EP0570498B1 - Method of reducing the risks of insulation breakdown on high voltage electrical cables and lines on ageing - Google Patents

Abstract

In a method for reducing the risks of breakdown of the solid dielectric insulator of transmission lines and cables operating at high voltage where said insulator normally separates the conductors of a multiwire line or the central conductor and external metal sheath of a coaxial line, small quantities of metal particles smaller than a micron are added into the dielectric insulating material.

Description

The present invention relates generally to cables and transmission lines called to operate under high electrical voltage.

It aims in particular to reduce the risks of breakdown inherent in all the solid dielectric insulators used in the preceding transmission devices, and especially the increase of these risks during the aging of the insulator.

Cables and transmission lines vary greatly depending on the range of voltages, currents and frequencies in which they are used. The transport of significant energies to a place of use is often carried out by lines with 2, 3 or 4 wires in low frequency, 400 Hz, 60 Hz or 50 Hz for example, or even in direct current.

The signal transmission is generally carried out by a two-wire line or by a coaxial line depending on the frequencies to be transmitted and the conditions of transmission.

For reasons of clarity and simplification, the description will be limited here to coaxial cables, but it is understood that the invention applies to all the insulators used in the different types of lines, to the portions of lines which constitute the "passages "insulators and high voltage devices incorporating solid insulators.

One of the major causes of failure of these high-voltage devices is electrical breakdown: a disruption occurs between the conductors through the insulators (sometimes preceded by a partial electrical discharge) leading to the piercing of the insulator and which can go as far as destruction. Failure occurs randomly, but we see that this risk worsens a lot over time, according to an aging process.

This problem of reliability and lifetime of insulators has preoccupied cable manufacturers for a long time. Until now, very limited knowledge of the physical phenomena involved has not made it possible to find effective solutions.

Because by definition the lines have a great length with respect to the wavelength, one cannot apply to them the approximation of the standing waves. The study of the phenomena of transmission, energy absorption and therefore aging can only be carried out using MAXWELL equations, the boundary conditions defined by the surfaces of separation between the different media, and specific relationships to the environments considered.

Without going back in depth on the theory of transmission lines, it will be noted that for a perfect conductor, the local charge density is zero, while for a dielectric the local electric field can be considered as the sum of the electrostatic field due to the charges. electric which develop there and of the field calculated by the equations of MAXWELL in uncharged medium.

If the environments were perfect, the electric and magnetic fields would be zero in the conductors, and would be in phase, not attenuated, in the insulators so that the energies electric and magnetic are equal. In practice, the conductors and insulators are not perfect and there are two types of faults: volume faults and interface faults.

The usual method for increasing the breakdown threshold of a coaxial cable consists in increasing the thickness of the dielectric material. But we are obviously limited by considerations of dimensions (size and price) and in any case the aging defects remain. Other ideas and several methods have been devised, all based on the fact that the existence of charges in the insulating dielectric mass is inevitable and that their harmful effects are limited by the application of other fields intended to deflect them, or by reduction of fields following special geometrical arrangements.

By way of example, the published patent FR-A-7703498 describes a transmission line comprising a device placed outside the dielectric region and intended to create a magnetic field so that the charged particles present in the dielectric undergo movement helical. Thus the probability for a charged particle to reach the energy level corresponding to ionization becomes very low. This process is undoubtedly effective in the case of a cable with gaseous insulator (SF₆ for example), but it is absolutely not justified for a solid insulator, which represents the most general case for industrial applications.

We have already sought, by various techniques, to improve the functioning of the dielectrics of electric cables and to make them more efficient with respect to the constraints they undergo. We can quote for example:

Patent FR-A-2 357 992 which describes an electric cable in which the insulator has a gradation of the permittivity from the central conductor to the external conductor. This gradation is obtained by using layers of insulation in which large quantities of oxides (80% to 100%) have been added. The object sought in this document is to minimize the harmful effects on the breakdown of a lack of symmetry or concentricity of the cable.

Patent CH-A-669,277 describes a cable comprising several layers of insulating material having different permittivities. The goal is to improve manufacturing constraints by reducing the thickness of the insulation.

Neither of these two documents mentions the means used to specifically combat the increased risk of breakdown as a function of aging.

The present invention specifically relates to a process for reducing the risk of breakdown of insulators in electrical transmission systems which makes it possible to considerably improve the preceding situation.

This process for reducing the risk of breakdown of the insulation of cables and lines with high electric voltage during aging, is characterized in that particles of elements with very high permittivity are added to the mass of the insulation. , the size of these particles being at most of the order of a micrometer and their abundance of the order of 3% of the mass of the insulator.

appliqué à un diélectrique et la polarisation

de ce même dielectrique qui résulte de l'application de ce champ, sont liés par la formule

formule dans laquelle ε est une grandeur sans dimension que l'on désigne sous le nom de permittivité du diélectrique. Elle traduit en fait la plus ou moins grande facilité du diélectrique à se polariser, c'est-à-dire à accepter des charges électriques, sous l'influence d'un champ électrique appliqué. Or, l'étude fondamentale précédente a précisément mis à jour le fait que cette permittivité locale du diélectrique jouait un rôle essentiel dans les phénomènes de claquages.A fundamental study of the breakdown phenomena carried out by the applicant made it possible to give a new interpretation by linking the breakdown phenomena at the polarization of the dielectric and at its relaxation. It will first be recalled that the electric field

applied to a dielectric and polarization

of this same dielectric which results from the application of this field, are linked by the formula

formula in which ε is a dimensionless quantity which is designated by the name of permittivity of the dielectric. It actually translates the greater or lesser ease of the dielectric to polarize, that is to say to accept electrical charges, under the influence of an applied electric field. However, the previous fundamental study has precisely brought to light the fact that this local permittivity of the dielectric plays an essential role in the breakdown phenomena.

