4 (60) 0.15 [83, 96] [351, 413] —- 589 --- 12 (::0) 0.16 (680) 6.2 (80) 0.4 [79, 112] [553, 623] --- 693 ---- 16 (L%) 0.28 (800) 7.8 (180) 1.6 [103, 143] [650, 734] —-- 117 801 --- 20 (280) 0.22 (880) 13.8 (200) 2.6 [79, 112] [770, 832] --- 158 ------m (200) 0.19 (170) 2.7 360 3.0 (reference) [117, 190] ------

TABLE 4.4. Resultsfrom thewatertreeanrdysis.Length of thelongesttreesobservedin the 10ex- aminedsamplegaregiveninbrackets(5 incaseof thereferencesamples).Thedottedlines indicatethattherearetoofew observationstocalculatetheaveragelongestwatertree(qT~o vrdue).Thecalctdationsof the qT~o -valuehavebeenperformedusingthelongesttrees foundin eachsample. 44 CHARACTERISATION OF THE ROGOWSIU TYPE TEST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

8 weeks of ageing upper semiconductor

Bow.-tie XLPE tree insulation

lower semiconductor

b) 1000 1 75 800 60 Vented water trees

600 ● 45

400 30

200 15 Bow-tie water trees

0 0 4 8 12 16 20

Ageing time [weeks]

FIGURE 4.6. Resultsfromwatertreeinganalysis.a)Photographsof typicrdwatertreesafterdifferent ageingtimes(insulationthickness1.3mm).b) &lcula~ expectedlongestwaterdees asafonctionof ageingtime(37% -value). CHAPTER4 45

a) 20 WE3&S 8 W< ks 99

63 50 Vented water trees 30

2 10 2 3 5 100 Non-treed insulation (d-~ )/d [%]

Bow-tie water trees

20 weeks 8 weeks I 10 2 3 5 100 Non-treed insulation (d-~ )/d [%]

FIGURE 4.7. Resultsfrom watertreeinganalysis.Lengthdistributionof thea) longestventedtrees andb) longestbow-tie treesusingGmnbels’ thirdasymptoticprobabilityfunctionof thesmallestvalue(remaininginsulationthickness).a) After8 and20 weeksof ageing. 46 CHARACTERISATION OF THE ROGOWSKI TYPE TEST OBJECTS ANO TRE SERVICE AGED CABLE SAMPLES

The bow tie treeing in the reference samples was similar to that observed in the samples containing salt inclusions. This was also found for the vented water trees initiated at the lower semiconductor. However, the density and length of the vented trees initiated from the upper semiconductor was strongly reduced.

4.4.3. AC breakdown strength

Unfortunately it was not possible to apply sufficient high AC voltage to measure the breakdown strength of the unaged and wet-preconditioned samples. In Figure 4.8 this is indicated as the dotted line at the external flash-overvoltage of 95kV (73kV/mm).

Results presented in Table 4.5 and Figure 4.8 a), show that the effect of increasing the ageing time from 8 to 20 weeks was to significantly reduce the AC breakdown strength from 46 to 34 kV/mm (rms). CH,4PTf3R 4 47

AC breakdown tests

Ageingtime Average breakdown Shapefactor, strength(63% vafue) $ [95% confidence interval (c.i.)] [weeks] IJA’lnun]

8 10.3 [4::0]

12 40 12.2 [37, 41]

16 15 [3?391

20 34 9.5 [32,36]

20 5.5 (reference) [4562]

TABLE 4.5. Resultsfrom theAC breakdowntest.

● 90

63 50

● 5

20weeks 8WEdCS Test limit

2 , 10 2 3 5 100 Breakdown stress wV/mm]

FIGURE 4.8. a) Distributionof breakdownstrengthusingWeibullsprobabilityfunction.Thedotted lineindicatesthe highestapplicabletest stressbeforeflashover. 48 CHARACTERISATION OF THE ROGOWSIU TYPE TSST OBJECTS AND THE SERVICE AGED CABLE SAMPLES

4.5. DISCUSSION

The results from water tree analysis show that the vented water treeing is significantly enhanced compared to the treeing from the reference test objects without salt inclusions. It is also demonstrated that this type of ageing generated very few and short bow-tie trees compared to other test methods using similar objects without NaCl particles [58].

Previous tests on unaged and wetted Rogowski type objects (0.7 mm thick XLPE insulation) show a typical breakdown strength of 90kVhnm [58]. As these samples were without impurities at the boundary between the insulation and the semiconductive screen, it is reasonable to assume that the unaged samples with NaCl inclusions had a breakdown strength somewhere between 73kV/mrn and 90kV/mm.

The results presented in Figure 4.8 show that an increasing length of the of the expected longest observed vented water trees was strongly correlated to decreased breakdown strength. This is consistent with observations made during the water tree analysis, where the breakdown channel was associated with vented water trees growing from the upper serniconductive screen, as indicated in Figure 4.6a). This was not the case when comparing the length of the longest observed bow-tie trees and the decreasing breakdown strength values.

●

Longest vented water tree

●

Longest bow-tie water tree

0 30 40 50 60 70 80

Breakdown stress, E63X ~V/mm]

FIGURE 4.8. Correlationbetween the longest observed water tree (37% -value) and breakdown stress(637a-vrdue). CHAPTER4 49

The examined service aged cable samples A and B had very low breakdown strength values and long vented water trees. By using Equation 4.3, the most probable value of the longest water tree (Zm) of 2.lm of cable A, was found to penetrate 98% of the insulation wall. This cable had the lowest residual AC breakdown strength. The estimated longest tree in case of cable sample B, with a total length of 5m, was calculated to bridge approximately 76% of the insulation wall.

4.6. CONCLUSIONS

The results show that the vented water trees strongly reduce the AC breakdown strength of both the laboratory aged test objects and service aged cable samples.

The method of inserting NaCl particles into the boundary between the upper semiconductive screen and the insulation was found to successfully enhance initiation and growth of vented water trees. In addition, low electric ageing stress and constant ageing temperature suppressed the growth of bow-tie trees, making the ageing comparable to that of the service aged cables. 50 CHARACTERISATION OF THE ROGOWSKI TYPE TEST OBJECTS ANO THE SERVICE AGEO CABLE SAMPLF.S CHAPTER 5

RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE

5.1. INTRODUCTION

The main purpose of this chapter is to present the results from the dielectric response measurements performed on both the service aged cable samples and the laboratory aged Rogowski test objects. In the previous chapter these samples were characterized by water tree and breakdown strength analysis.

Special attention has been paid to the examination of the effect of frequency and magnitude of test voltage. All applied voltages and electric fields are referred to as peak-values except where otherwise noted.

5.2. TIME DOMAINMEASUREMENTS

5.2.1. Time dependence

Typical results from measurement of depolarisation and polarisation currents of wet but unaged Rogowski type test objects are shown in Figure 5.1. The measured depolarisation currents as well as the polarisation currents follow the empirical Curie von Schweidler (CVS) - model (Z(t) - t-n), which appears as a straight line in a log-log plot. For this unaged sample, the parameter ‘n’ was found to be approximately 1.2 and independent of applied voltage levels. This was typically found for the unaged test objects. The results also demonstrate that for times less than 100 seconds there was no significant difference between polarisation and depolarisation currents. 52 CHAPTER5

~o-lo

5 3 2 L

~o-ll

5 3 2 6kV(4.6kV/rnm) ~o-12

5 3 2 2kV(1.5kVhnm) 10-13

5 3 2 r , ~-14 , v Lu 235 10235 100 2

Time [seconds]

FIGURE 5.1. Examplesof resultsfrom measurementsof polarisationanddepolarisationcurrentsper- formed on Rogowski XLPE samplesprior to ageing (wettedat 50°C in 24h priorto responsemeasurements).

Figure 5.2 shows the polarisation and depolarisation currents of a water treed test object subjected to ageing for 20 weeks. Comparing with Figure 5.1, it cart be seen that the effect of watertreeing is to increase the magnitude of the currents, and to deerease the parameter ‘n’ to a value close to 1. This parameter was found to be independent of applied voltage.

Figure 5.3 shows that this was also found in case of measurements performed on one sample of the service aged cables A and B. Results from measurements of return voltages are presented in Figure 5.4. Particularly high values of return voltages and depolarisation currents were observed in case of the service aged cable B.

It is also demonstrated that the measured polarisation currents on the Rogowslci test objects were higher in magnitude than the depolarisation currents. This feature of the response was typically found in case of polarisation currents measured on the test objects aged for 20 weeks, and as indicated in Figure 5.2, this difference was clearly seen both when applying a low and a high test voltage. RESULTS PROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE 53

10-’0~ I 5 3 2

~o-ll 5 3 2 ~o-lz 5 3 2

[ + Marisationcumnt ‘

J.u 235 10235 100 2

Time [seconds]

FIGURE 5.2. Polarisationand depolarisationcurrentsmeasuredon measuredon a Rogowski test object subjectedto 20 weeksof watertreeageing.

10-8 I 5 3 2 10-9

5 3 2 1o-1o

5 3 2 10kV ~o-ll 5kV 5 10kV 3 5 kV 2 ~o-12 1kV 1kV 5 1,,,,,,, , I 35 10 2351002351000

Time [seconds]

FIGURE 5.3. The depolmisationcurrents(normalised)measuredon one sampleof theservieeaged 24kV XLPE cablesA andB (5.6mminsulationthickness). 54 CHAPTER5

Cable sampleB 7 10kV 5 kV

10kV 10 5kV 1kV 0 1kV o 300 600 900

Time [seconds]

FIGURE 5.4. Returnvoltagesmeasuredon theserviceaged24kVcable(5.6mminsulationthickness) samplesA andB.

5.2.2. Voltage dependence

The results presented in Figure 5.2, demonstrate that the effect of increasing the applied test stress by a factor of 3 from 1.5 to 4.5kV/mm, is to increase the magnitude of the depolarisation currents by approximately a factor of 5. In case of the unaged sample, a corresponding linear increase was measured. This nordinearity is clearly shown in Figure 5.5, where the depolarisation currents measured after 20s are plotted as a function of the applied voltage. In case of the nontreed sample, the current is proportional to the charging voltage up to 6 kV (4.6kV/mm). The water treed sample aged for 20 weeks displayed a high degree of nonlinearity for test voltages above 1.5kV (1.2kV/mm). In case of this sample, the degree of nonlinearity as detined in Equation 3.1 is approximately 1.8.

The sample aged for 12 weeks displayed lower current magnitudes compared to the sample aged for 20 weeks. In addition, the departure from a linear voltage dependence occurred at a higher stress level. RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE 55

Depolarisationcurrents I 20 weeks of ageing 4 measuredafter20s

1 /

20 weeks of ageing (no salt, @)

12 weeks of ageing

Unaged

0123456

Chmging voltage WV] ~ical depoltisation currentsmeasuredon unagedand watertree aged Rogowski samplesasa fimctionof thechargingvoltageupto 6kV (4.6kV/mm).

