Very low pour point dielectric

Abstract

 A substantially biodegradable dielectric solution comprising a low wax content napthenic crude oil, a synthetic oxidation inhibitor and a flow modifier.

Description

FIELD OF THE INVENTION

[0001] This invention relates to a novel biodegradable dielectric fluid. More particularly, this invention relates to a biodegradable dielectric fluid that is refined from selected low wax content base stocks and which has a very low pour point.

BACKGROUND OF THE INVENTION

[0002] Dielectric liquids may be used in transformers as insulators and as cooling agents. Their ability to insulate is dependent upon certain properties such as dielectric strength, whereas their ability to effectively function as a cooling agent is premised, in part, upon a low viscosity of the fluid which ensures sufficient dissipation of the heat generated during the operation of the transformer.

[0003] The viscosity of a dielectric fluid is directly affected by low temperatures in that wax crystals form and interfere with essential heat transfer characteristics. For example, conventional transformer oil may be refined [by 1989 industry standard practice] from crude oils, such as napthenic and paraffinic oils, and generally exhibits a pour point of between 55°C and -40 C. However, at low temperatures, wax crystals form in the oil and prevent the low viscosities which are essential for the oils to function as heat transfer agents in a transformer. Specifically, low temperatures create poor flow properties in the transformer oil and prevent the proper extraction and discharge of heat from the core and coil assembly of transformers. This results in localized overheating and promotes degradation of the cellulose insulation and the oil itself. Consequently, there is premature dielectric breakdown and shortened transformer life.

[0004] The aforementioned wax crystals are unfortunately present in nearly all crude oils in refined transformer oil. Presently, there exist numerous techniques for reducing the wax content in these oils including, but not limited to, precipitation followed by centrifugation and screening or by selective solvent extraction. However, these processes are costly to perform and are not done on oil fractions that become transformer oil. Other methods for reducing the wax content in oil include exposing the fluid to ultra-violet light to reduce its pour point.

[0005] Yet another method of reducing the wax content in oil is disclosed in Mills et al. patent No. 3,912,617 in which a refrigerator oil comprises a blend of a hydrorefined napthenic oil component and wax-free paraffinic component. In Mills, the paraffinic component may be a wax-free hydrogenated polyolefin oil or a high viscosity index, hydrocracked lube or a mixture of such components. While the Mills oil composition includes a waxfree oil component, it is directed towards a high temperature compressor and does not resolve the problems which occur at lower temperatures and high viscosities.

[0006] Thus, this and other attempts have been made in the field to develop a process or a composition that results in a dielectric fluid having reduced wax content and greater viscosities at lower temperatures. However, these attempts are either extremely costly and only partially effective or not practical to perform at all. Moreover, these attempts to extract wax from the dielectric fluid have other adverse effects that include residual extraction solvents, such as furfural, which remain in the oil and thereby impair its electrical characteristics. Hence, the inventive wax-free oil solution described herein resolves the aforementioned problems by resulting in a dielectric fluid having improved heat transfer characteristics at temperatures well below those obtained with conventional transformer oil.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to provide a dielectric fluid that exhibits excellent electrical characteristics and low temperature flow properties.

[0008] Another object of the present invention is to provide a dielectric fluid that eliminates the high viscosity and solidification which is associated with conventional transformer oil in areas where the ambient temperature is very low.

[0009] Another object of the present invention is to provide a dielectric fluid that exhibits excellent heat transfer characteristics.

[0010] Another object of the present invention is to provide a dielectric fluid without residual extraction solvents and which has excellent electrical characteristics.

[0011] Another object of the present invention is to provide a dielectric fluid that eliminates overheating of the oil and degradation of the cellulose insulation.

[0012] Another object of the present invention is to provide a dielectric fluid that contains minimal wax content.

[0013] Another object of the present invention is to provide a dielectric fluid that prevents premature dielectric breakdown and shortened transformer life.

[0014] Another object of the present invention is to provide a dielectric fluid that utilizes oils having very low pour points.

