JPH06188053A - Forming method for cable connecting part - Google Patents

Abstract

PURPOSE:To provide a forming method for a cable connecting part, obtainable an excellent electric characteristic appliable to even a super-high-voltage cable exceeding 275kV, e.g. 500kV, and moreover excellent in workability having a short working time. CONSTITUTION:A conductor 12 is connected by a compressed sleeve 13, then a heat contracted tube, composed of fluoreresin and being astride a cable insulating body 14, is mounted on the outer periphery of the conductor 12, and the surface of the cable insulating body 14 is heated from above at a temperature of e.g. 400 deg.C or more to form a polyethylene layer 15 of non-bridge-formation or a low-bridge-formation degree on the surface layer part of the cable insulating body 14. Then the heat contracted tube is removed, a split die for forming a reinforcement insulating body is coveredly engaged on a conductor connecting part with the cable insulating body 14 strided, and heat-melted super-low-density polyethylene, having e.g. a melting point of 116 deg.C and a density of 0.890, is pressed in and hardened to form a reinforcement insulating body 16 composed of non-bridged polyethylene.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a cable connecting portion, and more particularly to a method of forming a cable connecting portion useful for forming a connecting portion of an ultra high voltage crosslinked polyolefin insulation cable having a voltage of more than 275 kV.

[0002]

2. Description of the Related Art Due to the recent increase in power demand, development of CV cables (cross-linked polyethylene insulation cables) exceeding 275 kV, for example 500 kV, has been promoted, and cable connection parts with excellent electrical performance applicable to such ultra-high voltage cables have been developed. There is a request.

Conventionally, the following extrusion molding method has been generally used to form the connecting portion of the CV cable.

That is, according to this method, as shown in FIG. 2, a split mold (not shown) for molding a reinforcing insulator is formed on the outer periphery of the conductor connecting portion 2 which is connected by stripping off the end portion of the cable 1 to be connected. ), And the insulating polyethylene composition containing a cross-linking agent that has been heated and melted is press-fitted into the split mold to form an insulator mold 3, and then the insulator mold 3 is heat-crosslinked to form a spindle. To form a reinforced insulator 4 in the shape of
It is characterized in that the reinforcing insulator 4 has excellent interfacial adhesion with the cable insulator 5, and a compact and highly reliable connection portion can be obtained.

However, in this method, since a cross-linking agent is used, its decomposition residue (for example, when dicumyl peroxide (DCP) is used as the cross-linking agent, acetophenone, cumyl alcohol, water, etc. are decomposed). Remains in the reinforced insulation as a residue.) Remains in the reinforced insulation to generate voids, which deteriorates the excellent electrical properties inherent to the crosslinked polyethylene. It takes time and the construction time is long.

On the other hand, as a method which requires a short construction time and is excellent in workability, there is known a method (prefabricated method) of on-site assembly using an epoxy unit and a rubber mold stress cone which are manufactured in advance in a factory. . However,
Although this method is excellent in workability, the adhesion of the interface is somewhat insufficient, and it is difficult to apply it to the connection part of a cable exceeding 275 kV, for example, 500 kV, in terms of electrical performance.

[0007]

As described above, there is a demand for forming a connecting portion for an ultra-high voltage CV cable exceeding 275 kV, for example, 500 kV, but in the conventional extrusion molding method, an insulating polyethylene composition containing a crosslinking agent is used. Therefore, voids may occur due to the decomposition residue of the cross-linking agent, and since a cross-linking step is required, there is a problem in workability. On the other hand, in the prefabricated method, the construction time is short and the workability is excellent, but the interface adhesion is poor and
It is difficult to apply it to the connection of ultra-high voltage cables exceeding kV.

The present invention has been made in consideration of such conventional circumstances, and has excellent electric characteristics applicable to an ultra-high voltage cable exceeding 275 kV, for example, 500 kV, and is formed in a relatively short time. It is an object of the present invention to provide a method for forming a cable connection portion which has excellent workability.

[0009]

The method for forming a cable connecting portion of the present invention is a non-crosslinking, melting point of 115 ° C. or more and crystallized over the cable insulator on the outer periphery of the conductor connecting portion of a crosslinked polyolefin insulated cable. It is characterized in that a reinforcing insulator made of polyolefin having a low degree of conversion is molded.

