Patent Publication Number: US-2012043505-A1

Title: Transformer oil composition, comprising at least one acid interceptor

Description:
The present invention relates to a transformer oil composition with improved stability. The present invention further provides for the use of at least one acid scavenger as a stabilizer in transformer oil compositions. The present invention further relates to the use of at least one acid scavenger for improving the oxidation stability of transformer oils. The present invention finally also relates to a process for producing corresponding inventive transformer oils. 
     A transformer is a component in electrical engineering which transfers electrical energy or information between inductively coupled circuits with low loss. Viewed from the outside, a transformer consists of a transformer tank manufactured from sheet steel. Within the tank is the active part of the transformer, namely the coils or windings thereof, applied to an iron core. The iron core consists of the limbs which bear the windings, and the two yokes which complete the magnetic pathway. The iron core consists of a coated magnetizable iron sheet. Eddy currents and associative losses and heating are prevented since an insulating layer, for example of waterglass, of the sheets lies transverse to the direction of the eddy current. For insulation reasons, the low-voltage winding is on the inside and the high-voltage winding on the outside of the iron core. The windings consist of retainers wrapped with oil-impregnated paper. 
     All cavities of a transformer are filled with a transformer oil for electrical insulation and for cooling of the coils. 
     Transformers are some of the most important and costly pieces of equipment in electrical power supply. The occurrence of faults in these components therefore leads not only to interruptions in the electrical power supply, but also to economic loss. The lifetime of a transformer depends in particular on the insulation material, which is obtained and produced from paper. These highly compacted papers, however, decompose in the course of operation of a transformer and form water. The effects of this moisture on the insulation of old transformers have become an important point of emphasis in the study of transformer failures. 
     A report published by the American company Hartford Steam Boiler Inspection and Insurance Co. (HSB), which offers insurance against equipment failure, arrived at the conclusion that damage to the insulation in the last 10 years constituted the second most frequent cause of failure of transformers. The mean average age of the transformers which failed due to damaged insulation was 17.8 years, and is thus well below the envisaged lifetime of 35 to 40 years. 
     The solid paper insulation used in transformers is usually based on cellulose. When cellulose ages, the polymer structure thereof decomposes and gradually releases water into the insulation oil, referred to as transformer oil. If the transformer fluid cannot absorb the water, it remains in the winding. 
     However, the occurrence of moisture is disadvantageous not only with regard to the reduced insulating action, since the water further accelerates the aging of cellulose and in turn gives rise to water in an autocatalytic process. The destruction of the transformers increases further as a result. 
     In “free-breathing” transformers, it is additionally possible for more moisture to be absorbed from the atmosphere. 
     This reduction in quality of the transformer cellulose reduces both the electrical and mechanical stability thereof. In general, the greater the water content in the transformer oil, the greater the reduction in the mechanical stability of transformer cellulose. 
     Mineral oil has only a very low capacity to absorb moisture. The majority of the water which forms in the course of aging of transformer cellulose remains in the windings and thus lowers the insulation resistance of the transformer. The moisture also reduces the resistance of the transformer to the mechanical and electrical stresses which occur in operation. 
     In addition, a high moisture level can significantly lower the dielectric strength of the mineral oil. In time, this leads to failures and/or to the necessity of reducing the power of the transformer, and can ultimately cause complete failure of the transformer. 
     The decomposition of cellulose in transformers is catalyzed by the presence of short-chain fatty acids. The short-chain fatty acids form initially as a result of decomposition from the cellulose used for the production of the papers. The higher the level of fatty acids present in the transformer oil composition, the more rapidly the fatty acid decomposes. Viewed in chemical terms, this process is thus autocatalytic. When the decomposition is advanced and causes paper fracture in a transformer, the transformer can even explode in the worst case. 
     Furthermore, the oils used in the transformers, called transformer oils, are expensive and should therefore be reused in the case of a loss of quality due to the formation of water and/or fatty acid. In the reprocessing of transformer oils, substances such as water and sediments are generally removed, and the transformer oil is typically heated up in a special machine, called an oil separation machine, and dried under reduced pressure by spraying. In the course of this, the transformer oil is filtered and freed of the existing sediments. 
