CA2594765A1

CA2594765A1 - Dielectric coolants for use in electrical equipment

Info

Publication number: CA2594765A1
Application number: CA002594765A
Authority: CA
Inventors: David Berthiaume; Josee Labrecque; Paul Angers
Original assignee: Individual
Current assignee: OLEOTEK Inc
Priority date: 2005-01-13
Filing date: 2006-01-13
Publication date: 2006-07-20

Abstract

The invention pertains to a dielectric coolant for use in electrical equipment, consisting essentially of a mixture of polyol esters. The coolant has a pour point of at most about -20 C, and preferably one that is even lower. The polyol esters are chosen from formula (X): R1 and R2 are selected from carbon chain length of C4 to C22 and an alkoxycarbonyl group of formula CR3R4C2H2OCOR5, wherein R5 a carbon chain lengths of C4 to C22. R3 and R4 are carbon chain lengths of C1 to C5 or alkoxycarbonyl groups of formula CH2OCOR6, wherein R6 is a carbon chain length of C4 to C22 or an alkoxycarbonyl group of formula CR3R4C2H2OCOR5. It is preferable that the polyol ester be selcted from NPG, TMP and HPN esters.

Description

DIELECTRIC COOLANTS FOR USE IN ELECTRICAL EQUIPMENT
FIELD OF THE INVENTION

The present invention relates to a synthetic insulating fluid for use as a dielectric and cooling medium in power and distribution electrical apparatus, such as transformers, oil bath breakers and attendant equipment.

BRIEF DESCRIPTION OF THE PRIOR ART

Oleochemistry is a broad science which finds application in multiple fields, such as lubricants, paints, plastics, fuels, cosmetics, food, and pharmaceutical products. Oleochemistry further finds an application in the field of transmission and distribution of electrical power, and particularly for transformers in which important quantities of dielectric coolant are used.
The dielectric coolants employed in the electric transformers are mostly of mineral and petrochemical origin. However, some electric transformers using vegetable oil based dielectric coolants are newly presented on the American and European markets. These new coolants have the advantage of being efficient, biodegradable and non-toxic according to the American regulation.
Until today, these coolants are mostly used for installations located nearby ecologically sensitive sites, such as rivers and other natural places, or indoor locations, such as commercial buildings and towers.
Vegetable oil based dielectric coolants are efficient and environmentally friendly as they are biodegradable and non-toxic. However, they are limited in their utilisation because of their relatively high temperature operating range (usually limited to operating temperatures higher than -20 C). This value, which corresponds to the coolant's pour point, is acceptable for the majority of the American states, but is insufficient for the cold climate found in northern countries where the temperature often drops below the general -20 C pour point. Thus, the pour point of standard dielectric coolants for cold climates is of about -40 C. The lowering of the pour point is limited by the nature of these coolants.
There are known dielectric coolants in the art such as those sold by ABB
inc (Biotemp ) and by Cooper Power Systems (Envirotemp ). These products are made of purified and treated vegetable oil combined with conservative agents and other additives. These oils are biodegradable, non-toxic and have increased flash points. However, as mentioned above, these oils have a pour point unsuitable for use in electrical equipments located in at most about - 40 C climates and usually require the presence of additives when intended for operation temperatures around -20 C. These treated vegetable oils also pose other problems - such as product variability instability, etc - when used as dielectric coolants in electrical equipment, depending on the source oil, fatty acid content and compositions. These property enhanced vegetable oils often undergo simple separation processes to remove solids and water, are subsequently enhanced with various additives and are finally mixed in certain proportions to give a desired mixture. These mixtures are often composed of triglycerides and traces of various free fatty acids of various saturations, and other compounds found in and/or modified from vegetable oils.
Various documents in the prior art describe dielectric coolants derived from natural sources such as vegetable oils. For example, the United States patent application US 2002/00272219 (OOMMEN et al.) describes high concentration oleic acid triglyceride compositions that also contain specific concentrations of diunsaturated, triunsaturated and saturated fatty acids.
Optionally, synthetic esters such as polyol esters are added to the composition in varying concentrations. This patent shows that the compositions have various properties, including a pour point that may be brought to about -38 C.
Also, the International patent application WO 2004/108871 A2 (HOANG
et al.) describes mixtures of natural triglycerides and fatty acid esters of 2-ethyl-1-hexanol. This dielectric coolant is a blend of a vegetable oil and the aforementioned fatty acid esters, and may contain a high concentration of the esters. Various physical properties are attainable with such blends, among which is a pour point between about -20 C and -30 C. Also, in order to reduce the pour point, this patent teaches that mono-esters are advantageous as well as the polyunsaturated fatty acids thereof should be used.
The United States patent application US 2002/0049145 Al (CANNON et al.) describes a vegetable oil based electrically insulating fluid which has been hydrogenated (e.g. partially hydrogenated soybean oil) to improve the stability of the oil, or has been fortified with high concentrations of oleic acid. The oil mixture may also be winterized, which implies the mixture is cooled and then filtered. This treatment may result in a pour point of about -25 C. Other additives such as solvents may also be added. Also, thinning agents are added to improve the mixture, and can be thinning agent esters such as methyl esters.
The International patent application WO 97/22977 describes a dielectric coolant including a vegetable oil, antioxidant and low temperature additive.
The vegetable based oil includes glycerides such as trigycerides having a variety of carbon chains, preferably those with at least one degree of unsaturation to mitigate oxidation and hydrogen gas release. To improve low temperature properties and notably to lower the pour point, additives are required and are mixed in with the composition; otherwise different sources of oils are mixed together.

The United States patent US 4,812,262 (SHINZAWA et al.) describes an electric device and an insulating oil for use therein, the oil being composed of a fatty acid polyol ester having a high fire point 300 C blended with a small amount of phenolic compound to improve antioxidant ability and epoxy compound to improve stability. In particular, this patent teaches a blend of phenolic compounds with an ester of trimethylolpropane (1) or, especially, with an ester of pentaerythritol (2) (thus esters having three or four alkoxycarbonyl groups). In the Examples, only pentaerythritol esters (four alkoxycarbonyl groups) are used in combination with the phenolic and epoxy compounds. This patent teaches that certain polyol esters may be used as insulating oil in combination with certain compounds to have adequate antioxidizing performance, and dwells on the fire point of the polyol esters as a determining property for the selection of polyol esters for this purpose.

