WO2007093631A1

WO2007093631A1 - Electrical insulation oil composition

Info

Publication number: WO2007093631A1
Application number: PCT/EP2007/051484
Authority: WO
Inventors: Yoshiharu Baba; Hiroyuki Takahashi; Hideto Terai; Norimitsu Tanaka
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2006-02-16
Filing date: 2007-02-15
Publication date: 2007-08-23
Also published as: ES2373577T3; EP1984927A1; BRPI0708062A2; JP5102452B2; JP2007220468A; CN101385095A; EP1984927B1; US20070208205A1; ATE533166T1

Abstract

An electrical insulating oil having a flash point (PMCC) of 130°C or more and a pour point of -40°C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt .% of bright stock oil, based on the total weight of the electrical insulating oil composition.

Description

ELECTRICAL INSULATING OIL COMPOSITION

The present invention relates to an electrical insulating oil composition.

Transformers, high-pressure circuit breakers and other high-pressure electrical devices are filled with electrical insulating oil compositions before use.

However, it must be possible for such electrical devices to be used maintenance-free for long periods after the start of operation. Similarly, such compositions are required to maintain stable physical properties and electrical properties over a long period.

It is known that when electrical insulating oil compositions are used for a long period, they deteriorate, causing a decrease in anti-corrosion properties and a decrease in insulating properties due to an increase in the acid value of the electrical insulating oil compositions and the formation of sludge. Hence, with the trend towards higher pressure, larger size and longer service life for high pressure electrical devices, the stability of electrical insulating oil compositions used therein has become more important.

The IEC (International Electrotechnical Commission) has specified the required performance for electrical insulating oil compositions used for filling transformers, high pressure circuit breakers and other electrical devices, and the performance required for mineral oil insulating oil compositions capable of withstanding long-term use as the standards IEC 296 and IEC 60296. Meanwhile, with the advances in mineral oil refining technology, the hydrorefining method has been widely adopted, making it possible to obtain oils from which polycyclic aromatics, unsaturated components, nitrogen content and sulphur content have been adequately removed. At first, it had been considered that it was better for electrical insulating oil composition to contain no nitrogen or sulphur content.

Subsequently, however, Japanese Laid-Open Patent Application No. 2000-345177 A disclosed that with regard to oxidation stability and other aspects it was preferable for electrical insulating oil compositions to contain a certain amount of these components. Thus, it has been proposed in JP 2000-345177 A that by reducing the resin content in an electrical insulating oil composition to ≤ 100 ppm, and also incorporating minute amounts of sulphide-type sulphur content and total nitrogen content, the stability of the electrical insulating oil composition is increased, and increases in electric charge are inhibited.

In fact, it is extremely difficult to refine a crude oil by petroleum refining processes to a state wherein the nitrogen content or sulphur content are such as to be suitable for use in an electrical insulating oil composition.

Accordingly, at present, although there have been attempts to adjust the components therein by adding an appropriate combination of additives or the like having a nitrogen content or sulphur content to highly purified refined oils, there are restrictions as to the additives which can be used, depending on the electrical insulating oil specification, moreover, there are cost problems, and production management complications .

It is therefore highly desirable to be able to obtain an electrical insulating oil composition having a balance of excellent electrical properties, low temperature performance, thermal oxidation stability and anti-corrosion properties by an economical and simple method .

In the present invention it has been surprisingly found that, through the addition of small quantities of bright stock oil to hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible simply and economically to increase the thermal oxidation stability and obtain electrical insulating oil compositions which are capable of fulfilling the various types of performance required of electrical insulating oil compositions in the IEC Standard 296 and the IEC Standard 60296.

Accordingly, the present invention provides an electrical insulting oil composition having a flash point (PMCC) of 13O⁰C or more and a pour point of -40⁰C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt . % of bright stock oil, based on the total weight of the electrical insulating oil composition .

In a preferred embodiment of the present invention, the electrical insulating oil composition has a pour point of -45⁰C or lower. In a preferred embodiment of the present invention, the electrical insulating oil composition has a flash point (PMCC) of 135⁰C or more. The hydrorefined mineral oil that may be present in the electrical insulating oil composition of the present invention may be refined by hydrogenating mineral oil. Examples of hydrorefined mineral oils that may be conveniently used include hydrorefined naphthenic mineral oil and hydrorefined paraffinic mineral oil.

