IEEE Transactions on Electrical Insulation. Vol. 24 No. 3, June 1080. 403. Electrical Properties of electrode/ p ol yet hylene/ elect ro de structures. T. Ditchi, C.
IEEE Transactions on Electrical Insulation
Vol. 24 No. 3, June 1080
403
Electrical Properties of electrode/ polyet hylene/ elect ro de structures T. Ditchi, C. AlquiC, J. Lewiner Ecole Supkrieure de Physique et de Chimie Industrielles, Paris, France
E. Favrie and R. Jocteur Silec, Montereau, France
ABSTRACT The pressure-wave propagation ( P W P ) method allows for the non-destructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyet hylene/electrode structures such as those involved in H V cables. In the work which is presented, we analyze in polyethylene samples, first the influence of the composition of the insulating resin itself and second that of the electrodes. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both, are observed. This application of the P W P method is of particular interest since it allows for a suitable choice of the materials and structures involved in insulator / conduc tor interfaces. Conductor (metal)
1. INTRODUCTION V power transmission cables using extruded synthetic insulation are already widely used in the world, essentially for alternative current links and, to a smaller extent, for direct current ones [l]. In both cases, a suitable choice of the materials constituting the cable structure is required in order to define equipment with increased performances.
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On Figure 1, the various parts of a cable structure are shown. The insulating material is generally made of low density polyethylene (LDPE), high density polyethylene (HDPE) or crosslinked polyethylene (XLPE). In order to provide electrical continuity between the inner metallic conductor and the insulator, a transition region is interposed. This so-called ‘semiconducting’ electrode
Insulator (polyethylene)
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Figure 1. Structure of a HV cable. is made of carbon-loaded polyethylene. Carbon concen-
0018-9367/89/0600-403$1.00
@ 1989 IEEE
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tration increases from the insulator to the metal, so that the region close to the metal is highly conductive. The shield electrode is made in a similar way on the outer surface of the insulator. Although this technology is now well controlled by cable manufacturers, problems arise when dealing with very high voltages, either for ac or dc currents. These problems are probably due to interface effects in which the impurities contained in the electrode/polyethylene structures play an important role [2]. As to the insulator, aithough extra-clean polyethylene resins are used, impurities or residues resulting from the manufacturing process, such as compressor oil or lubricant, can be found in small quantities. As to the electrodes, they usually contain large amounts of additives, as well as impurities. These impurities have a very large influence on the electrical properties of the structure, due to several phenomena. For instance, impurities contained in the insulating resin or diffusing from the electrodes into the insulator may be at the origin of the build up, under electric stress, of a space charge or dipole distribution. Charge transfer may also occur at the interfaces. The local increase of the internal electric field which results from this non-homogeneous charge distribution reduces the effective electric strength of the dielectric and may lead to disruptive phenomena. In the recent past, only empirical methods of investigation of these processes were available. The most widely used method was the measurement of the currents induced during or after the polarization process and the analysis of current us. voltage characteristics. More recently, local analyses has been used in order to identify the chemical species which may be responsible for these currents or for space charge effects [3-41. They have shown that many impurities or ions which are found in the insulator come from the semiconducting electrodes by a diffusion process, and that these impurities originate from the carbon black. However, these methods do not give any information on the impurities responsible for space charge effects, on the distribution of these charges through the insulator, or on the resulting distortion of the internal electric field. New perspectives are now opened by the pressure wave propagation ( P W P ) method [5-71 which allows for the direct measurement of the electric field distribution in the insulator. As this method is non-destructive, it makes it possible t o determine the charge distribution at different instants of the polarization process. Moreover, it can be applied to a sample under electric stress.
In this paper, after a brief description of the principle of the P W P method, we give some details about the experimental set-up used in order to measure the electric field and charge distributions in thick samples, under applied voltage. Then we present new results in which the influence of the composition of the insulating resin and of the electrodes on the electrical properties of the electrode/insulator/electrode structure, is analyzed. Only d a t a obtained in LDPE resins will be presented here.
2. THE PRESSURE WAVE PROPAGATION METH0 D E will simply recall the basic principle of this method which has already been presented elsewhere [571. short rise-time pressure wave, propagating a t the velocity of sound in a dielectric material, acts as a virtual probe sensitive to the electric field or to the charge density. It has been shown that the variation in time of the charges induced on measuring electrodes, during the transit of the wave through the sample under test, is directly related to the field and charge spatial distributions. Depending on the impedance of the load connected to these electrodes, this results in a short-circuit current or in an open circuit voltage from which the distributions can be deduced. In order to obtain a good spatial resolution, the rise-time of the pressure wave has to be short as compared to its transit time through the sample.