The United States patent US 6,726,857 (GOEDDE et al.) discloses dielectric coolants comprising relatively pure blends of compounds and that the coolant "mixtures consist of two or more compounds selected from the following classes: aromatic hydrocarbons, polyalphaolefins, polyol esters and triglycerides derived from vegetable oils", along with additives to improve pour point, increase stability and reduce oxidation rate. This patent specifically teaches that polyalphaolefins are blended with at least one of the aforementioned compounds to make the coolant composition. Thus, the polyol esters are blended with polyalphaolephins or aromatics to give the dielectric coolant blend. Some of these blends may pose problems involving biodegradability, renewability of resources required and the expense of producing at least two very different compounds of different chemical families.
Notably, the most preferred polyol ester is a pentaerythritol with four identical fatty acid chains of C9H2O. In the Examples, pentaerythritol is blended in varying amounts with phenyl ortho-xylyl ethane (Example VI), polyalphaolefin (Example VIII), and/or aromatics (Example IX), together with additives to optimize the performance of the compositions.
Most insulating oils used in the prior art comprise vegetable oils, which have pour points unsuitable for use in cold temperatures, and thus to further lower the pour point, pour point depressant additives are blended with the base vegetable oil. This gives rise to various inconveniences and inefficiencies, as two different compounds (i.e. vegetable oil and an additive) must be combined, which may imply increased processing costs, and depending on the additive, the biodegradability, stability, efficiency of the insulating fluid may be compromised. The resulting blend has disadvantages when used as a dielectric coolant, as its properties may not be up to certain standards of the industry or up to the requirements of certain applications.
Therefore, there is a need for innovating new dielectric coolants that are biodegradable, non-toxic, and efficient in cold climates, especially meeting operating temperatures colder than -20 C.

The prior art in dielectric coolants for electric equipment such as transformers has many inefficiencies and drawbacks, in the processing, storing and/or use of the dielectric coolants.
SUMMARY

5 The present invention provides a dielectric coolant that overcomes some of the disadvantages and drawbacks of known fluids used in the art, which are mentioned hereinabove. The inventive dielectric coolant is used in electrical equipment and presents numerous advantages over the prior art.
Notably, the dielectric coolant according to the present invention has a pour point of at most about -20 C for use in power distribution equipment for cold climates.

The inventor has unexpectedly found that a dielectric coolant consisting essentially of a mixture of polyol esters has advantageous properties enabling the mixture to be used as a coolant in low temperatures. More specifically, the dielectric coolant according to the present invention has at most a pour point of about -20 C.

Accordingly, the present invention provides a dielectric coolant for use in electrical equipment, consisting essentially of a mixture of polyol esters chosen from formula X:

R~ O,X, O ,r R2 0 O (X) ;
wherein:

R, and R2 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5, wherein R5 is selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22; and R3 and R4 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C, to C5 and alkoxycarbonyl groups of formula CH2OCOR6, wherein R6 is selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5i said dielectric coolant having a pour point of at most -20 C.

According to a preferred embodiment of the present invention, the dielectric coolant has at most a pour point of about -40 C and consists of a mixture of more than one compound chosen from the following:

a neopentylglycol (NPG) polyol ester of formula I:

R' )r O,,~ O r R2 O O
(I) .

wherein R, and R2 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22; and/or - a polyol ester of formula II:

O O
R
, R2 O

(II) .

wherein RI, R2 and R6 are the same or different and are selected from branched or unbranched alkyl groups with carbon chain lengths of C4 to C22;

- polyol esters of formula (III):

O O O
AO-""
O "'~ O JI"- R, (Ill);
wherein R, and R5 are the same or different and are selected from branched or unbranched alkyl groups with carbon chain lengths of C4 to C22.

According to another embodiment of the invention, the dielectric coolant consisting essentially of the mixture of polyol esters of formula X defined above, and having a pour point of at most about -20 C is manufactured by a process comprising the following steps:

providing at least a first carboxylic acid and a first polyol to react together to produce a first polyol ester selected from the polyol esters of formula X;
providing at least a second carboxylic acid and a second polyol to react together to produce a second polyol ester selected from the polyol esters of formula X, said second polyol ester being different from said first polyol ester;
mixing the first polyol ester with the second polyol ester to produce the mixture of polyol esters.

On one hand, it is advantageous that the preceding polyol esters are combined in proportions that enable a synergistic effect on certain properties of the dielectric coolant. Notably, the different polyol esters are combined to unexpectedly lower the pour point of the resulting polyol ester mixture. In a preferred embodiment of the present invention, the pour point of the mixture of polyol esters is lower than the proportional average of the pour points of the individual polyol esters used in the mixture. In other preferred embodiments, the mixture of the polyol esters has a pour point lower than any of the pour points of the individual polyol esters used in the mixture. A variety of polyol esters may be used to produce mixtures having variety of compositions. It should also be made clear that though the pour point of the mixture is preferabiy lower than the proportional average of the individual polyol esters, that this is not necessary to the functioning of the invention and other embodiments are envisioned by the inventors, and will be dicussed here below.

The present invention has the advantage of providing dielectric coolants having at least one of the following characteristics:

- no need for pour point depressant additives;
- more resistant to hydrolysis and oxidation;

- non-toxic;
- biodegradable;

- pour point down to -90 C; and - flash point of at least 200 C.

DETAILED DESCRIPTION OF THE INVENTION

The inventor has surprisingly found that the use of a mixture of polyol esters - of which neopentyl glycol ester (NPG ester), hydroxypivalic acid neopentylglycol ester (HPN ester) and/or trimethylolpropane ester (TMP ester) are preferred - offers an efficient fluid as a dielectric coolant useful in power distribution equipment, such as transformers.

The dielectric coolant of the present invention consists of a mixture of polyol esters and has a pour point of at most -40 C.