Through the hydrorefining of mineral oils, polycyclic aromatic substances, unsaturated components, sulphur compounds and nitrogen compounds are removed. Compared to the hydrorefined paraffinic mineral oils, the hydrorefined naphthenic mineral oils may have better low temperature flow properties and lower oxidation stability, and, conversely, compared to the hydrorefined naphthenic mineral oils, the hydrorefined paraffinic mineral oils may have inferior low temperature flow properties, but better thermal oxidation stability.

Examples of synthetic hydrocarbon oils that may be conveniently used in the electrical insulating oil composition of the present invention include polypropylene, polybutene, polypentene and other isoparaffinic synthetic hydrocarbon oils, poly-α-olefins (PAO) falling within Group IV in the API (American Petroleum Institute) base oil classification stated below in Table 1, and Gas to Liquid (GTL) synthetic oils. Synthetic hydrocarbon base oils sold by the Shell group under the trade designation "XHVI" may be conveniently used.

Japanese Laid-Open Patent Application 52-072706 A and Japanese Laid-Open Patent Application 07-228876 describe methods of manufacturing synthetic hydrocarbon oils that may be conveniently used in the electrical insulating oil composition of the present invention. The afore-mentioned hydrorefined mineral oils and synthetic hydrocarbon oils may be used alone. However, the pour point, kinematic viscosity, thermal oxidation stability and the like of the electrical insulating oil composition of the present invention may be optimally adjusted by appropriate combination of several substances .

The hydrorefined mineral oil and/or synthetic hydrocarbon oil used in the electrical insulating oil composition of the present invention each have a preferred kinematic viscosity at 4O⁰C in the range of from 8.0 to 24 mm²/s.

Preferred hydrorefined mineral oils may be selected from hydrorefined naphthenic mineral oils and/or API base oil classification Group II or Group III hydrorefined paraffinic mineral oils .

Preferred synthetic hydrocarbon oils may be selected from isoparaffinic synthetic oils, GTL synthetic oils and poly-α-olefins (PAO) belonging to API base oil classification Group IV or mixtures thereof.

Since the kinematic viscosity of bright stock oil is relatively high compared to hydrorefined mineral oils and/or synthetic hydrocarbon oils, it is particularly preferred that the intrinsic kinematic viscosity of the hydrorefined mineral oil and/or synthetic hydrocarbon oil at 40°C is 16 mm²/s or less, more preferably 12 mm²/s or less if hydrorefined mineral oil or synthetic hydrocarbon oil is used alone with bright stock oil.

If combinations of hydrorefined mineral oil and synthetic hydrocarbon oil are used with bright stock oil, the respective kinematic viscosity of the hydrorefined mineral oil and the synthetic hydrocarbon oil is each preferably in the range of from 8.0 to 24.0 mm²/s, more preferably in the range of from 8.0 to 20.0 mm²/s.

TABLE 1

The bright stock oil added to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil may be obtained by further distilling under reduced pressure the atmospheric pressure residual mineral oil obtained when paraffinic mineral oil is distilled at atmospheric pressure from crude oil, and performing deasphalting, solvent extraction and solvent dewaxing processes on that reduced pressure residual oil. That is to say, said bright stock oil may be a heavy mineral oil base oil fraction obtained by reduced pressure distillation of the residual paraffinic mineral oil from atmospheric distillation of crude oil, and deasphalting, solvent extraction and solvent dewaxing treatment of the residual oil obtained.

The bright stock oil used in the present invention is preferably a base oil belong to API base oil classification Group I, with total sulphur content in the range of from 0.4 to 2.0 wt . % , mercaptan sulphur content of 100 wt. ppm or less, sulphide-type sulphur content of 0.5 wt .% or less, total nitrogen content in the range of from 100 to 500 wt . ppm, basic nitrogen content of 300 wt. ppm or less, and total content of polar components (IP368) in the range of from 30 to 70 wt . % .