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Very reproducible pressure waves can be obtained by the impact of a short laser pulse on an absorbing target adjacent to the sample. The data which will be presented in the next Sections were obtained using a Nd/Yag laser emitting, at 1.06 p m wavelength, a 3 ns width pulse with an energy ranging from a few to 350 mJ. The P W P method also allows us to study the evolution of the charge distribution in a sample during the polarization process, while a HV is applied [8],or to investigate the phenomena occurring during a reversal of the polarity of the applied voltage. On Figure 2, a typical experimental set-up is shown. Such complete measuring equipment is now commercially available 191. In the experiments hereunder described, the samples are 2 mm thick LDPE plates, on which two semiconducting electrodes are hot-pressed. One of these electrodes is also used as a target for the laser beam. The samples are submitted to a 60 kV voltage a t 50°C. During this polarization process, the evolution of the charge distribution is measured, until stabilization is reached.
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Figure 2. Typical experimental set-up for the laserinduced pressure wave propagation method.
3. INFLUENCE OF THE INSULATOR ITSELF
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tained in the insulator itself, two different, extra clean, resins originating from two manufacturers are compared. In the following they are referred to as type A and type B resins. In this first step of the analysis, the electrodes have a standard composition already used for ac cables and are 1 mm thick.
On Figure 3a and 3b, the evolution in time of the charge distributions which develop during the polarization process, respectively in type A and type B samples, is presented. For type A resin, after 1 h, negative charges are observed close t o the cathode, corresponding to charge transfer a t this electrode, and a relatively low density of heterocharges is also present. Later on, the density of the charges of opposite polarity to that of the closest electrode increases, so that the injected negative charges are no longer observed, possibly because of the finite spatial resolution. After 40 h, stabilization is reached. Only charges of opposite polarity to that of the closest electrode can be seen, and their maximum density reaches a few pC/cm3. The electric field is maximum a t the interfaces and is approximately twice the applied field. Conversely, for type B resin, although heterocharges are observed close t o the electrodes a t the early stage of
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Figure 3 Evolution in time of the charge distributions in samples with 1 mm thick standard electrodes. Polarization conditions: 30 kV/mm, 50°C. (a) for type A insulating resin, (b) for type B insulating resin. the polarization, the dominant process is a charge trans-
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fer occurring at both electrodes, leading to an increase of the electric field in the bulk of the sample. In this case, after stabilization, the maximum value of the internal field is equal t o 110% of the applied field in the bulk and reduces to 70% of this field at the interfaces. The difference between these two materials is striking and rather unexpected. It implies that the insulating resins contain different types of impurities. In the next Section we will also take into account the composition of the electrodes and try to analyze the combined effect of products diffusing from the electrodes and of impurities contained in the resin itself.
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4. INFLUENCE OF THE COMPOSITION OF THE ELECTRODES HV cables contain several compounds, which may differ from one manufacturer to another, but generally include polyethylene, carbon black, plastifying agents and other additives in smaller quantities. In each of these constituents, impurities which vary with the origin of the product are always present [3]. For example 'standard' electrodes considered in Section 3 contain, among other chemical species, 33% of carbon black, less than 1% of zinc stearate, referred to as additive 1 (Ad#l) and a large amount (approximately 10%) of an aromatic plastifying agent, noted additive 2 (Ad#2).
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In order to determine the influence of the two additives on the electrical properties of the electrode/insulator/ electrode structure, specimens with electrodes without Ad#1 and others without Ad#2 were submitted to the polarization process previously described. The two insulating resins A and B were successively used in each case. Moreover, preliminary experiments have shown that the relative thicknesses of the electrodes and of the insulator, in some cases, influence the resulting charge distributions. Samples having respectively 0.2, 0.5, and 1.0 mm thick electrodes have been tested, the thickness of the insulator being unchanged and equal to 2.0 mm. The charge distributions measured after stabilization in these various sample configurations are presented on Figure 4 and 5. On Figure 4,we have summarized the results corresponding t o type A samples. Figure 4a corresponds to standard electrodes whereas Figure 4b corresponds to electrodes without zinc stearate and Figure 4c to electrodes without plastifying agent. Figure 5
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Figure 4. Comparison of the stabilized charge distributions obtained in type A samples, for various types of electrodes: (a) standard electrodes; (b) electrodes without zinc stearate; (c) electrodes without plastifying agent.
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IEEE Transactions on Electrical Insulation
Vol. 24 No. 3, June 1989
ples, for standard electrodes, and electrodes without zinc stearate. The reason why results are not shown for type B samples will be given later.
