JPH0268810A - Power cable - Google Patents

Abstract

PURPOSE:To make it possible to obtain an excellent water tree resistivity by using cross-linked ultra low density polyethylene as an insulator. CONSTITUTION:Ultra low density polyethylene(ULDPE) having density 0.91-0.88g/cm<2> is used as an insulating material and cross-linked and coated. Since ULDPE has a similar structure to rubber and has less crystallization, condensed water is hard to be centralized, so that the development of water tree is prevented. It is thus possible to have a power cable having remarkable effectiveness of restraining the generation of water tree.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a plastic electrical cable with improved water resistance.

<Prior Art> Cross-linked polyethylene cables (hereinafter referred to as XLPE cables), which utilize the excellent insulating properties of polyethylene and have improved thermal properties through cross-linking, are widely known.

<Problem to be solved by the invention> The weakness of this XLPE cable is that, as a phenomenon unique to the cable, water trees occur due to moisture in the insulator and the presence of a localized abnormal electric field, reducing the insulation performance of the cable. There's a problem.

In other words, it is believed that water trees in the XLPE insulating layer occur when water is concentrated in polyethylene, which is a hydrophobic polymer, when there is an abnormal electric field locally.

In particular, further improvement in water resistance is desired under severe conditions such as power distribution class power cables that are immersed in water.

Therefore, the present inventors investigated various insulating materials and found that low density polyethylene has a density of 0.91.
It has been found that when so-called ultra-low density polyethylene (hereinafter referred to as ULDPE), which has an extremely low density of ~0.88 g/cm', is crosslinked and coated, extremely good water resistance can be obtained.

The present invention has been made based on these facts.

<Means for Solving the Problems and Their Effects> In other words, the electric cable of the present invention uses crosslinked ULDPE (density 0.91 to 0.88 g/cm3) as an insulator.

The following are the reasons why the use of ULDPE improves water resistance.

First, water trees are thought to develop as water condensed in abnormal electric field areas in a polymer is accompanied by the formation of micropaths, cracks, etc. in the polymer due to Maxwell stress and the like. However, in the case of ULDPE, since it has a rubber-like structure with little crystallinity, condensed water is difficult to concentrate and tends to be uniformly diffused in ULDPE, making it difficult for micropasses and cracks to occur. It is thought that the development of trees is prevented.

In addition, as a means to prevent flow deformation of the insulator, which is a concern when this electric cable is used at high temperatures, crosslinking treatment is appropriately performed. The method for this crosslinking treatment is as follows:
Examples include chemical crosslinking using an organic peroxide crosslinking agent, irradiation crosslinking using electron beam irradiation, and silane crosslinking using a silane coupling agent. Here, as the organic peroxide crosslinking agent, dicumyl peroxide, ■, 3-bis(
1-Butylperoxy-isopropyl)benzene and the like are preferably used, and the amount thereof to be mixed is preferably about 1 to 3% by weight.

Furthermore, if necessary, as an anti-aging agent etc., for example, 4
, 4-thiobis-(6-L-butyl-3-methylphenol), pentaerythrityl-tetrakis (3-(3,
5-Di-1-butyl-4-hydroxyphenyl)propionate comethane etc. alone or in combination, 0.1~
It may be blended in an amount of about 0.3% by weight.

<Example> The composition of the insulator of the electric cable according to the present invention is as follows.

First, the density of the ULDPE used is 0.89g/Cm3
, the melt flow ratio (MFR) was 1°5, and this ULDPE was mixed with 2.0% by weight of dicumyl peroxide as a crosslinking agent and 4,4-thiobis-( as an antiaging agent).
6-t-butyl-3-methylphenol) was added in an amount of 0.25% by weight (Example 1).

For comparison, ordinary cross-linked polyethylene (XLPE)
) was prepared (Comparative Example 1).

The base polyethylene in this case has a density of 0.92 g/cm'
The melt flow ratio (MFR) was 1.2, and the crosslinking agent and antiaging agent were mixed in the same type and amount as in Example 1 above.

After kneading each of the above-mentioned blends under heating in a roll mill according to their blending amounts, each resin mixture was press-molded into a size of 10 cm x 10 cm.

