DE2341356C2 - Electric capacitor and method of impregnating the same - Google Patents

Description

(a) Arranging the capacitor winding in the housing,

(b) heating the capacitor under vacuum to a temperature of about 100 to 140 ⁰ C for at least 8 hours,

(c) filling the housing under vacuum and at elevated temperature with the impregnating agent, and

(d) Soaking the condenser with heat for at least 4 hours at a temperature of 85 to 140 ^° C.

7. The method according to claim 6, characterized in that the condenser after the heat soaking cooled down and the soaking is then repeated.

8. The method according to claim 6 or 7, characterized in that the aromatic ester by means of a Column cleaning process is cleaned with removal of all foreign materials and to the aromatic Ester at least 1 wt .-% of the epoxy is added and dissolved therein.

The invention relates to an electrical capacitor with at least one capacitor winding in one sealed housing, the impregnation agent is not halogenated and made of a liquid ester with an addition of less than 10 wt .-% of an epoxy compound stabilizing the ester, and a Method for impregnating this electrical capacitor.

Such a capacitor is already known from DE-OS 19 24 331, in which castor oil is used as the liquid ester or other derivatives of ricinoleic acid can be used.

Furthermore, it is already known from DE-GM 69 05 943, as an aromatic ester for phthalic acid ester To use impregnation of electrical capacitors. Diethylhexyl or dioctyl phthalate is also used as an example called.

AC tests on capacitors have shown that dioctyl phthalate (DOP) can be used as an impregnating agent is not suitable for high-voltage AC capacitors, although it has certain advantageous ones Has properties such as capacitor compatibility, high dielectric constant and biodegradability. These good properties are, however, by the instability at high electrical loads and overshadowed the resulting short service life. A sign of the short lifespan in one Capacitor is a rapidly increasing or high loss or power factor that is a measure of the electrical power Losses in a capacitor is. It is generally a critical point in high stress loads AC power capacitors. The electrical instability leads to an early breakthrough or failure.

Stabilization of the impregnating agent usually includes the addition of small amounts of another Materials for the impregnation agent, which improve the impregnation agent by removing the contamination, which are present in capacitors or are generated there and which cause degradation, neutralize.

Such a stabilizing additive to an aliphatic impregnating agent, namely castor oil or other derivatives of ricinoleic acid and an epoxy compound, is - as already mentioned above - already known from DE-OS 19 24 331.
The aromatic DOP (dioctyl phthalate) is a liquid material that, when used as an impregnation

The medium in capacitors exposed to elevated temperatures and high voltage stresses shows a rapid increase in the power factor and accordingly quickly leads to failure of the capacitor. For these reasons, the development of DOP is practical and commercial recognized impregnation agent for an electrical capacitor were omitted.
Starting from a generic capacitor, according to the preamble of the main claim, it is

<>. ·; The object of the present invention to provide a capacitor and a method for its impregnation, in the case of which, based on aromatic esters, the advantageous properties inherent in these products, such as Capacitor compatibility, high dielectric constant and biodegradability are retained, ! indcrerscits, however, the aforementioned disadvantages of these substances, namely the instability with high electrical

Load and the resulting short service life of the capacitor can be eliminated.

To solve the problem, the inventors have found that the above in connection with aromatic Problems identified with esters can be overcome by making the ester a liquid aromatic Ester to which more than 0.01% by weight of the epoxy is added.

The aromatic ester may advantageously be a derivative of phthalic acid, and for the purposes of the present invention Invention, the preferred ester is the reaction product of phthalic acid with 2-ethylhexyl alcohol, known as di (2-ethylhexyl) phthalate or simply as dioctyl phthalate - hereinafter referred to as DOP for short is.

The epoxy used in cbm impregnation agent is preferably a diglycicdyl ether of bisphenol-A. The capacitor winding is preferably made of polypropylene. iu

The process for impregnating the electrical capacitor is characterized by the following process stages:

(a) Arranging the capacitor winding in the housing,

(b) heating the capacitor under vacuum to a temperature of about 100 to 140 ⁰ C for at least 8 hours,

(c; Filling the housing under vacuum and at elevated temperature with the impregnating agent

and
(d) Soaking the condenser with heat for at least 4 hours at a temperature of 85 to 140 ° C.