     Irrespective of these reprocessing options, there is a great interest in improving the stability of transformer oils such that the lifetime of the transformer oil is prolonged overall, and reprocessing of transformer oils is required at a later stage or not at all. 
     The object of this invention is achieved by a composition which comprises at least one transformer oil and, as an additive, at least one acid scavenger. In the context of the present invention, the acid scavengers used may especially be carbodiimides; epoxidized triglycerides, such as epoxidized soybean oil and epoxidized rapeseed oil; trialkylamines; dialkylamines; pyridines or pyridine derivatives; diazabicyclooctane; and polyvinylpyridine. In a preferred embodiment of the present invention, the acid scavenger is at least one carbodiimide compound. The carbodiimide compound is preferably a monomeric, dimeric or polymeric carbodiimide. 
     It has been found in accordance with the invention that the addition of at least one acid scavenger, especially of at least one carbodiimide compound, leads to elimination of fatty acids already formed in transformer oils, which—as explained above—lead to catalysis of cellulose decomposition and hence to unwanted formation of water. Therefore, the addition of at least one acid scavenger, such as at least one carbodiimide compound, leads to an interruption in the autocatalytic decomposition of cellulose in a transformer, and hence also to a reduction in water formation in transformer oil compositions. The acid scavengers, such as carbodiimides, in the context of the present invention, thus stabilize the transformer oil compositions and hence increase the lifetime of the transformer and the service life of the transformer oil compositions. The acid scavengers provided in accordance with the invention, such as the carbodiimides, react selectively with fatty acids. They thus reduce the content of fatty acids in the aged transformer cellulose, which in turn catalyze the decomposition of the cellulose to water. 
     In the context of the present invention, an acid scavenger is therefore understood to mean any compound which is capable of suppressing the formation of fatty acid from cellulose, or is capable of at least keeping the concentration of fatty acid in a transformer oil constant, preferably of reducing it. 
     In the context of the present invention, stabilization of a transformer oil composition is especially understood to mean that the electrically insulating properties and the heat-removing properties of the transformer oil composition are maintained for longer than in the case of no additization. 
     More particularly, stabilization of a transformer oil composition by an acid scavenger is understood to mean that the acid scavenger, such as the carbodiimide, is capable of scavenging the fatty acids present in the composition, and of keeping the water content in the composition constant, preferably of reducing it. 
     In addition, stabilization of a transformer oil composition by an acid scavenger is also understood to mean that the transformer constituents are protected from corrosion. 
     The carbodiimide acid scavengers particularly preferred in accordance with the invention are first described in detail hereinafter. 
     Carbodiimide Compounds 
     The requirements on a carbodiimide compound as the inventive additization relate firstly to the solubility thereof in the transformer oils, which may, for example, be ester base oils or mineral oils. The carbodiimide compounds preferably have, at a temperature of 60° C., a solubility in the corresponding transformer oils of 0.001 to 2.5% by mass, more preferably 0.1 to 1% by mass, especially 0.2 to 0.5% by mass. 
     In addition, the carbodiimide compound should have a long-term stability in the transformer oil composition. The long-term stability should exist under the conditions of use of transformer oils, i.e. especially in the presence of moisture and at temperatures at which the transformer oils are also used. It is therefore especially preferred when the carbodiimide compound to be used in accordance with the invention has a long-term stability in a transformer oil which is used at 60° C. and has a water content of preferably at most 30 ppm in the case of mineral oils and 400 ppm in the case of ester-based insulating oils. 
     Furthermore, the carbodiimide compounds should be such that they react rapidly and selectively with short-chain acids, especially with lactic acid and/or butyric acid. 
     The carbodiimide compound to be used with preference in accordance with the invention, with regard to the chemical structure thereof, is not subject to any particular requirement at first, provided that the carbodiimide is capable of stabilizing the transformer oil composition for the purposes of the present invention, i.e. more particularly is capable of reacting with the fatty acids present in the transformer oil. 
     The carbodiimide compounds which can be used in the present invention may be those which are synthesized by commonly known processes. The compound can be obtained, for example, by means of performance of a decarboxylation condensation reaction of different polyisocyanates using an organophosphorus compound or an organometallic compound as a catalyst at a temperature of not less than about 70° C. without using any solvent or using an inert solvent. 
