TITLE

A high voltage corona grading ring and a high voltage device comprising such ring

TECHNICAL FIELD

The present invention relates to a high voltage corona grading ring for use with a high voltage device.

The invention also relates to a high voltage bushing comprising a bushing insulator.

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 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. The voltage grading can be obtained with or without a condenser body through which the electrical conductor passes.

An example of a bushing adapted for use with a high voltage DC transformer will now be described with reference to Fig. 1 showing the overall structure of the bushing, referenced 1 , and Fig. 2 showing a high voltage transformer 100 provided with bushings of Fig. 1 , which are electrically connected to a transformer housing 102.

A high voltage conductor (not shown) runs through the center of a hollow bushing insulator 2 that forms a housing around the high voltage conductor. A flange 4 is provided to connect the housing of the bushing to ground through a tank assembly housing or a wall. A ground potential grading shield (not shown) may be mounted to the flange.

The bottom end portion of the high voltage conductor forms a bottom contact 6, which is arranged to be connected to the internal components of the transformer. An upper outer terminal (not shown) is provided at the end of the bushing opposite the bottom contact end in order to electrically connect the transformer device to external sources.

A corona grading ring 10 is arranged radial outside of the outer terminal. This corona grading ring is provided to eliminate or at least minimize corona appearing close to the outer terminal. The existence of corona - partial discharge in air - generates ozone, which is highly corrosive. The combination of corona, moisture, and time will cause rapid decay of the equipment. Also, corona may cause sparking and radio interference. By providing an electrically conductive corona grading ring radial outside of the outer terminal, the risk of corona appearing is decreased.

A high voltage device 100, such as a high voltage transformer, provided with a plurality of bushings 1 is shown in Fig. 2.

Grading rings are normally made of aluminum or another type of electrically conductive metal. Small particles on such rings can lead to discharge activity, although the curvature of the grading ring itself should guarantee a more or less partial discharge (pd) free operation

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved high voltage corona ring for use with a high voltage device which decreases the susceptibility for discharge.

According to a first aspect of the invention a high voltage corona grading ring according to claim 1 is provided.

According to a further aspect of the invention, a high voltage bushing as defined in appended claim 8 is provided.

With the inventive arrangement, several advantages are obtained. When applying a layer having a reduced conductivity or being semi-conductive, the field enhancement of an external particle on such a grading ring would be reduced compared with a ring without such a layer as it will reduce the potential of the particle due to a potential drop in the layer. This reduced sensitivity for external particles leads to a reduction of discharge activity in the ambient atmosphere of the ring and is especially of interest in outdoor conditions, where small insects and other air born particle easily can be attached on the surface of the grading ring. This is especially, but not only, important for DC applications at higher voltage levels and larger rings.

According to a preferred embodiment, the layer in question can for instance be a paint with a certain fixed or varying resistivity, being significantly higher than the underlying body of the ring, but significantly lower than the ambient atmosphere, which in most cases is air.

In a preferred embodiment, the electrically conductive layer on the high voltage corona grading ring is an electrically conductive paint, which is easy to apply in a manufacturing process. The electrically conductive paint is by way of example a paint comprising carbon.

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 high voltage bushing provided with a corona grading ring;

Fig. 2 shows a high voltage device, such as a transformer, provided with corona grading ring bushings; and

Fig. 3 is by way of example a sectional view of an embodiment of a corona grading ring according to the invention;

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

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 already built or envisaged in the near future.

The present invention is applicable to the general description of the high voltage bushing arrangement 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. 3, showing a schematic sectional view of an embodiment of a corona grading ring according to the invention, wherein part of the hollow bushing insulator 2 and the upper outer terminal 8 are shown together with part of the corona grading ring 10. The corona grading

ring is circularly cylindrical or toroid shaped like prior art corona grading rings and a left part 1 Od thereof has been omitted in the figure for improved clarity.

The corona grading ring 10, which is electrically connected to the insulator 2 by means of for example brackets and screws, comprises a thin shell shaped body portion, referenced 10a made by way of example of aluminum or some suitable electrically conductive metal. The cross-sectional shape of the body portion is essentially circular and the space 10b inside of the shell shaped body portion is left empty to be filled with air.

The outer surface of the body portion is by way of example at least partially provided with an electrically semi-conductive layer 10c, shown with a dotted line in Fig. 3.

The layer can be provided as an electrically semi-conductive paint, preferably electrically conductive carbon paint. Alternatively, the paint is a semi-conductive paint comprising silver, for example as silver glass particles. Silver is highly conductive and has excellent anti-corrosive parameters. Preferred embodiments of a high voltage corona shield according to the invention and comprised in a bushing for a high voltage device has been described. A person skilled in the art realizes that this could be varied within the scope of the appended claims. Thus, the inventive corona shield is not limited to the configuration shown in the figures.

Although the high voltage device has been described as a transformer, it will be appreciated that this device can be anything else, such as a reactor, breaker, generator, or other device finding an application in high voltage systems. It is realized that the high voltage device also can be a wall of a building.