The condenser is expediently cooled down after the soak and the soak is then repeated.

It has proven to be particularly advantageous if the aromatic ester is purified by means of a column purification process is purified with removal of all foreign materials and to the aromatic ester at least 1% by weight of the epoxy is added and dissolved therein.

The invention is explained in more detail below with reference to the drawing. In detail shows 1 shows a capacitor winding in which paper is used as the dielectric,

FIG. 2 shows a complete condenser in the form of a sealed container which contains the coil according to FIG. 1 contains,

Fig. 3 is a cross-section of part of a capacitor winding in which a film of a synthetic Resin is used as a dielectric,

F i g. Figure 4 is a cross-section of part of a capacitor coil in which both paper and film are made a synthetic resin is used as a dielectric,

Fig. 5 is a cross-section of a portion of a capacitor coil in which a film of a synthetic Resin is used in a different arrangement in a capacitor, and

6 is a greatly reduced illustration of an example of a power capacitor which has a plurality of Includes winding, as is usually the case as a large phase shifter, high frequency capacitor or for induction heating is used.

Typical properties of the dioctyl phthalate ( DOP), which is preferably used as an aromatic ester, are listed in Table 1 below.

Tabel. 1

Typical properties of the liquid DOP

Di (2-ethylhexyl) phthalate C ₂₄ H ₃₈ O ₄ , molecular weight = 391

Density at 25 ° C 0.987 g / cm ³

Refractive index N - ^ - 1.4859

Boiling point 229 ° C at 5 mm Hg

Flow point -55 ⁰ C

Dielectric constant (DK) 5.24 at 25 ° C

% Loss factor 7.7%. 8.5% at 100 ⁰ C and 100 Hz, measured in about 200 1 (corresponding

55 US gallons) barrels

Viscosity 30 es at 38 ⁰ C (corresponding to 100 ⁰ F)

4.2 es at 99 ° C (corresponding to 210 ⁰ F)

In Fig. 1 is an exemplary capacitor winding! 10 with a pair of covering strips 11 and 12 and paper strips 13 and 14 shown as dielectric. The covering strips 11 and 12 can also be used as a metal coating on the Paper strips 13 and 14 may be formed or on separate and additional dielectric strips of different Materials including paper and plastics. Suitable electrical conductors or taps 15 and 16 are used to f-5 the covering foils 11 and 12 with suitable capacitor connections 18 and 19 associate. The roll 10 is in the container 17 of FIG. 2 arranged and after suitable drying and evacuation of the container, it is filled with a liquid impregnating agent and sealed. the Connections 18 and 19 of the container are connected to the taps 15 and 16 of the roll 10.

Each dielectric paper strip 13 and 14 can be replaced by a variety of paper strips in order to

to get a thicker dielectric or the electrical benefit of using a variety of strips to have. Also, each strip 13 and 14 can be replaced by one or more strips 20 and 21 of synthetic resin be replaced, as shown in Fig. 3, or by a mixed dielectric from a strip of paper 13 and a resin strip 20 as shown in Figs.

In these typical embodiments, the dielectric liquid impregnant is caused to im to penetrate essentially into all gaps, pores and spaces in and between the dielectric strips 13 and 14, to penetrate and fill them. This type of impregnation considered to be essentially complete Impregnation is necessary to prevent the occurrence of damaging corona discharges in Wech-Ki self-current capacitors to reduce their nominal or application voltage and the formation of arcing to avoid. The impregnation agent located in the electrical field between the coverings is highly electrical Stresses, corona discharges and elevated and changing temperatures and others subject to damaging environmental conditions. It is believed, however, that these conditions z. B. in power capacitors no failure of the capacitor during a service life from IO to Cause 20 years.

