GB1571630A

GB1571630A - Electrical capacitor

Info

Publication number: GB1571630A
Application number: GB4445/77A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1976-02-03
Filing date: 1977-02-03
Publication date: 1980-07-16
Also published as: JPS5296400A; SE7701128L; IT1081702B; DD129011A5; FR2340604A1; DE2604004A1

Description

(54) AN ELECTRICAL CAPACITOR (71) We, ROBERT BOSCH, G.m.b.H., a German Company, of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, 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: The invention relates to a self-healing capacitor which is impregnated with an impregnating agent and has mixtures of paper and synthetic material tapes in particular for a voltage range of less than 1000V.

Self-healing capacitors generally comprise metallized paper tapes alone or in combination with other non-metallized paper tapes (one to three-layer arrangement). They are primarily used for alternating-current loading and are impregnated with mineral oils or other non-polar fluids having a relative permittivity between 20 and 25.

It would be advantageous to use impregnating means having a greater relative permittivity since in this manner the capacitance of the capacitor could be increased without increasing the size of the capacitor or this size could be reduced yet still retaining the same capacitance. Moreover, such capacitors could be loaded electrically to a greater degree without causing harmful discharges in the impregnating agent, and so shortening the service life of the agent. Impregnating agents - having a greater relative permittivity have already been used for some time in capacitors other than self-healing capacitors. These are mainly chlorinated hydrocarbon fluids, for example polychlorinated biphenyls.

These substances have a relatively high relative permittivity of 5 5 to 6 and also have a favourable electrical conductivity and dissipation factor.

However these substances, which are suitable not only for capacitors, are hardly biologically degradable. Because of - en- vironmental pollution. either during operation when devices are not completely sealed or through improper scrapping thereof - manufacture and use is restricted to an increasing extent. In Japan and Scandinavia for example, the use of chlorinated hydrocarbons in small capacitors has recently been banned. For this reason, a variety of substitute substances has been proposed for impregnating capacitors, for example compounds of halogenized ethers, of diaryl sulphonate and halogen-free aromatic compounds, mixtures of halogen-free esters and ethers, or mixtures of esters with polar or with nonpolar substances.Certainly, compared with chlorinated hydrocarbons, these substitute substances have a lower relative permittivity of 4-1 to 4-9. However this would be no disadvantage for their use in selfhealing capacitors since in this field impregnating agents having a relative permittivity of 2 to 3 have had to suffice till now. What is more significant is the fact that the conductivity and the dissipation factor of these substitute substances are substantially poorer than those of the mineral or synthetic oils generally used at present for selfsealing capacitors.Experiments with single and double layer metallized paper capacitors, which were impregnated for the purpose of comparison either with mineral oil having a relative permittivity of about 2-5 or with a halogen-free ester having a relative permittivity of about 4.5, were therefore anything but encouraging.

It is therefore, an aim of the present invention to develop a self-healing capacitor which has a higher specific capacity and a higher flashover potential than the previously known constructions without adversely affecting its dissipation factor and conductivity. The capacitor according to the - invention should also be particularly ecologically - acceptable.

According to the present invention there is provided a self-healing capacitor impregnated with a halogen-free substance, which is fluid at room temperature and which has a relative permittivity of approximately 4 0 to 5 0,- and having at least one Iayer of syn thetic material disposed between layers of different polarity.

The invention is based on the surprising discovery that use of an intermediate layer of a synthetic material substantially reduces the adverse effect exerted by substances of a higher relative permittivity upon the dissipation factor and the conductivity of the capacitor. In using an ester as an impregnating agent, this being proposed as a substitute for the pollutant chlorinated hydrocarbon, an even lower dissipation factor is obtained than with mineral oil impregnation although this ester has only one-thou sandth of the conductivity and one-hundredth of the dissipation factor of the chlorinated hydrocarbon.

