JPS6367288B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oil-immersed power cable (hereinafter abbreviated as OF cable) with excellent electrical characteristics.

Conventionally, for OF cables, a laminate of polypropylene film and cellulose paper with a textured surface on the film side has been proposed as an insulating film wrapped around the conductor (for example, (Sho 54-66498).

However, since such a laminated film contains cellulose paper as a constituent component, it is impossible to make the ε and tan δ values of the film lower than those of a single polyolefin film that does not contain cellulose paper. Therefore, in a power cable using this, power transmission loss due to dielectric loss during long-distance power transmission cannot be ignored, and it is not practically preferable for ultra-high voltage or ultra-super high voltage power transmission of 275 KV or more, especially 500 to 1000 KV class.

Laminated paper uses an extruded polypropylene laminate layer as an adhesive layer, but since this layer is non-oriented, it has an elastic modulus that is only 1/5 to 1/3 that of biaxially oriented polypropylene film.
It does not contribute to the stiffness of the laminated paper at all. Therefore, the laminated paper is particularly useful in the present invention.
Compared to axially oriented polypropylene, it is inferior in rigidity and tends to wrinkle due to bending during cable installation, making it unsuitable for practical use. moreover,
Since the extruded laminate layer is non-oriented, components from this layer are eluted into the oil when immersed in oil, which is undesirable in that it deteriorates the tan δ of the composite dielectric.

On the other hand, plastic films, especially polyethylene, polypropylene, cross-linked polyethylene,
Polyolefin films such as 4-methylpentyl have low ε and tanδ values, and cables using them have low dielectric loss during power transmission, so they have long attracted attention as insulating materials for OF cables. However, the swelling property of the above film in hydrocarbon-based insulating oil is extremely large, and the thickness increases by 10 to 20% at the operating temperature of the cable (80 to 100°C). The swelling of the film causes an increase in the surface pressure between the insulating layers, resulting in poor oil flow between the film layers.Also, when cables are bent during cable installation, the slipperiness between the tapes increases. As a result, buckling wrinkles occur, and the current situation is that the cable cannot be put to practical use.

As a countermeasure against this problem, methods such as grooving the film surface, adhering powder to the film surface, and creating unevenness by embossing the film surface have been attempted, but grooving can damage the film. The powder adhesion method causes uneven adhesion and movement of the powder, and if the unevenness is applied by normal embossing, the unevenness will flatten when wound or used as a cable, reducing oil flow. The problem is that you end up doing it. To explain in more detail about unevenness, when OF cables are manufactured using insulating tape with ordinary embossed processing on plastic film, the unevenness increases when used as a cable.
Due to the temperature of around 100° C., the height of the unevenness on the surface of the insulating tape is drastically reduced, resulting in a disadvantage that the flowability of the insulating oil is significantly reduced.

An object of the present invention is to provide a film for OF cables that has excellent durability against cable bending and is not affected by swelling due to oil absorption.

That is, the present invention provides a film having a thickness of at least uniaxially oriented polyolefin resin.
Embossing is applied to a 60 to 300μ single-layer or laminated film, and the embossed film is wrapped around a conductor and impregnated with hydrocarbon-based insulating oil.
For OF cables, (a) Maximum roughness Rmax of at least one surface of the embossed film according to JIS BO601-1976 method
5μ≦Rmax≦50μ, and the number of protrusions with a height of 5μ or more is 2/2 in at least one direction.
5 mm or more, and (b) the apparent compressive elastic modulus P (Kg/cm ² ) when 10 sheets of the embossed film are stacked and a load of 1 Kg/cm ² is applied is 5≦P≦30. (c) The OF cable is characterized in that the rate of change in thickness is within ±4% when the embossed film is immersed in hydrocarbon insulating oil at 100°C for 24 hours. .

