CH642768A5 - CONDENSER WITH LAYERS OF METAL FILMS, ALTERNATING WITH DIELECTRIC SPACERS, IMPREGNATED WITH A DIELECTRIC LIQUID. - Google Patents

Description

The invention relates to a capacitor having layers of metal foils alternating with dielectric spacers impregnated with a dielectric liquid, the dielectric liquid preferably having a high content of diisopropylbiphenyl.

The recent developments of dielectric hydrocarbon based liquids for insulation purposes with high voltage loads, for example for use with capacitors, have been based on the use of high-grade aromaticity to withstand the effects of corona discharges producing overvoltages. These liquids were essentially alkylated aromatics such as isopropylbiphenyl or solutions of an aromatic in an alkyl aromatic ester such as diisononyl phthalate. The alkyl substituents give the liquid a wide temperature range in which it is flowable, especially at low temperatures and in processes with low vapor pressure. However, they cause the corona resistance to be poor in aromatic substituents because hydrogen is generated and retained, which leads to further discharges. This is probably due to the fact that hydrogen is not firmly bound in the dielectric liquid, so that the hydrogen is produced more easily and on a larger scale. In addition, these liquids do not absorb hydrogen, as aromatics obviously do. The alkyl content of a hydrocarbon-based dielectric liquid should therefore be just high enough to maintain the necessary properties of the liquid without the electrical properties being adversely affected. There is therefore a tendency to prefer liquids that contain low levels of alkyl but high levels of aryl. In order to optimize dielectric liquids made of isopropylbiphenyl for capacitors, 20% of diisopropylbiphenyl was considered to be the maximum tolerable amount of alio kyl, the rest being monoisopropylbiphenyl (see US Pat. No. 4,054,937). The corona quenching voltage at low temperatures has been found to begin to decrease at a diisopropylbiphenyl content of about 20%. It was also determined that the pour points increase with a diiso-15 propylbiphenyl content of about 20%.

U.S. Patent 4,054,937 discloses a condenser containing 20% diisopropylbiphenyl and 80% monoisopropylbiphenyl.

US Pat. No. 2,837,724 describes a dielectric liquid for transformers, which may be a methyl tert-butylbiphenyl.

A dielectric liquid sold under the trade name "MCS1238" was analyzed and a content of 17% sulfone was determined, while the remainder was isopro-25 pylbiphenyl, of which 31.1% was diisopropylbiphenyl and the remainder was monoisopropylbiphenyl. This liquid is covered by U.S. Patent No. 3,796,934.

It has now been discovered that, contrary to the assumptions of U.S. Patent 4,054,937, high concentrations of di-30-isopropylbiphenyl in a dielectric liquid containing isopropylbiphenyl intended for capacitors are not as harmful to the electrical properties as expected . In particular, the surge resistance and corona resistance are not as good as dielectric liquids that contain large amounts of monoisopropylbiphenyl, but are still within the limits that are acceptable for commercial use.

In addition, dielectric liquids made from iso-40 propylbiphenyl with high concentrations of diisopropylbiphenyl resist crystallization at low temperatures better than dielectric liquids made from isopropylbiphenyl with high concentrations of monoisopropylbiphenyl if the content of Me-45 taisomer in is not monoisopropylbiphenyl is also high.

Finally, dielectric liquids based on isopropylbiphenyl with high concentrations of diisopropylbiphenyl are cheaper than dielectric liquids based on isopropylbiphenyl with high concentrations of Mo-50 noisopropylbiphenyl.

The capacitor according to the invention is characterized in that the dielectric liquid contains 25 to 100 percent by weight diisopropylbiphenyl. It preferably contains 25 to 100 percent by weight diisopropylphenyl, 0 55 to 80 percent by weight monoisopropylbiphenyl and 0 to 20 percent by weight triisopropylbiphenyl.

The invention is explained in more detail below using an exemplary embodiment which is illustrated in the drawings.

60 The only figure in the drawings shows a sectional view of a capacitor according to the invention.

The capacitor consists of a container 1 which is hermetically sealed and contains one or more windings which consist of a straight conductive foil 2 and a conductive foil 3, which is narrower and has rounded edges. These foils alternate with insulation layers 4 which, according to the embodiment shown, consist of a film 5, a paper layer 6 and a

nem film 7 exist. A dielectric liquid 8 fills the container 1 and impregnates the winding. Electrode connections can be provided according to conventional procedures. The dielectric layers will also tend to adapt to the available spaces, so that in practice the large free space shown in the drawing will be considerably reduced.

