Transformer bushings are porcelain and contain dielectric oil similar to mineral oil. It may be classified in to condenser and non-condenser type. The condenser type bushing consists of oil impregnated paper insulation or resin bonded paper insulation with interspersed conducting layers. This type of bushing is usually made up of equal capacitance layers between the center conductor and ground flange. These capacitance layers provide equal voltage steps, which is a uniform voltage gradient. The non-condenser type bushing consists of solid core, alternate layers of solid and liquid insulation, homogeneous insulating materials or gas filled insulation. Bushings may be further classified as having capacitance taps or power factor taps. 

A high voltage bushing for a high voltage device containing insulating liquid, the high voltage bushing comprising:a hollow insulator housing comprising a first side insulator arranged to be provided outside of the high voltage device and a second side insulator arranged to be submerged in the insulating liquid of the high voltage device,a high voltage conductor provided in the hollow insulator housing; anda voltage grading shield provided between the high voltage conductor and the insulator housing.

The present invention relates generally to high voltage bushings and more particularly to a high voltage bushing partially submerged in an insulating liquid, such as oil. The invention also relates to a high voltage device comprising such bushing.

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 high voltage transformer 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 is the voltage grading performed with a condenser body comprising paper-oil or resin impregnated epoxy through which the electrical conductor passes, allowing to connect the inside of the device on which the bushing is fitted to the outside. Thus, the condenser core provides a smooth electric potential distribution between the high voltage and the grounded parts.

Common to transformer bushings with a condenser body is that the part of the bushing that is submerged in the transformer tank contains oil.

An object of the present invention is to provide a high voltage bushing which has good dielectric and thermal properties, which contains few parts and is easily adapted to different applications.

The invention is based on the realization that a bushing with a grounded shielding tube instead of a condenser core can be used in applications wherein part of the bushing is submerged in oil. This is the case in for example transformer bushings, which are submerged in transformer oil in a transformer tank.

According to a first aspect of the invention a high voltage bushing for a high voltage device containing insulating liquid is provided comprising a hollow insulator housing comprising a first side insulator arranged to be provided outside of the high voltage device and a second side insulator arranged to be submerged in the insulating liquid of the high voltage device, and a high voltage conductor provided in the hollow insulator housing; and being characterized by a voltage grading shield provided between the high voltage conductor and the insulator housing.

According to a second aspect of the invention a high voltage device comprising at least one such bushing is also provided.

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

In a preferred embodiment, an insulating gas, such as SF6 or N₂ or mixtures thereof, is used as insulating medium inside the part of the bushing 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 bushing.