The invention relates to a bushing for high direct voltages with a condenser body for field control of the connection between the bushing and a conductor connected to the bushing according to the precharacterising part of claim 1. The bushing is particularly designed for transformers which are connected to convertors in HVDC plants.

If in a vessel with transformer oil two energized electrodes are positioned at a certain distance from each other, at a certain voltage a flashover will occur between the electro­des. The flashover tendency may be minimized by inserting between the electrodes an insulator body which functions as a barrier.

Transformer bushings may comprise an upper insulator and a lower insulator of electric porcelain. At the joint between these there is a fixing flange which is connected to the transformer casing. In the centre of the bushing there is a tube on which is wound a condenser body to obtain a favou­rable electrical field distribution. The current can be con­ ducted through the tube or a flexible conductor drawn through the tube.

Condenser bodies for bushings are described in a number of patent specifications and publications of various kinds. In this connection, the following may, inter alia, be mentio­ned, namely EP-A-0 032 690 "Foil-insulated high voltage bushing with potential control", EP-A-0 032 687 "High-vol­tage bushing with layers of embossed insulating foils", EP-­A-0 051 715 "Safety device for high-voltage bushings", ASEA Journal 1981, Volume 54, No. 4, pages 79-84. Common and ty­pical for the design of the condenser bodies is that they have a central circular-cylindrical portion. From both ends this portion changes into outwardly-directed straight fru­strums of cones whose cross section areas have a decreasing radius.

A variant of the design of a condenser body is disclosed in GB-B-1 025 686, "Pothead for connecting oil-filled cables to transformers and other electrical apparatus". As above, the condenser body has a conical part terminating towards the transformer. However, towards the cable connection the con­denser body terminates in a cross section area which is equal to the cross section area of the circular-cylindrical portion.

Another variant of the design of a condenser body is disclo­sed in a MICAFIL publication MNJ 11/12 from June 1969, in which a so-called "Re-entrant type bushing" is described.

This bushing is also intended to be used only within the a.c. field. Electrically, it is built up in the same way as a conventional a.c. bushing with a condenser body made of oil-impregnated paper, bakelite paper or is impregnated with molded resin and has concentric layers of a conducting mate­rial. The principle of the manufacture is that the transfor­mer side of the body is first wound into an inward conical shape into a diameter where about 70% of the stress lies, whereupon the body is continuously wound into an outward co­nical shape into the final outer diameter with 0% of the stress. The advantage of such an embodiment is that a shor­ter bushing is obtained on the oil side. In addition, the shield may be omitted.

Power transformers which are used in convertor plants entail special problems from the point of view of insulation, which somehow have to be overcome in order to ensure a satisfac­tory function.

In high voltage direct current plants, so-called HVDC plants, there is often used at least one convertor per pole and station. Normally, also, each convertor comprises seve­ral bridges connected in series. One of the poles of one bridge is normally connected to ground and the other pole is connected to the next bridge, and so on, thus obtaining the series connection. The direct voltage potential of each bridge relative to ground is then increased according to the number of bridges which are connected between the respective bridge and ground.

Each bridge in the series connection is supplied with an al­ternating voltage from a separate transformer. With increas­ing direct voltage potential on the bridges relative to ground, the insulation on bushings and windings of the transformers which are connected to the bridges will also be subjected to an increasingly higher direct voltage potential with a superimposed alternating voltage. The insulation of these must therefore be dimensioned so that they are capable of withstanding the increasingly higher electrical stresses to which they are then subjected.

The increasing direct voltage potential leads to special problems which do not exist in transformers used for pure alternating voltage transformation.

For convertor transformers, the lower insulator of the bush­ing and the transition between the conductor of the trans­former winding and the bushing present areas of problems from the point of view of insulation technique. This is de­scribed, inter alia, in "Power Transmission by Direct Cur­rent", by E. Uhlmann, Springer Verlag 1975, pages 327-328.