6.10-10 Currentsmeasuredafter20s SampleB

~ Measured ---,- calculated 4.10-10

2.10-10 SampleA

1.10-10

Sample D (degassed) 0 o 2 4 6 8 10

Charging voltage WV] The depolarisationcurrents(normalised)measured(opensquares)andcalculatedfrom returnvoltagemeasurementsby usingEquation2.16 (filledcircles)after20s asa functionof appliedvoltagesup to 10kV on theserviceagedcablesamples.Thedottedlines aretheestimatedlinearresponsebaseduponcurrentvaluesupto lkV. 56 CHAPTER 5

The Rogowski test objects without salt particles, displayed a linear voltage dependence of the current values up to 6kV even after 20 weeks of water tree ageing.

Figure 5.6 shows the voltage dependence of the depolarisation currents and voltage return of the service aged cable sample A and B. The measured and derived from return voltage measurement depolarisation currents after 20s are plotted as a function of the charging voltage. The non-linearity was particularly clear for the service aged cable sample B, and the degree of nonlinearity at 10 kV was calculated to 1.4 and 1.3 in case of cable sample A and B respectively. A good agreement between measured depolarisation currents and the values determined from return voltage measurements are demonstrated, both in the linear and in the nonlinear regime.

5.2.3. Sensitivity analysis

Figure 5.7 presents results from measurements of depolarisation currents of a short section of cable sample B (2m) comected to the long and unaged cable sample D (50m). This experiment simulates a service situation where only a short section of a cable is water tree degraded. It is shown that the depolarisation currents were reduced to values only slightly above those measured on the unaged cable sample D. However, the voltage dependence and consequently the degree of nonhearity was approximately the same as that measured in case of sample B. At a DC voltage of 12 kV the degree of nonlinearity was calculated to 2 in case of cable sample B and 1.8 for cable sample B+D. Thus the short water tree degraded cable seetion caused the dielectric response of the long cable to become nonlinear.

5.2.4. Water and thermal treatment

The results presented in Figure 5.7, show that the effect of drying is to reduce the magnitude of the depolarisation currents to values slightly above that of unaged cable sample D. The results also show that after drying the dielectric response became linear.

By keeping the cable sample in water at 90°C the high values of the depolarisation currents were re-established to values higher than measured initially, but the non-linearity was not restored. RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE 57

CablesampleB 1000 Wettedat2~C

Wettedat9(YC

500

Cable sample B+D

80 Cable sample B Dried at 90”C 60

Cable sample D 20

0 o 4 8 12

Charging voltage ~Vl

FIGURE 5.7. The depolarisationcurrents(normalised)measured after 20s plotted as a fnnction of chargingvoltageof the cortneetedand disconnectedcable samples(filledcircles),and of cablesampleB afterwet anddry thermaltreatmentat 9@’C.The dottedlines arethe estimatedlinearresponses. 58 CHAPTER5

5.3. FREQUENCY DOMAIN MEASUREMENTS

5.3.1. Frequency dependence

Typical results from measurements of the dielectric loss factor, E‘‘(f),of the wet and unaged Rogowski type test objects containing salt inclusions are presented in Figure 5.8. In this case the loss factor was found to decrease with decreasing frequency from approximately 2.5.10-3 at 10Hz to 1.10-3 at O.lHz. Drying the sample for 12 hours at 60°C caused the loss factor to strongly decrease to values close to 5.10 4, values comparable to those measured on the unaged and dry samples without salt impurities. As indicated in Figure 5.8, wetting the nontreed sample (without salt impurities) for 12 hours at 60°C, did not affect the dielectric loss factor. A similar effect was observed for the apparent permittivity, As’(f). Figure 5.9. shows that the effect of drying was to reduce the apparent permittivity and frequency dependence.

Figures 5.10 and 5.11 show that the effeet of water tree ageing was to increase the dielectric loss factors and apparent permittivities, and making them less frequency dependent compared to the unaged samples.

The service aged cable B, had higher values of dielectric loss factors than cable A. This is shown in Figure 5.12, where typical measured values were found to be 8.10-4 and 4.10- 2 for sample A and B respectively. As can be seen, the loss factors are approximately independent of applied frequency. However, when testing cable A at 8.5kV, the dielectric loss factor strongly increased with decreasing frequencies. The slope was approach- ing -1 at the lowest frequencies, indicating DC conductivity. By assuming this, the conductivity of the insulation was calculated to 1.10-13S/m at O.lHz.

5.3.2. Voltage dependence

The effect of increasing the applied test voltage was to increase the dielectric loss factor and the apparent permittivity of the water tree degraded insulation. This is shown in Fig- ure 5.10, where results from measurements of a Rogowski test object subjected to water tree ageing for 20 weeks are presented. As can be seen, the loss factor increased by a factor of approximately 3.5, when increasing the test voltage from 1 to 6 kV at O.lHz. The corresponding nonlinear increase of the apparent permittivity was calculated to be 8. Results presented in Figure 5.13 and 5.14, show that a nonlinear increase was observed at voltages lower than lkV. RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE 59

Unaged and wetted 3 (lWth NaCl inclusion)

4 34 Umged andwetted 2 - anddry (Without NaCl inclusion)

Frequency ~z]

FIGURE 5.8. Measurementsof dielectriclossfactoron a nontreedsample.

10-2

5 \ Dried

3 Unaged anddry Unaged 2 andwetted y (MWhoutsaltparticles)

I , , , t , , ,,, , t 1d 10-2 2 510-1 2 5 ~ 2 5 10

Frequency ~z]

FIGURE 5.9. Measurementsof the apparentpermittivityas a functionof fkequencyon a nontreed sample. 60 CHAPTER 5

Stto 6kV 3

2 time 2kV2nd ------● 2kV Ist time 10-2 ● 0.5kV

3 t 2

lU 5 ~o-l 2 5 12 10

Frequency [Hz] FIGURE 5.10.Dielectriclossfactormeasuredon a samplesubjectedto 20 weeksof wetageing.The responseat2kV is measuredbeforeandafter(dottedline)theapplicationof 6kV.

~o-l ● r 6kV 5

I 2kV2ndtime 3 t ------

2 2kV lstdme

r===” lo-z + 0.5 5 kV

d ,n_321!_____l-u 5 5 1o-1 2 12 10

Frequency [Hz] FIGURE 5.11.The apparentpermittivitymeasuredon the samesample.Theresponseat2kV is meas- uredbefore andafter(dottedline)theapplicationof 6kV. RESULTS FROM MEASUREMENTS OF THE TliW3 AND FREQUENCYDOMAIN DIELECTRICRESPONSE 61

10-2

2.. 5 . . . ..,. . . . . 3 -... EI 8.5kV 2 ‘----- Znd &z------~kv ~o-3 E14kv [3 1st time 5 3 6- E12kV 3 SampleA 2

~o-4 , ,

1o-1 2 5 2 5 1 10

Frequency ~z]

FIGURE 5.12. Resultsfrom dieleetic loss factor measurementson one sampleof the service aged XLPE cablesA andB. The dottedline is the dielectricloss factor againmeasaredat 4kV afterthemeasurementat8.5kV.

In samples aged for 12 weeks, the dielectric loss factors were typically linear at applied voltages larger than 1.5kV (lkV/mm). However, this was generally not found in case of the apparent permittivity, where the magnitude was found to still increase at applied electric fields above lkV/mrn. As can be seen in Figure 5.15, the response measured on the samples aged for 20 weeks without salt inclusions, displayed typically low and linear dielectric loss factors and permittivities. The values were found to be comparable to those measured on the unaged samples.

The graphs presented in Figure 5.16, show that in particular cable A had a nonlinear response. When increasing the charging voltage from 2 to 8.5 kV, the loss factor increased by a factor of 6.5 at 2Hz. In case of cable B, the corresponding increase of the loss factor was calculated to be 3. 62 CHAPTER 5

~o-l

5 3 2

~o-2

3 2

~o-3 12345

Applied voltage WV]

FIGURE 5.13. Dielectricloss factor,&‘’, as a functionof appliedvoltage (peakvalue) at O.lHz and differentageingtimes.

1o-1

3 2

~o-z

3 2

123456

Applied voltage ~V]

FIGURE 5.14. Apparentpermitdvity,As’,as a fnnctionof appliedvoltage(peakvalue)atO.lHZand differentageingtimes. RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DDZECTRIC RESPONSE 63

3 Apparentpermittivity,A&’ 2’ - ● 10-3,?1 3 2

1 10-4 II I 123456

Applied voltage ~V] FIGURE 5.15. Apparentpermittivitiesand dielectriclossfactorsofreferenceRogowskitestobject subjectedto 20 weeksof watertree ageing.(ObjectswithoutNaClinclusions).

3 2

2 4 6 8

Applied voltage (peak value) &V] FIGURE 5.16. Dielectricloss factorsof the serviceaged cable samplesA andB at2Hz plottedas a functionof appliedvoltage.The number’2’ indicatesthe secondmeasurementof the dielectric loss factor at 4kV atter the measurementsat the highest test voltage (8.5kV). 64 CHA~ER 5

5.3.3. Hysteresis effect

Figure 5.10 also show that the response from the Rogowski object subjected to 20 weeks of ageing was characterized by a hysteresis effect. According to the described procedure, the loss factors were measured at different frequencies at increasing voltages up to 6kV. A subsequent measurement of the loss factor at 2kV showed a frequency independent increase, as indicated by the dotted line. A hysteresis effect was also observed when measuring the apparent perrnittivity of the water treed samples, but not in the nontreed samples.

Such a hysteresis effect was also observed in case of the service aged cables (see there- sults presented in Figure 5.12). However, at 8.5kV the frequency dependence of cable sample A was characterized by a DC conductivity (leakage) at frequencies below 1 Hz. A subsequent measurement at 4kV caused the dielectric loss factors to become strongly frequency dependent also at that voltage level (indicated by the dotted line in Figure 5.12.).

5.4. RELATION BETWEEN TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSES

5.4.1. Magnitude of the dielectric response

The results from Fourier transformation of the depolarisation currents of the Rogowski test objeets measured at 2kV are presented in Figure 5.17 and 5.18. The transformations were performed using measurements of both unaged and objects subjected to 20 weeks of water tree ageing. Included is also the frequency domain measurements of the dielectric loss factors and apparent perrnittivity at 2kV.

It can be seen that the loss factors deduced from the time domain data are lower than those measured in the frequency domain. The ratio between the measured and the deduced loss factors for the sample subjected to 20 weeks of ageing, is approximately 10. The corresponding ratio in case of the unaged and wet preconditioned samples is about 3. However, both measured and deduced values showed nearly frequency independent loss factors.

