MXPA99004840A - Thixotropic dielectric fluid for capacit - Google Patents

Abstract

The present invention relates to: An energy capacitor comprising: a housing, a capacitor coil, and a dielectric fluid, wherein the capacitor coil and the dielectric fluid are placed in the housing, said capacitor being characterized in that the dielectric fluid exhibits a thixotropic index of at least

Description

THIXOTROPIC DIELECTRIC FLUID FOR CAPACITORS FIELD OF THE INVENTION The present invention relates to a dielectric fluid to be used in electrical devices. More particularly, the present invention relates to a dielectric fluid for electric capacitors.

BACKGROUND OF THE INVENTION Electric capacitors are used in a variety of applications. Capacitors are generally referred to as large capacitors and small capacitors. Large capacitors include high voltage capacitors (greater than 600 volts AC) and low voltage capacitors, induction heated capacitors, and power factor correction capacitors. Small capacitors are typically found in application categories such as starting and running motor capacitors and lighting capacitors. A common configuration for electric capacitors includes a spiral winding configuration with a pair of capacitor roll sections. Each of the capacitor roll sections is typically a polypropylene strip having an aluminum or zinc coating. The appropriate electrical cables are adhered to the coil winding roll using known conventional techniques. The coil winding roll is then placed in a housing and the housing is filled with a capacitor fluid. Capacitor fluids must have a high dielectric constant, maintain a low dissipation factor, and be compatible with the other materials in the capacitor. Capacitor fluids must also withstand fluctuating and elevated temperatures, pressure and adverse voltage conditions to give the capacitor a long operating life. The capacitor fluid must also be relatively viscous to minimize capacitor fluid losses from the capacitor if the capacitor housing develops a leak. In addition, the components used to make the capacitor fluid must be biodegradable to minimize environmental damage if the capacitor fluid leaks out of the capacitor. Cichanowski, E.U.A. Patent No. 4,388,669, which was assigned to the assignee of the present application, describes electrical fluids based on propylene glycol. The dielectric fluids based on propylene glycol exhibit high normalization capacity by virtue of the low carbon to hydrogen ratio and the high weight percentage of molecular oxygen. The dielectric fluids based on propylene glycol also show a low swelling effect on the polypropylene layer and minimal penetration in the capacitor roll. An impediment to dielectric fluids based on propylene glycol is that they can drain from a fractured capacitor because dielectric fluids based on propylene glycol have only moderate viscosities in the range of 500 centipoise. Bentley, E.U.A. Patent No. 4,656,558, describes a dielectric fluid containing polybutylene with an average molecular weight of at least 800. Bentley, E.U.A. Patent No. 4,787,010, describes the formation of a dielectric material from a mixture of polybutene and polyethylene. Bentley '010 indicates that the dielectric material is semi-solid to prevent the dielectric material from escaping from a capacitor in which the dielectric material is used. Using dielectric materials formed from polybutene presents problems in obtaining the desired normalization results since polybutene lacks molecular oxygen.

BRIEF DESCRIPTION OF THE INVENTION A dielectric fluid to be used in a capacitor. The dielectric fluid contains a mixture of vegetable oil, transformer oil and a clay material. The dielectric fluid exhibits a thixotropic index greater than 20. Advantageously, the dielectric fluid exhibits a dielectric strength greater than 32 kilovolts. The dielectric fluid also exhibits minimal degradation of the components when used in the capacitor.