Accordingly, great care has been taken in the field of capacitors, highly pure and tolerable Materials such as paper and chlorinated diphenyl, and great efforts are being made to to remove gases and water vapor through high temperature drying and evacuation. Impurities, that may be present in the materials and structure of a capacitor are gases, water vapor and solid impurities, such as chemical elements and compounds present in other materials are present, such as B. in paper or in polypropylene film. These chemical elements and compounds can be of a strange kind or they can be used in the manufacture of the other materials and for this reason have got into this. One source of contaminants in the condenser is the catalytic converter, used in the manufacture of polypropylene. A typical catalyst can cause impurities

2; genes in the form of salts of aluminum and titanium. Such impurities can be from the Impregnation agents are released. So is a chlorinated diphenyl impregnant Solvent, which dissolves and transfers impurities and leaches impurities from the dielectric, which are harmful to the condenser. These contaminants react unfavorably with the Impregnation agent or combine in some other way to react with the impregnation agent what leads to a degradation, which in the case of a capacitor is first noticed by the increase in the power factor. The elevated temperatures with a condenser in operation and the low ones that occur electrical and corona discharges activate the impurities with subsequent impairment of the Capacitor. The main elements known to cause early capacitor failure are hydrogen and chlorine, with the formation of e.g. B. react with hydrogen chloride. Chlorine was previously called an element undesirable in capacitor impregnants or other capacitor materials viewed. Unfortunately, for other reasons, it is considered to be a critical component in a considerable number of cases Quantities in the best available impregnants for AC capacitors, ζ. B. polychlorinated Diphenyl, remained. For these reasons, there are many additives for chlorinated diphenyl impregnants has been suggested to serve to remove HCl, chlorine, or hydrogen and so be effective and extend the life of the capacitors. These additives include tin tetraphenyl, Anthraquinone and epoxies.

As mentioned earlier, DOP has been used to impregnate AC capacitors to use, thereby preventing such DOP-impregnated AC capacitors from a exhibited high power factor and rapid deterioration. Since DOP has no known chlorine components contained, there was no compelling reason to use the listed additives. Try on with DOP impregnated AC capacitors that have been exposed to high voltage loads, such as the in connection with the F i g. 1 and 2 initially showed quite good electrical results. However, in the case of accelerated endurance tests at elevated temperatures, inadequate ones occurred to an increasing extent Capacitor failures, which are primarily caused by increasing power factors and subsequent electrical

5ü see ^A .usfaü were displayed. The tests were repeated with the capacitor of FIGS. 3 and 4, which is different from the capacitor of FIG. 1 in that a synthetic resin film replaced or was used in conjunction with paper strips 13 and 14 of FIG. Failures similar to those of the paper capacitors occurred. In both cases, an investigation revealed neither the presence of HCl nor chlorine, as would have been expected in capacitors impregnated with chlorinated diphenyl.

It was surprisingly found that the addition of an epoxy compound to the DOP, the with DOP impregnated capacitor effectively stabilized against an earlier failure and only a short service life. Repetition of the above and other suitable tests using an epoxy showed one strong reduction in failures, as shown in the following examples. In these examples, DOP

6 (i purified by a column filtration process using alumina or Fuller's earth as the filter material. The impregnation process was generally carried out in the manner described in US Pat. No. 3,363,156, including drying the capacitors by heating them for several hours at elevated temperatures such as above 100 ⁰ C and usually below 125 ° C. During this cycle, the capacitors were under a vacuum of less than 200 μ mercury. After

(.5 impregnation with DOP, which was held at about 70-80 ° C, is composed as the capacitors, and gave them in the heat for several hours, eg. 4-16 hours, at about 100 ⁰ C. In the indicated Soak time is the time it took for the condenser to reach the desired temperature, not including the time to cool down to room temperature.The times given are the times at the stated temperature.

example 1

Two groups of capacitors of 10 pieces each, as described with reference to FIGS. 1 and 2, were assembled. The dielectric paper strips 13 and 14 each comprised a pair of paper strips, one about 2.5 cm (1 inch) wide and 0.008 mm (0.3 thousandths of an inch) thick and the other about 2 thousandths of an inch wide .5 cm and a thickness of 0.009 mm (0.35 thousandths of an inch). The unsealed capacitors of Fig. 2 was heated un'er vacuum for several hours at 125 ⁰ C. Thereafter, the Group 1 capacitors were filled with purified DOP and the Group 2 capacitors were filled with the same purified DOP, but to which 1% by weight of an epoxy known as the diglycidyl ether of bisphenol-A had been added. The following results were obtained in the endurance test with these capacitors:

Endurance test

380 volts alternating current, 100 ° C

Failed capacitors
after hours

Group 1 6-4200

Group 2 (epoxy) 0-4279

550 volts AC, 85 ° C

Group 1 7-1130

Group 2 2-4205

It can be seen from these results that the failures are considerably reduced and the service life is reduced such capacitors containing the epoxy additive is extended. The first life test were the capacitors tested under the harsh conditions of 100 ° C and 380 volts alternating current. Despite under these conditions there were no failures within 4279 hours of operation, while 6 of the capacitors, containing no epoxy failed in 4200 hours. Under the even stricter conditions of the second life test, the improved results were equally surprising.

