JPS6237485B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oil-immersed power cable using a composite insulating tape in which a plurality of insulating tapes are bonded together by melt extrusion of polypropylene.

A polypropylene laminated paper insulated OF cable is made of a composite insulation material made by bonding two pieces of kraft paper together by melt extrusion of polypropylene, so-called polypropylene laminated paper tape is wound to form an insulating layer, and the insulation layer is impregnated and filled with insulating oil. Because it has the advantages of low dielectric loss and high electrical breakdown strength, it is attracting worldwide attention as an ultra-high voltage or ultra-super high voltage power cable.

However, the biggest obstacle in putting this cable into practical use is that the film-like polypropylene layer formed by melt extrusion, which is a component of polypropylene laminate paper, is not compatible with hydrocarbon oils such as mineral oil or alkylbenzene, which are commonly used as insulating oils. The reason for this is that in the coexistence of the material, the material exhibits a so-called swelling phenomenon and exhibits an increase in thickness.

There are two important problems caused by this increased thickness.

First, due to the increase in the thickness of polypropylene, the expansion of the insulating oil due to temperature changes in the cable,
The problem is that the flowability of the insulating oil within the insulating layer, which is necessary to compensate for shrinkage, is reduced. In conventional kraft paper insulated OF cables, the kraft paper is a porous material, so the insulating oil can penetrate through the kraft paper in the thickness direction, but in polypropylene laminated paper, the film-like polypropylene layer Therefore, the insulating oil cannot penetrate through the thickness direction and has no choice but to flow through the voids caused by the unevenness of the surface of the kraft paper or the voids between the fibers inside the kraft paper, which increases the thickness of the polypropylene layer. Even in the absence of it, the flowability of insulating oil is essentially poor. However, if relatively low-density kraft paper is used, it is possible to sufficiently maintain the degree of distribution required for normal cables. However, when the thickness of the polypropylene layer increases, the kraft paper layer is compressed, the voids decrease, and the flow of insulating oil further decreases, eventually making it impossible to secure the required flow of insulating oil. .

A second problem is also caused by the kraft paper layer being compressed.

In other words, the compression of kraft paper inevitably increases the surface pressure between adjacent tapes, which makes it difficult for the tapes to slide against each other and causes wrinkles in the tapes when the cable is bent during installation. cause

These phenomena become more pronounced as the rate of increase in the thickness of the polypropylene layer increases, and as the proportion of the thickness of the polypropylene layer in the total thickness of the polypropylene laminated paper increases. In other words, the greater the rate of increase in the thickness of the polypropylene layer, the smaller the proportion of the polypropylene layer in the total thickness of the polypropylene laminated paper needs to be. However, from the point of view of the electrical properties of polypropylene laminated paper, the proportion of the polypropylene layer should be larger. Therefore, in order to obtain a power cable with excellent performance, it is necessary to minimize the proportion of increase in the thickness of the polypropylene layer. is important.

The present invention has been developed in view of these circumstances, and is characterized by using a mixture of silicone oil and alkylbenzene as the impregnating oil in order to suppress the increase in the thickness of the polypropylene layer.

When conventional polypropylene laminated paper insulation is impregnated with a hydrocarbon-based insulation layer such as mineral oil or alkylbenzene, it is impossible to avoid a certain degree of increase in the thickness of the insulation layer due to swelling. For example, techniques have been devised to carry out superheating treatment in a vacuum at a temperature below the melting point of polypropylene.

On the other hand, in order to prevent the plastic film from swelling, the difference between the solubility parameter (SP value) of the plastic film and the SP value of the insulating oil should be 2.6 or more, thereby preventing the insulating oil from dissolving into the plastic film. It is also known that
(Refer to Japanese Patent Publication No. 47-25951.) In this known technique, polyphenylene oxide, polycarbonate, polystyrene, polysulfone, etc. with a large SP value are used as the plastic film, and silicone oil with a small SP value is used as the insulating oil. In order to absorb gas (H ₂ ) generated from the silicone oil, polybutene with a small SP value is mixed with the silicone oil.

However, in the known technique, the difference in SP values is so large that the voids existing in the plastic film cannot be filled, and there is a risk that damage may occur due to corona discharge caused by the voids. Furthermore, since a plastic film with a high SP value must be used, polypropylene, which has good electrical properties (low SP value), cannot be used as the film or adhesive layer. Furthermore, since the types of plastic film and insulating oil that can be used are limited in combination, there are restrictions on material selection and there are problems in terms of cost.