The electric direct voltage field has a distribution diffe­rent from that of the alternating voltage field. The distri­bution of the direct voltage is mainly determined by the re­sistivity of the various insulating mediums present in the field. Though transformer oil, cellulose material and elec­tric porcelain are good electric insulators, a certain amount of electric current is flowing through these materi­als when subjected to an electric voltage. The relation bet­ween the resistivity of cellulose material and transformer oil is about 100. This means that when cellulose in series with oil is subjected to an electric voltage the cellulose is subjected to a considerably higher electric fields strength than the oil, which in turn, therefore, imposes de­mands for a sufficient amount of solid insulating material in order not to exceed the electric withstand strength. The field distribution as well as the direction of the field strength vector will thus be different from the case with alternating voltage. The current transport also entails a redistribution of charges in the insulating mediums used.

Because of the heavy dependence of the resistivity on moi­sture content, field strength, temperature, etc., the dis­tribution of direct current is difficult to predict. In ad­dition, the physical nature of the direct voltage, i.e. charge transport, charge, time-dependent behaviour, and so on, gives a picture of the insulation problems arising in connection with HVDC plants, which is very complex and dif­ficult to interpret. In "Space Charge and Field Distribution in Transformers under DC-stress" by U Gäfvert and E. Spicar, CIGRE Int. Conference on Large High Voltage Electric Sy­stems, 1986 Session, 12-04, the complexity of the direct voltage distribution is illustrated. As previously mentio­ned, problems have arisen at the connection between the transformer bushing and the conductor of the transformer winding. This has led to the removal of the lower insulator of electric porcelain on the bushing in order to manage the stresses at the HVDC terminal at the higher voltage levels.

No simple explanation of the above phenomenon has been pre­sented. However, there are reasons to suspect that the long surfaces which arise in connection with bushings for high voltages in combination with the direction of the field along the long surfaces are of importance in this conned­tion. Admittedly, also the alternating voltage field is di­rected along the surface of the lower porcelain, but its physical nature is different. One hypothesis is that the distribution of the direct voltage field runs the risk of becoming unstable and unevenly distributed along suffi­ciently long surfaces. Another interesting hypothesis is de­scribed in an article entitled "Effect of Duct Configuration on Oil Activity at Liquid/Solid Dielectric Interfaces" by R E James, F E Trick, R Willoughby in Journal of Electrosta­tics, 12, 1982, pages 441-447. In this article it is stated that increased charge transport at surfaces caused by turbu­lence and access to charge is the reason for low electric withstand strength.

One way of overcoming the above problems is disclosed in US-­A-539,209, "Barrier of condenser type for field control in transformer bushing terminals". In this case, the transfor­mer bushing comprises a lower insulator. To attain the de­ sired field control, a condenser type barrier is used which has internal cones which make contact, across a certain oil gap, with the outer conical part of the lower insulator of the bushing as well as with the conically formed insulation surrounding the conductor of the transformer.

The invention aims at designing a bushing of the above-men­tioned kind which withstand very high direct voltage stresses and enjoys relatively small outer dimensions.

To achieve this aim the invention suggests a bushing accord­ing to the introductory part of claim 1, which is character­ized by the features of the characterizing part of claim 1.

Further developments of the invention are characterized by the features of the additional claims.

As has been described above, the invention primarily relates to a transformer bushing with a condenser body for field control for transformers used in convertor plants. The task of the condenser body is to overcome the flashovers which - as it has proved - may arise in transformer bushing termi­nals. It is designed so as to function as a barrier with both capacitive and resistive control of the electric field and is dimensioned so that the condenser body withstands the electric field strengths occurring in this bushing and in particular in the sensitive region at the connection between the conductor of the transformer and the bushing.

It is assumed that the conductor which comes from the trans­former winding and is to be connected to the conductor of the bushing is surrounded by a conducting tube which is co­vered by wound electrical insulation. This insulation is formed, from the end of the conducting tube, as a straight frustrum of a cone with cross section areas with an increas­ing radius which than changes into a circular-cylindrical portion towards the transformer. The conductor of the bush­ing also often consists of a conducting tube.

The part of the condenser body which is situated on the air side of the transformer bushing is formed as a conventional condenser body. This means that, counting from the fixing flange of the transformer bushing, it has a circular-cylind­rical portion which changes into an outwardlydirected straight frustrum of a cone with decreasing diameter. Also other embodiments of this portion may be used.

