CA1079953A - Dielectric liquids comprising phthalate esters and halogen compounds - Google Patents
Dielectric liquids comprising phthalate esters and halogen compoundsInfo
- Publication number
- CA1079953A CA1079953A CA246,692A CA246692A CA1079953A CA 1079953 A CA1079953 A CA 1079953A CA 246692 A CA246692 A CA 246692A CA 1079953 A CA1079953 A CA 1079953A
- Authority
- CA
- Canada
- Prior art keywords
- impregnant
- ester
- dielectric
- capacitor
- additive
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Landscapes
- Organic Insulating Materials (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A dielectric liquid composition consists of a pre-determined mixture of an aromatic ester and certain chlorinated compounds. This mixture is used to impregnate an electrical capacitor, and provides an improved corona extinction voltage characteristic, an increase in the dielectric constant and decreased flammability. One working example utilizes a mixture of a liquid phthalate ester such as dioctyl phthalate, pr 2-ethyl-hexyl phthalate (DOP), and trichlorobenzene (TCB) as the impregnant for an electrical capacitor.
A dielectric liquid composition consists of a pre-determined mixture of an aromatic ester and certain chlorinated compounds. This mixture is used to impregnate an electrical capacitor, and provides an improved corona extinction voltage characteristic, an increase in the dielectric constant and decreased flammability. One working example utilizes a mixture of a liquid phthalate ester such as dioctyl phthalate, pr 2-ethyl-hexyl phthalate (DOP), and trichlorobenzene (TCB) as the impregnant for an electrical capacitor.
Description
This invention relates to a dielectric liquid composition suitable for use as an insulating medium in electrical devices, and more particularly, to an improved halogen containing ester type dielectric liquid composition particularly adaptable for use as a dielectric liquid impregnant for electrical capacitors.
U.S. Patent 3,363,156 dated January 9, 1968, Cox, discloses and claims various types of dielectric liquid impregnated electrical capacitors. The dielectric liquid composition described in Cox is a chlorinated hydrocarbon, more particularly a halogenated aromatic hydrocarbon and specifically a chlorinated diphenyl. The chlorinated diphenyl impregnants for electrical capacitors are commercially available under the trademark Aroclor, a trademark of the Monsanto Company, and a specific example being Aroclor 1242 or Aroclor 1016. The chlorinated diphenyls, referred to as PCs's,have recently been associated with ecological problems and their continued use in applications other than electrical has been limited.
Accordingly there is a continuing search for new and improved impregnants in the eIectrical capacitor field.
In U.S. Patent 3,754,173 dated August 21, 1973 Eustance, there is disclosed an epoxide stabilized liquid aromatic ester impregnant which does not have many of the PCB ecological disadvantages. Of the problem associated with the aromatic ester kind of impregnant is the fact that the flammability point, i.e. the level at which the liquid will sustain combustion, is relatively low and many of these ester materials are classified as flammable under some failure conditions of an electrical capacitor.
It has also been disclosed that the aromatic esters in accordance with the Eustance patent will provide very high corona start voltages ICSV) and therefore favourably compare with the Aroclor liquid for capacitors, However, a further important voltage level or criteria of a cap-acitor i~ referred to as the corona extinction voltage (CEV) of a capacitor, The corona start voltage and the corona extinction voltage are the voltages at which de-leterious corona discharge may commence in a capacitor and be extinguished respectively during a rising and de-creasing voltage level across the capacitor, In highvoltage power factor corxection capacitors, the corona ex-tinction voltage for the ester impregnants has been found to be significantly less than the corona extinction voltage for the Aroclor type impregnant , In addition to the foregoing problems~ many of the ester impregnants, particularly those of higher molecular weights, are otherwise desirable for capacitor impregnants but have an increased viscosity which creates a problem in essentially completely impregnating a capacitor in ac-cordance with the teachings of the above noted Cox patent, Additive liquids used to minimize the foregoing problems have been found in many cases to have adverse ef~ects on other characteristics of the impregnant such as lowering the dielectric constant (DK).
Accordingly it i~ an object of this invention to provide an improved ester dielectric liquid composition, It i8 a further object of this invention to provide an improved capacitor dielectric liquid ester impregnant having increased corona extinction voltage characterstics, It is another object of this invention to provide an improved capacitor dielectric liquid ester impregnant having improved dielectric constant characteristics, It is a specific object of this invention to provide ~079953 an electrical capacitor which ha~ been impregnant with a dielectric liquid composition compri~ing a mixture of a dielectric liquid aromatic ester, a chlorine containing compound, and an epoxide stabilizer, This invention will be better understtod when taken in connection with the following descriptions and the drawings in which--Fig, 1 is a curve showing the increase in the corona extinction voltage ~CEV) of capacitors containing DOP-TCB
mixtures, Fig, 2 is a curve showing the increase in dielectric constant of DOP-TCB mixtures, Fig, 3 is a curve showing the viscosity of DOP-TCB
mixture~, Fig. 4 is an exemplary roll section comprising al-ternate electrode fail strips and dielectric strips, Fig, 5 is a cross sectional view of a part of a capacitor roll section utilizing solely synthetic resin film as a dielectric, Fig, 6 is a view of a part of a capacitor roll section utilizing mixed synthetic resin film and paper as a die-lectric, Fig, 7 is a cross sectional view of a part of a capacitor roll section utilizing a syn~hetic resin film in a different dielectric paper and film arrangement in a capacitor, Fig, 8 is a complete capacitor in the form of sealed can containing the roll section of Fig, 5, Fig, 9 is a greatly reduced drawing of an exemplary power capacitor utilizing multiple rolls, a structure common to the large size power factor correction, induction heating, and high frequency capacitorC, 107~ 3 36-CA-3289 It has been discovered that the aromatic ester im-pregnants i~ accordance with the Eustance patent may be qreatly improved when a chlorine containing compound is added to the ester. The chlorine contribute-~ improved anti_flammable characteristics to the impreynant while at the same time advantageously raising its corona ex-tinction level, increasing its dielectric constant, and reducing its viscosity~
In high voltage capacitor applications the aromatic esters evidence a very high corona start voltage, a critical requirement of this impregnant. It is expected that capacitors~ during their operating life, are subjected to certain transient overvoltage conditions which could cause a corona discharge within the dielectric. However, with an impregnant such as a chlorinated diphenyl, this corona i8 extinguished as soon as the transient voltage has passed or as soon as the transient has reduced to a certain degree In an effective impregnant, such as chlorinated diphenyl, the voltage level at which the corona is ex-tinguished (CEV) i8 very close to that level at which corona originates This is known as a high corona ex-tinction voltage and a high corona ectinction voltage is required in order to prevent the lingering corona from ser~ou~ly injuring the capacitor~ but more importantly, to extinguish the corona when the voltage drops to the ordinary operating voltage of the capacitor When 1, 3, 4 trichlorobenzene ~TCB) is added to dioctyl phthalate (2_ethyl hexyl) impregnant (DOP), there is an increase in the corona extinction value of capacitors containing mixtures More specifically, it was found that the addition of at least about 10% of chlorine, by weight, to DOP, where the chlorine is an admixture and is not ir _ 4 --1073953 36-CA_328g combined with the DOP, is a ~ufficient amount to provide a significantly improved corona extinction voltage of the mixture which is close to that of Aroclor 1016: one of the more common chlorinated diphenyl impregnant~ utilized in the capacitor industry As low a8 an added amount of 5% chlorine by weight still results in a measured improve-ment to the corona extinction voltage of capacitors con-taining the phthalate.