Definitions It will be understood that a "dielectric coolant" of the present invention should possess at least one of the following characteristics. It should transfer heat effectively, have an appropriate dielectric strength, and/or should not include ingredients harmful to the environment. As mentioned above, it has been surprisingly found that a mixture of polyol esters, optionally including an HPN ester, and combinations thereof satisfy both the requirements for suitability as dielectric coolant in cold climates and the requirements relating to environmental compatibility. It will also be understood that a dielectric coolant contemplated by the invention comprises a mixture of the defined polyol esters, and thus encompasses a mixture of polyol esters of formula X, and more particularly and preferentially, of formula I and/or of formula II and/or of formula IIl.

It will be understood that a "polyol ester" of the present invention refers to an ester compound having at least two ester functional groups. Normally, an ester is produced by reacting a compound having a carboxylic acid functional group with a compound having an alcohol functional group. The polyol esters result from the chemical combination of polyalcohol compounds with organic acids containing a variety of alkyl groups. Though this reaction is the most common for producing polyol esters, the polyol esters of the present invention are not limited to those produced in this fashion. Esterification and transesterification are preferred reaction techniques, though others may be used. The polyol esters defined in formula X may be extracted from naturally occurring oils, or derived from other natural or synthetic sources. Thus, polyol esters of synthetic or natural origin may be used in the present invention.

It will be understood that the "mixture" of polyol esters of the present invention refers to a mixture that includes any combination of one or more 5 polyol esters defined in formula X. Of course, though the mixture is of polyol esters, it is understood that it may also include impurities. These impurities, which may be the result of the fabrication, reaction, transfer, transport, storage, etc, of the mixture, will be further discussed herebelow. The mixture may also be a substantially pure fluid composed of a polyol ester, thus having each Ri, 10 R2, R3, R4 R5 and/or R6 groups individually the same. An example of such a mixture is a substantially pure fluid consisting of the polyol ester NPG-dicaprylate (where R, and R2 are both CH3-(CH2)6-, though keeping in mind that there are often other carbon chain lengths such as C6 and/or C8 mixed in with this substantially pure mixture), with impurities. Nevertheless, it is preferable to have a certain range of carbon chain lengths, and thus that at least one of R, to R6 groups is different. Of course, the mixture may alternatively consist of a single class of polyol esters, such as for example NPG
esters. Such a preferable mixture may include NPG esters having a variety of R, and R2 groups. For example, NPG-dicaprylate and NPG-trioleate could be mixed together to give a pure NPG ester mixture, that may include impurities.
Another example of a mixture of polyol esters is where two or more polyol esters of different classes are mixed together. For example, NPG esters and TMP esters may be used to make up the mixture. However, the mixture of polyol esters may include those of different classes, each having a variety of R
groups, in order to make up the mixture.

It will be understood that the term "consisting essentially of' used to characterize the dielectric coolant in relation to the mixture, prescribes that the dielectric coolant is substantially made of the mixture of polyol esters but may include some other compounds in minimal amounts. This semi-closed terminology, as pertaining to the field of chemistry and more specifically to the field of dielectric coolants that include polyol esters, indicates that the dielectric coolant does not include substantial amounts of compounds commonly used in the art as the main base of the coolant fluid. However, this terminology does not exclude traces of esters, fatty acids, acids, alcohols, among others, found in the prior art. The prior art teaches, for instance, that esters, including polyol esters, may be used as additives in coolants comprising compounds such as polyalpha olephins, aromatics and vegetable oils, among others. The present invention may be distinguished from such blends in that the dielectric coolant is composed of select polyol esters of formula X, and that the dielectric coolant does not require substantial amounts of other compounds. Thus trace amounts of undesired compounds, as well as small quantities of other compounds such as biocides, stabilizers and antioxidants are permitted.

It will be understood that "impurities" referred to hereabove, may include a variety of compounds. The dielectric coolant may contain impurities common in the arts of dielectric coolants, polyol ester manufacturing, as well as chemistry in general. Also, depending on the given impurity, different levels may be acceptable. However, other impurities may be present in greater quantities.
Dielectric coolant impurities may be introduced into the coolant at a variety of stages of the production, transport, storing and/or use of the coolant. In the production of polyol esters, trace amounts of organic acids and alcohol may be present. There may also be traces of by-products of alcohol - acid reactions.
When the polyol ester mixture is manufactured using a solvent and a catalyst, trace amounts of these compounds may be present in the isolated polyol ester mixture. Also, depending on the source of the organic acids and alcohols (e.g.
if the acids are derived from a natural source comprising a complex array of compounds), small amounts of these miscellaneous compounds is permissible.
As some polyol esters may present a certain degree of degradation over time, these degradation products may be present in varying amounts according to the age of the dielectric coolant as well as the quality and conditions under which it was stored or used. In this context, an acceptable trace amounts will be understood to be an amount that does not significantly affect the functioning of the base mixture.

It will also be understood that the term "at most" used to characterize the pour point indicates that the pour point of the dielectric fluid is below the indicated temperature. In other words, the value of the pour point is a temperature equal to or below -40 C, such as -45 C, -55 C and -90 C.

It will be understood that by "about", it is meant that the value of said pour point or flash point can vary within a certain range depending on the margin of error of the method used to evaluate such pour point or flash point.
The margin of error may also include factors such as the measurement apparatuses, measurement standards and/or measurement method used to evaluate the pour point or flash point, and would be known to a person skilled in the art. For example, the method to measure the pour point, ASTM D97, measures in intervals of 3 C, and thus such a margin of error exists for these measurements.

It will be understood that the "pour point" of a fluid is the temperature at which the fluid stops flowing. It may be measured according to a variety of standards, including the ASTM D97 method and the ISO 3016 standard. This property is particularly important in the field of dielectric fluids for electrical equipment since the pourability of the fluid affects numerous aspects of heat transfer capacity. For instance, if a fluid can no longer flow, the convective heat transfer is all but eliminated. Convection is a very important heat transfer mechanism and is dependant on whether a medium can flow or not. Also, when handling dielectric fluids at low temperatures it is desirable to have an adequately low pour point so that the coolant may be changed, replenished, tested, etc. Furthermore, the dielectric properties of a coolant may change depending on its pourability, i.e. a dielectric coolant that is above its pour point has different dielectric properties than the same coolant at or below its pour point. It should also be noted that the pour point of a mixture is not necessarily an average of the pour points of the individual constituents, but, due to a variety of complex chemical interactions, the pour point of the mixture may be higher or lower. Also, the pour point is a function of a variety of molecular factors and may not be predicted alone by other properties of given chemical compounds such as the fire point, flash point, viscosity, molecular mass, etc.