It is particularly favorable if the dielectric liquid used is 25 to 80% (all percentages given here are percentages by weight, based on the weight of the dielectric liquid, unless expressly stated otherwise) diisopropylbiphenyl, 20 to 75% monoisopropylbiphenyl and 0 to 15% Contains triisopropylbiphenyl. Preferably, the dielectric liquid contains 35 to 80% diisopropylbiphenyl, 20 to 65% monoisopropylbiphenyl and up to 15% triisopropylbiphenyl. If less than 20% diisopropylbiphenyl is used, the dielectric fluid may no longer be practical for low cost capacitors. Dielectric liquids based on isopropylbiphenyl with 25 to 100% diisopropylbiphenyl have very good properties even at low temperatures.

Because of the special manufacturing process (alkylation of biphenyl), the dielectric liquid may contain some biphenyl. Biphenyl vaporizes and irritates the mucous membranes and should therefore be present in no more than about 1%. The biphenyl concentration would be significantly below this value if the proportion of diisopropylbiphenyl is high because biphenyl is largely implemented from the outset with a high degree of alkylation of the biphenyl, and it would be further reduced in the franking of the dialkyl component, because of the considerably greater volatility of the Biphenyls.

The dielectric liquid preferably contains up to about 1% of an antioxidant for thermal stability. The preferred amount is 0.01 to 0.2%, particularly favorable antioxidants being di-tert-butyl-p-cresol, di-tert-butylphenol or mixtures thereof.

The liquid preferably also contains up to about 2%, in particular 0.1 to 0.5%, of hydrogen acceptor in order to increase the resistance to corona discharges. An anthraquinone such as ß-methylanthraquinone, anthrachinone or ß-chloroanthraquinone can be used. Because of its easier availability and greater solubility, ß-methylanthraquinone is particularly cheap.

The antioxidants as well as the hydrogen acceptors seem to work together in such a way that they reduce the corona resistance when one of the two substances is used in high concentrations. Therefore, it is beneficial if none of these ingredients are used in an amount of more than 1%. A composition in which both components are effective contains about 0.2% di-tert-butyl-p-cresol and about 0.5% β-methylanthraquinone. Although this is not necessarily preferred, the liquid can still contain up to 2%, advantageously 0.05 to 2%, of epoxy resin such as glycidylphenyl ether in order to increase the corona strength.

The capacitors preferably contain dielectric spacers made of film (e.g. polypropylene) and paper or made of 100% film, because with capacitors of this type a high dielectric constant of the impregnation liquid is not so critical. However, the liquid can also be used for capacitors containing 100% paper dielectric spacers.

The invention will now be explained in more detail with reference to the following examples:

3,642,768

example 1

Experiments were carried out with small impregnated capacitors with a capacitance of about 12 microfarads, which contained polypropylene film and paper, which 5 same dielectric is also used in high-voltage line capacitors. The windings contained 75 gauge film and 45 gauge paper in a film-paper-film configuration with one of the films narrower than the other and folded at the edges. (With this film arrangement, the electrical load on the film edge is lower than if two films of the same width and films with sharp edges are used in the usual way.) A severe overvoltage test was carried out on the capacitors, in which they were continuously connected to i5 with a voltage of 2.7 kV, which is 10% above the nominal voltage, and were also subjected to 6 cycles of overvoltage of 8.1 kV, that is 3.3 times the nominal voltage, every 3 minutes. (The highest overvoltage to which a capacitor is subjected in practical operation is three times the nominal voltage, and such an overvoltage occurs very rarely.) The following table shows the results of this test on capacitors of this type which are used with a dielectric Liquid impregnated containing 80% diisopropylbiphe-25 nyl and on a currently accepted condenser containing less than 5% diisopropylbiphenyl and more than 95% monoilopropylbiphenyl. The table also included results obtained from capacitors impregnated with trichlorobiphenyl, which had been used until recently, and with mineral oil containing about 80% aliphatic hydrocarbons.

Impregnation agent

35

80% diisopropylbiphenyl, 14% triisopropylbiphenyl 40 and about 5% monoisopropylbiphenyl + 0.2% di-tert-butyl-p-cresol + 0.5% β-methylanthraquinone 96%) monoisopropylbiphenyl, 45 3% diisopropylbiphenyl, 0.2% Di-tert-butyl-p-cresol, 0.5% ß-methylanthraquinone trichlorobiphenyl + 0.3% ß-methylanthraquinone, mineral oil + 0.2%) di-tert.-butyl-p-cresol + 0.5% ) ß-methylanthraquinone (no rounded foils)

Number of overvoltage attempts until failure

603,700

> 1005,> 1475, 1058,> 1430

170.240 9.23

55

Regarding the above results, it can be seen that a high level of diisopropylbiphenyl, although not as good as a high level of monoisopropylbiphenyl, did lead to better results than 6o trichlorobiphenyl and is satisfactory for commercial high voltage capacitors.