Representative mixtures of this invention were utilized as impregnants in capacitor embodiments and their corona extinction values (CEV) were obtained as more cleaxly shown in Fig. 1. Referring to Fig.l, the curve of the CEV ver~us the weight percent of chlorine in a DOP/TCB
blend is illustrated as a curve which rises from about 1500 volts to about 2350 volts to about 2350 volts at the highest point. It is to be noted that the increase in CEV is most pronounced at about 8% through 12% with very little increase after about 20% of TCB has been added to the mixture The 10% chlorinate by weight provides a sîgnificant increase in the overall CEV, which approaches that of the Aroclor 1016. TCB which is a chlorinated fluid is electronegative and this characteristic may be of sign-i~icant assistance in the quenching of a corona arc. The character of the corona pulses noted during these tests is also improved by the addition of TCB In DOP capacitors, the corona pulses break in every suddenly at high level (ab. 100 picocoulombs). In capacitors with the DOP/TCB
blend, on the other hand, the corona breaks in at a low level which increases in a controlled fashion as the voltage is increased. As the voltage is decreased, the level of the DOP/TCB pulses decrease rapidly and extin-guish about the same point at which it appeared, while on iO79953 36--CA_3289 the other hand, the DOP pulses per~ist at a high level up to the extinguishing voltage. Corona extinction voltage i8 measured much in the same way and by the same equipment which is used to measure corona start voltage. It usually involves an amplified testing circuit which di~play~ the presence of electrical discharges within the insulation system on a cathode ray oscilloscope. The corona measuring equipment utilized in the invention has a sensitivity of one (1 0) picocoulomb.
DOP_TCB mixtures were also studied to determine their dielectric constants Dielectric constant (DK) is the ratio of the capacitance of a given structuxe with the dielectric liquid as compared to the capacitance of that structure with air as the dielectric medium The value obtained i~ qualified with reference to the conditions of measurement (voltage, frequency, and temperature). The dielectric constant ~DK) of the preferred aromatic esters, the Aroclors, and TCB
are desirably high so that the expected result of mixing these liquids would be a somewhat average DK Surprisingly enough, it was discovered that the mixture of this invention exhibited a dielectric constant which was significantly greater of the dielectric constant of either DOP alone or the TCB alone The dielectric constant of a mixture is usually calculated by the use of a linear dielectric constant mixing rule or a logarithmic dielectric constant mixing rule 1,2. However, the behavior of the mixtures of the present invention appears to be an exception to both rules This synergism which is noted to be most significant for the dioctyl phthalate and trichlorobenzene mixtures is not readily explained by previous experiences with dielectric-mixture~. The synergism of an increased dielectric constant ~'or~ ne~C
A above the expected mixing rule value w~s -~ef~hi~Kff~by testing 36-CA_3289 of DOP mixtures with other chlorinated compunds.
The linear dielectric constant mixing rule as defined in both cited reference~ by apprc>priate equations i8 in-tended to mean the straight line function, i.e. the dash A line of Fig 2 As hereinafter ~ to in the claims, Dielectric Constant Mixing Rule refers to the rules noted in the above cited references and in thi~ specification.
A DK higher than that calculated by the Dielectric Constant Mixing Rule is a DK whose value is above that indicated by the dash line regardless of the slope or initial height of the dash line. ~he basis for the straight line function is derived from the equations given and whether or not the line is precisely str3ight is of no importance. This in-vention describes a DK greater than that predicted by the noted equations.
Several mixtures were studied under conditions of 25C
f and at a frequency of 100 Hertz, and their dielectric con-stants are more clearly shown in Fig. 2 Re~erring now to Fig. 2, there is disclosed a volume percent TCB in DOP/TCB
mixtures as compared with the DK value of the mixtures It can be seen that at about 20% of TCB in the mixture a maximum DK of about 5.5 is reached This DK does not ~eem to be affected by further increases in the amount of TCB.
The DK expected, from linear or logarithmic mixing rules for these mixtures, would be about 5.15 as indicated by the dash line The high DK figure of 5.5 is compared to the DK of about 5.15 to 5 2 for either of the TCB or the DOP material alone.
The curve of Fig. 2 shows a DK higher than either com-ponent. However, when one of the components has an initial DK which is much lower than the other component, the DK
curve will be higher than the noted mixing rules predict 10799S3 ~ 6 -CA 3 2 8 9 although the mixture will not exhibit a DK higher than either component.
Variou~ mixtures of DOP ~ TCB were made up and utilized in representative type capacitors. It wa8 noted during the course of this work that the TCB reduced the viscosity of the DOP and thus facilitated impregnation particularly in all film capacitors Accordingly, where many of the estèrs are more undesirable because of their high viscosity, the addition of TCB may reduce that viscosity to more desirable and appropriate levels One example of the viscosity lowering ability of TCB
in DOP is shown in FIG. 3. In Fig. 3, the curves show that the viscosity of the mixture decreases quite rapidly with increasing amount~ of TCB at room temperature conditions or 2sC. At 70 C it is noted that the viscosity decreases less rapidly with increasing amounts of TCB since at this temperature the viscosity of the DOP is already low. By comparison, the viscosity of the more common chlorinated diphenyl impregnant, Aroclor 1016, is about 45 centistokes at 25C.
