Dielectric Fluid, Capacitor, and Transformer

Until a few years ago, polychlorinated biphenyls (PCB's) were widely used as dielectric fluids in transformers and capacitors. However, when it was discovered that PCB's persisted in the environment and had deleterious effects on wildlife and possibly people, government regulations began to prohibit their use. As PCB's were phased out, it was necessary to find other dielectric fluids of comparable electrical, chemical, and physical properties.

One of the most important electrical properties for a dielectric fluid is a high dielectric strength, particularly a high partial discharge onset and offset voltage. While many of the fluids that were substituted for PCB's were comparable in other properties, in these two properties they were often deficient.

We have discovered a dielectric fluid which has great resistance to partial discharges and has greater dielectric strength than prior fluids, particularly in non-uniform electrical fields such as occur around points and edges. The fluids of this invention tend to evolve less hydrogen under discharge, and the discharge pulse size is smaller.

U.S. Pat. No. 2,176,952 discloses a dielectric fluid which includes a combination of mineral oil and a compound such as diphenyl oxide.

U.S. Pat. No. 4,054,937 discloses a capacitor containing a dielectric fluid of a mixture of monoalkylated, and di-alkylated aromatic compounds.

FIG. 1 is an isometric view with an enlargement window illustrating the construction of a film-paper-film capacitor winding.

FIG. 2 is an isometric view partially cut away showing a certain presently preferred embodiment of a capacitor in accordance with the invention.

FIG. 3 is a side view in section of a certain presently preferred embodiment of a transformer containing a dielectric fluid stabilized according to this invention.

In FIG. 1, two layers of metal foil 1 and 2 are wound with layers of paper 3 and layers of insulating film 4. The insulating film is preferably polypropylene because it permits a capacitor to operate at a high KVAR (kilovolt amperes reactive) per unit volume due to its high electric strength and low dissipation factor. Also, it is not degraded by the dielectric fluids of this invention even under extreme operating conditions. Other insulating films such as high density polyethylene could also be used. Leads 5 and 6 are connected to foils 1 and 2, respectively. The capacitor winding 7 is placed in a can and is impregnated with a dielectric fluid according to this invention.

In FIG. 2, a capacitor unit 8 having a unitary housing 9 that encloses a plurality of wound capacitor sections 10 that are stacked on end in the housing. Each capacitor section 10 comprises electrode foil material and a dielectric material which consists entirely a plastic film such as polypropylene. Each section 10 is wound with dielectric material on each side of the two electrodes in a convolute winding. The unit is impregnated with a liquid dielectric fluid according to this invention. The plurality of capacitor sections are mutually interconnected by metal areas 11 deposited on the ends of the sections and contacting extended foil material that extends from the sections outside the zone in which the dielectric film is located. The plurality of interconnected sections are connected to terminals 12 extending from housing 9 by wire conductors at selected locations.

In FIG. 3, a transformer 13 is shown as comprising a sealed tank 14, a ferrous metal core 15 a primary coil 16, a secondary coil 17, and a dielectric fluid 6 which surrounds and covers the core and coils.

The organic dielectric fluid of this invention is a mixture of the principal insulating fluid such as oil or a propyl biphenyl with an arene compound, plus a polymerization inhibitor. The arene compound is a compound having at least one aromatic group and vinyl or ethylenic unsaturation, and contains only hydrogen, carbon, and possibly ether-oxygen atoms. Thus, the compound could be a vinyl aromatic hydrocarbon (such as styrene), a vinyl aromatic ether hydrocarbon, an ethylenic aromatic hydrocarbon, or an ethylenic aromatic ether hydrocarbon. The total number of non-ethylene and non-aromatic carbon atoms in the arene compound should be less than seven because compounds with more non-ethylenic non-aromatic carbon atoms tend to produce too much hydrogen in an electrical discharge. A certain presently preferred general formula for the arene compound is ##STR1## where each R group is independently selected from hydrogen, hydrocarbon, and ether hydrocarbon, and at least one of the R groups is aromatic. Vinylic unsaturation is preferred to ethylenic unsaturation because vinylic unsaturation is more reactive with hydrogen. Preferred vinylically unsaturated aromatic compounds include vinyl xylene, vinyl toluene, and vinyl isopropyl biphenyl, because these compounds have high aromaticity. Other vinyl arene compounds which could be used include vinyl methyl diphenyl, vinyl dimethyl diphenyl, vinyl ethyl diphenyl, vinyl phenyl tolyl ether, and vinyl phenyl xylyl ether. Ethylenically unsaturated arene compounds which are preferred because of their high aromaticity are di-tolyl ethene and isopropyl diphenyl ethene. Xylyltolyl ethene can also be used.

The principal insulating fluid component can be either mineral oil or an aromatic hydrocarbon or an aromatic ether which can have saturated aliphatic substituents of 1 to 3 carbons. Aromatic hydrocarbons or ethers are preferred to mineral oil. The preferred mineral oil is transformer oil as it is better suited for electrical applications. The aromatic hydrocarbon or ether is a compound which contains at least one aromatic group and which consists of only hydrogen, carbon, and possibly ether oxygen atoms. The aromatic hydrocarbon or ether should also have less than seven non-aromatic carbon atoms as more non-aromatic carbon atoms tend to result in the production of too much hydrogen in an electrical discharge. Examples of suitable aromatic hydrocarbons or ethers include isopropyl biphenyl, diphenyl oxide, diphenyl, diphenyl methane, diphenyl ethane, benzyl phenyl ether, isopropyl diphenyl ether, xylyltolyl diphenyl ether, di-tolyl diphenyl ether, and ethyl methyl diphenyl ether. Compounds which contain aliphatic ether linkages (--CH.sub.2 --O--CH.sub.2 --), such as dibenzyl ether, although suitable electrically, should be avoided as they tend to be chemically unstable. The preferred aromatic hydrocarbon or ether is isopropyl biphenyl because it is important commercially.

The arene compound and the diluent are selected so that the dielectric fluid has a boiling point greater than 150.degree. C., a freezing point less than -40.degree. C., and a dielectric constant of less than 3.5. That is, either the arene compound or the diluent by itself may exceed these limits, but the mixture as a whole must fall within the limits to be useful as a dielectric fluid.

To prevent polymerization of the arene compound, it is preferable to also include a free-radical polymerization inhibitor in the dielectric fluid. Suitable polymerization inhibitors include t-butyl pyrocatechol and pyrocatechol.

The dielectric fluid of this invention is a mixture of about 5 to about 40 molar percent of the arene compound, about 60 to about 95 molar percent of the diluent, and about 0.5 to about 5 molar percent of the polymerization inhibitor. If less of the arene compound is used, there is little enhancement of the electrical properties of the diluent, and if more than 40 molar percent of the arene compound is used, it may tend to polymerize.

The dielectric fluid of this invention can be used in capacitors, transformers, cables, and other electrical equipment. However, it may not be suitable for use in circuit breakers because it may have a tendency to carbonize in an electric arc.

The following example further illustrates this invention.

Partial discharge tests were performed with a vacuum impregnated 0.030" pressboard (cellulose) barrier between a needle and a large sphere electrode in the liquid.

The above-described experiments showed that the addition of 5% and 25% of a vinyl substituted aromatic (styrene) significantly increased the discharge onset and offset voltage discharge pulse size of isopropyl biphenyl liquid. Styrene itself may be too low a boiling liquid for some processing conditions or applications. A higher boiling vinyl compound would be preferred.