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Fothergill et al.: Electrical, Microstructural, Physical and Chemical Characterization

514

Electrical, Microstructural, Physical and Chemical Characterization of HV XLPE Cable Peelings for an Electrical Aging Diagnostic Data Base J. C. Fothergill Department of Engineering, University of Leicester, Leicester LEI lRH, U.K.

G.C. Montanari DIE-LIMAT, University of Bologna. vide Risorgimenta 2,40136 Bologna. Italy

G. C. Stevens Polymer Research Centre, University of Surrey, Guildford, G42 SXH, UK

C. Laurent, G. Teyssedre Universite Paul Sahatier, 118 route de Narhonnc, 31062 Toulouse, Cedex, France

L. A. Dissado Department of Engineering, University of Leicester, Leicester LEI 7RH, UK

U. H. Nilsson Borealis AB, 44486 Stenungsund, Sweden and G. Platbrood Laborelec, Rodestraat 125. B-1630, Linkebeek, Belgium

ABSTRACT The aim of the European project "ARTEM1S"is to develop a diagnostic system for assessing aging in power cable insulation. Its first task was to make a thorough characterisation of the cable insulation before aging. This is intended to provide a background against which any changes introduced by thermo-electric aging can be identified. The aging markers derived from this initial characterisation will be considered both a s diagnostic indicators in their own right, and also to develop a n aging model for predictive purposes, if and when possible. This stage of the ARTEMIS' programme is now complete and we will present a n analysis of the results, and show how they may he correlated with the concepts proposed in aging theories.

Index Terms nism.

1

- Insulating

materials, characterization, cables, aging mecha-

INTRODUCTION

HE requirement for extra high voltage (EH") underT g r o u n d power &Ies (400. 500 k v ) is increasing, The mean electric field in the insulation of such cables was raised recently to about 16 kV/mm and the most common insulation used is polyethylene (XLPE), Long.tem experience of X L ~ however, ~ , is limited to moderately stressed cables with mean fields of 5 to kV,mm, Many power cables have been operat. ing for 20 years and are approaching the end of their 30Monusoipt war mceiued on 25 Noember 2001, in p a l f o r m I8 March 2003.

year design life. Much of the related generation and transmission equipment (for example nuclear power stations and transformers) have a 40-year design life. If robust methodologies could be found for improving or/and evaluating the reliability of ac Power cables, it may be possible to continue to use them without compromising the reliability of the system. Such methodologies require considerable improvements in the understanding of degradation mechanisms of cable insulation and the establishment of a dependable life modd. Such Progress would also enable XLPE cables to be more competitive at EHV levels in comparison with oil-paper cables and, even, overheads h e S .

1070-9878/1/$17.00 0 2003 IEEE

ZEEE Transactions on Dielectrics and Electrical Insulation

Vol. 10, No. 3; June 2003

Recently there has been some convergence of agreement on theories of electro-thermal aging of semi-crystalline polymeric insulation. The three main proponents appear to he the Dissado- Montanari-Mazzanti (DMM) theory 111, that of Lewis et al [21 and the Crine theory [3]. It is not the purpose of this paper to contrast the models (see, for example, Griffiths [41) hut to examine experimental methods of characterising aging following.the theories proposed. In this context, aging is considered the change within the polymer, brought about by electrical and thermal stresses, that leads to macroscopic degradation, thus loss of serviceability, ultimately leading to partial discharges and electrical treeing. Experimental techn’iques are therefore described that can he used in a complementary manner and are expected to provide chemical-physical, microstructural and electrical characterisation. It is also important that these experimental techniques provide aging markers that can he used for the prognosis of the cable and the diagnosis of aging leading to both bulk and localised degradation, which can take place close to the design field.

transit of space charge across the sample and the onset field at which electro-luminescence and space charge can be detected. Other facets of the ARTEMIS programme, such as molecular computations, will he used to aid the interpretation and will he described elsewhere.

The work described here is part of the European ARTEMIS’ programme. The ARTEMIS partners include cable manufacturers, material suppliers, electricity distributors, and a number of universities throughout Europe. The programme is intended to provide an understanding of the electrical degradation process, through the development of aging models, as well as diagnostic methodologies for evaluating the degree of aging and the reliability of in-service power cables and improved design and manufacturing techniques. The aging model is to he developed through an analysis of samples of unaged and aged power cables. In the .programme, a variety of investigative techniques have been applied to lengths of cables, but mainly to samples peeled from full-sized XLPE-insulated cables. The use of peelings as specimens will be justified in terms of reproducibility of experimental data and equivalence to the full-size cable investigations also carried out. The techniques used range from those with micron and submicron resolution that are intended to determine sub-micron features, through to ‘hulk‘ measurements such as differential scanning calorimetry (DSC) that give information on larger characteristics, e.g. crystallinity, in addition to lamella thickness information. Spectroscopic techniques have been used to identify the chemical composition (e.g. in terms of cross-linking by-products such as acetophenone and cumyl alcohol) and aid the interpretation of the electro-luminescence processes. These techniques will be described and the results reported. The electrical properties of the material are characterised by dielectric measurements, conduction currents, electroluminescence, and a space charge accumulation and packet formation. Relationships between the various electrical properties are attempted taking into account the

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2 THEORETICAL BACKGROUND Electrical failure of polymeric cables is normally due to growth of damage starting from weak points such as contaminants, protrusions and voids (CPVs) [5,61. Improvements in manufacturing technology have resulted in a reduction of concentration and size of CPVs and this has permitted increases in the design field. In order to consider whether it is worthwhile investing further effort in improving manufacturing processes, it is necessary to.understand the aging mechanisms involved in the production of “macrodefects” ( > 10 p m ) from “microdefects” ( < 10 nm). High quality cables should not contain macrodefects. Thus such an aging mechanism will require the progressive growth of microdefects to the point at which degraddtion is possible, at normal design stresses, through hot electron and avalanche damage, followed by electrical tree growth. It is for this reason that work has been focussed in this project on the investigation of properties that may he associated with microstructural modifications, such as space charge, spectroscopy techniques, luminescence, microscopic observations (at micro and nanoscale). Moreover, aging models are investigated, which rely upon microstructural changes to explain damage growth. The three life models referred to earlier [1-3] differ slightly in physical details and somewhat more in their mathematical development, but their commonality can be found in considering the different size scales of the various processes involved. The Lewis model considers the breaking of chemical bonds as a starting point for the aging. The problem is stated more generally in thermodynamic terms in the other two ageing models. They consider that moieties, i.e. small regions, of the polymer may exist in either of two states. In an unaged polymer, most of the moieties are in state 1 whereas, as aging progresses; more and more moieties switch to state 2. An energy barrier exists between the states and moieties may switch between states by thermal activation. In the Lewis model, these states would comprise unbroken and broken bonds; the other two models are more general and may include molecular chain reconfigurations for example. The presence of a local electric field changes the dynamic equilibrium between the states, and, according to the DMM theory, causes irreversible changes when a threshold field is exceeded. It is assumed that changes from state 1 to state 2 will eventually strain, and possibly lead to, nano-metre and submicron sized voids when a sufficient concentration of moieties have switched to state 2. Areas that are mechanically weakened in this way will increase in size