[0015] Another object of the present invention is to provide a dielectric fluid that is compatible with the design and materials of the construction of transformers which are currently manufactured.

[0016] A further object of the present invention is to provide a dielectric fluid that is cost effective and simple to manufacture.

[0017] The objectives and advantages of the present invention are achieved by providing a transformer oil that is made, in part, from selected crude oils that naturally exhibit very low pour points and which contain little wax. These oils are normally used as lubricants in refrigeration compressors and in cold climates. These substantially wax-free oils are refined and processed into a dielectric fluid by conventional industry pratice.

[0018] Specifically, the inventive transformer oil comprises, in part, a petroleum fluid which is refined from selected low wax content base stocks and which may be used with napthenic or paraffinic transformer dielectric oils. The transformer oil further includes a synthetic oxidation inhibitor and a low temperature flow modifier.

[0019] In the preferred embodiment, the crude oils from which the petroleum stocks are refined are known as wax-free napthenic crude oils and contain very little or no wax. These crude oils may originate from many areas, including the fields in and around the Cottage Grove field in Oil City, Louisiana. These napthenic crudes are identifiable by the characteristics which are described below.

[0020] The napthenic crude oils exhibit a correlation between their boiling point and density. For example, the fraction of crude oil that boils at temperatures between 2500C and 2700C (at standard pressure) has a specific gravity above 0.860. Likewise, the oil fraction that boils at temperatures between 2750C and 300 0C (at standard pressure) exhibits a specific gravity above 0.934.

[0021] The napthenic crude oils contain fractions that exhibit a viscosity-gravity constant (VGC) equal to or above 0.876, where VGC is defined as:

VGC

= (10 d - 1.0752 log (V - 38))/(10 - log (V - 38))

where d

= the specific gravity of the oil 60F/60F

v

= Saybolt viscosty at 38.40C (1000F)

log

= log base 10.

[0022] The napthenic crude oils contain fractions that exhibit a Universal Oil Product (UOP) Characterization Factor or Watson K Factor between 10.5 and 12.5. The UOP Characterization Factor, K, is defined as:

where: Tb = the average boiling point expressed in absolute temperature Rankine d = the specific gravity of the oil 60F /60F The inventive transformer oil, in the preferred embodiment, has a pour point or melting point of less than about -600C as compared to an average pour point of approximately -480C for conventional transformer oil. The preferred flash point of the transformer oil is a minimum of about 1450C, as measured by ASTM standard method D92.

[0023] The ASTM method involves filling a test cup to a specified level with a sample of the transformer oil. The temperature of the sample is increased rapidly at first and then at a slow constant rate as the flash point is approached. At specified intervals a small test flame is passed across the cup. The lowest temperature at which application of the test flame causes the vapors above the surface of the liquid to ignite is the flash point.

[0024] The preferred viscosity of the transformer oil is a maximum of about 12 cSt. at 400C which is equal to approximately 66 SUS.

[0025] A variety of synthetic oxidation inhibitors may be used with the inventive transformer oil. In the preferred embodiment, the oil contains less than about 1.5% by weight, preferably about 0.08% to about 0.30%, of a hindered phenolic compound, such as 2,6 di-tert butyl phenol or 2,6 di-tert butylated paracresol. Alternatively, any one of a number of related compounds may be used which have the ability to increase the oxidation stability of petroleum oils. Examples of oxidation inhibitors that may be used herein include Ethyl 701 and Ethyl 702 which are manufactured by the Ethyl Chemical Corporation, Baton Rouge, Louisiana.

[0026] Likewise, a variety of flow modifiers may be used with the inventive transformer oil. In the preferred embodiment, the oil contains less than about 1.0% by weight, preferably about 0.3% to about 0.7%, of an alkylated polystyrene type of flow modifier, such as OA-100B, manufactured and sold by Argus Chemical Corporation. Argus also makes variations of OA-100B, such as OA101 and OA-100A. Other companies make similar compounds that may be used as flow modifiers which include a dialkyl fumarate/vinyl acetate copolymer which is known as Paraflow 387 and is manufactured by Exxon Chemical Company, and an alkyl ester copolymer that is named Lubrizol 6662 and is manufactured by Lubrizol Petroleum Chemical Company.

