SE1651123A1 - Use of certain aromatic compounds as additives to a dielectric liquid for reducing the viscosity thereof - Google Patents

Description

15 20 25 30 _2_ low enough to enable the liquid to start flowing; otherwise parts of the transformer will heat up to dangerous temperatures while cooler parts are clogged with cold and highly viscous liquid.

Having a clogged or slow flowing system at start up may lead to overheating and breakdown of the transformer.

Not only must a dielectric liquid be a good heat dissipater but it must also have good insulating properties. The constant development and optimization of transformer construction has led to compact structures with the conductors as close to each other as the physics allow without risk of discharges going from one conductor to the other. Oil and paper have been used as insulat- ing material in oil filled electrical equipment for nearly a century. The main technical reason for this is that oil and paper are effective insulators, especially in combination. ln a transformer the oil can, in some cases, be exposed to partial discharges due to poor im- pregnation ofthe solid insulation, or ifthe insulation is wet. Design or assembly error may be other factors causing partial discharges. With partial discharges, some oil molecules will break and the fragments may combine to form hydrogen and methane, which dissolve in the oil.

Some reactions in the oil can absorb dissolved hydrogen, this is tested with the industry stand- ard gassing tendency IEC 60626 or ASTIVI D3300. ln the gassing tendency standard the oil is sat- urated with hydrogen or nitrogen gas in a sealed container and exposed to discharges. The hy- drogen absorbing reactions occur mainly when aromatic structures are present, and, to some degree, are dependent on the amount of aromatic structures. insulating oils with high natural aromatic content absorb more hydrogen gas in the gassing tendency tests but this property can also be altered by certain additives.

The trend for power transformers is that they are built for higher voltages and run at higher average loads closer to their maximum capacity. All parts of the transformer need to be opti- mized and so must also the dielectric liquid. The dielectric liquid must also have excellent oxida- tion stability to last in the transformer for many years at high temperature. The dielectric liquid must also have good solubility properties to keep any impurities formed in solution where they will make no harm. 10 15 20 25 30 _3_ When choosing an insulating liquid for electrical equipment there are many material properties that need to be considered. The common requirements for mineral insulting oil are found in the specifications |EC60296 and ASTl\/l D3487.

To fulfil all demands of modern high voltage transformers naphthenic super grade oil is most often chosen as the insulating liquid. The transformer design must take the oil properties into consideration and with high quality oil the design can be optimised better.

Today less refined oils are commonly used to obtain the desired low gassing tendency effect, since they contain more aromatic compounds. ln other cases a well refined oil is used but with an additive. A commonly used additive is Tetralin (tetrahydronaphthalene), a volatile com- pound that also has some negative health issues, such as being suspected carcinogenic and be- ing irritating to skin and eye.

The use of alkyl(C1-C4)naphthalene as an additive to mineral oil to impart to the oil higher gas- absorbing qualities is known from i.a. DE 24 53 863 and WO 93/21641.

US 2010/0059725 teaches that addition up to 10 wt% of reformer distillate, containing 1- and 2-ring aromatic compounds, to a transformer oil improves the gassing tendency of the trans- former oil. Suitable 1- and 2-ring aromatics are e.g. alkylated benzene, naphthalene, alkylated naphthalenes, indanes, biphenyls and diphenyls.

US 3 932 267 teaches that addition of up to 5 % by weight of certain aromatic compounds con- taining two or more six carbon membered fused or unfused rings at least one of which is a ben- zene ring, e.g. biphenyl, can improve the gassing properties of an uninhibited transformer oil, which oil has been produced according to a specific process disclosed therein.

US 4 493 943 teaches the use of a combination of at least one diarylalkane, and at least one of mono- and and/or diolefin having two condensed or noncondensed aromatic nuclei, for obtain- ing an electrical insulating oil having i.a. good hydrogen absorbing capacity. Preferred diarylal- kanes are diarylmethane, 1,1-diarylethane, 1,2-diarylethane, and among these especially com- 10 15 20 25 30 _4_ pounds having a benzene ring, which is not substituted with an alkyl group, e.g. ar- ylphenylethane. Other conventional electrical insulating oils such as polybutene, mineral oils, alkylbenzenes, alkylnaphthalenes and alkylbiphenyls can be added to the oil.