The part of the condenser body which is covered by the in­vention, i.e. on the oil side of the transformer bushing, normally counting from the fixing flange of the bushing, is formed as a circular-cylindrical portion the end of which is provided with an inwardly-directed straight frustrum of a cone. The axial length of the circular-cylindrical portion which is located on the oil side of the bushing is largely adapted such that its end coincides with the transition from conical to circular-cylindrical portion of the insulation of the conductor coming from the transformer winding. The coni­city of the cone, which from that point is directed inwards, largely coincides (see, however, below) with the conicity of the insulation of the conductor of the transformer winding with space for an intermediate oil gap.

Such a design of a condenser body means that a conventional condenser body is integrated with a condenser type barrier. This causes the electric field to be controlled in the de­sired way while at the same time obtaining a shielding of the conductor of the transformer. In this way the condenser body for the bushing according to the invention serves as an insulation barrier both for direct voltage and alternating voltage fields.

Otherwise, the condenser body for the bushing according to the invention is built up as a conventional condenser body, i.e. it consists of wound insulating material with condenser layers of foil type concentrically inserted therein. The in­ner radius of the condenser body corresponds to the outer radius of the continuous current-carrying tube of the trans­former bushing.

As mentioned above, the condenser body is manufactured from an insulating agent alternating with conducting layers to obtain the desired capacitive control of the electric alter­nating field. The innermost condenser layer which is concen­tric with the conductor has an axial length which approxi­mately corresponds to the inner axial length of the conden­ser body. Outside of this innermost layer there are concen­tric layers alternating in length in the radial direction so that the stack formed by these layers tapers in either axial direction. The taper is made so that, concurrently with in­creasing radius of the condenser body counting from the first layer, the layers are laid in an axial direction such that their outer edges connect with the outwardly-directed straight frustrum of a cone of the condenser body on the air side and an evenly decreasing taper counting from the inner­most layer towards the fixing flange on the oil side. In ad­dition there are short layers which are laid such that, con­currently with increasing radius of the condenser body coun­ting from the innermost layer, they are laid in the axial direction such that their outer edges connect with the in­wardly-directed straight frustrum of a cone of the condenser body. The axial length of these short layers is adapted such that their areas are constant, i.e. the axial length de­creases with increasing radius of the condenser body.

To obtain the desired field control, the innermost layer is connected to the central conducting tube, to which high vol­ tage is applied, and the outermost layer at the fixing flange is connected to ground.

As mentioned above, the direct voltage field is controlled by several factors. Thus, for example, the medium that has the lowest resistivity is field-controlling. As mentioned above, an oil gap is formed between the insulator body of the conductor to the transformer winding and the surrounding inwardly directed straight frustrum of a cone of the conden­ser body. Since the oil has the lowest resistivity, most of the current is conducted in the oil gap, which therefore controls the field in parallel with surrounding surfaces. To obtain an even distribution of the field along these surfa­ces, it is therefore important that the width of the oil gap increases with decreasing radius. Otherwise, the field would be concentrated towards that part where the radius is smal­lest, i.e. where the cross section area of the oil gap is smallest. Therefore, the conicity of the inwardly directed straight frustrum of a cone of the condenser body and the conicity of the conical portion of the insulator body are suitably chosen such that the radial cross section area of the oil gap is approximately constant along the conical por­tion of the bodies.

Another field-controlling part is the radial distribution of the field in that part of the condenser body which does not contain any layers, i.e. around the innermost layer to which high voltage is applied. Between the oil gap towards the in­sulation on the conductor of the transformer winding and this region, the conducting layers function - in the direct voltage case - as equipotential plains which prevent the field from being concentrated to any part of the mentioned oil channel. With an accurately formed oil channel, the above-mentioned factors cooperate to obtain an even distri­bution of the field in the oil channel, the field being guided over, in the desired manner, to the insulation on the conductor of the transformer winding.

It is exceedingly desirable that the oil systems in the transformer and in the bushing consist of separate systems.

To achieve this, two different principal embodiments of the condenser body are suggested. These will be described in greater detail below in connection with the drawings, and therefore only a brief description of the principle will be given here. One alternative is that the condenser body is designed as a tight unit, for example impregnated and cured with some suitable cast compound. The second alternative comprises enclosing the condenser body in a tight casing.

This leads to the creation of two oil gaps at the transition between the insulation of the conductor of the transformer winding and the condenser body.