A sheet-like product with a thickness of 1 mm or 3 mm was obtained. The pressing conditions at this time were a temperature of 180° C. and 1 hour and 30 minutes.

Note that the gel fraction of the sheets made of each resin mixture thus obtained was all 85% or more.

This measurement is based on a method in which the material is immersed in warm xylene at 110° C. for 24 hours to melt only the uncrosslinked portion, and then dried and the gel fraction is measured.

Next, the number of water tree occurrences and the dielectric loss tangent (tan δ) of sheets made of these resin mixtures were measured and shown in Table 1.

A water tree test method and a dielectric loss tangent (tan δ) measurement method for measuring the number of water trees generated in this case are shown below. The water tree test method will be explained based on FIG. 1.

Water tree test method In FIG. 1, 1 is an insulating sheet used as a test sample. This insulating sheet 1 is a sheet formed by press-molding the above-mentioned resin mixture, and in this water tree test, a sheet having a thickness of 3 mm is used. A coating layer 2 of conductive paint is provided on the bottom surface of the sheet 1 to serve as a ground electrode, and a water tank 4 is provided on the top surface of the sheet 1 to form a water electrode, to which a high voltage of 10 kV, 1 kHz is applied. Electrode 3
The structure is configured so that more power can be applied. After applying the voltage between the electrodes for 30 days, the sheet 1 was boiled and water trees generated on the sheet 1 were observed. At this time, we focused only on water trees with a diameter of 50 μm or more, and measured the number of these.

Note that here, the number of occurrences is based on conventional cross-linked polyethylene (X
The number is expressed as a relative number when the number of water trees generated on a comparison sheet prepared with the intention of LP E) is set to 100.

Dielectric Loss Tangent (tan δ) Measuring Method For measuring the dielectric loss tangent (tan δ), a 1 mm thick sheet made of each of the above formulations is used as a sample. To this, 1
A voltage of kV and 50 kHz was applied, and the dielectric loss tangent (tan δ) was measured using a Schering bridge.

Table 1 From Table 1, it is clear that Example 1 of the present invention has a water tree suppression effect compared to Comparative Example 1.
Moreover, it can be seen that the insulation performance is equivalent to that of the conventional product (Comparative Example 1).

Next, an electric cable was created using the above-mentioned crosslinked ULDPE and XLPE as insulators. The structure of this cable has a three-layer structure in which an insulating layer with a thickness of 3 mm is provided on the conductor, and an inner semiconducting layer and an outer semiconducting layer are formed, and the shielding or sheath that is normally applied to the outside is omitted. At this time, copper was used as the conductor, and the cross-sectional area of the conductor was 100 mm2. A semiconductive mixture of ethylene-vinyl acetate copolymer and conductive carbon black was used for the internal and external semiconductive layers, and the coating layers were formed by extrusion coating.

For each of the above-mentioned power cables thus prepared, the following submergence electrification test was conducted to determine the dielectric breakdown voltage, and the results are shown in Table 2.

Water immersion charging test Each of the above cables was immersed in 70°C warm water, and 1
After applying a voltage of kV and 10 kHz for 90 days, the AC (50 Hz) voltage was further increased under step-up conditions of 5 kV/30 minutes, and the dielectric breakdown voltage was measured.

According to Table 2, the density is 0.91 to 0.88 g/cm'
Since the coating uses ULDPE that has an extremely low temperature and is cross-linked, the insulation performance (Lanδ, etc.) is on the same level as conventional XLPE, and the effect of suppressing water tree generation is large, so that it can be There is no decrease in dielectric breakdown voltage, and flexibility can be improved due to the low crystallinity of ULDPE.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram for explaining the water tree test method in the present invention. Patent Applicant: Fujikura Electric Cable Co., Ltd. From this Table 2, it can be seen that in the case of Example 2 of the present invention, the dielectric breakdown voltage after water immersion electrification is higher than that in Comparative Example 2. <Effect of the invention>

Claims (1)

[Claims] Crosslinked ultra-low density polyethylene (density 0.91-0.
A power cable using 88g/cm^3) as an insulator.