Power capacitors are important components in systems for the transmis- sion and distribution of electric power. Power capacitor installations are used pri- marily to increase the power-transmission capability through parallel and series compensation for voltage stabilisation by means of static var-systems and as fil- ters for the elimination of harmonics.

Second and third aspects of the invention relate to use of the type de- scribed in claim 21, and to a method of the type described in claim 23.

Capacitors have a phase angle close to 900, and therefore generate reac- tive power. By connecting capacitors in the vicinity of the components that con- sume reactive power, the desired reactive power can be generated there. Cables can thus be utilised to the full for transmitting active power. The consumption of reactive power in a load may vary and it is desirable to constantly generate a quantity of reactive power corresponding to the consumption. For this purpose, a plurality of capacitors are connected via series and/or parallel connection in a ca- pacitor bank. The number of capacitors required to correspond to the consumed reactive can be connected in. Compensating for consumed power by utilising ca- pacitors in the manner described above is known as phase compensation. For this purpose a capacitor bank in the form of a shunt battery is arranged in the vi- cinity of the components consuming reactive power. Such a shunt battery consists of a plurality of capacitors connected together. Each capacitor comprises a plural- ity of capacitor elements. The structure of such a conventional capacitor is de- scribed below.

A shunt battery usually comprises a number of chains of a plurality of ca- pacitors connected in series. The number of chains is determined by the number of phases, usually three. The first capacitor in a chain is thus connected to a cable for transmitting electric power to the consuming component. The cable for trans- mitting is arranged a certain distance from the ground or from points in the sur-

roundings with earth potential. This distance is dependent on the voltage of the cable. The capacitors are then connected in series from the first capacitor, which is connected to the cable, and downwards. A second capacitor arranged at the opposite end of the chain of series-connected capacitors is connected to earth po- tential or to a point in the electrical system having zero potential (e. g. non-earthed 3-phase system). The number of capacitors and their design are determined so that the permissible voltage (rated voltage) over the series-connected capacitors corresponds to the voltage of the cable. A plurality of capacitors are therefore se- ries-connected and arranged in stands or on platforms insulated from earth poten- tial. Such a capacitor bank thus includes a plurality of different components and requires relatively large quantities of material. It also requires a relative robust construction so that the stand/platform can withstand the effects of wind, earth- quakes, etc. Considerable work is thus required to construct such a capacitor bank. This problem is particularly noticeable when the capacitor bank consists of a large number of capacitors. The capacitor bank also takes up a relatively large area on the ground.

Long cables for alternating voltage are inductive and consume reactive power. Capacitor banks for series-compensation are therefore arranged with regular spacing along such a cable in order to generate the necessary reactive power. A plurality of capacitors is connected in series to compensate the inductive voltage drop. In a capacitor bank for series-compensation, as opposed to a shunt battery, the series-connection of capacitors usually only takes up part of the volt- age in the cable. The chains of series-connected capacitors included in the ca- pacitor bank for series compensation are also arranged in series with the cable to be compensated.

A conventional capacitor bank comprises a plurality of capacitors. Such a capacitor in turn comprises a plurality of capacitor elements in the form of capaci- tor rolls. The capacitor rolls are flattened and stacked one on top of the other to form a stack 1 m tall, for instance. A very large number of dielectric films with in- termediate metal layers will be arranged in parallel in the vertical direction of the stack. When a voltage applied over the stack increases, the stack will be com- pressed somewhat in vertical direction, due to Coulomb forces that act between the metal layers. For the same reason, if the voltage decreases the stack will ex- pand somewhat in vertical direction. The stack formed has a specific mechanical

resonance frequency or natural frequency, which is relatively low. The mechanical resonance frequency of the stack is amplified by specific frequencies of the cur- rent, which may produce a loud noise. The mains frequency constitutes such a frequency. However, amplification of the mechanical resonance frequency can also be effected by harmonics in the current.

