The characterization of a LDPE affected by different levels of water tree degradation using absorption current measurements

2001 IEEE 7th International Conference on Solid Dielectrics, June 25-29,2001, Eindhoven, the Netherlands 508 THE CHARACTERIZATION OF A LDPE AFFECTED BY DIFFERENT LEVELS OF WATER TREE DEGRADATION USING ABSORPTION CURRENT MEASUREMENTS M. I. F. Frutd, J.C. Filippini3, A. Jimknez', J.A. P6rez-Gbmez' and A. Jadraque' I) Departamento de Fisica Aplicada I, Fac. Informatica, Univ. de Sevilla, Avda. Reina Mercedes s/n 41012, Spain. 2, Dkpartement de Gknie Physique et de Gknie des Mate'riaux, Ecole Polytechnique de Montrkal, 2900 Bod EdouardMontperit, C.P. 6079, Succ. Centre-Ville, Montrkal, QuLbec, H3C 3A7, Canada. 3' Laboratoire d'Electrostatique et Mate'riaux Diklectriques de Grenoble, CNRS-UJF, 25 Avenue des Martyrs, B.P. 166, 38042 Grenoble, France. INTRODUCTION One of the most important applications of polymer dielectrics (EPR, HDPE, XLPE, LDPE ...) is their extended use as power cable insulations. Conduction phenomena are of great interest for both users and manufacturers of cables [l]. In this work, in order to characterize the conduction process in polymer dielectrics, we will focus our interest on the absorption current through a LDPE. As our current measurements were performed on laboratory specimens, we will thoroughly describe: a) laboratory specimens, b) experimental conditions and c) results of measurements. Actually, we have experimentally considered the influence of the degree of degradation or degraded width percentage and polarization voltage on the measurements of absorption current and capacitance. From the point of view of accelerated ageing, the typical electro-chemical degradation process of service power cables known as "water treeing" was simulated at laboratory. Then we measured absorption currents for specimens with a different degree of treed degradation and finally, a correlation between the degree of ageing and the fitting parameters of current experimental data to the Curie-von Schweidler law was proposed. EXPERIMENTAL Plane-plane LDPE specimens were obtained from sheets prepared with a "Carver" heating press at LEMDCNRS. A precise method for the fabrication of those sheets was developed which allowed us to cut disks of great homogeneity whose thickness/diameter were respectively &SO0 pm (Ak10 pm) and @=35 mm. A granulated mass of LDPE (PElS from Borealis), m=l g, was inserted in each of the 12 circular holes of a brass matrix and exposed to successive selected conditions of pressure, temperature and waiting periods. In this way we could attain the necessary homogeneity and quasiuniform width in our experimental specimens for ensuring a good (i) reDeatability / (ii) remoducibility in electric measurements which are defined as: (i) quality in the repetition (iteration) of a measurement on a unique specimen; (ii) quality in the repetition of a measurement on several specimens -identical specimens in principle-. From the point of view of accelerated ageing, the typical electro-chemical degradation of cable polymeric insulations known as "water treeing" was simulated at laboratory using a plane-plane electrode configuration, a 0.1 M 'NaC1 solution and an ageing voltage of 5 kV, at 1500 Hz. Once the laboratory specimens were assembled from disks, and previously to the measurement of absorption current with a Keithley 6517, the evolution of capacitance [2] and tg6 was also controlled for the same scheduled periods of accelerated ageing. In parallel with these experiences, a destructive microscopic analysis of the water tree aged specimens was also performed, which allowed us (once the water tree kinetics for the LDPE under study was completed) to establish a continued correlation between the evolution of the above electrical variables and the actual treed thickness. Such a correlation permits the usage of the related variables as an instrument of degradation diagnosis for any specimen under test. Growth and characterization of water trees In order to obtain a multitude of water trees we produced a wide dispersion of tree inception points in one of the sides of the cutted disks of LDPE. We applied a pressure of 372 bar, during an interval of 2 min with an abrasive paper P400 (used and replaced after its application) and we repeated this process twice for each disk [3]. This abrasive-paper method, at least in LDPE, revealed as less aggressive and more reproducible than the method of sandblasting [4], especially because we could choose a grain width which produces a kind of defect deep enough for initiating water trees without involving a direct risk of dielectric breakdown when applying the ageing voltage. The laboratory specimens [2] (see Fig. 1) were placed on a metallic recipient which acted both as ground electrode and as a container of a small quantity of silicon oil. It Figure 1. Specimen configuration: (1) platinum electrode, (2) silicon cap, (3) salt solution, (4) PE cylinder, (5) LDPE disk, (6) silver paint. 0-7803-6352-3/01/$10.00 0 2001 IEEE 509 helps to avoid partial discharges in the air by fulfilling the occasional small cavities that usually appear between the LDPE disk and the metallic plane of the ground electrode. As LDPE results much more affected by partial discharges than =PE, we got several very important advantages from the usage of the oil technique.