Various sulphur compounds and nitrogen compounds are concentrated in said bright stock oil by the distillation and solvent extraction processes. Through the use of in the range of from 0.5 to 10 wt . % of said bright stock oil in the electrical insulating oil composition of the present invention, preferably in the range of from 0.5 to 8.0 wt. %, based on the total weight of said electrical insulating oil composition, the afore-mentioned sulphur content and nitrogen content are incorporated in appropriate quantities in the electrical insulating oil composition. Hence, it is possible to markedly improve the thermal oxidation stability thereof. If the afore- mentioned mercaptan sulphur and sulphide-type sulphur contents become too high, then the corrosivity may become higher, which is undesirable. Further, if the basic nitrogen content exceeds 300 wt . ppm, then the oxidation stability performance may decrease. The electrical insulating oil composition of the present invention has a preferred kinematic viscosity at 40⁰C in the range of from 8.0 to 16.0 mm²/s, more preferably in the range of from 8.0 to 12.0 mm²/s. With a kinematic viscosity at 40°C of less than 8.0 mm²/s, a satisfactory flash point may not be obtained. If the kinematic viscosity at 40⁰C exceeds 16.0 mm²/s, then the circulation efficiency of the electrical insulating oil composition in a transformer may decrease, the cooling effect may become inadequate, and overheating may occur. The electrical insulating oil composition of the present invention may comprise one or more antioxidant additives. Antioxidants that may be conveniently used are phenolic or aminic antioxidants .

Examples of phenolic antioxidants that may be conveniently used in the electrical insulating oil composition of the present invention include 4,4'- methylenebis (2, 6-di-tert . -butylphenol ) , 4, 4' -bis (2, 6-di- tert .-butylphenol) , 4,4' -bis (2-methyl-6-tert . - butylphenol) , 2,2' -methylenebis (4-ethyl-6-tert.- butylphenol) , 2,2' -methylenebis ( 4-methyl-6-tert . - butylphenol), 4 , 4 ' -butylidenebis ( 3-methyl-6-tert . - butylphenol) , 4, 4' -isopropylidenebis (2, 6-di-tert. -butylphenol), 2 , 2 ' -methylenebis (4-methyl-6-nonylphenol) , 2,2'- isobutylidenebis ( 4 , 6-dimethyl-phenol) , 2,2'- methylenebis ( 4-methyl-6-cyclohexylphenol) , 2, 6-di-tert. - butyl-4-methylphenol, 2, 6-di-tert . -butyl-4-ethylphenol, 2, 4-dimethyl- 6-tert .-butylphenol, 2, 6-di-tert . -butyl-α- dimethylamino-p-cresol , 2, 6-di-tert.-butyl-4 (N, N' - dimethylaminomethylphenol) , 4,4'-thio-bis ( 2-methyl-6- tert. -butylphenol), 4,4'-thiobis (3-methy1-6-tert . - butylphenol), 2 , 2 ' -thiobis- ( 4-methyl-6-tert . - butylphenol), bis (3-methyl-4-hydroxy-5-tert . -butylbenzyl) sulphide, bis (3, 5-di-tert . -butyl-4-hydroxybenzyl) sulphide, 2,2' -thio-diethylenebis [3- (3, 5-di-tert . -butyl- 4-hydroxyphenyl) propionate], tridecyl-3- (3, 5-di-tert .- bύtyl-4-hydroxyphenyl ) propionate, pentaerythrityl- tetrakis[3-(3, 5-di-tert . -butyl-4-hydroxyphenyl) propionate] , octy1-3- (3, 5-di-tert .-butyl-4-hydroxyphenyl) propionate, octadecyl-3- (3, 5-di-tert . -butyl-4- hydroxyphenyl) propionate, octyl-3- (3-methyl-5-tert . - butyl-4-hydroxyphenyl) propionate.

The content of the one or more antioxidant additives is preferably less than 2 wt.%, more preferably less than 1 wt. %, even more preferably less than 0.6 wt . % and most preferably less than 0.4 wt.%, based on the total weight of the electrical insulating oil composition.

In a preferred embodiment of the present invention, the electrical insulating oil composition comprises in the range of from 0.01 to 0.4 wt.% of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.

In other embodiments of the present invention, it is preferred that the electrical insulating oil composition comprises in the range of from 0.01 to 0.08 wt.% or in the range of from 0.08 to 0.4 wt.% of phenolic antioxidant, based on the total weight of the electrical insulating oil composition. Further additives that may also be present in the electrical insulating oil composition of the present invention are dependent upon the specific application of the electrical insulating oil composition.

