JPS5848967B2 - Embossed film for electrical insulation and power cables - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an embossed plastic film for electrical insulation that provides a pressure-type power cable with excellent electrical properties, and a pressure-type power cable with excellent electrical properties using the film as an insulating layer.

Conventionally, OF (oil-filled) cables and GF (gas-filled) cables have been put into practical use as power cables, and cellulose-based insulating paper has been mainly used as the insulating tape for these cables, but in recent years insulating paper Plastic films, which inherently have superior insulating strength and dielectric properties and less hygroscopic properties, are being considered as the insulating layer.

Insulating fluids used here include mineral oil, silicone oil, synthetic oils such as polybutene and alkylbenzene,
There are various types such as nitrogen, sulfur hexafluoride, freon, etc.
Efforts are being made to obtain plastic film insulated power cables filled with these insulating fluids, which are easy to impregnate and circulate, and have high practical performance.

The present invention solves various problems encountered when attempting to use only plastic film as an insulating tape in such cables without using insulating paper together, and provides a pressure-type power cable with excellent electrical characteristics. The present invention provides an embossed plastic film and a pressure-type power cable using the same that has excellent electrical properties.

In other words, when replacing insulating paper with plastic film for the above-mentioned purpose, plastic film generally has a property that it is poor in containing and distributing insulating fluid compared to insulating paper, and it is difficult to reduce the ultimate strength of the cable. To avoid problems such as damage, etc., countermeasures include cutting grooves on the film surface, applying powder to the film surface, using cellulose paper with excellent air permeability, and making the film uneven (embossed). Methods such as processing have been attempted, but the grooving process damages the film, the powder adhesion method causes uneven adhesion and movement of the powder, and the use of cellulose paper in combination has a correspondingly high insulating force or dielectric properties. The effectiveness of using plastic films, which are excellent in quality, is reduced, and if the unevenness is applied, the size of the unevenness becomes smaller or disappears when being wound or used as a cable, reducing circulation. There was a problem with this.

To explain in more detail, when a pressure-type power cable is manufactured using an insulating tape made from a single piece of ordinary plastic film with a textured surface, it is exposed to temperatures of several tens of degrees Celsius to 1000 degrees Celsius when used as a power cable. Moreover, the height of the unevenness of the insulating tape decreases, and the insulating layer becomes loose, resulting in a decrease in dielectric strength.

In addition, the flowability of the insulating fluid is greatly reduced, and in cables in which the cooling medium flows through the insulating layer, the cooling effect is lost, the temperature of the cable increases, and the various types of cables used A problem arises in that the insulator deteriorates rapidly.

In addition, as a manufacturing procedure for pressure-type power cables, the tape is wound around the conductor and then heated and dried, for example, at 120°C for 2
It was found that using the method of vacuum drying for about 4 hours caused more serious problems such as decreased fluid flow, localized voids, movement of conductors, and decreased impulse breakdown voltage. .

In view of this problem, the present inventors conducted intensive research and found that by applying heat treatment to a plastic film, an embossed plastic film with a significantly improved shape retention rate of 95% or more, which will be described later, was obtained. Even if such embossed plastic film tape is wrapped around a conductor and then heated and dried, the cable according to the present invention cannot be used at a temperature of about 80°C to 100°C, which is the normal cable operating temperature. However, it has been discovered that an excellent pressure-type power cable can be obtained which does not have the above-mentioned disadvantages such as decreased fluid circulation, localized voids, conductor movement, and decreased impulse breakdown voltage.

That is, in order to obtain an embossed plastic film with good shape retention, according to research by the present inventors, it is effective to perform embossing at as high a temperature as possible. For example, biaxially oriented polyethylene terephthalate (hereinafter abbreviated as PET)
In the case of film, the melting point is as high as 263°C, so 18
En at about 0 0C? can be processed, 120
Even after drying for 24 hours at 0CX, the shape retention rate is 9.
9%, which is excellent and does not require any special heat treatment after embossing, but in the case of films with a low melting point of 160 to 170°C, such as biaxially oriented polypropylene (hereinafter abbreviated as PP) film, the embossing roll of the film Due to welding, shrinkage, etc., the embossing temperature can only be raised to about 140°C at most.
As an example, the shape retention rate by heat drying at 1200CX for 24 hours is as low as 30 to 50%.

