EP0494807B1 - High voltage binding post - Google Patents

Abstract

The invention relates to a high-voltage binding post for electrical apparatuses, such as a current or voltage transformer, having outsides moulded from insulating resin. The binding post comprises at least two portions, one of which, the head (1), is connected to a high-voltage line by means of a pin (2) and the other of which, the base (4), is connected to the earth of the apparatus. Both ends are separated by a tubular shedded insulator (5) which encloses a conduit (6) where the conductors connecting the head (1) to the output terminal of the apparatus are placed. According to the invention, the head (1) is covered with a metal layer (7) over its external portion and a metal ring (9) is interposed between the head (1) and the insulator (5). The invention concerns manufacturers of medium- and high-voltage electrical equipment. <IMAGE>

Description

The invention relates to an apparatus as defined in the preamble of claim 1 and comprising a high voltage terminal more particularly suitable for current transformers for outdoor use in molded resin. It is mainly of interest to manufacturers of electrical equipment.

Such devices are known in practice.

The high voltage connection terminal is a device intended to ensure the connection with an external high voltage line. Although the present invention has been developed especially in the context of application to current transformers, it can nevertheless be perfectly applied to other types of electrical apparatus.

To follow the example of dry transformers, these include windings wound around magnetic cores placed in a closed insulating mass molded around said core. These windings are connected, one to an external high voltage line, the other to a secondary low voltage network.

The realization of high voltage connection terminals poses some construction difficulties. In fact, they generally consist of two parts which are the head connected to the high voltage line and the base at ground potential. These two parts are separated by a tubular insulator which encloses a conduit in which are placed the electrical conductors which join the lower part of the device to the ground potential receiving the secondary output terminals and an upper part carrying the connection parts to the high voltage line. Consequently, between the head and the base there is a very significant potential difference and given their proximity, this creates intense electric fields which favor the formation of electric arcs and ionizing discharges.

The insulator used is of the fin type, that is to say a series of peripheral discs are arranged outside the insulator, the sides of the discs being inclined downward to prevent it from being able to have stagnant moisture on the underside of the fins. Many insulator profiles are known to those skilled in the art to lengthen the path from the high voltage part to the grounded part and create vanishing lines opposing the creation of electric arcs to prevent the path of sparks and the arcing of electric arcs along the insulator.

When it rains or when more or less conductive atmospheric deposits form on the electrical device, which promote conduction of the electrolytic type, good resistance to sparks and electric arcs is required from the material constituting the insulator. This is why porcelain, elastomers, synthetic resins or other similar materials are used to make the insulator.

Furthermore, the active parts of the electrical apparatus, constituted in the case of transformers of magnetic circuits, of low and high voltage windings, are coated by molding with solid synthetic resin, the assembly forming the body of the electrical apparatus. . Thus, this device is divided into two parts, one formed by all of the parts in contact with the high voltage, the other part being made up of all the parts in contact with the ground at ground potential. . The naturally insulating surface of the body of the electrical appliance will be subjected to the electric field prevailing between the high voltage part and the low voltage part connected to earth. It will therefore be the seat, like the insulator, of rampant electric shocks which, in the long run, can degrade the device and cause its destruction.

Indeed, the different regions of the surface of an insulating material are brought to unpredictable potentials, very difficult to determine, which depend in particular on the electric fields existing in the vicinity. When, as in the case of high-voltage electrical transformers, there are intense electric fields, these reverberate on the surface of the insulating materials without it being possible to determine a priori the places, hence the formation of discharges wild electrics which appear randomly on the surface of insulating materials and amplify under the effect of atmospheric humidity and pollution of the insulator.

This serious problem which is likely to destroy the electrical appliance could be solved by covering the body of the electrical appliance with a conductive metallic deposit. It is generally a relatively thick and solid layer of hot sprayed metal such as zinc, in order to form a conductive layer which equalizes the potential of the covered surface. The conductive layer is connected either to the earth potential or to the high-voltage terminal according to the chosen arrangement. In this way, the surface potentials are fixed and the influences of the external electric fields are dominated. The risks of creeping electrical sparks are completely eliminated. In addition, the conductive layer must be able to flow the electric current due to a possible arc along the insulator. Indeed, the latter then undergoes the effect of concentration of the electric field high voltage line / earth and therefore, it is at this level that measures must be taken to reduce the risk of accidental breakage, in the event of overvoltage instantaneous, external or internal insulation, by creating an arc and simultaneously to prevent the formation of ionizing electric discharges inside and in the vicinity of the insulator.

Under these conditions, the conductive layer, thin despite everything, ends in a sharp edge facing the insulator which will generate sparks in service and arcs in the event of overvoltage on the network, while the leakage currents along the insulator, which will arise on this edge, will gradually degrade it by eating away at the metallic layer.