The noticeable benefit of adding epoxy to paper capacitors is worth mentioning. Normally the well-known generation of water vapor by the paper and the well-known hydrolysis appears to be more ionizable DOP ester products incompatible. However, a very small amount of the epoxy appears to be more beneficial To effect conversion than expected based on the stoichiometric proportions of the reactants were.

In the following example, capacitors of FIG. 3, d. H. those using polypropylene films 20 and 21 as Dielectric contained, subjected to similar test conditions. 4 <i

Example 2

In this example, 2 groups of 10 unsealed capacitors each were assembled, as shown in Figs. 1, 2 and 3 are shown. The dielectric consisted of biaxially oriented isotactic polypropylene strips about 48 mm (corresponding to 1 7/8 inches) wide and a thickness of about 0.009 mm (corresponding to 0.35 thousandths of an inch). The capacitors were vacuum dried at room temperature and at room temperature impregnated. The group 2 capacitors contained the same purified DOP, where however, 1% by weight of diglycidyl ether of bisphenol A was added. The capacitors were sealed after vacuum drying and impregnation at room temperature and two hours at iÖÖ "C soaked in heat. The following results were obtained:

Capacity / power factor at 300 volts AC and 85 ° C

100 ⁰ C 85 ° C DC breakdown

strength in kilovolts per 0.025 mm thickness of the polypropylene film

Group 1 3.52 / 0.39 3.58 / 0.34 5.94

Group 2 3.54 / 0.28 3.60 / 0.26 5.57

The same capacitors were subjected to a lifetime value with the following results received:

Life test

300 volts alternating current 85 ⁰ C

Group 1 Group 2 (epoxy)

Hours to failure

after 256 hours, 3 capacitors remained
after 256 hours, 7 capacitors remained

Example 3

in this example the capacitors were compared with known similar capacitors which contained chlorinated diphenyl as an impregnating agent to which about 0.3% by weight epoxy had been added. 3 groups of 10 capacitors each were assembled according to the construction of FIG. The dielectric consisted of a polypropylene film about 48 mm wide and 0.009 thick, as in Example 2. The capacitors were dried at room temperature in a vacuum for several hours and then impregnated at room temperature with the specified impregnating agent, the 1% by weight diglycidyl ether of bisphenol A had been added. The capacitors were then sealed and subjected to heat soaking for 4 hours at 100 ^° C. Endurance tests and measurements of the dielectric strength at a rate of increase of 180 volts direct current per second at 85 ^° C. are listed below:

Original dielectric strength 85 ⁰ C in kilovolts on average

Number of failed capacitors per number of tested capacitors after hours

at 300 volts

Alternating current

at 550 volts

Alternating current

85 ⁰ C

Group 1

Purified 1.92

DOP without epoxy

Group 2

Purified 1.95

DOP with 1% epoxy

Group 3

Chlorinated 1.76

Diphenyl and epoxy

7 / 10-256

6 / 9-867
(first failure
after 394 hours)

8 / 9-256

6 / 9-256

7 / 9-835

9 / 9-177

The above results show that the stabilized DOP passed the high temperature endurance test better than the other capacitors. It should be noted that the first failure of a capacitor with DOP epoxy at 380 volts alternating current and 100 ⁰ C occurred after 394 hours of operation, while with the non-stabilized DOP capacitors 7 failures occurred in 256 hours and with the capacitors as well chlorinated diphenyl as impregnation agent failed 8 capacitors in 256 hours.

Additional capacitors were assembled and treated with DOP, which was up to 10% by weight of the Contained diglycidyl ethers of bisphenol-A. So was z. B. a small capacitor with the structure of FIG. 5 containing DOP as an impregnant, and good results were obtained.

