GB2032950A

GB2032950A - Electrical Insulating Oil

Info

Publication number: GB2032950A
Application number: GB7928935A
Authority: GB
Original assignee: Kureha Corp; Nissin Electric Co Ltd
Current assignee: Kureha Corp; Nissin Electric Co Ltd
Priority date: 1978-09-07
Filing date: 1979-08-20
Publication date: 1980-05-14
Also published as: DE2935211A1; FR2435785A1

Abstract

An electrical insulating oil comprising a compound having the structural formula of <IMAGE> wherein R is CH3 or C2H5. The invention also provides electrical device elements impregnated with either of the above insulating oils or a mixture thereof.

Description

SPECIFICATION Electrical Insulating Oil This invention relates to an electrical insulating oil and electrical devices using as the insulating or dielectric material insulating paper and/or plastic film impregnated with the insulating oil.

In various electrical devices such as capacitors, transformers, and power cables, insulating paper and/or plastic film are widely used as the dielectric or insulating material, and mineral oil is the commonest of the various kinds of insulating oil with which the devices are impregnated.

Mineral oil, however, has such a high pour point and a high viscosity that it is difficult to have insulating or dielectric material deeply impregnated with the oil. Therefore, the electrical devices impregnated with mineral oil are inferior in electrical characteristics such as the corona characteristic and the dielectric breakdown strength which chiefly affects the life of the devices. Since the viscosity increases as the temperature drops, the electrical characteristics of the electrical devices impregnated with an insulating oil having a high viscosity tend to be greatly deteriorated in a cold environment.

Accordingly, the primary object of the invention is to improve the electrical characteristics of oilimpregnated electrical devices.

Another object of the invention is to make oil-impregnated electrical devices compact in size and light in weight.

A particular object of the invention is to improve the electrical characteristics of oil-impregnated electrical devices using plastic film as the insulating or dielectric material.

Another object of the invention is to provide an electrical insulating oil to be used in oilimpregnated electrical devices for improvement of their electrical characteristics.

The electrical devices of the invention are impregnated with an insulating oil consisting of a compound having the structural formula of

wherein R is selected from the group consisting of CH3 and C2H5.

The compound in which R is CH3 is 1-phenyl-1-(methylphenyl) ethane, which will be referred to as MDE hereinafter for simplicity, while the compound in which R is C2H5 is 1- phenyl-1 -(ethylphenyl) ethane, which will be referred to as EDE.

The electrical devices of the invention are also impregnated with a mixture of EDE and MDE.

Thus, the invention is characterized by the use of either EDE or MDE, or a mixture of the two compounds as the insulating oil in electrical devices. The mixture of EDE and MDE will be referred to as E/MDE hereinafter for simplicity.

EDE, MDE and E/MDE are lower in the pour point and viscosity and higher in the flash point, visible gas generating voltage and dielectric constant than mineral oil. In other words, the compounds are superior as electrical insulating oil to mineral oil in the electrical characteristics.

In the electrical devices which use as the insulating or dielectric material the plastic film which is nonporous and has a smoother surface than insulating paper, it is difficult for the insulating oil to penetrate through the film. Low viscosity of oil increases its ability to penetrate through plastic film. In this respect, the fact that the oils of the invention are lower in viscosity than mineral oil improves the electrical characteristics of the devices impregnated therewith such as the dielectric breakdown strength, etc.

The oils of the invention are superior to mineral oil also with respect to the swelling property and solubility of plastic film. If plastic film has a high swelling property, that is, it absorbs much oil so as to be swollen thereby, the apparent volume of the film increases so that the gaps or passages between the film layers for the oil to pass through decrease with resulting reduction in circulation of the oil therethrough. If the plastic film absorbs much oil, voids are produced between the film layers. As a result, the temperature of the electrical device rises with simultaneous reduction of its dielectric breakdown strength. If a large amount of the plastic film is dissolved in the oil the electrical characteristics that the film inherently has are deteriorated.

By using the oil of this invention with plastic film, it is possible to suppress such temperature rise, improve the dielectric breakdown strength of the electrical device and prevent deterioration of the characteristics of the film.

The oil of this invention is particularly suitable as insulating oil for use in electrical capacitors. In a capacitor impregnated with the oils of the invention the energy density (which is expressed by the dielectric constant E multiplied by the square of the potential gradient (V/ym)) is remarkably increased as compared with the capacitor impregnated with mineral oil.

As is well known, since the volume of a capacitor is proportional to the inverse number of the energy density thereof, an increase in the energy density of a capacitor results in a decrease in the volume of the capacitor.

The capacitor of this invention on which an excess voltage is impressed for a long period of time is less likely to suffer from dielectric breakdown than the capacitor impregnated with mineral oil or the other conventional insulating oils.