The main object of the present invention is to present an improved apparatus in terms of insulation. In this regard, improvements have been made at two levels. On the one hand as regards the outside of the terminal, the surface of the head of the device will no longer be the seat of the formation of creeping sparks and therefore, its longevity is ensured. Furthermore, thanks to special constructive arrangements, the distribution of the electric field between the parts under voltage and ground inside the insulator is fully controlled to avoid any internal ionizing electrical discharge.

Other objects and advantages of the present invention will appear during the description which follows which is however given only for information.

The high voltage connection terminal for electrical devices such as a current or voltage transformer, comprising at least two parts, one of which, the head, is connected to a high voltage line and the other, the base, with earth , both separated by a tubular insulator which encloses a conduit where the conductors connecting the high-voltage connection piece to the internal circuit of the device are placed, is characterized by the fact that the head is covered with a metallic layer and that a metal ring is interposed between said head and the insulator.

• la figure 1 schématise en vue de coupe la structure interne de la borne haute tension de raccordement selon la présente invention,
• la figure 2 représente en vue de détail la liaison entre l'anneau et la broche haute tension.
• la figure 3 schématise en vue de coupe la structure interne d'une borne haute tension de raccordement à montage inversé.

The invention will be better understood on reading the following description, accompanied by attached drawings, among which:

• FIG. 1 shows diagrammatically in section view the internal structure of the high-voltage connection terminal according to the present invention,
• Figure 2 shows in detail the connection between the ring and the high voltage pin.
• Figure 3 shows schematically in section the internal structure of a high-voltage connection terminal with reverse mounting.

The present invention relates to a high voltage connection terminal for electrical devices, such as dry current or voltage transformer. It mainly concerns manufacturers of electrical equipment.

Although the present invention has been developed more particularly in the context of the construction of medium voltage current transformers, it may extend to all medium and high voltage devices comprising members placed in an insulating casing or coated with insulating resin.

The high voltage connection terminals are electrical components intended to be connected to an external high-voltage line. In the example of the current transformer taken here, all of the electrical components, that is to say magnetic core and high and low voltage winding, are embedded in a molded resin which is in the form of a head (1) in Figure 1. The head (1) is crossed by a pin (2) connected directly to the external high voltage line. The pin (2) is fixed on a hollow insulating support (3) which encloses the various electrical components. This constitutes the first part of the device which is brought to the high voltage potential.

The other part of the high-voltage terminal is formed by the fixing base (4) and which is brought to earth potential. The low voltage output terminals of the device are fixed to the base (4).

The head (1) and the base (4) are separated by a tubular insulator (5) with fins which encloses a conduit (6) where the conductors, not shown, of electrical connection are placed. A significant potential difference corresponding to the supply voltage of the high-voltage electrical device exists between the head (1) and the base (4).

The fin insulator (5) must have a sufficient creepage distance to prevent the passage of sparks due to the electric field prevailing between the head (1) and the base (4). When it rains or when more or less conductive atmospheric deposits form on the electrical device, good resistance to creeping sparks and electric arcs is required of the material constituting the insulator. These electrolytic conduction phenomena are very formidable and cannot be totally eliminated. Particular care must be taken in the choice of material used for the manufacture of the fin insulator (5) so that it cannot be destroyed by the action of either electric arcs or creeping sparks. For example, it is desirable to use porcelain, elastomer, synthetic resins or other materials of a different nature from that used to form the coating of the active parts of the device.

These same active parts which, in the case of transformers, consist of magnetic circuits, low and high voltage windings, are coated by molding with a solid synthetic resin, the assembly forming the body of the device. In this way, the surface of this naturally insulating body will be subjected to the action of external electric fields. By definition, the potentials of the different surface areas of these insulating materials will be floating and will vary depending on the construction geometries, the proximity of live conductors and also atmospheric conditions. Under these conditions, we will see on the surface of insulating materials totally uncontrollable electric fields. In certain circumstances, they may give rise to ionizing discharges which, in the long run, can seriously damage the device.

Specifically, to combat this kind of damage, the manufacturers cover the body of the device with a relatively thick and solid metallic deposit. Thus, the insulating surfaces are eliminated and the superficial electric fields disappear. These conductive layers are connected either to the high voltage potential or to the ground potential, in order to precisely define the live parts and the grounded parts and thus precisely locate the electric fields. In practice, the surface metal deposits consist of a projection of hot zinc onto the resin to form a conductive layer equalizing the potential of the covered surface.

In the example chosen from Figure 1, the outer part of the head (1) is covered with a conductive layer (7). This conductive layer (7) is connected to the potential of the pin (2) and therefore to the high voltage line. Under these conditions, the electric field in the outer part of the terminal is strictly limited to the part between the upper and lower ends of the fin insulator (5). The thickness of the conductive layer (7) must be sufficient to withstand the flow of an electric leakage current originating between the pin (2) and the base (4).