It can be seen from the above examples that the epoxy plays an important role in the service life of the capacitor plays. The epoxy stabilizer is characterized by the fact that it is in its special environment is able to interact with such chemical elements or compounds that usually present in electrical capacitors or generated during operation to prevent these connections from breaking down the DOP. These elements and compounds are those that can be generated in a capacitor that uses DOP as an impregnating agent, and this in the absence of any any materials that could generate HCJ. Most known epoxies made in a different way are compatible with capacitors appear to give the desired result to varying degrees. In Both water and acids are present or are formed in the condenser environment. The esters can split with the formation of acids and alcohols. It is believed that the epoxy with such Acids reacts and so prevents the acid from degrading the ester or causing the capacitor to fail cause. The epoxy also seems to reduce the hydrolysis problem by settling with the water in the System connects. The epoxy also appears to passivate or cover up any damage in the foil. In Capacitors with a paper dielectric can use the epoxy while stabilizing the system with the cellulose react. The function of the epoxy seems to be considerably different if only a film di-

clektrikum, d. H. there is no paper dielectric in the capacitor. One reason is that in the movie Various materials are present, such as stearates and antioxidants, that are not found in paper and it is also due to the low water content of the film. In this way, the stabilization of an ester-impregnated capacitor exclusively containing film as the dielectric can be achieved by means of a different mechanism which, however, is not yet fully known. s

While the function of the epoxy ultimately relates to an operating capacitor, it also plays an essential role in the ester in which it is dissolved. In this ester the epoxy works with the Degradation products of the ester and thus reduces the hydrolysis problem. The epoxy also combines with contaminants in the ester, which the ester prior to its use in a condenser, e.g. B. during storage, may be exposed to transport and handling. ι ο

The epoxy compound which can be used in the context of the present invention can generally thereby can be characterized as containing the following group:

-CH CH-

the z. B. is present in glycidyl ethers and derivatives of ethylene oxide. Specific examples of these compounds are phenoxypropylene oxide (phenyi-glycidyl-ether), glycidyl-allyl-ether, benzyl-ethylene oxide, styrene oxide, 1,3-bis (2,3-epoxy-propoxy) benzene and 4,4'-bis (2,3-epoxy-propoxy) diphenyldimethylmethane. Also are commercially available epoxies usable in the context of the present invention, such as di (2-ethylhexyl) -4,5-epoxy-tetrahydrophthalate, B ^ -epoxy-o-methylcyclohexylmethyl-S ^ -epoxy-o-methyl-cyclohexanecarboxylate and 1-epoxyethyKJ ^ -epoxycyclohexane. If desired, one can also use mixtures of two and use more such epoxy compounds.

Tests have shown that the specific type of epoxy is not critical. Different epoxies can be used or mixtures of epoxies can be used so long as an effective amount thereof is added. As large this effective amount depends primarily on the molecular weight and the rate of reaction and the solubility in the impregnation agent. Higher molecular weight epoxies are in larger ones Quantities to be used as such lower molecular weight epoxies. In general, are sets above 0.01% by weight and advantageously between about 0.01 and about 10% by weight of the epoxy is useful. the Epoxies act in a manner that is believed to be common to all epoxies because of their chemical structure is the same. Their response time and their effect are advantageous for the DOP in the capacitor. In the above Examples were given the main focus on comparative results of DOP with and without the use of Epoxies directed.

The epoxy can be introduced into the capacitor in a number of ways. It can be used during manufacture of the polypropylene dielectric material can be introduced into it or it can be converted to liquid Add DOP before or after inserting it into the capacitor case. Preferably the epoxy is combined with the DOP as a solution and this solution is used to impregnate the capacitor used. A main reason for this procedure is that the esters, including DOP, are sensitive are compared to increased temperatures and that they are subject to strong changes with increasing temperatures could be. Therefore, the addition of the epoxy to the ester stabilizes before heating, especially during of the impregnation process, the ester, not to mention the stabilization in the condenser.

