The intent of this paper is to cross-correlate the information obtained by space charge profile analysis and electroluminescence (EL) detection in cross-linked polyethylene samples submitted to dc fields, with the objective to make a link between space charge phenomena and energy release as revealed by detection of visible photons. Space charge measurements carried out at different electrical fields by the pulsed electro-acoustic method show the presence of a low-field threshold, close to 15 to 20 kV/mm, above which space charge begins to accumulate considerably in the insulation. Charges are seen to cross the insulation thickness through a packet-like behavior at higher fields, starting at about 60 to 70 kV/mm. EL measurements show the existence of two distinct thresholds, one related to the permanent excitation of EL under voltage, the other being transient EL detected upon specimen short-circuit. The former occurs at values of field corresponding to space charge packet formation, and the latter to the onset of space charge accumulation. The two techniques give therefore consistent information on space charge phenomena and associated energy release in the optical EM spectrum

Auge, J. L.

Dissado, L. A.

Fothergill, J.

Laurent, C.

Mazzanti, F.

Montanari, G. C.

Seeley, A.

Teyssedre, G.

City Research Online

Laurent, C., Teyssedre, G., Auge, J. L., Mazzanti, F., Montanari, G. C., Dissado, L. A., Fothergill, J. & Seeley, A. (2000). Space charge and associated electroluminescence processes in XLPE cable peelings. Paper presented at the 69th Annual Conference on Electrical Insulation and Dielectric Phenomena, 15th - 18th October 2000, Victoria, Canada. City Research OnlineOriginal citation: Laurent, C., Teyssedre, G., Auge, J. L., Mazzanti, F., Montanari, G. C., Dissado, L. A., Fothergill, J. & Seeley, A. (2000). Space charge and associated electroluminescence processes in XLPE cable peelings. Paper presented at the 69th Annual Conference on Electrical Insulation and Dielectric Phenomena, 15th - 18th October 2000, Victoria, Canada. Permanent City Research Online URL: http://openaccess.city.ac.uk/3165/ Copyright & reuseCity University London has developed City Research Online so that its users may access the research outputs of City University London's staff. Copyright © and Moral Rights for this paper are retained by the individual author(s) and/ or other copyright holders.  All material in City Research Online is checked for eligibility for copyright before being made available in the live archive. URLs from City Research Online may be freely distributed and linked to from other web pages. Versions of researchThe version in City Research Online may differ from the final published version. Users are advised to check the Permanent City Research Online URL above for the status of the paper.EnquiriesIf you have any enquiries about any aspect of City Research Online, or if you wish to make contact with the author(s) of this paper, please email the team at publications@city.ac.uk.  Space charge and associated electroluminescence processes in XLPE cable peelings C. Laurent1, G. Teyssedre1, J. L. Augé1, G. Mazzanti2, G.C. Montanari2,  L.A. Dissado3, J.C. Fothergill3, A. See3 1=LGET, Univ. Paul Sabatier, France, 2=DIE, Univ. di Bologna, Italy, 3= Univ. of Leicester, UK   Abstract The intent of this paper is to correlate the information obtained by space charge profile analysis and electro-luminescence detection in cross-linked polyethylene samples under DC\fields, with the objective to make a link between space charge phenomenon and energy re-lease as revealed by detection of visible photons. Space charge measurements carried out at different electrical fields by the pulsed electro-acoustic method. show the presence of a low-field threshold, close to 15-20 kV/mm, above which space charges begins to accu-mulate considerably in the insulation. Charges are seen to cross the insulation thickness through a packet-like behavior at higher fields, starting at about 60-70 kV/mm. Electroluminescence measurements show the existence of two distinct thresholds, one related to the excitation of electroluminescence under voltage, the other upon specimen short-circuit. The former occurs at values of fields corresponding to space charge packet formation, and the latter to space charge accumulation. The two techniques give therefore consistent informa-tion on space charge phenomenology and associated energy release in the optical EM spectrum.  