This phenomenon can be explained by combining a PET film with good shape retention and a PP film with high impulse rupture strength with PET/PP or PET film, as previously proposed by the present inventors.
The same is true for laminated and bonded embossed films such as /P P/PET.I) Due to problems such as curls and wrinkles caused by P films, the embossing temperature is at most about 1400C, which is the temperature when operated as a pressure-type power cable. Although it exhibits good shape retention 11- at a certain temperature of several tens of degrees Celsius,
Shape retention rate after heating and drying at 120°C for 24 hours is 40~
60% is low and bad.

As described above, for films that cannot be embossed at high temperatures, the present invention embosses them with low-temperature dust at 130 to 140°C, and then heat-processes the embossed plastic film in hot air at 130 to 140°C for several minutes. It has been surprisingly found that by carrying out this process, an embossed plastic tape with an excellent shape retention rate of about 95% after heating and drying at 120° C. for 24 hours can be obtained.

The reason why such heat treatment after embossing is effective is not clear, but it is presumed that the heat treatment stabilizes the deformation strain during embossing by restoring or fixing it.

Here, any method of embossing (unevenness) can be used, such as normal embossing, vacuum forming, compression molding, etc.
It is possible to continuously and efficiently process irregularities on long films.
In particular, ordinary embossed insulating tape is preferred, and embossed insulating tape is also effective in terms of insulation strength in the cable of the present invention.

The embossing temperature should preferably be as high as possible than the temperature encountered during cable use or manufacturing, but if a PP film with a relatively low melting point is present as mentioned above, the upper limit is about 140°C, and 130 to 130°C. 1400C is preferable and generally 20 to 30℃ below the lower melting point12
0'C or more is preferable.

Also, the higher the embossing pressure, the better, but 10 to 50K.
A line pressure of cr/crrL is practical.

Regarding the heat treatment conditions for the embossed film, first of all, if the temperature is too high, it will melt or the embossed unevenness will disappear, and if it is lower than the heating drying temperature encountered during cable manufacturing, for example 120°C, the effect will be small, and PP
When a film is present, the temperature is preferably from 130 to 140°C, and generally from 120°C to 120°C and below the embossing temperature.

As for the heat treatment time, in the case of hot air at 130° C., a few minutes is sufficient for the effect, and even if it is longer than that, the effect tends to be saturated.

The heat source for the heat treatment is arbitrary, as long as the embossed plastic film reaches a predetermined temperature, such as infrared rays, high frequency irradiation, contact with a heating medium, immersion, etc., but in particular hot air using air as a heating medium can be used. Uniform heating is possible, and there is no need to remove the heating medium, making it practical.

In addition, when heating the embossed plastic film, when it is wound into a roll, heating spots may occur due to the difference in temperature between the inner and outer layers, and the winding shape and winding tension may change, so the embossed plastic film should be heated as low as possible in sheet form. A method in which the product is conveyed in hot air is preferred; for example, a long product can be efficiently heat-treated by passing it through a hot air zone of a conventional coating device and winding it up normally.

There are no particular limitations on the plastic film to which the product of the present invention is applied; for example, polyester-based, polyolefin-based, polyfluorinated olefin-based, polysulfone-based, polycarbonate-based, polyphenylene oxide-based, polyamide-based, polyimide-based, etc. However, especially mechanical strength,
Polyester films with excellent thermal and chemical stability, especially biaxially oriented polyethylene terephthalate (PET) films, and polyolefin films with low dielectric loss and excellent dielectric strength, especially biaxially oriented polypropylene (PP). Film is preferred.