An example of a power capacitor of this known type is described in US 5,475,272. A high-voltage capacitor constructed from a plurality of capacitor ele- ments stacked one on top of the other and placed in a common container, is thus described here. The container is made of metal in conventional manner. The elec- trical lead-throughs are made of porcelain or polymer. The publication also de- scribes various alternative coupling for connecting the capacitor elements in se- ries or in parallel.

Description of the invention In known capacitors of this type the capacitor elements are impregnated with oil. The oil is also arranged to surround the capacitor elements and thus fill up the space between these and the wall of the container. Oil is satisfactory from the insulation aspect but entails a number of drawbacks. Damage to the container or defective sealing may result in oil leakage which may damage the function of the capacitor as well as contaminating the environment.

Against this background, the object of the present invention is to over- come the problem of oil leakage from a power capacitor of the type under consid- eration.

From a first aspect of the invention this object is achieved by a power ca- pacitor of the type described in the preamble to claim 1 comprising the character- istic features defined in the characterizing part of the claim. The insulating me- dium in the form of a dielectric fluid, e. g. an oil comprising a gelling component.

The dielectric fluid may be electrically insulating oil to which gelling components have been added. In this context it should be understood that the component may consist of a mixture of part-components. The gel surrounding the capacitor ele- ments in the container thus replaces the oil normally used for this purpose. Any damage to the container will not therefore result in oil leakage since no liquid oil is present. The consistency of the dielectric fluid prevents the formation of drops and it is therefore unable to leak out. Since the container is made of a polymer mate-

rial and therefore yields to a certain extent and is negligibly sensitive to cracking, it has properties of significance in combination with the enclosed gel. The material combines good insulation ability with other desired features such as strength, manageability and cost. A design in accordance with the invention also offers fa- vourable conditions for overcoming the problem of thermal conduction and insula- tion around the edges of the capacitor windings, which is a particular problem with power capacitors for high voltage.

It is known per se to gel an oil for use in electrical arrangements. PCT/SE 98/02314, for instance, describes the arrangement of an electrical arrangement comprising an electric conductor and an insulation system with a porous, fibre- based or laminated structure. The structure is impregnated with a dielectric fluid that is caused to solidify to a gel. The publication describes, inter alia, an applica- tion for impregnating a capacitor bank wound from metal and plastic foil. However, a capacitor element impregnated in this way does not eliminate the problem of leakage from the oil surrounding the capacitor elements in a container. This is be- cause said arrangement describes a gel system in which the oil is thermo-revers- ible, i. e. at high temperature it becomes fluid. Neither does the publication solve this type of problem.

Additional examples are described in JP 716 12 68 and JP 103 26 721.

However, this does not deal with power capacitors for high voltage either.

JP 103 26 721 shows a capacitor in which the gel is intended to suppress me- chanical vibrations. The object is thus completely different from that of the present invention, which is focused on the task of avoid an insulating fluid leaking out through the container. JP 103 26 721 shows a capacitor in which one side wall consists of urethane resin. The purpose is to prevent electrically conducting mate- rial penetrating out if the capacitor breaks, by avoiding cracks in the material through the addition of a more flexible material in the form of a gel. Here, too, it is a question of the gel being intended to achieve mechanical suppression.

In a preferred embodiment of the power capacitor in accordance with the invention the gel state of the dielectric fluid is thermostable throughout the entire temperature range occurring when the capacitor is in operation. Increased secu- rity against the occurrence of oil leakage is obtained by choosing the gelling com- ponent so that the gel state is retained even at relatively high temperatures.

