JPS6046762B2 - Resin composition for DC electrical insulation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resin composition for direct current electrical insulation used for insulating high voltage direct current wires, cables, their connections, terminals, and other direct current circuits.

Electric wires and cables with insulators mainly composed of polyolefin polymers such as polyethylene and cross-linked polyethylene are widely used for AC high-voltage power transmission because of their ease of maintenance.

However, since AC power transmission has dielectric loss, it is necessary to compensate for the line charging current, and it is widely known that DC power transmission is more advantageous for transmitting large currents over long distances. However, in an experiment in which a cable with cross-linked polyethylene as an insulator was used for 250KV DC power transmission, it was announced that the insulation performance lacked stability (IEEJ Electric Power Technology Joint Study Group Material PE-76-1) Insulation performance and testing methods for DC cables, December 14, 1976). Therefore, it has been desired to develop an insulating resin composition suitable for direct current power transmission by improving these drawbacks. The reason for the lack of stability in insulation performance is that the accumulation of space charge in the insulator caused by direct current causes tree-shaped discharge deterioration, so one solution is to use conventional insulation. It is known that the problem can be solved to some extent by creating a synthetic resin material layer that is difficult to inject into the material layer (
JP-A-51-110678). However, conventionally, many of these ultra-high voltage power cables have a structure consisting of a Ξ layer, and adding a new resin layer to eliminate space charges would require a new extruder. Since increasing the number of layers risks increasing the defective rate, we worked to develop technology that would make effective use of existing equipment. Then, when we conducted an experiment in which we added polyamide to a conventional polyolefin composition to evenly disperse a synthetic resin material that is difficult to inject charge into an insulator, we surprisingly succeeded in reducing this space charge without impairing the insulation properties. They discovered that the accumulation can be alleviated and succeeded in developing an insulating resin composition suitable for DC power transmission. The present invention relates to a resin composition for direct current electrical insulation that has the effect of suppressing deterioration due to tree discharge. It is known that polyesters such as polyethylene terephthalate are also useful as resins that are difficult to inject charges into (Japanese Unexamined Patent Publication No. 51
-110678), commonly available polyester resins have a high melting point (polyethylene terephthalate has a high melting point of 260
℃), kneading, extrusion, and crosslinking, and therefore it is considered unfavorable. According to the invention,
1 or a mixture of polyamides and polyamide copolymers selected from the group 1 to 100 parts by weight of polyolefin
A resin composition containing 0 to 5 parts by weight and, if necessary, a required amount of an antioxidant, a heat deterioration inhibitor, an ultraviolet stabilizer, a crosslinking agent, a crosslinking aid, a blowing agent, etc., is used. Then,
We succeeded in improving the stability of polyolefin-insulated DC cables and DC high-voltage insulation materials.

The polyolefins mentioned here include not only low-density polyethylene and high-density polyethylene, but also ethylene copolymers containing 50% or more of ethylene, such as ethylene. Refers to all polyolefin polymers that can be used for electrical insulation, such as monovinyl ester copolymers, ethylene-alkyl acrylate copolymers, ethylene-propylene copolymers, and various modified polyethylenes that are partially modified from polyethylene. It's so cold. Regarding polyamides and polyamide copolymers, those whose melting points are below the extrusion temperature of polyolefins are desirable for molding purposes. Polyamide copolymers are easy to handle especially when blended for crosslinked polyethylene, but polyamides whose melting points are below the crosslinking temperature are preferred. In the case of polyamide, it can be used by powdering it and kneading it into polyolefin. In addition, if the blending amount is less than 1 part by volume, the tree resistance effect will be insufficient, and if it is more than 5 parts by volume, the moldability will decrease and the wire
There are restrictions on mechanical properties such as bending properties and elongation required for cables. Stabilizers applicable to the compositions of this invention include 4-4'' thiobis(6-t-butyl-3-methylphenol) and 2,6-di-t-butyl-p
-cresol, etc., but the stabilizer is not limited thereto, and any stabilizer that is usually effective for polyolefins can be used as is.