The invention will be described in detail with reference to the accompanying drawings, wherein: Fig. 1 is a graph showing the percentage of the capacitors using insulating paper alone as the dielectric that survive continuous impression of alternating voltage thereon without dielectric breakdown; Fig. 2 is a graph showing the percentage of the capacitors using as the dielectric both insulating paper and polypropylene film that survive continuous impression of alternating voltage thereon without dielectric breakdown; Fig 3 is a graph showing the percentage of the capacitors using as the dielectric polypropylene film alone that survive continuous impression of alternating voltage thereon without dielectric breakdown:: Fig. 4 is a graph of characteristic curves showing the relation between the heating time and the corona starting voltage of the capacitors the dielectric of which is polypropylene film along; Fig. 5 is a graph of characteristic curves showing the relation between the heating time and the corona starting voltage of the capacitors the dielectric of which comprises both insulating paper and polypropylene film; and Fig. 6 is a graph of characteristic curves showing the temperature rise of transformers in temperature rise test.

The characteristics of the oils of the invention will first be explained. The dielectric characteristics of the oils of the invention are given in Table I, wherein the characteristics of mineral oil (to be referred to as MO hereinafter), and those of 1-phenyl-1-(dimethylphenyl)ethane (to be referred to as PXE hereinafter) and alkylbenzene (to be referred to as AB hereinafter) are also given for comparison. PXE and AB have recently been proposed in the field of art as insulating oil for use in oil-impregnated electrical devices.

As shown in the table, the oils of this invention can be distinguished from the prior art oils in that the viscosity of the oils of the invention is below 5 (cst) at 300C.

Table I EDE MDE E/MDE MO PXE AB Specific graviiy 0.980 0.983 0.982 0.879 0.988 0.870 Flash point (OC) 148 144 146 135 148 136 Pour point (OC) < -60 < -60 < -60 -32 -47.5 < -60 Viscosity3O0C(cst) 4.1 4.0 4.1 10 6.5 10 0 C (cst) 10.5 10.5 10.5 60.3 22.3 55.2 Visible gas generating voltage(kV/mm) 81 81 81 45 78 52 Dielectric constant (8000,0Hz) 2.45 2.45 2.45 2.20 2.49 2.17 Dielectric loss (80 C,60 Hz) (%) 0.01 0.01 0.01 0.005 0.01 0.007 As is obvious from the above, EDE, MDE and E/MDE are lower than MO and PXE in the pour point and viscosity particularly at low temperature, and lower than AB in viscosity.With respect to the dielectric constant and visible gas generating voltage, EDE, MDE and E/MDE are superior to MO, etc.

The swelling property and solubility of plastics film, particularly polypropylene film, (to be referred to as PP film), with respect to oil will now be explained. Table II shows the swollen amount and dissolved amount of PP film as well as those amounts caused by conventional oils.

As can be seen from the table, the oils of this invention can be distinguished from the conventional oils in that the swollen amounts by the former are less than 2% at 800C.

Table II EDE MDE E/MDE MO PXE AB Swollen amount (%) 1.4 1.4 1.4 8.6 2.9 8.5 Dissolved amount (%) 0.06 0.06 0.06 0.15 0.10 0.16 The values given above are all those measured after immersion of PP film in the oils at 800C for 30 days. As is obvious from the table, the oils of the invention are less likely to be absorbed by PP film than MO, PXE and AB, and PP film is less likely to be dissolved in the oils of the invention than in the prior art oils.

As previously mentioned, these characterisitcs have good effects on the electrical devices impregnated with the oils of the invention.

The improved electrical characterisitcs of the electrical devices impregnated with the oils of invention will now be described.

Example 1 Capacitors of type No. 1 to No. 3 were prepared and tested.

In type No. 1, four sheets of 25,um thick insulating paper having a density of 0.85 g /cm3 were wound to form the dielectric element of the capacitor. The capacitance was 1.5 yF.

In type No. 2, a sheet of 1 8 ym thick insulating paper having a density of 0.80 g/cm3 was sandwiched between two sheets of 1 8 ym thick PP film and the three sheets were wound to form the dielectric element of the capacitor. The capacitance was 1 2 juF.

In type No. 3, two sheets of 1 8 zbm thick PP film were wound to form the dielectric element of the capacitor. The capacitance was 10 4F.

The capacitors to be tested were impregnated with the oils of the invention and MO, PXE and AB, and the dielectric constant E, the potential gradient G (V/,um) and the energy density EG2 were measured. The results of the measurement are given in Table Ill.

The corona starting voltage (V/ym) was also measured at different temperatures. The results of the measurement are given in Table IV.