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strain, which may be defined in terms of the change in volume occupied by the polymer moiety in converting from state 1 to state 2. The crystallinity and density determine the number of moieties per unit volume; these may change locally. Aging would be expected to result in morphological changes. It is therefore important to measure crystallinity, free volume and morphological features. The electromechanical energy stored around trapped space charge centres drives chain rearrangements. In this case, the local field strength is dependent upon the quantity of space charge in the centre and thus only indirectly upon the applied field. In the presence of an applied voltage, space-charge trapping centres are formed that correspond to microscopic defects (e.g. clusters of chemical additives and impurities, cavities). Thus, a space-charge centre could be a charged cavity generating a concentrated electromechanical stress. A minimum “threshold” field is required (in the DMM model) to make this process irreversible. This leads to the growth of low-density regions

leading to super-micron sized voids and the more rapid degradation associated with hot electron injection, partial discharging and electrical treeing (e.g. [6D. For the DMM model, the essential steps that would give rise to measurable changes are outlined below. A similar (or partly similar) sequence of aging steps and features may be identified for the other two models and similar experimental techniques would be required to validate them. All three models assume that a section of the polymer can transfer between alternative local states by surmounting an energy barrier. The presence of an electric field alters the energies of the two states, thereby altering the relative proportions of the polymer in each of the alternative states at equilibrium. It also accelerates the rate of approach to the equilibrium distribution from any arbit r a q starting distribution. It is important to note that these processes are temperature dependent; aging results should therefore show an appropriate dependence on temperature.

2.2 THE GROWTH OF LOW-DENSITY 2.1

ELECTRO-MECHANICALSTRAINS

REGIONS Low-density regions will grow that contain an excess of voids in the nanometre to sub-micrometer size. Measur-

The structural change between alternative states of the model corresponds to the generation of an elemental

Table 1,

Features of model MICROSTRUCTURAL

Experimental measurements and techniques associated with aging processes Techniques Measureands Crystallinity

DSC

Raman and Micro-Raman

Spectroscopy

ELECTRICAL

FTlR

Amorphous content h n g period spacing Nano and meso voids

ATR - R I R

Lamella and c~ystal domain size.

AFM

Traps

PEA, TSM Dielectric spectroscopy,low field conductivity Charging - discharging currents

SAXS TEM with permanganic etching

(carrier mobility) PL, RIL, EL Molecular modelling

PHYSICAL

Chain reconfiguration

FTlR

Raman - pRaman Nan0 to microvoids

Free volume Traps CABLE STABILITY

’~

Chemical changes, additives

Oxidation L

.

General degradation

Molecular modelling TEM with permanganic etching SAXS AFM LSCM Optical microscopy PAS SAXS see above

PL, RIL FTIR FT- Raman

Chemometrics, PEA, TSM

FTIR FT- Raman HPOIT Breakdown voltage

IEEE Transactions on Dielectrics and Electrical Insulntion

able quantities associated with these changes would include chain reconfigurations, void size distribution and free volume. Voids would also be expected to act as charge trapping centres and so trap characterisation is important in distinguishing this part of the ageing process. Chemical impurities may act as recombination centres. The propensity of the material to trap charge as well as measurements of electrical charge transport and mobility are likely to be useful indicators of changes in trap number and depth distribution. Cumulative effects lead ageing at a macroscopic scale.

2.3 AGEING AT A MACROSCOPIC SCALE Degraded regions grow such that, when they become big enough, they provide a path to electrical failure via hot electron injection, increases in the local conductivity, electrical discharges and mechanical fracture. Failure (thermal, mechanical, electrical) can he said to have occurred when sufficient conversion takes place in a local region such as to change the energy (or potential) surface and destroy material integrity. The critical fraction of moieties that are required to change from state 1 to state 2 after which ageing can be said to have taken place should he established. Estimating this critical conversion factor is to be done by comparing measurable features of aged and unaged cable specimens. As well as thermo-electric ageing, there may be changes due to the instability of the cross-linked polyethylene cable insulation. It contains many chemical additives and impurities including: catalysts and other additives required during manufacture, reaction by-products, for example acetophenone remaining from the cross-linking reaction, stabilisers such as antioxidants, imperfections such as "ambers"(oxidised particles etc.) occurring during processing, ionic impurities that diffuse from the semicon screens and " naturally-occurring" chemical imperfections such as hydroxyl and ketone functions, double or triple bonds, and branching. The spatial distributions of concentrations of many of these chemicals may change during the life of the cable, and indeed during the life of test specimens taken from cables, and may contribute a complicating factor to the measurement of the ageing process.

3

51 7

Table 2. Meanings of abbreviations used in Table 1.

Abbreviation AFM ATR-FI1R DSC EL FTIR HPOIT LSCM PEA PL RIL

SAXS TEM TSM

Experimental Technique Atomic Force Microscopy Attenuated Total Reflection - Fourier Transform Infra-Red Spectroscopy Differential Scanning Calorimetly Electroluminescence Fourier-Transform Infra-Red spectroscopy High Pressure Oxidative Induction Time Laser scanning confocal microscopy Pulsed electro-acoustic space charge measurement Photoluminescence Recombination-induced luminescence Small-angle X-ray scattcring Transmission Electron Microscopy Thermal Step Method for space charge measurement

are listed variables that can be experimentally measured and techniques that may be used. Many of the techniques are referred to using their commonly used abbreviations; their full names are given in Table 2. The selection of techniques is necessarily pragmatic. Some techniques were considered too resource intensive, for example the measurement of the distribution of local strains. Some were removed from the programme as they provided only little or no extra information, for example modulated - temperature DSC. Some methods have been duplicated because they are considered especially important or/and straightforward to apply to cables in service. An example of this is the use of both the PEA and TSM techniques, which are intended to give similar information about space charge profiles. The techniques are described below according to the order in which they appear in Table 1.

EXPERIMENTAL

There were two considerations in the planning of experimentation. Firstly, it was necessay to identify techniques that could be used to characterise the stages of the aging described above. Secondly, a common specimen design, preparation and storage procedure had to be adopted that could he used, at least for most of the techniques, in the various participating European laboratories.