Note that the effect of the present invention is even more remarkable when a polyolefin film having a relatively low melting point is used.

Further, the film is not limited to a single film, but two or more films may be laminated and bonded and embossed.

The orientation of the film can be selected depending on the type of film, such as unstretched, uniaxially stretched, biaxially stretched, etc., but in general, from the viewpoint of mechanical strength and dielectric strength, in the case of laminated adhesives, It is preferable to incorporate at least one layer of biaxially stretched film, and in this case, the thickness of each film is desirably about 60 μm or less from the viewpoint of dielectric strength.

Also, laminated adhesive means that multiple sheets of plastic films are laminated and adhered to each other, but it is more difficult to bond over the entire surface than with spot-adhesive adhesives such as ordinary laminated insulating paper. In the case of the above-mentioned embossed plastic film, it is preferable to bond the film with the embossed plastic film because it has a higher impulse breaking strength. It may be laminated, thermocompressed with a hot-melt polymer adhesive, extrusion laminated, or a composite film may be formed in the first film forming process, or a seed layer may be bonded by an appropriate method at the same time as the uneven processing. However, the former method is particularly preferable in terms of impulse rupture strength.

Note that the form retention rate in the present invention refers to the thickness of the film 1 before embossing (unevenness) as shown in Fig. 1 (if it is divided into multiple sheets before embossing, the total thickness)
As shown in Fig. 2, the apparent thickness of the film 1 after uneven processing is B, and the thickness of the film 1 after the unevenness is reduced by some external processing is C as shown in Fig. 3. Then,
It is defined as

Next, to explain the power cable of the present invention in more detail, the embossed plastic tape for electrical insulation, which has been heat-treated after the above-mentioned embossing process and has a shape retention rate of 95% or more after heating at 120°C for 24 hours, is wrapped around a conductor. By turning the embossed plastic film tape and enclosing the insulating fluid under pressure, an excellent pressure-type power cable with no drop in impulse breakdown voltage can be obtained even if the cable is dried at 120°C for 24 hours after winding the embossed plastic film tape. This is what we discovered.

In addition, in the case of a film used for ordinary insulation, when considering how to deal with it when used as a power cable, it is important to observe the deformation behavior when a single film is heated under no tension or low tension. For example, a heating model test of about 5 minutes can be used to ensure that it corresponds to the situation in use.

However, when used in pressure-type power cables, such as the film of the present invention, which is wound over a conductor many times under great tension,
Moreover, in the case of films that require long-term durability of 10 years or more, various external conditions (pressure, winding tension, number of windings, overlapping ratio, film thickness, compatibility between oil and film, etc.) ), so in a short heating test of about 5 minutes as mentioned above, not only the deformation behavior but also the
Measures against dielectric breakdown are also not sufficient.

Therefore, in the present invention, in addition to the above manufacturing conditions, as a result of considering the correspondence with the insulation performance with the cable, we found that it is necessary to check the shape retention rate after heating at 120°C for 24 hours. It was found that the insulation performance almost matches the insulation performance of the above cable.

Traditionally, when manufacturing pressure-type power cables that are wrapped with electrically insulating paper or plastic film as an electrical insulating layer, tape is placed on the conductor to prevent deterioration of the insulator due to moisture and deterioration of the cable's electrical properties. After winding, the cable is placed in a vacuum drying oven and dried by heating at a high temperature, for example, 120° C., for a long time, for example, about 24 hours.

If an insulating fluid is then pressurized and sealed, an ideal pressure-type power cable containing almost no water can be obtained.

This cable manufacturing method can be easily applied without any particular problems when using conventionally used electrically insulating paper, but various problems have occurred when using embossed plastic film.

However, it has been discovered that by using the embossed plastic film of the present invention, such a cable manufacturing method can be easily applied.

