JPH08264036A - Dc power cable - Google Patents

Abstract

PURPOSE: To improve the space charge characteristic when a DC voltage is applied to an insulator and practically use a DC power cable as a tidal current power cable by providing the insulator made of cross-linked polyethylene having the prescribed cross-linking degree on the DC power cable. CONSTITUTION: The insulator of this DC power cable is made of cross-linked polyethylene, and the cross-linking degree is set within the range of 30-70% in the gel fraction display. For forming the insulator, a cross-linking agent made of an organic peroxide such as dicumyl peroxide is added and kneaded to low-density polyethylene having the density of 0.93g/cm<3> or below, it is pressed and covered on a conductor to form a semiconductor layer and a sheath, and low-density ethylene is cross--linked. When the type, added quantity, and cross-linking temperature of the cross-linking agent are changed, the cross- linking degree can be freely adjusted within the range of 30-70%. As another method, low-density polyethylene is extruded and covered on the conductor to form an insulator, then an electron beam is irradiated, low-density polyethylene is cross-linked, and the insulator of cross-linked polyethylene can be obtained. When the radiation quantity of the electron beam is changed, the cross-linking degree is set to 30-70%.

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 used for DC power transmission, in which the insulator is made of crosslinked polyethylene.

[0002]

2. Description of the Related Art Practically used power cables for DC power transmission are limited to oil-immersed insulation cables (OF cables). The oil-immersed insulated cable is a conductor in which insulating paper is wound in multiple layers on a conductor, and the insulating paper and the minute gaps between the insulating paper are constantly impregnated and filled with insulating oil under pressure to form an insulator. Widely used for AC and DC power transmission from high voltage to ultra-high voltage, because it has features such as no ionization phenomenon, no deterioration of the insulator, and reduction in the thickness of the insulator that can achieve a large current capacity. Is
In particular, it has become the mainstream of underground power transmission lines in the field of ultra high voltage cables.

However, in this oil-immersed insulated cable, it is necessary to constantly apply hydraulic pressure, it is necessary to analyze oil, there is the possibility of environmental pollution due to oil leakage, and safety is not guaranteed during a fire. There are drawbacks such as. Therefore, as a DC power cable without such a defect in maintenance, a crosslinked polyethylene insulated cable (CV cable) having an insulator made of crosslinked polyethylene having an insulator made of crosslinked polyethylene that has already been put into practical use for AC power transmission is used. ) Can be used.

However, when this crosslinked polyethylene insulated cable is used for DC power transmission, the amount of space charge accumulated in the insulator made of crosslinked polyethylene increases,
Space charge characteristics are inferior to other insulating materials such as uncrosslinked low-density polyethylene and high-density polyethylene and have a drawback that they cannot be put to practical use.

[0005]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to improve the space charge characteristics of an insulator made of crosslinked polyethylene of a crosslinked polyethylene insulated cable when a direct current is applied, so that the cable can be used as a DC power transmission cable. Especially.

[0006]

The object of the present invention is to increase the degree of cross-linking by the gel fraction of cross-linked polyethylene forming an insulator to 30 to 30.
It is solved by setting the range to 70%.

The present invention will be described in detail below. In the DC power cable of the present invention, the insulator is made of cross-linked polyethylene, and the degree of cross-linking is represented by gel fraction of 30 to 70.
It is in the range of%. As a method for forming an insulator made of cross-linked polyethylene, a low-density polyethylene having a density of 0.93 g / cm ³ or less is added with a cross-linking agent made of an organic peroxide such as dicumyl peroxide and kneaded, and the resulting mixture is placed on a conductor. There is a method in which the low-density polyethylene is cross-linked by extrusion coating on the above to form a semiconductive layer, a sheath, etc., and then heating this in a cross-linking tower or the like. In this case, the degree of crosslinking can be freely adjusted within the range of 30 to 70% by changing the type of crosslinking agent, the addition amount, the crosslinking temperature and the like.

As another method, low density polyethylene is extrusion-coated on a conductor to form an insulator, which is then irradiated with an electron beam to crosslink the low density polyethylene with an electron beam.
It can be an insulator of cross-linked polyethylene. In this case, the degree of crosslinking can be controlled within the range of 30 to 70% by changing the irradiation dose of the electron beam.

Further, as another forming method, a catalyst such as an organic silane compound or an organic peroxide is added to low density polyethylene and melt-kneaded in an extruder to graft the low density polyethylene with the organic silane compound. After extruding the graft on the conductor to form an insulator, the insulator is brought into contact with water such as hot water or steam in the presence of a catalyst such as an organotin compound to crosslink low-density polyethylene (silane crosslink). There is something to do. In this case, the degree of crosslinking can be set to 30 to 70% by changing the grafting ratio of the organic silane compound, the crosslinking conditions and the like.