     Examples of a monocarbodiimide compound which is present in the above-described carbodiimide compounds are dicyclohexylcarbodiimide, diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide, dioctylcarbodiimide, tert-butylisopropyl-carbodiimide, 2,6-diisopropylphenylenecarbodiimide, diphenylcarbodiimide, di-tert-butylcarbodiimide and di-β-naphthylcarbodiimide, particular preference being given to dicyclohexylcarbodiimide or diisopropylcarbodiimide with regard to industrial employability. 
     Corresponding processes for preparing carbodiimides and correspondingly suitable compounds are described, for example, in U.S. Pat. No. 2,941,956, JP-B-47-33279, J. Org. Chem. 28, 2069-2075 (1963) and Chemical Review, 1981, Vol. 81, No. 4, pages 619 to 621. 
     An organic diisocyanate as a starting material for preparing a polycarbodiimide compound includes, for example, aromatic diisocyanate, aliphatic diisocyanate, alicyclic diisocyanate or a mixture thereof, and includes especially naphthalene 1,5-diisocyanate, diphenylmethane 4,4′-diisocyanate, diphenyldimethylmethane 4,4′-diisocyanate, phenylene 1,3-diisocyanate, phenylene 1,4-diisocyanate, tolylene 2,4-diisocyanate, tolylene 2,6-diisocyanate, a mixture of tolylene 2,4-diisocyanate and tolylene 2,6-diisocyanate, hexamethylene diisocyanate, cyclohexane 1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane 4,4′-diisocyanate, methylcyclohexane diisocyanate, tetramethylxylylene diisocyanate, 2,6-diisopropylphenylene isocyanate and 1,3,5-triisopropylbenzene 2,4-diisocyanate. 
     In addition, in the above-described polycarbodiimide compound, the degree of polymerization thereof can be controlled to a suitable level using a compound, such as monoisocyanate, which is capable of reaction with a terminal isocyanate group of the polycarbodiimide compound. 
     The monoisocyanate for control of the degree of polymerization by protection of a terminal group of the polycarbodiimide compound includes phenyl isocyanate, tolyl isocyanate, dimethylphenyl isocyanate, cyclohexyl isocyanate, butyl isocyanate and naphthyl isocyanate. 
     In addition, the terminal protecting agent for controlling the degree of polymerization by protecting a terminal group of the polycarbodiimide compound is not limited to the above-described monoisocyanates, and includes compounds with an active hydrogen capable of reaction with isocyanate, for example (i) an aliphatic, aromatic or alicyclic compound with an —OH group, such as methanol, ethanol, phenol, cyclohexanol, N-methylethanolamine, oligo-, polyethylene glycol monomethyl ether and oligo-, polypropylene glycol monoalkyl ethers, fatty alcohols and oleyl alcohols; (ii) a compound with an ═NH group, such as diethylamine and dicyclohexylamine; (iii) a compound with an ═NH 2  group, such as butylamine and cyclohexylamine; (iv) a compound with a —COOH group, such as succinic acid, benzoic acid and cyclohexanecarboxylic acid; (v) a compound with an —SH group, such as ethyl mercaptan, allyl mercaptan and thiophenol; and (vi) a compound with an epoxy group. 
     The decarboxylation condensation reaction of the above-described organic diisocyanate is performed in the presence of a suitable carbodiimidation catalyst. Preferred carbodiimidation catalysts for use are an organophosphorus compound and an organometallic compound [a compound expressed by the general formula M-(OR) 4  in which M is titanium (Ti), sodium (Na), potassium (K), vanadium (V), tungsten (W), hafnium (Hf), zirconium (Zr), lead (Pb), manganese (Mn), nickel (Ni), calcium (Ca) and barium (Ba) and the like; R is an alkyl group or aryl group having 1 to 20 carbon atoms], and particular preference from an activity point of view is given to phospholene oxide among the organophosphorus compounds, and to alkoxides of titanium, hafnium and zirconium among the organometallic compounds. Mention should additionally be made of strong bases, for example alkali metal or alkaline earth metal hydroxides or oxides, alkoxides and phenoxides. 
     The above-described phospholene oxides include especially 3-methyl-1-phenyl-2-phospholene 1-oxide, 3-methyl-1-ethyl-2-phospholene 1-oxide, 1,3-dimethyl-2-phospholene 1-oxide, 1-phenyl-2-phospholene 1-oxide, 1-ethyl-2-phospholene 1-oxide, 1-methyl-2-phospholene 1-oxide and double bond isomers thereof. Among these, owing to easy industrial availability, particular preference is given to 3-methyl-1-phenyl-2-phospholene 1-oxide. 
     The carbodiimide compound is especially 4,4′-dicyclohexylmethanecarbodiimide (degree of polymerization=2 to 20), tetramethylxylylenecarbodiimide (degree of polymerization=2 to 20), N,N-dimethylphenylcarbodiimide (degree of polymerization=2 to 20) and N,N′-di-2,6-diisopropylphenylenecarbodiimide (degree of polymerization=2 to 20) and the like, and is not especially restricted provided that the compound has at least one carbodiimide group in a molecule with the function. 
     Carbodiimides suitable in the context of the present invention are especially monomeric, dimeric or polymeric carbodiimides. 
     Several preferred configurations of the carbodiimide compound are explained in detail hereinafter. 
     In a first configuration, in the context of the present invention, a monomeric carbodiimide is used. 
     In this configuration of the present invention, the carbodiimide preferably has the general formula (I) 
     