US 4 967 039 teaches that a silicone base oil for use as impregnant in an electric power cable for fire hazard conditions is rendered non-gassing by the addition of about 2-8% of an aryl al- kane having at least two benzene rings spaced apart by not less than one nor more than two aliphatic carbon atoms. According to US 4 967 039 the preferred additive is l-phenyl 1-(3,4 di- methylphenyl) ethane, also known as PXE.

US 5 601 755 teaches a dielectric composition comprising benzyltoluene, benzylxylene, (methylbenzyl)to|uene and (methy|benzyl)xylene. The composition can be mixed with mineral oils typically used in transformers.

Summary of invention The present inventor has surprisingly found that diphenylmethane, diphenylethane and similar compounds when added in a small amount to mineral oil will markedly reduce the viscosity of the oil. The extent of reduction of viscosity is unexpected. For example, at -40°C the viscosity of the fluid is almost reduced by 50% when 5% diphenylmethane is added thereto.

Accordingly, in one aspect the present invention relates to the use of a C12-C16 aromatic com- pound consisting of a naphthalene, biphenyl, biphenyl ether, or diphenylalkane structure, op- tionally substituted with one, two, three, or four C1-C4 alkyl groups, as an additive to a dielectric liquid for a transformer in an amount of 1-10 % by weight, for reducing the viscosity of the die- lectric liquid.

Accordingly, by means ofthe present invention, the cold start-up specification of a given dielec- tric liquid will be improved, and the compound can thus be used as an additive for improving the cold start-up specification of a given dielectric liquid. 10 15 20 25 30 _5_ The invention will thus enable a given dielectric liquid comprising the inventive additive to be used as having a cold start-up Classification corresponding to a lower temperature as compared to the cold start-up specification of the oil without the additive. ln one aspect the invention relates to the use of a dielectric liquid containing the inventive ad- ditive for improving the cold start-up performance and characteristics of a transformer. ln another aspect the invention consequently relates to the use of a dielectric liquid containing the inventive additive as having a cold start-up specification corresponding to a lower tempera- ture than that of the cold start-up specification of the dielectric liquid without the additive. ln a further aspect the present invention relates to the use, e.g. including start-up, of the in- ventive transformer at an ambient temperature or temperature of the transformer of be- low -20°C.

By virtue of the invention the viscosity dependent heat transfer coefficient ofthe system will also be improved, and hence also the overall heat transfer coefficient of the system.

By virtue of the reduced viscosity the invention allows for cooling of the transformer to a lower temperature. A lower temperature of the transformer will in turn extend the service life of the transformer. The improved heat transfer coefficient of the system will further enhance the cooling performance ofthe inventive dielectric liquid.

Also, the lower the temperature can be kept in a transformer, the lower the power losses will be. The lower power losses at lower temperatures are due to í.a. a lower resistance in the met- al conductors, and a lower dielectric dissipation factor in the oil at such lower temperatures.

A number of the compounds of the invention are known from the prior art to decrease the gas- sing tendency of an insulating oil. The compounds of the invention can thus be used as a multi- purpose additive to dielectric fluids to decrease both gassing tendency and viscosity thereof. 10 15 20 _6_ ln cold climate areas there is a need to have insulating oils with low viscosity at low tempera- tures in order to ensure safe start-ups. The additives of the present invention enables achieve- ment of a combination of a low viscosity at a vide temperature range, e.g. from -40°C to +100°C, and a negative gassing tendency, without affecting flash point negatively or having health and safety issues.

The present invention is especially intended for use in the power industry, particularly in power transformers. The cooling system of the inventive transformer can e.g. be of ONAN, ONAF, OFAN, OFAF, OFWF, ODAN, ODAF, Or ODWF type.

The terms "oil", "dielectric oil", "dielectric liquid", and "dielectric fluid" have been used inter- changeably herein.

