EP1347469A1 - Device for controlling a high electric field in synthetic insulating material containing at least one rigid electrode - Google Patents

Abstract

The central electrode is enclosed within a semiconducting sheath (3) which in turn is located in the insulating material (2). The semiconducting sheath is attached to the electrode by a zone of very strong adherence (5A) and a zone of very weak adherence (5B). After manufacture any void formation takes place at the weak adherence zone but discharge is prevented by the action of the semiconductor

Description

The invention relates to a device for controlling a high electric field in an insulating synthetic material, comprising at least one rigid electrode, part of which is completely surrounded by molded insulating synthetic material. We hear by high electric field a field created by an electrode set to an average potential or high voltage. The main applications of the invention relate to the field of current bushings for electrical equipment, and in particular bushings in which the electric field created by a first electrode has an average potential or high voltage is locally controlled by at least a second surrounding electrode this first electrode. Generally, the first electrode carries the current and is consisting of a cylindrical busbar, and a second electrode has a annular part which remotely surrounds this busbar. This second electrode, also called field control electrode or insert or flange, is maintained around the busbar by an insulating synthetic material which is molded to completely surround the annular part of the insert and to fill the space between the two electrodes. In addition to its electric field control function, this second electrode also often has a mechanical function of maintaining the current crossing: the electrode has a part which is located outside the insulating material and which can for example be welded to the tank of a gas-insulated electrical appliance.

In such devices, the field control electrode is generally connected to the earth potential, which implies that the electric field between the two electrodes is higher than in other longitudinal parts of the crossing. However, the distance between the electrodes is provided such that the local field values remain very lower than the field limits which are intrinsic to the synthetic material and beyond which would cause deterioration of the insulation provided by the material. In the known devices of the state of the art, it is necessary to avoid that cavities can be created in the insulating material, in particular between the two electrodes, because such cavities are inevitably occupied by gas under more or less low pressure and low electrical permittivity. Indeed, these cavities give rise to local concentrations electrostatic field which most often results in electric discharges partial, which leads to a deterioration of the insulation which can eventually cause the perforation of the synthetic material.

When synthetic insulating material is molded around the electrodes, especially if this material is an elastomer obtained by polymerization, there is a risk of cavity formation between the field control electrode and the material. Indeed, stresses are generated in the material during manufacture due to the reduction in volume of the material during its withdrawal from manufacture, i.e. its chemical withdrawal during of polymerization and its thermal shrinkage during cooling. Subsequently, others stresses can also be created due to mechanical, thermal or other than the current crossing may undergo during its use. Likewise, cannot exclude the risk that a cavity will form between the busbar and the elastomer.

In order to limit the risks of cavity formation in particular at the level of the field control electrode it is possible to use for manufacturing this electrode an elastic conductive material allowing stress relaxation in synthetic insulation. For example, patent document US5726390 describes an electrode control field of annular shape made of such a material. Another electrode tripod-based ring, having a capacitive voltage divider function, is fixed on this field control electrode and consists of a body of synthetic material covered with a conductive layer.

It is likewise known from the state of the art to use metal inserts to several rigid portions joined together by flexible portions, so as to obtain a field control electrode which generally has a certain flexibility to absorb most of the stresses on synthetic insulation material especially during its withdrawal from production. However, for mechanical maintenance of a current crossing, it is advantageous to have a field control electrode made of a rigid metallic material for example in the form of a flange retaining ring. It is usually desired to have very good adhesion between a rigid electrode and the insulating synthetic material, especially if this material is a elastomer.

The search for a perfect adhesion, that is to say without any risk of detachment, requires special surface treatments as well as excellent control of molding parameters. In addition, the geometry of the control inserts field must be adapted so as to minimize the risk of local detachment. In practice, this objective is difficult to achieve systematically: there is always a risk that cavities will form following local ruptures in adhesion, in particularly for the interface zones between insulator and conductor which are located between two rigid electrodes opposite. Indeed, the insulating material cannot perform freely withdraw its manufacturing in the space between the two rigid electrodes, which generates significant tensile forces on the interfaces with the electrodes. The current bushings comprising a conductive bar surrounded by an annular flange rigid field control thus have high probabilities of being affected by the formation of cavities giving rise to partial discharges.