In the present invention, the degree of crosslinking of the insulator is expressed by the gel fraction calculated from the weight of the insoluble matter obtained by dissolving the crosslinked polyethylene in boiling xylene. Further, in the present invention, when the degree of crosslinking of such an insulator is less than 30%, the heat resistance, chemical resistance, stress cracking resistance and the like of the insulator are reduced to the same level as that of uncrosslinked low density polyethylene. However, it becomes inconvenient. On the other hand, if the degree of crosslinking exceeds 70%, the effect of reducing the space charge amount cannot be obtained.

In the present invention, the degree of crosslinking is 3
If necessary, a small amount of carbon black, a cross-linking aid, an inorganic filler, a voltage stabilizer, etc. may be added to the insulator composed of 0 to 70% of cross-linked polyethylene.

In the DC cable having such a structure, as will be apparent from the results of experimental examples described later, the amount of space charge accumulated when a DC voltage is applied is improved and the space charge characteristics are improved. It can be put to practical use.

(Experimental Examples 1 to 6) Density 0.92 g / c
m ^3, was added at various amount of dicumyl peroxide in a low density polyethylene melt index 1.5, it was kneaded and extruded into a sheet-shaped test piece, and cross-linked by various crosslinking temperature, the crosslinking Test pieces having a degree of crosslinking of 85% (Experimental Example 1), a degree of crosslinking of 70% (Experimental Example 2), a degree of crosslinking of 50% (Experimental Example 3) and a degree of crosslinking of 30% (Experimental Example 4) were obtained. Further, for comparison, an uncrosslinked test piece (Experimental Example 5) made of only the low density polyethylene and an uncrosslinked test piece made of only high density polyethylene having a density of 0.94 g / cm ³ and a melt index of 1.0 ( Experimental example 6) was also prepared.

With respect to these six kinds of test pieces, the space accumulated charge amount was measured by the pulse electrostatic stress method. The applied DC voltage was 60 kV, the gap between the electrodes was 2 mm, and the voltage application time was 3 hours. The results are shown in FIGS. 1 to 6. FIG. 1 shows the results for a conventional test piece having a degree of crosslinking of 85% (Experimental Example 1), FIG. 2 shows the result for a test piece having a degree of crosslinking of 70% (Experimental Example 2), and FIG. The results for the test piece (Experimental Example 3) are shown in FIG. 4, the results for the test piece with a crosslinking degree of 30% (Experimental Example 4), and the results for the uncrosslinked low-density polyethylene test piece (Experimental Example 5). FIG. 6 shows the results of uncrosslinked high-density polyethylene test pieces.

From these results, the degree of crosslinking is 70 to 30%.
Therefore, it is clear that the amount of spatially accumulated charges is greatly reduced.

(Example, Conventional example) Density 0.92 g / cm
^3. Dicumyl peroxide was added to low density polyethylene with melt index 1.2 at various addition amounts and kneaded, and this was mixed with an inner semiconductive layer and an outer semiconductive layer on a conductor with an effective area of 60 mm ^2. By extrusion coating, an insulator having a thickness of 2.5 mm was formed and cross-linked to obtain three types of power cables. The DC breakdown voltage and the Imp breakdown voltage of these power cables were measured at a test temperature of 90 ° C. The results are shown in Table 1.

[0017]

[Table 1]

From the results in Table 1, it can be seen that the power cable of the present invention has better dielectric breakdown characteristics at high temperature than the uncrosslinked polyethylene insulated cable.

[0019]

As described above, in the DC power cable of the present invention, the insulator is made of cross-linked polyethylene having a cross-linking degree of 30 to 70%, so that it is accumulated in the insulator when a DC voltage is applied. The amount of space charge is significantly reduced. Further, the dielectric breakdown characteristic at high temperature is also improved as compared with a cable having an insulator made of uncrosslinked polyethylene.

[Brief description of drawings]

FIG. 1 is a graph showing the results of an experimental example.

FIG. 2 is a graph showing the results of an experimental example.

FIG. 3 is a graph showing the results of an experimental example.

FIG. 4 is a graph showing the results of an experimental example.

FIG. 5 is a graph showing the results of an experimental example.

FIG. 6 is a graph showing the results of an experimental example.

Claims (1)

[Claims] 1. The degree of crosslinking, expressed as a gel fraction, is 30 to 70%.
A DC power cable characterized by having an insulator made of the cross-linked polyethylene of.