As used herein the term "cold start-up specification" of an oil is intended to primarily refer to the Lowest Cold Start Energizing Temperature (LCSET) as defined in IEC 60296 of the oil. ln the table below the maximum viscosity, and the maximum pour point for different LCSETs are set out.

LCSET Maximum viscosity Maximum pour point °C mmZ/s °C 0 1 800 -10 -20 1 800 -30 -30 1 800 -40 -40 2 500 -50 Detailed description of the invention ln an attempt to support the modern transformers with top of the line insulating oil a multipur- pose additive has been tested for the application of both decreasing viscosity and decreasing gassing tendency. As a result C12-C16 aromatic compounds of a naphthalene, biphenyl, biphenyl ether, or diphenylalkane structure, optionally substituted with one, two, three, or four C1-C4 10 15 20 25 30 _7_ alkyl groups have been found to both decrease the viscosity and the gassing tendency of a die- lectric liquid. The extent ofthe reduction of the viscosity is however unexpected.

For example, at 40°C addition of 5% by weight of the additive to a dielectric fluid will typically produce a decrease in viscosity about twice the expected decrease, e.g. as estimated using Ref- utas equation. The decrease in viscosity according to the invention will generally be even great- er at lower temperatures. For example, at -40°C, the addition of 5% of the additive may even result in a reduction of the viscosity of the dielectric fluid by about 50%. Accordingly, the in- ventive additive is generally more efficient at lower temperatures, such as at 0°C, and below.

A reduced viscosity of a given dielectric liquid will correspond to an improved cold start-up specification ofthe dielectric liquid. The inventive dielectric liquid containing the additive can thus be used in new applications, requiring a cold start-up specification corresponding to a low- er temperature than that of previous specifications ofthe dielectric liquid, to which applica- tions the dielectric liquid previously has not been qualified.

As mentioned previously the function ofan oil in a transformer is cooling and insulation. The oil flows through the transformer and removes heat and therefore it is not only viscosity but also the viscosity dependent heat transfer coefficient that is interesting to look at. Heat transfer works in different ways depending on the design of the system to be cooled (e.g. ONAN, ONAF, OFAN, OFAF, OFWF, ODAN, ODAF, or ODWF). According to the present invention, addition of 5% by weight of the additive to a dielectric fluid can e.g. improve the heat transfer coefficient of the system at 40°C with about 14%.

Examples of suitable compounds for use as an additive according to the invention are diphe- nylether, diphenylmethane, biphenyl, both isomers ofdiphenylethane, all isomers of methylbi- phenyl, dimethylbiphenyl, ethylbiphenyl, dimethylnapthalene, trimethylnapthalene, ethylme- thylnapthalene, propylnaphthalene, isopropylnapthalene, methylpropylnapthalene, isopropyl- methylnaphthalene and diethylnapthalene, or a mixture of any of the previous compounds. An especially preferred compound is diphenylmethane. Another preferred compound is diphe- nylether. 10 15 20 25 30 The viscosity decreasing effect is been found to be greater when compounds in the range of C12-C14 are being used.

The Clz-Clö compounds of a diphenylalkane structure are preferably diphenyl Cl-Cg alkane com- pounds, and more preferably diphenyl Cl-CZ alkane compounds, i.e. diphenylmethane and di- phenylethane structure compounds.

The inventive compounds preferably exhibit O, 1, or 2 C1-C4 substituents, and more preferably no substituents. ln preferred embodiments, any C1-C4 substituents present are Cl-Cg substitu- entS.

The compounds used as additives according to the invention are preferably non-halogenated.

The flash point is another important property oftransformer oils, especially when fire hazard is a critical factor.

Compounds having a relatively high flash point are generally being preferred. For example, di- phenylmethane has a flash point around 130°C, as compared to 77°C for tetralin.

According to the invention the additive is mixed into an insulating fluid for the purpose of de- creasing the viscosity, especially the low temperature viscosity, and to decrease the gassing tendency of the fluid. The insulating fluid could be e.g. of the following kinds, naphthenic min- eral oil, paraffinic mineral oil, natural ester, synthetic ester, poly-alpha olefin, silicon oil, syn- thetic iso-paraffin or a mixture of any of said insulating fluids.