In accordance with a preferred embodiment the dielectric fluid is silicon-based, this applying in particular to the gelling silicon component. A capacitor is thus achieved which is extremely advantageous from the environmental aspect, for in- stance. A gel system that instead contains components such as polyurethane and/or isocyanates does not have such environmental advantages. Since these produce toxic gases in the event of a fire, they contribute to a hazardous working environment during manufacture and demands for safe waste management and destruction. Toxic gases are produced in the event of fire in a capacitor containing oil in accordance with said PCT/SE 98/02314. Furthermore, a gel system with such components has the drawback that these swell greatly and negatively influ- ence the metallising film. Since an embodiment with metallised film, i. e. metal- coated film, is advantageous, this is a considerable drawback. Tests have shown that the films may even be destroyed. These drawbacks are avoided with a sili- con-based gel system. This is therefore an embodiment of great significance.

The present invention is particularly advantageous for application in a power capacitor which is produced in known manner from capacitor elements in the form of rolled film of plastic and metal or a metal-coated film, wherein the gel is arranged to impregnate the wound capacitor element, possibly at its end por- tions, in order to avoid partial discharges. This thus constitutes a preferred em- bodiment of the power capacitor in accordance with the invention. Alternatively, such a winding can be performed dry.

In an alternative embodiment of such a power capacitor, a second dielec- tric fluid is arranged in the space between turns of the winding, which second di- electric fluid is in liquid form, i. e. not elated.

The gel surrounding the capacitor elements in the container should fulfil certain requirements. It should thus display high shearing strength in gelated state, good thermal conductivity, high electric strength, be sufficiently electrically insulating and be thermostable within the temperature range occurring during op- eration.

In accordance with a preferred embodiment the dielectric fluid comprises an electrically insulating oil. The fluid is thus of a type that in high degree is capa- ble of fulfilling said requirements. From this aspect, it is particularly suitable for the oil to comprise silicon oil.

In accordance with a preferred embodiment of the invention the gelling component comprises silicon, preferably polydimethyl siloxane with at least some vinyl substitutes, i. e. vinyl side groups.

In accordance with another preferred embodiment, the gelling component comprises silane-functional cross-linking agent. In a preferred alternative this cross-linking agent comprises silicon, suitably polydimethyl siloxane, with at least some silane substitutes.

The quantity of silane-functional cross-linking agent is preferably 1-80 per cent by weight.

The preferred gelling component, the preferred alternative thereof and the preferred content thereof contribute to the dielectric fluid acquiring favourable properties as regards the above requirements.

In a particularly preferred embodiment the dielectric fluid also comprises metal complex, which further contributes to satisfying the above requirements.

Here, too, the quantity of metal complex mixed in is 2-4000 ppm, preferably 10- 2000 ppm, which has been found to constitute a suitable amount.

In accordance with yet another preferred embodiment the dielectric fluid comprises silicon liquid of low molecular weight, preferably polydimethyl siloxane liquid. In this case also, a fluid is obtained that in gelled state satisfactorily fulfils the requirements set.

In accordance with another embodiment of the invention the dielectric fluid comprises an agent that retards gelation. This permits a well controlled and extended gelling process that facilitates manufacture and contributes to achieving good quality of the gel function.

A suitable quantity of the gelation-retarding component is 0.001-4 per cent by weight. In accordance with another preferred embodiment the composition of the dielectric fluid is 1-80 per cent by weight, preferably 20-50 per cent by weight, silane-functional cross-linking agent, 2-4000 ppm, preferably 10-2000 ppm metal complex, 0-60 per cent by weight, preferably 10-50 per cent by weight polydime- thyl siloxane of low molecular weight, 0-4 per cent by weight gelation-retarding agent and the remainder polydimethyl siloxane with vinyl substitutes.

With such a composition the fluid acquires very suitable properties for in- sulating medium that fulfils the necessary requirements.

In accordance with an alternative, also preferred, embodiment the dielec- tric fluid comprises a vegetable oil, possibly mixed with silicon oil.

In accordance with a further preferred embodiment the gelling component comprises a vegetable oil.

In accordance with yet another embodiment of the invention at normal operating temperature the pressure in the gel is at least equivalent to atmospheric pressure.