That is, in the drying method for each cable in the manufacturing method of such cables, in order to dry the tape after winding, a high temperature exceeding the normal operation temperature of the cable, that is, a temperature of 100° C. or more is applied, which results in poor shape retention. It has been found that problems such as abnormal temperature rise due to reduced flowability of the insulating fluid, generation of local voids, movement of the conductor, and reduction in impulse breakdown voltage and increased variation occur.

This phenomenon occurs even when the drying temperature is lowered to about the operating temperature of the cable, that is, about 80 to 100'C.

The present inventors believe that if an embossed plastic film, which has a shape retention rate of 95% or more when exposed to high cable operating temperatures or drying temperatures, is used as an electrical insulation layer, an excellent pressure-type power cable that does not have such problems can be created. It was discovered that this can be obtained.

The pressure-type power cable to which the present invention is directed is one in which an electrically insulating tape is wound around a conductor such as stranded copper wire as an electrically insulating layer, an outer layer is applied that can withstand pressure, and an insulating fluid is sealed or circulated. The insulating fluid to be sealed includes mineral oil, silicone oil, synthetic oil such as polybdenum and alkylbenzene, gas or liquid such as nitrogen, sulfur hexafluoride, and freon, but it may be used as appropriate depending on the material of the film insulating tape. Applicable.

This will be explained below using examples.

Example 1 A biaxially stretched PP film with a thickness of 50μ was vertically stretched. 6
6 MAIL width 066, height 0. The material was fed to a pair of embossing rolls consisting of a metal roll and a paper roll with 225 pyramid-shaped irregularities of 2 mm per ICriL, and embossed at a temperature of 130°C with a line pressure of 35K4/crrL to give an apparent thickness. A 120μ embossed PP film was obtained.

Next, as the heat treatment of the present invention, the film was passed through hot air at 120°C for 5 minutes in the form of a continuous sheet and wound up to a thickness of 9.
An embossed film of the present invention having a thickness of 1 μm was obtained.

The thickness of the film after drying at 120° C. for 24 hours was 89.5 μm, and the shape retention rate was excellent at 96%.

Note that if the embossing temperature is set to 140°C, the PP film will shrink, and if the heat treatment temperature is also set to 140°C, the PP film will shrink.
The shrinkage of the film and the reduction of embossed unevenness are increased, 1
The temperature was preferably 30°C or lower.

Comparative Example 1 When the embossed PP film that had been embossed in Example 1 was dried at 120°C for 24 hours without being subjected to heat treatment, the film had a thickness of 77 μm and a shape retention rate of 38
% was bad.

Example 2 On one side of a biaxially stretched PET film with a thickness of 12 μm, a solvent-soluble polymer adhesive was applied, in which the main ingredient was a linear saturated polyester resin and the curing agent was an incyanate resin in an equivalent ratio.
Apply a 0% toluene solution adhesive using a gravure coater so that the thickness after drying the solvent is 1μ, dry the solvent in a hot air oven at 80°C, and then apply a corona discharge to the adhesive surface in advance. Biaxially stretched PP films with a thickness of 30μ that have been treated to facilitate adhesion are superimposed and fed to a pair of laminating rolls to produce 5Kq/crfL at a temperature of IIO°C.
A composite film having a total thickness of 43 μm was obtained by thermocompression bonding at a line pressure of 12 μm and a 30 μm polyolefin film, one by one, laminated and bonded over the entire surface.

Furthermore, another biaxially stretched PET film with a thickness of 12 μm was laminated in the same manner to obtain a three-layer composite film with a total thickness of 56 μm, with both surface layers being polyester films and the middle layer being a polyolefin film. I got it.

When the film was embossed at a temperature of 140°C and a linear pressure of 35K4/cIrL, the apparent thickness was 128.
A three-layer composite embossed film of μ was obtained.

Next, the embossed film was subjected to the heat treatment of the present invention.
Passed through hot air at 0°C for 5 minutes and rolled up to a thickness of 87μ.
An embossed film of the present invention was obtained.

The thickness of the film after drying at 120°C for 24 hours was 85.
At 5μ, the shape retention rate was 95%, which was excellent.