One advantage of the condenser body with the integrated con­denser type barrier in a transformer bushing according to the invention in relation to the concept with a separate condenser type barrier disclosed in US-A-539 209 is that the outer dimensions of the system can be made smaller. Another advantage is that the extent of the interfaces which are subjected to a tangentially directed electric field strength is reduced.

By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figures 1 and 2 two alternative embodiments of a condenser body for the bushing according to the invention.

To show the invention in the best way, the proportions bet­ween the diameter and axial length of the condenser body are not according to scale. The same is true also of the coni­city of the cones. The principal embodiment where the condenser body is formed as a tight cast unit is shown in Figure 1. As has been de­scribed, the condenser body 1 is built up as a solid of re­volution which consists of wound insulating material with concentrically inserted foil-type condenser layer. In order to show the condenser body 1 to a certain extent in its pro­per context, Figure 1 also shows the central current-car­rying part 2, in the form of a tube, of a transformer bush­ing around which the condenser body 1 is centered, as well as the conductor of the transformer consisting of an inner energized tube 3 and insulating material 4 wound thereon, which material 4 is formed as a circular-cylindrical part 6 which changes into a conical taper 5 towards the end of the tube.

A transformer bushing in which the condenser body is to be included normally has an upper insulator of electric porce­lain acting towards the air side. On the oil side transfor­mer bushings normally also have a lower insulator of, for example, electric porcelain. In an embodiment according to the invention, on the other hand, there is no such lower in­sulator of conventional type.

The condenser body in the first alternative is impregnated with a suitable cast compound, for example epoxy. The con­denser body is then wound from, for example, an insulation paper which is impregnable by the cast compound used.

On the air side the condenser body is formed as a condenser body according to the state of the art, i.e. with a circu­lar-cylindrical portion 7 which changes into an outwardly-­directed straight frustrum of a cone 8. On the oil side the condenser body continues in a circular-cylindrical portion 9 with the same outside diameter as the circular-cylindrical portion on the air side. The axial length of the outer con­tour of the circular-cylindrical portion is adapted such that its end coincides with the transition of the insulation from the conical to the circular-cylindrical portion of the conductor of the transformer winding. From the end of the condenser body there extends an inwardly-directed straight frustrum of a cone 10 with a conicity which, according to the method previously described, somewhat deviates from the conicity of the conductor of the transformer winding. This leads to the creation of an oil gap between the condenser body and the conical portion 5 of the conductor of the transformer winding. It is important for the distribution of the direct voltage field that the oil gap between the in­wardly directed straight frustrum of a cone of the condenser body and the conical portion of the conductor of the trans­former winding should have largely the same radial cross section along the whole outer contour of the cones. The dif­ference in radius is therefore greatest at the smallest base surfaces of the cones.

The first and innermost condenser layer 11 is electrically connected to the current-carrying tube 2 of the bushing, as indicated at the point of connection 12. This first layer has an axial length which corresponds to the inner axial length of the condenser body. It is surrounded by concentric layers 13 which are laid one above the other in a radial di­rection and tapering relative to the first layer, in an axial direction. The taper is done by laying the layers, concur­rently with increasing radius, in an axial direction so that the outer edges on one side connect with the conical contour of the air side and with an evenly decreasing taper towards the fixing flange of the transformer bushing on the other side. The outermost of these layers is connected to ground potential.

Furthermore, the condenser body is provided with concentric short layers 14 which connect with the contour of the in­wardly directed straight frustrum of a cone. The axial length of these short layers is adapted so as to have a practically constant area independently of the radius on which they are situated.

An embodiment of a condenser body according to the above-­mentioned second alternative is shown in Figure 2. The field-controlling parts of the condenser body, i.e. the wound insulating material and the layers, are arranged, from the design point of view, in the same way as in Figure 1. However, in this embodiment the insulation part consists, for example, of oil-impregnated insulation paper. In this alternative the entire condenser body is surrounded by oil enclosed in a tight casing 15. This leads to the creation of two oil gaps between the inwardly directed cone of the con­denser body and the conical portion of the conductor of the transformer winding. i.e. inside and outside the tight ca­sing, respectively. Both of these gaps must now be dimensio­ned in the same way as the oil gap in Figure 1. The demands that need to be placed on the material in this casing are that it must have sufficient mouldability and that its resi­stivity is to be greater than or at least as great as the resistivity of the oil.