NO133949B - - Google Patents

Description

The present invention relates to the use of certain ester/hydrocarbon or ester/ether mixtures as dielectric fluid.

Dielectric fluids are used in capacitors, transformers, coaxial cables and other types of electrical devices to exclude gases from the device's insulating part and thereby produce insulation that can withstand higher voltages.

A good dielectric fluid should have a high electrical resistance

and be cheap and preferably have fire resistance. It should be easy to clean to achieve low conductivity or low loss factor. If it is used in capacitors, it should have a relatively high dielectric constant and should be able to impregnate the capacitor's dielectric barrier layer. It should have good corona properties, especially if it is used in capacitors. Good corona properties include (in case of partial discharge) high corona start-up and extinguishing voltages in an experiment where the voltage is increased until -corona sets in and then reduced. Furthermore, strong "corona" (which may occur sporadically in operation) should not produce permanent damage, such as persistent blistering, which would significantly reduce the corona quench and restart voltages. The dielectric fluid should retain its good electrical properties

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at high "temperatures in there._dielectric and metallic components

in the equipment. It should not be too volatile, as volatile liquids are difficult to handle and evaporate quickly during use. If a

dielectric fluid turns into solid form', ruptures, it and loses /dielectric strength. Therefore, a good dielectric fluid should have a low solidification or breaking temperature, so that it can be used at a low temperature. ,.;

Currently, chlorinated diphenyl compounds, such as trichlorodiphenyl (hereafter referred to as "TCDP"), are widely used commercially as dielectric fluids, as they provide a very good compromise regarding many desirable properties. But dielectric fluids are used in large quantities, and there is always a risk of accidental loss or leakage of the fluid. Many of the chlorinated compounds are not rapidly biodegradable, but remain for long periods and accumulate in the food chain. Despite their desirable properties, it is therefore possible that chlorinated diphenyls may soon be completely prohibited for use as dielectric fluids, or such precautions may be involved that their use will become very expensive. Since consideration of the environment has become a very important factor in the selection of dielectric fluids in recent years, it is very difficult to find satisfactory replacements for these halogenated dielectric fluids.

The mixture which according to the invention is used as a dielectric fluid contains from 20 to 95 percent by weight of a non-halogenated organic ester or ester mixture which is liquid between -20 and 150°C and has a loss factor of less than 10% at 100°C, and from 5 to 80% by weight of a non-halogenated aromatic hydrocarbon or a non-halogenated aromatic ether which is soluble in the ester or ester mixture and has 2 optionally fused benzene rings and a boiling point above 140°C and a loss factor of less than 10% at 100°C , preferably also a mixture-soluble antioxidant in an amount of 0.05

to 5.0% by weight calculated on the mixture of ester and hydrocarbon or ester and ether, and/or a hydrogen acceptor soluble in the mixture in an amount of 0.3 to 3.0% by weight calculated on the mixture of ester and hydrocarbon or ester and ether.

A mixture of the above esters and hydrocarbons or ethers is a very good dielectric fluid and, most importantly, it is biodegradable and does not pose a potential hazard to the environment. In electrical properties, such as electrical fastness, loss (or power) factor, dielectric constant as well as corona properties, it is comparable to TCDP. It can be easily cleaned to achieve a low loss factor. Its corona products do not persist as blisters. The preferred dielectric fluids are superior to TCDP in preserving the capacitor, indicating thermal stability and inertness to the capacitor components. Finally, the dielectric fluids used according to the present invention are cheaper than TCDP and can be used together with the same dielectric materials used in capacitors containing TCDP.

In order for the invention to be better understood, embodiments thereof will now be described by means of examples and with reference to the accompanying drawings, in which: Fig. 1 shows an isometric view with an enlarged part showing the construction of a film-paper-film -capacitor winding. Fig. 2 shows an isometric view with a part partially cut away, which shows the winding according to fig. 1 placed in a container to form a complete condenser.

According to fig. 1 is two layers of metal foil 1 and 2 wrapped with layer 3 of paper and layer 4 of plastic film. The plastic film is preferably polypropylene, as this enables the condenser to work at a high KVAR (reactive kilovolt-ampere) per volume unit due to its high electrical strength and low loss factor. It is also not degraded by the dielectric fluids used according to the present invention even under extreme operating conditions. Other plastic films, such as HD polyethylene, can also be used. Wires 5 and 6 are connected to foils 1 and 2. A capacitor winding 7 is placed in a container 8 and is impregnated with a dielectric fluid 9 (see fig. 2). Wires 5 and 6 are connected to connection points 10 and 11.