The cause of this surprising result seems to lie in the following: the electrical field strength in the impregnating agent, in the dielectric which is preferably paper and in the synthetic material (if this lies in the electrical field) is inversely proportional to the relative permittivity of the individual material. With oil impregnation the field strengths in the oil and in the synthetic material are approximately identical since the respective relative permittivities are approximately equal. In the case of impregnation with a substance of a higher relative permittivity, for example, between 4 and 5, the field strength in the impregnating agent drops while the field strength in the synthetic material increases.Since the synthetic material has the lower losses and the impregnating agent has the higher losses, the total dissipation factor of the dielectric decreases, the higher the electrical load in the synthetic material or the lower the field strength in the impregnating agent. Thus, the contribution of the heavier-loss impregnating agent to the total dissipation factor may be completely or even over-compensated.

With mineral-oil-impregnated metallized paper capacitors it is admittedly already known practice to use a mixed dielectric.

Thus for example, paper tapes which are damped on one side are wound together with undamped synthetic tapes or paper tapes which are metallized on both sides are separated by a layer of non-metallized synthetic material. What is common to these capacitors is the possibility, and also the economic necessity, of operating them with a field strength of 40 to 80V/slm which is maybe higher than for the previously mentioned capacitors.Because of the unfavourable characteristic values of impregnating agents, e.g. electrical conduc ti'ity, having a higher relative permittivity, the idea of impregnating such highly loaded capacitors with such a material which, even with capacitors loaded to a substantially ester degree, leads to an inadmissable de terioration must have been all the more improbable.

It is advantageous for the impregnating agent to be an ester stabilized with addi tives, in particular a dioctyl phthalate with additives of epoxide and hydroquinone.

The layers of synthetic material may, for example, be made from poly-carbonate or polypropylene.

To manufacture a roll capacitor of the above type it is advantageous for the roll to be separately impregnated and then in stalled in the capacitor housing and for the capacitor, after a short vacuum treat ment, to be filled through a filling opening in the housing and then tightly sealed.

The invention is described further, by way of example with reference to the ac companying drawings, illustrating test re sults, in which drawings: Fig. 1 is a graph illustrating the depen dence of the dissipation factor on tempera ture, in the cases of a conventional capaci tor impregnated with mineral oil, and an experimental capacitor impregnated with a halogen-free ester but not provided with intermediate layers of synthetic material; Fig. 2 is a graph showing the dissipation factors of the two capacitors at various temperatures; Fig. 3 is a diagram comparing the D.C.

resistances of the impregnation agents of the two capacitors at 800C; Fig. 4 is a diagram showing how the dis sipation factor of one capacitor according to the invention compares with that of a conventional capacitor, and a view of said capacitor according to the invention; and Fig. 5 is a diagram showing how the dis sipation factor of another capacitor accord ing to the invention compares with that of a conventional capacitor, and a view of said other capacitor according to the invention.

Fig. 1 shows the dependence of the dissi pation factor of the more favourable two layer construction in dependence upon the temperature at two different voltages. The results from the capacitors impregnated with the substitute material of a higher relative permittivity are, as was anticipa ted, considerably poorer than those of the oil-impregnated capacitors having a lower relative permittivity. Already at a working field strength of approximately 20V/m the higher specific capacity of the ester impregnated capacitor was gained at the cost of a deteriorated operating behaviour, in particular a higher operating tempera ture and a shorter service life. The reason for the higher dissipation factor lies in the poorer characteristic values of the impreg nating agent, e.g. electrical conductivity.

Fig. 2 shows the dissipation factor at dif- ferent temperatures, Fig. 3 shows the direct-current resistance at 800 C of the mineral oil and of the halogen-free ester.

The dissipation factor of the ester is higher by over two decimal powers and the directcurrent resistance is more than three decimal powers lower than is the case with the mineral oil. The minimum values according to the Bosch-Standard are shown by line A.

Fig. 4 shows the course of the dissipation factor in dependence upon the temperature for a capacitor which was impregnated with mineral oil (Shell K 8) and for a capacitor which was impregnated with an ester (Bayer OC 4200). The relative permittivity was in the first example approximately 2-3 and in the second example approximately 4.5. The capacitor construction is illustrated in the right-hand top corner of Fig. 4. It comprised two metallized papers 1, 2 which were damped on both sides and wound with two poly-carbonate layers 3, 4. The curve shows clearly that the dissipation factor of the ester-impregnated capacitor is on the whole lower than that of the mineral oli-impregnated capacitor.The, by orders of magnitude, poorer characteristic values relative to the dissipation factor are therefore no longer justifiable.

