WO2008027006A1

WO2008027006A1 - High voltage bushing

Info

Publication number: WO2008027006A1
Application number: PCT/SE2007/050598
Authority: WO
Inventors: Tommy L. Larsson
Original assignee: Abb Technology Ltd
Priority date: 2006-08-31
Filing date: 2007-08-30
Publication date: 2008-03-06
Also published as: CN101506911A; CN101506911B

Abstract

A high voltage DC bushing with a grounding flange has an external housing (13) with flange portions (13a) that extend at an angle pointing away from the grounding flange.

Description

HIGH VOLTAGE BUSHING

FIELD OF INVENTION

The present invention relates generally to high voltage DC bushings and more particularly to a high voltage bushing having an improved shed profile. The invention also relates to a high voltage transformer comprising such high voltage bushing.

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

Conventional bushings are constituted by an insulator made of ceramic or composite material, which is provided with sheds or flange portions 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, which is disclosed in the European patent publication EP 1 014 388 A2 will now be briefly described with reference to Fig. 1. The bushing comprises an insulating and supporting body 2 of a polymeric material enclosing an internal high voltage conductor 1. An insulator 3, usually made of silicone rubber, is molded on the insulating and supporting body 2 to obtain the chosen creepage distance, therefore ensuring high electrical strength characteristics even in highly polluted environments. The insulator is provided with a plurality of sheds or flange portions 3. In order to avoid accumulation of polluting material and to allow water to flow off the insulator, the shed flange portions are provided extending at an angle below horizontal from the supporting body. This, however, in DC applications results in a non-optimal electrical DC field, resulting in DC stress on the top of the shed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high voltage DC bushing and transformer with reduced DC stress on the bushing.

The invention is based on the realization that with sheds or flange portions extending from the supporting body of the DC bushing at an angle above horizontal, the flange portions will gentler follow the electrical DC field, resulting in less DC stress on the top of the flange portions .

According to a first aspect of the invention, a high voltage DC bushing is provided comprising an insulating and supporting body; a grounding flange (14) provided on the insulating and supporting body; an external housing arranged on the insulating and supporting body, wherein the external housing is provided with a plurality of flange portions; the bushing being characterized in that the flange portions extend from the supporting body at an angle from the longitudinal axis and pointing away from the grounding flange.

According to a second aspect of the invention, a high voltage DC device comprising such a bushing is provided. With the inventive bushing, the DC stress on the flange portions will be less as compared with conventional prior art bushings .

In a preferred embodiment, the flange portions extend at an angle in the interval of 0°< OC <90°, and more preferably in the interval of 75° < CC < 85°.

In a further preferred embodiment, the high voltage DC bushing comprises a grounding flange about midways along the longitudinal axis of the bushing, enabling wall mounting. The flange portions then preferably extend in different direction on the different sides of the grounding flange.

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 detailed view of a bushing according to the invention;

Fig. 3 is an overall view of a high voltage DC device provided with bushings of the kind shown in Fig. 2; and

Fig. 4 is a view of an alternative embodiment of a bushing according to the invention mounted to a wall.

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, references will be given to directions and orientations, such as horizontal. It will be appreciated that these reference are applicable during normal operation. Also, 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 1200 kV, are envisaged in the near future.

A prior art high voltage bushing arrangement has been described in the background section with reference to Fig. 1 and this figure will not be further discussed herein .

In Fig. 2, there is shown an enlarged view of the upper portion of a bushing according to the invention, wherein in this figure an example of the electrical DC field potential is shown as equipotential curves during DC operation .

The bushing, generally referenced 10, comprises an insulating and supporting body 12 of a polymeric material enclosing an internal high voltage conductor (not shown) . An external housing in the form of an insulator 13 made of relative low resistive silicone rubber is molded on the insulating and supporting body 12. The insulator comprises a plurality of flange portions or sheds 13a, which extend from the supporting body at an angle above horizontal .

The equipotential lines are essentially following the direction of the sheds 13a due to the electrically conductive nature of the material of the insulator 13. In the air outside the insulator 13, the equipotential lines extend generally more vertically. This means that there is a bend or curve in these lines.

On high voltage bushings it is important to control the electrical stress in air since excessive stress in air can attract charges radically changing the voltage distribution. By providing the insulator 13 with sheds 13a extending out from the supporting body 12 at an angle above horizontal, the sheds will follow the general more vertical extension of the equipotential lines better compared to if the sheds should extend at an angle below horizontal .