Mixtures of DOP and TCB were also tested for their flame retardant characteristics These tests show that the ability of the DOP to sustain combustion i~ lessened by the addition of the TCB, and, therefore, the TCB addition is quite ~avourable for capacitor operation where a high degree of inflammability is desired. For example, it was found that DOP has a fire point, the point at which DOP will sustain flame at about 240 C. However a 70% DOP + 30% TCB
mixture had no fire point up to about 265 C
Preferred capacitor structures embodying this invention are illustrated in Pigs. 4 through 9. Referring now to Fig 4 here is disclosed a typical roll section 10 comprising alternate electrode foils 11 and 12 and dielectric strips 13 and 14. Strips 13 and 14 may be single strip~ of paper or a synthetic resin, plural strip of the~e materials, or composite strips. The electrode foilR 11 and 12 may also be formed as metallized coatings on the strips 13 and 14, or on separate and additional stripc of various dielectric material~. Suitable electrical connectors in the form of tabs 15 and 16 are utilized to connect the electrode 11 and 12 to appropriate capacitor terminals.
The dielectxic structures for roll section 10 may include any of the composites of Fig. 5, 6 and 7 as illustrated Referring to Fig. 5, there is illustrated what may be referred to as an all ~ilm roll structure 17 In this structure, only a synthetic resin film such as polypropylene film is used as the 901e dielectric between electrodes 11 and 12. A typical all polypropylene film capacitor will utilize one or more polypropylene film strips 18 between electrodes 11 and 12.
Referring to Fig 6, there is illustrated one form of a mixed dielectric roll structure 19 or a capacitor using dissimilar dielectrics, such as a synthetic resin film and a paper strip, although different resin films may be employed as a composite, such as polypropylene film and a polyester film. As illustrated in Pig. 6, roll structure 19 has been described as a semi-~andwich construction and uses one or more paper strips 20 with one or more synthetic resin film ~trips 18.
Referring to Fig. 7, there is illustrated a further mixed dielectric roll structure 21. Structure 21 has been referred to as an inverted sandwich -~tructure whose primary characteri~tic is that a synthetic resin film is used ad-jacent each foil 11 and 12 and there is one or more in-_ g _ termediate dissimilar strips 20 l~hich are usually employed as a combined dlelectric and wiclcing #trip. In a preferred form of the invention as illu~trated, the intermediate strip i8 a single paper strip 20 and the synthetic resin strips are single polypropylene strips 18, One or more of the roll sections 10 of Fig. 1, in-A cluding the ~a~ structure of one or more structures ofFigs, 5,6, and 7, are assembled in cans or casings, im-pregnated with the impregnant of this invention, and then sealed. Typical capacitor constructions are illustrated in Figs, 8 and 9, Referring to Fig. 8, there is illustrated what may be referred to as a motor start or motor run capacitor 22, Such a capacitor usually includes a single roll section 10 of Fig. 4, which is inserted into a metal casing 23 and Realed therein. The tabs 15 and 16 of roll section 10 are connected to external capacitor terminals 24 and 25, The metal casing 23 is filled with the impregnant of this invention through fill hole means 26 which is illustrated as being solder sealed, Plural and larger roll sections are u~ed in capacitors referred to as power capacitors or power factor correction capacitors, One such typical capacitor is illustrated in Fig, 9. Refexring now to Fig, 9, there i~ illustrated a high voltage power factor correction capacitor 27, Capacitor 27 usually includes a large rectangular steel cas-ing 28 which may be form ~ to 1 meter in the longer dimen-sion shown, Casing 28 includes therein a row of longer roll sections 10 whose connectors 15 and 16 are suitably connected to external capacitor terminals 29 and 30, Casing 28 may be filled with the impregnant of this inven-tion in a manner similar to that as described for Fig, 9, .
A number of capacitors were made up for testing purposes of this invention. A typical te~t capacitor in-volved the r~ll section of Fig. 4 assembled a~ the motor run capacitor of Fig. 8 In a f:irst test capacitor, the strips 13 and 14 were composite strips comprising a pair of polypropylene strips 18 and intermediate paper strip 20 (Fig. 7). In the examples as made up in this application, the polypropylene comprised two strips 18 of about 0.47 mil. (12 mm) thick polypropylene and one sheet of 0 50 mil. (1.27 mm) paper. The overall capacitor height was about 4 in. (10.16 cm) In a second capacitor structure made up for the purposes of this invention~ the dielectric strips 13 and 14 each comprised a pair of paper strips 20, one of which was about 0.60 mil. (1 55 mm) thick paper and the other which was 0.65 mil. (1.68 mm) thick paper. These capacitors were impregnated in accordance with the general impregnation cycle as set out in the Cox U S. patent No.
3,363,156 dated January 9, 1968 which comprised drying the capacitors in a vacuum oven at temperatures up to and including 105 C and then cooling the capacitors to less than 100 so that they may be filled with a dielectric liquid at about 65 to 75C. Thereafter, the capacitors were sealed and permitted to heat soak overnight, i.e., about 14 to 16 hours, in a forced circulation oven at about 95 C
Testing of these capacitors of the first described kind in-dicated the following corona start voltage levels as æet out in Table I.
Corona Start Voltage CSV (avg of 5) Minimum 95% DOP/5 TCB 3020 2900 9~% DOP/10 TCB 2912 2850 80% DOP/20 TCB 2870 2750 60% DOP/40 TCB 2950 2850 ~79gS3 Ten of the capacitors from T~ble I which included the 60% DOP/ 40% TCB blend were put on life test at 2650 V~C
(volts, alternating current) at 70 C There were no failures after 400 hours ac compared wqth three ~ailures in DOP capacitors without TCB, and zero failures in cap-acitors impregnated with Aroclor 1016 It has been found in the practices of thi-q invention that some form of electrical stabilizer i~ de~irable in the impregnant mixture Recently, the epoxide compounds have been found to be particularly sffective with chlorinated diphenyl compounds and also with ester compounds although the reactions of neither addition are well understood. The epoxides are the preferred stabilizer~ for the present in-vention and they perform a stabilizing function in the presence of both an ester liquid and a chlorinated com-pound without any adverse effects from the component mixtures. The liquid impregnant from the capacitors of Table I above included about 0.3 percent of l_epoxyethyl_3, 4-epoxycyclohexane.