Introduction Measurements of electrical properties useful for insula-tion characterization and for aging diagnosis were car-ried out by different research groups in the framework of the European project “ARTEMIS” (Aging and Relia-bility Testing and Monitoring of Power Cables : Diag-nosis for Insulation Systems), with the aim to investi-gate the degradation mechanisms and to evaluate relia-bility of high voltage cross-linked polyethylene (XLPE) cables. Among these properties, space charge, charging-discharging current, polarization loss and electrolumi-nescence (EL) measurements are associated with the mechanism of charge injection and transport. There is a general agreement among researchers that electrical aging of synthetic insulation used in high vol-tage cables could be started by space charge accumula-tion. However, there is no clear picture of the nature of the degradation reaction that would be involved at a mi-croscopic level. Electroluminescence emission enables a link to be made between space charge phenomenology and the release of energy due, for example, to radiative charge recombination. It is therefore of importance to be able to define levels of fields and associated space charge profiles that are liable to give rise to lumines-cence excitation since EL is the evidence of the exis-tence of highly reactive excited states in the insulation that could promote accelerated aging [1]. This paper presents and discusses the results obtained from space charge and EL measurements, focusing on the limits of electrical field (thresholds) which promote space charge formation and luminescence excitation. Experimental procedures Measurements were performed on peels cut from un-aged cross-linked polyethylene (XLPE) cables produced specifically for the project [2], width 80 mm and mean thickness 150 m. Before testing, specimens were pre-treated in an oven for 48 h at 50°C in order to expel cross-linking by-products. Space charge observations were made by the pulsed electroacoustic technique (PEA) at various constant DC voltage levels from 1 - 150 kV/mm. The poling scheme consisted in a poling time (104 s) and a depolarization (volt-off) time when the specimen is short-circuited (lasting 2000 s). Electrodes were made of semicon (high voltage, anode) and aluminium (cathode). Tests with gold electrodes prepared by cold sputtering were also performed [3, 4]. Electroluminescence was detected through a photomul-tiplier (PM) working in photon counting mode, using specimens metallized on both surfaces with semi-transparent layers of gold deposited by cold sputtering. EL measurements were carried out during polarization and depolarization at various DC poling fields. The pro-cedure adopted for measuring EL under DC consisted in increasing the voltage in constant steps up to a maxi-mum voltage, and then decreasing the voltage with the same step. EL is recorded at constant voltage both in the increasing and decreasing cycle. The duration of each step is relatively short (300 s upon increase, 150 s upon decrease), the steady state value of the current is there-fore not reached. As no evidence of EL transient was detected, the values have been averaged over the whole time range at each step. A sampling time of one second was adopted for these measurements. Two voltage cycles were applied consecutively on the same sample:  0/120/0 kV/mm, with steps of 4.0 kV/mm  0/150/0 kV/mm, with steps of 5.3 kV/mm. The procedure adopted to measure EL during depolari-zation was slightly different. We have already reported that under short-circuit conditions, the EL is a transient process occurring immediately after grounding the sam-ple [4]. The analysis of the EL decay necessitates a shorter sampling time which has been chosen at 10 ms. Voltage was applied to the specimen by using a high voltage switch which can be used to ground the sample   after the polarization time. The length of the polariza-tion time was 3600 , which allows sufficient space charge accumulation in the sample. The length of the depolarization time is also 3600 . The voltage is further increased and the same experiment is repeated. Mea-surements were carried out starting from 10 kV/mm up to 90 kV/mm, with steps of 5 kV/mm.  Experimental results and discussion Fig. 1 shows the EL versus DC field characteristics ob-tained for the two voltage cycles. The EL during the de-creasing part of the voltage cycle (Figure 1a) is slightly lower at the same field than that during the increasing part which means that charge accumulation is the con-trolling factor in EL excitation. Experiments performed on different samples have shown a good reproducibility of the EL versus field characteristics. A threshold for EL detection (EPP) can be defined at field 60-70 kV/mm as seen in Figure 1b. The results obtained from depolarization measurements are summarized in Figure 2. The integrated EL counts recorded after electrode short-circuit and grounding are reported as a function of the poling field in Figure 2a. A threshold below which the EL emission is lower than the measuring system sensitivity and above which the EL signal is steeply increasing in magnitude is clearly detectable close to 20 kV/mm. The time dependence of the luminescence after short-circuit is also shown in Figure 2b at a field level of 80 kV/mm.  It is noteworthy that the threshold value detected by means of EL observations under short-circuit is signifi-cantly lower than the previously inferred permanent EL threshold estimates that were carried out under field. Indeed the above-described experimental procedure al-lows, through the EL measurements carried out during depolarization, the detection of signals coming from charge recombination forced by the short-circuit condi-tion, that occur at fields much lower than those required for permanent EL detection. In other words, this proce-dure is able to provide early and sensitive indications of the presence of space charges, and thus of the electrical field above which space charge is accumulated in the material under test.  