Through the addition of bright stock oil to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC (International Electrotechnical Commission) Standard 296, Class II, in that on maintaining for 168 hrs at 100⁰C in IEC oxidation stability test method 61125A, sludge formation is 0.1 wt.% or less, and the acid value is 0.4 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention provides an electrical insulating oil composition having a flash point (PMCC) of 130°C or more and a pour point of -40⁰C or lower, more preferably of -45°C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt . % of bright stock oil, and which composition conforms with IEC (International Electrotechnical Commission) Standard 296, Class II, in that after 168 hrs at 100⁰C in IEC oxidation stability test method 61125A, sludge formation is 0.1% or less, and the acid value is 0.4 mg KOH/g or less.

Further, through the addition of bright stock oil to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type U, in that on maintaining for 164 hrs at 120°C in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt . % or less, and the acid value is 1.2 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130⁰C or more, more preferably of 135°C or more and a pour point of -40⁰C or lower, more preferably of -45°C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt .% of bright stock oil, and which composition conforms with IEC Standard 60296, Type U, in that after 164 hrs at 120°C in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt . % or less, and the acid value is 1.2 mg KOH/g or less.

Moreover, in a preferred embodiment through the addition of bright stock oil, and the further addition of in the range of from 0.01 to 0.4 wt.%, more preferably in the range of from 0.08 to 0.4 wt.% of an antioxidant, preferably a phenolic antioxidant, to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type I, in that on maintaining for 500 hrs at 120°C in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt .% or less, and the acid value is 1.2 mg KOH/g or less .

Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130⁰C or more, more preferably of 135⁰C or more and a pour point of -40⁰C or lower, more preferably of -45⁰C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil, in the range of from 0.5 to 10 wt .% of bright stock oil and in the range of from 0.01 to 0.4 wt.%, more preferably in the range of from 0.08 to 0.4 wt.% of an antioxidant, preferably a phenolic antioxidant and which composition conforms with IEC Standard 60296, Type I, in that after 500 hrs at 120⁰C in IEC oxidation stability test method 61125C, ^• sludge formation is 0.8 wt.% or less, and the acid value is 1.2 mg KOH/g or less.

Further, in the same way, through the addition of bright stock oil, and the further addition of in the range of from 0.01 to 0.4 wt.%, more preferably in the range of from 0.01 to 0.08 wt.% of an antioxidant, preferably a phenolic antioxidant, to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type T, in that on maintaining for 332 hrs at 120⁰C in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt .% or less, and the acid value is 1.2 mg KOH/g or less . Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130⁰C or more, more preferably of 135°C or more and a pour point of -40⁰C or lower, more preferably of -45⁰C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil, in the range of from 0.5 to 10 wt .% of bright stock oil and in the range of from 0.01 to 0.4 wt.%, more preferably in the range of from 0.01 to 0.08 wt.% of an antioxidant, preferably a phenolic antioxidant and which composition conforms with IEC Standard 60296, Type T, in that after 332 hrs at 120⁰C in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt.% or less, and the acid value is 1.2 mg KOH/g or less. That is to say, the present invention provides an electrical insulating oil composition which conforms to one or more of the following standards:- IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T) .

The present invention further provides for the use of an electrical insulating oil composition as hereinbefore described in order to conform with one or more of IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T) . The present invention further provides the use of the composition as hereinbefore described as an electrical insulting oil for application in one or more of transformers, regulators, circuit breakers, power plant reactors, shunt reactors, switch gears, cables and electrical equipment.

The present invention will be described with reference to the following Examples which are not intended to limit the scope of the invention in any way. EXAMPLES

As the hydrorefined mineral oils, mineral oils and synthetic hydrocarbon oils, the base oils A-J and bright stock oil having the properties shown in Table 2, Table 3 and Table 4 were prepared. Base Oils A-J and Bright Stock Oil

The properties of the base oils A-J and the bright stock oil are stated in Table 2, Table 3 and Table 4 on the basis of the following criteria.

(1) Flash point: as per JIS K 2265 (PMCC: Pensky- Martens closed method, and COC: Cleveland open method) .

(2) Kinematic viscosity (40⁰C) , kinematic viscosity (100⁰C) : as per ASTM D445. ^■

(3) Pour point: as per ASTM D97.

(4) Acid value: as per ASTM D974. (5) PCA (PCA content): as per BS2000P-346.

TABLE 2

TABLE 4

Using the afore-mentioned base oils A-J and bright stock oil, Examples 1-7, which were electrical insulating oil compositions according to the present invention which not containing antioxidants, were prepared in accordance with the compositions shown in Table 5 and Table 6.