       
         
         
             
             
         
       
     
     in which the R 1  to R 4  radicals are each independently a straight-chain or branched C 2 - to C 20 -alkyl radical, a C 2 - to C 20 -cycloalkyl radical, a C 6 - to C 15 -aryl radical or a C 6 - to C 15 -aralkyl radical. 
     For the R 1  to R 4  radicals, preference is given to C 2 -C 20 -alkyl and/or C 2 -C 20 -cycloalkyl radicals. 
     For the R 1  to R 4  radicals, particular preference is given to C 2 -C 20 -alkyl radicals. 
     C 2 -C 20 -Alkyl and/or C 2 -C 20 -cycloalkyl radicals are understood in the context of the present invention to mean especially ethyl, propyl, isopropyl, sec-butyl, tert-butyl, cyclohexyl and dodecyl radicals, particular preference being given to the isopropyl radical. 
     C 6 - to C 15 -Aryl and/or C 6 - to C 15 -aralkyl radicals are understood in the context of the present invention to mean especially phenyl, tolyl, benzyl and naphthyl radicals. 
     A corresponding carbodiimide is commercially available from Rhein Chemie Rheinau GmbH under the names Additin® 8500, Stabaxol® 1 I or Stabaxol® I LF. The products sold by Raschig with the name Stabilizer 3000, 7000 and 7000 A can also be used in the context of the present invention. 
     In a second configuration, in the context of the present invention, a polymeric carbodiimide is used. 
     A corresponding polymeric carbodiimide has the general formula (II) 
       R′—(—N═C═N—R—) n —R″  (II)
 
     in which
         R is an aromatic, aliphatic, cycloaliphatic or araliphatic radical which, in the case of an aromatic or araliphatic radical in one ortho position, preferably in both ortho positions, to the aromatic carbon atom which bears the carbodiimide group, may bear aliphatic and/or cycloaliphatic substituents having at least 2 carbon atoms, preferably branched or cyclic aliphatic radicals having at least 3 carbon atoms, especially isopropyl groups,   R′ is aryl, aralkyl or R—NCO, R—NHCONHR 1 , R—NHCONR 1 R 2  and R—NHCOOR 3 ,   R″ is —N═C═N-aryl, —N═C═N-alkyl, —N═C═N-cycloaliphatic,   —N═C═N-aralkyl, —NCO, —NHCONHR 1 , —NHCONR 1 R 2  or NHCOOR 3 ,   where, in R′ and in R″ independently, R 1  and R 2  are the same or different and are each an alkyl, cycloalkyl or aralkyl radical and R 3  has one of the definitions of R 1 , or is a polyester or polyamide radical, and   n is an integer from 1 to 5000, preferably from 1 to 500.       