Turning now to the excitation mechanism of the EL, it is noteworthy that in the last stage of the process, light would always be generated upon annihilation of a pair of positive and negative charges, either trapped at dif-ferent sites, or bounded to the same center. As always in the EL process, the quest is to find the mechanism of generation of the pair. When both kinds of carriers are involved in charge transport, there is a strong probabili-ty that the light comes from recombination of the two kinds of charged particles if they are able to interact, i.e. one has to observe positively and negatively charged regions with frontiers in between. If only one kind of carrier is contributing to the transport mechanism, the EL should be excited when these carriers reach a kinetic energy able to generate the pair by impact excitation or impact-ionization (hot carrier effects). Correlating the space charge profiles with the two different EL regimes is of interest and is reported here. Consideration of the emission spectra of the EL can also give a first indication. Although it has not been possible to record the EL emission spectrum under DC stress, we have analyzed it under AC (previous studies have shown that for a given material, EL spectra have similar spectral features under DC and AC [5]). The AC spec-trum is shown in Figure 3, where it is compared with the spectrum of the light emitted by the same XLPE materi-al during the recombination of positive and negative charges, in the absence of an applied field (i.e. the exci-tation mechanism cannot be due to hot carriers)[6]. Even if all the spectral features of the EL spectrum can-not be fully interpreted on the basis of the so-called re-combination-spectrum, it is clear that the latter is a component of the former. Other indications that cannot be discussed in this paper point towards an EL con-trolled by the recombination of charges in the presence of two types of carriers (i.e. without the mediation of hot electrons). Further indications come from the space charge distribution. The PEA measurements show that below fields of the order of 15-20 kV/mm, the space charge magnitude is at the sensitivity limit of the system, close to 0.1 C/m3. Above 20 kV/mm charge begins to accumulate consi-derably as function of applied field. Qualitative exam-ples of this behavior are given by the space charge pro-files reported in Figure 4. They were obtained after 104 s of poling at 15 and 35 kV/mm, respectively. As can be seen, while no charge can be detected at 15 kV/mm, a small but appreciable amount of heterocharge appears at 35 kV/mm.  A quantitative evaluation of the field value above which charge accumulates can be obtained resorting to the cal-culation of QM, called the total stored charge, and de-fined as the integral of the absolute value of the charge profile, at a given depolarization time, e.g. 10 s, exclud-ing the image charges on the electrodes [3]. Once the obtained value of QM is plotted as function of poling (Laplacian) field, E, the plot of Figure 5 is obtained. From this plot (hereafter called the threshold characte-ristic), the threshold above which space charges begins to accumulate in insulation can be easily determined, i.e. above 20 kV/mm from Figure 5. It is noteworthy that this value is close to the one obtained for EL mea-surements (Figure 2) and that it does not change signifi-cantly for gold-coated or uncoated specimens. The good correspondence between the threshold esti-mates obtained by space charge and EL techniques shows that each technique provides consistent informa-tion regarding the space-charge formation phenomenon, thus space-charge accumulation observed by PEA and electroluminescence produced by trapped charges dur-ing depolarization constitute different aspects of the same process.    In order to investigate the recombination phenomena-giving rise to the permanent EL threshold, EPP, the 3D space-charge patterns providing space charge evolution with time should be considered. Figures 6 and 7 show space charge patterns obtained during polarization and depolarization at 60 and 90 kV/mm. The intensity of the charge is associated with the darkest tone of gray (with the exception of the electrodes, where the maximum in-tensity, saturating the tones to the white color, is achieved). As can be seen, at 60 kV/mm and, more clearly, at 90 kV/mm, positive and negative charge packets tend to leave electrodes and cross insulation. When the fronts of the packets meet each other, charges recombine and only one packet prevails, reaching the opposite electrode. This interpretation is not straightforward on the basis of space charge observa-tion, since PEA (as the other techniques for space charge measurements) provides information on the net charge, independently of real charge recombination. However, if the EL observations are considered, it comes out that above 60-70 kV/mm charge recombina-tion becomes significant, constituting the threshold for permanent EL.  