WO2008027009A1 - High voltage dc bushing and high voltage dc device comprising such bushing - Google Patents

Abstract

The hollow insulator of a high voltage DC bushing comprises a composite layer (12a) and an electrically isotropic inner insulator (12b) provided on the inside of the composite layer.

Description

HIGH VOLTAGE DC BUSHING AND HIGH VOLTAGE DC DEVICE COMPRISING SUCH BUSHING

FIELD OF INVENTION The present invention relates generally to high voltage bushings and more particularly to a high voltage bushing having a controlled internal dielectrical stress. The invention also relates to a high voltage device comprising such bushing.

BACKGROUND

It is known that electrical equipment and devices, such as high voltage DC transformers, are usually equipped with bushings, which are suitable to carry current at high potential through a grounded barrier, e.g. a transformer tank or a wall.

Conventional bushings are constituted by an insulator made of ceramic or composite material, which is provided with sheds and is generally hollow, and on the inside can the voltage grading be performed with or without a condenser body through which the electrical conductor passes, allowing to connect the inside of the device on which the bushing is fitted to the outside.

An example of a prior art bushing adapted for use with a high voltage dc transformer will now be described with reference to Figs. 1-3, wherein Fig. 1 shows the overall structure of the bushing, generally referenced 1, Fig. 2 is an overall cross-sectional view of the bushing mounted to a transformer housing, and Fig. 3 is a detailed sectional view of the area enclosed by the dashed line in Fig. 2. A high voltage conductor 10 runs through the center of a hollow bushing insulator 12 that forms a housing around the high voltage conductor. Inside the insulator housing there is provided a condenser core 14 for voltage grading which is build up around the high voltage conductor 10. A flange 16 is provided to connect the housing of the bushing to ground through a tank assembly housing, schematically shown as 18 in Fig. 2. A ground potential grading shield (not shown) may be mounted to the flange.

The bottom end portion of the high voltage conductor 10 forms a bottom contact 20 arranged to be connected to a mating internal contact 22 provided in the transformer housing 18. An upper outer terminal 24 is provided at the end of the bushing opposite the bottom contact end in order to electrically connect the transformer device to an external source.

An isolation gas, conventionally SF6, is provided in the space 26 inside the hollow bushing insulator 12.

A detailed sectional view of the bushing insulator 12 shown in area enclosed by dashed lines in Fig. 2 is shown in Fig. 3. The insulator 12 comprises a fiber reinforced composite main layer 12a, which encloses the space 26 filled with isolation gas.

The fiber reinforced composite layer 12a can be manu- factured in different ways. One manufacturing method is so-called wet winding, which is a filament winding process wherein the strand is impregnated with resin just prior to contact with a mandrel. This method is commonly used for manufacturing fiber reinforced thermosetting matrix composite cylinders. This manufacturing method results in an electrically anisotropic composite layer, i.e., the electrical conductivity varies with the direction along the composite layer.

For use in DC applications, such as in bushings for HVDC applications, material resistivity in the fiber reinforced composite 12a of the hollow bushing insulator can be very anisotropic. This can give an electrical field concentration needed to be considered inside the composite layer 12a. This electrical field concentration may become critical in the design.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a high voltage DC bushing and a high voltage DC device wherein the DC field in the composite layer of the bushing insulator can be reduced and controlled.

The invention is based on the realization that by providing an isotropic layer having a relatively low resistivity on the inside of the composite layer of the hollow bushing insulator, the DC field in the composite layer is reduced and controlled.

According to a first aspect of the invention a high voltage DC bushing is provided comprising a hollow insulator housing having a longitudinal axis and comprising a main layer having a first resistance per axial length unit, and a high voltage conductor provided in the hollow insulator housing; the bushing being characterized by an inner layer provided on the inside of the main layer and having a second resistance per axial length unit being lower than the first resistance per axial length unit. According to a second aspect of the invention a high voltage DC device comprising such a bushing is also provided.

With the inventive bushing, several advantages are obtained. The DC field in the composite layer of the hollow bushing insulator is reduced, resulting in a more controlled voltage distribution in the product. This more controlled voltage distribution gives possibilities to reduce the overall size of the bushing. Furthermore, static electricity inside the insulator is reduced which in turn reduces the risk for introducing dust during production inside the bushing.