     In a first preferred embodiment of these polymeric carbodiimides, R is an aromatic or araliphatic radical which may bear, in at least one ortho position, preferably in both ortho positions, to the aromatic carbon atom which bears the carbodiimide group, aliphatic and/or cycloaliphatic substituents with at least 2 carbon atoms, preferably branched or cyclic aliphatic radicals with at least 3 carbon atoms, especially isopropyl groups. 
     Particularly suitable carbodiimides are those of the general formulae (I) and (II) which are substituted by isopropyl in the ortho positions on this aromatic or araliphatic radical to the carbodiimide group, and which are likewise substituted by isopropyl in the para position to the carbodiimide group. 
     In a second preferred embodiment of these polymeric carbodiimides, R is an aromatic radical which is joined via a C 1 - to C 8 -alkyl radical, preferably C 1 - to C 4 -alkyl radical, to the carbodiimide group (—N═C═N—). 
     In addition, it is also possible to use polymeric aliphatic carbodiimides, for example based on isophorone diisocyanate or H12-MDI (hydrogenated MDI), which are obtainable from Nishinbo. 
     To prepare the carbodiimides and/or polycarbodiimides of the general formula (I) or (II), it is possible to use monoisocyanates and/or diisocyanates which are converted by condensation with elimination of carbon dioxide at elevated temperatures, for example at 40° C. to 200° C., in the presence of catalysts, to the corresponding carbodiimides. Suitable processes are described in DE-A-11 30 594 and in FR 1 180 370. Useful catalysts have been found to be, for example, strong bases or phosphorus compounds. 
     Preference is given to using phospholene oxides, phospholidines or phospholine oxides, and also the corresponding sulfides. In addition, the catalysts used may also be tertiary amines, basic metal compounds, carboxylic acid metal salts and nonbasic organometallic compounds. 
     All isocyanates are suitable for preparation of the carbodiimides and/or polycarbodiimides used, preference being given in the context of the present invention to using carbodiimides and/or polycarbodiimides based on C 1 - to C 4 -alkyl-substituted aromatic isocyanates, for example 2,6-diisopropylphenyl isocyanate, 2,4,6-triisopropylphenyl 1,3-diisocyanate, 2,4,6-triethylphenyl 1,3-diisocyanate, 2,4,6-trimethylphenyl 1,3-diisocyanate, 2,4′-diisocyanatodiphenylmethane, 3,3′,5,5′-tetraisopropyl-4,4′-diisocyanatodiphenylmethane, 3,3′,5,5′-tetraethyl-4,4′-diisocyanatodiphenylmethane, tetramethylxylene diisocyanate or mixtures thereof, and on substituted aralkyls, such as 1,3-bis(1-methyl-1-isocyanatoethyl)benzene. It is particularly preferred when the carbodiimides and/or polycarbodiimides are based on 2,4,6-triisopropylphenyl 1,3-diisocyanate. 
     When they have been prepared from isocyanates, polycarbodiimides may also still contain reactive NCO groups and complex-bound monomeric isocyanates. 
     These polycarbodiimides containing NCO groups can be modified in such a way that the isocyanate groups present are eliminated with reactive, hydrogen-containing compounds such as alcohols, phenols or amines (cf. DE 11 56 401 A and DE 24 19 968 A). In this regard, more particularly, reference should also be made to the use of polypropylene glycol monoalkyl ethers, and fatty alcohol and oleyl alcohol residues for end capping. 
     The polymeric carbodiimides of the general formula (II) may also be end-capped with isocyanate compounds. 
     Polymeric carbodiimides in the context of the present invention are commercially available from Rhein Chemie Rheinau GmbH under the names Stabaxol® P, Stabaxol® P100, Stabaxol® P200 and Stabaxol P400. It is also possible to use the products sold by Rasching with the name Stabilizer 2000, 9000 and 11000 in the context of the present invention. 