The embossing temperature of the three-layer composite film was 150.
At ℃, waving and wrinkles occur due to shrinkage of the PP film14
The temperature was preferably 0°C or lower, and the same was true for the heat treatment temperature.

When the relationship between the drying temperature and the shape retention rate of the embossed film of the present invention was determined, it was as shown by curve A in FIG. 4, and was excellent with almost no change.

Comparative Example 2 When the three-layer composite embossed film obtained in Example 2 was dried at 120°C for 24 hours without being subjected to heat treatment, it had a thickness of 85μ and a poor shape retention rate of 40%. Ta.

When the relationship between the drying temperature and the shape retention rate was determined in the same manner as in Example 2, it was as shown in the curved line in FIG. 4, and the change was extremely poor.

Example 3 An ultra-high pressure evaporative cooling cable using the three-layer composite film of the present invention as an insulating tape material, as shown in Example 3, will be described.

First, a model cable was created to determine the impact voltage breakdown strength (impulse breakdown strength) of the cable of the present invention.

A sectional view thereof is shown in FIG.

In the figure, 4 is a three-layer composite plastic tape according to the present invention which is slit to a width of about 19 mrnl, and is wound side-to-side on a stranded conductor 3 with an outer diameter of about 20 mm with a gap of about 1 mrrt, so that the upper layer and the lower layer overlap by 1/3. Thickness approx. 1
It is rolled up until it forms a mountain.

5 is a terminal processing section, and 6 is a low voltage side electrode.

The present inventors have been researching for some time an evaporative cooling type cable that uses the latent heat of vaporization of liquefied gas to cool conductor heat generation and dielectric heat generation in an insulating layer.
As shown in FIG. 5, the liquefied gas supplied from the terminal side to the refrigerant passage 2 in the stranded conductor 3 flows in the passage in the direction of the arrow, and at the same time evaporates as the stranded conductor 3 generates heat. , flows in the insulating layer 4 in the radial direction.

This method not only directly cools the conductor but also directly cools the heat generated by the dielectric in the insulating layer, so it has a large cooling effect and is expected to be applied to future ultra-high voltage, large capacity underground lines.

In addition, the liquefied gas used in this method is ``Freon'' 1 2 (
It has been found that CCt2F2 (corresponding to Freon 12) is superior.

Furthermore, the insulating layer used in this method needs to have high insulating performance (particularly impact voltage breakdown strength) and at the same time stable refrigerant flowability over a long period of time. As shown in No. 2, it was found that the latter was satisfied because it had excellent shape retention, and it was further confirmed in this example that it had high impact voltage breakdown strength.

First, an overview of the test method will be explained.

Assemble the created cable into a container and heat it at 120℃ for 24 hours.
Perform a vacuum drying process for an hour.

Next, when the conductor is energized, liquefied fluorocarbon 12 is supplied from both terminals into the refrigerant passage, and the gas-liquid mixture evaporates due to the heat generated by the conductor and flows inside the insulation layer in the radial direction of the cable. A system is constructed in which gas is recovered, liquefied in a condenser, and then supplied again into the refrigerant passage.

The cross-sectional area of the conductor used was approximately 100 m4, the current value was approximately 1400 amperes, and the refrigerant flow rate was approximately 40 liters per hour in liquid volume.

The gas-liquid mixing state was visually confirmed at the time of recovery.

Adjust the gas pressure so that the refrigerant evaporation temperature is 80℃,
After confirming that it was sufficiently stable, we started applying electricity.

A standard waveform shock voltage of 1×40 μS was applied with negative polarity on the conductor side from 50% of the expected breakdown value in 2 kV steps until breakdown occurred three times at each voltage.

As a result, the cable part broke during the first 150kV application.

At this time, the stress directly above the conductor is 155 kV/m
m, which was a sufficiently high value for use in supervoltage cables.