Further embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF DRAWINGS The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is an overall view of a prior art high voltage bushing;

Fig. 2 is a cross-sectional view of the bushing of Fig. 1 assembled to a transformer housing;

Fig. 3 is a detailed sectional view of the area enclosed by dashed lines in Fig. 2;

Fig. 4 is a detailed sectional view similar to that of Fig. 3 but showing a hollow insulator of a bushing according to the invention;

Fig. 5 is a view similar to that of Fig. 4 but showing an alternative embodiment of a bushing according to the invention; and Fig. 6 is a view similar to that of Fig. 4 but showing yet an alternative embodiment of a bushing according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following a detailed description of a preferred embodiment of the present invention will be given. In this description, the term "high voltage" will be used for voltages of 50 kV and higher. Today, the upper limit in commercial high voltage devices is 800 kV but even higher voltages, such as 1000 kV or more, are built or envisaged in the near future.

The present invention is applicable to the general description of the high voltage DC bushing given in the background section with reference to Figs. 1 and 2 and reference will in the following be made to these figures. However, reference will first be made to Fig. 4, showing a detailed sectional view similar to that of Fig. 3 but showing a hollow insulator of a bushing according to the invention. The insulator 12 comprises a fiber reinforced composite main layer 12a, which encloses the space 26 filled with isolation gas, see Fig. 2. An inner insulator layer 12b is provided on the inside of the composite layer 12a, i.e., on the surface of the composite layer facing the space 26 filled with isolation gas. The inner insulator layer 12b is preferably made of silicon rubber, composite material or suitable varnish. A preferred thickness range for the inner layer is 10-1000μm.

In order to balance the creation of DC fields in the composite layer 12a, the resistance per axial length unit of the inner insulator layer 12b shall be lower than the resistance per axial length unit for the composite main layer 12a and the outer insulator 12c.

Thus, by providing an insulator on the inside of the composite layer of the hollow bushing insulator, a bushing having reduced DC fields in the insulator is achieved.

The inner insulator 12b is preferably electrically isotropic but can also be electrically anisotropic as long as the resistance per axial length unit is lower than the resistance per axial length unit of the composite main layer 12a.

In an alternative embodiment shown in Fig. 4, an outer insulator 12c made of polymeric material, conventionally silicon rubber, is provided on the outside of the compo- site main layer 12a. This outer insulator 12c is provided for increasing creeping distance and for improving behavior in rain. This outer insulator layer 12c, which has relatively low resistivity, can contribute to an electrical field concentration needed to be considered inside the composite layer 12a. This electrical field concentration may become critical in the design, making the inner layer 12b even more important for decreasing DC stresses .

The inner layer 12b has been shown as a layer with uniform thickness. It could very well be so that this layer has a varying thickness along the axial direction of the bushing. An example thereof is shown in Fig. 6, wherein the thicker portions of the inner layer 12b are provided at highly DC stressed portions of the bushing. Preferred embodiments of a high voltage DC bushing and a high voltage DC device according to the invention have been described. A person skilled in the art realizes that these could be varied within the scope of the appended claims. Thus, although the high voltage device to which the inventive high voltage DC bushing is attached has been described as a transformer, it will be appreciated that this device can be other things, such as a reactor, breaker, wall, generator, or other device finding an application in high voltage systems.

Claims

1. A high voltage DC bushing comprising:

- a hollow insulator housing (12) having a longitudinal axis and comprising a main layer (12a) having a first resistance per axial length unit, and

a high voltage conductor (10) provided in the hollow insulator housing;

characteri zed by

an inner layer (12b) provided on the inside of the main layer (12a) and having a second resistance per axial length unit being lower than the first resistance per axial length unit.

2. The high voltage bushing according to claim 1, wherein the inner layer is electrically isotropic.

3. The high voltage bushing according to claim 1 or

2, wherein the inner layer is provided at electrically highly stressed portions of the hollow insulator housing (12) .

4. The high voltage bushing according to any of claims 1-3, wherein the inner layer (12b) comprises any of silicon rubber, composite material or varnish.

5. The high voltage bushing according to any of claims 1-4, wherein the inner layer has a thickness in the range of 10-1000μm.

6. The high voltage bushing according to claim 1, comprising an electrically isotropic outer layer (12c) provided on the outside of the main layer.

7. A high voltage DC device comprising at least one bushing, each of the at least one bushing comprising:

a hollow insulator housing (12) having a longitudinal axis and comprising

o a main layer (12a) having a first resistance per axial length unit, and

- a high voltage conductor (10) provided in the hollow insulator housing;

characteri zed by

an inner layer (12b) provided on the inside of the housing and having a second resistance per axial length unit being lower than the first resistance per axial length unit.