GB2094338A - Dielectrical insulating fluid for use in electrical capacitors - Google Patents

Abstract

A dielectric insulating fluid particularly suitable for use in electrical capacitors, for impregnating the sheets of insulating material contained therein, comprises at least one hydrocarbon of general formula [Rn-Ar-R']p-Ar-Rm, where Ar is a benzene, di- or polyphenyl, or naphthalene ring, R is H, alkyl or cycloaliphatic, R' is at least C2 alkylene group which may be substituted, n and m are 1-5, and p is at least 1.

Description

SPECIFICATION Dielectric insulating fluid particularly suitable for use in electrical capacitors, and a capacitor containing said fluid In the case of electrical capacitors comprising plates between which there are disposed sheets of dielectric material, such as paper or synthetic film, it is known to impregnate such sheets with dielectric fluids in order to give them the required insulating properties.

For this purpose, the industry of this sector has up to the present time mainly used dielectric fluids in the form of halogenated diphenyl derivatives, in particular oils known as Askerels.

However, the dangers of using such halogenated derivatives have been reported, including for example in certain EEC directives, in particular because of their toxicity and their tendency to accumulate in the environment, with serious damage to the ecosystem.

The industry of this sector has therefore had to seek replacement compounds for said halogenated diphenyl derivatives which possess low toxicity towards man and animals, and high biodegradability should they become dispersed into the environment.

The main object of the invention is therefore to provide a dielectric insulating fluid, in particular for use as an oil for impregnating electrical capacitors, as a replacement for the commonly known oils for this purpose, in particular for said halogenated diphenyl derivatives.

It must also be noted that a suitable dielectric oil for capacitors must possess low viscosity, together with good compatability with and chemical inertia towards the capacitor components.

Furthermore, the liquid must be able to tolerate the severe operating conditions of the capacitor without undergoing alteration, especially where the uniformity of the dielectric field is less, for example towards the plate edges, in which zones the insulation is subjected to particular stress. To this must be added the fact that possible limited partial discharges within the dielectric must not prejudice the capacitor life.

These are therefore the characteristics which the dielectric fluid according to the invention must possess.

A further object of the invention is to provide a dielectric fluid with the aforesaid characteristics by means of a preparation process which is substantially simple to carry out and of low cost, for example by starting with relatively inexpensive, easily obtainable reactants.

In attaining these objects, according to the present invention it has been surprisingly found that a dielectric insulating oil particularly suitable for use in electrical capacitors can be advantageously represented by at least one hydrocarbon of the following general formula: [Rn-Ar-R']p-Ar-Rm in which: Ar is a benzene, diphenyl or polyphenyl, or naphthalene-ring, R can be H, CH3, C2H5, isopropyl or other higher homologues, or a cyciic aliphatic group, R' is the group -CH2-CH2-, or its higher homologues, including branched, in which H can be substituted by other atoms or groups, n and m are equal or different whole numbers lying between 1 and 5, p is a whole number equal to 1 or more.

Furthermore, if n is greater than 1, the substituents R can then be different from each other. The same is valid for the value of m.

The structural formulas of some hydrocarbons according to the invention are given hereinafter by way of non-limiting example thereof:

and its higher homologue:

and its higher homologue:

According to the invention, the synthesis of these compounds is based essentially on the Friedel Crafts reaction between a dihalogen derivative of the type: X-R-X in which X is a halogen and R is a -CH2-CH2- group or its homologues, including branched, where H can be substituted by another atom or radical, and a hydrocarbon of the type: ArR in which Ar is a benzene, diphenyi or generally polyphenyl, or naphthalene ring, R is H or an aliphatic or cycloaliphatic group, and n is a whole number of between 1 and 5.

Furthermore, the substituents can be the same or different if more than one.

The reaction is carried out with classical catalysts of the type AICI3, FeCI3, BF3, at moderate temperature, so that isomerisation phenomena are minimised.

The molar ratio of the aromatic hydrocarbon to the halogen derivative can vary widely according to whether it is preferred to obtain mainly compounds of low molecular weight, or compounds of longer chain and thus higher-boiling.

In addition, the hydrochloric acid which is evolved can be recovered in the form of a concentrated aqueous solution, and reused as such.

On termination of the reaction, after destroying the catalyst with water or alkali, and washing and neutralising the reaction mass, the excess hydrocarbon is separated by rectification, and the final produce is recovered by distillation.

After subjection to dielectric purification by the traditional methods used in this field, low viscosity oils of high boiling point are obtained. According to the invention, said oils are suitable for use as dielectrics, in particular in capacitors. Some examples of the synthesis of oils prepared according to the present invention are given hereinafter.

EXAMPLE 1 55.4 kg of toluene are fed into a 60 litre reactor fitted with a stirrer, condenser, heater and a gaseous HCI removal system. 9.9 kg of toluene are distilled off in order to dehydrate the remaining 45.5 kg (corresponding to 493.8 moles).

It is cooled to 800C, and AIC13 (40 g) is added, after which 4.073 kg of 1,2-dichloroethane are dripped in slowly over a period of about one hour.

The reaction is complete after about six hours when HCI ceases to be evolved, and gas chromatograph analysis shows the disappearance of the dichloroethane. The mass is washed twice with equal volumes of warm water, and the organic phase is separated and subjected to rectification (20 plates) in order to completely remove the toluene.

The residual mass is distilled under vacuum to produce 7.250 kg of oily fluid product having a slight straw colour. Said product corresponds to a mixture of said hydrocarbons of formula (la) and (Ib).

After dielectric purification, said product shows the following characteristics: Dielectric constant at.900C 2.44 tgs x 100 (900C-50 Hz) 0.15 Viscosity at 200C 7.8 cst Dielectric strength 320 kV/cm Density at 200C 0.964 This oil was used experimentally as a dielectric in the construction of several capacitors. The tests gave a positive result.

EXAMPLE 2 A Friedel-Crafts reaction was carried out in a manner entirely similar to that described in example 1, but using xylene (mixture of isomers).

After removing the xylene by rectification, a clear oily fluid product was obtained by distillation under vacuum at 2 torr. Said product corresponds to a mixture of said hydrocarbons of formula (ill) and (llb). When purified for dielectric use, it shows the following characteristics: Dielectric constant at 900C 2.41 tgA x 100(900C--50 Hz) 0.20 Viscosity at 200C 1 8.3 cst On mixing the oil produced in said example 1 with the oil produced in example 2, mixtures were obtained having average dielectric characteristics, but possessing an exceptionally low pour point.

For example on mixing 80 parts by weight of the product prepared in example 1 with 20 parts by weight of the product prepared in example 2, an oil was obtained, which will be known as (I + II), having the following characteristics: Viscosity at 200C 9.7 cst Dielectric constant (900C) 2.43 tgA x 100(900C--50 Hz) 0.17 Dielectric strength 320 kV/cm After remaining for several days at a temperature of -250C, this liquid showed no crystal formation, thus proving suitable for use as an insulating oil even in rigid climates.

Examples are given hereinafter of the application of the oils according to the invention to capacitors, compared with a typical oil of the known art, namelytrichlorodiphenyl.

EXAMPLE 3 In this example, capacitors with a cylindrical element were used, the dielectric of which consisted of two papers of density equal to 1, with a total thickness of 1 8 Mm.

The plates consisted of an aluminium sheet of thickness 5 Mm, and of such a length as to produce capacitors having capacitance values which were similar whatever the impregnating fluid used.

Said oil (I + II) of the present invention and trichlorodiphenyl as the comparison oil were used for impregnating the capacitors.

The capacitors were subjected to a life test at 500 V.a.c., 50 Hz at 700 C. The results obtained are given in the following table.

Final measurements No. of Initial after capacitors Measurement measurements 1000 hours temperature negative Oil 0C gF tgd% F tgd% tested tests 1+11 20 7.32 0.33 7.37 0.42 10 2 Trichloro- diphenyl 20 7.11 0.39 7.17 0.62 8 6

EXAMPLE 4 Capacitors with a cylindrical element were constructed having a dielectric consisting of two polypropylene sheets with a total thickness of 30 m.

The polypropylene used was of the type suitable for impregnation.

The plates consisted of an aluminium sheet of thickness 5 ,um.

The capacitors were again impregnated with the oils already indicated in example 3.

The capacitors were subjected to a life test at 1950 V.a.c., 50 Hz at 70"C. The results obtained are given in the following table.

Final measurements No. of Initial after capacitors Measurement measurements t000 hours temperature negative Oil C F tgd% F tgd% tested tests I + 11 20 2.23 0.027 2.27 0.030 8 1 Trich loro diphenyl 20 2.43 0.033 2.48 0.039 8 3 From these examples, the comparison of the capacitors which failed during the test is particularly significant, showing the technical progress of the oils according to the invention.

The process proposed by the invention for preparing hydrocarbon compounds for use as dielectric fluids, as can be deduced from the aforegoing description, is substantially simple to carry out and in particular allows low-cost compounds of relatively wide availability to be used as the reactants.

It can therefore be seen that the invention attains the stated objects.

The dielectric fluid of the invention, which as stated is particularly suitable for use as an insulant in capacitors, is also suitable for transformers and electrical equipment in general. A further interesting application of the fluids of the invention is as a heat transfer liquid in heat exchangers.

In addition, the dielectric fluid of the invention can comprise other compounds of known type with dielectric properties in mixture with the aforesaid hydrocarbons.

Claims (9)

1. A dielectric insulating fluid, particularly suitable for use in capacitors, characterised by comprising at least one hydrocarbon of the following general formula: [Rn-Ar-R'jp-Ar-Rrn in which: Ar is a benzene, diphenyl or polyphenyl, or naphthalene ring, R can be chosen from H, CH3, C2Hs, isopropyl or higher homologues, or a cycloaliphatic group, R' is a group of the type -CH2-CH2- or its higher homologues, including branched, in which one or more hydrogens can be substituted by other atoms or groups, n and m are equal or different whole numbers lying between 1 and 5, p is a whole number equal to 1 or more, said R groups being the same or different.

2. A dielectric insulating fluid as claimed in claim 1, characterised in that said hydrocarbon has the following structural formula:

3. A dielectric insulating fluid as claimed in claim 1, characterised in that said hydrocarbon has the following structural formula:

4. A dielectric insulating fluid as claimed in claim 1, characterised in that said hydrocarbon has the following formula:

5. A dielectric insulating fluid as claimed in claim 1, characterised in that said hydrocarbon has the following formula:

6. A dielectric insulating fluid as claimed in claim 1, characterised by comprising a mixture of hydrocarbons of formula (la) and (Ib).

7. A dielectric insulating fluid as claimed in claim 1, characterised by comprising a mixture of said hydrocarbons of formula (Ila) and (llb).

8. A dielectric insulating fluid as claimed in claim 1, characterised by comprising a mixture of said hydrocarbons of formula (la), (Ib), (Ila) and (llb).

9. An electrical capacitor comprising at least one layer of dielectric material such as paper or synthetic material, characterised in that said layer is impregnated with a dielectric insulating fluid as claimed in claim 1.

1 0. A dielectric insulating fluid and electrical capacitor substantially as heretofore described, in particular in the examples.