It will be understood that a "transformer" is a device that transfers electric power from one circuit to another by electrical magnetic means. Transformers are used extensively in the transmission of electrical power, both at the generating end and the user's end of the power distribution system. A
distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.

The dielectric coolant of the present invention consists of a mixture of polyol esters chosen from formula X:

R11y0~_,><,0,,r R2 O O (X).

R, and R2 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5, wherein R5 is selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22.

R3 and R4 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C, to C5 and alkoxycarbonyl groups of formula CH2OCOR6i wherein R6 is selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5.

As will be appreciated, Rl, R2, R3, R4, R5 and R6 may be selected to give various preferred embodiments of the polyol ester of formula X, and more notably, NPG ester, HPN ester and TMP ester. The polyol ester mixture may also include pentaerythritol (PET) ester and/or di-hydroxypivalic acid neopentylglycol (dHPN), as will be further described herebelow.

The first preferred polyol ester is neopentyl glycol (NPG) having the following formula:

R~ 0,..~Oy R2 O O
(I) Thus, in this preferred embodiment, R3 and R4 of formula X have been selected to be the same and to be methyl groups. R, and R2 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22. R, and R2 preferably present chains of natural or synthetic origin that offer a good level of biodegradability. Even more preferably, R, and R2 present chains obtainable from a biological source such as vegetable or animal oils.

According to the present invention, R, and R2 of the NPG ester are both or independently:

CH3-(CH2)6-C H3-(C H2)7-C H=C H-(C H2)7-;

I

I
CH3 ; and/or I

According to a further preferred embodiment, R, and R2 of the NPG
ester are both or independently CH3-(CH2)6- or CH3-(CH2)7-CH=CH-(CH2)7-, 5 and still preferably are both CH3-(CH2)6-.

According to a second preferred embodiment, the dielectric coolant of the invention comprises a hydroxypivalic acid neopentyiglycol ester (HPN
ester). This HPN ester is of the following formula:

O O O
R5 A O O O~
pl, (III).
10 First, it should be noted that formula III corresponds to formula X that has been rotated about a planar axis and has the following characteristics. In this preferred embodiment, R3 and R4 of formula X have been selected to be the same and to be methyl groups. Also, R2 has been selected to be alkoxycarbonyl groups of formula CR3R4C2H2OCOR5, wherein R5 is selected 15 from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22. RT may be selected from branched or unbranched, saturated or unstaurated alkyl groups with carbon chain lengths of C4 t0 C22.

Preferably, R, and R5 have the same meaning and preferred embodiments as R, and R2 defined above.

According to a preferred embodiment, R1 and R2 are both CH3-(CH2)6- .
The third preferred polyol ester is trimethylolpropane (TMP) having the following formula:

O O
R ~O O~ R2 O

(II) In this preferred embodiment, R3 and R4 of formula X have been selected to be an ethyl group and an alkoxycarbonyl group of formula CH2OCOR6 respectively, and wherein R6 is selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22.

RI, R2 and R6 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22.

R1i R2 and R3 preferably represent chains of natural or synthetic origin that offer a good level of biodegradability. Even more preferably, R1, R2 and present chains obtainable from a biological source such as vegetable or animal oils.

Accordingly, R1, R2 and R3 may be each or individually:
CH3-(CH2)6-CH3-(CH2)7-CH=CH-(CH2)7-I

I

I
CH3 ; and/or I I

According to a preferred embodiment, RI, R2 and R3 are CH3-(CH2)6- or CH3-(CH2)7-CH=CH-(CH2)7-, and still preferably they are CH3-(CH2)6-.

Also, PET esters may be a constituent of the polyol ester mixture. In this case, R3 and R4 are alkoxycarbonyl groups, as referred to above. Also, dHPN
esters may be used, in which case R3 and R4 are selected to be the same and to be methyl groups, while R, and R2 are selected to be alkoxycarbonyl groups of formula CR3R4C2H2OCOR5. Furthermore, when R6 is selected as an alkoxycarbonyl group of formula CR3R4C2H2OCOR5, the resulting polyol esters are hydroxypyvalic derivatives of PET and/or TMP.

In a preferred embodiment of the pour point of the mixture of polyol esters is lower than the proportional average of the pour points of the individual polyol esters used in the mixture. In other preferred embodiments, the mixture of the polyol esters has a pour point lower than any of the pour points of the individual polyol esters used in the mixture.

However, in another preferred embodiment of the invention, the pour point of the dielectric coolant is about -20 C or less, but is not lower than the proportional average of the pour points of the different individual polyol esters making up the mixture. As dielectric coolants are chosen according to properties other than the pour point - such as fire-point, biodegradability, stability, cost, etc - other factors may come into play when choosing the combination of polyol esters to use to make up the mixture. For example, some fatty acids are more abundant than others and therefore are less expensive, which may prefer such acids in the production (with alcohols) of the corresponding polyol ester product. Still, the advantageous pour point of less than about -20 G is possible, even when using a variety of polyols having a variety of factors and properties influencing their selection and use.

Furthermore, a dielectric coolant consisting essentially of a mixture of polyol esters may be composed of certain polyol esters selected for certain properties they may have. On the one hand, the molecular mass, the pour point, the fire point, the flash point, the density, the stability, the viscosity, among others, could be used to help a person skilled in the art to select the polyol esters to be used. On the other hand, other factors may influence the choice of polyol ester. For example, different polyol esters have different production costs due to process design, reactant availability and market price of certain materials. Thus, different polyol esters may be selected for the dielectric coolant mixture on the basis of their cost and/or chemical/physical properties. The resulting mixture of polyol esters has the beneficial properties of being biodegradable, environmentally friendly, and able to be used in cold environments.

Alternatively, certain polyol esters may be used as the unique component of the mixture. In particular, it is advantageous to use NPG esters, TMP esters or HPN esters alone in the mixture to produce the dielectric coolant. In each case, a variety of carbon chains may be used for the above mentioned polyol esters.