A generally preferred group of dielectric fluids according to the invention is the group compris- ing naphthenic mineral oil, paraffinic mineral oil, natural ester, synthetic ester, and mixtures the reof. 10 15 20 25 30 _9_ At low temperatures, e.g. at below 0°C, natural esters may be less suitable as the dielectric liq- uid, due the relatively low pour point.

Preferred insulating fluids for use at low temperatures are naphthenic mineral oil, paraffinic mineral oil, synthetic ester, and mixtures thereof.

The means of addition of the additive to the dielectric fluid and the mixing thereof is not critical as long as an adequate mixing of the components can be accomplished. For example, ifthe ad- ditive is solid when added to the dielectric fluid, the mixture should be heated to a temperature above the melting point of the additive, in order to enable adequate mixing. A suitable temper- ature of addition of the additive to the dielectric fluid is a temperature at which both additive and dielectric fluid are in a liquid state.

The additive should be added in an amount low enough in order that crystallization of the addi- tive in the dielectric fluid is avoided also at the low temperature end of the intended service temperature range. lf crystallization of the additive occurs, the dielectric solution may freeze, or become unduly high in viscosity.

For example, depending on the specific oil, diphenylmethane may crystallize at -40°C when used in an amount greater than 5% in naphthenic insulating oil.

The solubility of the additive may also vary for different oils. For example, a naphthenic oil will probably be able to dissolve more additive than a paraffinic oil.

Examples of specific compounds for use in the invention are diphenylether, diphenylmethane, 1,2-diphenylethane, 1,1-diphenylethane, and benzyltoluene. ln the table below values of certain properties for two compounds used in the invention, viz. diphenylmethane, and diphenylether, and for tetralin, respectively, are provided for compari- SOn. 10 _10- Tetralin Diphenylmethane Diphenylether Ox. Stab. 500 h (1 0/0 in naph- 500 h (2 0/0 in naph- 500 h (5 0/0 in naph- IEC67T25C thenic oil) - no im- thenic oil) - no im- thenic oil) - no im- pact on ox. stab. pact on ox. stab. pact on ox. stab.

Flash point p.m. 77°C 130°C 115°C ISO 2779 Boiling point 206-208°C 264-266°C 259°C Of pure substance Effect on Gassing ten- - 15 ml / min - 11 ml / min - 17 ml / min dency when 10/0 added ASTM D2300 - m: Causes seri- ous eye irritation. - ?ll: Toxic to aquatic life With long lasting effects Hazard statements - m: Suspected of causing cancer - m: Causes skin irritation. - m: Causes serious eye irritation. - m: Toxic to aquatic life With long lasting effects - m: May be fatal if sWalloWed and enter airWays - LOU: Very toxic to aquatic life.

- LN: Very toxic to aquatic life With long lasting effects.

The above inventive compounds in the table have different hazard statements, but the more serious hazard statement of cancer suspicion that tetralin has is not stated for any ofthem.

The present invention will now be described in more detail by means ofthe following examples. ln the Examples the following dielectric fluids were used.

O_il Type Pour Point, PP (ISO 3016) A Naphthenic Mineral Oil -63 B Naphthenic Mineral Oil -57 C Naphthenic Mineral Oil -66 D Paraffinic Mineral Oil -45 E Synthetic Ester -63 F Natural Ester -21 10 15 20 25 30 _11- Example 1 - addition of 5 % by weight of diphenylmethane 5 % by weight of diphenylmethane, which has a melting point of about 25°C, was added to 200 cm3 of naphthenic mineral oil (i.e. Oil A above) having a viscosity of 7.60 mmZ/s at 40°C. The oil was heated to 40°C, and mixed using a magnetic stirrer.

Using Refutas equation, and a value of the viscosity of diphenylmethane at 40°C of 2.14 mmZ/s, the resulting viscosity of the mixture at 40°C was estimated to be 7.02 mmz/s.

The actual viscosity of the resulting mixture was measured to be 6.53 mmz/s using ASTM meth- od D7042. ln other words, the resulting decrease in viscosity was a factor of about two larger than expected.