Table III Capacitor Oil type No. E G EG2 1 3.70 27 2697 EDE 2 2.67 40 4272 3 2.25 41 3782 1 3.70 27 2697 MDE 2 2.67 40 4272 3 2.25 41 3782 1 3.70 27 2697 E/MDE 2 2.67 40 4272 3 2.25 41 3782 1 3.70 27 2697 PXE 2 2.67 38 3855 3 2.25 37 3080 1 3.2 17 925 MO 2 2.62 32 2683 3 2.23 32 2283 Table IV Capacitor Oil Type No. --40 OC 20 OC BC 0C 1 47 65 90 EDE 2 65 90 120 3 65 95 120 1 47 65 90 MDE 2 65 90 120 3 65 95 120 1 47 65 90 E/MDE 2 65 90 120 3 65 95 120 1 35 63 90 PXE 2 50 88 110 3 47 93 110 1 17 55 70 MO 2 15 65 75 3 15 75 75 As can be seen from Table III, the capacitors impregnated with EDE, MDE or E/MDE had their energy density remarkably improved over those impregnated with MO or PXE.To take type No. 1 for example, the energy density of the capacitors impregnated with the oils of the invention is about three times that of the capacitor impregnated with MO. As previously mentioned, the volume of a capacitor is proportional to the inverse number of its energy density. In this respect, therefore, impregnation of a capacitor with EDE, MDE or E/MDE reduces the volume of the capacitor to about one-third of that of a capacitor impregnated with MO.

It can be seen from Table IV that in the capacitors of the invention the corona starting voltage and consequently the dielectric breakdown strength become higher than in the capacitor impregnated with MO or PXE particularly at low temperature. This is believed to be due to the fact that as compared with MO, etc. the oils of this invention have lower pour points and lower viscosity values particularly at low temperature.

The spreading characteristics of the oils of the invention and those of MO, etc. were also measured on PP film. The area MO spread over was 7x 103 mm2 and the area PXE spread over was 1 8x 103 mm2, whereas the areas, EDE, MDE and E/MDE spread over were all as 23 x 103 mm. This shows that the oils of the invention have better spreading characteristic than MO, etc. This is believed to be due to the fact that the oils of the invention penetrate smoothly through PP film layers to immerse them, with resulting improvement in the above-mentioned characteristics.

The area the oils spread over was measured in the following manner: 0.02 cc of each of the oils was applied onto a sheet of PP film, onto which another sheet of the film was quietly dropped from 2 cm above. After 20 minutes the area of the oil drop spread over in the film sheet was measured.

Figs. 1 to 3 show the percentage of the capacitors that survive a continuous impression thereof of an alternating voltage one and a half times higher than the rated voltage while the capacitors are kept at 800C. Line A indicates the result obtained from the capacitors impregnated with EDE, MDE and E/MDE individually; the stepped line B indicates the result obtained from the capacitors impregnated with MO; and the bent line C indicates the result obtained from the capacitors impregnated with PXE.

In Fig. 1 the capacitors are of the previously mentioned type No. 1, in Fig. 2 they are of the type No. 2 and in Fig. 3 they are of type No. 3. Ten capacitors for each kind of the oils and for each of the three types were tested.

In Fig. 2, for example, dielectric breakdown occurred in 20% of the capacitors impregnated with MO on the 100th day, and in additional 30% on the 120th day and in additional 20% on the 130th day.

After ail, only 30% of the capacitors survived.

On the other hand, none of the capacitors impregnated with each of EDE, MDE and E/MDE suffered from dielectric breakdown upon lapse of 140 days. The same is true with the capacitors of the other types. Thus, the capacitors impregnated with the oils of the invention can enjoy a longer life than the prior art capacitors.

Fig. 4 and 5 are characteristic diagrams wherein the corona starting voltage (Vum, 250C) of the capacitors impregnated with the oils and then heated at 7O0C is plotted against the heating time. In Fig. 4 the capacitors are of the previously mentioned type No. 3 and in Fig. 5 they are of the type No.

2. In both Figs. 4 and 5, the curve A is obtained from the capacitors impregnated with EDE, MDE and E/MDE, the curve B is obtained from the capacitors impregnated with MO and the curve C is obtained from the capacitors impregnated with PXE.

To take Fig. 5 for example, in the capacitors impregnated with MO the corona starting voltage reaches a saturated level upon heating for 40 hours, whereas in the capacitors impregnated with the oil of the invention the voltage reaches a saturated level upon heating for only 1 5 hours. A similar tendency is seen in Fig. 4. This is believed to be due to the fact that the oils of this invention have good permeation characteristic caused by good spreading characteristic on PP film as well as low viscosity.