The techniques known by the applicant to achieve an adhesion which provides good adhesion does not systematically guarantee the good resistance of membership. In practice, however, it is not always possible to prevent detachments occur either during manufacture or during the use of the crossing current. The manufacturers of bushings use various techniques of adhesion between a electrode and insulating material to avoid detachment as much as possible. It is known to use a layer of synthetic material to improve adhesion by creating a double interface, for example a polymer layer interposed between an electrode and the insulating material if the latter is thermosetting. However, the risk of detachment and cavity creation at an interface is not completely discarded, especially if the adhesion surface between the layer and the electrode is large.

It should be noted that there exists in the prior art a particular category of current crossings for which the risk of partial discharge between the conductor central and the insulating material can be removed at least locally. This category concerns bushings for which an empty or gas-filled space is provided between the conductor and the insulation. For example, patent document US6339195 describes a current crossing structure in which the central conductor is surrounded at a distance by a hollow cylindrical insulator whose internal wall is covered with a layer conductor at the same high voltage potential as the conductor. So the field electrostatic is zero in the space between the central conductor and the inner wall of the insulation.

However, the current crossing structure described in this patent document US6339195 would not completely eliminate the risk of detachment and partial discharge at an annular field control electrode which would be inserted into the material of the insulator and potentialization of the tank of an apparatus electric, in particular if this electrode were to provide a holding function mechanics of the crossing in another type of application than that described in the document.

On this subject, it can be noted that the use of conductive or semiconductor layers as a coating of insulating material to avoid discharges partial in gas-filled spaces is well known in the art. Through example, patent document US6060791 describes capacitors using a dielectric solid in the form of a ceramic covered with a metallic layer. Ceramic is intended to be brought into contact with a metal electrode, and the metal layer cancels the electric field in small cavities that may be present at the interface between this layer and the electrode. Furthermore, the use of semiconductor layers to create cavities with equipotential walls is also known in the field of interconnection systems between medium or high voltage cells, where a axial conductor joining two cells is typically surrounded at a distance by a sleeve made of insulating synthetic material. Such a sleeve can be covered on its internal wall a semiconductor layer in electrical contact with the conductor so that the volume of air in the sleeve is delimited by equipotential surfaces.

The main objective of the invention is to provide a device for controlling a electric field in an insulating synthetic material molded around a rigid electrode and safely prevent partial discharges from appearing at the interface between the electrode and the insulating material. The invention succeeds in achieving this objective without however seek to obtain perfect adhesion and prevent any appearance of cavity at this interface, contrary to what is usually sought in the state of the technique.

To this end, the invention relates to a device for controlling an electric field high in an insulating synthetic material, comprising at least one rigid electrode with a part around which the insulating material is molded, a layer of another synthetic material being disposed between at least one rigid electrode and the material insulating so as to produce a double interface, characterized in that this layer is made of a semiconductor material whose interface with the insulating material everywhere has a first type of adhesion corresponding to strong adhesion and whose the interface with the electrode has zones of this first type of adhesion as well as areas of a second type of adhesion corresponding to weak or zero adhesion.

Such a semiconductor layer has a low but sufficient conductivity to form an equipotential surface with the same electrical potential as the electrode that it covers. If a separation between a semiconductor layer and an electrode creates a cavity, the walls of this cavity are equipotential and the electrostatic field inside is zero, which implies that there is no risk of partial discharges in such a cavity.

In one embodiment of the field control device according to the invention, the areas of poor adhesion between an electrode and a semiconductor layer form a continuous surface which is located opposite another electrode connected to a other electrical potential.

The invention also relates to a current crossing comprising a device for field control according to the invention with a first electrode consisting of a bar conductive under medium or high voltage and at least one other electrode which surrounds distance the bar and which is set to earth potential or to an intermediate potential lower than that of the bar.