DETAILED DESCRIPTION OF THE INVENTION While the present invention is susceptible to modalities in various forms, it is shown in the drawings and the preferred embodiments herein will be described hereinafter with the understanding that the present description should be considered as an exemplification of the invention and not it is intended to limit the invention to the specific embodiments illustrated. A dielectric fluid produced in accordance with the present invention generally includes a mixture of vegetable oil, transformer oil and a clay material. The present invention also relates to a capacitor including a housing, a capacitor coil, and a dielectric fluid produced in accordance with the present invention. The dielectric fluid exhibits a high thixotropic index that allows the capacitor to be easily filled with the dielectric fluid and the leak potential of the dielectric fluid outside the capacitor is minimized. As used herein, the term thixotropic index is a ratio of the viscosity of dielectric fluid at a low cutting speed to the viscosity of dielectric fluid at a high cutting speed. The dielectric fluid exhibits a high normalization capacity due to the low carbon to hydrogen ratio of the fluid and the high percentage by weight of molecular oxygen. The dielectric fluid of the present invention has a low swelling effect on the polypropylene film and therefore both minimize the capacitance losses due to the interaction with the fluid. In addition, the dielectric fluid has minimal penetration in the capacitor coil due to the high surface tension of the dielectric material. Vegetable oil is a low viscosity hydrocarbon based oil. The vegetable oil is selected with a high stability at pressure and at elevated temperature. The vegetable oil is further selected to be substantially inert with respect to the other components in which the dielectric fluid is used. For example, vegetable oil does not react with the polypropylene film used to make the other portions of the capacitor. The vegetable oil is preferably soybean oil. A preferred vegetable oil can be obtained from Hunt-Wesson, Inc. of Fullerton, California. However, a person skilled in the art will appreciate that it is possible to use one or more different vegetable based oils alone or in conjunction with the soybean oil without departing from the present invention. The concentration of vegetable oil in the mixture is between 40 and 85% and preferably between 40 and 60%. Preferably, the concentration of the vegetable oil in the mixture is approximately 50%. As used herein, all references to percent are in percent by weight unless otherwise indicated. The transformer oil is preferably an insulating mineral oil. The transformer oil is selected with a stability to the pressure and elevated temperature. While the transformer oil is typically more aggressive towards the other components in the capacitor than the vegetable oil, the transformer oil is selected to minimize the degradation of the other components in which the dielectric fluid is used. A lightweight insulating mineral oil can be obtained from Exxon Corporation of Houston, Texas, under the designation mineral oil UNIVOLT 60. A person skilled in the art will appreciate that it is possible to use one or more different mineral oils in conjunction with the insulating mineral oil. light without departing from the present invention. The concentration of the transformer oil in the mixture is between 15 and 60% and desirably between 40 and 50%. Preferably, the concentration of the transformer oil in the mixture is approximately 48.5%. The clay material is incorporated into the mixture to increase the viscosity of the mixture. The clay material is preferably in the form of a finely divided powder. Depending on the selected clay material, it will be necessary to use an activator in conjunction with the clay material to produce a desired level of viscosity increase. A clay material suitable for use in the present This is the montmorillonite clay. A preferred montmorillonite-based clay is sold under the designation CLAYTONE 40 by Southern Clay Products, Inc., of Gonzales, Texas. Other clay material appropriate to be used in the present invention is bentonite clay. A preferred bentonite clay is sold under the designation BENTONE 1000 by RHEOX, Inc., of Highstown, New Jersey. The concentration of the clay material in the mixture is up to 6% > , preferably between 2 and 5%. Preferably, the concentration of the clay material in the mixture is about 3%. Preferably an activator is used together with the montmorillonite clay. The activator increases the dispersion of the clay material in the mixture of vegetable oil and transformer oil. In addition, the activator reduces the sedimentation of clay material in the fluid of the capacitor. It is believed that the activator produces these results by binding to the surface of the clay material. A suitable activator for use with the present invention is the polyethylene-polypropylene copolymer. A preferred polyethylene-polypropylene copolymer is sold under the designation surfactant of block copolymer PLURONIC 31 R1 by the BASF corporation of Parsippany, New Jersey. Another suitable activator for use in the present invention is a tetrafunctional polyoxypropylene / oxyethylene block copolymer, with ethylenediamine base, which is sold under the designation TETRONIC 150R1 by BASF Corporation of Parsippany, New Jersey.