Comparative Example 3 Using the three-layer composite embossed film tape shown in Comparative Example 2 that was not subjected to heat treatment, a cable was created in exactly the same manner as in Example 3, and after drying under the same conditions, The impact voltage breakdown strength was determined in the energized and cooled state.

The evaporation temperature was the same as 80° C., and the current value and refrigerant flow rate were also the same as in Example 3.

The charging method is also the same as in the third embodiment. The result is 76kV2.
The cable part broke the second time.

At this time, the stress directly above the conductor was approximately 80 kV and 4 m, which was a much lower value than in Example 3.

When we dismantled the breakdown point, we found that most of the breakdown path from the conductor to the low-voltage electrode had creeping connections between the layers of the tape, and only 10% had broken through the insulation layer. .

This seems to be because the degree of unevenness in the uneven processing became smaller, which increased the gap between the tapes and reduced the insulation strength between the layers.

In Example 3, the shortest distance from the conductor to the low-voltage electrode was linearly penetrated, indicating that the power cable using the insulating tape material with excellent shape retention provides excellent withstand voltage performance. It was revealed.

Example 4 FIG. 6 is an example of a 550 kV GF (gas filled) cable using a laminated adhesive textured plastic tape according to the present invention.

The insulating fluid is supplied from the supply path 2 through the stranded conductor 3 to the insulating layer 4.
is supplied inside.

In this case, since the insulating layer is made of a tape with a textured surface, it has excellent flowability, and the insulating fluid is sufficiently filled in the insulating layer 4.

Note that the insulating fluid in this case is "Freon" 12 (CCt
2F2, equivalent to Freon 12), and the sealing pressure is 2
0 Ky/cyytG.

The conductor 3 is a copper stranded wire with a cross-sectional area of 2000 mm and an outer diameter of 60 mm.

The cable has an AC working stress of 15kV/+
+ m, and the impact voltage breakdown strength against the impact voltage withstand voltage standard is 8 7. 9 kV/mm or more is required, but the plastic tape shown in Examples 2 and 3 of the present invention has a voltage of 155 kV/mm, which fully satisfies the aforementioned standards and is suitable for use with 550 kV class cables. It turned out that it is possible to manufacture.

In addition, considering that the impact voltage breakdown strength of an actual cable made in a factory is generally about 10% lower than the value obtained for a model cable, it can be said that the performance is sufficient.

As described above, the embossed plastic zinc film of the present invention provides a pressure-type power cable with excellent electrical properties, but it can also be used for other purposes such as interlayer insulation of transformers and spacers for various electrical devices. .

[Brief explanation of drawings]

FIG. 1 is an example of a cross-sectional view of a laminated film before being subjected to uneven processing. FIG. 2 is an example of a cross-sectional view of a laminated film after the entire layer has been subjected to uneven processing. FIG. 3 is an example of a cross-sectional view of the film shown in FIG. 2 after the unevenness of the film has been reduced. FIG. 4 is a graph showing the relationship between drying temperature and form retention rate of the embossed film. FIG. 5 is an example of a cross-sectional view of a model cable for determining the impact voltage breakdown strength. FIG. 6 is an example of a cross-sectional view of the plastic tape insulated pressure type power cable of the present invention. DESCRIPTION OF SYMBOLS 1...Film, 2...Insulating fluid supply channel, 3...Twisted conductor, 4...Insulating layer, 5... Terminal processing section, 6...low voltage side electrode, 7... steel pipe.

Claims (1)

[Scope of Claims] 1. An embossed plastic film for electrical insulation that is heat-treated after embossing and has a shape retention rate of 95% or more after heating at 120'C for 24 hours. 2. A pressure-type power cable in which a tape made of plastic film is wound around a conductor to form an insulating layer, and an insulating fluid is pressurized and sealed, and the plastic film is embossed and then heat-treated. A pressure-type power cable made of an embossed plastic film for electrical insulation that has a shape retention rate of 95% or more after heating at ℃ for 24 hours.