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The most striking result is the large decrease of the charge densities measured in samples without Ad#l (compare Figure 4a and 4b, and Figure 5a and 5b). This decrease was observed on all samples, whatever the insulator used. The charge densities are reduced by a factor 4 in resin A, and by a factor 5 to 10 in resin B samples with thick electrodes (1.0 and 0.5 mm). In resin B, the strong injection of negative charges at the cathode, which is observed for 1 mm thick standard electrodes, decreases when the thickness of the electrodes is reduced (curve 3 of Figure 5a). Moreover this charge transfer completely disappears when Ad#l is suppressed. In the case of type A samples (Figure 4a), although negative charge transfer at the cathode is not observed for 1 mm thick standard electrodes in the stabilized state, this phenomenon appears at the beginning of the polarization (Figure 3a). It is also observed for thinner standard electrodes. In this case, the spatial resolution is improved because the broadening of the pressure pulse during its propagation through the front electrode is reduced. As shown on Figure 4a, this allows for the observation of negative injected charges in the case of thin standard electrodes. Similarly to what was observed for type B samples, such charges no longer exist when Ad#l is suppressed (Figure 4b).
A possible interpretation is that zinc stearate diffuses in the insulator and is highly responsible for the development of space charges observed with the standard electrodes. Particularly, the injection of negative charges at the cathode can be attributed to this additive. As the interaction of this ionizable impurity with resins A and B leads in one case to hetero-charges and in the other case to homo-charges, it seems that these two resins contain also impurities which react differently with zinc stearate. In type A, hetero-charges are generally predominant, whereas in type B, injection of charges a t the anode is typical of this insulating resin.
When the plastifying agent Ad#2 is suppressed from the composition of the electrodes, the behavior of samples made with resin B is rather fluctuating, especially when dealing with thick electrodes. This effect is attributed to the controlled diffusion of Ad#l in the insulator by the plastifying agent so that, when Ad#2 is suppressed, the behavior of the interfaces is not well controlled. Similarly, with resin A, when Ad#l is present in the electrodes but Ad#2 is suppressed, the measured charge densities are intermediate between those obtained with the standard electrodes and those corresponding to electrodes without Ad#l.
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In summary, it seems that the combination of Ad#l and Ad#2 present in standard electrodes is largely responsible for the space charge distributions which develop, under electric stress, in the insulator. When zinc stearate is suppressed, the charge densities are greatly reduced, specially in resin B, and in this case the distortion of the field does not exceed 7% of the applied one, at 50’C. Ad#2 seems to facilitate the diffusion of zinc stearate in the insulator.
5 . CONCLUSION HE P W P method was used to analyze the influence of the various chemical species present in the insulator and in the electrodes used in HV cables, on their electrical properties. Modifications of the composition of these materials have shown that the diffusion of zinc stearate in the insulator plays an important role in the build up of a space charge and that this diffusion is made easier by other additives contained in the electrodes.
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This application of the P W P method provides a direct method to define materials for the electrode/insulator/ electrode structures used in HV insulation. It should help in designing highly performing equipment.
REFERENCES [l]J . Perret, R. Fournii, “IEEE 1975 Conference on Di-
electric Materials Measurements and Applications” , Cambridge 1975.
[2] A. W. Pattullo, D. K. Das-Gupta and D. E. Cooper, “Charge injection and storage in cross-linked polyethylene insulated cable”, Conference on Electr. Insul. and Diel. Phenomena, p. 160, 1988 Annual Report.
[3] J . P. Crine, S. Pilissou, H. St-Onge, “Elemental and ionic impurities in cable insulation and shields”, edited by Soc. des Electriciens et des Electroniciens, Conference JICABLE, Versailles, Paris France, pp. 206-213, Sept. 1987. [4] J. P. Crine, S. Pilissou, Y. McNicoll and H. St-Onge, “Evaluation of analytical techniques for the characterization of dielectric materials”, Second International Conference on Properties and Applications of Dielectric Materials, Beijing, China, pp. 9-12, 1988. [5] P. Laurenceau, G . Dreyfus, J . Lewiner, “New principle for the determination of potential distributions in dielectrics”, Phys Rev. Lett. Vol. 38, pp. 46-49, 1977.
[6] C. AlquiC, G. Dreyfus, J. Lewiner, “ Stress-wave probing of electric field distributions in dielectrics” , Phys. Rev. Lett. Vol. 47, pp. 1483-1487, 1981. [7] J . Lewiner, “Evolution of experimental techniques for the study of the electrical properties of insulating materials”, IEEE Trans. Elect. Ins. 21, pp. 351-360, 1986. [8] F. Chapeau, T. Ditchi, C. Alquii, J. Lewiner, J . Perret, B. Dalle, ‘‘ Comparative study of the behavior of two polyethylene types under dc voltage, by the pressure wave method” , Conference JICABLE 1987, Versailles, Paris France, edited by S.E.E., pp. 91-97, Sept. 87. [9] HLP Technologies, Paris, France. This paper is based on a presentation given at the 6th International Conference on Electrets, Ozford, England, 1-3 September 1988. Manuscript was received on 2 Mar 1989