Even another activator to be used in the present invention is propylene carbonate, which is sold by Arco Chemical Company of Newton Square, Pennsylvania, under the designation ARCONATE 1000. The concentration of the activator used to formulate the dielectric fluid of the present invention It is up to 3%. The concentration of the activator is preferably selected based on the concentration of clay material. A ratio of clay material to activator is preferably between 3: 1 and 6: 1. A preferred concentration of the activator in the dielectric fluid is about 0.5%. Preferably the dielectric fluid is prepared by mixing the transformer oil, the clay material and the activator if it is used. To ensure that a homogeneous mixture is produced, mixing is continued for about one hour. The vegetable oil is then added to the mixture and mixing is continued for another additional hour. The dielectric fluid exhibits a trixotropic index of at least 20 and desirably between 20 and 500. Preferably, the trixotropic index is between 50 and 200. As indicated above, the thixotropic index is a ratio of the viscosity of the dielectric fluid at a rate of low cut with respect to the viscosity of the dielectric fluid at a high cutting speed. For demonstration purposes, the viscosity at low cutting speed is preferably obtained by using a needle rotation speed of approximately 0.6 revolutions per minute. Preferably the viscosity is measured in a Brookfield viscometer. The viscosity with speed High cutting preferably is obtained using a needle rotation speed of about 60 revolutions per minute. Typically the viscosity with low cutting speed is at 500 centipoise and preferably at least 10,000 centipoise. Preferably, the viscosity with low cutting speed is between 15,000 centipoise and 75,000 centipoise. The viscosity with high cutting speed is typically less than 500 centipoise. Preferably, the viscosity at high cutting speed is between 100 centipoise and 500 centipoise. Because the dielectric fluid exhibits a high thixotropic index, the dielectric fluid easily flows into the capacitor and fills the voids between the sections of the capacitor coil. The high thixotropic index of the dielectric fluid further reduces the potential for spillage of the dielectric fluid from the capacitor because the dielectric fluid exhibits a high viscosity once it is placed in the capacitor.

EXAMPLE 1 A thixotropic dielectric fluid was prepared from vegetable oil, transformer oil, montmorillonite clay and activator. The transformer oil (UNIVOLT 60, Exxon Corp.) was mixed with the montmorillonite clay (CLAYTONE 40, Southern Clays Products, Inc.) and the activator (PLURONIC P123, BASF Corp.) at respective concentrations of 93%, 6% and 1%. The mixture was liquefied for about one hour to produce a substantially homogeneous mixture. After the mixture was mixed with an amount approximately equal of soybean oil (Hunt-Wesson, Inc.) and then liquified for about one hour to produce a substantially homogeneous dielectric fluid. The high temperature stability of the dielectric fluid was evaluated in conjunction with a capacitor film based on zinc-coated polypropylene. The performance was compared with the performance of dielectric fluid based on propylene glycol and dielectric fluid based on polybutene. The polybutene-based dielectric fluid was sold by the Amoco company of Chicago, Illinois under the designation INDOPOL H-300. The physical characteristics of the dielectric materials are indicated in Table 1. The dielectric strength of the dielectric fluids was analyzed in accordance with ATSM document D877.

TABLE 1 Invention Polypropylene Polybutene Present glycol Dielectric strength, kV Average 30.4 36.5 28.3 Standard deviation 2.7 1.8 0.9 Viscosity, Brookfield, cP 60 RPM 402 500 62,000 6 RPM 2,090 500 62,000 0.6 RPM 16,000 500 62,000 The dielectric fluid of the present invention was vacuum pre-dried to a moisture content of approximately 290 parts per million. The dielectric material based on polypropylene glycol was also pre-dried in vacuum. The porebutene-based dielectric fluid was obtained from a capacitor manufactured by Aerovox, Inc. of New Beford, Massachusetts and had a moisture content of about 1.5 parts per million. Long, approximately 61 cm long strips of zinc-coated polypropylene-based capacitor film (Toray 05AEHA0296 (ZN) 75.0 x 2.57.500) were placed in approximately 59 ml capacity glass containers together with approximately 25 grams of each dielectric fluid. Additional sample vessels were prepared in which the dielectric fluid was only soybean oil (Hunt-Wesson, Inc.) or transformer oil (UNIVOLT 60, Exxon Corp.). The containers were then sealed with lids lined with thin foil. The sample containers were maintained at a temperature of about 117 ° C for about 74 hours. The sample containers were then allowed to cool to room temperature in a desiccator. The capacitor film in each sample container was then evaluated for weight gain or loss and retention of the zinc coating. The results of the evaluation are reported in table 2.