It is important to notice that the dielectric loss factors measured on a nontreed sample are in good agreement with the deduced time domain values. A good agreement is also obtained between the dielectric loss factors deduced using the Fourier integral analysis RESULTS FROM MEASUIU?MENTS OF THE TIME AND FREOUENCY DOMAIN DIELECTRIC RESPONSE 65

~o-l -E- Frequency domain -o- Time domain 5 — Harnon approximation 3 2 [3 ~o-2 20weeks

5 r 3 weeks 0 ❑ 2

~o-3 +-0 \

5 New ) 3 ,o.42t—____J ~o-z 2 3 5 ~o-l 2 3 5

Frequency ~z] FIGURE 5.17. Measureddielectricloss factors and calculatedfrom depolarisationcurrentsat 2kV

using Equation2.10 (DIE.L@) andtheHamonapproximation.

1o-1 -E- Frequency domain 5 + Tme domain

3 3 E 2 20 weeks

10-2 [3

5 0weeks 4 3 2

10-3

3 2 \

, ~–4 , ,

FIGURE 5.18. Measuredapparentpermitdvityand calculatedfrom depolarisationcurrentsat 2kV. The calculationshave beenperformedby usingEquation2.10 @ZE.L”). 66 CHAFTER 5

and the Hamon approximation.

As indicated in Figure 5.18, the differences between the measured and calculated apparent permittivities are similar to that found in case of the dielectric loss factor.

5.4.2. Degree of nonlinearity

Results presented in Figure 5.19 show the average values of the nonlinearity factor of the 10 Rogowski test objects measured in the frequency domain. It is demonstrated that in case of considering the apparent permittivity the nonlinearity factor was increased from 1 to 7 after 20 weeks of ageing. The corresponding degree of nonlinearity associated with the dielectric loss factor show an increase of 2.6. The increase was found to be particularly high after 8 weeks of ageing.

The calculated time domain nonlinearity factor (Equation 3.2) increased with ageing from a value of 1 (linear) before ageing to approximately 1.7 after 20 weeks of ageing.

Time domain measurements were characterized by a linear response at low applied voltages. At a certain charging voltage Ui the transition to a nonlinear response started. In Figure 5.20, it is shown that this ‘initiation’ voltage decreased from approximately 5kV after 8 weeks to 2kV after 20 weeks of ageing. RESULTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DIELECTRIC RESPONSE 67

10 a

6 T ~

~ 4 ~ ~ .- 2 ~2 2 E

1 0 4 8 12 16 20

Ageing time weeks]

FIGURE 5.19. Averagevaluesof thenonlinearityfactorof 10testobjectsin frequencydomain(Equa- tiOllS3.2 and3.3).

10 10 8 8

6 6

4 4

2 2

1 1 o 4 8 12 16 20

Ageing time weeks] FIGURE 5.20. Calculationsof tie averagenonlinearityfactorof 10testobjectsin timedomain(Equa- tion 3.1), andinitiationvoltage, Uj,of thenonlinearincrease(fNed circles)as a functionof ageingtime. 68 CHAPTER 5

5.5. DISCUSSION

The time and frequency domain dielectric responses measured on the service aged cable samples and the laboratory aged Rogowski type test objects containing vented water trees, have been found to increase and to become nonlinear with ageing time. In an ear- lier time domain examination of service and laboratory aged XLPE cable samples the appeammce of nonlinearity was assigned to the presence of bow-tie trees in the cables [8]. The results presented in this thesis clearly demonstrate that also vented water trees causes the dielectric response to become nonlinear.

However, the results indicate that it maybe insufficient to base condition assessment on dielectric loss level only. A long cable containing a short water treed section, had low loss values comparable to that of a new and nontreed cable. The degree of non-linearity, on the other hand, remained approximately constant, even when the water @eedegraded section of the cable constituted only 490 of the total cable length. Similar results have also been reported in case of frequency domain measurements on short water tree degraded cable sections [54]. This demonstrates the great importance of using the nonlinear feature for condition assessment of the cable insulation.

The appearance of a nonlinear response was particularly clear in case of the service aged cable samples and the Rogowski type test objects subjected to water tree ageing for 20 weeks. However, the responses measured in the time and the frequency domain revealed a different voltage dependence. The response in time domain was linear at low charging voltages, and became nonlinear attest voltages typically above 2kV. The measured loss factors became linear at test voltages above 2kV, generally observed on Rogowski objects subjected to ageing for less than 12 weeks. Such feature of the loss factor can therefore be assigned to a less severe water tree ageing.

A practical consequence of the observed frequency independence is that the measurement time required to assess the condition of the insulation can be reduced. However, results from measurements of the dielectric loss factor of the severely water treed cable sample A, show that precautions must be taken. At a high test voltage, a voltage induced change of the insulation properties occurred making it conductive. Such a change can only be detected when measuring the loss factors at several frequencies, or by time domain measurements of both the polarisation and depolarisation current.

A hysteresis effeet was observed both during frequency and time domain measurements. Infrequency domain this was characterized by an increase of the loss factors when it was measured a second time at a lower test voltage. In time domain this hysteresis effect was observed as higher values of the polarisation currents than the depolarisation currents at the same test voltage. This was typically measured at all test voltages, and was particularly clear for Rogowski objects subjected to ageing for 20 weeks. This striking differ- RE.WJLTS FROM MEASUREMENTS OF THE TIME AND FREQUENCY DOMAIN DD?.LECTRICRESPONSE 69

ence between the polarisation and depolarisation currents of severely water tree degraded samples, can be utilized as an indicator of a highly nonlinear response also at low applied voltages. Similar results have been obtained by measurements of polarisation and depolarisation currents on laboratory water tree aged LDPE samples [65].

The simplified transformation of the return voltages to depolarisation currents was found to agree well with measured currents. This indicate that the nonlinear time domain dielectric response can easily be obtained by measurements of voltage return.

Fourier transformation of the nonlinear return voltages and depolarisation currents is questionable. However, the responses were found to be linear up to a certain voltage (’initiation voltage’). Limiting the Fourier-transformations of the time domain measurements to below this voltage level, the transformations should be valid.

The results from depolarisation current measurements show that the content of water and/or thermal treatment strongly influence the nonlinear response of a cable containing water trees. Thus, information of the thermal history of the cable can be very important for a correct condition assessment.

5.6. CONCLUSIONS

The frequency and voltage dependence of the dielectric response of the laboratory aged Rogowski test objects are similar to that measured on the service aged XLPE cable samples.

The measurements indicate that both the frequency and the time domain methods can be used for assessing the status of the XLPE insulation. 70 CHAPTER 5 CHAPTER 6

COMPUTER SIMULATION OF PROPOSED MECHANISM FOR NONLINEAR DIELECTRIC RESPONSE

6.1. INTRODUCTION

This chapter presents results of electric field and dielectric loss calculations, performed in order to test the hypothesis of nonlinear dielectric response presented in Chapter 2. Both the permittivity and conductivity of the XLPE insulation and the water tree region have been taken into account.’f’he water tree structure is considered to consist of a string of waterfilled voids interconnected by a thin cylindrical channel of crazed polymer. In these calculations the interconnecting channel is considered to be either insulating or conducting due to high content of water. Analytical calculations of the electrostrictive forces at the surface of a waterfilled and elongated void are also included.The calculations were performed by applying a sinusoidal potential in the frequency range from 10mHz to 10Hz, focusing on the effect of frequency, conductivity and shape of the mod- elled treed region upon the field distribution and losses.

During DC voltage application a transient polarisation current will flow within the water tree structure. The resulting electric field distribution and dielectric loss of such a voltage application was, however, not included in the calculations.

6.2. METHOD OF CALCULATION

6.2.1. The water tree model

The calculations were performed on the two simplified models of ‘water tree units’ 72 COMPUTER SIMULATION

a) b)

2pm

3pm

FIGURE 6.1. A simp~led sketchof a watertree showingellipsoidalcavitiesforming a ‘stringof pearls;interconnectedby smallchannels.b) The watertreeunitshowingvoids interconnectedby an openedcrazingzone. The ellipticityof the voids is e = 0.87, with a ratioof dh = 2.

b) c)

)

1 axisof Sy letry

FIGURE 6.2. Cylindricalmodelof a watertreeunit.a) Model A andb) modelB usedin thecrdcula- tions of electricfields andlosses.Due to symmetryonly a quarterof the ‘water-tree unit’ is neededfor numericalcalculations.e) The correspondingFEMmeshof model B. CHAPTER 6 73

shown in Figure 6.1 as model A and B. In presence of an electric field these water tree units are consider to consist of ellipsoidal water filled voids connected by a thin water filled cylindrical channel.

In Model A this tree unit is placed within a cylinder of XLPE insulation, as indicated in Figure 6.2. Initially the thickness of the cylindrical channel and the other distances and sizes of the model, shown in Figure 6,1, were chosen according to what have been reported to be typical for real water tree structures described in Section 2.2.1. The volume of the channel initially constitute less than 0.01’% of the total insulation.

Model B take the screening effect of neighboring tree branches into account. The water tree unit is considered to be surrounded by a conductive cylinder lmm away from the surface of the voids. It’s thickness was chosen to be equal to the diameter of the thin interconnecting cylindrical craze channel, i.e. 0.08pm.

A sensitivity analysis was performed to study the effect of different parameters of the models. For instance the volume of voids and channels, number of connected ‘water tree units’, and different conductivity of water tree regions.

6.2.2. Methods for calculation of electric field and losses

The electric field distributions and dielectric losses of the models were numerically calculated using the Finite Element Method (FEM) program FLUX2D@ made by CEDRAT [66]. In these calculations, both the complex permittivity, the conductivity of the XLPE insulation and the water tree region have been taken into account. In this case, the conductivity of the liquid inside the voids as well as the channel is considered initially to be that of pure (de-ionized) water [49]. Results from frequency domain measurements presented in Section 6.3.2, show that the dielectric loss factor, (&’‘(w)), of unaged XLPE insulation is approximately frequency independent, and has values of typically 104. This has been included in the model by introducing a frequency independent tarii value of 5.10-5for the XLPE insulation (see details in Appendix 2).

As shown in Figure 6.2, only a quarter of the model is needed for numerical calculations due to the axial symmetry. The upper eleetrode is initially set to a potential of 28V, and the lower line of symmetry is kept at OV, thus resulting in an average electric field of 4kV/mm. Here the total number of nodes generated was 11869 resulting in 5726 trian- gular elements. To obtain sufficient accuracy the meshgrid was made dense in regions where the electrical field may become strongly inhomogenious.