The dielectric fluids with the best properties contain a mixture of from 75 to 90 percent by weight of the organic ester and from 10 to 25 percent by weight of the aromatic hydrocarbon or aromatic ether.

The organic ester preferably contains a benzene ring, since aromatics have better corona resistance and have better thermal stability. Either the ester's acid part or its base part can be aromatic. Phthalic acid esters seem to have better properties than other esters, and especially diisononyl phthalate (hereinafter referred to as "DINP") has excellent properties. Other suitable esters include dibutyl adipate, diethyl adipate, diisodecyl adipate, ethyl benzoate, n-butyl benzoate, isopropyl benzoate, isobutyl benzoate, n-propyl benzoate, ethyl capronate, ethyl caprylate, methyl caprylate, diethyl malonate, dimethyl malonate, di-ethyl oxalate, dibutyl phthalate, diethyl phthalate, dimethyl phthalate, dioctyl phthalate, diethyl succinate, butyl valerate etc. Mixtures of suitable esters can be prepared to satisfy the temperature requirements and can also be used when the individual esters do not satisfy the temperature requirements.

The aromatic hydrocarbon or the aromatic ether is

a compound which is soluble in the ester, has one or two benzene rings which may be condensed, has a loss factor of less than 10% at 100°C. They should have a low vapor pressure, as compounds with a low boiling point are difficult to handle and can evaporate in the condenser, and its boiling point is above 14 0°C. Two-ring compounds are used as these have a boiling point of over 140°C. Diphenyl oxide (hereinafter referred to as DPO) is preferred as it has excellent electrical properties and does not soften the polypropylene film or react with it in the capacitor. Other suitable aromatic hydrocarbons or ethers include acenaphthene, dibenzfuran, diphenyl, diphenylmethane, fluorene, naphthalene, phenetol, etc. Mixtures of aromatic hydrocarbons and/or ethers are also contemplated.

Preferably, the dielectric fluids also comprise from 0.05 to 5 percent by weight (of the ester plus the hydrocarbon or ether) of an antioxidant. The antioxidant is a free radical inhibitor which is soluble in the solution of the ester and the hydrocarbon or ether. It helps to keep the loss factor low for a long time both for the liquid itself and in the condenser. The preferred amount of antioxidant is from 0.1 to 3%, and a preferred antioxidant is di-tert-butyl-paracresol (hereinafter referred to as DTBPC)

which has been found to work very well. Other suitable antioxidants include phenols of the formula

where the R groups are typically hydrogen or alkyl. Other known antioxidants which do not increase the loss factor of the dielectric fluid can also be used. Mixtures of antioxidants can also be considered.

The dielectric fluid preferably also comprises from 0.3 to 3% (of the ester plus the hydrocarbon or ether) of a corona resistant additive to increase the resistance of the dielectric fluid to corona, by reducing the time required to recover from a surge which causes corona. This additive is a compound which reacts with and takes up hydrogen under discharge conditions and which is soluble in the solution of the ester and the aromatic ether or hydrocarbon. It is believed to work by absorbing hydrogen that is formed at the corona and thereby reduces the formation of blisters. A preferred additive is g-methylanthraquinone, as this is easy to handle and is stable. Other suitable additives include azobenzene, anthraquinone and other substances that can take up hydrogen. Mixtures of additives can also be used.

The invention will now be described using the following examples.

Example 1

A mixture civ 80% DINP - 20% DPO was prepared and several of its properties were compared with those of TCDP and mineral oil:

The above data show that the DINP-DPO blend has satisfactory characteristics for capacitor fabrication (ie, low enough viscosity and low enough vapor pressure and high enough flash point) and capacitor performance, which will be shown more fully in the following examples. As the flash point is the same as for mineral oil, corresponding precautions should be taken for this

mixture as those developed and accepted for mineral oil.

Example 2

Small experimental capacitors were made, each containing a winding of a flattened roll of sheets of 6 micron aluminum foil, 12.5 micron polypropylene, 13 micron paper of density 0.7, 12.5 micron polypropylene, 6 micron aluminum foil, 12.5 micron polypropylene, 13 micron paper and 12.5 micron polypropylene. These windings were approximately 76.2 mm high and 50.8 mm wide and

9.5 mm thick and had a capacitance of about 0.3 u F. Some capacitors were impregnated with DINP and some with 80% DINP - 20% DPO. The treatment was carried out according to methods that are known and used by professionals in the field. The impregnation was carried out by adding the dielectric fluid under vacuum, increasing the pressure to atmospheric pressure and then performing a vacuum-pressure cycle several times. The device impregnated with 80% DINP - 20% DPO required fewer such cycles to become impregnated than those impregnated with DINP.

