D E S C R I P T I O N

POWER CAPACITOR

Field of the invention

The invention concerns a power capacitor according to the generic part of claim 1. Such capacitors which usually contain windings of the self-healing type consisting essentially of metallized plastic films, are used, e.g., for phase correction in power distribution grids.

Prior art

A power capacitor according to the generic part of claim 1 is known from US 6 313 978 Bl. The winding is accommodated in a chamber within a plastic casing closed by a pressure sensitive bistable cover carrying first and second electrical contacts which are connected to the winding by conductor bands with weak points. The chamber is filled up with an insulating filler liquid. When the winding begins to fail the consequent out-gassing causes gas bubbles to form and rise through the filler liquid to the top of the chamber and increase the pressure acting on the cover. If the said pressure reaches a threshold the cover snaps into a convex shape and breaks the conductor bands at the weak points. The housing further contains an air volume which is compressed in case of a sudden pressure build-up which might otherwise damage the casing. Due to the filler liquid, e.g, oil, even a minor rupture of the casing may result in liquid spill with the risk of fire and damage to the environment.

A similar power capacitor is known from US 4 580 189 A where a casing surrounds a chamber containing a winding and filled

in part by a soft potting material like plasticized urethane and in part by gas. Low rates of out-gassing as occur when the failing of the winding causes the same to melt may, due to the compressibility of the gas, fail to cause a rise of the pressure large enough to ensure a timely response of the disconnector .

A power capacitor comprising a winding or several windings in a chamber filled up with a gel as an insulating filler, in particular, oil mixed with a gelling component, is known from US 2003/0 133 255 Al. The capacitor, however, lacks a disconnector .

A further power capacitor is disclosed in EP 0 256 348 Al. It comprises a winding which, apart form an upper face, is surrounded by resin as an insulator. The upper face is covered by a deformable membrane separating the winding from a gas-filled compartment. The small volume between the membrane and the winding is filled with an insulating paste. If, due to out-gassing of the winding, sufficient pressure builds up under the membrane it assumes a convex shape and breaks a lead connecting the winding with an electrical contact. As the winding is surrounded mostly by a solid insulating filler, gas forming away from the upper face may be trapped and have no effect on the membrane. In this case the disconnector remains inactive and the capacitor may fail catastrophically .

Summary of the invention

It is the object of the invention to provide a power capacitor according to the generic part of claim 1 which is a dry capacitor with no risk of liquid spill. This object is

attained by the feature in the characterizing part of claim 1.

As a gel is used as an insulating filler in the chamber containing the winding even a relatively large crack in the casing will not lead to spill as the deformation of the gel under the impact of mechanical forces and pressures is limited. On the other hand, it has been found that gels are sufficiently deformable to transmit pressure building up due to gas formation anywhere within the chamber to the pressure sensitive actuator although gas cannot permeate the gel in bubbles as in the case of a liquid but only by diffusion which is too slow to be relevant here. As gels are also, like liquids, virtually incompressible, even a failure of the winding where relatively small amounts of gas are produced, e.g., a failure due to melting of the plastic film, will cause the disconnector to be activated.

Brief description of the drawings

In the following, the invention will be explained in more detail with reference to the following figure which shows an embodiment.

Fig. 1 schematically shows an axial section through a power capacitor according to the invention.

Description of the preferred embodiments

The power capacitor comprises a rigid casing comprising a cylindrical container 1 consisting of plastic or metal, e.g., polyethylene, aluminium or steel, and a round cover 2 consisting of hard plastic, e.g., PU. The casing surrounds a chamber 3 containing a winding 4 with a first electrode and

a second electrode electrically conductively connected by leads 5, 6 to a first electrical contact 7 and a second electrical contact 8, respectively, which are arranged on top of the cover 2. The winding 4 is preferably of the self- healing type and may consist, e.g., of two plastic foils wound together, each covered by a metal film forming one of the electrodes as described, e.g., in EP 1 341 195 Al. It may, however, have a different structure, e.g., containing additional electrodes not connected to electrical contacts as disclosed in WO 2007/071 097 Al. The chamber 3 also contains a compressible cushion 9 consisting, e.g., of PU foam and is otherwise filled up essentially completely with an insulating gel 10 as a filler. The cushion 9 serves to compensate differences in the thermal expansions of the casing on the one hand and the winding 4 and the gel 10 on the other hand. If the said thermal expansions match, i.e., if the thermal expansion of the gel is gauged so as to compensate a difference between the thermal expansion of the container 1 and the thermal expansion of the winding 4 - the former being usually somewhat larger - the cushion 9 is not required.