3.1

Vol. 10, No. 3; June 2003

EXPERIMENTALTECHNIQUES

A summary of the principle processes occurring in ageing is given in Table 1. Against each of these processes

3.1.2RAMAN SPECTROSCOPY Raman measurements were made on a F T N I R Raman

PE 2000 spectrometer, with 1000 mW of 1.064 p m laser radiation incident on the sample in c.a. 100 p m spot. Res-

SI8

Fothergill et al.: Electrical, Microstnrctural, Physical and Chemical Characterization

olution was 4 cm-I, and 32 scans were averaged. MicroRaman spectra were conducted on a Renishaw 1000 spectrometer with 5 mW of 780 nm laser radiation incident on 0.5pm spot. Resolution was 4 cm-’ and the integration time was 16 s. The spectrometer was operated in the confocal mode, and the depth resolution was estimated at 5 pm. Spot spectra were taken at several points on both sides of the samples. The same relative orientation hetween sample and laser was preserved. The crystalline, a,, amorphous, ma, and interphase (or “rigid-amorphous”), ab, fractions can he determined [SI as follows

In equation (1) I,,,, is the intensity of the CH, bending band at 1416 cm-’, I,,,, is the total intensity in the twisting region at 1295 cm-’ (an internal standard that is independent of crystallinity) and 11303is the intensity of the twisting CH, hand of liquid-like material at 1303 cm-I. The scaling factor of 0.46 is determined from fully crystallised PE: further work is required to confirm the applicability of this factor for XLPE. For low-density PE these calculations always lead to Raman crystallinity values lower than those obtained by DSC (by about 5.10%). The correspondence is better with high-density polyethylene. In practice Raman and DSC are complementary. Raman provides a direct measure of the amorphous phase content and of the pure orthorhombic crystalline content. In the latter case only those PE chains in the orthorhombic crystal form can contribute to the Raman scattering peak. In contrast, DSC will measure “ordered” PE stems which may he just outside the true orthorhombic phase but which none the less participate in crystal melting. DSC cannot provide an independent measure of the amorphous phase hence Raman is needed to be able to assess the interphase (or rigid amorphous) content ab as the difference between the orthorhombic crystal contribution and the amorphous phase contribution. 3.1.3 FOURIER TRANSFORM INFRA RED SPECTROSCOPY AND MULTIVARIATE

STATISTICAL ANALYSIS (CHEMOMETRICS) ATR-ETIR spectra were conducted on a Perkin Elmer Spectrum 2000 spectrometer equipped with a diamond single reflection ATR accessory. The resolution was 4 cm-’ and 16 scans were signal averaged. The CH, rocking band found in the ATR-FTIR spectra of polyethylene enables calculation of the amorphous content a“. This band is composed of a peak at 730 cm-’ due to the crystalline phase only and a peak at 720 cm-’ representing the contributions from both crystalline and amorphous phases. [91. A multivariate statistical analysis (chemometrics) [Ill was undertaken on the FTIR results to reduce the wealth

of spectral information to a few potential markers for spatial variation of both unaged and later aged conditions. This would enable us to observe the dependence of chemical changes in the system to cable test conditions (voltage stress, temperature and time), and to explore relationships of the behaviour of other measured properties (e.g. voltage breakdown, space charge) with specific changes in the IR spectra. The spectra obtained from the investigation of the specimens in ARTEMIS were baseline-corrected and normalised to the C-H wagging bands of PE before principle component regression (PCR) analysis. PCR is a two-stage process. In the first stage, it can minimise the number of independent components required to describe the variations between spectra (or chromatograms, etc.). This technique enables several thousand spectral points (X-data) to be reduced to a few principal components (PCs), where the PCs describe the (spectral) X-variance across all of the samples (data compression). In the second stage, these Pc‘s are regressed against known Y-property data (for example, distance from cable conductor or aged condition) and any trends that are present can be identified and a correlation or predictive model constructed. 3.1.4 MICROSCOPY

Atomic Force Microscopy (AFM) observations were conducted with a Digital Instruments Nanoscope I11 operating in the “tapping” mode. Voids were observed using ultra-cryo-microtomed sections of the cables. Transmission electron Microscopy (TEM) samples were prepared by a conventional permanganic etching process followed by replication with cellulose acetate film, metal and carbon coating and dissolution of cellulose acetate LIZ].

Laser Confocal Scanning Microscopy (LSCM) can he used to image voids directly with a resolution of approximately 200 nm. Optical microscopy of stained specimens was used also for void detection for super-micron voids.

3.1.5SMALL ANGLE X-RAY SCAlTERING SAXS can be used to characterise void sues in the nano range, thus complementing E M and AFM. Measurements were made using a SIEMENS X-Ray diffractometer D5000 equipped with a solid silicon-lithium Si-Li detector, which allows a beam monochromatisation without signal losses. The interpretation of the experiments was carried out according to the methods described in [13-151. The scattering results are influenced by both clystalline domain scattering which is able to provide information on the long-period spacing of the microstructure (i.e. amorphous plus lamella thickness forming a periodic structural repeat unit) and by nano to mesa void scattering at low

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. IO, No. 3; June 2003

scattering angles. No attempt has been made to separate these two components.

ing current measurements can also be transformed from the time to the frequency domain to produce a result similar to that of dielectric spectroscopy, hut allowing much larger poling fields. For this purpose long charging times should be used, e.g. > 5/(~rf,,,,J;these should also be used to ensure a quasi-steady state condition for the charging current from which the conduction current is derived.

3.1.6 SPACE CHARGE MEASUREMENT The pulsed electro-acoustic (PEA) apparatus for space charge measurement can perform space charge observations with and without field applied to the specimen, so that it is particularly appropriate for investigation electrical properties such as charge injection and transport [16,171. Measurements are made on film specimens in a controlled temperature environment O-IOOk 0.1 “C in air, under dc poling field ranging from 3 to 150 kV/mm. The electrodes can be made from either a semicon material or aluminium; specimens with gold-coated surfaces may be used to ensure a good surface contact. The system resolution is better than 0.5 C.m-’ in charge density and 10 p m spatially. In order to extract quantitative information from space charge profiles, an estimation of mean absolute stored charge density was carried out according to the procedure described in [161. The (log) mean stored charge density (preferably measured at a given depolarisation time) can then be plotted as a function of (log) poling fields, and an estimate can be made of the dc charge threshold, i.e. the level of field above which space-charge begins to accumulate significantly [161. Another quantity derived from space charge measurements (using the depolarisation characteristic) was the trap-controlled apparent mobility will decrease with time since deeper and deeper traps will be involved in releasing charges as time elapses after the beginning of depolarisation. Hence, mobility trend evaluation as function of time under stress is investigated referring to a given depolarisation time (e.g. 1000 SI.

3.1.7 CHARGING-DISCHARGINGCURRENT MEASUREMENTS AND DIELECTRIC SPECTROSCOPY Charging-discharging current measurements and dielectric spectroscopy can provide sensitive indicators of changes in density, clystallinity, electric strength and microstructure all of which might be expected to change if the material ages (e.g. [191). Dielectric spectroscopy enables measurements of the real and imaginary parts of the capacitance to be made over a range of frequencies, temperatures and voltages. In addition, even if dielectric spectroscopy is inherently an A C technique, it is an effective way to estimate conductivity in this application [191. Charging current measurements provide a method for establishing the conduction current as a function of voltage, including the higher voltages at which the characteristic is expected to become non-linear, as well as the lowfrequency polarisation characteristics. It bas been suggested (e.g. [161), that this so-called “threshold voltage” for dc conduction is a useful indicator of aging. Discharg-

519

The dielectric measurements were performed using Solartron CDI interface to a guarded electrode system comprising a Keithley 6165 “resistivity adapter”cel1. Measurements were made by averaging at least four readings at each frequency over a range 1 mHz to 1 MHz in air at controlled temperatures of 20-100+0.1 “C.Only low voltages were used (typically 1 VrmJ Charging-discharging current measurements were carried out by a three-terminal cell, with gold electrodes, in N atmosphere at 300 kPa and different fields (from 1 to 90 kV/mm) and temperatures (20 to 80 “C). Poling time and depolarisation time were lasting up to 24 and 10 h, respectively.