In accordance with a preferred embodiment each capacitor element is substantially circular-cylindrical in shape and the inside of the container has cor- responding circular-cylindrical shape so that the container closely surrounds each capacitor element, the axial direction of each capacitor element being oriented to coincide with the axial direction of the container.

Since the inside of the container has a circular-cylindrical shape corre- sponding to the cylindrical shape of the capacitor elements so that the container closely surrounds the capacitor elements, a capacitor is obtained that is as com- pact as possible and suited to an advantageous and electrically favourable shape of the elements from a manufacturing point of view.

In accordance with another embodiment the container is made of an elec- trically conducting material. The insulation between the capacitor elements and the container can therefore be simpler without risk of discharge between capacitor elements and container. Furthermore, the electrical connections of the capacitor can be made extremely simple and the creepage distance necessary between them can be provided by the container itself. With the simplification of the insula- tion and elimination of the lead-throughs, the capacitor will also be relatively com- pact, thereby enabling compact capacitor banks to be built.

A second aspect of the invention relates to the use of a gelled dielectric fluid to insulate capacitor elements arranged in a container. In preferred embodi- ments the gel has a composition corresponding to that stated above for the power capacitor in accordance with the invention. Similar advantages as those described above with regard to the invented power capacitor are gained with the use in ac- cordance with the invention.

From a third aspect the object is achieved by means of the method de- fined in claim 23. A power capacitor in accordance with the invention is obtained in a practical way by means of this method.

In accordance with a preferred embodiment of the method in accordance with the invention the dielec. ric fluid is degassed before being introduced into the container. This increases the functional reliability of the capacitor since air bub- bles are avoided in the gel, which might cause the appearance of surface glow.

Such surface glow can cause erosion in the long run.

The above and other preferred embodiments of the power capacitor in accordance with the invention, the invented use and the invented method are de- fined in the sub-claims to respective claims 1,21 and 23.

Brief description of the drawings Figure 1 is a schematic view in perspective of a capacitor in accordance with a first embodiment of the invention, Figure 2 illustrates a detail from Figure 1, Figure 3 constitutes a graph illustrating the heat development in the capacitor element shown in Figure 2, Figure 4 is an enlarged radial part section through the detail in Figure 2, Figure 4a is a section corresponding to Figure 4, but illustrating an alternative embodiment, Figure 4b is a section corresponding to Figure 4, but illustrating another alter- native embodiment, Figure 5 is a longitudinal section through a capacitor element in accordance with an alternative embodiment, Figure 6 shows two capacitor elements as shown in Figure 5, connected to- gether, Figure 7 is a view in perspective of a capacitor in accordance with another embodiment of the invention.

Advantageous embodiments of the invention Figure 1 shows schematically the design of a capacitor in accordance with the invention. It consists of an outer container 1 of polyethylene which encloses, in this case, four capacitor elements (2a-2d). The container 1, like the capacitor elements 2a-2d, is circular-cylindrical. The capacitor elements 2a-2d are con- nected in series. Connection terminals 3,4 are arranged at each end of the ca- pacitor. Each terminal consists of a conducting foil mounted in the material of the

container and extending therethrough. A gel 10 is arranged between the capacitor elements 2a-2d and the container. The gel serves as electrical insulation and thermal conductor.

Figure 2 shows an individual capacitor element comprising metal-coated polymer films tightly rolled to a roll. The capacitor element 2 has an axially running hole 6 running centrally through it, which may be used for cooling the element.

Typical dimensions for such a capacitor element are a diameter of 100-300 mm, a hole diameter of 20-90 mm, preferably at least 30 mm, and a height of 50- 800 mm. Such a capacitor element is intended for a voltage of about 1-15 kV. A capacitor element with a diameter of 200 mm, a hole diameter of 60 mm and a height of 150 mm, for instance, is intended for a voltage of about 4-10 kV. Up to 40 kV is thus obtained with four of these connected in series, as shown in Figure 1, and 80 kV is obtained with eight capacitor elements, etc.