For calculation of the electrical potential and electric field, the following equations were solved at each node: 74 COMPUTER SIMULATION

div(o. VU+imOzr. VU) = O (6.1.)

E = –V(U) (6.2.) where ois the conductivity of the material, U is the potential, &ris the permittivity of the material and E the electric field.

The dielectric losses were numerically calculated by FEM according to

(6.3.)

where Pi is the dielectric losses of region i considered, with the corresponding volume Vi, cois the angular frequency and tarz~ is the dielectric loss factor. The current density, J, at the electrode is approximately given by:

(6.4.)

where EOis the uniform applied electric field, and Ee is the electric field at the electrodes. The overall dielectric loss factor was then calculated according to the following relation:

(6.5.)

where Ui is the applied voltage between the electrodes. It is assumed that there is no space charge present inside or at the boundary between the different geometric parts, and that the different domains are homogeneous, linear and isotropic. All calculations of electric fields have been performed along the vertical axis of symmetry of the models before and after opening of the channel. When the channel is assumed to be open and conducting, it is considered to have the same material properties as the voids. The calculations were performed in the frequency range of 10mHz to 10 Hz.

To calculate the electric field in the channel requires knowledge of its permittivity and conductivity. Two extreme situations can be considered 1) The channels are filled with water with a relative permittivity of approximately 80 and 2) The channels are partly filled with polymeric fibrils and partly with water. If most of the volume is filled with fibrils, the relative permittivity might approach that of polyethylene (2.3). The relevant material parameters used are summarized in Table 6.1. CHAPTER 6 75

Geometricpart Permittivity Conductivity [(fkn)-l]

Voids 80 5.5.10”’5

Channelopened 5.5.10-6 (closed) (::) (7.10-16(0.lHz))

Matrix(XLPE) 2.3 7.10-16(0.lHz)

Thincond.cyl. 80 5.5.10”6

TABLE 6.1. Materialconstantsof ModelA andB usedinthecalculations.

6.2.3. Calculation of Maxwell forces

The waterfiiled cavity was subjected to a DC or an uniform AC (O.1 Hz) stress of 4kV/ mm, using material constants as described in Table 6.1.

The appearance of electrostrictive pressure (Maxwell forces) caused by the application of the electric field was calculated for the water tree unit presented in Figure 6. lb). The channel is considered to be insulating, with a long distance between the voids. As no mechanical stresses can exist inside the waterfiied cavity, the Maxwell force must be bal- anced by mechanical stresses in the surrounding XLPE insulation. During application of an electric field, the pressure p at the water filled void surface can be expressed as [67];

(&’ Jaj)-E’ ~(co)). ~ E’ l(~). p. ~,) (6.6.) 2 ( ‘1+&’ Jo) where En is the normal electric field componen~ Et the tangential electric field compo- nent. e ‘l(o) and E >(a) are the complex permittivity of water (material 1) and polyethylene (material 2) respectively. The electric field depend upon the applied stress and the field enhancement, expressed by [68];

El = EO. q(e, &((a)), et)) (6.7.) where EO is the applied uniform field, and @e field enhancement factor, which is a function of the ellipticity, e, the perrnittivity E and the angle cz determine the position of the electric field as indicated in Figure 6.9. 76 COMPUTER SIMULATION

6.3. RESULTS FROM NUMERICAL CALCULATIONS

6.3.1. Electric field distribution

Examples showing potential distributions of Model A and B before and after opening of the water tree channel are presented in Figure 6.3. When the channel becomes conductive the equipotential lines will be squeezed to the eleetrode sides of the voids. As expected the effect of introducing a conductive cylinder in the vicinity of the water tree unit (Model B), is to move the potential lines upwards resulting in a more homogeneous potential distributions.

Figure 6.4 shows results from electric field calculations considering Model A with a closed and non-conducting interconnecting channel at an applied electric field of 4kV/

a) b) c)

..-.-—.-. .. ..—....-.— .... .-—.--... .-.—-...... ------... ..---.—,------., =---- R -—%___ _.—- ‘\,, ...... ------—..‘-.. \ /~ ‘L L- j\i $., ----- \\ ...——- /’- 1 /’ ,/ ,,’ //—-—---”- ) { ..._— ), ‘..__ .

Model A Model A ModelJB U Channel conducting Channel insulating Channel conducting

FIGURE 6.3. Potential distribution at0.1 Hz a) before openingof thewatertreechannel(Model A), b) afteropeningof the watertree channel(Model A) andc) Model B. The distance betweenthepotentiallinesis 2.74 V. CHAPTER 6 77

a) 6 . { I 5 m

4 E= 80,f= O.lHZ

3 E~ 2.3,f= 10Hz------’”

2 e~ 2.3,f= O.lHZ \

1 -m

10-16 10-1410-12 10-10 l(J-8 10-6

Conductivity inside voids [S/m]

b) n% @

“\ E= 80,f = O.lHZ

E- 2.3,~= 10Hz

E- 2.3,f = O.lHZ

10-16 10-1410-12 lo-lo lo-8 lo-6

Conductivity inside voids [S/m]

FIGURE 6.4. a) Electricfield enhancementat the upper void tip as a functionof the conductivityof the waterinsi& the voids,b) Averagemagnitudeof the electricfieldinsidethe voidsas a function of the conductivityinside the voids (E#.lOcV/m). The valueshave been calculatedalong the verticalaxis of symmetryinside the voids before openingof the channel. 78 COMPUTER SIMULATION

mm and a frequency of 0.1 Hz

To illustrate the effect of the permittivity, Figure 6.4 a) shows how the field enhancement at the tip of the void varies with conductivity, frequency and perrnittivity. As can be seen, the field enhancement becomes independent of permittivity value for conductivity values exceeding 10-7(Chn)-l and frequencies below 10Hz. At conductivity values below 10-7(C?III)-1,the electric field at the void tip is strongly influenced by the permittivity. If the void is considered to consist of pure water, the choice of value of the water permittivity is not decisive regarding the electric field distributions. However, Figure 6.4 b) show that for a conductivity wdue of 5“10-6 (Chn)- 1, the maximum electric field inside the voids is reduced by a factor of 109 compared to the uniform applied electric field at O.lHz. A value approximately independent of the permittivity.

Considering Model A, the field enhancement at the void tips near the electrodes before opening of the channel is calculated to be 5.7.E0. This is in good agreement with previous analytical calculations of electric fields with such geometry [68]. When the crazing zones of the channel is filled with liquid water making it conducting, the electric field enhancement at the void tips was calculated to 10.4.E0 in Model A and to 6.5.E0 in Mod- el B.

Result presented in Figure 6.5 show the current density of the channel calculated along the horizontal axis of symmetry at an applied stress of EOand at O.lHz. The current density in the channel was found to be approximately 6 pA/mm2, a value significantly higher than that of the void. The same effect can be seen in Figure 6.6 b), where the electric field has been calculated along the vertical axis of symmetry. The opening of the channel causes the electric field in the channel to decrease by a factor of approximately 103 and the electric field inside the void to increase by a factor 104. The electric field inside the channel will be approximately 102 higher than that within the waterfilled voids.

6.3.2. Dielectric loss

The results presented in Figure 6.8, show that the dielectric losses inside the channel strongly increase when the channel becomes conductive. Considering the water tree unit enclosed by the conductive cylinder (Model B), the losses at O.lHz increased to values 103 higher than before making the channel conductive. This increase was even larger at higher frequencies. Figure 6.8 b) shows the overall resulting dielectric loss factor before and after opening of the channel when 4kV/mm is applied. Calculations considering Model A, show that the effect of making the channel conductive is to increase the overall dielectric loss factor at O.lHz 25 times. The value was also found to increase with increasing frequency. In case of Model B, the screening effect of the surrounding cylinder reduced both the resulting dielectric loss factor and its frequency dependence, particu- CHAPTER6 79

0 0.5 1 1.5 2

Length of symmetry-axis [pm]

FIGURE 6.5. Calculatedcurrentdensitiesafteropeningof thechannel(at 1OI-IZ).

Channel Void XLPE 108 J ------‘------106

104

102 ------afteropening

10°

lefore opining *------.-

01234567

Length of symmetry-axis [pm]

FIGURE 6.6. Electric field rdongthe axis of symmetrycalculatedbefore and afteropeningof the channel(atO.lHZ). 80 COMPUTER SIMULATION

Model A, channelclosed

~0-23 I

~o-z ~o-l 10° 101 Frequency [Hz] FIGURE 6.7. Dielectriclossesas a fimctionof frequencyinsidethechannelcalculatedbothbefore andafteropeningof thechannel.

10° 5 3 2

Medel A, opened channel/ 7“

5 3 2 Model A, daonel clnsed ~o-5 ,,8, lo-z 1o-1 10° 101 Frequency ~]

FIGURE 6.8. Dielectriclossfactoras a functionof frequencycalculatedbothbefore andafteropen- ing of the channel.The dottedline shows dielectricloss factor in case of Model B, includingthedielectriclossesof thethinconductivecylinder. CHAPTER 6 81

larly at frequencies below 1 Hz.

6.3.3. Sensitivity analysis

Model A was modified in order to investigate how the calculated losses depended upon the shape and material parameters of the water tree unit. Ordy one parameter was varied at the time. The relative change of the total losses was compared to the total losses of the reference model A with a conducting channel. All values were calculated at O.lHz, and a sumrmmy of the results are presented in Table 6.2.

The calculations showed that deereasing the channel radius from 80 to 20 nm, caused the losses to increase by a factor of 13. An increase in losses was also observed when increasing the number of connected water tree units, and increasing the number of units from 1 to 4, increased the losses by a factor of approximately 7. The effect of decreasing the volume of the voids was to reduce the dissipated power.

The opened and conducting channel was made discontinuous by making 40nm of the 3.5pm long cylindrical channel insulating halfway between the voids, having the same material properties as the surrounding insulation. This caused the total losses to decrease by a factor of 10. By changing the value of the conductivity of the water in the opened channel horn 5.10-6 to 1 [C?m]-1, caused the calculated losses to decrease by a factor of 1.105.

In all cases the calculated losses were found to be higher than the values for model A with insulating channel (case 9).