While the power factor stabilizer is preferably an epoxide material, other such stabilizers such as the anth-roquinones may be utilized but with apparently less effect than the epoxide as disclosed in the Eustance patent. On the other hand, some of the anthrcquinones~ such as the monochloroanthroquinones and the dichloroanthroquinones, include a significant amount of the chlorine which appears to be the necessary ingredient in the impregnant of this invention.
The dielectric liquid c~mpo~ition of this invention i5 more specifically related for use in electrical devices such as electrical capacitors. For that reason, the com-ponents, particularly the halogen component, chould be - 12 _ iot79953 chosen to be capacitor compatible with various capacitors whether of the small motor run kind or the large power factor correction kind. Compatibility means a material A which is-~ct~ and stable not only under the operating conditions and environment of the capacitor but also with respect to capacitor materials such as copper, aluminum, steel, paper, and synthetic re~ins as example~.
This invention may be separated into certain well-defined categories, each of which relate to the addition of a halogen containing compound to an ester ba~ed die-lectric liquid impregnant, particularly adaptable as an electrical capacitor impregnant. In a preferred embodiment the ester is an aromatic, but it may be an aliphatic ester, and it may also be a partially halogenated ester to which a further halogen-containing compound i8 added. me further halogen-containing compound i8 in the mixture in an ad-mixture state, i.e., the added halogen-containing compound is not combined in the ester molecule.
For example, best results have been obtained in the practice of this invention when an aromatic ester, pre-ferably a phthalate ester is the major component and a separate component, such as trichlorobenzene, is mixed therewith The chlorine is attached to the mixing component and not to the base ester so that the chlorine compound is in admixture relationship to the e~ter.
The halogen-containing compounds to this invention may be suitable compounds from the chlorinated, fluorinated, brominated, and the like, compounds Examples of such halogenated compounds may include, for example, dichloro-benzene, difluorobenzene, dibromobenzene, and mono-chlorobenzene, including chlorinated aromatic and non aromatic compounds .
- 13 _ ~0~7~95~
It i~ a preferred concept of this invention that the ester be the predominate liquid or component, i e., the basic liquid for impregnation purposes Thi~ means that ordinarily the ecter is the greater amount by volume or weight in the mixture as compared to any other component.
In a capacitor environment, the basic electxical characteris-tics, such as the dielectric constant and dissipation factor, would be primarily based on the ester as the predominate liquid. One or more ester~ make up one or more components and a non ester chlorinated compound, such as TCB, is another component. A stabilizer i8 not considered as a component. While some mixtures may contain three or more components, the foregoing description of a two com-ponent system remains the preferred one.
In a preferred embodiment of the invention, the ester based dielectric liquid is an aromatic ester, such as a phthalate ester, and more particularly a branch chain phthalate ester such as di-2_ethylhexyl phthalate and the di-isophthalates. Corresponding to this preferred embodi-ment, the halogen-containing compounds are the chlorinated, and more particularly, the chlorinated benzenes such as trichlorobenzene.
By the use of the preferred embodiment of this in-vention, the particular desired, of increased dielectric constant, a higher corona extinction voltage, and increa~ed flame retardance, can all be programmed to the desirable function of the capacitor While this invention has been disclosed with respect to particular embodiments thereof, numerous modifications may be made by those skilled in the art without departing from it~ true spirit and scope. Therefore, it is intended that the appended claims over all such modifications and variations _ 14 -36-cA_32as iO799~3 which come within the true 8pirit and scope of the pre~ent invention.
1 C P Smyth, "Dielectric Behavior and Structure", page 23, McGraw-Hill, 1955. ,~
( ~ - 1) (2 ~ + 1) M = 4 ~ ~o +
g L d 3 3KT
Where ~ i8 the dielectric constant, M is the molecular weight~
N is the number of molecules per mole, ~ o i8 the polariz-ability, ~ is the molecular dipole moment in the liquid, is the sum of the molecular dipole moment and the moment induced as the result of hindered rotation, and d is the density
U.S. Patent 3,363,156 dated January 9, 1968, Cox, discloses and claims various types of dielectric liquid impregnated electrical capacitors. The dielectric liquid composition described in Cox is a chlorinated hydrocarbon, more particularly a halogenated aromatic hydrocarbon and specifically a chlorinated diphenyl. The chlorinated diphenyl impregnants for electrical capacitors are commercially available under the trademark Aroclor, a trademark of the Monsanto Company, and a specific example being Aroclor 1242 or Aroclor 1016. The chlorinated diphenyls, referred to as PCs's,have recently been associated with ecological problems and their continued use in applications other than electrical has been limited.
Accordingly there is a continuing search for new and improved impregnants in the eIectrical capacitor field.
In U.S. Patent 3,754,173 dated August 21, 1973 Eustance, there is disclosed an epoxide stabilized liquid aromatic ester impregnant which does not have many of the PCB ecological disadvantages. Of the problem associated with the aromatic ester kind of impregnant is the fact that the flammability point, i.e. the level at which the liquid will sustain combustion, is relatively low and many of these ester materials are classified as flammable under some failure conditions of an electrical capacitor.
It has also been disclosed that the aromatic esters in accordance with the Eustance patent will provide very high corona start voltages ICSV) and therefore favourably compare with the Aroclor liquid for capacitors, However, a further important voltage level or criteria of a cap-acitor i~ referred to as the corona extinction voltage (CEV) of a capacitor, The corona start voltage and the corona extinction voltage are the voltages at which de-leterious corona discharge may commence in a capacitor and be extinguished respectively during a rising and de-creasing voltage level across the capacitor, In highvoltage power factor corxection capacitors, the corona ex-tinction voltage for the ester impregnants has been found to be significantly less than the corona extinction voltage for the Aroclor type impregnant , In addition to the foregoing problems~ many of the ester impregnants, particularly those of higher molecular weights, are otherwise desirable for capacitor impregnants but have an increased viscosity which creates a problem in essentially completely impregnating a capacitor in ac-cordance with the teachings of the above noted Cox patent, Additive liquids used to minimize the foregoing problems have been found in many cases to have adverse ef~ects on other characteristics of the impregnant such as lowering the dielectric constant (DK).