Also accommodated in the container 1 is a gas-filled compartment 11 which is separated from the chamber 3 by a flexible membrane 12 and a rigid wall 13. Fixed to the centre of the membrane 12 is a blade-like separator 14 with a cutting edge directed against a conductor band 15 which forms a part of the lead 5 accommodated inside the compartment 11. The separator 14 forms, together with the conductor band 15, a switch which can be actuated by the membrane 12. The latter is in contact with the gel 10 so as to monitor the pressure in the chamber 3. If the pressure reaches a predetermined threshold the membrane 12 will cause the switch to disconnect the winding 4 from the first

electrical contact 7 as will be described in more detail below.

If the winding 4 reaches a condition where it is likely to fail in the near future, e.g., if it approaches the end of its life span or has otherwise been damaged, it will usually release gas which accumulates around the winding 4 and increases the pressure there. Due to the plasticity of the gel 10 this pressure increase is transmitted to the boundary of the chamber 3 and, due to the rigidity of the remaining parts of the same causes a marked deformation of the flexible membrane 12 which protrudes into the compartment 11 and causes the cutting edge of the separator 14 to disrupt the conductor band 15 and disconnect the winding 4 from the first electrical contact 7 when the pressure reaches the threshold. As the activation of the disconnector as described above has no effect on the shape or size of the casing it can not be obstructed by external mechanical constraints, e.g., the way the capacitor is built into a larger assembly.

Instead of designing the above described switch for disconnecting the first electrode from the first electrical contact, the above switch can be modified to directly connect the first electrical contact 7 to the second electrical contact 8.

The cushion 9 can delay the action of the disconnector to some small extent which is, however, acceptable. The compressibility of the cushion 9 must, of course, be gauged so as to ensure that thermal expansion during normal operation does not cause the pressure to reach the threshold.

The gel 10 has a storage modulus of between lOPa and 10'000Pa and also a loss modulus of between lOPa and 10'000Pa. Preferably, both storage modulus and loss modulus are between lOOPa and l'OOOPa. The gel may be a mixture of a silicone gel as a gelling component and silicone oil as a plasticizer as described in US 2003/0 133 255 Al, p. 4, that is, Silgel® from Wacker-Chemie GmbH and "Dow Corning® Silicone Transformer Liquid" from Dow Corning, mixed with a mass ratio of 60-70% to 40-30%. A lower cost alternative is a self-extinguishing polyurethane gel formed by mixing two components which react to form the gel, namely a mixture of long-chain polyols with an average molecular mass of at least 1000, preferably between 2000 and 4'000, e.g., polyether and/or polyester polyols and up to 5% (mass) of plasticizer consisting of, e.g., mineral oil and/or hydrocarbon polymers as a first component, and an isocyanate hardener, e.g., technical diphenyl methane-diisocyanate, as a second component. Both are available from ABB Schweiz AG, Micafil as resin X1321 Rl and hardener P980, respectively.

There are many possible modifications of the embodiment described above which are well inside the compass of the invention. In particular, the switch can short-circuit the winding in order to activate an over-current protection device, e.g., a fuse or interruptor in series with the capacitor. The membrane can be of the bistable type. The chamber may contain several windings connected in series or in parallel etc..

List of reference symbols

1 container 2 cover 3 chamber

4 winding

5, 6 leads

7, 8 electrical contacts

9 cushion 10 gel

11 compartment

12 membrane

13 wall

14 separator 15 conductor band