• une double interface par une couche semi-conductrice est réalisée entre l'électrode de contrôle de champ et le matériau isolant, et une adhésion directe est réalisée entre la barre conductrice et le matériau isolant, ou
• une double interface par une couche semi-conductrice est réalisée entre la barre conductrice et le matériau isolant, et une adhésion directe est réalisée entre l'électrode de contrôle de champ et le matériau isolant, ou
• une double interface par une couche semi-conductrice est réalisée d'une part entre l'électrode de contrôle de champ et le matériau isolant et d'autre part entre la barre conductrice et le matériau isolant.

In an embodiment of a current crossing according to the invention, a second so-called field control electrode consists of a metallic annular flange which has a tubular part completely surrounded by molded insulating synthetic material as well as a part annular located outside of this insulating material and connected to the metal casing of an electrical apparatus. The space between the field control electrode and the busbar can be completely occupied by the insulating material. At least the following three alternatives are possible for making a current crossing in this mode:

• a double interface by a semiconductor layer is produced between the field control electrode and the insulating material, and direct adhesion is produced between the conductive bar and the insulating material, or
• a double interface by a semiconductor layer is produced between the conductive bar and the insulating material, and a direct adhesion is produced between the field control electrode and the insulating material, or
• a double interface by a semiconductor layer is produced on the one hand between the field control electrode and the insulating material and on the other hand between the conductive bar and the insulating material.

In an advantageous embodiment of a current crossing according to the invention, the synthetic insulating material is of the EPDM type possibly loaded with Alumina or Silicone, the field control electrode is made of stainless steel, and a double interface between the field control electrode and the insulating material is produced by an elastomeric material or semiconductor EPDM.

The invention is described in more detail in the following, with reference to the drawings attached which illustrate embodiments by way of non-limiting examples.

FIG. 1 schematically represents a half-section view of a device for field control according to the invention comprising an annular electrode also shown in figure 3.

Figure 2 schematically shows the field control device of the Figure 1 after a reduction in volume of the insulating material molded around the electrode annular.

Figure 3 schematically shows a sectional view of a current crossing comprising a field control device according to the invention and in particular a annular field control electrode as shown in Figure 1.

FIG. 4 schematically represents a half-section view of a crossing of current comprising a field control device according to the invention disposed between the central conductor and the insulating material of the bushing.

FIG. 5 schematically represents a half-section view of a crossing of current according to the invention, combining the innovative characteristics of the devices shown in Figures 3 and 4.

Figure 6 schematically shows a sectional view of a current crossing of a particular type, comprising a field control device according to the invention in level of an electrode.

Figure 1, a field control device according to the invention is shown schematically in longitudinal half-section along the axis of revolution A of the electrode rigid 11 that includes this device. As shown in Figure 3, this electrode consists of an annular flange which is arranged in an insulating synthetic material 2 molded around. The device is represented at a time corresponding to the end of the demoulding of this insulating material, while the material has not yet removed it from manufacturing. The rigid electrode 11 coaxially surrounds another rigid electrode 10 that constitutes the central conductor of a current crossing 1. Besides the electrodes 10 and 11, the field control device comprises a layer 3 made of a material synthetic semiconductor and interposed between the electrode 11 and the insulating material, so as to achieve a double interface. This semiconductor layer 3 is directly at the contact of electrode 11 and is therefore electrically connected to the latter.

In practice, the semiconductor layer can be deposited on the rigid electrode 11 before the insulating material 2 is molded around. According to the invention, the realization of the interface 5 between the semiconductor layer 3 and the rigid electrode 11 is provided for that this interface has zones 5A of a first type of adhesion corresponding to strong adhesion as well as 5B zones of a second type of adhesion corresponding to little or no adhesion. In addition, the interface 4 between this semiconductor layer and the insulating material is produced so as to exhibit everywhere a strong adhesion adhesion.