Furthermore, Examples 8-11, which are electrical insulating oil compositions according to the present invention containing antioxidants, were prepared in accordance with the compositions shown in Table 7.

Further, for comparison with the afore-mentioned examples, Comparative Examples 1-15 were prepared in accordance with the compositions shown in Tables 8-10.

With regard to the "clay treatment" referred to in Tables 5-10, activated clay in the stated quantity relative to the oil component was used and these were mixed together by stirring for 15 mins at 30-40°C, and the oil layer separated by pressure filtration. Processing was performed by the usual refining methods for electrical insulating oil compositions for the decolourisation, deodorisation, dewatering and stability improvement of electrical insulating oil compositions.

TABLE 5

* wt. % relative to amount of oil components used.

TABLE 6

wt . % relative to amount of oil components used

TABLE 7

Note: The antioxidant "BHT" is 2 , 6-di-tert . -butyl-4- methylphenol

* wt . % relative to amount of oil components used. "BHT' added to oil components after clay treatment.

TABLE 8

* wt . % relative to amount of oil components used. TABLE 9

wt . % relative to amount of oil components used. IEC standard values were used as the standard reference values for the performance of the electrical insulating oil compositions. If these standard reference values were satisfied, then it was considered that long- term stable use over a wide temperature range was possible .

Accordingly, for the afore-mentioned Examples 1 and 2 (not containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 11 together with the standard reference values for IEC Standard 296, Class II.

Further, for the afore-mentioned Examples 1, 2, 6 and 7 (not containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 12 together with the standard reference values for IEC Standard 60296, Type U.

The measurement of the values of the properties, other than (1) to (5), described for the afore-mentioned base oils, was as stated below.

(6) Surface tension: as per ASTM D971.

(7) Corrosive sulphur: as per ASTM D1275.

(8) Water content: as per ASTM D1533.

(9) Oxidation test (168 hrs at 100⁰C) : as per IEC method 61125A.

(10) Oxidation test (164 hrs at 120⁰C) : as per IEC method 61125C.

(11) Insulation breakdown voltage: as per ASTM 1816 (VDE) . For the afore-mentioned Examples 3-5 (not containing antioxidants), the values of the density and oxidation stability on the basis of the aforesaid IEC Standard 296, Class II (168 hrs at 100⁰C, IEC method 61125A) were measured, and those results are shown in Table 13.

For the afore-mentioned Examples 8 and 9 (containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 14 together with the standard values for IEC Standard 60296, Type I.

The measurement of the values of the properties, other than (1) to (11), described for the afore-mentioned base oils and Examples (not containing antioxidants), was as stated below.

(12) Oxidation test (500 hrs at 12O⁰C) : as per IEC method 61125C.

Examples 10 and 11 (containing antioxidants) had lower antioxidant contents than Examples 8 and 9, and the values of their electrical insulating oil properties were measured, and those results are shown in Table 15 together with the standard values for IEC Standard 60296, Type T. The measurement of the values of the properties, other than (1) to (11), described for the afore-mentioned oils and Examples (not containing antioxidants), was as stated below.

(13) Oxidation test (332 hrs at 120⁰C) : as per IEC method 61125C.

For Comparative Examples 1-15, the afore-mentioned oxidation stability on the basis of the afore-mentioned IEC Standard 296, Class II (168 hrs at 100⁰C, IEC method 61125A) was measured, and those results are shown in Tables 16-18. Discussion

For the non-antioxidant-containing electrical insulating oil compositions of Examples 1-7, the values relating to oxidation stability (IEC oxidation test) , which is regarded as an important characteristic value, were substantially lower than the IEC Standard 296, Class II, standard reference values (maximum values) or the IEC Standard 60296, Type U standard reference values (maximum values), and said compositions were judged to be excellent as electrical insulating oil compositions.

With Examples 1 and 2, the values from the oxidation test by the IEC method 61125A (168 hrs at 100⁰C) in the IEC Standard 296 and from the oxidation test by the IEC method 61125C (164 hrs at 120⁰C) in the IEC Standard 60296 were substantially lower than the standard reference values (maximum values) in either of these standards, and the compositions were excellent.

