MXPA98005747A - Control of electrical effort - Google Patents

Abstract

A composition for use as an electrical stress control layer (12) for use with cable joints and terminations has a non-linear V-1 characteristic. The composition comprises a polymeric matrix filled with varistor powder of contaminated zinc oxide. The particle filling is screened at 125 microns, calcined at 1100 ° C and ground slightly to maintain the spherical shape of the particles. More than 50% by weight of the particles has a maximum dimension between 5 and 100 micr

Description

ELECTRICAL EFFORT CONTROL * This invention relates to the control of electrical stress, and in particular to a composition of matter for effecting the control of electrical stress, and its application to the joining or termination of electric power cables, for example. Electrical equipment, which includes electric cables, operating at medium voltage, say 10 kV and above, can be subjected to electrical stresses that may not be sufficiently controlled by a material that is essentially electrical insulator only. Stress control matepal is known particularly for such applications. Such material can be classified as "linear" and "non-linear". A linear stress control material follows Ohm's law: I = kV, where I = current V = voltage, and K is a constant A non-linear material obeys a generalized form of this equation: I = kV? where ? is a constant greater than 1, whose value depends on the material under consideration. jtt; EP-B-0121986 (Raychem), for example, describes the termination of a high-voltage cable in which the layer of the linear stress control material is used in combination with a non-linear stress control matepal layer. The material for each layer can be selected from a variety of materials comprising filled polymers. A document published in the Proceedings of the 8th CIMTEC Ceramic Congress and Forum on Materials Symposium, June 29 to July 4, 1994, by Strümpler, Kluge-Weiss and Greuter of ABB, entitled Smart Varistor Composites, describes a material comprising contaminated ZnO varistor powder as a filler in a polymeric matrix. The proposed applications of the material are exemplified as the suppression of overvoltages or field gradient in insulating sleeves and cable terminations. The filling is prepared by concreting the powder under conditions ranging from 935 ° C to 1320 ° C and from 4 hours to 20 hours respectively. The shape of the filler particles is almost spherical. It is indicated that the agglomerates can have diameters from about 3 microns to about 300 microns and that, by cutting before or after concretion, it can select a fraction of special size. The prepared sample is cut to a particle size of < 200 microns. The resulting material has non-linear current / voltage characteristics. FR-A-2547451 (Electricité de France) describes a non-linear resistance material for use in the distribution of voltage in a cable transmission. The material comprises a B bond or agent, which may be a polymeric material, and a ceramic powder based on contaminated zinc oxide. In this publication, the mass percentage of the powder (zinc oxide) and its particle size are shown to be two essential characteristics of matepal, with zinc oxide 5 forming at least 50% of the total mass and , at least 50% of the dust grains having a diameter of more than 100 μm. The material is shown to be non-linear, with the field resistance varying uniformly with the current density. The ceramic powder is obtained by crushing a pellet of the material that has been concreted at high temperature (800-1500 ° C). That is, the initial particles of zinc oxide are compressed at high temperature into a coherent solid body, or pellet, of the type used as a varistor in high-voltage illumination protectors, which is crushed by means of an agate grinder. planetary marble to form dust. Bliss Crushing results in particles of irregular and usually distorted shape. The lack of linearity of the resulting powder is shown to improve as its particle size increases, at a constant percentage (85%) of the powder in the total composition. A particle size between 140 μm and 200 μm is preferred up to a size of 100 μm to 140 μm μm. Samples that have a particle size of less than 100 μm are shown to be significantly non-linear. US-A-4 297 250 (Westinghouse) describes a method for making a ZnO powder composition that exhibits non-linear V-l characteristics. A mass of agglomerated particles is pressed to provide a cohesive pressured raw body which is then heated between 1050 ° C and 1400 ° C for a time to make the particles together. The concreted body is then ground to provide finely divided powder particle fragments. The fragments are cut and heated between 500 ° C and 1050 ° C and subsequently separated to provide a finely divided powder exhibiting non-linear V-l characteristics. It will be appreciated that the pressure of the subsequent concreting and grinding steps will significantly change the shape of the particles from their initial regular shape to an irregular shape, as shown in Figures 1 and 2 of the patent and the subsequent heating step covers only the irregular shapes. It is an object of the present invention to provide an improved electrical stress control composition that is based on zinc oxide and exhibits a non-linear electrical behavior. According to one aspect of the present invention, there is provided an electrical stress control composition, comprising: (a) a polymer matrix, and (b) a particle filler comprising varistor powder of contaminated zinc oxide; wherein (i) the filler particles are calcined at a temperature of 800 ° C and 1400 ° C, and subsequently separated so that substantially all the particles retain their substantially spherical, original shape. (ii) at least 85% by weight of the filler comprises zinc oxide, (iii) more than 50% by weight of the filler particles have a maximum dimension of between 40 and 100 microns, so that the composition exhibits electrical behavior non-linear so its specific impedance decreases by a factor of 10 when the electric field is increased by less than 5 kV / cm in a region within an electric scale of 5 kV / cm to 50 kV / cm, and (iv) the filling comprises between 5% and 60% of the volume of the total composition. The teaching of the prior art with respect to non-linear zinc oxide-filled polymeric material is that a powder obtained from a concreted body and having a particle size of more than 100 micrometers (EdF) and less than 200 microns (ABB ) is preferred for stress control purposes. The present invention, in contrast, requires a smaller particle size, and executes that grinding of a compressed block to provide the powder resulting in particles of an irregular shape, which can be disadvantageous. To this last respect, it is observed that the calcination of the present invention is heating at high temperature under gravity without any compression. The resultant stable body needs little force to separate, for example by light grinding and this minimizes the percentage of particles, say 5%, which is caused to have an irregular shape instead of the shape, generally spherical, uniform of the particles, which is the common form of contaminated zinc oxide varistor powder as originally supplied by the manufacturers.

