GB1582312A

GB1582312A - Impregnated electrical capacitor

Info

Publication number: GB1582312A
Application number: GB11213/77A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1976-03-24
Filing date: 1977-03-16
Publication date: 1981-01-07
Also published as: SU841613A3; BR7701487A; DE2704458A1; JPS52125754A; SE7703351L; FR2345800A1

Description

(54) IMPREGNATED ELECTRICAL CAPACITOR (71) We, GENERAL ELECTRIC COMPANY, a corporation organised and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady, 12305, State of New York, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to an improved electrical capacitor containing dielectric liquid impregnants.

Oil filled electrical capacitors are usually impregnated to a high degree with the intention that all voids and interstices be essentially completely filled with a dielectric liquid. Corona discharges may easily occur where voids are present in the system under a high dielectric stress because of ionization of gas in these voids. In spite of vacuum impregnation and improved dielectric fluids, troublesome corona discharges occur all too frequently. Impregnation to fill every conceivable void in a capacitor has not been achieved. High stress conditions and dielectric breakdown continue to cause hydrogen gas evolution and void formation which in turn become corona sites. In the capacitors of this invention the voids and interstices are filled with a dielectric liquid impregnant to which are added two additional components.

More specifically, the present invention provides an electrical capacitor comprising: a. a casing member b. a capacitor section in said casing c. and a liquid impregant filling said casing, said impregnant comprising a base liquid together with two additives, namely 1. a capacitor compatible liquid halogenated benzene, toluene or naphthalene, said liquid being different from said base liquid and having a boiling point lower than said base liquid, and 2. a monoolefinic liquid which is different from said base liquid and said halogenated liquid, and which has a hydrogen gas negative value higher than that of the base liquid.

In one preferred form of this invention, the additives are trichlorobenzene and a decene (as hereinafter defined), more particularly, tetradecene-1.

This invention will be described further with reference to the accompanying drawings in which Figure 1 illustrates a single roll capacitor of the present invention and Figure 2 illustrates a high voltage power capacitor including a plurality of capacitor roll sections of the present invention.

Referring now to Figure 1 there is shown a typical small industrial or motor run capacitor 10 which utilizes the liquid impregnant 11 used in this invention. Capacitor 10 comprises a casing 12 in which there is a capacitor roll section 13 connected by appropriate leads (not shown) to external terminals 14 and 15 in a cover 16. Cover 16 is sealed to casing 12 and the capacitor roll section 13 is impregnated through fill hole 17 with the impregnant 11, and may be submerged in the impregnant 11 which fills the casing 12. Accordingly, cover 16 is tightly sealed to the casing 12 to prevent leakage of the impregnant and fill hole 17 is solder sealed.

Referring now to Figure 2 there is shown a power capacitor 20 which utilizes the impregnant of the present invention. Power capacitors which are inclusive of power factor correction capacitors are usually rated in kilovars and may be on the order of 150 to 200 kilovars each. Such a capacitor 20 as illustrated includes a very large casing 21 in which there is positioned a serial row of individual capacitor roll sections 22. These roll sections are suitably electrically connected to terminals 23 and 24. The casing is filled with the impregnant 11 of this invention which essentially completely impregnates the capacitor roll sections.

A long time satisfactory impregnant for the capacitors of the kind illustrated in Figures 1 & BR< 2 is chlorinated diphenyl which is characterized by having very good impregnation characteristics and excellent corona start and corona extinction voltages. However, recent environmental problems with the chldrinated diphenyls have spurred a search for a substitute impregnant.

Some substitute impregnants have been found to have significant problems in satisfying the corona start and extinction voltages, required for high voltage capacitors, particularly power factor correction capacitors in the ratings of above about 100 kilovar. Other substitutes are not as sufficiently corona resistant or insensitive to corona as desirable.

When, for example, trichlorobenzene (hereinafter sometimes abbreviated at TCB) and a decene are both added to a base impregnant there is an improved capacitor life and a further marked increase in the corona start voltage. (The term "decenes" as used in this application is intended to mean those aliphatic monoolefins having from 10 to 20 carbon atoms per molecule such as for example, decene, undecene, dodecene, tridecene, tetradecene, and so on through the progression). These unexpected results occur in spite of the fact that the addition of a decene to, for example, a TCB dioctyl phthlate (DOP) mixture (the DOP being the base impregnant) reduces the increase in dielectric constant of the mixture obtained by the same, and further reduces the TCB content of the mixture. The decene material with its lower dielectric constant is replacing TCB which has a higher dielectric constant.