Further, for Examples 1, 2, 6 and 7, favourable results were obtained for all of the characteristic values stated in Table 11 and Table 12, in particular the kinematic viscosity at 4O⁰C and -30°C, the pour point and acid value, and all compositions satisfied the standard reference values for IEC Standard 296, Type II and/or IEC Standard 60296, Type U. Moreover, for the antioxidant-containing electrical insulating oil compositions of Examples 8 and 9, in the IEC method 61125C (500 hrs at 120⁰C) oxidation test relating to IEC Standard 60296, Type I, the values relating to oxidation stability, which is regarded as an important characteristic value, were each substantially lower than the standard reference values (maximum values) therein, and they were judged to be excellent electrical insulating oil compositions. Further, favourable results were also obtained for the other characteristic values stated in Table 14, in particular the kinematic viscosity, with both compositions satisfying the standard reference values for IEC Standard 60296, Type I.

Furthermore, with the electrical insulating oil compositions of Examples 10 and 11, which contained small quantities of antioxidant, in the IEC method 61125C (332 hrs at 12O⁰C) oxidation test relating to IEC Standard 60296, Type T, the values relating to oxidation stability, which is regarded as an important characteristic value, were each substantially lower than the standard reference values therein, and said compositions were judged to be excellent electrical insulating oil compositions. Favourable results were also obtained for the other characteristic values stated in Table 15, in particular the kinematic viscosity, and both compositions satisfied the standard reference values for IEC Standard 60296, Type T. However, with the substances of Comparative Examples 1-15, in which bright stock oil was not used, in the IEC method 61125A (168 hrs at 100⁰C) oxidation test relating to the afore-mentioned IEC Standard 296, none of the compositions satisfied the standard reference values for IEC Standard 296, Class II. In other words, the substances of Comparative Examples 1-9 and 15 substantially exceeded the standard reference values (maximum values), and it was judged that the comparative compositions were unsuitable for use as electrical insulating oil compositions. Further, although the compositions of Comparative Examples 10-14 satisfied the standard reference values as regards acid value, said compositions did not satisfy the standard reference values as regards sludge, and it was judged that said comparative compositions could not be used as suitable electrical insulating oil compositions.

TABLE 11

K)

TABLE 12

KJ -4

TABLE 12 continued

CC

TABLE 13

K)

TABLE 14

O

TABLE 14 continued

TABLE 15

TABLE 15 continued

TABLE 16

TABLE 17

TABLE 18

Ul

Claims

C L A I M S

1. An electrical insulating oil having a flash point

(PMCC) of 13O⁰C or more and a pour point of -4O⁰C or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt . % of bright stock oil, based on the total weight of the electrical insulating oil composition.

2. Electrical insulating oil composition according to Claim 1, wherein said composition has a pour point of -45⁰C or lower.

3. Electrical insulating oil composition according to Claim 1 or 2, wherein said composition has a flash point (PMCC) of 135°C or more.

4. Electrical insulating oil composition according to any one of Claims 1 to 3, wherein said composition comprises in the range of from 0.01 to 0.4 wt . % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.

5. Electrical insulating oil composition according to any one of Claims 1 to 4, wherein said composition comprises in the range of from 0.01 to 0.08 wt . % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.

6. Electrical insulating oil composition according to any one of Claims 1 to 4, wherein said composition comprises in the range of from 0.08 to 0.4 wt . % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.

7. Electrical insulating oil composition according to any of Claims 1 to 6, wherein the hydrorefined mineral oil has a kinematic viscosity in the range of from 8.0 to 24 mm²/s at 40⁰C and is selected from a hydrorefined naphthenic mineral oil and/or an API base oil classification Group II or Group III hydrorefined paraffinic mineral oil.

8. Electrical insulating oil composition according to any one of Claims 1 to 7, wherein the synthetic hydrocarbon oil has a kinematic viscosity in the range of from 8.0 to 24 mm²/s at 4O⁰C and is selected from an isoparaffinic synthetic oil, a GTL synthetic oil, a poly-α-olefin (PAO) belonging to API base oil classification Group IV or a mixture thereof.

9. Electrical insulating oil composition according to any one of Claims 1 to 8, wherein the bright stock oil is a heavy mineral oil base oil fraction obtained by reduced pressure distillation of the residual paraffinic mineral oil from atmospheric distillation .of crude oil, and deasphalting, solvent extraction and solvent dewaxing treatment of the residual oil obtained.

10. Electrical insulating oil composition according to any one of Claims 1 to 9, which composition conforms to one or more of the following Standards:- IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T) .

TS8091FF.doc