The role of local permittivity is all the more important as the volumes concerned are on a submicrometric scale. On this scale the macroscopic notion of permittivity no longer has any meaning and we must consider both the electric charge present in the dielectric trapped in a potential well, and the surrounding medium consisting of a building of a hundred 'atoms. The whole constitutes what is called a polaron.

As the dielectric is subjected on the one hand to electric and magnetic fields and that it has on the other hand a non-zero charge density, one must seek either to reduce this charge density, or to better distribute it. This is the superposition of the volume charges (characteristics of the nature, and of the state of the material) and of the charges injected at the interfaces by the polarity of the conductors. We can therefore play on the nature of the material and on the modifications of its properties.

This is why, according to the invention, the properties of the dielectric material are modified by adding to it small quantities of particles made up of elements with very high permittivity and dimensions less than one micron. On this scale, there is a strong interaction between the electric charges in the potential well and the surrounding atoms, which ensures a very high level of trapping of charges whose mobility is the essential cause of aging. It is found that the material thus doped, not only has a higher breakdown threshold, but also has a high resistance to aging under normal service conditions, that is to say in the presence of an electric field. . The best results have been obtained with metallic particles.

According to a first embodiment of the invention, when the dielectric insulator is refractory, for example constituted by a ceramic, the electrical properties are modified by doping the insulating material using particles of an oxide of a metal chosen from chromium, titanium and manganese in the submicron state, which is deposited in the form of a paint on at least one of the surfaces of the insulator and the conductor and which is then diffuses into the insulation by heating at a high temperature, of the order of 1000 ° C. to 1500 ° C. At this temperature, the metal oxide is dissociated and the metal diffuses in the grain boundaries of the ceramic.

According to a second embodiment of the invention, when the insulator consists of a polymer or an organic solid, the electrical properties are modified by doping the insulator with metallic impurities. In the latter case, doping can be done, in accordance with the invention, by injection of metal powder during the polymer casting operation, the particles of the powder having a size necessarily less than a micrometer.

In any case, the present invention will be better understood by referring to the description which follows of several examples of implementation as well as to FIG. 1 showing the compared properties of a treated cable and an untreated cable.

In general, we will begin by recalling that all the insulating dielectrics known to date can be used in the context of the present invention. In other words, these dielectric insulators can be either polymers for making cables, or ceramics for making electrical passages and connectors, or more generally any other solid, organic or mineral insulator.

As already specified above, the dielectric insulating material chosen conditions the doping treatment used. Thus, for example, in a first example of implementation of the invention, a coaxial insulating passage is produced using an Al₂O₃ alumina ceramic of 99% purity and consisting of a sintering of grains from 1 to 7 microns . This ceramic is obviously refractory, doping can be done at high temperature. In practical terms, this involves diffusing a metal into the grain boundaries of the ceramic. In this example, the metal being chromium, a chromium oxide paint is deposited either on the surface of the conductor, or on the surface of the insulator, or on both at the same time. After mounting the conductor in the insulation, the chromium thus deposited is diffused by a heating operation at a temperature of the order of 1000 ° C to 1500 ° C. The temperature and duration of heating depend on the dimensions of the ceramic. As an indication, for a passage of one centimeter diameter and a continuous electrical voltage of 100 kV, the diffusion takes place at 1200 ° C for 5 minutes. The metal content in the joints is around 3%.

The effect of this treatment was measured over a 12 month period. The measured parameter is the value of the electric shocks that the insulation can withstand in service without cracking. Whereas for the untreated ceramic the breakdown resistance is 800 kV / cm at the start of the use and 400 kV / cm after 12 months, the ceramic doped according to the process object of the invention, supports 1.5 MV / cm at the start of use and still holds 1.2 MV / cm after 12 months of service.

According to a second embodiment of the invention, the dielectric insulator used is a polymer which is doped at a relatively moderate temperature of the order of 200 to 300 ° C. for example with metallic impurities. These impurities are powders and doping takes place at the same time as the polymer is poured into the molding device in the form of an injection of metal particles which can for example be copper or iron and which dimensions are chosen. below the micrometer.

By way of example with a cable in which the polymer is commercial polyethylene, 3% of iron particles with dimensions between 0.1 and 0.5 micrometer were added. The effect of this treatment was measured over a 12 month period. The result is shown in Figure 1. In this figure 1, the time in months is plotted on the abscissa and the resistance to electric shock in MV / cm on the ordinate. Curve 1 is that of an untreated cable and curve 2 that of a cable treated according to the invention.

Claims (4)

1. Method of reducing the risks of breakdown of the insulation of cables and lines at a high electrical voltage during their ageing, characterized in that particles of elements having very high permittivity are added throughout the bulk of the insulation, the size of these particles being at most of the order of one micrometre and their quantity of the order of 3% by weight of the insulation.
2. Method of reducing the risks of breakdown according to Claim 1, characterized in that the particles are metallic.
3. Method of reducing the risks of breakdown according to Claim 1, characterized in that the additive is a metal which is made to diffuse into the grain boundaries of a sintered material, the grains of which have sizes of the order of one micron, using a metallic-oxide paint which is heated at a high temperature.
4. Method of reducing the risks of breakdown according to Claim 2, characterized in that, the insulation being a polymer, the doping is performed by injection of metallic powder during the operation of causing the polymer to flow, the particles of the powder having a size less than one micrometre.

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