[0027] The present invention contemplates preparing a biodegradable dielectric fluid comprising a petroleum fluid which is refined from wax-free napthenic crude oils and combined with a synthetic oxidation inhibitor and a low temperature flow modifier. The fluid may be used with conventional napthenic or paraffinic transformer dielectric oils.

[0028] The following example illustrates the inventive composition.

EXAMPLE I

[0029] A biodegradable dielectric fluid was prepared from low wax content base stocks.

[0030] Approximately ten (10) liters of substantially wax free napthenic crude oil, such as the oil found in Northwest Louisiana, measuring ten (10) centistokes at 400C is refined by conventional industry practice which involves two stages of high vacuum distillation. This oil should have a molecular weight range and molecular structure such that the material possesses the following characteristics. First, the viscosity of the oil should be between 9.0 and 12.0 cSt. at 400C, and include a viscosity index of less than 15. Second, the oil should have a pour point of below -550C.

[0031] A predetermined amount of a hindered phenolic compound, specifically, 2,6 di-tert butyl phenol is then added to the refined crude oil. The amount of phenolic compound that is added to the oil is consistent with the required level for oxidation stability. The hindered phenol oxidation inhibitor that is used herein is Ethyl 701 which is manufactured by Ethyl Chemical Corporation.

[0032] Finally, a predetermined amount of a low temperature flow modifier, which in this case is An alkylated polystyrene, is also added to the wax-free oil composition. The amount of flow modifier that is added to the oil is consistent with the required flow properties of the fluid. The flow modified that is used herein is OA-100B and is manufactured by Argus Chemical Corporation.

[0033] The oil composition is mixed. The result is a biodegradable dielectric fluid which has a very low pour point.

[0034] It will now be appreciated that the present invention provides for a novel dielectric fluid that is refined from low wax content base stocks and which has a very low pour point.

[0035] The foregoing is for purpose of illustration rather than limitation of the scope of protection accorded this invention. The latter is to be measured by the following claims, which should be interpreted as broadly as the invention permits.

Claims

1. A substantially biodegradable dielectric solution comprising a low wax content naphthenic crude oil, a synthetic oxidation inhibitor, and a flow modifier.

2. A solution according to claim 1, wherein the naphthenic crude oil is substantially wax-free.

3. A solution according to claim 1 or claim 2, wherein the naphthenic crude oil has an index of less than 15, a viscosity between 9.0 and 12.0 CSt at 400°C and a pour point of less than -550°C.

4. A solution according to any preceding claim, which has a pour point of less than -600°C and a flash point greater than 1450°C.

5. A solution according to any preceding claim, which has a viscosity of no more than 12 CSt at 400°C.

6. A solution according to any preceding claim, wherein the naphthenic crude oil boils at between 2500 and 3000°C at standard pressure.

7. A solution according to any preceding claim, wherein the naphthenic crude oil has a specific gravity of at least 0.860.

8. A solution according to any preceding claim, wherein the naphthenic crude oil has a Universal Oil Products Characterisation Factor of between 10.5 and 12.5.

9. A solution according to any preceding claim, wherein the synthetic oxidation inhibitor comprises a hindered phenolic compound.

10. A solution according to claim 9, which comprises up to 1.5% by weight of the hindered phenolic compound.

11. A solution according to claim 9 or claim 10, wherein the hindered phenolic compound is 2,6-di(tert-butyl)phenol and/or 2,6-di(tert-butyl)-p-cresol.

12. A solution according to any preceding claim, wherein the flow modifier is an alkylated polystyrene.

13. A solution according to claim 12, which comprises up to 1.0% by weight of the alkylated polystyrene.