An example of the OF cable of the present invention is the single-core oil-immersed cable shown in the accompanying drawings. That is, 1 is an oil flow path, and 2 is a hollow helical tube forming a central oil conduit in the metal conductor 3. An insulating layer 4 formed by winding an insulating film in multiple layers, which is the object of the present invention, is provided on the metal conductor 3.
The cable is additionally completed with a semiconducting shielding layer 5, a metal sheath 6, and a corrosion protection layer 7. However, the OF cable of the present invention is not limited to this single-core cable, but also includes a multi-core cable.

The film used in the present invention is a polyolefin film. Specifically, it is a film of polyethylene, cross-linked polyethylene, silane-cross-linked polyethylene, cis-1.2-polybutadiene, cross-linked cis-1.2-polybutadiene, polypropylene, polybutene, poly-4-methylpentene, polystyrene, etc.; Alternatively, it may be a laminated film of different polymer films. A film particularly suitable for the purposes of the present invention is a biaxially oriented polypropylene film.

It is more preferable to use a film made by re-stretching a biaxially oriented polypropylene film to make it stronger. Although the film of the present invention is mainly composed of polyolefin, it may contain a polar polymer other than polyolefin in an amount not exceeding 10% by weight. Examples of the polar polymer include polyethylene terephthalate, polysulfone, polycarbonate, polyphenylene sulfide, and polyphenylene oxide. A small amount of a crosslinking agent, a commonly used heat stabilizer, an antioxidant, and a lubricant may be added to the film within a range that does not impair the electrical properties of the film.

In addition, as a constituent layer of the laminated film of the present invention, a film containing a polar polymer other than polyolefin (for example, polyethylene terephthalate, polyphenylene oxide, polysulfone, polycarbonate, polyamide, polyimide, etc.), nonwoven fabric, or paper containing a total thickness of 10 It may be included within %.

The thickness of the film is preferably in the range of 60 to 300μ, more preferably 100 to 200μ.
In order to avoid the occurrence of buckling wrinkles in the cable, it is desirable that the tape be thicker, but on the other hand, impulse rupture strength generally tends to decrease as the thickness increases. Therefore, taking both characteristics into consideration, the thickness of the film needs to be in the range of 60 to 300 microns, preferably 100 to 200 microns.

At least one side of the film of the present invention has
Maximum roughness Rmax value according to JIS B0601−1976 method,
It is desirable that there be irregularities due to embossing in the range of 5μ≦Rmax≦50μ. More preferably 10μ
A range of ≦Rmax≦20μ is preferable. When Rmax is less than 5μ, the oil flow between the insulation layers is insufficient, and the occurrence of wrinkles when the cable is bent exceeds an acceptable level, making the cable unusable.

When the Rmax value exceeds 50μ, the impulse breakdown voltage decreases significantly, and the film no longer exhibits satisfactory performance as a film for ultra-high voltage OF cables.

Regarding the density of protrusions on the film surface, the height
The number of protrusions of 5μ or more is at least in one direction,
It is necessary to have 2 pieces/5mm or more. If the number of protrusions is less than 2/5 mm, sufficient flowability will not be obtained to ensure the movement of insulating oil in response to bending changes in the cable, making it difficult to make the cable practical.

The compressive elastic modulus of the embossed film needs to be 5 Kg/cm ² or more and 30 Kg/cm ² or less, particularly preferably in the range of 10 Kg/cm ² or more and 20 Kg/cm ² or less. The compressive elastic modulus mentioned here is expressed as an apparent compressive elastic modulus value calculated from the rate of change in thickness when 10 films are stacked and a load of 1 kg/cm ² is applied. If the compressive elastic modulus is less than 5 Kg/cm ² , it is insufficient to absorb the wrinkles that occur when the cable is bent; on the other hand, if it exceeds 30 Kg/cm ² , the wrinkles that occur when the cable is bent cannot be ignored.

When the embossed film is immersed in a hydrocarbon insulating oil at 100° C. for 24 hours in a free state, the rate of change in thickness must be within the range of ±4%. More preferably, it is within the range of ±2%. If it is less than -4%, the gap between the insulation layers of the cable becomes larger than an allowable level, which is not preferable because it causes problems such as a decrease in impulse breakdown voltage and an increase in thermal resistance. On the other hand, if it exceeds 4%, the surface pressure between the insulating layers increases, oil flowability deteriorates, the slipperiness between the tapes deteriorates, and wrinkles are likely to occur when bending the cable, which is not preferred in practice.