DOP is compatible with both the polypropylene film dielectric alone and with paper. The preferred mode of impregnation is described in US Pat. No. 3,363,156 and is referred to as essentially complete impregnation. One form of essentially complete impregnation includes heating the impregnated capacitor to an elevated temperature, preferably above 85 ^° C. for a longer period of time, in order to not only cause the DOP to penetrate into the molecular structure of the polypropylene, but also this Also converting polypropylene into a type of semi-permeable membrane for the DOP, allowing the DOP to penetrate the film. DOP penetrates the polypropylene film more slowly than chlorinated diphenyl. The best results were obtained when more intensive evacuation or drying was used. A 24-hour drying cycle with temperatures of 130 to 140 ^° C. was used successfully, the DOP being introduced into the condenser at 100 ^° C.

Soaking can be continued as a specific cycle after the condenser has been impregnated and sealed, and it is an important criterion. The heat-soaking can take place at temperatures above 85 ° C and preferably in the range of 100-120 ⁰ C. The best results were obtained with the larger and more difficult to impregnate capacitors when a variety of post-impregnation heat soaks were used. A first heat soak was carried out by placing the condensers in an oven and increasing ^{the temperature to around HO 0 C.} After 8 hours at this temperature, the capacitors were allowed to cool to about room temperature. The temperature of the furnace was then increased to 110 ^° C. and this temperature was maintained for a further 8 hours. Comparative tests have shown that the multiple hot drinks described leads to better results than a single hot drink carried out over 16 hours. A preferred method for the production of DOP-impregnated capacitors in which paper is used includes a method performed at about 120 to 14O ⁰ C Trock- <> 5 then NEN under vacuum, impregnating at above 100 ⁰ C, followed by repeated heat soaking , as described. If only films are used as the dielectric, drying can be carried out at lower temperatures and for shorter times.

The polypropylene film, as described in US Pat. No. 3,363,156, is a stereoregular crystalline, biaxially oriented film, which is also preferred in the context of the present invention and in all examples was used. By crystalline it is meant that the material is substantially crystalline Proportion and the crystallinity determines the physical properties of the material. The usage of DOP is not limited to the listed dielectrics and other members of the Polyfm group, as well as other synthetic resins such as polycarbonates polysulfone and polyester are used as dielectrics will. The significant feature is the use of the DOP together with an epoxy stabilizer. The stabilized impregnation agent, especially DOP, is an improved impregnation agent for those capacitors that are exposed to high voltage loads and high temperatures. A high

ι υ voltage load for the dielectric, if this is a film made of a synthetic resin, such as polypropylene, is about 750 volts per 0.025 mm (equivalent to 1/1000 of an inch) thickness of the polypropylene up to 1200 volts per 0.025 mm, with the more critical part of the high load being in a range around about Starts 900 volts per 0.025 mm and extends up to about 1400 volts per 0.025 mm. In addition, there will be such Stress-exposed capacitors are impregnated in a manner described in US Pat. No. 3,363,156 is described and referred to as essentially complete impregnation and which has a certain reproducibility of the results, the z. B. brings a high corona start voltage with it, the relationship is related to the thickness of the dielectric. In high voltage power capacitors for shunt applications, in which the total thickness of the dielectric between the electrodes is of the order of 0.025 mm, the corona starting voltage at room temperature must generally be above

2½ 2000 volts and in many cases it must be higher than 2500 volts. For applications with low voltages, where thinner dielectrics are used, the corona starting voltage can be lower. the Corona starting voltage is usually 1 1/2 to 2 1/2 times the highest voltage load in the dielectric at the application voltage of the capacitor at room temperature and it is under variable operating conditions of the capacitor stable.

DOP is useful in many different types of single dielectric systems Material, such as only paper, only film or a mixture of these. An example of a mixed dielectric System is shown in the arrangement of Fig. 4, in which a sheet of paper 13 adjacent the one Electrode 11 is located. The embodiment of Figure 5 can be seen that other mixed dielectrics two films 20 and 21 with a sheet of paper 13 in between, or vice versa, two

3d sheets of paper with a film in between to be used.