Thermal losses arise in the capacitor element 2, resulting in internal heat- ing of the element. The maximum temperature is critical for the electrical dimen- sioning. Higher temperature forces lower stress, which leads to lower output per volume unit, i. e. it has considerable influence on the consumption of material and the cost. In a cylindrical volume with homogenous heat generation, and with no opening at the centre, the temperature profile in radial direction will acquire an as- ymptotic appearance as indicated by the broken curve in Figure 3. If the capacitor element is provided with a central opening 6 with radius Ri, the temperature pro- file will follow the unbroken curve in Figure 3. Forced cooling is also possible if necessary. The temperature profile obtained will then be as indicated by the dot- ted line in Figure 3.

Figure 4 shows an enlarged radial part section through the capacitor ele- ment in Figure 2. The part section shows two adjacent turns of the metal-coated film. The film 8a and 8b, respectively, is approximately 10 urn in thickness and the material is polypropylene. The metal layer 9a, 9b is approximately 10 nm thick and consists of aluminium or zinc or a mixture thereof, which has been vaporised onto the polypropylene film prior to rolling. With such a metallised film an electric stress E in the order of 250 V/um can be reached. The technique of manufactur- ing a capacitor element in this way is already known and a more detailed descrip- tion is therefore superfluous. Alternatively the capacitor elements can be built up using film foil technology where propylene film and aluminium foil are rolled up to-

gether. However, the use of metallised film has the advantage of self-healing and allows higher electrical stress and higher energy density than with the film foil technology.

The metal layer covers the plastic film from one side edge up to a short distance from its other side edge. A random area 16a of the film 8a thus lacks metal-coating. In similar manner a random area 16b of the film 8b lacks metal coating. The exposed random area 16b of the film 8b, however, is at the opposite end edge from that on the film 8a. Electrical connection for the layer 9a is ob- tained at the upper end of the element, seen in the drawing, and at the lower end of the layer 9b so that a plus electrode is obtained in one direction and a minus electrode in the other. To ensure efficient electrical contact the end portions may be sprayed with zinc.

In the modified embodiment shown in Figure 4a the capacitor element has inner series-connection. Here the metal layer 9a, 9b on each plastic film 8a, 8b is divided into two portions 9a', 9a"and 9b', 9b", respectively, separated by an uncoated part 17a, 17b, respectively. It is also possible to divide the metal layers into more than two portions. Each pair of metal-layer portions, e. g. 9a'and 9b', forms a part capacitor element which is connected in series.

Figure 4b shows a variant of the modified embodiment, where the metal layer 9a on only the one plastic film 8a is divided into two parts 9a', 9a"separated by an uncoated part 17a, whereas the metal layer 9b on the other plastic film 8b is undivided. Each of the parts 9a'and 9a"extends right out to the edge of the film 8b so that the electrical connection in this case occurs to the same film 8b. The metal layer 9b on the other plastic film terminates on both sides a short distance 16a, 16b from the edge of the film and is thus not electrically connected in any di- rection.

The gel between the capacitors elements (2a-2d) and the container con- sists of a component sold under the trade name Silgel0612 from Wacher-Chemie GmbH and comprises gel-forming components. Low-viscous silicon oil is mixed into this component. A component sold under the trade name"Inhibitor PT 88", also from Wacher-Chemie GmbH, constituting a gelation-retarding component, is mixed in in an alternative embodiment. A suitable silicon oil may be an oil sold under the trade name"Dow Corning (DSilicone Transformer Liquid"from Dow Corning.

By way of example the liquid mixture that is to form the gel may be com- posed of about 60-70 % of the gelling component, half consisting of Sitge ! @612A and half of Silgel0612B. The basic component, i. e. the silicon oil constitutes about 30-40 %, the lower proportion being applicable if an inhibitor is used. The remainder, i. e. up to a few per cent, consists of the gelation-retarding component.

