JPH04368719A - Dc power cable - Google Patents

Abstract

PURPOSE:To improve a DC power cable used for a high-voltage power transmission line such as a submarine cable. CONSTITUTION:In a DC power cable formed with an insulator made of a polymer on the outer periphery of a core conductor, the insulator is made of polyethylene or cross-linked polyethylene added with magnesium oxide surface-treated with a vinyl silane coupling agent. Coagulated coarse grains adversely affecting the breakdown characteristic can be reduced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in DC power cables suitable for use in high voltage DC power transmission lines such as submarine cables.

0002

[Prior Art] Currently, power cables made of polyethylene or cross-linked polyethylene as insulators are being used as power cables for AC power transmission because they have many advantages in terms of excellent insulation properties, ease of maintenance, and disaster prevention. CV cables are widely used, and combined with significant advances in manufacturing technology in recent years, they have now come into practical use as 500kV cables.

As described above, CV cables have many excellent features and a proven track record as AC cables, but when they are applied to high-voltage DC power transmission, problems specific to DC insulation become apparent, and domestic Of course, there are still no examples of its application to actual railway lines anywhere in the world. It is generally known that a typical problem is the presence of space charges that are formed within the insulator when a DC voltage is applied to the cable.

For example, it is known that when a negative DC voltage is applied to a cable, a negative space charge is formed near the conductor side, and conversely, a positive space charge is formed near the shield side. In such a case, although the electric field directly above the conductor electrode and at the shielding electrode is relaxed, it not only generates a localized high electric field inside the insulator, but also reduces the effective insulation thickness of the cable. This is also known. Furthermore, if a lightning impulse voltage of opposite polarity (in this case positive) to the DC voltage enters such a state, or if the polarity of the DC voltage is suddenly reversed, the electric field directly above the conductor electrode, which had been relaxed by the space charge, will be weakened. This results in an unexpected decrease in breakdown voltage.

[0005] Therefore, in order to apply a cable made of polyethylene or cross-linked polyethylene as an insulator for direct current use,
It is a necessary condition to suppress the formation of the above-mentioned space charge as much as possible, and various proposals have been made so far as measures for suppressing it.

For example, as shown in Japanese Patent Publication No. 57-21805, 20 to 80 parts by weight of a polar non-flat shaped inorganic insulating powder having a particle size of 50 microns or less, that is, aluminum silicate, is added to polyethylene. One example is a cable that has a cross-linked insulator containing calcium silicate, calcium carbonate, magnesium oxide, etc., and a water-blocking layer provided around its outer periphery. This is to prevent a decrease in DC dielectric strength due to

However, while the above-mentioned conventional DC cable has improved DC breakdown characteristics, the polar inorganic insulator added to polyethylene acts as a foreign substance, which is another characteristic required for cables. There is a problem in that the impulse rupture properties are lower than that of additive-free (unfilled) crosslinked polyethylene. In particular, such inorganic insulating powders are not uniformly dispersed in polyethylene,
If particle agglomeration occurs in the insulator, it results in defects that are undesirable in terms of electrical insulation, significantly reducing the breakdown characteristics.

[0008] In addition, particle agglomeration is likely to occur even in the powder state before being mixed with polyethylene due to the absorption of moisture contained in the air, causing clogging of the screen mesh of the compound manufacturing machine. There is a problem that it cannot cope with mass production. For mass production, it is possible to make the screen mesh coarser, but this impairs the original function of the screen mesh, and inevitably there is a problem that it cannot remove foreign substances sufficiently as an insulator.

[0009] The purpose of this invention was to solve the above-mentioned problems, and the above-mentioned effect of suppressing space charge formation is impaired in polyethylene or cross-linked polyethylene insulated cables to which polar inorganic insulating powder is added. The object of the present invention is to provide a DC cable that has improved lightning impulse breakdown characteristics, enables rational insulation design, and is highly reliable and economical.

0010

[Problems to be Solved by the Invention] The present invention provides a DC cable made of polyethylene or cross-linked polyethylene insulator to which magnesium oxide, which has been surface-treated with a vinyl silane coupling agent, is added among the above-mentioned polar inorganic powders. It is something.