6.3.4. Increase of temperature

As shown in Figure 6.6 the current density of the channel was calculated to 6pA/mm2 at 10Hz. By assuming adiabatic heating of the channel, the corresponding temperature increase caused by this current could be calculated to 1.5°C. However, decreasing the channel diameter from 80 to 40nm caused the current in the channel to increase, and the corresponding maximum increase of the temperature was calculated to 27°C (see details in Appendix 3). 82 COMPUTER SIMULATION

Ratio between reference loss-value and value after the changes in the model. Case Changes in model Numeric values Increase:value>1 Decrease:value<1

1 Decreasing channel 80 [pm] 1 2 radius 40 [pm] 3 3 20 [pm] 13

1 Increasing number 1 1 4 of ‘units’, indicated 2 2.6 5 by number of chan- 4 6.6 nels.

1 Decreasing void - ah= 0.5/1 1 6 size - ah= 0.25/0.5 0.3 7 - void as channel 0.2

8 Discontinuous 0.11 channel

9 Conductivities in l.lo-lbb) 9.6.10-8 10 channel and voids. 1.10-8b) 3.3 (Except b)where the 1 1 5.5 .10-6 conductivity of the 6.10-4 11 1.10-2 voids was 5.5 .10-6) 12 1 6.10-6

TAELE 6.2. Results from sensitivity analysis.Case 1 denotes the reference value; Model A- with conductive channel. CHAPTER 6 83

6.3.5. Analytic calculations of electrostrictive pressure

Analytical calculations of the electrostrictive pressure exerted along the boundary between a waterfilled void and its surrounding polyethylene are shown in Figure 6.9. The figure illustrates the decrease in pressure at the void/craze tip as the ratio between the elliptical semiaxes a and b becomes smaller.

When the waterfilled void is elongated the electrostrictive pressure is strongly increased at the void tip. When the ratio between the axes db = 30/1, the electrostrictive pressure at the void tip is calculated to approximately 14 N/mm2.

The calculated values are only slightly decreased when applying a DC stress instead of the 0.1 Hz AC signal.

N + AC, alb=3011, O.lH

‘“ O 10 20 30 40 50 60 70 80 90 Angle,Ct [degrees]

FIGURE 6.9. Eleetrostrictivepressurealong the surface of an ellipsoidal waterfiied cavity sur- roundedby aXLPEmatrix,asafnnetionof theangle,eg definedin thefigure.Thefig- ure illustratesthe increasein pressureat the voidkraze tip as the ratio betweenthe ellipticalserniaxesa andb becomeslarger,andtheanglecxsmaller.Inthesecalculations E.= 4kV/mm,~= O.lHz, and~~ = 5.10-6[S2m-1]. 84 COMPUTER SIMULATION

6.4. DISCUSSION

The calculations strongly support the hypothesis that the non-linear dielectric response can be result of reopening of initially collapsed crazing zones, causing liquid water to enter into sections of the water tree structure making it more conductive.

The electric field enhancement at the tip of the elongated water tree structures, will be strongly dependent upon their shape and the existence of neighboring water tree branches. The effect, however, of elongating and re-opening crazing zones is to increase the electric field at the tips of this structure. This will result in higher magnitude of the mechanical Maxwell stresses acting on the boundary between the opened water tree structure and the collapsed crazing zones. Thus channels requiring higher fields to be opened, may then become conductive. The result of this cascade effect is to increase the dielectric losses with increasing applied stress. Time and frequency of the applied voltage is also probably of importance. The dielectric losses are mainly caused by the current flowing and the length of the water tree channel.The magnitude of the current is determined by the frequency of the voltage and the electric field stress at the tip of the water tree. This means that the dielectric losses will depend upon the distribution of water within the treed insulation, and consequently the degree of non-linearity will increase by increasing length of water trees.

At low frequencies the electric field inside the waterfilled voids is very low. At a conductivity of 5.10-6 (pure water) the electric field has decreased to 10-8 EO. It has been proposed [69] that field enhanced dissociation of the solution inside the water tree stmcture may contribute to the non-linear dielectric response, This is rather unlikely at such low electric fields. At higher frequencies however, the electric field inside the channels may become significant.

Also the rather simple model including a water tree branch in the vicinity of the tree unit indicate less frequency dependent loss factors. However, this also causes the losses inside the channel to decrease. This result indicates that the effect of increase of losses due to re-opening of closed channels is more likely in regions where the water-fikxi voids are more separated from each other, thus decreasing the macroscopic permittivity [70]. This is equivalent to water tree regions with low water contents. This mechanism will be experimentally examined in the next chapter.

Although the calculated total losses of the model are dependent upon values of material properties and geometrical dimensions, the sensitivity analysis show that in all cases an increase of the losses was observed when water enters the initially closed and insulating channel.

Local heating of the structure may occur, in particular when the channels are thinner than CHAPTER 6 85

the diameter of the voids. In the literature, heating of liquid water within water-tree channel have been found to be likely in case of impulse voltage conditions, where the induced transient currents have been calculated to cause the water inside such a tree channel to boil using high values of the water-conductivity [71]. Such Joule heating in has not, however, up to now been experimentally detected [72].

The fracture limit of polyethylene is approximately 10-20N/mm2, and the results from the analytical calculations of electrostrictive pressure show that the ratio a/b between the semiaxes must typically be 30 in order to exert such high pressures. In case of waterfilled voids embedded in the polymer, weak sections of crazed insulation are likely to be present at regions close to the tips. The forces required to open up such structures are likely to be smaller than the fracture limit of polyethylene. Thus the Maxwell forces exerted at the tip of a waterfiiled void may be sufficiently high to force water into the crazed insulation channels.

6.5. CONCLUSION

The calculations strongly support the hypothesis that the nonlinear dielectric response is a result of re-opening of initially collapsed crazing zones. This will then cause the water to penetrate into sections of the water tree structure causing the losses to strongly increase. 86 COMPUTER SIMULATION CHAPTER 7

CONTENT OF WATER WITHIN WATER TREES

7.1. INTRODUCTION

In Chapter 2 the nonlinear dielectric response of watertreed XLPE cable insulation was supposed to be explained by voltage assisted ingress of water into watertreed regions. The numerical calculations presented in Chapter 6, showed that this mechanism can be present in water treed regions with low water content.

The purpose of this chapter is to experimentally examine the physical state and amount of the water absorbed within water trees. The measurements were performed using lWIR spectrometry of thin XLPE slices rnicrotomed from the service aged cables characterized in Chapter 4.

Secondly, anew method was developed in order to study in-situ the permeation of water into initially dried watertrees. The aim of this was to test the proposed hypothesis by studying the effect of AC and DC electrical fields upon the permeation of water into water treed sections. These experiments were performed using helically sliced sections of cable sample C.

7.2. EXPERIMENTAL METHODS

7.2.1. Equipment for measure ment of water content

IR-spectroscopy measurements of distribution of water content were performed on 100pm thick microtomed XLPE slices. To prevent drying, the thin slices were placed between lmm thick glass-plates of quartz as indicated in Figure 7.1. Measurements of short duration were, however, performed without the glass-plates (see Appendix 4). 88 CONTENT OF WATER WITHIN WATER TREES

a) b) to the IR detector

I 10 II?beam

t I ] dried pathl_ 1atmosphericti

lmm quartz ‘ glass-plates / 100I.uuthick watertreedXLPE sample

FIGURE 7.1. a) Photographshowinga typical ventedwatertreeexamined(2.2mmin length).The straightlinesindicatethelocationsof watercontentmeasurements.b) Samplearrange- mentshowinga thinwatertreedslice placedbetweentwo lmm quartzglass-plates.

Liquid nitrogen cooled MCT detector dryair= r.m-.i.— 0—-0w~

.Test sample 1 [ IR source Michelson tetierometer

Li~ht/ lR- Evacuated chamber M~croscope

FIGURE 7.2. The principleof micro-Infrared(Ill) measurements.The sampleswere placed in a smallchamberin the microscope andpurgedwith dry air.The IR-beamwas finally detectedby a liquidnitrogencooled MCT detector. CHAPTER 7 89

FTIR-microspectrometry of water treed XLPE cable samples was carried out on a BRUKER A-590 micro-FTIR instrument schematically shown in Figure 7.2. The infrared microscope and the sample chamber were continuously purged by dry air. The microscope was operated in transmission mode using a circular aperture of 30 pm in diameter which converged the IR beam into a MCT detector. The IR mirror and source system was evacuated to pressures below 5mbar before the measurements started. The system was capable of measuring transmittances at wavenumbers ranging from 400 to 6000 cm-l. In order to achieve high sensitivity of the IR detector system, the detector was cooled by liquid nitrogen for 1hour before the measurements started. A maximum signal was achieved by automatically adjusting the interferometer by a controlling PC using a spectroscopic software.

The water content at a specific water-tree location was calculated using Lambert-Beers law;

–log(t) = –log : = U. c. d=A (7.2.) ()0

The Lambert-Beer law describes the linear relationship between the transmittance T, the absorbance A, and the concentration c in a sample of thickness d [73]. etis the absorption coefficient, which in case of degassed and unaged XLPE has a value of 3.85.10-4 [rnmppm]-l at wavenumber 3420 cm-l [17]. Zand ZOis the band intensities of the sample and the reference spectra respectively. The reference spectra is in this case nontreed and dry XLPE slice with the same thickness as the measured sample. In case of the permeation experiments, the reference spectra also contained the absorbance due to the thin silicone layer (see Appendix 5). The C-H deformation peak at 1895cm- 1,which is found to be directly proportional to the sample thickness, was used as a reference when making corrections for small variations in sample thickness [74].

In order to saturate the samples with water, the microtomed slices were immersed in water for more than 1 week at 20°C before performing the FTIR micro-spedrometry measurements. The slices were then kept in water for 12 hours at different temperatures up to 80°C and the saturated content of water in the selected slices was measured. Finally, the samples were dried at 40 and thereafter at 80°C in a ventilated oven for 12 hours.

All measurements were performed by moving the narrow I.R-beam (30pm in diameter) along a straight line corresponding to the central water tree branch (path 2) or perpendicular to the branches (path 1), as indicated in Figure 7.1. All the micro-FTIR measurements were performed at room temperature (20°C). 90 CONTENT OF WATER WITHIN WATER TREES

7.2.2. Measurements of water permeation

The permeation of liquid water into initially dried water treed samples was performed using 0.5mm thick slices cut from the service aged cable sample C. This cable was found to have few but long water trees. Therefore a lm long cable section was helically sliced making it easy to locate the vented water trees growing from the insulation screen.

As indicated in Figure 7.3, the slice was covered with a thin layer of transparent silicone glue, and then placed between two lmrn quartz-plates. One end of these glass plates was cut circular adjusted to the curvature of the XLPE slice schematically shown in Figure 7.4a). They were also equipped with a centred hole, encircled by a vacuum evaporated ahmninium electrode. The diameter of this ground electrode was equal to the diameter of the conductor screen, ensuring electrical contact between the screen and the metal electrode. Figure 7.4b) shows a photo of the experimental setup for the diffusion measurements.

The glass plates and the slice were placed between two Lexan blocks. The water tree was placed in a position where a semi-circle was cut away, thus avoiding IR absorption of the blocks. The slice was finally squeezed between the glass plates and the blocks by tightening the screw, The silicone rubber was allowed to cure for 24 hours. Any error introduced due to the thin silicone layers was examined by performing an experiment using a sample without water trees.