3.1 .E BREAKDOWN VOLTAGE Breakdown voltage tests are widely used to test for general degradation of insulation. The technique is, however, affected by macrodefects, since breakdown will generally initiate at the weakest point. In tests in this project, 20 peeling specimens were tested with linearly-increasing ac voltage (at a rate of 2 kV/s) and the results analysed using Weibull statistics (e.g. [61).

3.1.9 MOLECULAR SIMULATION A clearer idea of the nature of the ageing reaction can be gained by investigating the nature of the traps, and this has stimulated current research via molecular simulation. This work has concentrated upon polyethylene with and without physical/chemical defects [21-241. Molecular simulation is a natural complement to this experimental work. 3.1.10 LUMINESCENCE TECHNIQUES AND MOLECULAR MODELLING Luminesccncc techniques include photoluminescence (PL), recombination induced luminescence (RIL) and electroluminescence (EL). Photoluminescence will detect both extrinsic chromophores such as additives and reaction by-products and intrinsic chromophores such an oxidised and unsaturated groups. The photoluminescence may be excited by UV. Room temperature and liquid nitrogen temperature measurements allow observation of fluorescence and phosphorescence respectively. Recombination induced luminescence observes light emitted from the relaxation of charges implanted at the sample surface using a plasma [251. It will detect “electrically active” chromophores, i.e. those which act as trapping and recombination sites. Electroluminescence (EL) refers here to

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light emitted intrinsically by the material under the influence of an electric field (excluding gaseous discharges). It is an important phenomenon to monitor as its onset might define a threshold for space charge formation, and possibly also for chemical degradation [26]. EL tests were performed at room temperature, in vacuum, on specimens with semi-transparent gold-coated electrodes, using ac and dc voltages. Test methodologies are described in [241. The test voltage range is similar to that used for space charges, in order to get crossed information.

3.2 SPECIMEN PRODUCTION AND

THERMAL PRE-TREATMENT Two different 63/90 kV power cables were manufactured, by two different European companies. Both of these used the same crosslinkable super-clean polyethylene to produce the insulation, which was 14 mm thick. The semiconductive material was super-smooth carbon-hlack doped ethylene butyl acrylate. For the purpose of this paper, data taken from different cables are reported without referring to the manufacturer, as the results for the two cables did not normally vary significantly. Peelings were cut from the cables using a lathe equipped with a specially designed knife to get optimum surface smoothness. They have a nominal width of 8 mm and a thickness of 150 pm. From a pre-screening study of the field distribution, it was decided to focus the investigation on the insulation situated between 2 and 4 mm in a radial direction from the inner semicon.

Table 3. Concentration (in ppm) of cross-linking by-products as a function of treatment time (50 "C, ambient pressure) by HPLC and confirmed by FTIR. Substance 0 hours 5 hours 24 hours

Cumyl alcohol Acetophenone a-methylstyrene

13350 3700 116

Cumene

i1

2420 88 0 0

0 0 0 0

It was decided to subject specimens to a thermal pretreatment before testing in order to eliminate the effects of crosslinking by-products. This decision was a compromise: it is clear the crosslinking by-products will not completely leave the cable during ageing and thus could contribute to degradation processes. on the other hand it was necessary to ensure repeatability between laboratories and stability with time in order to avoid the risk that real degradation processes are partially hidden (for some of the test techniques mentioned above) by by-product concentration variation. The thermal treatment was carried out at a temperature of 50 "C,which is well below any significant crystallite melting as revealed by WSC measurements. Measurements of the concentration of crosslinking by-products by FTIR and HPLC (High Precision Liquid Chromatography) revealed that they were below the detection limit of these techniques after two days at 50 "C and ambient pressure, Table 3. The time for thermal pre-treatment was therefore fixed at 48 h. Measurements of space charge, charging current and electric strength clearly emphasise the importance of the thermal treatment. Without the treatment, measurements varied from specimen to specimen depending on the time elapsed between peeling preparation and measurement, even when the specimens were kept in aluminium envelopes and/or under a mild vacuum. The photoluminescence properties of XLPE materials are partly due to the crosslinking by-products; this was confirmed by the change in luminescence yield with thermal treatment. The influence o f thermal treatment on photoluminescence is described in detail in [271. The by-products are known to play a role as charge traps. T h i s was indeed observed since the thermal treatment strongly influences the charge distribution in the samples. Space charge profiles of treated and untreated peelings are shown in Figure 1. As can he seen, thermal treatment strongly reduces the amount of accumulated charge. Extension of the treatment to 96 h

I' \'

TREATED

TREATED FOR 48hr48h

UNTREATED -30 40 -50

0

m

IW

1%

200

Thickness [m?

Figure 1 . Examples of space charge profiles after poling at 90 kV/mm, 20 "C far 10,000 s. Untreated and treated peelings.

0

m

IM

im

200

0

m,dnes 1 3. Figure 2. Examples of space charge profiles recorded 2 s from the beginning of depolarisation, after poling at 90 kV/mm, 20 "C for 10,000 s. Peelings treated for 48 11 and 48+48 h.

IEEE Transactions on Dielectrics and Electrical Insularion


does not, however, produce any further change (see Figure 2). The thermal treatment was therefore a key process in stabilising the material properties.

Table 4. Atomic Force Microscauv measurements of rouehness of " the peel.faces. RMS rouehness Peel Face Tvuical neak to Maximum ueak

4 RESULTS 4.1 GENERAL OBSERVATION ON QUALITY OF PEELINGS Investigations were made of the surfaces of the peelings since these might affect measurements hut were not a property of the cable. Because of the cutting procedure, the surfaces were not as smooth as if compressionmoulded specimens had been used. Measurements using an atomic force microscope (AFM) indicated that the two surfaces of the peelings have a different surface roughness. The typical and maximum peakto-trough heights were measured and the RMS roughness estimated. In addition, ATR-FTIR (and micro-Raman) results show that the degree of crystallinity at the smoother (concave) face of the peelings is 1-2 % higher than the Crystallinity at the other face. These artefacts however are quite small compared to the overall variations of crystallinity through the insulation. .The PEA technique was used to investigate the effect of the roughness on charge injection and storage at the surface. Four combinations of specimen surface and electrode were considered, changing polarity of test voltage and specimen orientation (smooth face in contact with either top or bottom electrode). The top electrode is covered with semicon whereas the bottom electrode is a polished aluminium plate. Tests were performed at different field levels, voltage polarity, with, and without gold-coated electrodes. On the whole, only small differences between the various configurations were found at low applied fields ( < 40 kV/mm dc) but significantly larger injection activity was detected for peelings having the smooth face in contact with positive top (semicon) electrode (configuration C) at high fields. In general, the configuration with smooth face, gold coated, in contact with the top electrode poled with positive field provided larger amount of space charge; this appears to be a general result. Electroluminescence (EL) imaging was performed to examine whether surface roughness influences the spatial distribution of light emitted by the peeling, through possible effects of field enhancement along the stripes on the surfaces. Measurements were made of the light distribution over the plane of the surface, i.e. with the optical axis perpendicular to the plane of the peeling. Although the EL excited under ac conditions exhibited some localisation, there was no correlation between the emitting zones and the macroscopic roughness features of the surface. This appears to he a general characteristic of EL excited by ac on metallised polymer films; the non-homogeneous distribution of EL in the sample plane does not appear to be a characteristic of the surface roughness.