The polyol ester mixture may be manufactured using various processes and starting materials. Esterification and/or transesterification are preferred reactions, which are known in the art. Also, animal and/or vegetable oils, from a variety of sources may be used as a starting material. Furthermore, a selection of the fatty acids to be used in the esterification and/or transesterification may be performed to include specific fatty acids or a select range thereof, to react with alcohols. On the other hand, it is also possible to have an extremely wide, non-selective range of fatty acids. Also, in the case of transesterification, triglycerides having a variety of carbon chains may be reacted with polyols to yield polyol esters. Also, specific carbon chain lengths, ramifications and saturations may be chosen for specific applications.

It is useful to note that the pour point of a fluid such as a dielectric coolant depends on the inability of the molecules to configurationally fit together to form crystals. It has been surprisingly found that this inability of fitting polyol esters together may be taken advantage of to produce a dielectric coolant consisting essentially of polyol esters that may have functional properties at very low temperatures.

This is advantageous in cold climates and northern countries, where the temperatures to which the electrical equipment is subjected are harshly low.
Furthermore, there are other applications in which a dielectric coolant may find use. For example, in the aeronautic industry, electrical equipment is subjected to extremely low temperatures, depending on the speed, height, among other factors.

As it will be understood by one skilled in the art, the pour point of a dielectric coolant affects its usefulness particularly with regards to electrical equipment in cold climates. In this connection, a dielectric coolant of the present invention has preferably a pour point of at most about -40 C, more preferably of at most about -60 C, and even more preferably of at most about -90 C.

The pour point of a mixture depends on the individual constituents of said mixture and also on their interaction (synergistic effects). These interactions are often very complex, in terms of the chemistry in play. The pour point of mono-ol-esters is often lower than that of polyol ester, and such esters are often used as additives to lower the pour point of a coolant. In the case of the present invention, it has been surprisingly found that certain polyol esters are advantageous for constituting a dielectric fluid with a very low pour point.
5 Furthermore, the pour point may also depend on the length, ramification (branching) and saturation of the carbon chains of the polyol ester. However, the combination of different polyol esters having different numbers of ester functional groups and/or different carbon chain lengths of different saturations, is often unpredictable. Thus, according to the present invention, mixtures of 10 polyol esters have been found that have a very low pour point, at most -20 C
and as low as -90 C, and are advantageous to be used as dielectric coolants.
Preferably, the dielectric coolant of the present invention contains no additional pour point depressants but consists simply of the polyol esters of formula X. Alternatively, and especially in extreme applications such as 15 aeronautic, aerospacial or arctic industries, small amounts of compounds that may be considered pour point depressants may be included. However, they are included only to decrease the pour point below the already surprisingly low value of at most -20 C.

According to another preferred embodiment, a dielectric coolant of the 20 present invention has a flash point of at least about 200 C. It will be understood that the flash point is the lowest temperature at which a flame will set light to the vapour of the oil or its degradation product. It represents a qualification test and is employed on a pre-industrial scale. The most common standard methods for measuring the flash point are ASTM D92 and ASTM D93.

The power factor is another preferred property of the invention in applications in electrical equipment as a dielectric coolant. As a general rule, the higher the power factor, the higher power losses. Thus, lower percentages are preferred. According to a preferred embodiment, the power factor at 100 C
of the dielectric coolant is equal to or lower than 14%. More preferably, the power factor of the inventive dielectric coolant is lower than 11 %, still preferably lower than 5%, and even more preferably equal to or lower than 2.85%. The power factor depends on a variety of factors including impurities in the fluid.
Also, different isolating oils have different power factor standards. For instance, according to Canadian norm CAN/CSA-C50-97 (class A), cold weather minereal oils should have a power factor equal to or lower than 0.5%, and according to American norm ASTM D6871, vegetable oils should have a power factor equal to or lower than 4.0%. Power factor standards depend on the type of fluid being used. The power factor of the present invention is acceptable for its application. For instance, the 50-50 mixture of TMP-trioleate and NPG-tricaprylate formulation was subjected to standard purification steps and an advantageous power factor was obtained. Thusly, purification steps commonly known to someone in the art may be used after production of the polyol ester mixture in order to improve its power factor to down to a certain degree.

According to the present invention, the polyol esters are preferably produced by reacting organic acids with alcohols. The organic acids are preferably chosen from natural sources such as vegetable or animal oils.
These natural sources often include a variety of triglycerides, fatty acids of varying saturations, and may undergo various processing steps to produce the polyol esters of the present invention. The organic acids may also be chosen from synthetic organic acids taken or derived from industrial greases, oils, distillation products and by-products, or manufactured synthetically according to any number of reaction processes.

According to the present invention, additives can be further added to the dielectric coolant described above in relatively small amount. These additives are selected from the group consisting of antioxidants, stabilizers, and biocides.
It should be understood that additives are used for a variety of reasons to improve the properties of dielectric coolants. Antioxidants, for example, increase the stability of the coolant with regards to oxidation, which may be present. On another example, biocides may prevent the growth of bacterias feeding on certain oil components during utilisation. Other additives may capture degradation products that inevitably occur during use of the coolant.
In many cases, the properties of the coolant may be altered or improved by adding such additives, even beyond what the surprising and advantageous results are obtained without such additives.

Preferred antioxidants include, but are not limited to, phenolic antioxidants, with 2,6-di-tert-butyl-paracresol (DBPC), BHA and BHT being particularly preferred antioxidants, having the formula:

/ I I \ \ \
HO OH I / I

H

Preferred stabilizers include, but are not limited to, epoxide additives.
Examples of epoxide additives used to prevent degradation of the esters are:

o,\ o V \o ~ ~ ~ ~ o/

There exists a very wide variety of biocide compounds in the art.
Preferred biocides include, but are not limited to, pyrithione derivatives, halogenated pyridine derivatives, triazole derivatives, and tetrahydrofuran derivatives.