Example 2

This example shows that dielectric fluids based on isopropylbiphenyl containing high concentrations of diisopropylbiphenyl are more fire resistant than those containing high concentrations of monoisopropylbiphenyl. The following table gives the

642 768

Results of experiments again, which carry the designation "Cleveland Open Cup Test" and which show the flash point and the fire point of different mixtures of mono- and diisopropylbiphenyl.

Liquid Cleveland Open Cup test, ° C

Composition Flash point focus

(Flash Point) (Fire Point)

80% di, 14% tri, 5%

180

186

Monoisopropylbiphenyl

53% di, 10% tri, 36%

160

170

Monoisopropylbiphenyl

27% di, 5% tri, 65%

157

171

Monoisopropylbiphenyl

1% di-, 95% mono-iso-

150

160

propylbiphenyl

Example 3

Attempts have been made to determine the low temperature stability of dielectric fluids based on di-isopropylbiphenyl containing different amounts of diisopropylbiphenyl.

A series of test tubes containing liquids with different amounts of mono- and di-isopropylbiphenyl were subjected to 6 repeated cycles of 2 hours each at temperatures of -30 ° C and 2 hours at -60 ° C and the development of opaque crystals was observed . Resistance to crystallization, characterized by remaining transparent in a glassy state, is necessary for liquid stability at low temperatures, since gas-filled cavities can form during the crystallization and break through at high voltages. The cyclic treatment at - 30 ° C and - 60 ° C accelerates the crystallization effects in liquids that are sensitive to these effects. One of the two stock liquids contained 97.2% monoisopropylbiphenyl, with 54.8% para isomer and 42.4% meta isomer, which caused the solution to crystallize easily because of the larger proportion of para isomer . The other stock liquid consisted of 80% di-, 14% tri- and 5% io mono-isopropylbiphenyl. The following table shows the observations made with these liquids after 6 low temperature cycles.

Liquid composition appearance at - 30 ° C

i5 and at - 60 ° C

% Mono-% di-% tri- after 6 cycles with isopropylbiphenyl at these temperatures

97.2 2 - all opaque

2088 10 - lower 2/3 opaque,

transparent above 79 17.5 3 lower 1/3 opaque,

transparent above 61 34 5 everything transparent

25 24.5 64.5 11 everything transparent

6 80 14 everything transparent

It can be seen that low temperature crystallization of monoisopropylbiphenyl is effectively suppressed 30 when about 25% or more diisopropylbiphenyl is used, and that the liquid containing 80% diisopropylbiphenyl also continues to resist crystallization.

C.

1 sheet of drawings

Claims (9)

642 768 PATENT CLAIMS 1. capacitor with layers of metal foils alternating with dielectric spacers which are impregnated with a dielectric liquid, characterized in that the dielectric liquid contains 25 to 100 weight percent diisopropylbiphenyl. 2. Capacitor according to claim 1, characterized in that the dielectric liquid contains 25 to 100 percent by weight diisopropylbiphenyl, 0 to 80 percent by weight mono-isopropylbiphenyl and 0 to 20 percent by weight triisopropylbiphenyl. 3. A capacitor according to claim 2, characterized in that the amount of diisopropylbiphenyl is 25 to 80 percent by weight, the amount of monoisopropylbiphenyl is 20 to 75 percent by weight and the amount of triisopropylbiphenyl makes up to 15 percent by weight. 4. Capacitor according to one of claims 1 to 3, characterized in that the dielectric liquid contains up to 1 percent by weight of an antioxidant and up to 1 percent by weight of a hydrogen acceptor compound. 5. A capacitor according to claim 4, characterized in that the antioxidant is di-tert-butyl-p-cresol and the hydrogen acceptor is ß-methylanthraquinone. 6. Condenser according to one of claims 1 to 5, characterized in that the dielectric liquid contains 0.05 to 2 percent by weight of an epoxy resin. 7. Capacitor according to one of claims 1 to 6, characterized in that the dielectric spacer consists of paper and film, only of film or only of paper. 8. Capacitor according to one of claims 1 to 7, characterized in that every second layer of the metal foil is narrower and has rounded edges. 9. Condenser according to one of claims 1 to 8, characterized in that the amount of diisopropylbiphenyl is 35 to 80 percent by weight, the amount of monoisopropylbiphenyl is 20 to 65 percent by weight and the amount of triisopropylbiphenyl makes up to 15 percent by weight.