In Fig. 2, the angle at which the sheds 13a extend is denoted CC . In other words, the angle CC is the angle of the direction in which the sheds 13a extend and the longitudinal axis of the bushing 10 in the "up" direction. With the previously given definition, "up" is in the direction of increasing potential.

This angle CC is preferably the same angle as the equipotential lines in air but for outdoor applications slightly less than the bushing mounting angle to assure water drainage. It is preferred that the angle CC is in the interval of 0 ° < CC <90°, and more preferably in the interval of 75° < CC <85°.

Since the sheds 13a of the described bushing can gather for example rain water on the insulator, the above- described vertical bushing is preferably arranged to operate indoors. If the bushing is tilted so that for example rainwater on the shed can drain, the bushing can be used both indoor and outdoor. Since the bushing in Fig. 2 is oriented in an upright position, the definition of "horizontal" is straightforward. This definition should be related to the orientation of the bushing, not to the orientation of the ground. Thus, in case the bushing is tilted, "horizontal" is the direction of a plane extending perpendicularly to the longitudinal axis of the bushing. "Up" is the direction from the grounded portion of the bushing and towards an end of the bushing, i.e., in the direction of increasing voltage potential. This will be further discussed below with reference to Fig. 4.

A high voltage DC device in the form of a transformer 20 provided with bushings 10 of the above-described kind is shown in Fig. 3. The bushings are electrically connected to internal components of the transformer, such as transformer core and windings. Since the bushings in Fig. 3 are arranged essentially vertically, this transformer is preferably provided indoors.

An alternative embodiment of a high voltage DC bushing according to the invention will now be described with reference to Fig. 4. This bushing, generally referenced 10', is mounted so that it extends through a wall 30 of a building. Thus, this bushing 10' exhibits two air sides, one indoors to the left in the figure and one outdoors to the right in the figure.

A grounding flange 14 provided about midways along the longitudinal axis of the bushing functions as a grounding means. With the above given definition of the angle OC, it is realized that this angle is calculated in different directions on the different sides of the grounding flange. On the indoor side of the bushing, the direction "up" is pointing essentially to the left while on the outdoor side of the bushing, the direction "up" is pointing essentially to the right. This means in turn that the sheds 13a point in different direction on the two sides of the wall 30.

It is not necessarily so that the angles CC on the different sides of the wall 30 are the same; it could be so that the environment calls for different shed angles indoors and outdoors .

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, the exact shape of the sheds can vary as long as the flange portions extend in a direction above horizontal .

All flange portions 13a of the described embodiment extend in essentially the same direction. It will be appreciated that different sheds 13a can extend at different angles CC to better follow the equipotential lines in air.

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 thing, such as a reactor, breaker, generator, or other device finding an application in high voltage systems.

Claims

1. A high voltage DC bushing comprising:

- an insulating and supporting body (12) having a longitudinal axis;

a grounding flange (14) provided on the insulating and supporting body;

an external housing (13) arranged on the insulating and supporting body, wherein the external housing is provided with a plurality of flange portions (13a);

characteri zed in that

the flange portions extend from the supporting body at an angle (CC) from the longitudinal axis and pointing away from the grounding flange (14) .

2. The high voltage DC bushing according to claim 1, wherein the angle (OC ) is in the interval of 0 ° < OC <90° .

3. The high voltage DC bushing according to claim 2, wherein the angle (CC) is in the interval of 75° < CC <

85° .

4. The high voltage DC bushing according to any of claims 1-3, wherein the bushing is arranged to operate indoors .

5. The high voltage DC bushing according to any of claims 1-4, wherein a grounding flange (14) is provided about midways along the longitudinal axis of the bushing.

6. The high voltage DC bushing according to claim 5, wherein flange portions (13a) extend in different direction on the different sides of the grounding flange

(14) .

7. The high voltage DC bushing according to any of claims 1-6, wherein different flange portions (13a) extend at different angles (CC) to better follow the equipotential lines in air.

8. The high voltage DC bushing according to any of claims 1-7, wherein the insulator (13) is made of silicon rubber .

9. A high voltage DC device comprising a bushing according to claim 1.

10. The high voltage DC device according to claim 9, wherein the device is provided indoors.