JP2002251921A - Dc power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plastic insulated DC power cable having light weight, having a small finishing outer diameter, suppressing a cost, and enhancing heat radiation property. SOLUTION: As an insulator 13, maleic anhydride grafted high density polyethylene having a density of 0.925 g/cm<3> or more and a maleic acid anhydride grafted amount of 0.01-5 wt.% is used. By using the insulator, drop in DC characteristics of the insulator caused by submergence is prevented, and an impervious layer such as a lead shield is made unnecessary. The thermal expansion of the insulator is made small, and a cushion layer for absorbing the thermal expansion is made unnecessary. The DC power cable having light weight, a small diameter, a low cost, and high electric characteristics can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC power cable, which uses an insulator made of maleic anhydride-grafted polyethylene, thereby eliminating the need for a water-proof layer for preventing water from entering, such as lead coating, and thereby reducing the cable. Lightweight and
The finished outer diameter is reduced to improve heat dissipation.

[0002]

2. Description of the Related Art A DC power cable has advantages over an AC power cable in that it has no dielectric loss and does not require a facility for compensating for an ineffective component with respect to a charging current. Also, in general, the insulation withstand voltage of insulators is larger in DC than in AC, and there is no need to consider stability.Therefore, the longer the transmission distance and the larger the capacity of transmission, the more the merit of a DC power cable appears. Is known to come.

[0003] DC power cables are often used as submarine cables. For this reason, oil-immersed insulated cables such as OF cables and solid cables that are currently in practical use are provided with a thick water-impervious layer made of metal such as lead coating to maintain oil pressure and prevent oil leakage. ing. In addition, despite the use of solid insulators made of cross-linked polyethylene or modified cross-linked polyethylene, plastic-insulated DC cables currently being developed are still made of thick metal, such as lead sheath, for the purpose of preventing inundation. The impermeable layer is provided.

FIG. 2 shows such a plastic insulated DC power cable, wherein reference numeral 1 denotes a conductor.
On the conductor 1, an inner semiconductive layer 2, an insulator 3, an outer semiconductive layer 4, a cushion layer 5, a lead coating (water shielding layer) 6, and an exterior 7 are sequentially provided. The insulator 3 is made of a crosslinked low-density polyethylene, and has a thickness of about 15 to 25 mm. The cushion layer 5 is made of foam rubber or the like and has a thickness of 2 to 3 mm. Further, the thickness of the lead sheath 6 as a water-blocking layer is 3 to 5 mm.

In such a plastic insulated DC power cable, since the thermal expansion coefficient of the crosslinked low-density polyethylene is relatively large, it is necessary to absorb the thermal expansion of the insulator 3 in the radial direction. ing. However, since the cushion layer 5 is made of a foam, it has good heat insulation properties, and has a drawback that heat dissipation from the conductor 1 is prevented. In addition, since the cushion layer 5 is made of a foam, it is easy to absorb water, and therefore, it is necessary to apply a thick lead cover 6 to increase the water shielding. Further, the provision of the cushion layer 5 has a disadvantage that the finished outer diameter of the cable is increased.

As described above, the plastic insulated DC power cable having the insulator 3 made of cross-linked low-density polyethylene has problems that the weight is heavy, the finished outer diameter is large, the cost is high, and the heat dissipation is poor. Was.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a plastic insulated DC power cable which is lightweight, can have a small finished outer diameter, can reduce costs, and has good heat dissipation. It is in.

[0008]

This problem is solved by using a maleic anhydride-grafted polyethylene as the insulator. Further, the density is set to 0.
It is preferably 925 g / cm ³ or more, and the grafting amount of maleic anhydride is preferably 0.01 to 5 wt%.
Further, the conductor preferably has a watertight structure.

[0009]

[Action] It has been clarified that the maleic anhydride-grafted polyethylene hardly deteriorates in electrical characteristics even in a state of being immersed in seawater, fresh water or the like under a DC voltage. That is, it has been confirmed by the present inventors that there is almost no occurrence of water trees under DC, and the DC breakdown voltage and DC insulation resistance tend to be rather improved.

The reason is that the polyethylene portion serving as the base of the maleic anhydride-grafted polyethylene is hydrophobic and hardly absorbs water. It is considered that the injected charge and the ionic carrier during the application of the DC high voltage are trapped (trapped) on the basis thereof, thereby improving the insulation resistance and the breakdown voltage.