Conditions are much more favourable in Fig. 5 where two papers 5, 6 which are metallized and damped on one side are wound with two polypropylene layers 7, 8.

The impregnating agents were the same as those in Fig. 4. The dissipation factor with ester impregnation is on average approximately 25% below that with mineral oil impregnation, as Fig. 5 clearly demonstrates.

The combination of features according to the invention therefore makes it possible to operate with impregnating agents whose high relative permittivity permits a higher specific capacity and a higher electrical loading of the capacitor, the values for the dissipation factor being as good as or better than those with mineral oils.

It is also within the framework of the present invention and the advantages to be achieved thereby to use synthetic rolls which are absorbent in such a manner that there is no need for paper layers. Synthetic strips having a roughened surface or synthetic fleece may be considered for this purpose.

WHAT WE CLAIM IS:- 1. A self-healing capacitor impregnated with a halogen-free substance, which is fluid at room temperature and which has a relative permittivity of approximately 4-0 to 5.0, and having at least one layer of synthetic material disposed between layers of different polarity.

2. A capacitor as claimed in claim 1, which is a metallised paper capacitor having interspersed strips of paper and synthetic material.

3. A capacitor as claimed in claim 1 or 2, in which the impregnating agent is an ester stabilized with additives.

4. A capacitor as claimed in claim 3, in which the ester is a dioctyl phthalate.

5. A capacitor as claimed in claim 3 or 4, in which the additives are of epoxide and hydroquinone.

6. A capacitor as claimed in any of the previous claims, in which the layer or layers of synthetic material is or are made of polycarbonate.

7. A capacitor as claimed in any of claims 1 to 5, in which the layer or layers of synthetic material is or are made of polypropylene.

8. A capacitor as claimed in any of the previous claims, in which the substance used as an impregnating agent also serves as a filling agent.

9. A method of manufacturing a capacitor as claimed in any of the previous claims, in which a capacitor roll is impregnated and is installed in a capacitor housing which, after a short vacuum treatment is filled through a filling opening therein and is tightly sealed.

10. A self-healing capacitor constructed substantially as herein particularly described.

11. A method of manufacturing a capacitor substantially as herein particularly described.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. mineral oil and of the halogen-free ester. The dissipation factor of the ester is higher by over two decimal powers and the directcurrent resistance is more than three decimal powers lower than is the case with the mineral oil. The minimum values according to the Bosch-Standard are shown by line A. Fig. 4 shows the course of the dissipation factor in dependence upon the temperature for a capacitor which was impregnated with mineral oil (Shell K 8) and for a capacitor which was impregnated with an ester (Bayer OC 4200). The relative permittivity was in the first example approximately 2-3 and in the second example approximately 4.5. The capacitor construction is illustrated in the right-hand top corner of Fig. 4. It comprised two metallized papers 1, 2 which were damped on both sides and wound with two poly-carbonate layers 3, 4. The curve shows clearly that the dissipation factor of the ester-impregnated capacitor is on the whole lower than that of the mineral oli-impregnated capacitor.The, by orders of magnitude, poorer characteristic values relative to the dissipation factor are therefore no longer justifiable. Conditions are much more favourable in Fig. 5 where two papers 5, 6 which are metallized and damped on one side are wound with two polypropylene layers 7, 8. The impregnating agents were the same as those in Fig. 4. The dissipation factor with ester impregnation is on average approximately 25% below that with mineral oil impregnation, as Fig. 5 clearly demonstrates. The combination of features according to the invention therefore makes it possible to operate with impregnating agents whose high relative permittivity permits a higher specific capacity and a higher electrical loading of the capacitor, the values for the dissipation factor being as good as or better than those with mineral oils. It is also within the framework of the present invention and the advantages to be achieved thereby to use synthetic rolls which are absorbent in such a manner that there is no need for paper layers. Synthetic strips having a roughened surface or synthetic fleece may be considered for this purpose. WHAT WE CLAIM IS:-

1. A self-healing capacitor impregnated with a halogen-free substance, which is fluid at room temperature and which has a relative permittivity of approximately 4-0 to 5.0, and having at least one layer of synthetic material disposed between layers of different polarity.