Hydrocarbon insulating oils preferably used in the present invention include mineral oil, paraffin oil, dodecylbenzene oil, naphthenic oil, diarylalkane oil, and polybutene oil.

In the combination of plastic film and insulating oil used in the present invention, the absolute value of the difference in solubility parameters between the film and the insulating oil is preferably 0 or more and 2.5 or less, more preferably 0 or more and 1.5 or less.

From the point of view of dielectric properties, it is desirable that the cable insulating material consists of a non-polar polymer, and therefore it is desirable that the difference in solubility parameter with the hydrocarbon insulating oil is not excessively large, and the difference is 2.5 or less. It's good. If a combination in which this difference exceeds 2.5 is used, the affinity between the film and the insulating oil will be poor, resulting in a decrease in the impulse breakdown voltage of the cable.
The solubility parameters (hereinafter referred to as SP values) of plastic films and insulating oils can be determined experimentally, but a simpler method is to calculate them using Small's method, which utilizes the additivity of the cohesive energy constant. I can do it. According to this method, the SP value of polyethylene is 8.0, polypropylene is 8.2, and polystyrene is 9.1. In addition, the SP value of insulating oil is 8.0 for dodecylbenzene oil, 7.1 for polybutene oil, 6.1 for silicone oil, 7.5 for naphthenic oil,
SAS oil is 9.7.

The film having the specific surface morphology described in the present invention is a group of a pair of rolls consisting of a metal roll with an engraved surface and a paper roll, or a group of two rolls having convexities on the surface and capable of interlocking with each other. It can be manufactured by passing a film between metal rolls. The temperature for embossing is preferably at least 60℃ or higher and 140℃ or lower, and the linear pressure between the rolls is at least 10Kg/ ^cm2 or higher, preferably 30℃.
Kg/cm ² or more is better. The film conveyance speed depends on the roll diameter and roll heat capacity, but it is usually 5~
The range is 50m/min. Regarding the uneven pattern on the surface of the metal roll, it is necessary that the density of the unevenness is 2 pieces/5 mm or more in at least one direction.
The uneven pattern can be selected from various shapes such as silk, tortoiseshell, satin, and lattice. The height of the protrusions on the surface of the metal roll is preferably 5μ or more and 500μ or less in terms of Rmax value. More preferably, 50μ or more,
180μ or less is desirable. The Rmax value of the convex side of the embossed film is usually lower than the Rmax value of the metal roll. For example, a 90μ biaxially stretched polypropylene film (abbreviated as PP-BO) is heated at 130°C and
When embossed with a linear pressure of Kg/ ^cm2 , the Rmax value of the film is 20-30μ, and under the same conditions, the PP-
15-25μ, 200μ when processing BO film
10~20μ when processing PP-BO film.
becomes. If the Rmax value of the film after embossing is excessively large (for example, 50μ or more), heat-treating the film at 60 to 120℃ for 5 seconds to 5 minutes will result in
It is also possible to reduce the Rmax value of the film.

The OF cable of the present invention uses a film with a special shape and characteristics as an insulator, so it has excellent effects such as no wrinkles due to bending of the cable and no adverse effects of swelling of the film due to oil absorption. It is something that can be demonstrated.

The embodiments and effects of the present invention will be described below with reference to Examples, but the present invention is not particularly limited thereto.

The evaluation method was as follows.

(a) Roughness density: 5μ on the paper roll surface side
The number of these protrusions was counted.

(b) Compressive elastic modulus of embossed film: According to the method described in "Electrical Insulating Paper" edited by Yuichiro Take, 10 films were stacked one on top of the other, and the value was expressed as the value when a load of 1 kg/cm ² was applied.