Accordingly it i~ an object of this invention to provide an improved ester dielectric liquid composition, It i8 a further object of this invention to provide an improved capacitor dielectric liquid ester impregnant having increased corona extinction voltage characterstics, It is another object of this invention to provide an improved capacitor dielectric liquid ester impregnant having improved dielectric constant characteristics, It is a specific object of this invention to provide ~079953 an electrical capacitor which ha~ been impregnant with a dielectric liquid composition compri~ing a mixture of a dielectric liquid aromatic ester, a chlorine containing compound, and an epoxide stabilizer, This invention will be better understtod when taken in connection with the following descriptions and the drawings in which--Fig, 1 is a curve showing the increase in the corona extinction voltage ~CEV) of capacitors containing DOP-TCB
mixtures, Fig, 2 is a curve showing the increase in dielectric constant of DOP-TCB mixtures, Fig, 3 is a curve showing the viscosity of DOP-TCB
mixture~, Fig. 4 is an exemplary roll section comprising al-ternate electrode fail strips and dielectric strips, Fig, 5 is a cross sectional view of a part of a capacitor roll section utilizing solely synthetic resin film as a dielectric, Fig, 6 is a view of a part of a capacitor roll section utilizing mixed synthetic resin film and paper as a die-lectric, Fig, 7 is a cross sectional view of a part of a capacitor roll section utilizing a syn~hetic resin film in a different dielectric paper and film arrangement in a capacitor, Fig, 8 is a complete capacitor in the form of sealed can containing the roll section of Fig, 5, Fig, 9 is a greatly reduced drawing of an exemplary power capacitor utilizing multiple rolls, a structure common to the large size power factor correction, induction heating, and high frequency capacitorC, 107~ 3 36-CA-3289 It has been discovered that the aromatic ester im-pregnants i~ accordance with the Eustance patent may be qreatly improved when a chlorine containing compound is added to the ester. The chlorine contribute-~ improved anti_flammable characteristics to the impreynant while at the same time advantageously raising its corona ex-tinction level, increasing its dielectric constant, and reducing its viscosity~
In high voltage capacitor applications the aromatic esters evidence a very high corona start voltage, a critical requirement of this impregnant. It is expected that capacitors~ during their operating life, are subjected to certain transient overvoltage conditions which could cause a corona discharge within the dielectric. However, with an impregnant such as a chlorinated diphenyl, this corona i8 extinguished as soon as the transient voltage has passed or as soon as the transient has reduced to a certain degree In an effective impregnant, such as chlorinated diphenyl, the voltage level at which the corona is ex-tinguished (CEV) i8 very close to that level at which corona originates This is known as a high corona ex-tinction voltage and a high corona ectinction voltage is required in order to prevent the lingering corona from ser~ou~ly injuring the capacitor~ but more importantly, to extinguish the corona when the voltage drops to the ordinary operating voltage of the capacitor When 1, 3, 4 trichlorobenzene ~TCB) is added to dioctyl phthalate (2_ethyl hexyl) impregnant (DOP), there is an increase in the corona extinction value of capacitors containing mixtures More specifically, it was found that the addition of at least about 10% of chlorine, by weight, to DOP, where the chlorine is an admixture and is not ir _ 4 --1073953 36-CA_328g combined with the DOP, is a ~ufficient amount to provide a significantly improved corona extinction voltage of the mixture which is close to that of Aroclor 1016: one of the more common chlorinated diphenyl impregnant~ utilized in the capacitor industry As low a8 an added amount of 5% chlorine by weight still results in a measured improve-ment to the corona extinction voltage of capacitors con-taining the phthalate.
Representative mixtures of this invention were utilized as impregnants in capacitor embodiments and their corona extinction values (CEV) were obtained as more cleaxly shown in Fig. 1. Referring to Fig.l, the curve of the CEV ver~us the weight percent of chlorine in a DOP/TCB
blend is illustrated as a curve which rises from about 1500 volts to about 2350 volts to about 2350 volts at the highest point. It is to be noted that the increase in CEV is most pronounced at about 8% through 12% with very little increase after about 20% of TCB has been added to the mixture The 10% chlorinate by weight provides a sîgnificant increase in the overall CEV, which approaches that of the Aroclor 1016. TCB which is a chlorinated fluid is electronegative and this characteristic may be of sign-i~icant assistance in the quenching of a corona arc. The character of the corona pulses noted during these tests is also improved by the addition of TCB In DOP capacitors, the corona pulses break in every suddenly at high level (ab. 100 picocoulombs). In capacitors with the DOP/TCB
blend, on the other hand, the corona breaks in at a low level which increases in a controlled fashion as the voltage is increased. As the voltage is decreased, the level of the DOP/TCB pulses decrease rapidly and extin-guish about the same point at which it appeared, while on iO79953 36--CA_3289 the other hand, the DOP pulses per~ist at a high level up to the extinguishing voltage. Corona extinction voltage i8 measured much in the same way and by the same equipment which is used to measure corona start voltage. It usually involves an amplified testing circuit which di~play~ the presence of electrical discharges within the insulation system on a cathode ray oscilloscope. The corona measuring equipment utilized in the invention has a sensitivity of one (1 0) picocoulomb.
DOP_TCB mixtures were also studied to determine their dielectric constants Dielectric constant (DK) is the ratio of the capacitance of a given structuxe with the dielectric liquid as compared to the capacitance of that structure with air as the dielectric medium The value obtained i~ qualified with reference to the conditions of measurement (voltage, frequency, and temperature). The dielectric constant ~DK) of the preferred aromatic esters, the Aroclors, and TCB
are desirably high so that the expected result of mixing these liquids would be a somewhat average DK Surprisingly enough, it was discovered that the mixture of this invention exhibited a dielectric constant which was significantly greater of the dielectric constant of either DOP alone or the TCB alone The dielectric constant of a mixture is usually calculated by the use of a linear dielectric constant mixing rule or a logarithmic dielectric constant mixing rule 1,2. However, the behavior of the mixtures of the present invention appears to be an exception to both rules This synergism which is noted to be most significant for the dioctyl phthalate and trichlorobenzene mixtures is not readily explained by previous experiences with dielectric-mixture~. The synergism of an increased dielectric constant ~'or~ ne~C
A above the expected mixing rule value w~s -~ef~hi~Kff~by testing 36-CA_3289 of DOP mixtures with other chlorinated compunds.