We define by adhesion with strong adhesion an adhesion to an interface between two materials for which the tensile force required to detach the interface is same order or greater than the cohesive forces of the less resistant of these two materials. Thus, a traction exerted with a force which exceeds the limits of cohesion of a material is supposed to cause a rupture in this material rather than a detachment of the interface for which strong adhesion is achieved.

Conversely, we define by adhesion with low adhesion an adhesion to an interface between two materials for which the tensile force necessary for the detachment of the interface is significantly less than the cohesive forces of the less resistant of these two materials. A break in one of these materials following a traction is excluded with this second type of adhesion, because excessive traction necessarily causes a separation of the interface and therefore the creation of a space between the two materials.

In the device represented in FIG. 1, the area 5A of the interface 5 has a adhesion with strong adhesion, of the same type as adhesion to the interface 4. However, the respective adhesion coefficients of these two adhesions of the same type are not necessarily identical or neighboring. For example, the coefficient of adhesion to the interface 4 can be provided larger than that at the interface 5, so as to permanently discard any risk of detachment and therefore of partial discharge at this interface 4 between the layer semiconductor and insulating material.

Figure 2, the device of Figure 1 is shown at a later time, when the insulating material 2 has withdrawn from production and has therefore been subjected to stresses until reaching a state of equilibrium. Furthermore, between the release of the insulating material and this later instant, the current crossing 1 may possibly have undergone mechanical, thermal or other stresses. Applied to the device of Figure 1, these constraints and possible stresses can have the consequence of taking off locally the interface 5 between the electrode 11 and the semiconductor layer 3 in regions of the 5B interface zone for which a low adhesion adhesion is as shown in Figure 2.

A small empty space 9, which we also call cavity, is thus created between the electrode and the semiconductor layer. This cavity 9 is occupied by gas under more or less low pressure, and is formed by equipotential walls so that no discharge partial is only possible in gas. At its annular part substantially cylindrical, the region of the semiconductor layer that is located between the two electrodes 10 and 11 of the current bushing has moved slightly towards the electrode central 10, while remaining approximately parallel to this electrode 10. Thus, the distribution of the electric field in the insulating material between the two electrodes 10 and 11 is only very slightly modified compared to a case where no separation takes place would produce at the interface 5 between the electrode 11 and the semiconductor layer 3. The field electric in the substantially cylindrical region of the inter-electrode space remains in in all cases an essentially radial field with respect to the axis of the bar.

It follows from the above that a field control device according to the invention avoids any risk of partial electric discharge between an electrode of the device and the insulating material that surrounds it, without significantly disturbing the distribution of the electric field in the material.

Figure 3, the current crossing 1 mentioned above is shown schematically in longitudinal section along its axis of revolution A. The device for field control partially shown in Figure 1 is visible here in its completeness and has complete symmetry of revolution along the axis A of the crossing. As in Figure 1, the current crossing is shown at a time corresponding to the end of the release of the insulating material 2 and before its removal from manufacturing. Conventionally, the annular field control electrode 11 is extended in the air by a cylindrical part intended for example to be welded at a connection to the metal tank of a gas-insulated medium-voltage switchgear.

Direct adhesion with strong adhesion is carried out at the interface 6 between the electrode central 10 and the insulating material 2. Given the geometry of the electrodes and that of the molding of the insulating material, the most important constraints on the material when it is removed from production is located in the space where the distance between the electrodes is the smallest. Because the 5B interface area with low adhesion represented in FIG. 1 comprises a substantially cylindrical annular region which delimits this inter-electrode space, detachment at this interface zone allows the material to carry out its manufacturing withdrawal while relaxing its constraints.

For a current crossing with a field control device such as shown in FIG. 3, the geometry of the field control electrode 11 as well as the parameters for molding the insulating material are provided so that a localized separation between the semiconductor layer 3 and the electrode 11 allows a sufficient relaxation of stresses in the material. By sufficient relaxation of constraints during the withdrawal of manufacture, it is understood in the present case that these constraints remain moderate enough not to risk a separation at the interface 6 between the central electrode 10 and the insulating material 2 in the region which is opposite with the electrode 11.