A reservoir of liquid water was carefully put in contact with the insulation screen close to the water tree. Inside the water-filled polyethylene tube, a thin metallic wire was inserted allowing a high voltage (DC or AC) to be applied. The strippable insulation screen was used as a HV electrode.

Prior to the permeation experiments, the outer water-tree boundary was marked, followed by drying for 3 days at 60°C. The water remaining in the water tree regions prior to the permeation experiments was used as reference values. The liquid water was then applied to the insulation screen, and the amount of water absorbed was measured after 3 and 5 days of wetting. Afterwards, the sample was removed from the test setup, and dried again for 3 days at 60°C. The same procedure was used when the sample was subjected to 4kV (1 .3kV/mm) AC (peak) or DC voltage in addition to the liquid water. CHAPTER7 91

to theIll detector

I m

FIGURE 7.3. Testsetupfor thewaterpermeationexperiments(sideview).

a) b)

lmm thickquartz glass-plate \ Insulationscreen \

Drilledhole

Vacuumevaporated _ ahrniniumelectrode

Conductorscreen Drilledhole for IR beam Ventedwatertree Appliedliquid water Reservoirfor liquidwater “ / Polyethylenetube / withwaterandmetalwire

FIGURE 7.4. a) XLPE slice placed on thequartzglass-plates.b) Photographof the setupfor diffu- sionmeasurements(topview). 92 CONTENT OF WATER WITRIN WATER TREES

7.3. EXPERIMENTAL RESULTS

7.3.1. Condition and amount of water in vented water trees

Figure 7.4 shows atypical FTIR transmission spectrum measured of a vented water tree cut from cable sample C examined without using the quartz glass plates. Strong absorption peaks due to water were observed at 3417 and 1647 cm-*. An additional absorption was found at 1110 cm-l. In case of cable B, additional absorption were found at 1732 and 1589 cm-l.

Speetral subtraction was carried out between a wet water treed and a dried non-treed region, and Figure 7.5 shows the resulting difference spectra scaled to the same amplitude. It is shown that the difference spectrum of the vented water tree region, is in good agreement with the spectrum of liquid water enclosed between two XLPE plates [75]. Drying the vented water tree at 400C caused the broad absorption peak at 3417cm-1 to shift to lower wavenumbers.

Results presented in Figure 7.6 summmizes the temperature effect of the saturated water

100

r“%.. --, 80

20 /’ 3417cm-1

,U o 4000 3200 2400 1600 800 Wavenumber [cm-l]

FIGURE 7.4. Transmissionspectraof a ventedwatertree aged (solid line) and dry and unaged region (dotledline) in O.lmm thickXI-FE slicesmierotomedfrom tbe servieeaged 24kV cablesampleC. CHAPTER 7 93

contents measured at two positions within the water treed region. For comparison, the saturation concentration of water in non-treed XLPE is shown as a solid line. The results show a high and approximately temperature independent volubility of water within water treed regions.

Results presented in Figure 7.7 show the measured distribution of water along the water tree structure. After being kept in water at 200C for 1 week high concentrations of water was present in the water tree region. Up to 0.3mrn from the insulation screen the water content was typically 0.5%, however, at about lrnrn from the insulation screen, the water content was increased to values as high as 2!%. This high value was found to coincide with the inception of tree branches. From about 0.3mm within the tip of the tree, the amount of water decreased to values comparable to that of non-treed XLPE. This region extended into the insulation ahead of the visible tree tip, thus indicating a lower density of water filled micro voids in this region.

The effect of drying at 40 and 800C, was to strongly reduce the water content of the water tree region. However, as shown by the results presented in Figure 7.7, significant amounts of water remained within the tree branches even after drying at 800C.

Figure 7.8 show results from measurements of water content along a path perpendicular to the branches. The low amounts of water between the tree branches indicate that these regions can be considered to be non-treed.

7.3.2. Distribution of water close to a water tree tip

In order to ensure that the entire water trees were enclosed in the examined samples, the sample thickness was increased to 0.5mm. Figure 7.9 show results from measurements of a 2.5mrn long vented water tree cut fi’om cable sample B. The measurements were performed by moving the narrow ir-bearn 2mm ftom the insulation screen to the tip of the tree.

As can be seen, high values of water content was measured 2mm from the insulation screen. However, the water content decreased gradually from values higher than 5000ppm to approximately 120ppm measured at the tip of the tree, which are comparable to those measured in case of unaged and wetted XLPE.

The results clearly demonstrate that such a gradual decrease was not observed along the side of the water tree branch. By moving the Ill-beam perpendicularly less than 50prn along path 1 caused the water content to typically decrease from 4000 to 100ppm. 94 CONTENT OF WATER WITHIN WATER TREES

50 Wet water tree ,, 40 ,’ Dried water tree 30 ------Liquid water ,’ (Data from ref. [75]) 20

3800 3700 3600 3500 3400 3300 3200 Wavenumber [cm-l]

FIGURE 7.5. OH-absorptionof liquidwaterbetweenplatesof XLPE insulation(datatakenfrom ref. [75])-. comparedto thatobtainedfrommeasurementsof watertreedinsulation.Resultof spectralsubtractionbetweena watertreedanda driednontreedregion.

80 60 40 20 [“c 105 1

10t=-z-=21 103 I 51

.- 1 I J-u 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

UT [.10 -3 K-1]

FIGURE 7.6. Arrheniusplot of the saturatedcontent of water measuredwithin the water treed regionsatthetwo positions:0.1 andlmm from theinsulationscreen. CHAPTER 7 95

L Wetconditioned 1 week at 20”C }

1~ Dried 12h at 4LT’C

om,i0.4 0.8 1.2 1.6 2 2.4 Distance from initiation site [mm]

FIGURE 7.7. Measureddistributionsof wateralonga straightline correspondingto thecentralwater treebranch(path2). The samplethicknesswasO.lmm.

102

5 I o 0.3 0.6 0.9 1.2 1.5 1.8 2.1 Length of path 1 [mm]

FIGURE 7.8. Watercontentmeasuredalongpath 1perpendicularto the tree branches.The measure- mentswereperformedon a ventedwatertree tim cablesampleC. 96 CONTENTOFWATERmm WATER~

2000 4000 1 2200 3000 2300 I

2000 2400 1000 I~d 25M 2450 o -200 -1oo 0 100 200 300 Length of path 1 [~m]

Watercontentproffle measuredon a 0.5mmthinslice rnicrotomedfrom cable B. The numbersaddedin thefigureindicatesthedistance( in pm)fromtheinsulationscreen.

7.3.3. Water permeation and effeet of applied voltage

The results from measurements of water permeation presented in Figure 7.10 to 7.11 were obtained without any application of voltage, and taken after 5 days of wetting. As can be seen, drying the 0.5mm thick slice at 60°C for 3 days was not sufficient to remove all the water within the treed regions. Figure 7.10 show that the largest values of water content were measured at the centre of the water tree, having a value of approximately 3700ppm 0.2mm away from the insulation screen. The water content measured 400pm from the centre of the water tree, decreased to values typically as low as 100ppm. After 5 days of permeation the largest values of water content were increased to approximately 0.9%, indicating a high volubility of water iwithin the water tree region.

The measured water content of the sample without watertrees was found to be less than 100ppm. These results also demonstrate that the permeation of water into regions between the quartz glass-plates can be neglegted.

Comparing Figure 7.10 and 7.11, it can be seen that the water content along path 1 is more homogeneous closer to the insulation screen than lmm away. This indicates that CHAPTER 7 97

0.2 mm

6000

1/ o days

3000 -

~ Nontree&5 days o -600 -400 -200 0 200 400 600 Length ofpathl [pm]

FIGURE 7.10. lWIR measuredwater contentprofile at a distanceof 0.2mm from the insulation screen.Thewaterprofdesweremeasuredon a 2.5mmlong ventedwatertreehelically cut fromcable sampleC.

9000 I 5 days lmm

6000 -

3000 -

Nontree~ 5 days o J -600 -400 -200 0 200 400 600 Length of path 1 [pm]

FIGURE 7.11. FITR measuredwater contentprofile at a distanceof 1 mm from the insulation screen. 98 CONTENT OF WATER WITHIN WATER TREES

15000 ● OkV 0.5 mm @ 4kVAC (Desk-value) ~ 12000 1.-.x

9000

6000

3000 ..,. .... -..

~600 -400 -200 0 200 400 600 Length of path 1 [pm]

FIGURE 7.12. FTJRmeasuredwatercontentprofilesof watertrees0.5mmfromtheinsulationscreen. Thewaterprofides.weremeasuredatvoltagelevels of Oand4kV (peakvalue).

4000 2mm ● OkV @ 4kVAC(peak-value) X 4kVDC --- 0 days A

0 -800 -600 -400 -200 0 200 400 600 Length of path 1 [pm]

FIGURE 7.13. Measuredwater content 2mm from the insulation screen. The water profiles were measuredat voltagelevelsof Oand4kV (peakvalue). CHAPTER7 99

the water tree structure are more branched closer to the water tree tip. A significant increase of water content was also observed lmrn away from the insulation screen. After 5 days of wetting the water content increased from approximately 4000ppm to 7500ppm.

Figure 7.13 presents the permeation results during the application of an electric field of 1.3kV/mm (AC and DC) are presented. As in case of no voltage the water content was significantly increased after 5 days of wetting. Voltage application did not significantly increase the amount of absorbed water. Anyhow, the largest values of water content were found when applying the AC electric field. This was found when measuring the water content both 0.5 and 2mm away from the insulation screen. 100 CONTENTOF WATER WITHIN WATER TREES

7.4. DISCUSSION

The good agreement between results from measurements of the vented water treed region and a spectrum corresponding to liquid water between two XLPE slices [75], strongly imply that water dissolved within the treed region is dominantly free liquid water. This is consistent with results from calorimetric measurements on water treed XLPE insulation, where most of the water within the treed structure was found to be liquid water [20].

However, drying the water treed sections at temperatures up to 80°C, revealed that significant absorption remained in the treed region. This water is likely to be bound to the polymer structure which may explain why the broad absorption peak at 3417cm-1 shifted to lower wavenurnbers [78]. In the literature it is proposed that hydroperoxide and car- boxylic acid groups bind water most strongly, thus acting as traps [50]. This is supported by the strong absorption peak observed at 1100 cm-* in the vented water tree regions, an absorption peak possibly caused either by presence of hydroperoxide or ionic sulphur- oxygen group [79].