Example 3

Capacitors were prepared as according to example 2, with 80% DINP - 20% DPO and with TCDP. The following table gives the results of dielectric tests for the capacitors and includes extrapolated data for mineral oil-impregnated capacitors.

The table above shows that capacitors impregnated with DINP - DPO have dielectric constants and power factors comparable to capacitors impregnated with TCDP. It also shows that mineral oil is a relatively poor impregnating agent for such capacitors as it gives a low corona cut-off voltage, which would give the capacitors a low cut-off voltage, as well as a low average dielectric constant.

Example 4

Using capacitors prepared as in Example 2, with different dielectric fluids, the corona properties were measured. The following table indicates the results.

The above test is considered to be extremely harsh in terms of the six seconds of continuous overvoltage. These overvoltages of 5300 and 4300 volts are the highest expected in power capacitor operation, in series and shunt, respectively, for dielectrics of this composition, and would occur for only a fraction of a cycle (<L1/60 second) at a -every time. This experiment shows, with a large margin of safety, whether a given dielectric will recover from the effects of such overvoltages. The table shows that the addition of DPO to DINP significantly increases corona resistance. The antioxidant DTBPC

had no further effect on this, but a hydrogen acceptor such as B-methylanthraquinone or azobenzene improved it further.

Example 5

As capacitors as described in example 2 impregnated with different dielectric fluids were used, the life of the capacitors was determined by aging at 125°C with 1600 volts applied voltage. The end point of the life of the capacitor was considered to be reached when the loss factor increased to a level where uncontrolled thermal change can occur in a large power capacitor. The following table indicates the results, where "relative lifetime" is the ratio between the lifetime of the capacitor and the lifetime of the capacitor impregnated with

TCDP:

The above-mentioned data indicate that the mixtures used according to the present invention are particularly good impregnation agents for film-paper capacitors and all-paper capacitors, and that capacitors impregnated with such mixtures containing antioxidants have a much longer service life than capacitors that are impregnated with TCDP. The data indicate that a service life of at least approximately 40 years can be expected for 150 KVAR power capacitors (with a preheater temperature of 85°C for 10% of the time) if the dielectric fluid is 80% DINP-20% DPO with 0.26% DTBPC.

Example 6

Capacitors of 0.8 µF were made with two layers of 12 micron paper with a density of 0.9 between aluminum foil. The capacitors were impregnated with 80% DINP- 20% DPO. The power factor was determined to be 0.26% at 85°C and 0.38% at 125°C with 440 volts applied, Corona onset voltages were over 800 volts. These capacitors could be used for lighting ballasts, motor starting, coromutators as well as low voltage power factor corrections.

Example 7

One liter volumes of 80% DINP - 20% DPO and 8u% DINP - 20% DPO containing 0.15% DTBPC antioxidant were prepared. Both liquids were exposed to ambient air at 100°C. After four days, the loss factor for the liquid that did not contain the antioxidant had increased to more than 10%. After two weeks, the loss factor for the liquid containing the antioxidant had increased by less than 1%.

Example 8

Experimental capacitor sections were produced according to example 5, where 0.15% DTBPC was used, some with tinned copper wires and others with aluminum wires, and placed in steel containers as shown in fig. 2. The capacitors were operated at 115°C at 1600 volts. Both groups had the same operating stability without harmful effects due to the presence of copper. Copper catalyzes the breakdown of the organic compounds with a high percentage of hydrogen, such as this impregnating agent, but the antioxidant is believed to have inhibited this effect.

Claims (2)

1. Use of a mixture of from 20 to 95% by weight of a non-halogenated organic ester or ester mixture which is liquid between -20 and +150°C and has a loss factor of less than 10% at 100°C, and from 5 to 80% by weight of at least one non-halogenated aromatic hydrocarbon or a non-halogenated aromatic ether which is soluble in the ester or ester mixture and has 2 possibly condensed benzene rings and a boiling point of over 140°C and a loss factor of less than 10% at 100°C, preferably also an antioxidant soluble in the mixture in an amount of 0.05 to 5% by weight calculated on the mixture of ester and hydrocarbon or ester and ether, and/or a hydrogen acceptor soluble in the mixture in an amount of 0.3 to 3.0 weight percent calculated from the mixture of ester and hydrocarbon or ester and ether - as dielectric fluid. 2. Use in accordance with claim 1 of a mixture where the ester occurs in an amount of from 75 to 90 percent by weight and the aro matic hydrocarbon or the aromatic ether occurs in an amount of from 10 to 25 percent by weight.