     The carbodiimides can also be used in the context of the present invention as a mixture of several carbodiimides. 
     In a further embodiment of the present invention, it is also possible that a mixture of different carbodiimides is used. When a mixture of carbodiimides is used, the carbodiimides used may be selected from the group of the monomeric, dimeric or polymeric carbodiimides, reference being made to the above remarks regarding the compounds of the general formulae (I) and (II) with regard to the monomeric carbodiimides and the polymeric carbodiimides. 
     In addition, it is preferred in the context of the present invention when carbodiimides which contain a reduced content of free isocyanates are used. Preferred carbodiimides contain preferably less than 1% by weight of free isocyanates. 
     In addition, it is possible also to use other acid scavengers as well as carbodiimides. 
     The inventive transformer oil composition may comprise the at least one carbodiimide in a wide range of variation of the amount added. For reasons of efficacy, it is, however, preferred that the at least one carbodiimide is present in the composition in an amount of at least 0.01% by weight, based on the inventive transformer composition. For economic reasons, there will be no interest in the amount of the at least one carbodiimide in the transformer oil composition exceeding 10% by weight, based on the inventive transformer composition. 
     In a particularly preferred embodiment of the present invention, the amount of the at least one carbodiimide in the inventive composition is 0.01 to 10% by weight, more preferably 0.05 to 5% by weight, especially 0.1 to 1.5% by weight, based in each case on the inventive transformer composition. 
     It is also possible to use the acid scavenger, especially the carbodiimide, as a masterbatch, in which case the acid scavenger may be dissolved in a high concentration in a transformer oil and is diluted in application. 
     There now follows a more detailed description of the transformer oil which preferably finds use in the inventive composition: 
     Transformer Oils 
     In the context of the present invention, it is possible to use any transformer oil. It is preferred when the transformer oil meets the specification IEC 61099: 1992. This document “Specifications for unused synthetic organic esters for electrical purposes” is a general standard in which the demands on new synthetic organic esters for electronic purposes are defined. 
     In general, the transformer oil is selected from the group consisting of oils based on mineral oil, oils based on esters and oils based on triglycerides. 
     When the transformer oils used are mineral oils, the mineral oils are preferably mobile naphthenic low-sulfur base oils of group III according to the ATIEL-API classification of base oils, with good low-temperature and oxidation properties. Corresponding mineral oils are available, for example, from Nynaes (Sweden) under the trade name Nytro® or from Shell under the trade name Shell Diala D. These transformer oils meet IEC 60296: 2003—“Fluids for electrotechnical applications—Unused mineral insulating oils for transformers and switchgears”. 
     When there is a requirement for transformer oils which have low flammability and are biodegradable and exhibit a relatively high water absorption, preference is given to using transformer oils based on branched or linear saturated mobile polyol esters with preferably low acid numbers. Examples thereof are the systems sold by M &amp; J under the trade name Midel®. Also suitable in this case are soybean oil, rapeseed oil or sunflower oil. 
     In a preferred embodiment of the present invention, the transformer oil is a trimethylolpropane ester (TMP) of the general formula (III) 
     