(c) Oil flowability: DDB oil (hard type) was flowed through the model cable (with the configuration shown in the figure) at a hydraulic pressure of 1 kg/cm ² at 80°C, and the oil flow resistance value was
Less than 10 ¹¹ cm ^-2 (equivalent to kraft paper) is good; 10 ¹¹ cm ^-2 or more and 10 ¹³ cm ^-2 or less is acceptable.
Those exceeding 10 ¹³ cm ^-2 were ranked as defective.

Example 1 One side of a biaxially stretched polypropylene film (hereinafter abbreviated as PP-BO) with a thickness of 60 μm was subjected to corona discharge treatment, and on top of this was applied an OH-polyester polyester Desmophen 800 (manufactured by Bayer) and an isocyanate compound (Japan Polyurethane). , Coronate L) (using butyl acetate as the solvent) is applied with a gravure coater to a dry thickness of 1 μm. Next, a 90μ thick PP-BO film that had been previously subjected to corona discharge treatment to facilitate adhesion was superimposed on this adhesive surface and fed to a pair of laminating rolls to form a 5Kg/cm ² wire at a temperature of 110℃. A composite film with a thickness of 151 μm was obtained by thermocompression bonding using pressure.

The length of this film is 0.66mm, the width is 0.66mm, and the height is
40 with 225 100μ pyramid-shaped irregularities per ^1cm2
The material was fed to a pair of embossing rolls consisting of a mesh metal roll and a paper roll, and embossing was performed at a temperature of 130°C with a line pressure of 35 kg/cm to obtain an embossed PP film with an apparent thickness of 171 μm.

When the surface roughness of this film on the paper roll side was examined using the JIS-BO601-1976 method, it was found that
Rmax=20μ, and the density of protrusions was 8/5 mm in both the longitudinal and width directions. Further, the apparent compressive elastic modulus was 20 Kg/cm ² .

This film was coated with dodecylbenzene oil (below)
The rate of change in thickness after being immersed in (abbreviated as DDB) at 100°C for 24 hours was measured with a dial gauge and was +1.5%. This film was slit to a width of 22mm and wound to a thickness of 20mm on a 40mmφ conductor (tension at the time of winding was 1Kg/22mm) to fabricate a prototype cable as shown in the figure. After being degassed at 100°C for 50 hours and impregnated with DDB oil, it was maintained at 100°C and changes in oil flowability over time were examined. Although the oil flow resistance after 3 days was 1.2 times the initial value, it remained at a level that was satisfactory for an OF cable. Oil immersion 3 at 100℃
When we performed a 900D two-way bend test in the opposite direction on the model table after a few days, there were no tape wrinkles, tape breaks, or gap disturbances, and the condition was exactly the same as in the initial stage of taping. Also, the impulse breakdown voltage of this model table is
It was extremely excellent at 145KV/mm.

Example 2 A 750μ thick unstretched sheet of high-density polyethylene containing 2% by weight of a crosslinking agent (triallyl isocyanurate) was formed. Subsequently, this sheet was stretched 5.0 times in the longitudinal direction at 120°C. This uniaxially stretched film (thickness: 150 μm) was embossed in the same manner as in Example 1, and then irradiated with an electron beam of 10 Mrad using a Van de Graaf type electron beam irradiation device. The height of the protrusions on the surface of this film was 20 μ, the density was 8/5 mm in both the longitudinal and width directions, and the apparent compressive modulus was 18 Kg/cm ² . When the embossed film was immersed in DDB at 100℃ for 24 hours, the thickness change rate was +2.0%, and the oil flowability and bending resistance of the model cable made in the same manner as in Example 1 were OF It maintained a satisfactory level as a cable. When the same bending test as in Example 1 was conducted, wrinkles were observed in the 2nd to 3rd layer directly above the conductor.

Example 3 A 300μ thick unstretched sheet of poly-4-methylpentene was formed, and then this sheet was stretched twice in the longitudinal direction at 140°C. This uniaxially stretched sheet was embossed in the same manner as in Example 1. However, the embossing roll temperature was set at 160°C. The height of the protrusions on the surface of this film was 20 μm, and their density was 8 protrusions/5 mm in both the longitudinal and width directions. Compressive elastic modulus of film (10 layers stacked, load 1
Kg/cm ² ) was 25Kg/cm ² . The thickness change rate during DDB oil immersion was +1.2%, which was excellent.
The oil flowability and bending resistance of the model cable are
It was at a satisfactory level as an OF cable.