Process of fabrication The dielectric coolant according to the present invention may be fabricated by a variety of methods. Principal among these preferred methods are the "formulation" mixture method and the "synthesis" mixture method. The formulation mixture is made by producing a first polyol ester by esterification and/or transesterification, followed by producing a second polyol ester. These first and second polyol esters are thus produced separately (two different batches), after which they are mixed together to produce the mixture of polyol esters.
The "synthesis" mixture may be produced by producing a variety of polyol esters in a same batch. Thus polyol esters and carboxylic acids are provided and reacted together to produce -a range of different polyol esters that are already mixed together.
Furthermore, the synthesis method and the formulation method may be combined, thus providing a synthesis mixture to which another polyol ester or another synthesis mixture is mixed to produce the dielectric coolant mixture.
Also, though batch processes are a preferred to produce the present invention, continuous processes may also be used and could be readily adapted by a person skilled in the art.
Alternatively, the polyol esters for the dielectric coolant mixture may be synthesized according to various other production methods. For example, they may be synthesized by reacting an alcohol with an acid halide, optionally in the presence of at least one certain catalyst; an alkyl halide with a carboxylic acid, optionally in the presence of at least one certain catalyst; an ester with an alcohol, optionally in the presence of at least one certain catalyst; and/or an alcohol with an acid anhydride, optionally in the presence of at least one certain catalyst. The aforelisted reactions, in particular, are described in the following reference: "Comprehensive Organic Transformations: A Guide to Functional Group Preparations", 2nd Ed., Richard C. Larock, 1999, especially at pages 1639, 1952, 1955 and 1969.
The present invention will be more readily understood by referring to the following examples. These examples are illustrative of the wide range of applicability of the present invention and are not intended to limit its scope.
Modifications and variations can be made therein without departing from the spirit and scope of the invehtion. Although any method and material similar or equivalent to those described herein can be used in the practice for testing of the present invention, the preferred methods and materials are described.
EXAMPLES

Producing the preferred dielectric coolants The polyol ester mixtures of the present invention were obtained by esterifying the appropriate polyol compoundor mixture with the appropriate carboxylic acid compound or mixture with an acid catalyst in a Dean Stark apparatus and/or formulation of separate batches thereof.
The polyol esters and HPN esters mixtures of the present invention could also be obtained by esterifying the appropriate polyol compound or mixture with the appropriate carboxyiic acid compound or mixture with a determined amount of hydroxypivalic acid with an acid catalyst in a Dean Stark apparatus and/or formulation of separate batches thereof.

Caprylic acid (667,3 g; 4,53 mol) and trimethylolpropane (202,3 g; 1,46 mol) are placed in a Dean Stark apparatus with toluene (solvent; 3,2 liters) and p-toluenesulfonic acid (catalyst; 14,2 g; 73 mmol) and heated at reflux under mechanical agitation for 5 hours. The solvent is then evaporated and the remaining oil is washed with a sodium carbonate solution and water to remove catalyst and unreacted caprylic acid.

Ri O

R2--/\\O O>--R3 Where RI, R2 and R3 are CH3-(CH2)6-.

Caprylic acid, oleic acid, and HPN are placed in a Dean Stark apparatus with toluene and benzenesulfonic acid (catalyst) and heated at reflux under mechanical agitation for 6 hours. The solvent is then evaporated and the 5 remaining oil is quickly washed with a diluted sodium hydroxide solution and brine.

RI-~( C
\O O O~

O .. :) Where R, and R2 independantly are CH3-(CH2)6- or CH3-(CH2)7-CH=CH-(CH2)7-EXAMPLE 3 (synthetic mixture) Caprylic acid, oleic acid, HPN and neopentylglycol are placed in a Dean Stark apparatus with toluene and benzenesulfonic acid (catalyst) and heated at reflux under mechanical agitation for 6 hours. The solvent is then evaporated and the resulting ester mix is quickly washed with a diluted sodium hydroxide solution and brine.

Rq O
\O O 0--~( O

O O O' R3 Ra Where Ri, R2, R3, and R4 independently are CH3-(CH2)6- or CH3-(CH2)7-CH=CH-(CH2)7-.

EXAMPLE 4 (synthetic mixture) 2-Ethylhexanoic acid, neopentyl glycol and hydroxypivalic acid are placed in a Dean Stark apparatus with toluene and p-toluenesulfonic acid (catalyst) and heated at reflux under mechanical agitation for 6 hours. The solvent is then evaporated and the remaining ester mix is washed with a sodium carbonate solution and water.

R1--~( O RS- 0 0 \O O 0---'( 0 O 0 \R2 Re 0y \/' 0-\yi~\~ /O 0 ~

Where Rl, R2, R3, R4, R5 and R6 are independently:
CH3-(CH2)4-CH (2-ethyl)-.

75 weight percent of neopentyl glycol dicaprylate is blended with 25 weight percent of HPN-dioleate. To the resulting blend is added 0.5 weight percent of an oxidation inhibitor such as 2,6-di-tert-butyl-paracresoi (DBPC).

To the blend obtained in EXAMPLE 5 is added an epoxide stabilizer.
Physical and chemical properties of preferred dielectric coolants of the invention shown in Table 1 were determined by the following methods:
- oxidative stability : method EN 14112 - kinematic viscosity : method ASTM D445 - flash and fire points : method ASTM D92 - pour point : method ASTM D97.
COMPARATIVE EXAMPLES
The physical and chemical properties of the preferred dielectric coolants In the following comparative examples, the especially advantageous and surprising results of mixing different polyol esters together may be readily appreciated. Individual polyol esters were produced (see Examples here above) and their individual pour points were measured, and then compared with the pour points of the polyol ester mixtures.

Comparative Example 1 The polyol ester mixture of TMP-tricaprylate and HPN-dicaprylate was produced according to various volume ratios of the individual polyol esters, ranging from 25%
to 75% v/v TMP-tricaprylate. In particular, the following pour point results were obtained:
100% TMP-tricaprylate -63 C
100% HPN-dicaprylate -54 C
50% TMP-tricaprylate - 50% HPN-dicaprylate mixture -61 C.
Note that the mixture has a pour point that is surprisingly lower than the proportional average of the individual polyol esters.

Comparative Example 2 The polyol ester mixture of TMP-tricaprylate and NPG-dicaprylate was produced according to various volume ratios of the individual polyol esters, ranging from 25%
to 75% v/v TMP-tricaprylate. In particular, the following pour point results were obtained:
100% TMP-tricaprylate -63 C
100% NPG-dicaprylate -66 C
50% TMP-tricaprylate - 50% NPG-dicaprylate mixture -72 C.

Note that the mixture has a pour point that is surprisingly lower than the proportional average of the individual polyol esters, and even more surprisingly lower than the pour point of either of the individual polyol esters.

Comparative Example 3 The polyol ester mixture of HPN-dicaprylate and NPG-dicaprylate was produced according to various volume ratios of the individual polyol esters, ranging from 25%
to 75% v/v HPN-dicaprylate. In particular, the following pour point results were obtained:
100% HPN-dicaprylate -54 C
100% NPG-dicaprylate -66 C
50% HPN-dicaprylate - 50% NPG-dicaprylate mixture <-74 C.

25% HPN-dicaprylate - 75% NPG-dicaprylate mixture -78 C.
Note that these mixtures have pour points that are surprisingly lower than the proportional average of the individual polyol esters, and even more surprisingly lower than the pour point of either of the individual polyol esters.

Comparative Example 4 The polyol ester mixture of TMP-trioleate and HPN-dicaprylate was produced according to various volume ratios of the individual polyol esters, ranging from 25%
to 75% v/v TMP-trioleate. In particular, the following pour point results were obtained:
100% TMP- trioleate -45 C
100% HPN-dicaprylate -54 C
50% TMP- trioleate - 50% HPN-dicaprylate mixture -54 C.
Note that the mixture has a pour point that is surprisingly lower than the proportional average of the individual po(yol esters, which would be about -49 C.

In the following Tables, the pour point and other properties of the individual polyol esters and the polyol ester mixtures (various v/v ratios) may be appreciated.

'p 0 N LO
3:
O_ i ~ CD LO

p N ~
~ v W L o N
A N C) q I.~n C.0 0. 0 o 0 ~
0-0 U ~ (0 o m v > = m uO cw "P Ct' 00 Q o w u9 T r.
z ~ )>, >=. C', ao (n E U -,t r ~ "' (/) M cr Orn 0) a) rn a) ON) W N o 0 0 0 0 0 a ~ c ~j ~ , N c 0 c +~
0 L- = N N N N
a .~ .~
..I o o M ~ a ~ ~
U Q ~ N cV N CV

W 0 ~ CO ~ OD LO
V N ~ N ~
0 ~
z a .~
in U
v (,a,1 ~ db ' ~ N ~ ~- ~
U) >

LO Od ti ~ 0) OC) O
v- X~ CJD
W o -j +
N ~ (U
m ..~+ I llfl1 N N IL EZ E
~- CD
CL I- Z vo d~- o _ I- Z = Z = , LD ~ N

co LU
x . ~--z p =
o p 00 e- r- o0 d ti(~O
-o f- 1 Cfl CO CU [LC~ O I ~ Lfl LC1 ln ln tn tn I 1.n tS~
I ~,I ~~~ V I - -~ ~
U O
a.
~--W
.J

W <o cv cu ca ns m ca cu c6 ~
CU - - -~ L7. C2 a Ll a L2 Q i2 aT~ f4 RS (B cu ci ~~~ n. Q a n~. Q ss~.
zs a'v ~ L co c~ co ~s cv ns ca cv C7 C~ C~ cL a. a. a a ct :Q o'r? cc-) v Ua o~
W [L r1 d u. zzzE--r-~-t-t-f-(9 c)aaQZZZ
o 0 0 0 0 0 0 0 0 0 z ' - z G. 2 m Q cZ a f- h 2==
a ~ ~~ N ti~ N~~~ o o o 0 0 0 0 0 C-LJ. 5 'CS T3 "a Y3 'i3 'd tS 'a 'Q Lf) OLO t.() OLp t1) OL[) Z
p c c c c c C c c c I- Lf) f- ln N f~ ~ N o ~ N c0 cB (lS cII tQ t6 tQ ca 'a -o a~-~ ~~} C3 Z3 ~ l1 (a N N N (II N N N N ~
LU !Q (B R7 qT _N S6 cQ c0 fQ C0 0 -0 ~ ~f 0 ~ _ N ~
[l. Ci LL G1 0. 0. 0- ~. L2. N~~ Q~ N N N Q (U 3 W ~ Q-W ~ c ~ icm i ~U~ ~ ~ c ~0 i o oo o o 0 0 0 0 o c~
. v z3 '~s p a ~ ~ ~ ~
zzzzzzcDaaaQaaaaaz a aaact.a(L aaa2 2 g:2 2 gZ> gga = T T= 2 z Z Z I- I- f- I- I- I- F- h- h- F--0 o 0 0 0 o o ~ 0 p o 0 0 o ~
lC) O Ln Ln O Ln Lo O k) Ln O 4A Lo O I . C ) tt) C D O to i N ln - N i - r i f ' Lr') N NLo t ' N N I' Lo N
LL
! -' p (D N M d tC) (O f' Od O~ m V- lA- CO ~ 00 O
U

W
.J
m

Claims

1. A dielectric coolant for use in electrical equipment, consisting essentially of a mixture of polyol esters chosen from formula X:

wherein:

- ~R1 and R2 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5, wherein R5 is selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C4 to C22; and - ~R3 and R4 are the same or different and are selected from branched or unbranched, saturated or unsaturated alkyl groups with carbon chain lengths of C1 to C5 and alkoxycarbonyl groups of formula CH2OCOR6, wherein R6 is selected from branched or unbranched, saturated or unsaturated alkyl groups with a carbon chain length of C4 to C22 and alkoxycarbonyl groups of formula CR3R4C2H2OCOR5.

said dielectric coolant having a pour point of at most -20°C.

2. The dielectric coolant according to claim 1, having a pour point between about -20°C and about -90°C.

3. The dielectric coolant according to claim 1 or 2, having a pour point between about -40°C and about -80°C.

4. The dielectric coolant according to any one of claims 1 to 3, having a pour point between about -45°C and about -60°C.

5. The dielectric coolant according to any one of claims 1 to 4, wherein the polyol esters of formula X comprise a neopentyl glycol ester (NPG ester).

6. The dielectric coolant of claim 5, wherein R1 and R2 of the NPG ester , both or independently are:

7. The dielectric coolant of claim 6, wherein R1 and R2 of the NPG ester, both or independently, are CH3-(CH2)6- and/or CH3-(CH2)7-CH=CH-(CH2)7-.