[0011] Here, the reason why magnesium oxide is used as an additive will be explained. Among the many polar inorganic insulating powders that are effective in suppressing space charge accumulation in insulators, magnesium oxide was specifically selected for the following reason.

(a) The impulse rupture properties of polyethylene or crosslinked polyethylene to which magnesium oxide is added are smaller than those of other polar inorganic insulating powder additives. That is, in a comparison of various polar inorganic insulating powder-filled polyethylenes that are thought to have a space charge suppressing effect, the product containing magnesium oxide has the highest impulse rupture strength.

(b) Since polar inorganic insulating powders such as talc and clay, which are widely used industrially as fillers, are natural ores, they contain many impurities that are harmful to electrical insulation, such as iron oxide, and their removal is difficult. There are also industrial limits. On the other hand, magnesium oxide can be obtained from magnesium-containing natural ores in terms of resources, but it can also be produced from artificially synthesized raw materials using magnesium salts in seawater, and is much more abundant than the natural ores mentioned above. It has the advantage of being able to supply materials with high purity and stable quality and physical properties.

Next, the reason why the surface treatment agent for magnesium oxide is limited to a vinyl silane coupling agent will be explained. Among the many surface treatment agents for filler materials, we selected a vinyl silane coupling agent because of its ability to form a stable chemical bond between polyethylene and magnesium oxide, and its inherent insulation resistance properties as cross-linked polyethylene filled with magnesium oxide. This is because it has been confirmed through experiments that the insulator is highly stable, has good dispersibility during kneading, and can produce a uniform insulator.

Furthermore, the reason why the surface treatment method is limited to a dry method will be explained. As a surface treatment method for filler materials, (
Examples include a) dry method, (b) wet method, and (c) integral blend method. Among these methods, the dry method was selected because it is superior in mass production compared to the other two methods, can be processed uniformly, and can yield homogeneous cable insulation. This is because it was confirmed that the powder itself has excellent moisture absorption resistance and is difficult to cause agglomeration of the powder itself.

[0016]

[Example] First, in order to find the optimal surface treatment agent for magnesium oxide as a filler for polyethylene, we treated it with various surface treatment agents that are generally used industrially for inorganic fillers. Magnesium oxide was produced, and its hygroscopicity and ability to pass through a screen mesh (indicators of cohesiveness as a powder) were investigated. Table 1 shows the types of surface treatment agents investigated and the test results. Here, the hygroscopicity is determined from the weight increase rate when magnesium oxide treated with various surface treatment agents is left at room temperature for 10 days in an atmosphere with a humidity of 95%. In addition, the mesh passing property was evaluated by placing the treated magnesium oxide in a container equipped with a 325-mesh screen mesh and evaluating the passing state by the tapping method. In the table, they are displayed in the order of ◎→○→△→× from the one with the best passability. For comparison, the results of a sample without surface treatment (sample J) are also shown.

Incidentally, the determination of mesh passability is as follows.

◎ Most of the particles passed through the mesh and no aggregated particles remained in the container.

○ Most of the particles passed through the mesh, but some aggregated particles remained in the container.

△ Approximately half of the particles passed through the mesh, but the rest remained in the container as aggregated particles.

× A small amount passed through the mesh, but most remained in the container as aggregated particles.

[0022]

[Table 1]

As is clear from Table 1, Samples A to I, which were subjected to surface treatment to varying degrees, had improved hygroscopicity and mesh permeability compared to Sample J, which had not been surface treated.

In particular, the silane coupling agents (Samples D to I) were generally good in terms of hygroscopicity, but among them, the vinyl silane coupling agent (Sample D) had very little moisture absorption.

On the other hand, in terms of mesh penetration, samples A and D
, G, and I were relatively good. With the exception of sample A, all of the samples had relatively low hygroscopicity as described above, suggesting that moisture absorption tends to promote agglomeration of the powder. In addition,
Although the reason for sample A, which is classified as rather high in hygroscopicity, is not clear, it was very good in terms of mesh penetration, similar to sample D.