TABLE 2 Weight Weight ppm H20 ppm H20 Initial change (g) final (g) final initial weight Invention 0.286 0.930 426 798 325% present Polypropylene 0.293 0.447 90 1, 270 153% glycol Polybutene 0.291 0.347 > 450 648 119%, Vegetable oil 0.292 0.489 70 702 168% Oil for 0.292 0.344 58 34 118% transformer EXAMPLE 2 Another thixotropic dielectric fluid was prepared from vegetable oil, transformer oil, montmorillonite clay and activator. The transformer oil (UNIVOLT 60, Exxon Corp.) was mixed with the montmorillonite clay (BENTONE 1000, Rheox, Inc.) and the activator (TETRONIC 150R1, BASF Corp.) in the following concentrations of 92%, 6% and 2%. % respectively. The mixture was liquefied for about one hour to produce a substantially homogeneous mixture. The mixture was then mixed with an approximately equal amount of soybean oil (Hunt-Wesson, Inc.) and then liquefied for about one hour to produce a substantially homogeneous dielectric fluid. The thixotropic characteristics of the dielectric fluid were analyzed using a Brookfield viscometer with a # 2 needle. The viscosity at 60, 6 and 0.6 revolutions per minute is 115, 325 and 1, 625 centipoise, respectively. The average dielectric strength of the dielectric fluid was 32,828 kilovolts with a standard deviation of 1,819 kilovolts.

EXAMPLE 3 Another thixotropic dielectric fluid was prepared from vegetable oil, transformer oil, montmorillonite clay and activator. The transformer oil (UNIVOLT 60, Exxon Corp.) was mixed with the montmorillonite clay (BENTONE 1000, Rheox, Inc.) and the propylene carbonate-based activator (ARCONATE 1000, Arco Chemical Co.) in the following concentrations 92%, 6% and 2% respectively. The mixture was liquefied for about one hour to produce a substantially homogeneous mixture. The mixture was then mixed with an approximately equal amount of soybean oil (Hunt-Wesson, Inc.) and then liquified for about one hour to produce a substantially homogeneous dielectric fluid.

The thixotropic characteristics of the dielectric fluid were analyzed using a Brookfield viscometer with a # 4 needle for the test at 60 revolutions per minute and with a # 2 needle for the tests at 6 and 0.6 revolutions per minute. The viscosity at 60, 6 and 0.6 revolutions per minute is 860, 3,885, and 31, 800 centipoise, respectively. The average dielectric strength of the dielectric fluid was 5,551 kilovolts with a standard deviation of 1,030 kilovolts.

EXAMPLE 4 The performance test for the dielectric fluid performance indicated in Example 1 and reported in Table 3 was repeated using a different capacitor film based on zinc-coated polypropylene (Bolmet ZMPPHE 2.5M Lot M3797-117 # 4). The results of the tests are reported in table 3.