Furthermore, a technique is known in which a highly airtight insulating paper and thick biaxially oriented polypropylene are bonded and integrated with thin molten polypropylene, and this insulating layer is impregnated with insulating oil to make a cable tape. (Unexamined Japanese Patent Publication 1973)
(Refer to Publication No. 11583, hereinafter referred to as Prior Application 1) This known technology improves working stress, but is insufficient in terms of mechanical cable application.

Furthermore, the amorphous portion is reduced to 3% or less, and the molecular weight is 10 ⁴
There is also a known technology for impregnating silicone oil into a polypropylene film containing 0.3% or less of the following: (Refer to Japanese Patent Publication No. 48-15751, hereinafter referred to as Prior Application 2
In this known technology, hydrocarbon insulating oil dissolves and swells polypropylene, so the dissolution of polypropylene is prevented by using silicone oil alone, but the amount of swelling and oil flow resistance are It was a big one.

In contrast, the present invention aims to alleviate the above-mentioned problems from the perspective of impregnated insulating oil.

That is, by using a mixed oil in which alkylbenzene and silicone oil are mixed in a predetermined ratio, it is possible to prevent the polypropylene layer from swelling, improve gas absorption, prevent electrical breakdown due to corona discharge, and further improve the polypropylene layer. The purpose is to fill voids and prevent corona discharge.

In other words, the feature of the present invention is that the swelling caused by the mixed oil is not completely prevented, but is caused to swell to the extent that it fills the voids.

The figure shows silicone oil (KF manufactured by Shin-Etsu Chemical Co., Ltd.) mixed with hard type alkylbenzene, which is a hydrocarbon-based insulating oil.
-96 50 cst), immersed polypropylene laminate paper in it, and heated it at 100°C. The thickness increase rate is shown in relation to the mixing ratio of silicone oil and alkylbenzene. The polypropylene laminate paper used in this experiment has a thickness of 43 μm.
Thickness made by bonding together two sheets of kraft insulating paper for power capacitors with molten polypropylene.
It is 125 μm.

As shown in the figure, as the ratio of silicone oil increases, the increase in thickness due to swelling of polypropylene laminated paper decreases, and in extreme cases, that is, in the case of only silicone oil, this increase in thickness becomes zero. It's summery. Therefore, it is preferable to use only silicone oil as the impregnating oil only from the standpoint of reducing the increase in thickness due to swelling of the target polypropylene laminated paper. However, silicone oil has poor gas absorption properties, poor corona resistance, and is relatively expensive, making it inappropriate to use only silicone oil as an impregnating oil for high-voltage cables.

On the other hand, hydrocarbon-based insulating oils such as alkylbenzene have the disadvantage of swelling polypropylene, but they have excellent gas absorption and corona resistance, and they also penetrate into polypropylene and eliminate defects such as voids within it. It has the effect of filling and improving breaking strength. Therefore, it is necessary to use a mixture of hydrocarbon-based insulating oil and silicone oil as the impregnating oil for the power cable whose insulating layer is formed by winding polypropylene laminate paper.

In the case of a hydrocarbon oil such as hard type alkylbenzene, the amount of silicone oil to be mixed is preferably in the range of about 5% to 75% by weight. In other words, the silicone oil ratio is 5
If it is less than 75%, it will not be very effective in suppressing swelling, which is the main objective of the present invention, and if it is more than 75%, it will have unfavorable properties in terms of gas absorption or corona resistance.

The characteristics of the present invention and the known technology are compared and shown in Table 1 of the appendix.

As can be seen from Table 1, the present invention is the only one that is good in terms of swelling amount, mechanical strength, and electrical performance.

Although the above description has been made using an example of an insulating tape in which two sheets of kraft paper are bonded together with melt-extruded polypropylene, the effects of the present invention are not limited to kraft paper, but can also be applied to synthetic fiber paper such as polypropylene fiber paper, or fiber Similar results can be expected in tapes made by bonding paper and plastic film together with melt-extruded polypropylene, or laminated paper made from three or more sheets of fiber paper or plastic films.

[Brief explanation of the drawing]

The figure shows the relationship between the mixing ratio of hard type alkylbenzene and silicone oil (KF-96 50cst, manufactured by Shin-Etsu Chemical Co., Ltd.) and the thickness increase rate of polypropylene laminated paper. (Temperature 100℃).

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【table】

Claims (1)

[Claims] 1. A composite tape made by bonding together fiber paper or fiber paper and plastic film by melt extrusion of polypropylene is wound around a conductor, and a composite tape made of silicone oil and alkylbenzene mixed with silicone oil in a weight ratio of 5 to 75% is wound around a conductor. An oil-immersed power cable characterized by being impregnated with a mixture.