The linear dielectric constant mixing rule as defined in both cited reference~ by apprc>priate equations i8 in-tended to mean the straight line function, i.e. the dash A line of Fig 2 As hereinafter ~ to in the claims, Dielectric Constant Mixing Rule refers to the rules noted in the above cited references and in thi~ specification.
A DK higher than that calculated by the Dielectric Constant Mixing Rule is a DK whose value is above that indicated by the dash line regardless of the slope or initial height of the dash line. ~he basis for the straight line function is derived from the equations given and whether or not the line is precisely str3ight is of no importance. This in-vention describes a DK greater than that predicted by the noted equations.
Several mixtures were studied under conditions of 25C
f and at a frequency of 100 Hertz, and their dielectric con-stants are more clearly shown in Fig. 2 Re~erring now to Fig. 2, there is disclosed a volume percent TCB in DOP/TCB
mixtures as compared with the DK value of the mixtures It can be seen that at about 20% of TCB in the mixture a maximum DK of about 5.5 is reached This DK does not ~eem to be affected by further increases in the amount of TCB.
The DK expected, from linear or logarithmic mixing rules for these mixtures, would be about 5.15 as indicated by the dash line The high DK figure of 5.5 is compared to the DK of about 5.15 to 5 2 for either of the TCB or the DOP material alone.
The curve of Fig. 2 shows a DK higher than either com-ponent. However, when one of the components has an initial DK which is much lower than the other component, the DK
curve will be higher than the noted mixing rules predict 10799S3 ~ 6 -CA 3 2 8 9 although the mixture will not exhibit a DK higher than either component.
Variou~ mixtures of DOP ~ TCB were made up and utilized in representative type capacitors. It wa8 noted during the course of this work that the TCB reduced the viscosity of the DOP and thus facilitated impregnation particularly in all film capacitors Accordingly, where many of the estèrs are more undesirable because of their high viscosity, the addition of TCB may reduce that viscosity to more desirable and appropriate levels One example of the viscosity lowering ability of TCB
in DOP is shown in FIG. 3. In Fig. 3, the curves show that the viscosity of the mixture decreases quite rapidly with increasing amount~ of TCB at room temperature conditions or 2sC. At 70 C it is noted that the viscosity decreases less rapidly with increasing amounts of TCB since at this temperature the viscosity of the DOP is already low. By comparison, the viscosity of the more common chlorinated diphenyl impregnant, Aroclor 1016, is about 45 centistokes at 25C.
Mixtures of DOP and TCB were also tested for their flame retardant characteristics These tests show that the ability of the DOP to sustain combustion i~ lessened by the addition of the TCB, and, therefore, the TCB addition is quite ~avourable for capacitor operation where a high degree of inflammability is desired. For example, it was found that DOP has a fire point, the point at which DOP will sustain flame at about 240 C. However a 70% DOP + 30% TCB
mixture had no fire point up to about 265 C
Preferred capacitor structures embodying this invention are illustrated in Pigs. 4 through 9. Referring now to Fig 4 here is disclosed a typical roll section 10 comprising alternate electrode foils 11 and 12 and dielectric strips 13 and 14. Strips 13 and 14 may be single strip~ of paper or a synthetic resin, plural strip of the~e materials, or composite strips. The electrode foilR 11 and 12 may also be formed as metallized coatings on the strips 13 and 14, or on separate and additional stripc of various dielectric material~. Suitable electrical connectors in the form of tabs 15 and 16 are utilized to connect the electrode 11 and 12 to appropriate capacitor terminals.
The dielectxic structures for roll section 10 may include any of the composites of Fig. 5, 6 and 7 as illustrated Referring to Fig. 5, there is illustrated what may be referred to as an all ~ilm roll structure 17 In this structure, only a synthetic resin film such as polypropylene film is used as the 901e dielectric between electrodes 11 and 12. A typical all polypropylene film capacitor will utilize one or more polypropylene film strips 18 between electrodes 11 and 12.
Referring to Fig 6, there is illustrated one form of a mixed dielectric roll structure 19 or a capacitor using dissimilar dielectrics, such as a synthetic resin film and a paper strip, although different resin films may be employed as a composite, such as polypropylene film and a polyester film. As illustrated in Pig. 6, roll structure 19 has been described as a semi-~andwich construction and uses one or more paper strips 20 with one or more synthetic resin film ~trips 18.
Referring to Fig. 7, there is illustrated a further mixed dielectric roll structure 21. Structure 21 has been referred to as an inverted sandwich -~tructure whose primary characteri~tic is that a synthetic resin film is used ad-jacent each foil 11 and 12 and there is one or more in-_ g _ termediate dissimilar strips 20 l~hich are usually employed as a combined dlelectric and wiclcing #trip. In a preferred form of the invention as illu~trated, the intermediate strip i8 a single paper strip 20 and the synthetic resin strips are single polypropylene strips 18, One or more of the roll sections 10 of Fig. 1, in-A cluding the ~a~ structure of one or more structures ofFigs, 5,6, and 7, are assembled in cans or casings, im-pregnated with the impregnant of this invention, and then sealed. Typical capacitor constructions are illustrated in Figs, 8 and 9, Referring to Fig. 8, there is illustrated what may be referred to as a motor start or motor run capacitor 22, Such a capacitor usually includes a single roll section 10 of Fig. 4, which is inserted into a metal casing 23 and Realed therein. The tabs 15 and 16 of roll section 10 are connected to external capacitor terminals 24 and 25, The metal casing 23 is filled with the impregnant of this invention through fill hole means 26 which is illustrated as being solder sealed, Plural and larger roll sections are u~ed in capacitors referred to as power capacitors or power factor correction capacitors, One such typical capacitor is illustrated in Fig, 9. Refexring now to Fig, 9, there i~ illustrated a high voltage power factor correction capacitor 27, Capacitor 27 usually includes a large rectangular steel cas-ing 28 which may be form ~ to 1 meter in the longer dimen-sion shown, Casing 28 includes therein a row of longer roll sections 10 whose connectors 15 and 16 are suitably connected to external capacitor terminals 29 and 30, Casing 28 may be filled with the impregnant of this inven-tion in a manner similar to that as described for Fig, 9, .
