In known support insulators of the type shown in Figure 1 the electric field strength is very high precisely in the vicinity of the electrode at high potential, where electrons can be easily emitted which may initiate a total flashover across the insulator.

The above-mentioned drawback has been realized, and, inter alia, in German patent 356 513 attempts have been made to shape the outer contour of the support insulator such that the voltage distribution along the insulator becomes a straight line under dry conditions. This results in a support insulator which has the largest diameter at the top at the high voltage electrode.

In case of moist weather or rainfall, however, there is a risk that the relatively high field strength influences water running along the surface of the insulator. It is known that a rivulet holds together longer if subjected to an electric field in the direction of the water flow, which is the case on ordinary insulators . Since this rivulet communicates with the high-voltage electrode, the risk of flashover across the insulator increases . Also with insulators according to German patent 356 513, a relatively high field strength is obtained in the vicinity of the high- voltage electrode with an ensuing increased risk of flashover in case of moist weather or rainfall.

The invention now relates to a design which comprises reducing the electric field E further in the vicinity of the high-voltage electrode and, in addition, designing this electrode such that, even in case of rainfall, it will withstand flashovers in a better way than prior art support insulators . Such a support insulator can be made considerably lower than known support insulators at the same voltage and may withstand a considerably higher voltage at the same overall height, respectively.

The invention relates to a support insulator which comprises at least one electrode lying at high-voltage potential at one end of the insulator. The insulator has been given an outer shape with a largest diameter in the vicinity of this electrode. The invention is characterized in that the insulator comprises a continuous, supporting rod of insulating material, at one end of which the electrode positioned at high-voltage potential is arranged. This electrode is composed of a first and a second electrically conducting part, arranged around the rod somewhat spaced from each other. The parts are electrically connected to each other by means of a conductor. The second part is arranged on the rod nearer to the other end of the insulator than the first part . The parts and the conductor are encased in insulating material, the insulating material around the first part forming a first solid of revolution substantially in the form of a ring and around the second part a substantially cup-shaped second solid of revolution with a convex outer surface facing the first solid of revolution. The second solid of revolution is designed with a considerably larger diameter than the first solid of revolution. The axes of rotation for these solids have thereby been arranged substantially parallel to the longitudinal axis of the rod. The insulating material around the conductor forms a tubular part which connects the first and second solids of revolution together. This tubular part has an outside diameter which is smaller than the outside diameter of the first solid of revolution so that the first solid of revolution forms a roof over part of the convex surface of the second solid of revolution.

By the above embodiment of the upper part of the support insulator, a pocket is created between the first and second solids of revolution, in which pocket a field weakening is created. This results in the support insulator according to the invention becoming less sensitive to precipitation than other known support insulators. Also, the electrical

strength is increased, under dry conditions, compared with prior art insulators .

The invention will be best understood with reference to the accompanying drawings .

Figure 1 shows a known type of support insulator and Figure 2 shows the field strength E along the insulator surface in Figure 1, S designating the distance along the insulator surface.

Figure 3 shows the field strength along an insulator according to the known German patent 356 513.

Figure 4 shows a conceived pear-shaped insulator with a field distribution according to Figure 5.

Figures 6 and 7 show show an insulator according to the invention? in which Figure 7 is a section through Figure 6.

Figures 8 and 9 show equipotential surfaces around the insulator according to Figure 7 in dry state and in wet state, respectively.

Figures 10 and 11 show an assembled insulator.

With reference to Figures 4 and 5, it can be determined that a pear-shaped insulator provides a field strength which is lowest on the electrodes . This reduces the probability of a so-called leader being initiated there and growing into a regular flashover. On the other hand, of course, the probability of local discharges arising at the centre of the insulator is increased, where E has been made deliberately larger. As long as these discharges are local and not connected to the electrodes via a leader, however, the holding value of the insulator is higher than in the case with even field distribution along the insulator, or a field

distribution where the field is high at one of the electrodes .