Preferably all the particles of the filler have a maximum dimension of less than 100 microns, preferably less than 125 microns. Preferably, not more than 15% by weight of the filled particles have a maximum dimension of less than 15 microns. The particle size is especially relevant when the composition is formed into a sheet or tubular sleeve for application as a stress control layer in electrical applications such as electrical appliances, cable joints, cable terminations, etc. Such a layer would typically be of the order of 1 mm in thickness, and if the particles have a dimension comparable to the thickness, the surface may be rough and air spaces may be formed, especially with irregularly shaped particles. In these circumstances, partial electrical discharges can occur to a degree unacceptable A powder material sieved to a particle size of < 200 microns, ie 0.2 mm as proposed by the ABB document, would significantly increase the risk of undesirable electrical discharges compared to a material sieved to a particle size of < 125 microns, that is 0.125 mm. 20 It is also noted that the ABB document does not provide teaching on the relevance of the size and / or size distribution for the electrical behavior of the composition. In accordance with the present invention, it has been found that, in addition to the requirement that all particles have a size of less than 125 microns, the size of The average particle of the ignited powder as measured for a Gauss distribution (either a real Gaussian distribution, or a transformed Gaussian distribution), should be between 20 and 60 microns, preferably between 25 and 50 microns and, more preferably between 35 and 45 microns. It is further preferred that less than 15% and, preferably less than 5% by weight, of the particles should have a maximum dimension of less than 25 microns. If this condition is not met, then it has been found that the resistivity of the final composition is very high in the high electric field voltage, with the effect that the point of * 10 switching (for a lower specific impedance) takes place at too high a value to be of practical use for voltage control applications typical of the composition of the present invention. The calcination of the filling takes place between 800 ° C and 1400 ° C and, preferably between 850 ° C and 1250 ° C, being the nominal temperature more preferred that of 100 ° C. It will be appreciated that the optimum temperature of the calcination process will depend on the particular polluters • present in zinc oxide dust. A minimum temperature is necessary to ensure that all polluters are active, that is to say that the fusion and diffusion of polluters takes place in a that the resulting powder exhibits an acute change in its impedance. However, too high a temperature can result in the adverse chemical decomposition of the stress control material. The powder needs to be kept at a calcination temperature for a sufficient time to ensure the uniform electrical properties through all the particles.

It is considered that the calcination process results in individual particles that effectively exhibit a "varistor effect". That is to say that the material in particle not only is not linear with respect to the variation of its electric impedance characteristic c.a. (the relationship between the ac voltage applied to the material and the resulting current flowing through it), but also exhibits a switching behavior, in which the graph of the voltage versus current shows an abrupt transition, which is quantified by the ratio that the specific impedance of the material decreases by at least one factor of 10 when the electric field is increased by less than 5 kV / cm (in some region within an electric field scale of 5kV / cm to 50 kV / cm, and preferably between 10 kV / cm and 25 kV / cm, being a typical operation scale of the material when used in the joining or termination of a power cable). Preferably, the transition is such that the specified decrease occurs when the electric field is increased by less than 2kV / cm within the range of 10 and 20 kV / cm. The lack of linearity occurs in the impedance of the material and also in its volume resistivity. The lack of linearity of the filler particles can be different on each side of the switching point. It is also important that at the switching point the material only significantly changes its lack of linearity, and does not lead to electrical separation or scintillation as the electrical stress increases. The smaller the particle size for a given composition, the lower the probability of separation occurring beyond the switching point.

In comparison with the material according to US-A-4 297 250, substantially all, ie at least 80% and preferably at least 90%, of the particles of the composition of the present invention retain their shape, substantially spherical, initial with the advantage of improved electrical performance. The point, ie the electrical stress, in which the composition changes between its values of higher and lower temperature, can be selected to adapt to the application of the resulting product. This feature can be varied by selecting, for example, (i) the particular polymer matrix, or its relative permissiveness, say within the range of 4 to 60, (ii) a suitable particle size filling, (ii) a suitable filler particle size, (ii) the volume content of the filler, and (iv) the temperature and / or duration of the calcination. The position of the switching point with respect to the value of the electrical effort will affect the boosting performance of a cable tie or termination, for example, to which the matepal is applied, and may be required to be different in different circumstances, such as between a joint or a termination, for example. In the last aspect, the pulse efficiency is improved if the switching point occurs at lower voltages of electric field applied. However, this means that matepal is more conductive and the resulting heating of the material must be considered, which increases the voltage rating of the cable. The use of zinc oxide contaminated in particles treated in this way allows the necessary amount to be included in the matrix polymeric between 5% and 50%, and preferably between 10% and 40%, by volume, and results in the particle size which is less than in the known composition referred to above, being such that more than 50% by weight of the particles of the filler have a maximum dimension of between 5 μm and 100 μm, and preferably between 25 μm and 75 μm. Advantageously, such particle dimensions are applied to at least 60% by weight of the particles. The volume content of the filler with respect to the polymer matrix can be selected to provide the required impedance wl of the composition, for example being greater for the material used to control the stress on the termination of an electric power cable that for a material used for stress control of a union of the same cable operating at the same voltage scale. The particle size is achieved by passing the particle material through a suitably sized screen, preferably 125 mins. Advantageously, the process of particle formation ^, calcined results in that they maintain a uniform external surface in They may be deformed or pointed and may be substantially oval or preferably spherical, rather than significantly elongated. The particle filler comprises at least 85% and preferably, at least 90% by weight of zinc oxide. The remaining material, the pollutants, may comprise part or all of the following example, as would be known to someone with experience in the technique of contaminated zinc oxide varistor materials: Bi2O3, Cr2? 3, Sb2O3, Co203, Mn03, Al203, CoO , Co30, MnO, Mn02, S? O2 and trace amounts 25 of lead, iron, boron and aluminum. t - The polymeric matrix may comprise elastomeric materials, for example silicone or EPDM; thermoplastic polymers, for example polyethylene or polypropylene; adhesives, for example those based on ethylene vinyl acetate; thermoplastic elastomers; gels; thermo-hardened materials 5, for example epoxy resins; or a combination of such materials, including copolymers, for example a combination of polyisobutylene and amorphous polypropylene. The total composition can also comprise other additives either -fl | known for those materials, for example to improve their ability to process and / or its adaptability for particular applications. In this last aspect, for example, materials for use as power cable accessories may be necessary to withstand external environmental conditions. Suitable additives can therefore include processing agents, stabilizers, anti-oxidants and plasticizers, for example oil. ? Contaminated zinc dust comprises between 5% and 60% of the The volume of the total composition preferably exceeds 10%, more preferably 20% and more preferably 25%, and preferably is less than 50%, and more preferably is less than 40% of the total volume.

The relative permissiveness of the total composition is preferably within the range of 4 to 60, preferably exceeds 6, and more preferably exceeds 8, and preferably is less than 40 and more preferably is less than 25, measured in field strength low electric per say of approximately 2 v / mm, at 50 Hz. In addition, the permissiveness should preferably not change by more than a factor of 2 in measurements up to 250 kHz. The specific impedance of the total composition is advantageously within the range of 108 ohm-cm to 1010 ohm-cm, measured at low electric field resistance (about 2V / mm) at 50 Hz. In accordance with another aspect of this invention, the electrical equipment is provided, for example a connection or termination of electric power cable, which has applied thereon a layer of material comprising a stress control composition as previously described with reference to an aspect of the present invention. The composition may be formed in a layer such as tape or a sheet that may be wrapped around the equipment. Alternatively, the layer may be in the form of a tubular sleeve. The layer can be also provided as part of a co-extrusion, for example being a # inner layer. A layer of material formed from the composition of the invention can typically have a thickness of about 1 mm. In one application, the composition of the present invention can form the envelope controlling the stress of the cable termination described in International Patent Application Publication Number WO 91/16564, the total contents of which publication are included in this by this reference. According to another aspect of the present invention, there is provided a method of manufacturing an electrically non-linear composition, wherein (a) a particle filler comprising contaminated zinc oxide varistor powder is calcined under gravity and subsequently separated in a manner that substantially all the particles retain their original shape, preferably substantially spherical, 5 (b) the powder is passed through a 125 micron sieve, and (c) the calcined and sieved powder is dispersed as a filler in a polymeric matrix . Advantageously, the steps of the method, the filling and the • composition, are as heretofore established herein with reference to said one aspect of the invention. A stress control composition and a terminated power cable using the composition, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: FIGS. and 2 show sections through two forms of power cable terminations employing the stress control composition; Figure 3 is a graph of a typical particle size distribution of a contaminated zinc oxide filler; Figure 4 is a graph of the impedance of the filled powder for various particle sizes; and Figure 5 is a graph of the specific impedance of the composition showing the effect of the calcination of the filler. Figure 1 shows a terminated 20 kV polymeric cable 2. The dielectric cable has been pulled back to expose the conductor 6 for connection to other electrical equipment (not shown), such as a commutator, by means of a folded tab 7. The outer insulating cable sheet 8 has been cut back to expose the casing cables 10 that have been bent backwards in preparation for being connected to ground. The semiconductor screen 12 extends beyond the sheet 8 although it is cut backward to partially terminate along the cable dielectric 4. The termination of the cable 2 is complemented by the application thereto of a stress control layer 14. and an external heat shrinkable insulating layer 16. The layers 14 and 16 overlap the semiconductive screen 12 adjacent the end of the sheet 8, extend along the dielectric 4 to its trimmed rear end, and overlap the base of the tongue 7. At one end, therefore, the stress control layer 14 electrically contacts the screen 12 and is in potential to ground and, at its other end, in the The operation is at the 20kV potential of the conductor 6 and the tongue 7. * The layers 14 and 16 are formed as an individual co-extruded pipe, although they can be formed and applied as separate pipes. It is also contemplated that in an alternative configuration, the stress control layer, whether tubular or not, may be shorter that the outer insulating layer and can then extend from the screen 12 and partially finish along the dielectric. Figure 2 shows an alternative cable termination, in which a cable 20 has its external insulating sheet 22, ground wires 24, semiconductor screen 26 and dielectric 28 cut away to expose the conductor 30, towards which a connecting tab 32 is folded. A rectangular sheet 34 of stress control material is wound once around the cable 20 to overcome the screen 26 and to extend partially along the dielectric 28. A cover external elastomeric insulator 36 comprising a generally cylindrical core portion 38 with a plurality of rain shields 40 extending therefrom, is applied to the cable 2 to extend longitudinally over the ground wires 24 at one end and over the tongue 32 at its other end. The cover 36 can be applied to being progressively released radially on the cable 2 from an extension arrangement as described in International Patent Application Publication No. WO 91/16564. The material of the stress control pipe 14 of Figure 1 and the sheet 34 of Figure 2 comprise an electrically non-linear stress control composition according to the present invention. The material in these embodiments comprises a matrix comprising silicone elastomer and a particle filler comprising contaminated zinc oxide. The contaminated zinc oxide comprises approximately 90% by weight of zinc oxide and approximately 10% of Bi2O3 + Cr203 + Sb2O3 + CO2O3 + MnO3. The powder was calcined in an oven at a temperature of about 100 ° C, before being mixed with pellets from a polymer matrix and fed into an extruder to produce the final required shape. The calcined filler comprised approximately 30% of the volume of the total composition comprising the filler and the polymer matrix.