TABLE 3 Weight Weight ppm H20 ppm H20 Initial change (g) final (g) final initials weight% Invention 0.286 0.974 426 903 263% present Polypropylene 0.293 0.519 90 1, 283 142% glycol TABLE 3 (CONTINUED) Polybutene 0.291 0.480 > 450 2,000 128% Vegetable oil 0.292 0.875 70 634 239% Oil for 0.292 0.405 58 29 113% transformer From the foregoing it will be noted that numerous modifications and variations can be made without departing from the essence and field of the novel concepts of the present invention. It will be understood that no limitations are intended with respect to the specific modalities illustrated or to be inferred. The description is intended to cover, through the appended claims, all modifications that fall within the scope of the claims.

Claims (24)

1. NOVELTY OF THE INVENTION CLAIMS 1. - A dielectric fluid composed of a mixture of vegetable oil, transformer oil and a clay material.
2. 2. The dielectric material according to claim 1, further characterized in that the dielectric fluid exhibits a thixotropic index of at least 20.
3. 3. The dielectric fluid according to claim 1, further characterized in that the dielectric fluid has a resistance dielectric of at least about 32 kilovolts.
4. 4. The dielectric fluid according to claim 1, further characterized in that the vegetable oil has a concentration between 40 and 85% by weight.
5. 5. The dielectric fluid according to claim 1, further characterized in that the transformer oil has a concentration of between 15 and 60% by weight.
6. 6. The dielectric fluid according to claim 1, further characterized in that the clay material has a concentration of up to 6% by weight. 7. - The dielectric fluid according to claim 1, further characterized in that the clay material is montmorillonite clay, bentonite clay or combinations thereof. 8. The dielectric fluid according to claim 1, further characterized in that it includes an activator. 9. The dielectric fluid according to claim 8, further characterized in that the activator is a polyethylene-polypropylene copolymer, a tetrafunctional polyoxypropylene / oxyethylene block copolymer, propylene carbonate or combinations thereof. 10. The dielectric fluid according to claim 8, further characterized in that the activator has a concentration of up to 3% by weight. 11. The dielectric fluid according to claim 8, further characterized in that the ratio of clay material to activator is between 6: 1 and 3: 1. 12. A dielectric fluid to be used in an energy capacitor, the dielectric fluid of vegetable oil having a concentration of between 40 and 85% by weight being composed; Transformer oil having a concentration of between 15 and 60% by weight; and a clay material having a concentration of up to 6% by weight, further characterized in that the dielectric fluid exhibits a thixotropic index of at least 20. 13. - The dielectric fluid according to claim 12, further characterized in that the dielectric fluid has a dielectric strength of at least about 32 kilovolts. 14. The dielectric fluid according to claim 12, and further characterized in that it includes an activator. 15. The dielectric fluid according to claim 14, further characterized in that the activator is polypropylene-polyethylene copolymer, tetrafunctional copolymer of polyoxypropylene / oxyethylene block, propylene carbonate or combinations thereof. 16. The dielectric fluid according to claim 14, further characterized in that the activator has a concentration of up to 3% by weight. 17. The dielectric fluid according to claim 14, further characterized in that the ratio of clay material to activator is between 6: 1 and 3: 1. 18.- An energy capacitor composed of: a housing; a capacitor roll; and a dielectric fluid, characterized in that the capacitor coil and the dielectric fluid are placed in the housing and further characterized in that the dielectric fluid exhibits a thixotropic index of at least 20. The power capacitor of claim 18, further characterized because the dielectric fluid has a dielectric strength of at least 32 kilovolts. 20. A method for producing a dielectric fluid, the method comprising: mixing transformer oil and a clay material to produce an intermediate mixture; and mix vegetable oil with the intermediate mixture to produce the dielectric fluid. 21. The method according to claim 20, further characterized in that the dielectric fluid exhibits a thixotropic index of at least 20. 22. The method according to claim 20, further characterized in that the dielectric fluid exhibits a dielectric strength of at least about 32 kilovolts. 23. The method according to claim 20, and further characterized in that it comprises mixing an activator with the intermediate mixture. 24. The method according to claim 21, further characterized in that a ratio of the concentration of clay material to the concentration of activator is between 3: 1 and 6: 1.