Due to the high melting point of diphenylmethane it not possible to measure the viscosity thereof at lower temperatures. The reduction in viscosity of a mineral oil, however, as com- pared to the viscosity of the oil itself at a given low temperature, and that of the resulting mix- ture at same temperature, is extensive. The lower the temperature, the higher the observed relative viscosity reduction, until the solidification point ofthe mix occurs. At -40°C the viscosity can be reduced by almost 50% when 5% diphenylmethane is added to mineral oil having a vis- cosity within the viscosity range normally used for a transformer oil.

The heat transfer coefficient to the walls of a long pipe with the diameter of 5 cm and a fluid flow speed of 1 m/s was estimated. The heat transfer coefficient for the pipe was calculated based on the the Nusselt-number, a friction factorf= 0.34, a dimensionless number describing turbulence in a system that in itself relies on viscosity, density, thermal conductivity and heat capacity. Adding 5% of diphenylmethane to Oil A will increase the heat transfer coefficient at 40°C with about 14% in the previously described pipe-system.

The following values were used for the heat transfer coefficient calculation.

Density of the oil, d = 0.867 (kg/ms) 10 15 _12- Thermal conductivity of the oil, k = 0.13 (W/m-K) Specific heat capacity of the oil, CP = 1.875 Density of diphenylmethane, d = 0.872 (kg/ms) Thermal conductivity of diphenylmetha ne, k = 0.135 (W/m-K) Specific heat capacity of diphenylmethane, Cp = 1.59 Heat transfer coefficient, hwith = 184.5 with diphenylmethane Heat transfer coefficient, hwithout = 161.9 without diphenylmethane hwith/hwithout z 1-14 Example 2 - addition of diphenylmethane to different dielectric fluids ln this Example, using the mixing procedure in Example 1, diphenylmethane as an additive was mixed with different dielectric fluids to demonstrate the effect on the viscosity of the fluids.

The gassing tendency of the resulting mixtures was established according to ASTM D2300. Six different dielectric fluids (A-F, as defined above) were used. additive Gassing Visc -40°C Visc -30°C Visc -20°C Visc 0°C Visc 40°C Visc Tendency ASTM ASTM ASTM D445 ASTM ASTM 100°C ASTM D445 D445 (cSt) (cSt) D445 D7042 ASTM D2300 (cSt) (cSt) (cSt) D7042 (ml/ min) (cSt) Oil A 33.5 2666 682.3 226.8 46.26 7.623 2.070 2.5% additive 5.9 1992 520.5 181.3 39.66 7.044 1.985 5.0% additive -13.5 1463 411.3 149.4 34.63 6.535 1.910 Oil B 7.0 5343 - - - - - 2.5% additive -14.1 3818 - - - - - 5.0% additive -29.9 2756 - - - - - Oil C 31.8 2418 - - - - - 2.5% additive 47 1774 _ _ _ _ _ 5.0% additive -15.8 1377 - - - - - Oil D - -*** 268.5 - - - - 25% - - 219.3 - - - - 10 15 _13- - Not measured *** Temperature very close to the pour point of the oil. Viscosity ofthe oil at the relevant tem- peFatUfe ?Ot meaSU red. 5.0% additive _ _ 1891 _ _ _ _ Oil E - - - 1376 247.3 28.56 5.186 25.04) - - - 1125 203.4 25.47 4.852 add1t1ve 59% - - - 911.6 175.0 22.76 4.546 add1t1ve Oil F - < PP < PP -*** 198.6 34.61 8.279 2.5% additive - - - - 177.0 31.68 7.790 59% - - - - 157.5 29.02 7.347 add1t1ve The above results demonstrate that diphenylmethane significantly can reduce viscosity, cold temperature viscosity and gassing tendency. Other tests performed have shown that diphenyl- methane has no detectable impact on oxidation stability and very little impact on the flash point of the solution.

