TITLE

A high voltage insulation system and a method of manufacturing same.

TECHNICAL FIELD

The present invention concerns a high voltage insulation system for high- voltage direct current, comprising a bushing with a conductor, a connection to a transformer conductor, a conductive shielding electrode shielding the connection between the bushing and transformer and a surrounding insulation system immersed in transformer oil,

The invention also refers to a method of manufacture a high voltage system.

BACKGROUND ART

The current connection between transformer bushing and transformer/reactor in an HVDC (High Voltage Direct Current) converter transformer or smoothing reactor is usually protected by an insulation system.

A high voltage insulation system for bushing connections of HVDC transformers and smoothing reactors is for example known from the European Patent No. 0285895. The patent discloses a bushing with its conductor connected to the transformer conductor inside a screen (a shielding electrode).

The current connection inside the shielding electrode is enclosed by solid insulation barriers situated in the transformer oil, which makes up the enclosing insulation system.

The method of increasing electrical withstand strength against AC stress in transformer oil by subdividing the oil volume around an electrode is also well known.

According to a first aspect the present invention seeks to provide an improved insulation system for very high voltages. According to a second

aspect the invention seeks to provide an improved method of manufacturing such a system.

SUMMARY OF THE INVENTION

These and other objectives have, according to the first aspect or the invention, been achieved by an insulation system as described in the in the characterizing part of claim 1 .

Further preferred embodiments of the invention are described in the subclaims 2 - 6.

An objective according to the second aspect of the invention has been achieved by a method of manufacture a high voltage insulation system according to the characterizing part of claim 7.

The present invention thus relates to a design of an insulation system for bushing connections in HVDC converter transformer and smoothing reactors, which combines two insulation structures, one cylindrical barrier enclosing the bushing, the transformer side and the bushing connection shielding electrode and a barrier system fastened in the shielding electrode itself.

Further, the invention relates to a method to manufacture an insulation system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the schematic design of the insulation system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is, by way of example, described in the following with reference to the attached drawing, where 1 is a bushing with a conductor 2 and a bushing insulation 3. The conductor 2 connects to a transformer conductor 4 at a connection 5. Transformer insulation 6 is arranged outside the transformer conductor 4. A conductive shielding electrode 7 is shielding the connection 5. The entire bushing connection is enclosed with a cylindrical solid insulation barrier 8, which encloses the bushing 1 , the shielding electrode 7 and some of the transformer side insulation material 6. 9 is a grounded turret wall and 10 is a grounded bushing flange. The insulation system is transformer immersed in transformer oil 1 1 or dielectric fluid with similar properties..

The bushing 1 connects to the transformer inside the shielding electrode 7. In accordance with the invention, the shielding electrode is provided with a barrier system fastened on it, which consists of solid insulation barriers 12. As illustrated in the drawing, the solid insulation barriers 12 are arranged radially outwards from the shielding electrode 7 with a distance 13 in between each barrier 12.

According to an embodiment, the solid insulation barriers 12 extend in an axial direction outside the axial direction of the shielding electrode 7 and the insulation barrier 12 closest to the shielding electrode has an axial extension which is shorter that the adjacent insulation barrier 12.

The solid insulation barriers 12 fastened on the shielding electrode end at a substantial distance 75 mm - 200 mm, typically 80 mm, from the insulation material 3 of the bushing and insulation material 6 of the transformer, and are thus not in direct contact with solid insulation material on either side.

According to the invention, the barriers 12 fastened on the shielding electrode have the task to subdivide the oil volume close to the shielding electrode 7 into smaller oil volumes, which have a higher dielectric strength against AC stress than larger volumes of oil.

During DC stress, which arises due to the HVDC operation, the barriers 12 are subjected to DC stress themselves, the amplitude of which is determined

by how much the barriers constrain the leakage current from ground to high voltage in every direction.

Ground 10 is situated at the bushing flange and the turret wall 9, which means that currents to ground flow axially along the bushing and transformer side, as well as in radial direction through the solid insulation barrier system 12.

In the direction tangential to the bushing and transformer side, the barriers 12 are not constraining the current flow, which allows for a very small amplification of the stress in those directions, compared to the stress obtained if they were absent.

In the direction perpendicular to that, radial direction outwards, the concentration of voltage stress due to restrictions of the current flow induced by the barriers 12 on the shielding electrode 7 and the cylindrical barrier 8 are divided between the cylindrical barrier 8 and the shielding electrode barriers 7, which makes up a reasonable voltage stress on average in the solid insulation material.

The insulation system with design in accordance with the invention as described above therefore can combine a high AC-withstand strength close to the shielding electrode 7 with a rational handling of the DC stress by the cylindrical barrier 8.

The dimension of the cylindrical barrier 8 is depending on the DC voltage level, but is always enclosing the complete length of the bushing and has an overlap of several hundreds of millimetres with the transformer side, the length of which is determined by the DC voltage stress. The barrier 8 is made from solid insulation and oil, typically being the combination of oil ducts and solid pressboard.

The solid insulation barriers 12 fastened on the shielding electrode subdivide oil volumes that have an extension of 2 mm - 30 mm, preferably 3 mm - 20 mm per duct, and where the number of ducts 13 may vary from one to several, typically being two or three. In the embodiment shown in the drawing, the number of barriers is two, forming two ducts 13. The barriers 12

subdividing the oil around the shielding electrode are made of solid insulation, typically pressboard, with a thickness between 1 mm and 5 mm, typically being 3 mm thick.

One advantage of using the cylindrical barrier 8 is that the production of it is independent of the production of transformer side insulation material 6 and therefore can be handled in parallel to the production of the transformer itself. It also provides easy assembly in the production process and at site and simple insulation system solutions compared to for example European Patent No. 0285895, where plenty of complex insulation barriers have to be manufactured and assembled with great care.

Another property of the solution used in that prior art patent is that the barriers close to the shielding electrode have to be designed to withstand the full DC voltage, since it does not provide a free current path between the shielding electrode at high potential and ground.

The combination according to the invention of the cylindrical barrier 8 and the shielding electrode barrier 7 combined with solid insulation barriers 12 gives the opportunity to handle very high voltages (AC-strength increased by the shielding electrode barrier system and DC-stress handled by cylindrical barrier) while maintaining a rational production process with easy, parallel manufacturing and assembly.

The high voltage insulation system according to the invention is designed for very high voltages, such as AC/DC voltages over 500 kV, preferably 800 kV and up to 1000 kV.

Although favourable, the scope of the invention must not be limited by the embodiments presented but also contain embodiments obvious to a person skilled in the art. For instance the insulation system can be immersed in dielectric fluid with similar properties as transformer oil. Further, the insulation system principle is applicable for all voltage levels. Further, the insulation system could be used for HVAC transformers and reactors, since it inherently possesses the suitable properties for that.