Thus, as compared with the capacitors impregnated with MO or PXE which latter has been proposed in recent years, the oils of this invention enable the capacitor impregnated therewith to be made compact in size and improve the dielectric breakdown and corona characteristics and help achieve a longer life. In particular the characteristics of the oils of the invention are excellent at low temperature and suitable for impregnation of capacitors to be used in cold environment.

Also the oils of the invention have good interaction characteristics on PP film, so that PP film can be used as the dielectric of a capacitor or part thereof with excellent effects. The oil of the invention can be used in not only power capacitors but also those for direct current.

Example 2 Power cables were prepared and tested in this example.

Power cables were two meter long and insulated by winding thereabout a stripe of insulating paper laminated with PP film. The cables were kept at 700C while a voltage one and a half times the rated value was impressed on the cables. After 100 days dielectric breakdown occurred in eight out of ten such cables impregnated with MO and in six out of ten such cables impregnated with AB. On the contrary, after 100 days no dielectric breakdown occurred in all of the cables impregnated with EDE, MDE and EiMDE.

It is believed that in the cables impregnated with MO and AB, the PP film was dissolved in the oils to increase the viscosity of the oils so that convection was blocked to effect rapid rise of the oil temperature and consequently dielectric breakdown. In the cables impregnated with the oils of the invention, however, a less amount of the PP film was dissolved so that the viscosity of the oils remained unchanged and no dielectric breakdown occurred. Thus, it is quite reasonable that the cables impregnated with the oils of the invention can enjoy a long life in practical use.

Example 3 Transformers were prepared and tested in this example.

Transformers were rated at 5000 kVA, 77KV with the coils being insulated by PP film. Table V shows the corona starting voltage of the transformers impregnated with the oils mentioned therein.

Table V EDE MDE E/MDE MO -400C 11 OkV 11 OkV 11 OkV < 80kV 200C 120kV 120kV 120kV 100kV Fig. 6 shows the temperature rise characteristic of the above transformers in temperature rise test. In the graph, the temperature of the transformer impregnated with MO reaches 500C in about 10 hours as shown by curve B, while in the transformers impregnated with EDE, MDE and E/MDE the temperature rises to 450C in about 7.5 hours (curve A).

The reasons why the transformers impregnated with the oils of the invention have better corona starting voltage and temperature rise characteristics than those impregnated with MO are believed tp be because the oils of the invention are lower in the pour point and viscosity so that they can penetrate deeply into the dielectric structure and as the temperature rises a convention occurs to suppress the temperature rise.

Thus, the invention has successfully provided the electrical devices which are compact in size, have a long life and excellent corona and dielectric breakdown characteristics, especially at low temperature. With PP film used as the dielectric or insulating material of the electrical devices, the oils of the invention do not deteriorate the excellent properties of the film. The oils are bio-degradable so that no problem of environmental pollution will occur.

Claims

1. An electrical device comprising an electrical device element impregnated with an oil consisting of a compound having the structural formula of

wherein R is selected from the group consisting of CH3 and C2H5.

2. The device of claim 1, wherein said element is a capacitor element.

3. The device of claim 1 wherein said element is a cable element.

4. The device of claim 1, wherein said element is a transformer element.

5. The device of claim 1, wherein said element includes at least plastic film as the insulating material.

6. The device of claim 5, wherein said plastic film is polypropylene film.

7. The device of claim 2, wherein the dielectric of said capacitor element comprises polypropylene film.

8. The electrical device of claim 2, wherein the dielectric of said capacitor element comprises a combination of polypropylene film and insulating paper.

9. The electrical device of claim 2, wherein the dielectric of said capacitor element comprises insulating paper.

10. An electrical device comprising an electrical device element impregnated with an oil consisting of a mixture of a first compound having the structural formula of

and a second compound having the structural formula of

11. The device of claim 10, wherein said element is a capacitor element.

12. The device of claim 10, wherein said element is a cable element.

13. The device of claim 10, wherein said element is a transformer element.

14. The device of claim 10, wherein said element includes at least plastic film as the insulating material.

1 5. The device of claim 14, wherein said plastic film is polypropylene film.

1 6. The device of claim 11, wherein the dielectric of said capacitor element comprises polypropylene film.

1 7. The device of claim 11, wherein the dielectric of said capacitor element comprises a combination of polypropylene film and insulating paper.

1 8. The electrical device of claim 11, wherein the dielectric of said capacitor comprises insulating paper.

1 9. An electrical insulating oil comprising a compound having the structural formula of

wherein R is selected from the group consisting of CH3 and C2Hs.

20. An electrical insulating oil comprising a mixture of a first compound having the structural formula of

and a second compound having the structural formula of

21. An electrical device substantially as hereinbefore described

22. An electrical insulating oil substantially as hereinbefore described.