Comparative Example 1 The 151μ thick PP-BO laminated film of Example 1 was embossed using a 50μ high, 200 mesh embossing roll. The Rmax value of this film was 10μ, and the protrusion density was 40/5mm in both the longitudinal and width directions. The compression modulus of this film was as high as 35 Kg/cm ² . The thickness change rate of this film after 24 hours at 100℃ during DDB is +
At 2.5%, the oil flow resistance of the model cable after 3 days of oil impregnation (maintained at 80℃) was as high as 2×10 ¹⁴ cm ^-2 , which is 20 times that of immediately after oil impregnation.
It was not practical as an OF cable.
When the cable was subjected to a bending test, the occurrence of buckling wrinkles was significant.

Comparative Example 2 The 151μ PP-BO laminated film of Example 1 was embossed with a depth of 10μ and 40 meshes. The Rmax value of this film is 2μ, the roughness density in the longitudinal and width directions is 8/5mm, and the compressive modulus is 28
It was Kg/ ^cm2 . When this film was immersed in DDB at 100℃ for 24 hours, the thickness change rate was +5.0%.
It was hot. The flow resistance of the model cable is that the internal pressure between the layers increases over time as the insulation layer absorbs oil, and the oil flow resistance after 3 days at 100℃ after oil impregnation is 10 times that of immediately after oil filling. 5×10 ¹⁴ cm ^-2 , which was an unsatisfactory characteristic for an OF cable.

Comparative Example 3 The 151μ PP-BO laminated film of Example 1 was embossed with a depth of 400μ and a 10 mesh embossing roll.
It has been given a textured finish. The Rmax of the film on the paper roll side is 80μ, and the density of protrusions over 5μ in height is 2/5mm in the longitudinal direction and 2/5mm in the width direction.
The apparent compressive elastic modulus was as large as 5Kg/cm ² . When this film was immersed in DDB at 100°C for 24 hours, the thickness increase rate was -5.0%. The oil flowability of the model table created using this film was insufficient for an OF cable. Further, when a buckling test was conducted under the same conditions as in Example 1, wrinkles were observed in several layers immediately above the conductor after the cable was dismantled. In addition, the impulse breakdown voltage of the model cable was at an extremely low level of 50 KV/mm, making it unsuitable for use as a film for ultra-high voltage cables.

As explained above using the examples, the OF cable using the film of the present invention shows no deterioration in oil flow due to swelling of the film, and has good durability against bending of the cable. It also has excellent electrical properties (impulse breakdown voltage of 140 KV/mm or more and good dielectric loss).

[Brief explanation of drawings]

The figure is an explanatory cross-sectional view showing an embodiment of the oil-immersed power cable of the present invention. 1: Oil flow path, 2: Hollow spiral pipe, 3: Conductor,
4: Insulating layer made of the embossed film of the present invention,
5: Shielding layer, 6: Metal sheath, 7: Corrosion protection layer.

Claims (1)

[Scope of Claims] 1. Embossing is applied to a single layer or laminated film with a thickness of 60 to 300 μ made of a polyolefin resin film oriented in at least one axis, and the embossed film is wound on a conductor to form a hydrocarbon In an oil-immersed power cable impregnated with insulating oil, (a) the maximum roughness Rmax of at least one surface of the embossed film according to the JIS B0601-1976 method;
5μ≦Rmax≦50μ, and the number of protrusions with a height of 5μ or more is 2/2 in at least one direction.
5 mm or more, and (b) the apparent compressive elastic modulus P (Kg/cm ² ) when 10 of the embossed films are piled up and a load of 1 Kg/cm ² is applied is 5≦P≦30. (c) An oil-immersed power cable characterized in that the rate of change in thickness when the embossed film is immersed in hydrocarbon insulating oil at 100° C. for 24 hours is within a range of ±4%.