- (a) There is a drastic diminution of the risk of dielectric breakdown for short ageing periods -for thin layers of water trees-. It means not only a reduction in the number of lost test objects but the fact that the process of tree growth for the rest of specimens is much more continued and reliable, as water tree retractions attributed to periods of absence of applied electric field are mainly avoided [5]. (b) The possibility of applying a stronger ageing electric field (Eoildz10 kV/m vs KO,- oileds3 kV/mm). Consequently, deeper and thicker degraded layers are developed for much shorter ageing periods. It permits us their detection through very significant changes for the values of electrical variables in measurements { C,I(t),tgG}. Once the ageing period is finished, the active part of the test object (LDPE disk with water trees) is extracted and stained with rodhamine during 48 h at a temperature of 60°C. Afterwards, five 2OOpm-width slices are cut from each disk with a microtome and their coloured water trees can already be visualized by using an optical microscope. Water tree average length was determined for each scheduled ageing time from equation (l), Where li and xi are respectively the length and width of each individual water tree, n is the total number of trees per slice and x,, is the total length of each slice. Experimental procedures for electric measurements Absorption current technique The LEMD experimental set-up for absorption current measurement is shown in Fig.2. Current measurement and DC stabilized voltage 6 8. ll.l....l.l..ll..“ “...“ .... ”..“ ..I*.I-.............” z “j Figure 2. Experimental set-up for current measurements ((1)-(8) components described in the text). supply were both performed by using a Keithley 6517 (3 and 4 respectively). Data acquisition was done by means of a GPIB cable (2) and a IEEE-488 card inserted in a PC586 (1) with a software developed from Keithley Testpoint packet. A special goal for this kind of very low current measurements consists in minimizing noise. Electromagnetic noise could be reduced by providing a especially designed test cell (3), triaxial connections (43) between Keithley and test cell, a metal box for the protection resistance (6) and an additional continuous Cu Faraday cage which confined all the previous elements ). The physical components of the test cell are schematically drawn in Fig.3. A special attention must be paid to the design of its upper electrode (7), which must make contact at a very well defined area on every LDPE specimen. A first screwed version of this 47 t 2 3 4 Figure 3. Test cell: (1) PE cylinder, (2) guard ring, (3) LDPE sample, (4) lower electrode, (5) protective screen, (6) PTFE insulator, (7) upper electrode, (8) contact spring, (9) Cu wire. upper electrode was substituted by a more weighted one whose contact was simply done by its weight. In this way, repeatability and reproducibility were greatly improved because of the constant pressure at the contact and by avoiding the generationlbuild-up of charges by a kind of mechanical stirring on the surface of the polymer [6]. AnalyticaVgraphical processing of experimental data was performed with Kaleidagrph 3.07 and Sigmaplot 5.0 programs. Capacitance measurements An Irlab apparatus (model LDTRP-2) and a 1621 General Radio bridge successively switched to the specially designed cell mentioned above were used to get the values of capacitance, C, and dissipation factor for every scheduled ageing period. Parasitic effects Before each measurement, the salt solution is removed and the sample surface is dried out. Nevertheless, after this operation, a waiting period , bz20 min, is absolutely required for avoiding a huge variation in capacitance (and the rest of electrical variables) with the instant chosen for performing the measurement -particularly in thick treed layers-. Concerning absorption current measurements, residual currents can be measured in the absence of applied polarization voltage, Vm when the ageing period is over. They can endure for periods ranging from 10 min up to 1 h depending on the LDPE volume affected by water trees. An adequate method for minimizing these 5 10 34 residual currents was applied by short-circuiting the entire test object support -once the ageing period is finishedduring a period of about, &,=30 min. 22.3 1 4.11S2.10-10 I 1.0566 EXPERIMENTAL RESULTS 64 146 250 The growth kinetics and the capacitance of water trees 34 3.5889. lo-’’ 1 a893 56.3 1.2905.10-9 1.2491 70.7 6.1215*10-’0 1.3001 The evolution of water trees with the ageing time in a LDPE is shown both graphically and from the variations of the average water tree length -water tree kineticsin Fig.4. 60 . 