In the present invention, a polyolefin having a melting point of 115 ° C. or higher and a low crystallinity has a melting point of 115
Linear low-density polyethylene (LLDPE: a copolymer of ethylene and α-olefin having a linear molecular structure) at a temperature of ℃ or higher and a density of 0.930 or lower, or a melting point of 115 ℃ or higher and a density of 0.910 or lower. Ultra low density polyethylene (V
In addition to LDPE), MILASTOMER (trade name) manufactured by Mitsui Petrochemical Co., Ltd., which is known as a thermoplastic polyolefin-based elastomer, can be mentioned. Among them, the melting point is 115 ° C.
As described above, VLDPE having a density of 0.910 or less is preferably used because it forms a reinforcing insulator having excellent interfacial adhesion with the cable insulator.

In the present invention, prior to the molding of the reinforcing insulator, the molecular chain of the crosslinked polyolefin at the portion is cut on the surface of the cable insulator and in the vicinity thereof to reduce the molecular weight, that is, non-crosslinked or low crosslinked. It is desirable to carry out a surface treatment of a polyolefin. As a result, the adhesion of the interface between the reinforcing insulator and the cable insulator can be improved, and the electrical performance can be improved. That is, by performing such surface treatment, concentration of stress at the interface due to heat shrinkage or crystallization shrinkage can be avoided, and occurrence of voids at the interface can be prevented. As a surface treatment method for this cable insulator, plasma reduction treatment or heat treatment in an inert gas atmosphere such as nitrogen gas can be used.

[0012]

In the method of the present invention, since a reinforced insulator made of a specific polyolefin having good heat resistance of non-crosslinking and low crystallinity is molded, the heat resistance is almost equal to that of the crosslinked polyolefin, and the electrical characteristics are An improved reinforced insulator is formed. That is, since the decomposition residue of the cross-linking agent does not remain in the reinforcing insulator, the DC withstand voltage characteristics and the impulse withstand voltage characteristics are superior to those of the cross-linked polyolefin, and the decomposition residue of the cross-linking agent is also improved. The generation of voids due to this is also prevented. Further, although the adhesive strength at the interface with the cable insulator is slightly weaker than that of the cross-linked polyolefin, since the crystallinity of the polyolefin is small and stress is less likely to be applied to the interface, the interface does not become an electrical weak point. On the other hand, the working time is significantly shortened because the crosslinking step is not performed.

In particular, prior to the molding of the above-mentioned reinforcing insulator, the surface of the cable insulator and its vicinity are subjected to a surface treatment for cutting the molecular chains of the cross-linked polyolefin to reduce the molecular weight thereof. In this case, the adhesion of the interface between the reinforcing insulator and the cable insulator can be further improved, and the electrical characteristics can be further improved.

[0014]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a vertical cross-sectional view of a CV cable connection part in the process of being formed according to an embodiment of the present invention. In FIG. 1, in this embodiment, first, the end portion of the CV cable 11 is stripped off and the exposed conductor 12 is connected by a conventional method, for example, a compression sleeve 13. Here, if necessary, although not shown, an internal semiconductive layer is formed on the conductor connecting portion by a conventional method, although not shown. Next, a heat-shrinkable tube (not shown) having good heat resistance, for example, made of fluororesin is attached to the outer periphery of the cable insulator 14 and the surface of the cable insulator 14 is placed on the heat shrinkable tube, for example, 400 ° C. Heat at the above temperature. By this heat treatment, the molecular chains of the cross-linked polyethylene in the surface layer portion of the cable insulator 14 are partially cut to form the polyethylene layer 15 having a non-crosslinked or low crosslinked degree.