Following a detachment as shown in Figure 2 at a large part of the interface zone 5B with low adhesion, mechanical maintenance of the crossing 1 is always ensured by the annular flange 11 due to the indirect strong adhesion adhesion which is produced between this flange 11 and the insulating material 2 on the part flange device.

Figure 4, a current crossing of the same shape as the previous one is shown schematically in longitudinal half-section along its axis of revolution A, at an instant which precedes the withdrawal of manufacture of the insulating material. Unlike the embodiment previous, an indirect adhesion is made between the central electrode 10 and the material insulator 2 thanks to a semiconductor layer 3 'which is arranged so as to create a double interface between this electrode and the insulating material, while direct adhesion with strong adhesion is produced at the interface 7 between the field control electrode 11 and the insulating material.

In a field control device according to the invention, the interface between a layer semiconductor and the insulating material is necessarily made so as to present everywhere a type of adhesion with strong adhesion, so as to avoid any risk of delamination and partial discharge at this interface. The interface 4 ′ shown in FIG. 4 responds to this need. On the other hand, the interface between a semiconductor layer and a device electrode is characterized by two types of adhesion. In Figure 4, this 5 'interface consists of a 5'B area where adhesion is of the low adhesion type and a 5'A zone where the adhesion is of the high adhesion type. Zone 5'B forms a surface continuous cylindrical corresponding to the surface of the central electrode 10 which is located in with respect to the field control electrode 11. The insulating material located between the two electrodes 10 and 11 around the 5'B zone undergoes strong stresses when it is removed from manufacturing, and the low adhesion to the 5 ′ interface in this zone makes it possible to separation of the semiconductor layer 3 '. This allows sufficient relaxation of the stresses in the insulating material so as not to risk separation at the interface 7 between the electrode 11 and the insulating material. Once the manufacturing withdrawal of the insulating material, the cavity which is created between the electrode 10 and the layer 3 'has equipotential walls making partial electrical discharge impossible in this cavity.

The 5'A zone corresponds to the remaining part of the 5 'interface and is not opposite no field control electrode. The geometry and molding parameters of the insulating material are provided so as to obtain sufficient relaxation of the stresses in the material around this 5'A zone when the material is removed from production, which means that these constraints remain moderate enough not to risk a separation of the 5 'interface at this level.

Figure 5, a current crossing of the same shape as the previous one is shown schematically in longitudinal half-section along its axis of revolution, at an instant which precedes the withdrawal of manufacture of the insulating material. The embodiment shown offers the particularity of combining the innovative characteristics of the devices according to the invention shown in Figures 3 and 4. Indeed, as in Figure 3, a indirect adhesion is achieved between the field control electrode and the insulating material thanks to a semi-conductive synthetic layer 3. As in FIG. 4, a indirect adhesion is achieved between the central electrode and the insulating material thanks to a semi-conductive synthetic layer 3 'identical or of the same kind as layer 3.

The interface between the semiconductor layer 3 and the field control electrode has two adhesion zones 5A and 5B of types with high adhesion and poor adhesion, as shown in Figure 1. The interface between the layer semiconductor 3 'and the central electrode has two adhesion zones 5'A and 5'B of types with high adhesion and low adhesion respectively, as shown in Figure 4.

In comparison with the embodiments of FIGS. 3 and 4, the mode of embodiment shown in Figure 5 is advantageous only in the case where the geometry of the field control electrode as well as the parameters for molding the insulating material cannot be planned in order to obtain sufficient stress relaxation in the material following a localized separation between a single semiconductor layer 3 or 3 'and an electrode 11 or 10. The use of a semiconductor layer on each electrode then makes it possible to obtain detachment at each interface between a layer 3 or 3 'and an electrode in the inter-electrode space when the material is removed from production insulating in this space. A relaxation of the stresses in the material can thus be obtained without risking creating a cavity whose walls are not equipotential.