4020 - 0- % (a) (b) Figure 4. Water tree evolution in LDPE (a) three photographs taken after ageing times (from top to bottom) of 4 h, 31 h and 272 h; (b) average water tree length dependence on ageing time. A quasi-linear dependence for the variations of the average capacitance of the samples with the ageing time is plotted in FigS. 19 18 17 g 16 U” 15 14 13 J 0 50 100 150 200 250 300 t,”(h) Figure 5. Electric capacitance vs ageing time. Using an equivalent plane capacitor model for the water treed samples and taking into account the variations of capacitance, C, and water tree length, 1, with the ageing time, the average relative permittivity of water trees, can be derived [2] from, where ~~d.3 is the relative permittivity of nondegraded PE and Ci is the non-degraded capacitance of the sample. A maximum value, ~~~d.1, was attained. Current measurements Evolution of Iab.(t) with deaadation Fig.6 presents the absorption current experimental data for a measuring period of 10 min, a polarization voltage Vp300 V and different periods of accelerated ageing. Series K , V&OO V 10’0 1 . . h I 4 1013 - 10 100 lo00 t(d Figure 6. Absorption current measurements for different water tree ageing periods and a polarization voltage, Vp300V (averaged values from samples of Series K): (1) Oh, (2) 15h, (3) 34 h, (4) 64 h, (5) 146 h, (6) 250 h. The experimental data were fitted to the potential Curievon Schweidler law, I=IOt-“ . Their corresponding parameters &,m} and degraded width percentage, g, were summarized in Table 1. We can obtain a first version of diagnostic method -estimation of the degree of degradation from a unique Iabs(t) measurementby simply plotting and adjusting the dependence g=g(m), which is shown in Fig.7. Table 1. Fitting parameters { Io,m} for each ageing time and corresponding degraded width. tenv(h) I g (%) I IOW I 0 0 I h.2S1.10-’2 1 80 1 1 I ....... I 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 rn Figure 7. The dependence g=g(m) as a first version of diagnostic tool for LDPE treed samples. 51 1 Evolution of Iah9(t) with polarization voltage Fig.8 shows the absorption current measurements in aged samples (Series J) as a function of the polarization voltage, V,. A similar study was performed for new samples (Series M) and their corresponding fitting parameters { 10,m) are summarized in Table 2. Series J IO" 3 10" ! I I 10 100 1000 t(s) Figure 8. Absorption current measurements and their linear curve fitting for different values of the polarization voltage, V, (averaged values from aged samples of Series J, g=63%): (1) 100 V, (2) 300 V, (3) 500 V, (4) IOOOV. Table 2. Absorption current fitting parameters { bm} new samples (Series M) and aged samples (Series J) as a function of VDc. A prevailing trend to saturation in the evolution of exponent m with polarization voltage, V,, for both aged and new samples is observed. DISCUSSION The relative permittivity of water trees in LDPE continuously increased from its value for non-degraded PE, c2=2.3, up to a maximum value qm,=4.1, for an ageing period of 272 h and an average water tree length of 373 pm (g=74.6%); it supplies a permittivity amplification factor A=1.8, a little bit higher than the value found for XLPE in [2], A=1.6 . From absorption current measurements we could determine the evolution of coefficients {Io,m} from the Curie-von Schweidler law with the ageing time. A progressive increase was obtained for both coefficients: 9897% for Io and 95% for m, when the degraded width grew from g=O% (new sample) up to g=70.7% (sample with a high level of degradation). We could establish, as V, got from 100 V up to IO00 V: (Io) 2137% increase in new samples and 202% increase in aged samples; (m) 63% increase in new samples, 25% decrease in aged samples and in both cases appear a remarkably trend to saturation with VDC. CONCLUSIONS A LDPE (PElS -base polyethylene for compounds for cable insulation with antioxidants and stabilizersfrom Borealis) with different levels of water tree degradation was characterized by means of measurements of water tree kinetics, capacitance and absorption current. The consecution of an acceptable level of repeatability/reproducibility in electrical measurements depends greatly on the optimization of the experimental procedures: from the fabrication of LDPE sheets / water tree inception points and method for accelerated ageing to the techniques for minimizing the parasitic effects. Particularly, an adequate design for an optimal coupling between the test cell and the laboratory specimen should be taken into account. Combining water tree kinetics and capacitance measurements, the maximum average value for the relative permittivity of water trees in LDPE was calculated, clm=4.1 . Absorption current measurements were fitted to the potential Curie-von Schweidler law in order to study the evolution of its parameters { bm) with ageing time and polarization voltage. Such a study is the base of two possible methods of LDPE-water tree degradation diagnosis from measurements of absorption currents. REFERENCES [l] J.M. Urtubi, G. Reolid, Iberdrola, Bilbao, Spain, J. Sarda, Asinel, Madrid, Spain, F. Valls, Cables PireIli, Barcelone, Spain, "The behaviour of dry insulation medium voltage cables in a moist environment", Jicable'95, pp. 366-369, 1995. [2] I. Radu, M. Acedo, J.C. Filippini, P. Notingher and F. Frutos, "The Effect of Water Treeing on the Electric Field Distribution of XLPE: Consequences for the Dielectric Strength", IEEE Trans. on EI, Vol. [3] F. Ciuprina, A. Ifrim, P.V. Notingher, G. 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