-

Smoother Mat

521

-1 -2

0.3 -1

-150

- 350

'

Based on these results, the roughness of the peeling surface appears to affect electrical properties only at medium to high fields; it is not significant at low fields, i.e. those close to the design stress. This supports the choice of peelings for insulation characterisation and subsequent ageing investigations (which were performed, however, referring to the worst configuration regarding space charge accumulation).

4.2 MICROSTRUCTURAL

CHARACTERISATION Crystallinity was estimated using both DSC and FTIRmicroRaman from peelings taken at various radial distances through the insulation from the inner semicon. Measurements were also made (not shown here) on cable cuts directly (i.e. not on a peeling) and the crystallinity of the peeled sample appears to be a few percent higher than the crystallinity of the cable cut. By using ATR-FTIR, it is possible to measure the amorphous fraction of the surface and therefore examine the difference of crystallinity between the two faces. Results show that the surface amorphous content is lightly higher (by 1-2 % ) o n the smoother side than on the mat side. The peeling process may therefore have some effect on the crystallinity of the specimens, probably due to surface heating.

In the region situated very close to the inner screen (between 0 and 0.01 mm) the micro-Raman crystallinity cuwe shows a reduction of roughly 25%. The DSC does not allow for such precise study because of the limits of manual sampling. Raman microscopy proves to be the most prac10

i l r

I

2

z :'

0

!i 6

~.

B~ 0 0

a

*

0.1

5

e

T 8 e m

B

a

a .

50

sJroso+aaca

E l m FbM [kvlmm]

Figure 3. Charge vs. poling field from PEA measurements (threshold characteristic) obtained from cable peels. The threshold field for space charge accumulation is indicated by an arrow ( z20 kV/mm).

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4.3 ELECTRICAL CHARACTERISTICS

1 01

1

1D

Im

?WO

toom

time [I]

Figure 4. Mobility calculations from PEA data on two unaged

ca-

bles (poling field 150 kV/mm).

tical way of detecting changes at such high levels of spatial resolution. At the opposite end of the radial position (i.e. close to the outer screen), the change in crystallinity is not so great with micro-Raman; only a slight decrease is perceptible. The crystallinity values obtained vary around 0.4 (+0.05), depending on the technique (for DSC : 0.42f 0.03). The DSC thermograms (not shown here) suggest the presence of two coexisting crystalline populations, the first one represented by a melting onset point between 73 and 75 “C and the main population presenting a peak maximum at about 105 “C. Measurements on un-crosslinked polyethylene received from the materials manufacturer showed a similar, although less pronounced, double population. The cable peelings contain a surface deformation layer due to cutting at room temperature, with characteristic deformation features aligned along the cutting direction. By increasing the permanganic etching time by 20 % to 3 3.5 h it was possible to remove the deformation layer to reveal the underlying morphology and voids. Lamellar micro-domains with some “V” shaped lamellae were revealed in the underlying microstructure. Micro-Raman spectroscopy reveals differences between the shiny and matt faces. By normalising the spectra to the CH, twist near 1300 cm-’, it is found that the intensities of some bands, all of A,, and/or B,, symmetry, are significantly greater on the smoother side than on the matt side. Given that the relative orientation bctween the exciting laser polarisation and the sample axis remains constant, this indicates that the smoother face is more oriented than the mat one. A study of the influence of the orientation angle of the peelings with respect to the incoming laser polarisation showed significant variations of the 1440 cm-’ CH2 bend, 1415 cm-l orthorhombic CH, bend and the 1130 cm-’ C-C stretch, all of which are sensitive to the orientation of polymer chains.

A combined approach using space charge distributions, charging - discharging currents and electroluminescence measurements was used. Figure 3 shows a typical space charge threshold characteristic, obtained plotting the values of (log) mean absolute charge density (equation (3))measured 10 s after the beginning of depolarisation as a function of poling field In this way, an estimate can be made of the “dc space charge accumulation threshold”, i.e. the level of field above which space-charge accumulates. Preliminary checks have been made to find out whether this dc charge threshold is likely to be useful in the assessment of ageing by subjecting some of the peelings themselves to thermo-electric ageing. The results of this assessment do appear to be promising showing that the D C charge threshold decreases with aging extent [16, 281. Calculations of trap-controlled apparent mobility relevant to two unaged cables, XLPE insulated, produced by the same manufacturer (one for ARTEMIS, the other hefore the beginning of the project), are shown in Figure 4. It is clear that the mobility is reproducible for the two types of cables. It is expected that this sensitive indicator will decrease with ageing as the concentration and depth of traps increases. An example of low-field low-frequency dielectric spectroscopy measurements is shown in Figure 5. From this the conductivity can be deduced, when a low enough frequency, corresponding to a long enough settling time, has been reached. Using this technique at different temperatures, it is straightfonvard to plot Arrhenius plots, such as that shown in Figure 6. This plot shows measurements from peelings taken from the two unaged cables produced by two different manufacturers in ARTEMIS. It can be seen that excellent agreement is found. A value of activation energy of about 0.88 eV is obtained. Preliminary measurements, not reported here, indicate that the Arrhenius plots for aged materials can change noticeably. Charging and discharging current measurements were carried out for up to 24 and 10 h, respectively, in order to 1.E-Og

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search for quasi steady-state conditions for charging current. Such experiments were repeated at different fields and log current density, J (A,”*), was plotted versus log electric field, E. Using this technique, a bend of the J vs E characteristic from low to high fields indicates the occurrence of space charge accumulation (according to the space charge limited current theory, SCLC [16,291) which, in general, corresponds well to the threshold for space charge accumulation detected by plots such as that of Figure 3. Previous work (e.g. [161) speculates that the threshold for space charge accumulation can constitute an interesting ageing marker, since trapping phenomena are associated with bulk degradation processes (giving rise to additional localised states, e.g. through oxidation, internal interface modifications). An example is shown in Figure 7 for an unaged specimen and one that was aged in the lahoratoly at 130 “C for 500 h with no field applied. The threshold values indicated by arrows and appear to be approximately 20 kV/mm and 10 kV/mm respectively. This compares closely to dc charge thresholds found using the space charge technique (Figure 3) and may confirm that both thresholds correspond to fields required for space charge accumulation.

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Figure 7. Log I YS. log E plots for an unaged peeling and one that had been aged in the laboratory. The threshold values are indicated by arrows.