Whatever the embodiment chosen for a current crossing comprising a field control device according to the invention, limiting the constraints that allows the device has the advantage of improving the aging resistance of the material insulating. For example, on a current bushing manufactured according to the embodiment illustrated in Figure 1, thermal aging tests at 110 ° C have shown that there is no degradation of the insulating material even after hundreds of hours in these conditions. This has been verified for a test duration exceeding one thousand hours, which is greater than twenty times the average duration observed before degradation of the material in a current crossing of identical geometry but carried out in a conventional manner with direct adhesions between the electrodes and the material. Flexural creep tests at 90 ° C also showed an absence of degradation of the insulating material even after thousands of hours.

On the other hand, it turns out that the use of a semiconductor layer to achieve indirect adhesion between an electrode and the insulating material makes it possible to increase adhesion compared to conventional direct adhesion. In particular, this property has could be verified with a Nitrile type semiconductor elastomeric material arranged between a field control electrode made of an austenitic stainless steel of type 316L and a synthetic insulating material of type EPDM loaded with tri-hydrated Alumina.

The applications of a field control device according to the invention are not limited to current crossings in which the inter-electrode space is completely occupied by the insulating synthetic material, even if it is in this type application that the advantages provided by the invention are the most notable.

In Figure 6 is shown a current crossing of a particular type, the insulating synthetic material is molded with a substantially different shape than the conventional form used for a current crossing as shown in FIG. 3.

The molding is carried out so as to leave a space between the bar or conductor central 10 and the insulating material 2 on a certain longitudinal part of the bar. Sure this part, the insulator of the crossing has an internal wall whose shape is in mostly cylindrical and coaxial with bar 10 to define a space of fixed thickness between the bar and the insulator. This internal wall is covered with a conductive layer or semiconductor 8 electrically in contact with the bar 10, so as to have a equipotential surface and therefore a zero electric field between the bar and the insulator to avoid no risk of partial discharge in this space. Insulating material 2 is molded around a field control flange or electrode 11 having a substantially annular shape with a cylindrical part coaxial with the bar 10. Thus, the electric field between the equipotential surface of the inner wall of the insulator and this cylindrical part of the flange is a radial field with respect to the axis of the bar.

The annular flange 11 has a longitudinal half-section in the form of an elbow with approximately rounded right angle, the part of the flange outside the insulator being intended to be connected to the tank 12 of an electrical apparatus insulated with gas. The open end of the inner wall of the insulator flares with a rounded curvature similar to that of the surface of the elbow of the flange 11 opposite, so as to distribute the electric field between layer 8 which covers this wall and the flange. This end of the insulator is extended by a fin to avoid any risk of electric arc in the gas between the layer 8 which is at the potential of the bar 10 and the flange 11 which is at the potential of the tank 12.

For this type of current crossing, the manufacturing shrinkage of the synthetic material insulator can be made freely in the inter-electrode space because this material is not adhered to the center electrode in this space. The relaxation of constraints in the insulating material may thus be sufficient to envisage direct adhesion between a electrode 10 or 11 and the material without risk of detachment during manufacturing withdrawal. A field control device according to the invention is therefore not systematically essential in this type of crossing. However, it may be advantageous to carry out a indirect adhesion between the field control electrode 11 and the synthetic material insulator, or even between the central electrode 10 and this material, using a layer made of a semiconductor synthetic material as described in the embodiments preceding.

Indeed, according in particular to the type of synthetic material insulating from the crossing of current as well as the geometry of the electrodes, mechanical, thermal or others that the crossing can undergo once installed can lead to detachments rooms at the interface between an electrode and the material in the case of direct adhesion, with a risk of partial discharge in the space created at this interface. The use of a layer in synthetic material, such as an elastomer disposed between an electrode and the insulator, allows an indirect adhesion which can present a better adhesion than direct adhesion and thus limit the risk of rupture of adhesion. Yes however, a risk of detachment remains, the risk of partial discharge may be annihilated by producing a double interface in accordance with the teachings of the invention. A layer of semiconductor synthetic material is used, and the interface between this semiconductor layer 3 and an electrode 10 or 11 is provided to present zones of a first type of adhesion corresponding to a strong adhesion as well as zones of a second type of adhesion corresponding to weak or zero adhesion.