From the above results, sample D was judged to be promising as a surface treatment agent. Furthermore, in order to evaluate from the viewpoint of electrical properties, we took Sample D and four samples A, G, and I, which had good mesh passability, and used crosslinked polyethylene sheets treated with four types of surface treatment agents and added with magnesium oxide. was created, and the specific insulation resistance and occurrence of agglomerated particles in the crosslinked polyethylene sheet were investigated.

The samples were prepared by adding various surface treatment agents A, D, G, and I shown in Table 1 to 100 parts by weight of low-density polyethylene.
Adding 5 parts by weight of magnesium oxide treated with
m sheets.

The specific insulation resistance was measured at a temperature of 90° C. and by applying a DC voltage of 4 kV in order to check the stability at high temperatures. In addition, aggregated particles were observed using an optical microscope.

The results of these comparisons are shown in Table 2.

[0030]

[Table 2]

As is clear from Table 2, sample D showed the highest specific insulation resistance among the four samples compared, and the aggregated particles observed in the crosslinked polyethylene were also small. It should be noted that Sample A, which had good mesh passing properties in Table 1, had more aggregated particles than expected in polyethylene. From the above, it was determined that the vinyl-based silane coupling agent of Sample D was the most suitable as a surface treatment agent for magnesium oxide.

[0032] In order to maximize the effect of surface treatment, it is thought that there is a close relationship not only with the type of surface treatment agent but also with the surface treatment method. Therefore,
The optimal surface treatment method for Sample D described above was further investigated.

Surface methods for inorganic powder include (1) dry method, (
There are 2) wet method, (3) integral blend method, etc.
They are used differently depending on the use and purpose.

The dry method (1) above is a dry method in which the filler to be surface treated is stirred at high temperature and the surface treatment agent or its solution is sprayed. This method is suitable for mass production as long as you have the equipment, but
Since it is sprayed and adsorbed onto the surface, the treatment may become uneven. Incidentally, the surface treatments of the samples shown in Tables 1 and 2 were all performed by (1) dry method.

In the wet method (2) above, the filler is slurried with water or a solvent, a surface treatment agent is added thereto, stirred, and then dried. It is said that it is not suitable for mass production because it requires a large amount of heat to remove the solvent, but uniform processing is possible.

[0036] The integral blend method (3) above is as follows:
The surface treatment agent is directly added to the mixed form of filler and resin and mixed by stirring, eliminating the need for surface treatment of the filler, but it also makes the treatment of the powder itself more uniform and reduces the agglomerated particles that are the problem here. Its effectiveness is questionable compared to the two mentioned above.

From the above, using the surface treatment agent of sample D described above, the surface treatment methods include (1) dry method;
(2) A comparative evaluation of hygroscopicity and mesh permeability was performed in the case where a wet method was adopted.

Table 3 shows the evaluation results for both.

[0039]

[Table 3]

[0040] As can be seen from Table 3, the wet method, which was expected to achieve uniform surface treatment, unexpectedly resulted in poor moisture absorption and mesh passability, and the size of the largest aggregated coarse particles, which was the most problematic, also increased. was confirmed.

From the above, it was determined that the dry method (1) is suitable for the surface treatment method.

[0042]

[Effects of the Invention] As explained above, according to the DC power cable of the present invention, a vinyl silane coupling agent is used as the insulator formed on the outer periphery of the core conductor,
In addition, by using dry-processed surface-treated polyethylene with added magnesium oxide or cross-linked polyethylene as the insulator, it is possible to reduce agglomerated coarse particles that have a negative effect on fracture characteristics, and improve DC and lightning impulse fracture characteristics. We were able to achieve improvement. At the same time, problems during cable manufacturing caused by clogging of the screen mesh have been resolved.

Claims (2)

[Claims] 1. A DC power cable in which an insulator made of a polymer is formed on the outer periphery of a core conductor, wherein the insulator is made of polyethylene added with magnesium oxide or cross-linked polyethylene that is surface-treated with a vinyl-based silane coupling agent. A DC power cable characterized in that: 2. The DC power cable according to claim 1, wherein the surface treatment of the magnesium oxide is carried out by a dry method.