The improvements achieved by the use of the two additives in accordance with this invention together with an ester liquid impregnant are shown by the following examples and comparative tests. In these examples an epoxide was also used in the impregnant, specifically 0.7 % by weight of dicyclohexane-diepoxy carboxylate. Also utilized was an.antioxidant in the handling and purification of the impregnant prior to impregnation. The antioxidant used was 2, 6 di-t-butyl paracresol which is commercially available as "Ionol" (registered Trade Mark) antioxidant from Shell Chemical Co. The "Ionol" antioxidant appears in the final capacitor impregnant.

EXAMPLE I In this example the ester base liquid is the liquid described in U.S. Patent 3754173 Eustance, assigned to the applicants of the present invention. For example, the liquid is a phthalate ester known as 2-ethylhexyl phthalate and sometimes referred to as dioctyl phthalate or DOP. To this base liquid there was added about 20% trichlorobenzene by volume and about 10% tetradecene-l by volume. The mixture referred to as ETD (estertrichlorobenzene-decene) was purified and used to impregnate ten capacitors similar to the one shown Figure 1 where the dielectric system comprises two sheets of 0.47 mil polypropylene and an intermediate sheet of 0.40 mil paper. The capacitors are rated at 0.8 microfarads and 1400 volts in a 1-1/4 inch by 4 inch oval case.Similar groups of ten capacitors each were impregnated with DOP or with DOP and TCB and with a commercially available polychlorinated diphenyl known as "Monsanto" 1016 of the Monsanto Company, USA ("Monsanto" is a registered Trade Mark").

Corona Extinction Liquid Impregnant Corona Start Voltage Voltage ETD Above 3200 volts DOP 2100 volts 500 1016 2750 volts 1620 DOP + TCB 2600 volts 1270 As can be seen from the above data of Example I, the use of the ETD mixture provides a marked increase in the corona start voltage of the noted capacitors. The comparison values are predicted on the fact that the capacitors are similar in construction and processing so that the differences obtained are valid for comparative purposes. This indicates that by the use of this invention smaller more economical and more efficient capacitors can be produced. No corona extinction value is given for ETD because no corona appeared at the 3200 volt level to extinguish.

EXAMPLE II In this example the capacitors were similar to those of Example 1. As a comparison, "Monsanto" 1238 ("Monsanto" is a registered Trade Mark) was used as an impregnant. This impregnant supplied by Monsanto U.S. comprises an isopropyl biphenyl blended with a sulfone material as noted in U.S. Patent 3796934. "Monsanto" 1238 was used with 20% by volume of TCB and 10% by volume tetradecene-l.

Corona Extinction Liquid Impregnant Corona Start Voltage Voltage ETD 3100 2300 1016 2830 2100 "Monsanto" 1238 2600 1800 "Monsanto" 1238 +TCB +DOP 2900 1900 As can be seen from the above Example II the impregnant taught by this invention provides a higher corona start voltage than either the 1016 material (which is treated as a representative standard) or "Monsanto" 1238.

In the selection of the monoolefinic liquid additive, the choice should be one which is capacitor-compatible, particularly where it constitutes as much as about 30% by volume of the impregnant. Further it should have a hydrogen gas negative value (which is a measure of the degree to which it absorbs hydrogen) higher than the hydrogen gas negative value of the host impregnant and remain that way over the operable temperature range of the capacitor.

For example, hexadecene has a hydrogen gas negative value of - 16 to - 18 in the 40"C to 600C range while dioctyl phthalate has a hydrogen gas negative value of about -4.4 to about -3.9 over the same range. In this specification hydrogen gas negative values are expressed in microliters per minute and may be determined by the procedures described in "Evaluation of Dielectric Fluids by Gassing Cell Tests" - Black Proc. IEE Vol. 119 No. 4, April, 1972. A difference of from -5 to -10 is a preferred range between the two materials. This difference value also depends on the kind and amount of gas negative material used; for example for a pure olefin or single component material such as the decenes at least 5 % by volume should be employed.

Trichlorobenzene which is a preferred additive of the present invention is also representative of the other halogenated additives having boiling points lower than the base liquid may be used herein. TCB as a commercial grade material is a mixture of isomers and is generally referred to as 1,2,4 trichlorobenzene. The dielectric constant (DK) is about 5 measured at 25"C and 100 HZ. When chlorinated materials are used as the halogenated additive in the impregnant, it is preferred that an epoxide also be used, as known in the art.

Both the decene material or other monoolefinic liquid additive and the halogenated hydrocarbon additive should be capacitor compatible, particularly in the operating temperature range of the capacitor, e.g., between about 40"C and 100"C. Since the halogenated hydrocarbon is generally used in relatively large quantities, e.g., 30% to 10% by volume, it preferably should have electrical characteristics similar to or better than those of the base liquid. For example, DOP has a dielectric constant of about 5.2 and TCB about 5.2. While the 'mixture of TCB and DOP provides an enhanced dielectric constant, the addition of 10% by volume of decene to the mixture reduces the overall dielectric constant to that contemplated by the Linear Volume Mixing Rule.

One reason for the improved results obtainable according to the teachings of the present invention is that gaps or voids at a corona site or elevated temperature site tend to become filled by vapor of the halogenated hydrocarbon additive but this vapor tends to suppress or minimize corona and arcing; ordinarily, gas molecules which find their way into such voids become ionized under high voltage conditions and corona can commence.

As known in the capacitor environment, the high stress, high temperature conditions lead to the evolution or generation of hydrogen gas which is quite deleterious to the capacitor.

Hydrogen gas tends to combine with other elements to form acids and water which lead to capacitor failure. More importantly, the hydrogen gas is easily ionized and corona discharge occurs. With the onset of corona, dielectric materials degrade, gases are evolved and corona tends to avalanche. This is especially significant where the hydrogen finds its way into a gap or airspace, and the conditions are not satisfactorily mitigated by the presence of TCB or other halogenated hydrocarbon vapor in these voids. In this invention, gas formed is absorbed by the decene or other monoolefinic material, while at the same time a void generated at the beginning of corona is filled with halogenated hydrocarbon vapor which is not absorbed by the monoolefin but which suppresses corona, diminishes ionization of hydrogen and gains time for the hydrogen to be absorbed.Accordingly, by the use of the two additives in accordance with the present invention the hydrogen gas is absorbed or essentially nullified to prevent or minimize corona. Thus concerted action is applied against voids and hydrogen gas whether they occur together at a given time or happen at different times and places.

The additives taught by this invention are added to a base dielectric liquids which is suitable for electrical capacitor use. The base material is broadly defined as one where, in the capacitor environment, voids or gaps are likely to exist or be caused to exist by operation of the capacitor. and where hydrogen gas is likely to be evolved. Typical base liquids are the mineral oils, askarels, esters, silicones, and synthetic hydrocarbons, particularly those known to have some capacitor impregnant characteristics. The base liquid may be a mixture of liquids, for example a mixture including a sulfone e.g. a mixture of a non-halogenated biphenyl and a sulfone.

The readers attention is directed to our copending application No. 11212/77 Serial No.

1582311 which describes and claims in claim 1 a composition for use as a dielectric liquid under electric stress in electrical apparatus, comprising: a. a dielectric liquid ester of an aromatic acid, which ester is hydrogen gas negative between 40"C and 100"C, b. and an olefin material dissolved therein, said olefin material having a higher hydrogen gas negative value than said component (a).

Also described and claimed in application No. 11212/77 [Serial No. 1582311] is an electrical capacitor, comprising a roll section comprising alternate layers of a dielectric strip and metallic electrode, a casing having said roll section contained therein, and electrical connection means connecting said electrodes to terminals on said casing, characterised by a dielectric liquid impregnant in said casing impregnating said roll section which comprises the composition as defined in claim 1 of the said application No. 11212/77 [Serial No. 1582311].

WHAT WE CLAIM IS: 1. An electrical capacitor comprising: a. a casing member b. a capacitor section in said casing c. and a liquid impregnant filling said casing, said impregnant comprising a base liquid together with two additives, namely 1. a capacitor compatible liquid halogenated benzene, toluene or naphthalene, said liquid being different from said base liquid and having a boiling point lower than said base liquid, and 2. a monoolefinic liquid which is different from said base liquid and said halogenated liquid, and which has a hydrogen gas negative value higher than that of the base liquid.

2. A capacitor according to Claim 1, wherein said halogenated liquid is a chlorine compound.

3. A capacitor according to Claim 1 or Claim 2, wherein said halogenated liquid includes a halo-benzene.

4. A capacitor according to any preceding claim, wherein said olefinic liquid is a decene (as hereinbefore defined).

5. A capacitor according to Claim 4, wherein said additives are trichlorobenzene and a decene.

6. A capacitor according to Claim 5, wherein said trichlorobenzene is in the range of 10% to 30% by volume of said impregnant and said decene is in the range of 10% to 20%volume of said impregnant.

7. A capacitor according to Claim 2, or any one of Claims 3-6 when appendant to Claim 2, wherein the liquid impregnant also comprises an epoxide.

8. A capacitor according to any preceding claim, wherein said base liquid includes an ester.

9. A capacitor according to Claim 8, wherein said ester is a phthalate ester.

10. A capacitor according to Claim 9, wherein said ester is dioctyl phthalate.

11. A capacitor according to any one of Claims 1-7, wherein said base liquid is a mixture of liquids, one of which is a sulfone.

12. A capacitor according to Claim 11, wherein said base liquid is a mixture of a non-halogenated biphenyl and a sulfone.

13. An electrical capacitor, according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings or substantially as described in either of the Examples herein.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

liquids, for example a mixture including a sulfone e.g. a mixture of a non-halogenated biphenyl and a sulfone.

The readers attention is directed to our copending application No. 11212/77 Serial No.

1582311 which describes and claims in claim 1 a composition for use as a dielectric liquid under electric stress in electrical apparatus, comprising: a. a dielectric liquid ester of an aromatic acid, which ester is hydrogen gas negative between 40"C and 100"C, b. and an olefin material dissolved therein, said olefin material having a higher hydrogen gas negative value than said component (a).

Also described and claimed in application No. 11212/77 [Serial No. 1582311] is an electrical capacitor, comprising a roll section comprising alternate layers of a dielectric strip and metallic electrode, a casing having said roll section contained therein, and electrical connection means connecting said electrodes to terminals on said casing, characterised by a dielectric liquid impregnant in said casing impregnating said roll section which comprises the composition as defined in claim 1 of the said application No. 11212/77 [Serial No. 1582311].

WHAT WE CLAIM IS: 1. An electrical capacitor comprising: a. a casing member b. a capacitor section in said casing c. and a liquid impregnant filling said casing, said impregnant comprising a base liquid together with two additives, namely 1. a capacitor compatible liquid halogenated benzene, toluene or naphthalene, said liquid being different from said base liquid and having a boiling point lower than said base liquid, and 2. a monoolefinic liquid which is different from said base liquid and said halogenated liquid, and which has a hydrogen gas negative value higher than that of the base liquid.
2. A capacitor according to Claim 1, wherein said halogenated liquid is a chlorine compound.
3. A capacitor according to Claim 1 or Claim 2, wherein said halogenated liquid includes a halo-benzene.
4. A capacitor according to any preceding claim, wherein said olefinic liquid is a decene (as hereinbefore defined).
5. A capacitor according to Claim 4, wherein said additives are trichlorobenzene and a decene.
6. A capacitor according to Claim 5, wherein said trichlorobenzene is in the range of 10% to 30% by volume of said impregnant and said decene is in the range of 10% to 20%volume of said impregnant.
7. A capacitor according to Claim 2, or any one of Claims 3-6 when appendant to Claim 2, wherein the liquid impregnant also comprises an epoxide.
8. A capacitor according to any preceding claim, wherein said base liquid includes an ester.
9. A capacitor according to Claim 8, wherein said ester is a phthalate ester.
10. A capacitor according to Claim 9, wherein said ester is dioctyl phthalate.
11. A capacitor according to any one of Claims 1-7, wherein said base liquid is a mixture of liquids, one of which is a sulfone.
12. A capacitor according to Claim 11, wherein said base liquid is a mixture of a non-halogenated biphenyl and a sulfone.
13. An electrical capacitor, according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings or substantially as described in either of the Examples herein.