       
         
         
             
             
         
       
     
     Corresponding trimethylolpropane esters are known from German patent application DE 10 2004 025 939 A1. In the above general formula (III), the R 1 , R 2  and R 3  radicals are the same or different and are each a linear or branched alkyl group having 5 to 11 carbon atoms. In a further preferred embodiment of the present invention, the R 1 , R 2  and R 3  radicals are the same or different and are each a linear or branched alkyl group having 7 to 9 carbon atoms. 
     The trimethylolpropane ester (IMP) of the general formula (III) features a viscosity of less than 23 mm 2 /s at 40° C. and a combustion point of higher than 300° C. These esters are thus outstandingly suitable as a dielectric insulating fluid for transformers. 
     The low viscosity can be achieved by selected acid components in the esterification. The R 1 , R 2  and R 3  radicals in the formula (I) consist of linear or branched alkyl groups having 5 to 11 carbon atoms. Preference is given to the use of radicals with linear or branched alkyl groups having 7 to 9 carbon atoms. The radicals must be saturated in order to achieve the oxidation stability necessary. It is possible for all radicals in a polyol ester to be the same, for only two radicals to be the same or for all of them to be different. Preference is given to a distribution of radicals with 7 to 9 carbon atoms which arise in the esterification of trimethylolpropanol with an acid mixture of C 8 - to C 10 -fatty acids, where the combustion point must be above 300° C. and the viscosity attains the preferred ranges described, of less than 23 mm 2 /s at 40° C. The higher the number of carbon atoms, the higher the combustion point will be, but the higher the viscosity will be. Since these values run in opposite directions, there is an optimum carbon chain length distribution of the R 1 , R 2  and R 3  radicals for each pair of values. 
     This class of trimethylolpropane esters meets the demands of standard IEC61099 and they are classified by the German Federal Environment Agency (UBA, Berlin) as not hazardous to water (NWG). 
     In a further embodiment of the present invention, the transformer oil may also be a pentaerythrityl ester. Such ester-based transformer oils are known on the market, for example, as Midel® 7131. In unadditized form, these transformer oils, however, lead to acid formation from insulation paper and thus lead to formation of water, which in turn leads to severe damage in transformers. 
     In addition to the already mentioned stabilizing effect due to the acid scavenger action of the carbodiimides in transformer oils, the carbodiimides also act synergistically in relation to copper, lead, tin and zinc, by preventing the corrosion of these metals used in the transformers and thus protecting the transformer oils in contact with these metals from aging. This inventive synergistic effect occurs especially when there are defects in the paper insulation and the windings come into contact with the transformer oil. This inventive synergistic effect is advantageous especially in the case of windings which comprise copper as material. 
     The inventive transformer oil composition may additionally comprise further additives customary for this field of use. For example, these may be antioxidants or metal deactivators. 
     In a further embodiment, the inventive composition therefore additionally contains 0.005 to 1.0% by weight of an antioxidant and/or 0.01 to 2.0% by weight of a metal deactivator, based in each case on the transformer oil. 
     The preferred amount of antioxidant is between 0.1 and 0.5% by weight and is especially 0.1% by weight, based on the transformer oil. 
     The preferred amount of metal deactivator is between 0.1 and 1.0% by weight and is especially 0.1% by weight, based on the transformer oil. 
     The antioxidant is preferably selected from the group consisting of bishydroxytoluene, hydroquinone, 4-tert-butylcatechol, naphthol, phenylnaphthylamines, diphenylamines, phenylic thioethers, tocopherols and mixtures of the substances listed. A suitable antioxidant is especially 2,6-di-tert-butylhydroxytoluene (BHT), which is sold under the Baynox® trade name by Lanxess Deutschland GmbH. 
     The metal activator is preferably selected from the group consisting of organic hetero compounds such as triazoles, tolyltriazoles, dimercaptothiadiazoles and mixtures of the substances listed. 
     The present invention additionally relates to a process for producing a corresponding inventive transformer oil composition by mixing the transformer oil with at least one acid scavenger, especially the at least one carbodiimide. For this purpose, the acid scavenger is added to the transformer oil, the resulting composition is optionally heated and the acid scavenger is stirred into the transformer oil with standard equipment. At the customary operating temperature of transformer oils (˜60° C.), the acid scavenger is generally likewise dissolved in the transformer oil, and so it is also possible to dispense with heating and/or stirring of the additive into the transformer oil before the use thereof. 
     The present invention additionally relates to the use of at least one acid scavenger, especially a carbodiimide, as a stabilizer in transformer oil compositions. With regard to the carbodiimide and the transformer oil composition, reference is made to the above remarks. 
     The present invention additionally relates especially to the use of at least one carbodiimide as an acid scavenger in transformer oil compositions. With regard to the description of the at least one carbodiimide and of the transformer oil, reference is made to the above remarks. 
     The present invention further relates especially to the use of at least one carbodiimide for protection of transformers from corrosion. With regard to the description of the at least one carbodiimide and of the transformer oil, reference is made to the above remarks. 
     The carbodiimides provided in accordance with the invention can be added either to new transformer oils which are being used for the first time or to transformer oils which are already in use. More particularly, it is also possible to add the acid scavengers provided in accordance with the invention, especially the carbodiimides, as additives to already regenerated transformer oils. 
     Corresponding processes for regenerating used transformer oils are known per se to those skilled in the art, and reference is made to the extensive known prior art on this subject. 
     The present invention further provides transformers which comprise the above-described transformer oil composition. The transformers may be power transformers, distribution transformers, mast transformers, tap changers or changeover switches. 
     The invention is illustrated by the examples below, without being restricted thereto. 
    
    
     WORKING EXAMPLES 
     The following samples were examined in relation to the aging characteristics: 
     Sample 1: 100% by weight of Shell Diala D (comparative example) and 
     Sample 2: 100% by weight of Shell Diala D+1% Additin® RC 8500 (a carbodiimide of the formula I where R 1 , R 2 , R 3  and R 4 =isopropyl, obtainable from Rheinchemie Rheinau GmbH) as an inventive example. 
     Samples 1 and 2 were aged at 145° C. in the presence of 20 weight of insulating paper (from Weidmann Plastics Technology AG) in a closed vessel over a period of 40 days. The aging-associated depolymerization (determined to DIN EN 60450) of the insulating paper was taken as a measure for the advancing aging.  FIG. 1  shows the results in graphic form. The degree of polymerization was measured over time. It is evident that the addition of carbodiimide slows the depolymerization of the insulating paper from the start.