This provision gives satisfaction to users at first. Indeed, the conductive layer (7) is perfectly able to meet the requirements and in the event of accidental arcing, the creeping spark travels the outer surface of the fin insulator (5) from the bottom of the layer (7), that is to say the crown for supporting the head (1) on the fin insulator (5), up to the base (4). Specifically, the surface layer (7) stops at the sharp edge (8) of the head support ring (1) on the insulator (5). The sharp edge promotes the formation of sparks in service and the creation of arcs in the event of overvoltage on the network. Leakage currents along the insulator (5) will also arise at this edge (8) and will gradually degrade it by gnawing the metal layer (7) at this level.

According to the present invention, a ring (9) is interposed between the head (1) and the insulator (5) as shown in Figure 1. Under these conditions, the sharp edge (8) of the support ring of the head (1) rests on the metal ring (9) and it will be precisely this metal ring (9) which will be the part closest to the base (4) and where the maximum electric field will be concentrated. If arcs are created, the spark will arise at the level of the ring (9) to go to the base (4) and the sharp edge (8) of the head (1) will be completely protected .

Figure 2 shows in detail the establishment of the ring (9) at the top of the insulator (5). This metal ring (9) has a profile with rounded edges. This limits the creation of electric arcs which seek, to take birth, angular or pointed forms.

The outer diameter of the ring (9) is greater than that of the head support ring (1). This makes it possible to protect the sharp edge (8) of the metal layer (7) in the event of the creation of an electric arc whose trajectory must pass through the ring (9).

To ensure that the ring (9) is at high voltage potential and can flow a current important, advantageously the ring (9) is connected to the pin (2) by a braid (10) or other conductive element as illustrated in FIG. 2.

Care must also be taken not to create or at least encourage ionizing discharges which can cause arcing inside the high voltage connection terminal. For this, one solution consists in interposing an insulating screen (11) between the conduit (6) and the ring (9).

The ring (9) can also be extended inside the insulator (5) by a cylindrical surface (12) as illustrated in FIG. 1 terminated by a flourishing (13).

We can also set up an electric insulating cylinder (14) extending the insulating screen (11) in the inner part of the insulator (5), which is interposed between the conduit (6) and the ring (9) provided with its extension (12).

To seal and in particular prevent communication to the outside of a liquid or gaseous dielectric, bathing the interior of the insulator, a series of seals (15) is put in place between the insulating cylinder (14) and the insulating part (16) of the base (4). The two previous parts are coaxial, which allows the part (14) to slide into the part (16).

We can also install an O-ring in the ring (9) to seal the gas or insulating liquids between the head (1) and the insulator (5).

In the example of the previous figures, the body of the device containing the various electrical components such as high voltage, low voltage and core winding, was enclosed in the body of the device, i.e. the head ( 1) molded resin placed at the top of the device. However, it is perfectly possible to envisage the reverse construction, as illustrated in FIG. 3 in which the head (1) containing the various electrical members is located in the lower part of the device. In this case, the high voltage connection pin (2) is replaced by studs (17 and 18) fixed on an insulating support (19) placed at the top of the insulator (5) and connected to the head (1) by electrical conductors introduced in the central connection conduit (6) .

The magnetic cores, primary and secondary windings, are embedded in a molded case (3) placed in the head (1) which is covered with a conductive layer (7) connected to earth.

To avoid that arcs can arise between the high voltage studs (17 and 18) and the external surface (7) of the head (1), a ring (9) is interposed between the insulator (5) and the head (1) at the base of said insulator, as illustrated in FIG. 3. This ring (9) is set to earth potential and therefore forms a protective screen against the conductive film (7).

Other implementations of the present invention, within the reach of ordinary skill in the art, could also be envisaged without departing from the scope thereof, as defined by the claims.

Claims (9)

1. Medium- or high-voltage device which includes a head (1) which contains the windings and the magnetic core located in an insulating support (3), said device including at least two parts, one of which is connected to the high voltage and the other one is earthed, both being separated from each other by a tubular insulator (5) with fins which contains a duct (6) in which are located the leads for linking the head (1) with the external portions of the device, characterized in that the insulating support (3) of the head (1) is covered with a conductive metal layer (7) at the external portion and in that a metal ring (9) is interposed between the head (1) and the insulator (5).
2. Device according to claim 1, characterized in that the ring (9) has a profile with rounded edges.
3. Device according to claim 2, characterized in that the ring (9) has an outer diameter larger than that of the crown supporting the head (1) on the insulator (5).
4. Device according to claim 1, characterized in that the ring (9) is connected, through a braid or another conductive element, to the high-voltage portion, in particular to the connecting pin (2).
5. Device according to claim 1, characterized in that a screen (11) is interposed between the ring (9) and the duct (6).
6. Device according to claim 1, characterized in that the ring (9) is extended, inside the insulator (5), by a cylindrical surface (12) ended by a widening (13).
7. Device according to claim 1, characterized in that a seal is placed in a groove provided for in the metal ring (9).
8. Device according to claim 1, characterized in that an insulating cylinder (14) is interposed, inside the insulator (5), between the duct (6) and the ring (9) and its extension (12).
9. Device according to claim 8, characterized in that the insulating cylinder (14) has seals (15) at the level of its connection by fitment onto the insulating portion (16) of the base (4).

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