High content of water was generally found in the middle of the vented trees, with lower content of water close to the insulation screen and within the tree tip. The amount of water in regions close to the insulation screen was in good agreement with previous measurements on short vented water trees [80], The high content of water within the tree section showing nearly temperature independent volubility, strongly indicated that these water tree regions probably contained voids that easily can become filled with liquid water even at 20”C.

The water content in this study was measuredly using an absorption coefficient calibrat- ed to XLPE, containing uniform distributed content of water. The distribution of water within the water tree regions was found to be strongly non-uniform. Thus an error is introduced when using the Lambert-Beers to calculate the water content.

The new experimental setup for in-situ examination of permeation of water into dried sections of vented water trees was proven to be successful. The results from the permeation experiments show that liquid water may easily re-enter the treed regions, indicating an open structure with high permeability for water. The application of AC and DC electric fields up to 1.3kV/mm did not significantly increase the rate of water permeation. This experimental result is in agreement with previous permeation experiments on water treed insulation [81]. This is not in accordance with the proposed theory, but maybe explained by diffusion beeing the dominating process.

However, these results only applies to sections closer to the insulation screen. Regions within the tip of the trees remains to be tested. CHAPTER 7 101

7.5. CONCLUSIONS

The FTIR-microspectrometry showed high amounts of free liquid water within the vented water trees. After drying low contents of bound water was found to be present particular within the water tree branches.

The permeation experiments showed that liquid water easily recentered the treed regions, indicating a an open structure with high permeability of water. The application of voltage did not significantly increase the rate of permeation in regions closer to the insulation screen. 102 CONTENTOFWATERWITHINWATER~ CHAPTER 8

DISCUSSION

The main purpose of this work has been to study the nonlinear feature of the dielectric response observed in watertreed XLPE insulation. This was performed by dielectric response measurements in time and frequency domain, numerical calculations of losses of simplified water tree models, and finally water content and water permeation measurements on single water trees.

In this chapter the hypotheses presented in the introduction will be discussed based upon the results obtained.

I. Condition assessment of water tree degraded XLPE cables can be performed by measurements of the dielectric response either in the time or the frequency domain.

In Chapter 5 it was showed that both the frequency and the time domain dielectric responses increase due to water tree degradation. The XLPE cable having high density of water trees, has larger dielectric loss factors than the cable sample with lower density of trees. Typically the values of the dielectric loss factors in the frequency domain are larger than those calculated from the time domain measurements. Increasing length of water trees also causes the degree of nonlinearity to increase. However, this increase is larger in the fk-quency domain than in the time domain. This was observed for both the service aged XLPE cables and the laboratory aged Rogowski test objects.

Frequency independence is another typical aspect of the dielectric response from samples containing water trees. An important practical consequence of this is that diagnostic measurements can be performed at short times or at a high frequency in order to reduce the measurement time.

The cable sample containing long vented water trees with a high probability to bridge the insulation w@l, displayed a DC conductivity starting from 8.5kV. This leakage current remained when measuring the response the second time at 4kV. Unfortunately, measurements of polarisation currents were not performed in this case, but it is likely 104 CHAPTER 8

that a transition to a leakage current (or equivalently a steady DC-current) would have been observed when measuring the polarisation current on this sample [82]. This illustrates the importance of measuring the polarisation currents (or resistance) of the water treed insulation.

Frequency domain measurements of unaged and wet Rogowski type test objects with salt inclusions, revealed that the dielectric loss factor is significantly increased compared to that of the wet and unaged objects without inclusions. This increase can probably be explained by the fissures observed close to the salt particles. Such fissures filled with liquid water are likely to cause an increase in tie dielectric losses measured in the frequency domain, as the ohmic losses are added to the capacitive current. This increase was not measured in the time domain, indicating that the time domain method is less sensitive.

2. Vented watertrees, which is considered the most detriment upon service fife, causes both the time and the frequency domain diehctric responses to become nonlinear

The results presented in Figure 9.1 clearly demonstrate that the nonlinearity factors increase with increasing length and density of the vented water trees. This increase is especially strong for vented water trees longer than 600pm, penetrating more than 46’% of the insulation wall of the test object.

The test objects without salt inclusions aged for 20 weeks, displayed a low and linear dielectric response both in frequency and in time domain. The bow-tie treeing of these samples was similar to that of samples with salt particles. This strongly indicates that the nonlinear increase is due to the vented water treeing. It is, however, difficult to determine whether the observed nonlinear increase is due to increasing lengths or increasing density of vented water trees.

The measurements of the time domain dielectric response of the long cable sample containing a short water tree degraded section cable sample, indicate that the nonlinear feature of the dielectric response is more sensitive to the lengths of the longest water trees than to the density.

3. The nonlinear feature of the didecbic response is caused by voltage assisted ingress of water into watertreed sections of the cable insulation

The proposed mechanism for the nonlinear dielectric response is based upon the assumption that at low or no applied electric stress the water treed region is characterised by spherical micro voids filled with liquid water and separated by channels of crazed insu- DISCUSSION 105

a) 10

8 T@&’ 6 /

q&”

‘nId

. o 200 400 600 800

Expected largest water tree [pm] (37%value)

b) 10

1 o 5 10 15

Density of vented trees, n [cm-z]

FIGURE 9.1. Relationbetweena) the expectedlargestwatertree andb) the densityof the vented watertreesandthecalculatednonlinearityfactors.Thesearebasedupontimedomain measurementsof depolarisationcurrents(qj andfrequencydomainmeasurementsof thecomplex pennittivity(@e’ andw‘’) of thewatertreeagedRogowski typeXLPE insulatedsamples. 106 CHAPTER 8

lation. Increasing the test voltage will result in Maxwell mechanical tensile stresses strong enough to elongate the water droplets, causing the crazes to open up, and water to penetrate into the mechanically weak crazing zones.

Results from the Finite Element Method (FEM) calculations showed that the effect of re-opening of crazing zones caused by an increased test voltage strongly increases the dielectric loss of the water treed insulation. This is qualitatively in good agreement with the dielectric measurements on water treed insulation, where the losses are increased above a certain test voltage. The calculated frequency dependence of the dielectric loss factor is, however, not in agreement with typical experimental results. The model is not, however, expected to be quantitatively correct, since the main purpose of these studies was to show the effect of water entering small channels of crazed insulation using a simplified approach. In the model the conductivities and permittivities were considered to be independent of temperature and electric field stress. During measurements the increased dielectric losses may generate local heating of the thin water filled channels causing the conductivity of water to increase [11]. This may result in decreased electric field in the channels, and lower and less frequency dependent loss.

Maxwell mechanical tensile stresses are considered essential for reopening of the initially collapsed channels. However, surface tension may also contribute to capillary ingress of water. This effeet is likely to take place in hydrophilic regions. The micro-FTIR measurements revealed that such hydrophilic substances were present in the treed regions.

When removing (or reducing) the electric field mechanical relaxation causes the channel to collapse and to slowly recover its former spatial structure. It is therefore likely that the water ingress processes are likely to be associated with hysteresis effects.

According to the proposed theory, a certain electric field is required to open up the channels and making them conductive. The frequency domain measurements of the cable sample becoming conductive at a test voltage of 8.5kV support this theory. The results indicate that an electric field of approximately 3kV/mm is required to create a conductive path through the insulation wall. This conductive path remained when reducing the voltage to a lower level. However, the first measurements at the lower level revealed no conductivity as the loss factor was frequency independent.

However, a hysteresis effect was also observed in the case of the polarisation and depolarisation current measurements on the water treed Rogowski type objects having high values of the nonlinemity factor. The polarisation currents were significantly larger than the depolarisation currents indicating that the materkd properties of the insulation are changed during the application of the high DC voltage. It is rather unlikely that this effect can be explained by long vented water trees bridging the insulation wall, as the estimated longest vented water tree was likely to cross approximately 70% of the insulation. DISCUSSION 107

Water permeation and water content measurements, indicated that elongated and opened structures are likely to exist within the water treed structures, as permeation of water into the partly dried structures was found to be independent of the applied electric stress (up to 1.3kV/mm). Unfortunately, the permeation experiments did not last long enough to study a possible voltage assisted water ingress into the tip regions. Measurements of capacitance on single vented water trees presented in the literature supports this interpretation. Based upon considerable observed difference in permittivity perpendicular (2.7) and in parallel (3.6) to the electric field, it is concluded that vented water trees consist of rather elongated substructures [83].

However, results from the numerical calculations of losses, indicated that the mechanism of voltage assisted ingress of water is more likely in treed regions with lower contents of water, as higher electric fields are necessary in order to open up the collapsed regions. The micro-FTIR measurements on single service aged vented water trees indicated that such regions were likely to be present 3-400pm within the tip, and close to the insulation screen. Due to high and voltage independent water permeation rate into the root region of the water trees, the results indicate that the formation of the nonlinear dielectric response, is caused by regions close to the tip of the water trees. Microscopy studies of samples after electrical tree initiation, revealed that electrical trees were initiated within the water tree structures, typically 300pm within the water tree tip. A typical example is shown in Figure 8.2 [84]. This indicate that high electric fields are likely to be present within these regions, a condition of Maxwell forces to be active.

Resistance measurements of single service aged vented watertrees bridging the insulation wall supports this interpretation [82]. These measurements show that application of a certain electric field is required before any value of the resistance is detectable. From

FIGURE 8.2. Electric tree initiatedapproximately300prnwithinthe tip region of a service aged ventedwatertree[84]. 108 CHAPTER8

a value, typically 2-3kV, the resistance decreased nonlinearly with increasing test voltage. A short electrical tree initiated within the tip of the bridging water tree, and reaching the conductor screen, caused the measured resistance to become linear from voltage levels as low as 10V, indicating that the short electric tree short-circuited the region making the resistance nonlinear.

After drying at 90°C the response of the service aged cable sample B became linear. The IR measurements revealed that the liquid free water in the treed regions diffused out after the thermal treatment. However, signitlcant amounts of bound water was found to re- main within regions closer to the insulation screen. This indicate that free water is essential for the nonlinear increase of the dielectric response. By re-wetting the cable at 90°C, the high dielectric loss factor was re-established to values higher than measured initially, but the non-linearity was not restored. At such high temperatures the water tree structures is likely to become completely filled by absorbed liquid water. The channels will open up and the fibrillar structure of the water tree will adjust to this high water conten~ causing the conductivity and permittivity of the treed region to increase. Thus no additional water penetration is obtained by increasing the applied test voltage. This effect may explain the apparent linear dielectric response after re-wetting.