Figure 8 shows the space charge profiles during poling, in steady conditions, at 60 and 90 kV/mm. Comparing with Figure 4, it can be seen that while at 30 kV/mm heterocharges are located only close to the electrodes, at 60 kV/mm and, more, at 90 kV/mm charges begin to penetrate into specimen thickness, so that charges of opposite sign are faced in the insulation bulk. This would support the appearance of recombination pheno-mena. Conclusions Both space charge and electroluminescence (EL) mea-surements can provide significant information about the processes of space charge formation and transport. A threshold for space charge formation can be detected through the threshold characteristics obtained from space charge magnitude vs. field and EL intensity vs. field measurements, once the latter experiments are car-ried out in the way indicated in this paper, i.e. measur-ing EL during depolarization. At higher fields, space charges occupy insulation bulk and would tend to cross insulation thickness through charge packets. The infor-mation coming from space charge observations is, again, in agreement with that given by EL measure-ments, which provide a threshold for permanent electro-luminescence at the fields at which space charges tend to cross insulation. Electroluminescence spectra, more-over, complement space-charge measurement informa-tion indicating that the occurrence of packet charges is associated with recombination phenomena. Acknowledgements This work was carried out under the ARTEMIS pro-gram EU Contract Number BRPR-CT98-0724. The au-thors thank the ARTEMIS publications committee for permission to publish. References [1] C. Laurent, F. Massines, and C. Mayoux, "Optical emission due to space charge effects in electrically stressed polymers", IEEE Trans. on DEI, Vol. 4, N°6, pp. 585-603, October 1997.  [2] L. Dissado et al., “Characterizing high-voltage XLPE cables through electrical, chemical, and microstructural measurements per-formed on cable peelings: the effect of surface roughness, thermal treatment and peeling location”, this conference. [3] G.C. Montanari, G. Mazzanti, D. Fabiani, M. Albertini, G. Perego, “Investigation of dc threshold of polyethylenes as a tool for insulation characterization”, IEEE CEIDP, pp. 559-563, Austin, USA, October 1999. [4] J.L. Augé, D. Fabiani, C. Laurent, and G.C. Montanari, “Investi-gating DC thresholds in polyethylene by space-charge, current and electroluminescence measuerements”, to be published in IEEE DEI. [5] C. Laurent, "Optical prebreakdown warning in insulating poly-mers", IEEE ICSD, pp. 1-12, Vasteras, Sweden, June 1998. [6] G. Teyssedre, L. Cissé, C. Laurent, F. Massines, and P. Tiemblo, "Spectral analysis of optical emission due to isothermal charge re-combination in polyolefins", IEEE Trans. on DEI, Vol. 5, N°4, pp. 527-535, July 1998. 0 40 80 1200510Field (kV/mm)EL (cps)80 40 0Noise Level(1)0 50 100 15001020100 50 0(2) 10 100110Field (kV/mm)EL (cps)Noise Levelslope 3.5 Fig. 1(b): EL versus DC field characteristics obtained during voltage increases for the two cycles    0 20 40 60 80 1000102103104105Estimated thresholdField (kV/mm)Integrated EL after short circuit (counts) Figure 2(a): :EL at the short-circuit after one hour of DC polarization. Integrated electroluminescence versus field level. (Data corrected for the PM background noise)  0 10 20 30 40 5010-1100101102103104105Short-circuit timeTime (x 10ms)Electroluminescence (counts/10 ms) Figure 2(b): :EL at the short-circuit after one hour of DC polarization. Time dependence of the EL at the short circuit for a poling field of 80 kV/mm. (Data cor-rected for the PM background noise   Figure 3: EL spectrum under AC compared to the lumi-nescence spectrum due to charge recombination (RIL)  Thickness [m-6]-50 0 50 100 150 200 250Charge [C/m3 ]-40-30-20-100102030POLING FIELD 15kV/mmPOLING FIELD 35kV/mm Figure 4: Space charge profiles after 10000 s of polari-zation. Poling fields 15 and 35 kV/mm    Electric Field [kV/mm]56789 20 30 405060708090  10   100Q M [C/m3 ]0.1110 Figure 5: Threshold caracteristic (total stored charge versus poling field) obtained from space charge mea-surements. The estimated threshold is indicated by a ar-row.  CATHODE  ANODEPOLARIZATION DEPOLARIZATIONSPECIMEN Figure. 6: Space charge 3D patterns obtained during po-larization and depolarization at 60 kV/mm. The intensi-ty of the charge is associated with the darkest tone of gray (the white color, however, is used for the elec-trodes, where the charge intensity is maximum).  CATHODEANODEPOLARIZATION DEPOLARIZATIONSPECIMEN Figure 7: Space charge 3D patterns obtained during po-larization and depolarization at 90 kV/mm.  Thicknes [m-6]-50 0 50 10 150 20 250Charge [C/m3 ]-10-80-60-40-2002040608010POLING FIELD 90kV/mPLI FIEL 60kV/ Figure 8: Space charge profiles after 1000 s of poling at 60 and 90 kV/mm.  

Space charge and associated electroluminescence processes in XLPE cable peelings

Space charge and associated electroluminescence processes in XLPE cable peelings

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