523

Space charge measurements carried out at higher field levels show that charges are able to cross the insulation thickness through a packet-like behaviour, starting at about 60 - 70 kV/mm. EL measurements show the existence of two distinct thresholds, one related to a continuous excitation of EL under voltage, the other being transient EL detected upon specimen short-circuit. The former occurs at values of field corresponding to charge packet formation and the latter to onset of space charge accumulation. The correspondence between pertinent values of the electric field obtained through space charge and EL analyses provides a further tool to investigate the existence of degradation thresholds in insulating materials. A detailed analysis of this correlation between electroluminescence onset and space charge distribution is proposed in [30]. Because charge traps can act as recombination centres, their involvement in charge trapping can be checked in specially designed luminescence experiments. In experiments carried out within the ARTEMIS programme, charges of both polarities are generated at the surface of the material under study by using non-reactive cold plasma in helium gas (plasma-induced luminescence, PILI. The analysis of the decay kinetics and emission spectrum of the subsequent luminescence allows unambiguous determination of the time range in which charge recombination is the dominant excitation process of the luminescence [251. The emission spectra obtained within this time range provide the optical fingerprint of chromophores acting as deep traps in the material. These spectra are likely to he affected by aging and may, therefore, be quantitative “aging matters”. Figure 8 shows and example of PIL decay for peels taken from one of the ARTEMIS cables, before and after thermal treatment. A full interpretation of the PIL decay and associated emission spectra can he found in [271. Electrical breakdown results on peelings taken from the two cables gave almost identical results. In each case 20

Figure 8. Plasma-induced luminescence decay ( - 185 “C)fallowing excitation of an Artemis sample by a silent discharge. Experimental points (open circle) are fitted to an expression separating the diffcrent contrihutions of the luminescence [251. Charge recombination (dashed line) dominates for t > 30 s. P, preconditioned by thcrmal treatmcnt; NP, as receivcd.

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specimens were teated between brass electrodes in oil. Weihull statistics were used for the analysis. The two cables gave characteristic breakdown strengths of 136+2 kV/mm with shape parameters of 15-21.

4.4 PHYSICAL CHARACTERISTICS Conventionally, optical microscopy is used in searching for micro-voids. In this project, voids were optically observed from peelings stained with methylene blue in a solution containing ethanol and a surfactant. The occurrence of these observations, however, was quite rare. The study of the peels was thus complemented with cryogenic ultramicrotomy of cable material with or without subsequent etching. TEM observations of unaged XLPE show that void sizes vary from about 1 p m (microvoids) down to nearly the thickness of individual lamella (nanovoids) and that a majority of observations lie in hetween (mesovoids) (see Figure 9 - left image). The ohserved voids generally have a spherical shape, i.e. one that would he inconsistent with particles. A statistical study of AFM observations of voids from 500 nm thick ultra-cyromicrotomed sections of an unagcd cable enabled an estimate of the concentration of 0.5 to 1.0 p n sized voids of approximately 1.6x This corresponds to a volume fraction of 0.004% for this type of void. This low value does not account for the total void fraction. However, the smallest voids are more numerous and are likely to contribute more to the total void number than to the total void volume fraction. The largest voids being less numerous, is consistent with an aggregation mechanism. Occasionally the coalescence of microvoids has been observed, as depicted in the right image of Figure 9. Figure 10 shows the apparent volumetric concentration of voids, for peelings taken from unaged cables. Since this technique measures voids that are between the nanoscopic sizes that can be accessed by SAXS and the microscopic sizes measured by optical techniques, these voids are called "mesoscopic". In this example there appears to be larger voids 4 mm away from the inner semicon than next to it. This may he the result of different thermal condi-

Figure 10. Apparent void concentration distribution fof unaged cablcs. r = 0, next to inncr semicon; I = 4, 4 mm into insulation from inner semican.

tions during manufacture or it may be related to a radial concentration of extractables (mainly di-cumyl alcohol and acetophenone). If the latter is the case, then voids may he partially or totally filled with these chemicals. After aging we would expect there to be spatial differences since the electric field and temperature are higher next t@the inner semicon then elsewhere in the insulation. This spatial resolution will therefore he useful. Using SAXS it is found that near the outer part of the cables the scatterers have characteristic lengths of 21.2 0.8 nm. For samples from the inner part of the cable, the characteristic length is found to he 22.7 1.4 nm. It is likely that this scattering is dominated by crystalline domains which yield a coherent long-period spacing, the observed sizes are consistent with other long-period spacings measured in low density polyethylenes. If this is the case there is only a small difference between the two locations. Nano and meso-voids scattering cannot be ruled out but no attempt has been made at this point to remove the crystal phase scattering to determine what the void contribution might he.

4.5 CABLE STABILITY The concentration of chemicals has also been investigated as a function of radial position mainly by spectroscopic (FTIR or micro-Raman) mapping. FTIR measurements made in transmission mode on peelings can provide absolute Concentration using the Lambert - Beer law calibrated using a series of solutions of pure compounds. An example of a plot of radial mapping from FTIR measurements made on aged and unaged peeling is shown in Figure 11 for cumyl alcohol. Data was calculated from the integrated absorbance of the 859 cm-' band with a k of 1 . 0 7 lo6 ~ cm*/g for this hand. This plot shows that cumyl alcohol is more concentrated in the centre of the insulation away from the semicon screens; this is likely to he due to the diffusion promss of this volatile compound during the high temperature crosslinking process. It also correlates with radial variation in the void volume fraction as mentioned above.


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Figure 11. Radial mapping of cumyl alcohol concentration by mIR and plots of the integrated volume fraction of cumyl alcohol and acetophenone (CA and Ac).

Similar profiles were obtained for acetophenone although its concentration profile is generally slightly flatter; this is because it has a higher diffusion coefficient. The total volume fraction of both cumyl alcohol and acetophenone was estimated by integration of their concentration plots (Figure 11) and knowledge of their density. A volume fraction of around 1% to 1.6% for a number of samples is estimated which is consistent with the initial dicumyl peroxide (DCP) content introduced by the manufacturer ( 2% by mass). These volume fractions are large compared to the estimates of void volume fraction, indicating that only a small fraction of these chemicals could fill the voids. If chemical products are associated with the voids, this would arise from phase separation of these products in the XLPE polymer melt before crystallisation. It was noticed by l T I R that short chains of ethylenebutyl acrylate co-polymer may he diffusing from the semicon screens into the XLPE to form an approximately exponentially decreasing profile of concentration from the interface. This concentration is very high (up to 60 kg/m3) near the interface hut it is possible that other species may be contributing to the observed profile. Initial comparative studies of stressed and unstressed specimens using PCR (chemometrics) suggests that the latter may prove to be a useful tool. By plotting the first two key PCs (PC4 and PC3 in Figure 12), which account for more than 90% of the variance between the property

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525

data being considered, the stressed samples were separated from unstressed (virgin) samples, which (as confirmed.by other measurements to he published) would allow evaluation of stressed condition by FTIR coupled with chemometrics, thus the possible identification of ageing markers associated with IR measurements. This stressed/unstressed separation is illustrated in Figure 12, which is a 'score' plot of principal component PC4 against principal component PC3. The stressed samples are represented as open circles and the unstressed as filled circles.

5 CONCLUSIONS n order to assess ageing in ac extruded power cables, a thorough characterisation of the cable insulation before ageing has been carried out is presented here. The experimental techniques chosen in this study have been justified in terms of current electro-thermal ageing theories. Several of them are thought to he useful in deriving "ageing markers". It is intended to use thesc markers both as diagnostic markers in their own right and also for prognosis of the health of the cable.