It is possible that nonlinear dielectric effects of the liquid present within the water tree regions can contribute to the nonlinear dielectric response. The solvated ions and the water within the treed structure are subjected to electric stress and possibly also heating. Electric properties such as perrnittivity and conductivity of liquid water becomes nonlinear due to different mechanisms and processes dependent upon frequency, temperature and type and amount of ions or other contaminants present [85, 86]. CHAPTER 9

CONCLUSIONS

Themore detailed results from this study can be summarized as follows;

A new mechanism for the nonlinear dielectric response has been proposed. This is based upon the assumption that at low or no applied electric stress the water treed region is characterised by spherical micro voids filled with liquid water and separated by channels of crazed insulation. Increasing the test voltage will result in Maxwell mechanical tensile stresses strong enough to elongate the water droplets, causing the crazes to open up, and water to penetrate into the mechanically weak crazing zones.

The numerical calculations strongly supports the hypothesis presented in Section 2.4, showing that the nonlinear dielectric response is a result of re-opening of initially collapsed crazing zones. This will then cause the water to penetrate into sections of the water tree stmcture causing the losses to strongly increase.

The results show that it is the vented water trees that reduce the AC breakdown strength of both the laboratory aged test objects and service aged cable samples.

The method of inserting NaCl particles into the boundary between the upper serniconductive screen and the insulation was found to successfully enhance the vented water tree initiation and growth. In addition, low electric ageing stress and constant temperature supressed the growth of bow-tie trees, causing ageing similar to that of the service aged cables.

Vented water treeing cause the time domain dielectric response to become nonlinear.

Both time and frequency domain measurements of the dielectric response can be used to assess the state of the insulation. However, the measured ffequency domain dielectric response have higher magnitudes, and degrees of nordinearities compared to those measured in the time domain.

The FI’IR-microspectrometry showed high amounts of free liquid water within the vented water trees. Low contents of bound water was found to be present particularly 110 CHAPTER 9

within the water tree branches.

The permeation experiments showed that the applied liquid water easily recentered the treed regions, indicating a significant connectivity of the treed structure. The application of voltage did not significantly increase the rate of permeation in regions closer to the insulation screen.

Thus from this study, it can be concuded that

The measurements indicate that both the frequency and the time domain methods can be used for assessing the status of the XLPE insulation, and therefore the first hypothesis was accepted.

Vented water treeing was found to cause the dielectric response measured in the time and frequency domain to become nonlinar, and therefore the second hypothesis was accepted.

The hypothesis.- that the nonlinear dielectric response was caused by voltage ingress of water into water treed sections was theoretically and numerically confirmed. The experimental results showed that such a mechanism is most likely present in regions within the water tree tip. However, more experimental work is needed before this hypothesis can be verified. APPENDIX 111

Appendix 1

SIMPLIFIED CALCULATIONS OF DEPOLARISATION CURRENTS FROM RE- TURN VOLTAGE MEASUREMENTS

The response function can be calculated ilom return voltage measurements using Equa- tion 2.5. Such calculations has been applied successfully in tie case of mass impregnated cables [39, 40]. However, XLPE insulated cables have a neglible conductivity (as long as no water trees penetrates the insulation wall), and the return voltages are found to be low compared to the charging voltages (see for example Figure 5.4).

Figure A. 1 show results from calculations of the dielectric response function from return voltage measurements at 2.5kV of the 24kV service aged cable sample B. The numerical calculations (MATLAB) were done by using Equation 2.5 with two different values of the conductivity and then compared to an approximated expression (Equation 2.6) [39].

~o-3

------Equation 2.5, o= 10”4 [L?m-l] 5 i

3 — Equation 2.5,o=10’18 [Qm-l] \ 2 ---- Equation 2.6 (Approximation)

~o-4 ~ Meas~ed (~uation 2.3)

3 2

10-5

3 2

10-6 1 , , , , ,s, I 35 10 23510023 5 1000

Time [seconds]

FIGURE Al. Dielectricresponsefunctionmeasuredandcalculatedfromretmnvoltagesmeasure- mentperformedat2.5kV on the24kV serviceagedcablesampleB. Thenumericalcal- culations(MATLAB)weredoneby usingEquation2.5 withdifferentnumericalvalues of theconductivitiesandthencomparedto anapproximatedexpression(Equation2.6) [39]. 112 APPENDIX

As can be seen, both the calculation based upon Equation 2.5 and the approximated expression fits well to the measured dielectric response function.

Thus, increasing the conductivity of the insulation to G= 10-14 [fAn-l], causes the calculated response function to deviate significantly from the measured response at times longer than 100 seconds. However, results from measurements of the conductivity of XLPE insulation (room temperature and low electrical fields) show that the conductivity is likely to be less than o= 10 ‘I* [Qrn-l] [87]. Thus in case of water treed XLPE cables were no trees are bridging the insulation wall, this value of the conductivity is likely to be applicable. APPENDIX 113

Appendix 2

RELATiON BETWEEN THE CONDUCTIVITY AND TANt5

The dielectric losses are estimated by FEM by using Equation 6.3, and all calculations are made with assumptions of sinusoidal applied voltages. At a certain angular frequency cothe losses have to be described either by a resistivity or tan~, The conductivity can be transformed to tad by the following relation:

(Al.) tanti=--J--ox’(o$p

In general the dielectric losses also depend upon the ratio between the imaginary and real parts of the permittivity. The total dielectric loss factor can then be considered to constitute a conduction and a dipolar term, where the term &‘’(o$ must be determined experimentally:

1 + E“(m) tana=— — (A.2.) QE’(ol)p E’(O$

In order to solve this problem, the relation between the dielectric loss factor and there- sistivity were defined as:

O.E’’(OI) 1 (A.3.) tan?i=~+-=W’((I$ w’(o)) m’(ol)p(o)

The only difference between Equations A.1 and A.3 is that the resistivity is considered to be frequency dependent. 114 APPENDIX

Appendix 3

ADIABATIC HEATING OF A WATER FILLED CHANNEL

Assuming adiabatic heating for one second, the temperature of the water in the channel will approximately increase by [88];

TC. A. AT=R~ (A.4.)

4T=~ (A.5.) ~“ ‘YC

Where; y: density of water (lkg/L) c: heat capacity of water (4190 J/kgK) A: area of channel cross-section AT: temperature increase R: resistance of channel L current flowing through the channel J current density a water conductivity (5.6 10-6 S/m) APPENDIX 115

Appendix 4

IRABSORPTION IN THE QUARTZ GLASS-PLATES

Measurements of long duration were performed by placing the thin microtomed watertreed slices between lmrn thick quartz glass plates. This was done in order to prevent drying during examination.

Figure A.2 show that the quartz glassplates absorb approximately all the infrared radia- tion at wavenumbers less than 2000 cm-l. Thus the small corrections for sample thickness based upon measurements at 1895 cm-l, were made without using the glass-plates. The quartz plates were nearly transparent in the water band region close to 3500cm-1.

100

4 llnrnquartz glass

‘:~ 4500 4000 3500 3000 2500 2000 1500 1000 Wavenumber [l/cm]

FIGURE A.2. Backgroundspectraof a lam quartzglassplate.The absorptionby thequartzglass- platesis particularlyhighatwavenumberslowerthan2000cm-1. 116 APPENOIX

Appendix 5

IR ABSORPTIONIN SILICONE

As mentioned in Section 7.2.3, a thin layer of silicone rubber was inserted on both sides of the 0.5mmthick XLPEslice in the region of the marked water tree. As this layer can influence the transmission spectra by absorbing the incident ir-beam, FTIR-measurements were conducted on a nontreed XLPE slice and placed between two quartz glass plates.

The results presented in Figure A.3, show that the thin silicone layer decrease the transmission spectra by 5~0 in the range from 3400 to 2900cm-1.

In Figure A.4, the result from the FTIR measurements of a 0.5mm silicone-layer squeezed between two glass-plates, and subjected to liquid water for 5 days is presented. As can be seen, only minor water absorption is observed at 3400cm-l. At other wavenumbers the spectra seemed to be unchanged. Thus systematic measurement errors caused by water absorption of the silicone glue can be neglected.

0 4500 4000 3500 3000 2500 Wavenumber [l/cm]

FIGURE A.3. Absorptionspectraof a 0.5mm tick XLPE slice coatedby a very thinlayer of silicone. APPENDIX 117

I “~ 4500 4000 3500 3000 2500 Wavenumber [l/cm]

FIGURE A.4. IR-spectraof wettedanda dry0.5mmthicksiliconelayer. 118 ‘APPENDIX

Appendix 6

WATER ABSORPTION IN THE STRIPPABLE INSULATION SCREEN

As the water has to permeate through the initially dried strippable screen before entering the treed regions, the water absorption properties of the screen are of interest. The experiments were performed to investigate the absorption rate of the screen material by weight analysis. Two samples (- 3g) were cut from the 0.5mm thick insulation screen of cable C and dried for 3 days at 60°C, equal to the procedure of the thermal treatment of the XLPE slices used for the water permeation experiments.

The screen samples were carefully dried by using blotting-paper prior to weight measurements, and the weight change was measured after 30 seconds of air exposure.

Results from measurements of water absorption by the insulation screen are presented in Table A. 1.

Time of wetting [days] O 0.2 1 2 5 21 180

Weight gain [%] -. 0.5 1.2 1.8 2.8 5.3 16.4

TAELE A.1. Measuredweightgaindueto absorptionof waterby thestrippableinsulationscreen.The sampleswerecutfromtheXLPE cablesampleC.

The results presented in Table Al show that the semiconductive screen absorbed high amount of water. After 5 days of wetting at room temperature, the insulation screen had absorbed approximately 370 of water, without reaching saturation. After 6 months of wetting the water content increased to 16~0. Thus the vented water trees will therefore have easy access to liquid water. APPENDIX 119

Appendix 7

LISTOF SYMBOLSANDABBREVIATIONS

Most of the symbols used in the text are included in the list.

A area a absorption coefficient B shapeparameterin the Weibull distribution c capacitance c heat capacity co geometric capacitance d insulation thickness &’ apparentpermittivity AP(t) polarisation E“ dielectric loss factor E electric field &’ real partof the complex permittivity co vacuum permittivity relative permittivity ;’ frequency induced tensile stress {t) dielectric response function r gamma function Y density ~d nonlinearityfactor in the time domain % nonlinearityfactor in thefrequency domain I current I band intensity Id depolarisationcurrent 10 band intensityof the reference spectra % polarisation current J currentdensity L length 1- longest observed watertree v expected largestvalue (37%-value) P probability Pi dielectric losses R resistance a conductivity T temperature T transmittance dielectric loss tangent u voltage U. ratedvoltage 120 APPENDIX

u,(t) returnvoltage v volume (0 angularfrequency K’ real partof the dielectric susceptibility K“ imaginary partof the dielectric susceptibility

AC alternatingcurrent DC direct current ESC environmental stresscracking FEM fiite element method fast fourier transform fourier-transforminfrared spectroscope SEM scanning electron microscope TEM transmissionelectron microscope XLPE cross-linked polyethylene REFERENCES

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