From such a viewpoint, by using maleic anhydride-grafted polyethylene as the insulator, even if moisture enters the insulator, the electrical characteristics of the insulator do not deteriorate, and moisture enters the cable. There is no need to dare to provide a water barrier layer for preventing water leakage. The density is 0.925
g / cm ³ or more of maleic anhydride-grafted polyethylene reduces the thermal expansion of the insulator in the radial direction,
A cushion layer for absorbing thermal expansion of the insulator is not required.

Further, by setting the grafting amount of maleic anhydride to 0.01 to 5% by weight, the DC insulation characteristics are also improved. Also, by using a water-tight structure as the conductor, if there is no water impervious layer in the event of inundation in the cable,
Although there is a possibility that water may penetrate into the conductor, if the conductor has a watertight structure, water does not run in the conductor, and damage due to flooding can be suppressed to a narrow range.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
FIG. 1 shows an example of a DC power cable of the present invention, which is a specification for a submarine cable. 1 in FIG.
1 indicates a conductor. On this conductor 11, the inner semiconductive layer 1
2, an insulator 13, an outer semiconductive layer 14, a shielding layer 15, an anticorrosion layer 16, and an exterior 17 are sequentially provided.

The conductor 11 is a segmental conductor obtained by using a number of rectangular conductors as strands and twisting them in an aligned state. The conductor 11 has a high conductor space factor and has almost no gap between the strands. is there. The inner semiconductive layer 12 and the outer semiconductive layer 14 are made of a semiconductive resin composition obtained by blending carbon black with an ethylene-vinyl acetate copolymer, an ethylene-ethyl acrylate copolymer, or the like.

The insulator 13 has a density of 0.925 g / cm ^3.
As described above, it is preferably made of maleic anhydride-grafted high-density polyethylene having a density of 0.940 g / cm ³ or more. In general, polyethylene has such a characteristic that as its density increases, the degree of crystallinity increases and its coefficient of thermal expansion decreases. For example, the thermal expansion of high-density polyethylene with a density of 0.95 g / cm ³ is from room temperature to 80 ° C. as compared to the thermal expansion of low-density polyethylene with a density of 0.92 g / cm ^3.
Between about 30 to 40%. For this reason, density 0.
Even maleic anhydride-grafted polyethylene of 925 g / cm ³ or more has a small thermal expansion.

It is known that maleic anhydride-grafted polyethylene having a density of 0.925 g / cm ³ or more has a small accumulation of space charge at the time of charging, and has extremely excellent DC insulation performance (Japanese Patent Publication No. 6-1994). No. 16366). This is because the carbonyl group present in the molecule of the maleic anhydride-grafted polyethylene is rich in electron affinity or electron resonance, and the presence of an appropriate amount of this carbonyl group reduces trapping of injected space charges. is there. Therefore, the insulator 13 has a small thermal expansion and excellent DC insulation characteristics.

The grafted amount of maleic anhydride in the maleic anhydride-grafted polyethylene constituting the insulator 13 is 0.01 to 5% by weight, preferably 0.2 to 1% by weight.
When the content is less than 0.01% by weight, the DC insulation characteristics are not improved. When the content is more than 5% by weight, the degree of crystallinity is reduced, and the dielectric breakdown strength is reduced. In addition, the melt flow rate is 0.0
It is preferably 5 to 10 g / min. This maleic anhydride grafted polyethylene is produced by adding a maleic anhydride and a radical generator such as an organic peroxide as a catalyst to polyethylene such as low-density polyethylene, linear low-density polyethylene, or high-density polyethylene, heating, Kneading and graft polymerization are performed.

The shielding layer 15 is formed by winding a metal tape having excellent corrosion resistance, for example, a stainless steel tape having a thickness of 0.05 to 0.5 mm, a tin-plated copper tape or the like on the outer semiconductive layer 14 in a horizontal direction. It is.

The anticorrosion layer 16 is formed by extrusion-coating a thermoplastic resin having excellent corrosion resistance such as low-density polyethylene and high-density polyethylene on the water-blocking layer 15.
Further, the exterior 17 is formed by densely winding a galvanized steel wire on the anticorrosion layer 16 and applying an anticorrosion paint such as a tar epoxy resin to the cable to protect the cable from external damage at the time of laying. There is something.

The DC cable having such a structure lacks the water-impervious layer and the cushion layer.
It is lightweight, has a small finished outer diameter, and has good heat dissipation. In other words, there is no thick metal waterproof layer such as lead coating, and even if the cable is immersed in the cable, the DC electrical characteristics of the insulator do not decrease as described above, so that there is no practical inconvenience. .

Further, since the thermal expansion of the insulator due to energization is small, the cushion layer becomes unnecessary, and the heat barrier is eliminated, so that the heat of the insulator can be radiated to the outside. further,
If the cushion layer is no longer needed, there is no danger that water will accumulate here. In addition, since a watertight segmental conductor is used, even if water enters the conductor, there is almost no gap between each of the segmental conductor wires, so water may run inside the conductor. Therefore, damage due to inundation can be locally suppressed.

In the DC power cable of the present invention, in addition to the segmental conductor, a normal round wire may be twisted as a strand. In addition, the density and the graft amount of the maleic anhydride-grafted polyethylene are not limited to the above ranges, but it is realistic to set the above ranges.

Hereinafter, specific examples will be described. Conductor cross-sectional area 1000
on segmental conductor mm ^2, the inner semiconducting layer, an insulator of a density 0.95 g / cm thickness 12mm made of ³ maleic anhydride-grafted high density polyethylene (about 0.2 wt% grafted maleic anhydride content), 3 external semiconductive layers
The layers were co-extruded into a cable core.

On this cable core, a thickness of 0.1 mm
A stainless steel tape is wound around to provide a shielding layer, a polyethylene anticorrosive layer is extrusion-coated on this, and a galvanized steel wire is further wound around this, and a tar epoxy-based anticorrosive paint is applied to provide an exterior, and the seabed is provided. Cable.

This cable was subjected to a long-term direct current application test under seawater immersion under the following conditions. That is, 10 cycles of + DC 540 kV, 10 cycles of −DC 540 kV, and 10 cycles of ± DC polarity inversion 480 kV were performed under a heat cycle in which the conductor temperature was from room temperature to 90 ° C., ON for 8 hours, and OFF for 16 hours.

After the application test, the residual breakdown voltage at 90 ° C. was measured. As a result, DC → 140 kV / mm and impulse → 130 kV / mm, which were equivalent to the initial characteristics (before the application test). Further, as an insulator, a density of 0.9
A similar cable was produced using maleic anhydride-grafted low-density polyethylene (maleic anhydride graft amount: 0.03% by weight) of 30 g / cm ³ , a similar direct current application test was carried out, and residual breakdown at 90 ° C. As a result of measuring the voltage, D
C → 120 kV / mm, impulse → 110 kV / mm
And was equivalent to the initial characteristics (before the current application test). From this result, it was found that even a cable without a water barrier layer exhibited excellent DC electric characteristics.

[0027]

As described above, the DC power cable of the present invention has an insulator made of maleic anhydride-grafted high-density polyethylene and does not have a water barrier layer. Therefore, the cable is lightweight, has a small finished outer diameter, and is inexpensive. If a maleic anhydride-grafted polyethylene having a density of 0.925 g / cm ³ or more is used, a cushion layer is not required, and the weight and diameter can be further reduced. Further, the graft amount of maleic anhydride is 0.01 to
If 5 wt% is used, the DC electrical characteristics are further improved. In addition, if a conductor having a watertight structure is used, it is possible to prevent water running and to minimize damage caused by inundation.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of a DC power cable of the present invention.

FIG. 2 is a schematic sectional view showing a conventional DC power cable.

[Explanation of symbols]

 13 ... an insulator.

Claims (4)

[Claims] 1. A DC power cable comprising a conductor and an insulator made of maleic anhydride-grafted polyethylene provided on a conductor, wherein the DC power cable does not have a water barrier layer for preventing water from entering the cable. 2. The DC power cable according to claim 1, wherein the density of the maleic anhydride-grafted polyethylene is 0.925 g / cm ³ or more. 3. The DC power cable according to claim 1, wherein the maleic anhydride-grafted polyethylene has a grafted amount of maleic anhydride of 0.01 to 5 wt%. 4. The DC power cable according to claim 1, wherein the conductor has a watertight structure.