A number of capacitors were made up for testing purposes of this invention. A typical te~t capacitor in-volved the r~ll section of Fig. 4 assembled a~ the motor run capacitor of Fig. 8 In a f:irst test capacitor, the strips 13 and 14 were composite strips comprising a pair of polypropylene strips 18 and intermediate paper strip 20 (Fig. 7). In the examples as made up in this application, the polypropylene comprised two strips 18 of about 0.47 mil. (12 mm) thick polypropylene and one sheet of 0 50 mil. (1.27 mm) paper. The overall capacitor height was about 4 in. (10.16 cm) In a second capacitor structure made up for the purposes of this invention~ the dielectric strips 13 and 14 each comprised a pair of paper strips 20, one of which was about 0.60 mil. (1 55 mm) thick paper and the other which was 0.65 mil. (1.68 mm) thick paper. These capacitors were impregnated in accordance with the general impregnation cycle as set out in the Cox U S. patent No.
3,363,156 dated January 9, 1968 which comprised drying the capacitors in a vacuum oven at temperatures up to and including 105 C and then cooling the capacitors to less than 100 so that they may be filled with a dielectric liquid at about 65 to 75C. Thereafter, the capacitors were sealed and permitted to heat soak overnight, i.e., about 14 to 16 hours, in a forced circulation oven at about 95 C
Testing of these capacitors of the first described kind in-dicated the following corona start voltage levels as æet out in Table I.
Corona Start Voltage CSV (avg of 5) Minimum 95% DOP/5 TCB 3020 2900 9~% DOP/10 TCB 2912 2850 80% DOP/20 TCB 2870 2750 60% DOP/40 TCB 2950 2850 ~79gS3 Ten of the capacitors from T~ble I which included the 60% DOP/ 40% TCB blend were put on life test at 2650 V~C
(volts, alternating current) at 70 C There were no failures after 400 hours ac compared wqth three ~ailures in DOP capacitors without TCB, and zero failures in cap-acitors impregnated with Aroclor 1016 It has been found in the practices of thi-q invention that some form of electrical stabilizer i~ de~irable in the impregnant mixture Recently, the epoxide compounds have been found to be particularly sffective with chlorinated diphenyl compounds and also with ester compounds although the reactions of neither addition are well understood. The epoxides are the preferred stabilizer~ for the present in-vention and they perform a stabilizing function in the presence of both an ester liquid and a chlorinated com-pound without any adverse effects from the component mixtures. The liquid impregnant from the capacitors of Table I above included about 0.3 percent of l_epoxyethyl_3, 4-epoxycyclohexane.
While the power factor stabilizer is preferably an epoxide material, other such stabilizers such as the anth-roquinones may be utilized but with apparently less effect than the epoxide as disclosed in the Eustance patent. On the other hand, some of the anthrcquinones~ such as the monochloroanthroquinones and the dichloroanthroquinones, include a significant amount of the chlorine which appears to be the necessary ingredient in the impregnant of this invention.
The dielectric liquid c~mpo~ition of this invention i5 more specifically related for use in electrical devices such as electrical capacitors. For that reason, the com-ponents, particularly the halogen component, chould be - 12 _ iot79953 chosen to be capacitor compatible with various capacitors whether of the small motor run kind or the large power factor correction kind. Compatibility means a material A which is-~ct~ and stable not only under the operating conditions and environment of the capacitor but also with respect to capacitor materials such as copper, aluminum, steel, paper, and synthetic re~ins as example~.
This invention may be separated into certain well-defined categories, each of which relate to the addition of a halogen containing compound to an ester ba~ed die-lectric liquid impregnant, particularly adaptable as an electrical capacitor impregnant. In a preferred embodiment the ester is an aromatic, but it may be an aliphatic ester, and it may also be a partially halogenated ester to which a further halogen-containing compound i8 added. me further halogen-containing compound i8 in the mixture in an ad-mixture state, i.e., the added halogen-containing compound is not combined in the ester molecule.
For example, best results have been obtained in the practice of this invention when an aromatic ester, pre-ferably a phthalate ester is the major component and a separate component, such as trichlorobenzene, is mixed therewith The chlorine is attached to the mixing component and not to the base ester so that the chlorine compound is in admixture relationship to the e~ter.
The halogen-containing compounds to this invention may be suitable compounds from the chlorinated, fluorinated, brominated, and the like, compounds Examples of such halogenated compounds may include, for example, dichloro-benzene, difluorobenzene, dibromobenzene, and mono-chlorobenzene, including chlorinated aromatic and non aromatic compounds .
- 13 _ ~0~7~95~
It i~ a preferred concept of this invention that the ester be the predominate liquid or component, i e., the basic liquid for impregnation purposes Thi~ means that ordinarily the ecter is the greater amount by volume or weight in the mixture as compared to any other component.
In a capacitor environment, the basic electxical characteris-tics, such as the dielectric constant and dissipation factor, would be primarily based on the ester as the predominate liquid. One or more ester~ make up one or more components and a non ester chlorinated compound, such as TCB, is another component. A stabilizer i8 not considered as a component. While some mixtures may contain three or more components, the foregoing description of a two com-ponent system remains the preferred one.
In a preferred embodiment of the invention, the ester based dielectric liquid is an aromatic ester, such as a phthalate ester, and more particularly a branch chain phthalate ester such as di-2_ethylhexyl phthalate and the di-isophthalates. Corresponding to this preferred embodi-ment, the halogen-containing compounds are the chlorinated, and more particularly, the chlorinated benzenes such as trichlorobenzene.
By the use of the preferred embodiment of this in-vention, the particular desired, of increased dielectric constant, a higher corona extinction voltage, and increa~ed flame retardance, can all be programmed to the desirable function of the capacitor While this invention has been disclosed with respect to particular embodiments thereof, numerous modifications may be made by those skilled in the art without departing from it~ true spirit and scope. Therefore, it is intended that the appended claims over all such modifications and variations _ 14 -36-cA_32as iO799~3 which come within the true 8pirit and scope of the pre~ent invention.
1 C P Smyth, "Dielectric Behavior and Structure", page 23, McGraw-Hill, 1955. ,~
( ~ - 1) (2 ~ + 1) M = 4 ~ ~o +
g L d 3 3KT
Where ~ i8 the dielectric constant, M is the molecular weight~
N is the number of molecules per mole, ~ o i8 the polariz-ability, ~ is the molecular dipole moment in the liquid, is the sum of the molecular dipole moment and the moment induced as the result of hindered rotation, and d is the density
2 A Von ~ipple, "Dielectrics and Waves", page 231, John Wiley & Sons, 1954.
Log km" = ~1 loq Kl + e2 log K2 ' Where km' is the dielectric constant of the mixture of k 1 and k2' and el and e2 are the volume ratio8 of the compon_ ents.