F i g. Figure 6 depicts a high voltage phase advancing capacitor that requires a low power factor. The condenser 22 comprises a large container 23 with a volume of approximately 23 liters (corresponding to 0.8 US feet ³ ) in which a large number (10-40) of elongate coils 10 are arranged. These coils 10 can be about 25-64 cm (10-25 inches) in length. To be effective, the DOP impregnant and additive must have penetrated through each roll 10, since failure of only one roll 10 will cause failure of the entire capacitor. Therefore, these power capacitors 22 are subjected to exceptional drying conditions, e.g. B. they are exposed to ^{temperatures of 100 to 150 0} C at a pressure of about 200 μ of mercury for 15 to 30 hours. While they are still under vacuum and at an elevated temperature, they are filled with the impregnating agent. Typically, the impregnating agent is filled into the housing at the well at a temperature of about 70-80 ⁰ C. Then, the capacitor is usually sealed and again exposed to elevated temperatures of about 80- 120 ⁰ C door longer times, that of the capacitor of the size of and the type of dielectric material used. All paper capacitors require a minimum time, but not subsequent heating. All film capacitors may require soaking for 16-24 hours.

Example 4

Investigations were made with other aromatic esters as impregnating agents, the epoxy additives contained. Such a test was carried out with typical capacitors, as in the examples above

5 (i carried out in which the dielectric was polypropylene and used as the impregnation agent dicapryl phthalate became. The epoxy was used in an amount of 1% by weight. These capacitors were at 550 volts alternating current and 85 ° C with an extremely high film load of 1570 volts per 0.025 mm polypropylene thickness tested. Surprisingly, these capacitors worked after 9500 hours of operation just as good as control capacitors that use chlorinated diphenyl as an impregnant was used.

Example 5

Capacitors with the configuration according to FIGS. 2 and 5 produced. The dielectric is bestanc

One strip of about 0.012 mm (0.5 / 1000 ") thick paper about 92 mm (3.625") wide and two strips of about 0.017 mm (0.7 / 1000 ") thick polypropylene film of the same width , μί. 5 the capacitors were dried for 24 hours under vacuum at 130-140 ⁰ C and then impregnated bc 100 ⁰ C. were investigated 2 groups of capacitors. the first group contained as Imprügnic agent DOP with 1 wt .-% and the epoxide other group used as impregnation agent

(ö 80 Gcw .-% DOP and 20 wt .-% dodecyl benzene. To this mixture were added 1 wt .-% of the epoxide. After that the capacitors were sealed and heat soaked for 8 hours at 100 ⁰ C. The capacitors WUR to a corona - Subjected to the starting voltage test and soaked in heat for a further 8 hours. Then a corona starting voltage test was carried out again on the capacitors

the corona starting voltage. This means that the corona starting voltage was determined by the second heat soaking process noticeably increased what was required to bring the capacitor to a satisfactory level of the corona starting voltage and that for the impregnating agent made from DOP and dodecylbenzene was higher than for the DOP alone as an impregnating agent.

Not all dielectric fluids are satisfactory as impregnating agents for capacitors. A dielectric liquid should have the following general properties; it should be in a cleaned or cleanable form, have a boiling point and freezing point outside the operating temperature of the condenser and a flash point above 175 ° C. In addition, the liquid should have a vapor pressure below atmospheric pressure at temperatures up to 200 ⁰ C and preferably up to 300 ⁰ C and a dielectric constant above 2, especially with synthetic film dielectrics such as polypropylene, and preferably 4 and more for paper Dielectrics. In addition, the liquid should have a relatively low viscosity, namely, less than 1000 remain centistokes at 25 ° C to -40 ⁰ C a liquid.

Power factor or dissipation factor is an extremely important criterion for a capacitor, particularly an AC phase shifting capacitor, because it typically operates at elevated temperatures and is normally exposed to such temperatures during manufacture. The power factor increases rapidly with temperature. The power factor of the purified impregnant itself should be considerably less than 10% and preferably less than 5%, measured at 100 ^° C. and 100 Hz, so that the power factor in the capacitor can be reduced to less than about 1%. This low power factor must be maintained for long lifetimes extending up to several years.

In addition, the impregnation agent should be compatible with the other materials of the capacitor and it should be able to withstand the changing operating temperatures at high voltage loads. Ease of handling, impregnation, and other physical properties are desirable. Additionally it is very desirable that the impregnating agent be readily biodegradable as compared with chlorinated ones Diphenylene, and has a low level of toxicity.

The aromatic esters most suitable for the purposes of the present invention meet these requirements mentioned criteria and are therefore preferred in the context of the present invention. Preferred esters found in otherwise suitable dielectric tests are the reaction product of an aromatic acid with an alcohol. A typical compound has the following formula:

where R is, for example, the aromatic substituent or radical of pyromeilitic acid, terephthalic acid, phthalic acid, trimellitic acid, trimesic acid or one of the groups phenyl, naphthyl, bisphenyl, tolyl, etc., X can represent an integer from 1 to 4 and where R 'is an alkyl or aryl group such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, etc. These are straight chain alkyl groups. However, branched alkyl groups such as 2-ethylhexyl, isopropyl, isobutyl, isooctyl, etc. can also be used. Examples of aromatic esters of phthalic acid useful in the present invention include dimethyl, diethyl, dipropyl esters, and so on . An example of an ester of benzoic acid is butyl benzoate.

In addition to the esters of organic acids, the reaction products of other acids, in particular the Phosphoric acid can be used with the above alcohols. Specific examples include tricresyl phosphate and triphenyl phosphate. They are also amenable to degradation by hydrolysis or oxidation therefore, the presence of the epoxides also has an advantageous effect on them. The aromatic esters are in the present invention unique in that the response properties are predictably similar, especially for the phihalate ester, so if the other component, if any, is otherwise a good one Capacitor impregnant is useful, the combination of the aromatic ester and the epoxy is.

The esters can be mixtures of esters or mixtures of esters and others otherwise satisfactory Include waterproofing agents. It is most preferred that the impregnating agents used contain the ester of the present invention as a main ingredient. For the purposes of the present Invention may comprise a mixture of mixtures of esters according to the present invention, for example DOP and dibutyl phthalate as well as DOP and didodecyl phthalate. The mixture can also be an ester according to the present invention together with an aliphatic ester such as dibutyl sebacate or castor oil, include. Further, the mixture can contain the ester of the present invention and a hydrocarbon in general, such as a mineral oil, alkyl naphthalenes, polybutenes, and the like. Specific examples of this Mixtures are DOP and mineral oil as well as DOP and dodecylbenzene. The mixtures can also be used by other (> u Esters such as phosphates e.g. B. tricresyl phosphate and triphenyl phosphate. The mixtures are used in order to provide the impregnant with properties which are different from those of the esters of the present invention Invention are different, such as an increased dielectric constant. The added material can also be in the form of a diluent or an impregnation aid, e.g. B. a wetting agent be used. An example of a material that performs a wide variety of functions is doclecylbenzene, which acts as a wetting agent and as an impregnating agent and is therefore used in relatively large quantities can be. While it is preferred that the ester of the present invention be in the mixture so far the electrical properties are affected, dominates, it is also within the scope of the present invention,

that lesser amounts of the ester of the present invention can be used. So can 10 to 40% by weight of the ester can be added to other impregnating agents in order to achieve their properties change. In the context of mixtures in the present invention, the mixture can be used in large amounts a suitable epoxy, the epoxy serving both functions, i. H. a stabilizer and a 5 to be impregnation agent.

The invention is specifically carried out with aromatic esters as described, which are the most common show improved results. An aromatic ester stabilized with epoxy, such as DOP, makes an impregnating agent, that is unexpectedly the same or in some cases better than the current best Impregnation agent, chlorinated diphenyl.

1 sheet of drawings

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Claims (6)

Patent claims: 1. Electrical capacitor with at least one capacitor winding in a sealed housing, wherein the impregnating agent is not halogenated and consists of a liquid ester with an addition of consists of less than 10% by weight of an epoxy compound stabilizing the ester, characterized in that that the ester is an aromatic ester to which more than 0.01% by weight of the epoxy is added are. 2. Capacitor according to claim 1, characterized in that the aromatic ester is a phthalic acid ester is. 3. Capacitor according to claim 2, characterized in that the phthalic acid ester is an ester of 2-ethylhexyl alcohol is. 4. Capacitor according to one of claims 1 to 3, characterized in that the epoxy is a diglycidyl ether of bisphenol-A is. 5. Capacitor according to one of claims 1 to 4, characterized in that the capacitor winding made of polypropylene. 6. A method for impregnating an electrical capacitor according to any one of claims 1 to 5, characterized by the following process steps: