GB1602092A - Fluid insulated electrical apparatus - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FLUID INSULATED ELECTRICAL APPARATUS (71) We, THE MICANITE AND INSULATORS COMPANY LIMITED, of Westinghouse Road, Trafford Park, Manchester, M17 IPR, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to fluid insulated electrical apparatus, that is to say electrical apparatus which, for operation, is impregnated with or immersed in an electrically insulating fluid, the invention being more particularly concerned with the provision of improved dielectric fluids for use for such insulation purposes.

In some electrical installations, equipment such as transformers, capacitors, cables and switchgear are required to be insulated by impregnation with or immersion in a dielectric liquid. Petroleum oils have been used for this purpose, but are disadvantageous in that they are highly inflammable and are liable to undergo decomposition at high temperatures. Some non-inflammable chlorinated hydrocarbons have been emploved as substitutes. but the manufacture of these materials involves a toxicity risk. It has also been proposed to use silicone oils, but these substances are somewhat inflammable and suffer from the additional disadvantage that they readily polymerise when subjected to a partial electric discharge, forming gelatinous deposits.

It is an object of the present invention to provide electrical apparatus insulated with a liquid which has good dielectric properties and is less subject to the abovementioned disadvantages than are the fluids hitherto proposed.

According to the invention, in fluid insulated electrical apparatus, as hereinbefore defined, the insulating fluid consists of an aliphatic ester formed by the reaction between methylol-substituted methane, ethane or propane, and an aliphatic straight chain mono-carboxylic acid having from seven to ten carbon atoms in the molecule, or of an aromatic ester formed by the reaction between trimellitic acid and one or more aliphatic monohydric alcohols having from two to nine carbon atoms in the molecule, or of a mixture of two or more of said esters.

Preferred aliphatic esters for use in accordance with the invention are tetraesters of pentaerythritol and tri-esters of trimethylol-propane, formed by the reaction of pentaerythritol and trimethylol-propane respectively with acids such as, for example, caprylic acid, heptanoic acid and pelargonic acid. The preferred aromatic esters are tri-esters formed by the reaction between trimellitic acid and either straight chain or branched aliphatic monohydric alcohols, especially the higher alcohols having seven, eight or nine carbon atoms; mixtures of alcohols may be employed.

The aliphatic esters of the class specified above are low viscosity, high boiling point liquids having markedly improved fire resistance in comparison with the equivalent grade of petroleum oil. These liquids also possess good electrical properties, in particular having adequate permittivity, low loss tangent, high electrical resistivity, and high breakdown strength; the electrical properties, especially the loss tangent and the resistivity, improve with increasing molecular weight and increasing symmetry of the molecule. The aromatic esters specified have higher viscosity and high boiling points, and also have good fire resistance and good electrical properties.

We have also found that these esters do not polymerise on exposure to high energy electron bombardment such as occurs in the presence of a partial electric discharge, so they will not form gelatinous deposits when subjected to such a discharge. Additional advantages of these materials are that they are non-toxic, are relatively inexpensive and can readily be disposed of, when necessary, by combustion or by bacterial action, so that they will present no ecological hazard.

Two or more of the esters of the classes specified herein can be blended together in any desired proportions, for obtaining insulating fluids having specific properties required for any particular application. We have found that particular advantages can be obtained by mixing an aromatic ester with an aliphatic ester: by varying the relative proportions of aromatic and aliphatic esters in such mixtures, a range of insulating fluids having controlled degrees of aliphatic/aromatic nature, as well as controlled electrical properties, viscosity and fire resistance, can be produced.

Some specific examples of typical esters suitable for use in accordance with the invention are listed below, with details of their properties. In each case. the breakdown strength was determined by the method specified in British Standard No. 148, the fire point was determined by the method specified in the Institute of Petroleum Handbook No. 36/63, and the autoignition temperature was determined by the method specified in the American Society for Testing Materials Standard No. D 286-58T.

Example 1 Trimethylol-propane triheptanoate, formed by reacting trimethylol-propane (C113 . CH2 . C(CH2OH)3) with heptanoic acid (C6H13 . CO2H).

Properties: Viscosity at 300C 24 centistokes Boiling point at 1 torr pressure 225"C Tan a 0.026 Resistivity lox 1012 ohm cm Permittivity 3.3 Breakdown strength 55 kV Fire point 530"F Autoignition temperature 835"F Example 2 Trimethylol-propane tricaprylate, formed by reacting trimethylol-propane with caprylic acid (C7H,5. CO2H).

Properties: Viscosity at 300C 30 centistokes Boiling point at 1 torr pressure 240"C Tan 8 0.003 Resistivity 6x 1012 ohm cm Permittivity 3.2 Breakdown strength 70 kV Fire point 560"F Autoignition temperature 840"F Example 3 Pentaerythritol tetrapelargonate, formed by reacting pentaerythritol (C(CH2OH)4) with pelargonic acid (C8H17 . CO2H).

Properties: Viscosity at 40"C 34 centistokes Boiling point at 1 torr pressure 255"C Tan a 0.002 Resistivity 10x 1012 ohm cm Permittivity 3.1 Breakdown strength 30 kV Fire point 635"F Autoignition temperature 850"F Example 4 A trimellitate formed by reacting trimellitic acid (benzene - 1:2:4 - tricarboxylic acid) with a mixture of the three straight chain alcohols n-heptanol (C7H15OH), n-octanol (C8Hl,OH) and n-nonanol (C9H19OII).

Properties: Viscosity at 100 C 7.5 centistokes Viscosity at 400C 53 centistokes Pour point -45 C Boiling point at I torr pressure 250"C Specific gravity at 200C 0.995 Fire point 300"C Flash point 276"C Tan 8 0.02 Resistivity 2x 1012 ohm cm Permittivity 5.0 Breakdown strength 55 kV Satisfactory insulating fluids are produced by mixing this trimellitate ester with pentaerythritol tetraheptanoate, in any proportions up to 50% trimellitate, by volume.

If desired the insulating fluids of the invention can be mixed with conventional anti-oxidants, for example substituted phenols or alkylated secondary amines, which give improved thermal stability without having any appreciable detrimental effect on the electrical properties of the fluid.

We have also found that the resistance of these esters to ignition by a flame can be markedly improved by the addition of a flame-retardant fluid. Examples of such additives are trichlorobenzene, hexachlorobutadiene, tritolylphosphate, and other inorganic compounds containing phosphate, chloride, bromide or fluoride substituents. Such compounds are effective as flame retardants if added to the insulating fluid in a proportion between 10% and 40% by volume of the volume of ester or mixture of esters.

WHAT WE CLAIM IS: 1. Fluid insulated electrical apparatus, as hereinbefore defined, wherein the insulating fluid consists of an aliphatic ester formed by the reaction between methylol-substituted methane, ethane or propane, and an aliphatic straight chain monocarboxylic acid having from seven to ten carbon atoms in the molecule, or of an aromatic ester formed by the reaction between trimellitic acid and one or more aliphatic monohydric alcohols having from two to nine carbon atoms in the molecule, or of a mixture of two or more of said esters.

2. Apparatus according to Claim 1, wherein the said ester, or each ester in said mixture, is a tetra-ester of pentaerythritol or a tri-ester of trimethylol-propane, or a tri-ester of trimellitic acid.

3. Apparatus according to Claim 2, wherein the said ester, or each ester in said mixture, is one formed by the reaction of pentaerythritol or trimethylol-propane with caprylic acid or heptanoic acid or pelargonic acid, or one formed by reaction of trimellitic acid with one or more of the said aliphatic monohydric alcohols having seven, eight or nine carbon atoms in the molecule.

4. Apparatus according to Claim 1, 2 or 3, wherein the said insulating fluid consists of a mixture of at least one said aliphatic ester and at least one said aromatic ester.

5. Apparatus according to Claim 1, wherein the said insulating fluid consists of one or more of the esters described in the foregoing specific examples.

6. Apparatus according to any preceding claim, wherein an anti-oxidant is mixed with said insulating fluid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. Example 4 A trimellitate formed by reacting trimellitic acid (benzene - 1:2:4 - tricarboxylic acid) with a mixture of the three straight chain alcohols n-heptanol (C7H15OH), n-octanol (C8Hl,OH) and n-nonanol (C9H19OII). Properties: Viscosity at 100 C 7.5 centistokes Viscosity at 400C 53 centistokes Pour point -45 C Boiling point at I torr pressure 250"C Specific gravity at 200C 0.995 Fire point 300"C Flash point 276"C Tan 8 0.02 Resistivity 2x 1012 ohm cm Permittivity 5.0 Breakdown strength 55 kV Satisfactory insulating fluids are produced by mixing this trimellitate ester with pentaerythritol tetraheptanoate, in any proportions up to 50% trimellitate, by volume. If desired the insulating fluids of the invention can be mixed with conventional anti-oxidants, for example substituted phenols or alkylated secondary amines, which give improved thermal stability without having any appreciable detrimental effect on the electrical properties of the fluid. We have also found that the resistance of these esters to ignition by a flame can be markedly improved by the addition of a flame-retardant fluid. Examples of such additives are trichlorobenzene, hexachlorobutadiene, tritolylphosphate, and other inorganic compounds containing phosphate, chloride, bromide or fluoride substituents. Such compounds are effective as flame retardants if added to the insulating fluid in a proportion between 10% and 40% by volume of the volume of ester or mixture of esters. WHAT WE CLAIM IS:

1. Fluid insulated electrical apparatus, as hereinbefore defined, wherein the insulating fluid consists of an aliphatic ester formed by the reaction between methylol-substituted methane, ethane or propane, and an aliphatic straight chain monocarboxylic acid having from seven to ten carbon atoms in the molecule, or of an aromatic ester formed by the reaction between trimellitic acid and one or more aliphatic monohydric alcohols having from two to nine carbon atoms in the molecule, or of a mixture of two or more of said esters.

2. Apparatus according to Claim 1, wherein the said ester, or each ester in said mixture, is a tetra-ester of pentaerythritol or a tri-ester of trimethylol-propane, or a tri-ester of trimellitic acid.

3. Apparatus according to Claim 2, wherein the said ester, or each ester in said mixture, is one formed by the reaction of pentaerythritol or trimethylol-propane with caprylic acid or heptanoic acid or pelargonic acid, or one formed by reaction of trimellitic acid with one or more of the said aliphatic monohydric alcohols having seven, eight or nine carbon atoms in the molecule.

4. Apparatus according to Claim 1, 2 or 3, wherein the said insulating fluid consists of a mixture of at least one said aliphatic ester and at least one said aromatic ester.

5. Apparatus according to Claim 1, wherein the said insulating fluid consists of one or more of the esters described in the foregoing specific examples.

6. Apparatus according to any preceding claim, wherein an anti-oxidant is mixed with said insulating fluid.

7. Apparatus according to any preceding claim, wherein a flame-retardant

fluid is mixed with said insulating fluid, the proportion of flame-retardant fluid being from 10% to 40% by volume of the volume of the said ester or mixture of esters.