I

It was found necessaty to include measurements that would give physical information over a range of size scales spanning nanometric to micrometric. Electrically based measurements WCTC found to be most useful in determining space charge accumulation and charge transport mechanisms, as well as trap concentrations and distrihutions. These can now be related to the model using molecular simulations. Principal component analysis (chemometrics) has been developed for assessing FTIR spectra. A new TEM technique has been developed for measuring void distributions. Luminescent techniques are also potentially very powerful for identifying chromophores that exist in the material and for providing complementary evidence to space charge measurements, for example the obsewation of light emission as positive and negative charge fronts meet. The use of peelings was justified as the most useful specimen that could be used for the variety of experiments required. Most of these measurements should be affected by chemical-physical and microstructural modifications induced by bulk ageing. Localised degradation effects would not he detected as well (rather, techniques as partial discharge measurements would be needed). However, purpose of ARTEMIS project was to investigate and model bulk degradation processes, rather than localised phenomena associated with macrodefects. The most likely candidates for aging markers, thus, may include: Measurement of the change in interphase fraction using FTIR Raman and long period spacing by SAXS. Voltage thresholds for space charge accumulation, increased conductivity and electroluminescence (and possibly for charge packet onset). Changes in apparent mobility.

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. . . .


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Fothergill et al.: Electrical, Microstivctural, Physical and Chemical Chnracterization

Changes in the Arrhenius plot (particularly the activation energy) of conductivity using low frequency, low field dielectric spectroscopy. Changes in mesovoid size distribution using TEM on replicas. Quantitative chemical characteristics extracted from FTIR spectra. Relative band strengths in the fluorescence spectra. It will be necessaty to review this list in the light of data that will h e available from aged cable specimens, which will be the purpose of a forthcoming paper.

ACKNOWLEDGMENTS This work was carried out under the ARTEMIS program EU Contract Number BRPR-CT98-0724. The authors thank the ARTEMIS publications committee for permission to publish. JCF gratefully acknowledges the provision of study leave from the University of Leicester.

REFERENCES [I]L. A. Dissado, G. Mazranti and G. C. Montanari, "The Role of

Trapped Space Charges in the Electrical Aging of Insulating Materials", IEEE Trans. DEI, Vol. 4, pp. 496-506, 1999. (21 T. J. Lewis, P. Llewellyn, M. J. van der Sluijs, J. Freestone and R. N. Hampton, "A New Model for Electrical Ageing and Breakdown in Dielectrics", 7th DMMA, UK, pp. 220-224, 1996. [31 J. P. Crine, "A Molecular Model to Evaluate the Impact of Aging on Space Charger in Polymer Dielectrics", IEEE Trans. DEI, Vol. 4, pp. 487-495, 1997. L41 C. L. Griffiths, S. Betteridge and R. N. Hampton "Thermoelectric ageing of cable grade XLPE in dry conditions",IEEE ICSD, PP. 279-282, Vasteras. Sweden, 1998. [6] L. A. Dissado and J. C. Fothergill, Electrical Degradation and Breakdown in Polymm, ed. G.C. Stevens, Peter Peregrinus, London. 1992. L71 T. Tikuisis, P. Lam and M. Cossar, "High Pressure Oxidative Induction Time Analysis by Diffcrential Scanning Calorimetly" Application Note TA-085, Novacar Chemicals Ltd., Canada [XI G. R.Strobl and W. Hagedorn, "Raman Spectroscopic Method for Determining the Crystallinity of Polyethylene", AIP Conf., Vol. 16, pp. 1181-1193, 1978. [9] 1. B. Huang, J. W. Hong and M. W. Urban, "Attenuated Total Reflectance Fourier Transform Infra-red Studics of Crystallineamorphous Content on Polyethylene Surfaces", Polymer, Vo1.33, p. 5173, 1992. [lo] G. Zerbi, G. Gallino, N. Del Fanti and L. Baini, "Structural Depth Profiling in Polyethylene Films by Multiple Inlernal Reflection Infra-red Spectroscopy" Polymer, Vol. 30, n. 12, pp. 2324-2327, December 1989. L111 L. Markey, G.C. Stevens, L.A Dissado and G.C. Montanari, "Multi-stress Electrical and Thermal Ageing of HV Extruded Polymeric Cables: Mechanisms and Methods" IEE Conf. Puh., No. 473, pp. 413-418, 2000 see also Multivariate Calibration by H. Martens and Tormod Naes, John Wiley, 1989. [I21 L. Markey and G. C. Srevens, "Microstructural Characaterisatian of XLPE Electrical Insulation in Power Cables: Determination of Void Size Distributions Using TEM', submitted to J. Phys. D.: Appl. Phys. (To be published). 1131 B. V. Zemb, Neutron, X-Ray and Light Scattering, p.247, Elsevier Science Publishers., 1991 L14l C. Servant, "Small-Angle X-Ray Scattering Study of Precipitation in a Cu-2 at.% CO Alloy", Acta Cryst., Vol. 849, pp. 458-463, 1993.

[I51 A. Guinier, Th&e et technique de la m d i o c ~ s t ~ l l o p p h i Ch. e, 14, Ed. Dunod, 1956. [161 G.C. Montanari, "The Electrical Degradation Threshold of Polyethylene Investigated by Space Chargc and Conduction Current Measurements", IEEE Trans DEI, Vol. 7, pp. 309-315, 2000.

[I71 J. M. Alison, "A High Field Pulsed Electro-acoustic Apparatus for Space Chargc and External Circuit Current Measurement Within Solid Insulators", I. Mrasuremenl Sci. Tech"., Vol. 9, pp. 1737-1750, 1998 [18] J. M. Alison, G. Mauanti, G. C. Montanari and F. Palmimi, "Mobility Estimation in Polymeric Insulation Through Space Charge Profiles Derived by PEA Measurements",lEEE CEIDP, Cancun, Mexico, pp. 35-39, 2002. [19l A. Motori, F. Sandrolini and G. C. Montanari, "Degradation and Electrical Behavior of Aged XLPE Cable Models", IEEE ICSD, pp. 352-358, Trondheim, Noway, 1989. [201 A.K. Janscher, Dielectric Rehuarion in Solids, Chelsea Dielectric Press, London, 1983. [21] M. Meunier, N. Quirke and D. Binesti, "The Calculation of the Electron Affinity of Atoms and Molecules", Mol. Sim., Vol. 23, pp. 109-125, 1999. [22] S . Serra, E.Tosatti, S . larlori, S . Scandolo, G. Santoro and M. Albertini, "Interchain States and the Ncgative El~ctronAffinity of Polyethylene", IEEE CEIDP, pp. 19-22, Atlanta, USA, 1998. 1231 M. Meunier, A. Aslanides and N. Quirke, "Molecular Modelling of Electron Traps in Polymer Insulators: Chemical Defects and Impurities", l. Chem. Phys., Vol. 115, pp. 2876-2881, 2001. [24l G. Teyssedre, C. Laurent, A. Aslanides, N. Quirke, L. A. Dissado, G. C. Montanari, A. Campus and L. Martinotto "Decp Trapping Centers in Crass-Linked Polyethylene Investigated by Molecular Modelling and Luminescence Techniques", IEEE Trans. DEI, Vol. 8, pp. 744-752, 2001. [251 G. Tardieu, G. Teyssedre and C. Laurent, "Role of Additives as Recombination Centres in Polyethylene Materials as Probed by Luminescence Techniques", I. Phys. D Appl. Phys., Vol. 35, pp. 40-47, 2002. I261 C. Laurcnt, "Optical Pre-breakdown Warnings in Insulating Polymers", IEEE ICSD, pp. 1-12, Vasteras, Sweden, 1998. [27l G. Teyssedre, G. Tardieu and C. Laurent, "Characterization of Cross-linked Polyethylene Materials by Luminescence Techniques", J. of Mat. Science, in press. [28l B. Garros, C. Audry, H. Schadlich, G. C. Montanari, I. Ghinello and L. Bcncivenni, "Evaluation of insulation degradation of stressed XLPE cables", Jicable, pp. 441-445, Vcrsaillcs, Francc, 1999. [29l A. Many and G. Rakay, "Thcoly of Transient Space-charge Limitcd-current in Solids in the Presence of Trapping", Phys. Rev., Val. 126. pp. 1980-1988, 1962. L30l G. Tcyssedre , C. Laurcnt, G. C. Montanari and F. Palmirri. In L. A. Dissado, and J. C. Fothergill. "Charge Distribution and Electroluminescence in XLPE under DC field", J. Phys. D: Appl. Phys., Vol. 34, pp. 2830-284.1, 2001.