After the non-crosslinked or low crosslinked polyethylene layer 15 is formed on the surface layer portion of the cable insulator 14 in this manner, the heat-shrinkable tube that has been attached is removed, and the surface is washed with alcohol or the like. Then, a split mold (not shown) having a spindle-shaped cavity for forming a reinforcing insulator is fitted over the conductor connecting portion across the cable insulator 14. Then, a polyethylene composition having a melting point of 115 ° C. or higher and a low crystallinity, for example, an ultra-low density polyethylene having a melting point of 116 ° C. and a density of 0.890, which has been melted by heating and is not melted, is press-fit into the mold, When the inside is sufficiently filled, press-fitting is stopped and cooling is started. As the temperature decreases, the in-mold composition gradually hardens to form the reinforcing insulator 16 made of non-crosslinked polyethylene having good electrical characteristics and being in close contact with the cable insulator 14. After that, the mold is opened, the outer shape is adjusted by finishing, and if necessary, an outer semiconductive layer or the like is further provided on the outer periphery thereof to complete the cable connecting portion of the present invention.

Since such a method does not include a crosslinking step, it can be applied in a short time. Moreover, the reinforced insulator 16 formed has good heat resistance, and since it does not contain decomposition residues of the crosslinking agent, it has excellent DC withstand voltage characteristics and impulse withstand voltage characteristics. Further, there is no fear of generation of voids due to the decomposition residue of the cross-linking agent, and the adhesion with the cable insulator is good. Therefore, it can be sufficiently applied to the formation of a connection portion of an ultrahigh-voltage cross-linked polyolefin insulation cable having a voltage of more than 275 kV, for example, 500 kV, and is very useful as a method for forming a cable connection portion having excellent electrical characteristics and workability.

By the way, Table 1 shows the characteristics of the reinforced insulator formed by applying the above method to the connection portion of the 6.6 kV CV cable (conductor cross-sectional area 250 mm ² ). In addition, in Table 1, examples formed in the same manner as above except that the surface treatment of the cable insulator was not performed, except that plasma reduction treatment in a nitrogen gas atmosphere was used instead of heat treatment as the surface treatment method. Shows examples formed in the same manner as above (above, Examples 1 to 6). Furthermore, for comparison, an example using general-purpose polyethylene having a melting point of 110 ° C. and a density of 0.922 (Comparative Example 1), 2% of DCP as a cross-linking agent was added to the general-purpose polyethylene.
An example is also shown in which the added material is used to form a film by a conventional extrusion molding method (Comparative Example 2).

[0018]

[Table 1] The above examples are examples in which the extrusion molding method is used as the method for molding the reinforced insulator, but the present invention is not limited to such examples, the so-called tape winding molding method, that is, a crosslinking agent. Unblended melting point is 115 ° C
A method of winding a tape made of polyolefin having a low crystallinity as described above, winding a pressing tape on the winding layer and heating while applying pressure, and fusing the winding layers together may be used. Good.

[0019]

As described in the above examples, according to the method of the present invention, the heat resistance is excellent, the influence of the cross-linking agent is not exerted, and the adhesion with the cable insulator is excellent and the electrical characteristics are remarkably high. An excellent reinforcing insulator can be formed in a short time. Therefore, compared to the conventional method, it is possible to manufacture a small and reliable cable connection in a short working time, and it is also useful for forming a connection of an ultra-high voltage cross-linked polyolefin insulation cable exceeding 275 kV, for example, 500 kV. Is.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a CV cable connection part in the process of being formed according to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing a CV cable connecting portion in the process of being formed by a conventional extrusion molding method.

[Explanation of symbols]

 11 ... CV cable 12 ... conductor 13 ... compression sleeve 14 ... cable insulator 15 ... non-crosslinked or low crosslinked polyethylene layer 16 ... non-crosslinked reinforced insulation made of polyethylene

Claims (3)

[Claims] 1. A non-crosslinked, reinforced insulator made of polyolefin having a melting point of 115 ° C. or higher and a low degree of crystallinity is formed on the outer periphery of a conductor connecting portion of a crosslinked polyolefin insulated cable across the cable insulator. A method for forming a cable connecting portion, characterized by: 2. Prior to molding of the reinforced insulator,
2. The method for forming a cable connecting part according to claim 1, wherein the surface of the cable insulator and its vicinity are subjected to a surface treatment for cutting the molecular chains of the cross-linked polyolefin of the part to reduce the molecular weight. 3. The method for forming a cable connecting portion according to claim 2, wherein the surface treatment is plasma reduction treatment or heat treatment in an inert gas atmosphere.