Example 3 - diphenylether ln this example, using the mixing procedure in Example 1, the effect of diphenylether on the viscosity at different temperatures, and gassing tendency, respectively, were tested using Oils A, D, E, and F, respectively. additive Gassing Visc -40°C Visc -30°C Visc -20°C Visc 0°C Visc 40°C Visc Tendency ASTM ASTM ASTM ASTM ASTM 100°C ASTM D445 D445 D445 (cSt) D445 D7042 ASTM D2300 (cSt) (cSt) (cSt) (cSt) D7042 (ml/ min) (cSt) Oil A 33.5 2666 682.3 226.8 46.28 7.623 2.070 25.04) -10.6 2134 555.2 190.1 - 7.123 1.999 add1t1ve 50% . . -34.0 1717 460.4 162.6 36.51 6.704 1.936 add1t1ve Oil D - -*** 268.5 118.2 34.73 7.375 2.188 50% - - - 95.51 29.40 6.627 2.056 10 _14- - Not measured *** Temperature very close to the pour point of the oil. Viscosity ofthe oil at the relevant tem- perature not measured.

Oil E - - - 1376 247.3 28.56 5.186 59% - - - 974.2 187.0 23.28 4.6276 add1t1ve Oil F - < PP < PP -*** 198.6 34.61 8.279 5.0% - - - - 1 . 2 . . additive 65 O 9 44 7 368 The above results demonstrate that diphenylether significantly can reduce viscosity, cold tem- perature viscosity and gassing tendency. Other tests performed have shown that diphenylether has no detectable impact on oxidation stability and very little impact on the flash point of the solution.

Example 4 - biphenyl ln this example, using the mixing procedure in example 1, the effect of biphenyl on viscosity at different temperatures, and gassing tendency, respectively, were tested using Oils A, D, E, and F, respectively.

Gassing Visc -40°C Visc -30°C Visc -20°C Visc Visc Visc Tendeney ASTM ASTM ASTM D445 (est) ß 40°c 100°c ASTM D445 (est) D445 (est) ASTM ASTM ASTM D2300 D445 D7042 D7042 (m1 min) (est) (est) (est) 011 A 33.5 2666 682.3 226.8 46.28 7.623 2.070 aäïve _18.2 2078* 540.6 185.9 40.06 7.100 2.001 aíáišve _58.7 _** _** ft* 36.41 6.648 1.923 0111) _ _ 268.5 118.2 34.73 7.375 2.188 25% additive - - - 106.Û 31.87 - - ašáäšve - - -** -** 29.07 6.552 2.Û45 oi1 E _ _ _ 1376 247.3 28.56 5.186 ašàsišve - - - 1182 220.1 - - 5.0% additive _ _ _ _ _ 23.59 4.654 10 15 _15- Oil F _ < PP < PP -*** 198.6 34.61 8.279 50% - _ _ additive - 165.0 29.27 7.347 - Not measured * Value probably not stable, additive slowly precipitated.

** Additive precipitated.

*** Temperature very close to the pour point of the oil. Viscosity ofthe oil at the relevant tem- perature not measured.

The above results demonstrate that biphenyl significantly can reduce viscosity, cold tempera- ture viscosity and gassing tendency. Other tests performed have shown that biphenyl has no detectable impact on oxidation stability and very little impact on the flash point of the solution.

Example 5 - bibenzyl (also referred to as 1,2-diphenylethane or dibenzyl) ln this example, using the mixing procedure in example 1, the effect of bibenzyl on viscosity at different temperatures, and gassing tendency, respectively, were tested using Oils A, D, E, and F, respectively.

Gassing Visc -40°C Visc -30°C Visc -20°C Visc 0°C Visc 40°C Visc Tendency ASTM ASTM ASTM ASTM ASTM 100°C ASTM D445 D445 D445 (cSt) D445 (cSt) D7042 ASTM D2300 (cSt) (cSt) (cSt) D7042 (ml/ min) (cSt) Oil A 33.5 2666 682.3 226.8 46.28 7.623 2.070 2.5% additive 20.0 2127 556.2 191.5 41.05 7.184 2.008 ašáiëlve 8.5 1741 460.9 163.7 37.10 6.817 1.956 Oil D - - 268.5 118.2 34.73 7.375 2.188 27% - - - 106.2 31.97 6.954 2.121 additive Oil E - - - 1376 247.3 28.56 5.186 2.5% additive - - - - - 25.61 4.904 Oil F - 2.5% additive - - - - 178.8 31.45 7.736 - Not measured *** Temperature very close to the pour point of the oil. Viscosity ofthe oil at the relevant tem- perature not measured. _16- The above results demonstrate that bibenzyl significantly can reduce viscosity, cold tempera- ture viscosity and gassing tendency. Other tests performed have shown that bibenzyl has no detectable impact on oxidation stability and very little impact on the flash point of the solution.