John C. Fothergill (SM'99 was born in Malta in 1953. He graduated from the University of Wales, Bangor. in 1975 with a Batchelor degree in Electronics. He continued at the same institution, working with Pethig and Lewis, gaining a Master degree in Electrical Materials and Devices in 1976 and a doctorate in the Electronic Properties of Biopolym e r ~in 1979. Following this he worked as a senior research engineer leading research in electrical power cables at STL, Harlow. UK. In 1984 he moved to the University of Leicester as a lecturer. He now has a personal chair in Engineering and is currently Dean of Science.

IEEE Transactions on Dielecfrics and Elechical Insulation

Vol. 10, No. 3; June 2003

Gian Carlo Montanan (MWSM'90-FW) was born on 8/11/55. In 1979, he obtained the Master degree in Electrical Engineering from the University of Bologna. He is currently a Professor of Electrical Technology at the Department of Electrical Engineering at the University of Bologna, and teaches courses on reliability and statistia for electrical systems and innovative electrical technologies. He has worked since 1979 in the field of aging an d endurance of solid insulating materials and systcms, OS diagnostics of electrical systems and Fnnovative electrical materials (magnetics, electrets. superconductors). He has been also engaged in the fields of power quality and energy market, power elcctronia, reliability and statistics of electrical systems. He is a member o i AEI and the Institute of Physics. Since 1996 he is the Italian representative on CIGRE and President of the Italian Chapter of the IEEE DEIS. He is convener of the Statistics Committee and member of the Space Charge, Multifactor Stress and Meetings Committees of IEEE DEIS. He is Associate Editor of IEEE Transactions on Dielectrics and Electrical Insulation. He is Sounder and President of the spin-off Techlmp, established on 1999. He is author and co-author of about 380 scientific papers.

GilbertTeyssedre was born May 12th, 1966 in Rouergue, France. He received his Engineer Degree in Materials Physics in 1989 at the National Institute for Applied Science (INSA) and graduated in Solid State Physics the same year. Then he joined the Solid State Physics Lab in Toulouse and obtained a Ph.D. degree in 1993 for work on transition phenomena and electroactive properties of fluorinated ferroelectric polymers. He enand has been working since then at the Electrical Engineering Lab in Toulouse. His research activities are aimed at the development of luminescence techniques in insulating polymers with focus on chemical and physical structure, degradation phenomena, space charge and transport properties. He is currently leading a team working on Space Charge, Luminescence and Durability of Insulating Materials.

Gary Stevens was barn on 21 October 1950. He graduated in physics from London University in 1972 and obtained a Ph.D. in solid state polymer physics from the University of London in 1975. He is the Director of the Polymer Research Centre at the University of Surrey, a past he has held since founding the Centre in 1994 following a research career in the UK power industry first with the CEGB and then with National Power. He is the Chairman of the Dielectrics Group of the UK Institute of Physics and is a member of the Institule. His research interests include structure-property relationships in electrically insulating and conducting polymeric materials and composites, particularly at molecular and nanoswpic to microscopic length scales, and the development and application of methods to study these. He is also active in both atmospheric and vacuum plasma research, surface science and spectroscopic methods and in understanding dielectric breakdown.

Christian Lauren1 was born in Limoges, France. in 1953. He studied solid state physics at the National Institute for Applied Sciences in Toulouse and received his engineering degree in physics in 1976. He joined the Electrical Engineering Laboratory at the Paul Sabatier University in 1977 to study electrical treeing and partial discharge phenomena, which were the topics of his Dr. Eng. Degree (1979). He entered the CNRS (National Centre for Scientific Research) in 1981 and got his Docks Sc. Phys. in 1984. In 1985, he spent one-year as a post-doctoral fellow with the IBM Almaden Research Center, where he studied plasma-polymerized thin films. Back to Toulouse he developed an approach of electrical aging in polymeric materials based on luminescence analysis. He is now dealing with experimental and modelling activity in relation with charge transport and aging. He is currently a Research Director at CNRS and Director of the Electrical Engineering Laboratory in Toulouse.

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Leonard A Disaada (SM'96) was born in St. Helens, Lancashire, U.K. o n 29 August 1942. He was educated in Thomas Linacre Technical School, Wigan, Lanacashire, 1953-1960, gaining a State Scholarship for University Entry in 1959. He graduated from University College London with a 1st Class degree in Chemistry in 1963 and was awarded a Ph.D. in Theoretical Chemistry in 1966 and DSc. in 1990. After rotating between Australia and England twice he settled in at Chelsea College in 1977 to carry out research into dielectrics. His interest in breakdown and associated topics started with a consultancy with STL begun in 1981. Since then he has published many papen and one book. together with John Fothergill, in this area. In 1995 he moved to The University of Leicester, and was promoted to Professor in 1998. He has been a visiting Professor at The University Pierre and Marie Curie in Paris, Paul Sabatier University in Toulouse, and Nagoya University, and has given numerous invited lectures, the most recent of which was the Whitehead lecture at CElDP 2002 in Cancun. Mexico. Currently he is an Associate Editor of IEEE Transactions DEI, co-chair of the Multifactor Aging Committee of DEIS and B member of DEIS Administrative Committec.

Gerard Platbrood was born in Forchies-LaMarche on 28 April 1949. He obtained the degree of Doctor es Sciences (PhD) in 1975. Since 1975 he has developed a laboratory at Laborelec (the technical Competence Center in energy processes and energy use, Rodestraat, 125 - 8-1630 Linkebeek. Belgium). This laboratory is multipurpose (classical chemical analyses of various alloys, plastics, materials from power plants and transformer oil analysis). Since 1995 he has studied new activities for electTica1 insulating materials: medium voltage, cables, insulator, envimn mental aspects of cables and batteries, irradiated cables from nuclear power plants, and HV (150 and 70kV) aging.