A typical particle size distribution of relative numbers ^ - of contaminated zinc oxide particles calcined from a suitable powder, after being passed through a 125 micron sieve, is shown in Fig. 3, from which it can be seen that there is a sharp 5 peak. in the particle size of approximately 40 micras, with the great majority of the particles that are between 60 and 60 micras. The switching behavior of the calcined contaminated zinc oxide particles, which shows the abrupt change in the non-linear specific impedance as a function of the resistance of electric field (at 50 Hz), is shown in Fig. 4 for three particle size scales. Curve i refers to a particle size of less than 25 μm, Curve II to a particle size of 25 μm to 32 μm and Curve ll to a particle size of 75 μm to 125 μm. It is observed that the switching point occurs at higher field resistance electric as the particle size is reduced. Figure 5 shows a comparison between the electrical behavior, mainly the variation of the specific impedance of a composition of stress control material as a function of the specific field at 50 Hz, of the particles that have been calcined, the20 Curve IV and particles that are otherwise identical but have not been calcined, Curve V. In those samples, the zinc oxide powder formulation given above formed 35% by volume of a silicone elastomer that provided the polymer matrix. It is evident that switching behavior does not occur, even in field resistances < 4É electrically significant, with the material whose particles have not been calcined, although this material exhibits non-linear behavior. The stress control composition of the present invention thus exhibits significant advantages over known stress control materials.

Claims (9)

1. An electrical stress control composition, comprising: (a) a polymer matrix, and (b) a particle filler comprising varistor powder of contaminated zinc oxide; wherein (i) the filler particles are calcined at a temperature between 800 ° C and 1400 ° C and subsequently separated so that substantially all the particles retain their original form, (ii) at least 85% of the weight of the filler comprises zinc oxide, (iii) more than 50% by weight of the particles of the filling have a maximum dimension of between 5 and 100 microns, so that the composition exhibits non-linear electrical behavior so that its specific impedance decreases by at least a factor of 10 when the electric field is increased by less than 5 kV / cm in a region within the electric field scale of 5 kV / cm to 50 kV / cm, and (iv) the filler comprises between 5% and 60% of the volume of the total composition.

2. A composition according to claim 1, wherein all the particles of the filler have a maximum dimension of less than 100 microns, preferably less than 125 microns.

3. A composition according to claim 1 or claim 2, wherein no more than 15% by weight of the filler particles have a maximum dimension of less than 15 microns.

4. A composition according to any one of the preceding claims, wherein the particles of the filler are calcined at a temperature between 950 ° C and 1250 ° C, preferably at about 1 100 ° C.

5. A composition according to any one of the preceding claims, wherein at least 90% of the weight of the filler comprises zinc oxide.

6. A composition according to any one of the preceding claims, wherein more than 50% by weight of the filler particles have a maximum dimension of between 25 and 75 microns.

7. A composition according to any one of the preceding claims, wherein the filler comprises between 10% and 40% of the volume of the total composition.

8. A layer of material comprising a stress control composition according to any one of the preceding claims.

9. A connection or termination of electrical power cable having applied thereto a layer of material according to claim 8.