Figures 6 and 7 show how a support insulator with these properties is obtained in practice. Figure 6 shows the support insulator 1 seen from the side. The support insulator 1 is composed of an upper first electrically conducting part 2 at high-voltage potential and a solid of revolution 3 of insulating material surrounding the part 2. Below this a second solid of revolution 4 is arranged, also of insulating material and surrounding a concealed second electrically conducting part connected to the first part. The solid of revolution 4 has a convex outer surface facing the first solid of revolution 3. The term "convex" here covers all conceivable cupped surfaces from the shape of a truncated cone to a more evenly curved surface. Further, numeral 5 in Figure 6 shows a shell of insulating material.

Figure 7 shows, as mentioned, a cross section of the support insulator 1 with the upper first electrode at high-voltage potential. This electrode is composed of a first conducting part 2 and a second conducting part 6, which are electrically interconnected by means of a conductor 7. The first part 2 may be solid whereas the conductor 7 and the - second part 6 may be produced in various ways, for example by metallic coating of the surface in a cavity or metallic coating on a foamed body which fills up the cavity inside the solid of revolution 4. The cavity ("electrode") is then suitably formed such that the second conducting part 6 is given the shape of a corona ring around the rod 8. The rod 8, which may be tubular or solid, has a supporting function and is suitably manufactured of fibre-reinforced resin. At the other end of the rod 8, the second suitably solid electrode 9 is placed and around this an insulation 9a of cast epoxy is placed. Around the rod 8 the shell 5 is arranged. The shell 5 surrounds a space 10 which may suitably contain an insulating agent, such as the gas SFβ. The shell 5 is provided with a number of parallel annular

folds 11 around the rod 8. The folds are arranged such that a number of drip edges 12 are formed along the shell 5. Such drip edges 13 and 14 are also produced at the solids of revolution 3 and 4, the one associated with 4 being more salient .

As mentioned above, Figures 8 and 9 show equipotential surfaces around the support insulator 1. The electric field lines will run perpendicularly to these surfaces. From both Figures 8 and 9 it is quite clear that the field between the solids of revolution 3 and 4 is considerably weaker than, for example, the field around the drip edge 14 and immediately below this. Thus, the field has been given a configuration as shown in Figure 5.

In the case of a wet insulator, water running along an insulator in the direction of the electric field is influenced by the field to produce a continuous rivulet or water jet'along the insulator, the insulating resistance of the insulator thus being deteriorated. The stronger the field, the greater the probability that such a jet will be produced. By the field weakening, produced by the invention, between the solids of revolution 3 and 4, rainfall will cause the water to fall from the surface of the solid of revolution 3 in the form of water droplets from the edge 13 down onto the solid of revolution 4, and not in a continuous rivulet which could have been the case if the field between the solids of revolution 3 and 4 had been stronger. Then when the water has collected on the upper side of the solid 4, this water has therefore no electrical connection with the high-voltage electrode. Further, the strong field around the drip edge 14 has the advantage that water running over the edge 14 is influenced by the field so that it is sprayed out in a direction away from the insulator and does not drip down onto the next insulating surface which is formed by the shell 5. Further, the field distribution curve, which is implicitly shown in Figures 8 and 9, indicates that the field in case of rainfall is

amplified around each drip edge 12, which means that the water, also where it is thrown out of the insulator and also in case of heavy rain, is prevented from forming a continuous jet along the insulator.

In the design described, the rod 8 is intended to function as a supporting member and is suitably manufacturd from reinforced plastic. The solids of revolutions 3 and 4 and the shell 5 are thus relieved and may be cast in a suitable material, for example epoxy resin.

In larger support insulators, as is clear from Figure 10, an insulator according to the invention may be connected together with an additional such insulator, or, according to Figure 11, with a conventional straight insulator 15. It is, of course, most important to have a field reduction in the vicinity of the high-voltage electrode where the field, for natural reasons, is greatest. Nearer to ground the stresses are already low, and a field optimization is less important there, which makes it possible to use simple, straight insulators. In an insulator of this type, it is suitable that also the joints 16 be made of insulating material .