GB2091037A - Metallized film capacitor - Google Patents

Abstract

A metallised film capacitor consists of a film winding which is enclosed within a sealed container. The container is filled with a mixture of gases which suppress electrical discharges in the winding so as to reduce capacitance loss, a suitable mixture being nitrogen and Sulphur Hexafluoride.

Description

SPECIFICATION Metallised film capacitor This invention relates to metallised film capacitors such as for example metallised polypropylene and metallised polyester film capacitors.

A metallised film capacitor may include a winding formed by two insulating films each of which is metallised on one side. The two films are wound together so that the metallisations on the two films are separated by the insulating films which act as the dielectric layer of the capacitor.

Electrical contact is made to each metallisation and the wound assembly is encapsulated within a container and sealed to the atmosphere.

Metallised film capacitors are useful as high voltage capacitors but a problem does arise above about 270V where electrical discharges start to occur which evaporate (or oxidise) portions of the metallisation which causes a loss of the effective area of the dielectric and subsequent loss in capacitance.

This invention seeks to provide a metallised film capacitor in which the above problem is mitigated.

According to one aspect of this invention there is provided a metallised film capacitor comprising a metallised insulating film winding enclosed within a sealed container which contains a mixture of gases to suppress electrical discharges in the winding and reduce capacitance loss.

The mixture of gases may include an inert gas which may be nitrogen.

The mixture of gases may include Sulphur Hexafluoride (SF6).

The mixture of gases may be at atmospheric pressure within the container and the proportion of SF6 may be between 10% and 50% of one atmosphere.

According to a second aspect of this invention there is provided a method of manufacturing a metallised film capacitor in accordance with the first aspect including the steps of placing a metallised insulating film winding within a container; introducing at least two gases into the container to form a mixture of gases which suppress electrical discharges in the winding, and sealing the container.

The winding may be baked under vacuum in the container before the introduction of the at least two gases.

In a preferred method in accordance with the invention the container and winding are flushed with an inert gas following the baking.

The inert gas may be nitrogen.

Following the flushing a further period of baking under vacuum may be provided before the introduction of the two gases into the container.

An exemplary embodiment of the invention will now be described with reference to the accompanying single figure drawing which illustrates a method of producing a capacitor in accordance with the present invention.

Referring to the drawing a capacitor 1 has a winding 2 formed by two metallised polypropylene films which are wound together so that the metallisations on the two films are separated by a layer of polypropylene which acts as the dielectric layer of the capacitor 1. Electrical connection to the metallised layer of one film is made at one end of the winding 2 by means of a connecting wire 3 whilst electrical connection is made to the metallised layer of the other film by means of a connecting wire 4 which contacts the winding 2 at an opposite end from the connecting wire 3.

The winding 2 is placed within a metal can 5 and the connecting wires 3 and 4 are connected to electrical terminals 6 and 7 respectively which are supported by means of an insulating disc 8 which locates within the can 5.

A tube 9 also passes through the disc 8 to provide an air channel between the atmosphere and the inside of the metal can 5. A layer of resin 10 is provided on top of the insulating disc 8 and around the terminals 6 and 7 and the tube 9 so as to seal the interior of the metal can 5 from the atmosphere with the exception of the air channel provided by the tube 9.

The capacitor 1 is placed at this stage within a vacuum oven 11 which is coupled by means of branching tube 12 to gas cylinders 13 and 14 which contain sulphur hexafluoride (SF6) and nitrogen respectively. Flow of SF6 gas from the cylinder 13 through the tube 12 into the oven 11 is controlled by means of a valve 15 whilst the flow of nitrogen from the cylinder 14 is controlled by a valve 16.

The oven 11 is evacuated to a medium vacuum and it is heated to a temperature of between 85 and 900C. The heating and the vacuum are maintained for a period of about 4 to 5 hours so as to drive out and remove oxygen from the winding 2 of the capacitor 1. At the end of this period the valve 1 6 of the cylinder 14 is opened so as to Introduce nitrogen gas into the oven 11 so that this gas flushes through the winding 2 via the channel provided by the tube 9.

After the winding 2 is flushed with nitrogen the oven 11 is re-evacuated and re-heated and the capacitor is baked once again for a period of about 1 hour. At the end of this period the valve 1 5 of the cylinder 13 is opened so as to allow SF6 into the oven 11 to fill the can 5 of the capacitor 1 to a pressure of 10% of one atmosphere. The valve 1 5 is closed and the valve 1 6 of the cylinder 14 is opened so as to bring the pressure within the oven 11 and within the can 5 up to atmospheric pressures with nitrogen gas.

The capacitor 1 is then removed from the oven 11 and the tube 9 is sealed to the atmosphere so as to leave the mixture of SF6 and nitrogen gases at atmospheric pressure within the can 5 of the capacitor 1.

In the resulting capacitors produced in accordance with the present invention the loss of capacitance during life tests due to electrical discharges within the winding is found to be greatly reduced.

The invention has been described with reference to the drawing by way of example only and modifications may be made without departing from the scope of the present invention. For example the invention is applicable to any form of metallised film capacitors and not just to metallised polypropylene capacitors.

The described embodiment utilises nitrogen in the mixture of gases but nitrogen may be replaced by any inert gas.

The proportion of SF6 gas used in the described embodiment is 10% but this is notessential and the proportions of the two gases may be adjusted accordingly. The applicant has found that a proportion of SF6 of up to 50% is particularly advantageous.

For ease of explanation of the present invention only a single capacitor has been shown within the oven 11 and this has been shown increased in size with respect to the size of the oven 11. In practice a large number of capacitors would be processed within the oven 11 at any one time.

Claims (12)

1. A metallised film capacitor comprising a metallised insulating film winding enclosed within a sealed container which contains a mixture of gases to suppress electrical discharges in the winding and reduce capacitance loss.

2. A metallised film capacitor as claimed in claim 1 in which the mixture of gases includes an inert gas.

3. A metallised film capacitor as claimed in claim 2 in which the inert gas is nitrogen.

4. A metallised film capacitor as claimed in any preceding claim in which the mixture of gases includes Sulphur Hexafluoride (SF6).

5. A metallised film capacitor as claimed in any preceding claim in which the mixture of gases is at atmospheric pressure within the container.

6. A metallised film capacitor as claimed in claims 4 and 5 in which the proportion of SF6 is between 10% and 50% of one atmosphere.

7. A method of manufacturing a metallised film capacitor as claimed in any preceding claim including the steps of placing a metallised insulating film winding within a container; introducing at least two gases into the container to form a mixture of gases which suppress electrical discharges in the winding, and sealing the container.

8. A method of manufacturing a metallised film capacitor as claimed in claim 7 in which the winding is baked under vacuum in the container before the introduction of the at least two gases.

9. A method as claimed in claim 8 in which the container and winding are flushed with an inert gas following the baking.

10. A method as claimed in claim 9 in which the inert gas is nitrogen.

11. A method as claimed in claim 9 or 10 in which following the flushing a further period of baking under vacuum is provided before the introduction of the two gases into the container.

12. A method of manufacturing a metallised film capacitor substantially as herein described with reference to the drawing.