8. The dielectric coolant according to claim 6 or 7, wherein R1 and R2 of the NPG ester are CH3-(CH2)6-.

9. The dielectric coolant according to any one of claims 6 to 8, wherein the mixture comprises at least about 5% v/v NPG ester by weight relative to the total weight of the mixture.

10. The dielectric coolant according to any one of claims 6 to 9, wherein the mixture comprises at least about 25% v/v NPG ester by weight relative to the total weight of the mixture.

11. The dielectric coolant according to any one of claims 1 to 10, wherein the polyol esters of formula X comprise a hydroxypivalic acid neopentylglycol ester (HPN ester).

12. The dielectric coolant of claim 5, wherein R1 and R5 of the HPN ester , both or independently are:

13. The dielectric coolant according to claim 12, wherein R1 and R5 of the HPN

ester both or independently are CH3-(CH2)6- and/or CH3-(CH2)7-CH=CH-(CH2)7-

14. The dielectric coolant of claim 12 or 13, wherein R1 and R5 of the HPN
ester are CH3-(CH2)6-.

15. The dielectric coolant according to any one of claims 11 to 14, wherein the mixture comprises at least about 5% v/v HPN ester by weight relative to the total weight of the mixture.

16. The dielectric coolant of claim 15, wherein the mixture comprises at least about 25% v/v HPN ester by weight relative to the total weight of the mixture.

17. The dielectric coolant according to any one of claims 1 to 16, wherein the polyol esters of formula X comprise a trimethylolpropane ester (TMP ester), R3 being an ethyl and R4 being an alkoxycarbonyl group of formula CH2OCOR6.

18. The dielectric coolant according to claim 17, wherein RI, R2 and R6 of the TMP ester both or independently are:

19. The dielectric coolant according to claim 18, wherein R1, R2 and R6 of the TMP ester both or independently are CH3-(CH2)6- and/or CH3-(CH2)7-CH=CH-(CH2)7-.

20. The dielectric coolant of claim 18 or 19, wherein R1, R2 and R6 of the TMP

ester are CH3-(CH2)6-.

21. The dielectric coolant according to any one of claims 17 to 20, wherein the mixture comprises at least about 5% v/v TMP ester by weight relative to the total weight of the mixture.

22. The dielectric coolant of claim 21, wherein the mixture comprises at least about 25% v/v TMP ester by weight relative to the total weight of the mixture.

23. The dielectric coolant according to any one of claims 1 to 22, further comprising at least one additive selected from the group consisting of an antioxidant, a stabilizer and a biocide.

24. The dielectric coolant of claim 23, wherein the antioxidant is selected from the phenolic antioxidants.

25. The dielectric coolant of claim 23 or 24, wherein the antioxidant is selected from the following:

2,6-di-tert-butyl-paracresol (DBPC);
BHA; and BHT, which have the following formulas respectively:

26. The dielectric coolant any one of claims 22 to 25, wherein the stabilizer is selected from epoxide compounds.

27. The dielectric coolant of claim 26, wherein the stabilizer is selected from the epoxide compounds of the following formula:

28. The dielectric coolant according to any one of claims 22 to 27, wherein the biocide is selected from pyrithione derivatives, halogenated pyridine derivatives, triazole derivatives, and tetrahydrofuran derivatives.

29. The dielectric coolant according to any one of claims 1 to 28, wherein the coolant has a flash point of about 200°C or higher.

30. A dielectric coolant for use in electrical equipment consisting of a mixture of polyol esters selected from at least one of the following:

- polyol esters of formula (I):

- polyol esters of formula (II):

polyol esters of formula (III):

wherein R1, R2, R5 and R6 all or independently are CH3-(CH2)6- and/or CH3-(CH2)7-CH=CH-(CH2)7-; and the dielectric coolant has a pour point of at most about -40°C.

31. A dielectric coolant as defined in claim 1 obtained by the process comprising the steps of:

- providing at least a first carboxylic acid and a first polyol to react together to produce a first polyol ester selected from the polyol esters of formula X;

- providing at least a second carboxylic acid and a second polyol to react together to produce a second polyol ester selected from the polyol esters of formula X, said second polyol ester being different from said first polyol ester;
and - mixing the first polyol ester with the second polyol ester to produce the mixture of polyol esters.

32. The dielectric coolant according to claim 31, having a pour point between about -20°C and about -90°C.

33. The dielectric coolant according to claim 31 or 32, having a pour point between about -40°C and about -70°C.

34. The dielectric coolant according to any one of claims 31 to 33, having a pour point between about -45°C and -60°C.

35. The dielectric coolant according to any one of claims 31 to 34, wherein the first polyol ester is selected from NPG esters, HPN esters and TMP esters.

36. The dielectric coolant according to any one of claims 31 to 35, wherein the second polyol ester is selected from NPG esters, HPN esters and TMP esters.

37. The dielectric coolant according to any one of claims 31 to 36, further comprising at least one additive selected from the group consisting of an antioxidant, a stabilizer and a biocide.

38. The dielectric coolant of claim 37, wherein the antioxidant is selected from the phenolic antioxidants.

39. The dielectric coolant of claim 37 or 38, wherein the antioxidant is selected from the following:

2,6-di-tert-butyl-paracresol (DBPC);
BHA; and BHT, which have the following formulas respectively:

40. The dielectric coolant according to any one of claims 37 to 39, wherein the stabilizer is selected from epoxide compounds.

41. The dielectric coolant of claim 40, wherein the stabilizer is selected from the epoxide compounds of the following formula:

42. The dielectric coolant according to any one of claims 37 to 41, wherein the biocide is selected from pyrithione derivatives, halogenated pyridine derivatives, triazole derivatives, and tetrahydrofuran derivatives.

43. The dielectric coolant according to any one of claims 31 to 42, wherein the process comprises the addittional step of adding additives to the mixture after the mixing of the first and second polyol esters.

44. The dielectric coolant according to any one of claims 31 to 43, wherein the process comprises the addittional step of mixing at least a third polyol ester to the mixture, the third polyol ester being selected from the polyol esters of formula X and being different from the first and second polyol esters.