On the field control electrode 11, the adhesion zones 5B with weak adhesion or zero does not necessarily cover the entire surface of the electrode which is opposite of the inner wall of the insulator. There may be areas 5A on this surface where the semiconductor layer 3 strongly adheres to the electrode, from the moment a detachment of this layer 3 is estimated to be impossible in these areas. Well understood, the interface 4 between the layer 3 and the insulating material 2 has everywhere a adhesion with strong adhesion, as in the realizations in connection with the figures preceding. Zones 5B can be located so that they are the only ones that can be affected by a risk of layer 3 delamination in the event of significant stress on crossing. The presence of these areas of low adhesion is therefore a security for prevent any risk of partial discharge occurring during the service life of the crossing.

A single annular zone 5B of poor adhesion is shown in FIG. 6. sectional view of a detail is shown in Figure 6A, showing part of the double interface in the vicinity of such an adhesion zone 5B. The structure of this double interface is similar to that shown in Figure 1, and the same references are times. Of course, other areas of low adhesion adhesion can be provided. on the surface of electrode 11.

On the other hand, a 3 'layer of semiconductor synthetic material can cover part of the bar 10 to form a double interface I between this central electrode 10 and the insulating synthetic material. Such a layer is not essential for this location, but can increase adhesion over direct adhesion between the electrode and the insulator.

The applications of the invention are not limited to devices for controlling field for current crossings. For example, in the field of vacuum interrupters, it is possible to overmold certain parts of elastomeric insulation metal of a vacuum interrupter which are external to the ampoule and are at potential contact of a bulb. The creation of a field control device according to the invention at the interface between the insulating material and such a metal part can be advantageous to avoid the appearance of partial discharges at this interface at the case where the material is subjected to strong stresses.

Claims (8)

Device for controlling a high electric field in an insulating synthetic material, comprising at least one rigid electrode (10, 11) with a part around which the insulating material (2) is molded, a layer (3) of another synthetic material being disposed between said rigid electrode and said insulating material so as to produce a double interface, characterized in that said layer (3) consists of a semiconductor material, the interface (4) between said layer (3) semi -conductive and the insulating material (2) has everywhere a first type of adhesion corresponding to a strong adhesion, and the interface (5) between this semiconductor layer (3) and said electrode (10, 11) has zones (5A) of said first type of adhesion as well as zones (5B) of a second type of adhesion corresponding to weak or zero adhesion. Field control device according to claim 1, in which the zones low adhesion adhesion between an electrode and a semiconductor layer form a continuous surface which is located opposite another electrode connected to another electric potential. Current crossing comprising a field control device according to one of claims 1 and 2, wherein the device comprises a first electrode consisting of a medium or high voltage busbar and at least one other electrode which remotely surrounds the bar and which is set to earth potential or to a intermediate potential lower than that of the bar. Current crossing according to claim 3, wherein a second electrode so-called field control consists of a metallic annular flange which comprises a tubular part completely surrounded by molded insulating synthetic material as well as an annular part located outside of this insulating material and connected to the metal casing of an electrical appliance. Current crossing according to claim 4, in which the space between the field control electrode and the busbar is completely occupied by the insulating material. Current crossing according to claim 5, in which a double interface a semiconductor layer is produced between the field control electrode and the insulating material, and direct adhesion is achieved between the busbar and the insulating material. Current crossing according to claim 5, in which a double interface a semiconductor layer is produced between the conductive bar and the material insulator, and direct adhesion is achieved between the field control electrode and the insulating material. Current crossing according to claim 5, in which a double interface a semi-conductive layer is produced on the one hand between the control electrode field and the insulating material and secondly between the busbar and the material insulating.

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