Claims (10)

10 15 20 25 30 _17- CLA|l\/IS

1. Use of one or more Clz-Clö aromatic compounds selected from compounds con- sisting ofa naphthalene, a biphenyl, a diphenyl ether, or a diphenylalkane structure, optionally substituted with one, two, three, or four C1-C4 alkyl groups, as a viscosity-reducing additive to a dielectric liquid for a transformer in a total amount of 1-10 % by weight.

2. The use of claim 1, wherein said compounds are selected from diphenylether, diphenylmethane, biphenyl, diphenylethane, methylbiphenyl, dimethylbiphenyl, ethylbiphenyl, dimethylnapthalene, trimethylnapthalene, ethylmethylnapthalene, propylnaphthalene, iso- propylnapthalene, methylpropylnapthalene, isopropylmethylnaphthalene and diethylnaptha- lene.

3. The use of claim 1, wherein said compounds are diphenylmethane and/or diphe- nylether.

4. Use of one or more Clz-Clö aromatic compounds selected from compounds con- sisting of a naphthalene, biphenyl, diphenyl ether, or diphenylalkane structure, optionally sub- stituted with one, two, three, or four C1-C4 alkyl groups, as a cold start-up specification improv- ing additive to a dielectric liquid for a transformer in an amount of 1-10 % by weight improving the cold start-up specification of the dielectric liquid in terms of the LCSET of the liquid as es- tablished according to IEC 60296.

5. Use of a dielectric liquid containing 1-10 % by weight of an additive comprising one or more Clz-Cl? aromatic compounds consisting of a naphthalene, biphenyl, biphenyl ether, or diphenylalkane structure, optionally substituted with one, two, three, or four C1-C4 alkyl groups, in a transformer as a cold start-up performance improving dielectric liquid improving the cold start-up performance ofthe transformer.

6. Use of a dielectric liquid selected from naphthenic mineral oil, paraffinic mineral oil, synthetic ester, poly-alpha olefin, silicon oil, synthetic iso-paraffin or a mixture thereof, con- 10 15 20 _18- taining 1-10 % by weight of an additive comprising one or more C12-C16 aromatic compounds consisting of a naphthalene, biphenyl, biphenyl ether, or diphenylalkane structure, optionally substituted with one, two, three, or four C1-C4 alkyl groups, in a transformer at a start-up or ambient temperature of below -20°C.

7. A dielectric liquid selected from the group consisting of naphthenic mineral oil, paraffinic mineral oil, natural ester, synthetic ester, poly-alpha olefin, silicon oil, synthetic iso- paraffin, or a mixture of any thereof for a transformer containing 1-10 % by weight of an addi- tive comprising one or more compounds selected from diphenylether, diphenylmethane, bi- phenyl, diphenylethane, methylbiphenyl, dimethylbiphenyl, ethylbiphenyl, dimethylnapthalene, trimethylnapthalene, ethylmethylnapthalene, propylnaphthalene, isopropylnapthalene, methylpropylnapthalene, isopropylmethylnaphthalene and diethylnapthalene.

8. The dielectric liquid of claim 7, wherein said compounds are diphenylmethane and/or diphenylether.

9. The dielectric liquid of claim 7 or 8, wherein the dielectric liquid is selected from the group consisting of naphthenic mineral oil, paraffinic mineral oil, natural ester, and synthet- ic ester, or a mixture of any thereof.

10. The dielectric liquid of claim 9, wherein the dielectric liquid is a naphthenic miner- al oil.