Log km" = ~1 loq Kl + e2 log K2 ' Where km' is the dielectric constant of the mixture of k 1 and k2' and el and e2 are the volume ratio8 of the compon_ ents.
Claims (21)
1. A dielectric liquid impregnant for electrical devices comprising a dielectric halogen-free liquid phthalate ester having as an additive thereto to increase the corona extinction voltage of said devices impregnated therewith a halogenated benzene compound.
2. A dielectric liquid impregnant for electrical devices comprising a dielectric halogen-free liquid phthalate ester having as an additive thereto a chlorinated benzene compound thereby forming a mixture having a dielectric constant greater than the calculated dielectric constant of said mixture determined by the Dielectric Constant Mixing Rule.
3. The impregnant of claim 2 wherein said ester is dioctyl phthalate.
4. The impregnant of claim 1 wherein said halogen is chlorine.
5. The impregnant of claim 1 wherein said additive is a chlorinated benzene.
6. The impregnant of claim 2 or 3 wherein said additive is a chlorinated benzene.
7. The impregnant of claim 1, 2 or 3 wherein said additive is a chlorinated benzene having from 1 to 3 chlorine substituents.
8. The impregnant of claim 1, 2 or 3 wherein said additive is trichlorobenzene.
9. The impregnant of claim 1, 2 or 3 wherein said additive is trichlorobenzene and comprises from about 5 to about 40 percent by volume based on the volume of said ester and said additive.
10. The impregnant of claim 1, 2 or 3 further comprising a stabilizer material.
11. The impregnant of claim 1, 2 or 3 further comprising an epoxy containing stabilizer material.
12. A dielectric liquid impregnant for a capacitor or the like comprising di 2-ethylhexyl phthalate ester having as an additive thereto in sufficient amount to provide a chlorine content of not less than about 5 percent by weight based on said ester and said additive, trichlorobenzene.
13. The impregnant of claim 12 wherein said trichloro-benzene is present in an amount to provide a chlorine content of between about 5 and 20 weight percent based on said ester and said additive.
14. The impregnant of claim 13 further comprising an epoxide stabilizer therefor.
15. An AC capacitor comprising in combination a) a sealed casing, b) a capacitor section in said casing comprising a pair of spaced electrodes and a solid synthelic resin film dielectric spacer therebetween, c) and a dielectric liquid impregnant as defined in claim 1, 2 or 12, in said casing, said capacitor having a significantly increased corona extinction voltage as compared to the same capacitor when using as said dielectric the phthalate ester component of said impregnant in the absence of the additive component thereof defined in said claims.
16. A process of improving the corona extinction voltage of an AC resin film capacitor having as a dielectric impregnant therein a halogen free liquid phthalate ester which comprises admixing with said phthalate ester prior to impregnating said capacitor a chlorinated benzene to provide a chlorine content of not less than about 5 parts by weight in said admixture.
17. The process of claim 16 wherein said ester is dioctyl phthalate.
18. The process of claim 16 wherein said ester is di 2-ethyl hexyl phthalate.
19. The process of claim 16, 17 or 18 wherein said chlorinated benzene has from 1 to 3 chlorine substituents.
20. The process of claim 16, 17 or 18 wherein said chlorinated aromatic hydrocarbon is trichlorobenzene.
21. The process of claim 16, 17 or 18 wherein said chlorinated benzene is trichlorobenzene and wherein the chlorine content of said admixture is at least about 10 parts by weight.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA246,692A CA1079953A (en) | 1976-02-27 | 1976-02-27 | Dielectric liquids comprising phthalate esters and halogen compounds |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA246,692A CA1079953A (en) | 1976-02-27 | 1976-02-27 | Dielectric liquids comprising phthalate esters and halogen compounds |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1079953A true CA1079953A (en) | 1980-06-24 |
Family
ID=4105330
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA246,692A Expired CA1079953A (en) | 1976-02-27 | 1976-02-27 | Dielectric liquids comprising phthalate esters and halogen compounds |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1079953A (en) |
-
1976
- 1976-02-27 CA CA246,692A patent/CA1079953A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4054937A (en) | Capacitor | |
| Borsi et al. | Properties of ester liquid midel 7131 as an alternative liquid to mineral oil for transformers | |
| CA1154503A (en) | Electrical capacitors | |
| US4266264A (en) | Meta isopropyl biphenyl insulated electrical apparatus | |
| CA2078808A1 (en) | Dielectric liquids | |
| US4276184A (en) | Dielectric fluids comprising non-halogenated mixtures of organic esters and aromatic compounds | |
| US4744000A (en) | Electrical capacitor having improved dielectric system | |
| US4566994A (en) | Flame-retardant electrical insulating oil composition | |
| US2566208A (en) | Dielectric composition of halogenated aromatic hydrocarbon and organic antimony compound as a corrosion inhibitor | |
| US4482478A (en) | Blends of branched chain phthalate esters and halogenated benzene compounds | |
| CA1079953A (en) | Dielectric liquids comprising phthalate esters and halogen compounds | |
| US4146491A (en) | Electrical devices containing improved dielectric fluids | |
| US4108789A (en) | Dielectric compositions containing benzyl esters | |
| US3959162A (en) | Insulating oil | |
| US3948789A (en) | Electrical devices containing improved dielectric fluids | |
| US4097913A (en) | Electrical capacitor having an impregnated dielectric system | |
| US4104184A (en) | Electrical devices containing select vinylsiloxane dielectric fluids | |
| US2627504A (en) | Dielectric containing halogenated aromatic hydrocarbon and mono ortho tolyl biguanide as corrosion inhibitor | |
| CA1339673C (en) | Perchloroethylene dielectric fluid containing aliphatic hydrocarbons | |
| RU2195729C1 (en) | Insulating compound | |
| GB1602092A (en) | Fluid insulated electrical apparatus | |
| Garg et al. | Compatibility of ester oil with transformer components and comparison with mineral oil | |
| CA1079951A (en) | Electrical devices containing improved dielectric fluids | |
| US2757141A (en) | Dielectric materials | |
| DE2548339A1 (en) | ELECTRIC CAPACITOR |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |