HIGH VOLTAGE CABLE CONNECTION

FIELD QF INVENTION The present invention relates generally to high voltage cable connection for connection of a high voltage cable to a high voltage oil immersed device containing insulating liquid.

BACKGROUND It is known that oil immersed high voltage devices normally connected to the cable system by cable terminations to air and an oil to air bushing on the device or with an oil insulated cable box and a rather complicated cable to oil interface. On lower voltage also rubber/epoxy/oil interfaces or even simpler is used.

High voltage 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.

The present invention seeks to provide a high voltage cable connection which has good dielectric and thermal properties, which contains few parts and is easily adapted to different applications.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a cable connection as specified in claim 1.

The invention is based on the realization that a cable connection with a grounded shielding tube instead of a oil insulated cable box and complicated oil interface can be used in applications wherein part of the device is submerged in oil. This is the case in for example in connections to transformers, which are submerged in transformer oil in a transformer tank.

With the inventive cable connection, several advantages are obtained. By using a shielding tube, the cable connection can be made completely dry, i.e., it contains no oil. Also, it has been shown that the electric field pattern in the cable connection is almost identical for both AC and DC applications, making the connection device suitable for both AC and DC.

In a preferred embodiment, an insulating gas, such as SF6 or N ₂ or mixtures thereof, is used as insulating medium inside the part of the cable connection that is connected to the high voltage device. This provides good thermal properties due to the insulating gas and the open design allowing the gas to circulate inside the cable connection.

The invention makes it possible to use a conventional GIS cable termination. An advantage is that a completely dry arrangement can be made with very few parts in a compact design.

All grounded parts can be made as one part if the detail design shows that this is advantageous.

The design will on the device side work similar in both DC and AC applications, this due to the material

relations (between SF6, epoxy, oil) for both permeability and resistivity are similar.

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 drawing, in which the sole Fig. 1 is a sectional view of a high voltage cable connection mounted to a high voltage device.

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.

Also, in this description the term "voltage grading shield" should be read to exclude condenser cores conventionally found in bushings arranged to be submerged in insulating liquid.

Reference is now being made to the figure.

The connection device, generally designated 1, comprises a high voltage conductor 2 that extends through the center of a hollow gas filled cable connection insulator

4a, 4b that forms a housing around the high voltage conductor. The cable connection has two sides, a first side or air side outside the high voltage device to which the cable connection is mounted, and a second side or transformer side submerged in an insulating liquid in the high voltage device to which the cable connection is fitted, in the present example a transformer, generally- designated 20. The transformer contains insulating liquid 22, such as transformer oil, which is enclosed by a tank, designated 24.

The air side of the transformer cable connection is mainly consisting of the high voltage conductor 2 and an air side insulator 4a separating the gas inside the cable connection from the surrounding air. Further, the transformer side of the cable connection is separated from the oil 22 in the transformer by a transformer side insulator 4b.

The insulator, which is preferably made of a composite material, such as epoxy, but could also be made of porce- lain, thus comprises two portions: an air side insulator 4a on the air side of the cable connection and a transformer side insulator 4b on the transformer side of the cable connection.

A flange 6 is provided to electrically connect the housing of the cable connection to ground through the tank 24 of the transformer 20.

A so-called throat shield or voltage grading shield in the form of a concentric grounded tube 8 is provided inside the hollow cable connection insulator 4a, 4b around the portion of the cable connection going into the

tank 24. This shield 8, which is made of a suitable conductive material, such as aluminum, accomplishes grading of the electrical field in the connection device and is used instead of a condenser core. The voltage grading shield 8 is surrounded by the insulating gas, such as SF6 or N ₂ or mixtures thereof, which is provided in the space 10a inside of the air side insulator 4a and the space 10b inside of the transformer side insulator 4b. It is preferred that these two spaces 10a, 10b are in communication with each other to provide circulation of the insulating gas, thereby improving cooling of the transformer side of the cable connection 1.

In DC applications, the inside of the transformer side insulator 4b, i.e., the surface of the transformer side insulator facing the insulation gas inside the insulator, may be covered with a dielectric material (not shown) with a relatively low resistivity, such as silicone rubber, composite material or varnish. Since the resistivity of silicone rubber is almost of the same order of magnitude as that of the oil inside the transformer, improved electric field distribution is obtained. This layer minimizes internal radial field stresses in the transformer side insulator 4b separating the gas in the cable connection 1 from the oil 22 in the transformer 20 and provides a smooth grading of the potential along the transformer side insulator 4b between the high voltage and the grounded flange 6 and increases thereby the dielectric strength of the insulator 4b.

Optimal performance is obtained by a geometrical design of the transformer side insulator 4b. In the preferred embodiment, the transformer side insulator has an essentially frusto-conical shape. This could be

supplemented by the thickness of the coating on the inside of the connection device or the thickness of the insulator 4b housing. In order to further improve the performance, the thickness of the coating can vary along the transformer side of cable connection.

A shielding ring 12 provided at the end of the transformer side of the cable connection and a corresponding barrier system 26 in the transformer connection can further enhance the performance.

In both AC and DC applications, in order to achieve a smooth grading of the potential along the transformer side insulator 4b between the high voltage and the grounded flange, the geometry of the transformer side insulator 4b is optimized. Also, in DC applications the geometry of the barrier system 26 in the transformer is taken into account when optimizing the cable connection.

A preferred embodiment of a high voltage cable connection and a high voltage device according to the invention has 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 cable connection is attached has been described as a transformer, it will be appreciated that this could be other devices containing insulating liquid, such as reactors or breakers.

The inventive cable connection has been described as an air-oil connection device, i.e., wherein the first side of the connection device is surrounded by air outside a transformer, for example. It is realized that this first

side can be provided in other environments, such as in gas in a gas-oil connection device.

The transformer 20 has been described with a barrier 26. It is realized that this barrier is optional.