Experiment has shown that at a treating temperature of 23°C solidification occurs in about an hour if no gelation-retarding component is present. With a mix- ture of 0.5 per cent by weight"Inhibitor PT 88", the solidification time is extended to just over 10 hours. With 1 % a solidification time of about 100 hours is achieved, and with more than 2% the time will be over 150 hours. The inclusion of about 1 % gelation-retarding component is probably suitable and gives sufficiently long solidification time at 60°C treating temperature.

The liquid mixture is permitted to penetrate between the film layers so that the capacitor element becomes impregnated at least at the side edges. The liquid with the various components is degassed and combined to a mixture. The mixture is introduced through an inlet in the container by means of a pressure difference achieved by means of a pump or a vacuum, for instance.

Figure 2 illustrates how a power capacitor in accordance with the present invention can be constructed for various types of capacitor elements. In all cases a capacitor element 2 is surrounded in a container 1 by the dielectric fluid 10 comprising gelling component, and is in gel form in the container.

In principle, the capacitor element 2 may be constructed in accordance with three different alternatives as regards the present of dielectric fluid inside the element. In accordance with a first alternative the capacitor element 2 may be dry, i. e. no dielectric fluid at all is present inside its winding. According to a second al- ternative the capacitor element contains a dielectric fluid that is gelled in equiva- lent manner to the surrounding gel 10. This may be particularly relevant in the end regions A. According to a third alternative the capacitor element 2 is impregnated with a dielectric fluid such as an oil which does not gel. Here, too, it may be a question of only the end regions A being impregnated.

The first alternative is primarily of interest in the case of tightly wound ca- pacitor elements, particularly of the type having a metal-coated plastic film. The other two alternatives are primarily of interest for loosely wound capacitor ele-

ments, particularly of the type in which separate plastic films and metal foils are used in the winding.

Figure 5 shows a longitudinal section of an alternative embodiment of a capacitor element 2'in accordance with the invention. The capacitor element is divided into three sub-elements 201,202,203 which are concentric with the common axis designated A. The outermost sub-element 201 is substantially tubu- lar, with an inner side 204 surrounding the intermediate sub-element 202 with a small space. The intermediate sub-element similarly has an inner side 205 that closely surrounds the innermost sub-element 203. The innermost sub-element 203 has a central channel 206 running through it. The three sub-elements have different radial thickness, the outermost having the smallest thickness. The sub- elements thus have substantially the same capacitance. Insulation 207 is ar- ranged between the sub-elements.

The sub-elements are connected in series. Two radially adjacent sub- elements have one of their coupling points at the same end. The outermost sub- element 201 is thus connected by the coupling member 210 to the intermediate sub-element 202 at one end of the capacitor element 2', and the intermediate sub-element 202 is connected by the coupling member 211 to the innermost sub- element 203 at the other end of the capacitor element 2'. This means that the connections 212,213 for the capacitor element 2'are located at opposite ends thereof.

If the number of sub-elements is greater than three, e. g. five or seven, connection of the coupling points at the ends of the sub-elements is continued al- ternately in the same way.

Figure 6 illustrates how a plurality of capacitor elements of the type shown in Figure 5 can be connected together in series. The figure shows two such ca- pacitor elements 2'a, 2'b. The connection 212 of the lower capacitor element 2'b at the upper end of the inner sub-element 203 is coupled to the connection 213 of the upper capacitor element 2'a at the lower end of the outermost sub-element 201. Insulation 214 is arranged between the capacitor elements in order to deal with the potential differences occurring in this type of capacitor element.

Figure 7 shows another example of a power capacitor in accordance with the invention. In this example the design of the container 301 and capacitor ele- ment 302 is of conventional type. The capacitor container 301 is thus box-shaped

and the capacitor element 302 is wound to flattened units stacked one on top of the other. The electrical connection terminals 303,304 are directed the same way. A gel 310 is arranged in the space between the capacitor elements 302 and container 301 in similar manner to the embodiments described above.