Examples of crosslinking agents that can be applied to this invention include dicumyl peroxide, 2,5-dimethyl-2,5,-bis(t-butylperoxy)hexane, and t-butylcumyl peroxide, which have a half-life of 100% per minute. ~200℃
Preferably.

Examples and comparative examples will be explained below based on Table 1.
5T! by breath molding various compounds shown in Table 1! r
l! Sample 1 was punched into a 25Tfn square sheet with a thickness of l, and the radius of curvature at the tip was 5 x 10-3Tm as shown in Figure 1.
I! A needle-like electrode 2 polished to a thickness of 1 or less is inserted, silver paint is applied to the surface of the sample facing the needle end, and the insulation thickness between the two is set to 5Tm.The apparatus shown in FIG. The generation of trees was observed by applying a DC voltage. Sample 1 was placed in a tank 8 filled with silicone oil, a DC voltage generated by DC voltage generator 3 was applied to the sample, and the length of the tree was measured using microscope 5. 4 is a light source, 6 is a filter,
7 is a vacuum switch.

The effect of polarity reversal, which is a phenomenon unique to DC insulation materials such as DC cables, is that after applying negative polarity of -40KV for one time,
The application was stopped, and after 1 second, a positive impulse voltage of +40 KV was applied, and the length of the tree generated at the needle end was measured and determined. The 50% DC tree generation voltage and the 50% impulse tree generation voltage are 50% of the sample voltage after 1 hour of application.
This is the voltage at which tree occurs at 0%.

Mechanical properties are 1 due to press molding? Thick sheet JIS
Judgment was made by measuring tensile elongation in accordance with K676O.

As shown in Comparative Examples 1 and 4, polyethylene has a polarity effect, so there is a large difference between the 50% DC tree generation voltage and the 50% impulse tree generation voltage between positive and negative polarities, and when the polarity is reversed, The extended length of the tree is also large, which confirms that its insulation performance lacks stability in DC power transmission, but as shown in Examples 1 to 9, polyamide and/or polyamide copolymer are blended with polyolefin. By doing so, we were able to reduce this polarity effect and alleviate the accumulation of space charge, which is thought to be a factor in the generation and elongation of trees, making it possible to obtain an excellent DC electrical insulating material.

In Comparative Example 2, the relaxation of space charge accumulation was insufficient, and in Comparative Example 3, the mechanical properties were significantly deteriorated and the material could not function as an insulating material. In Example 9 and Comparative Example 4, 2 parts by weight of dicumyl peroxide was added to cause chemical crosslinking, but as shown in Examples 2 and 9, the presence or absence of crosslinking had a negative effect on the effect of adding polyamide, etc. not give. Furthermore, in all of Examples 1 to 9, it was observed that the AC tree generation voltage increased by 50%, but since the dielectric loss increases due to the increase in dielectric constant and dielectric loss, it is difficult to use long-distance high voltages and large currents. For AC power transmission, the application of this composition is not advantageous.
In addition, when only HDPE, EVA, and EEA are used without the addition of polyamides, when only LDPE is used (Comparative Examples 1 to 3)
) showed a similar trend. The method for producing DC high voltage cables and DC high voltage insulation materials using these DC electrical insulation resin compositions can be carried out by conventional methods, and only a partial modification of the extrusion processing conditions is sufficient.

The structure of the cable is also based on replacing the conventional insulating material layer with the composition of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape of a sample and the arrangement of electrodes used for observing the state of tree formation, and FIG. 2 is a schematic diagram of an experimental apparatus for tree observation.

Claims (1)

[Claims] 1. A direct current electrical insulation characterized by blending 100 parts by weight of polyolefin with 10 to 50 parts by weight of one selected from polyamides and polyamide copolymers or a mixture of several of them. Resin composition for use. 2 Antioxidants, thermal deterioration inhibitors, ultraviolet stabilizers and/or
or other stabilizers in the required amount.
Compositions as described in Section. 3. The composition according to claims 1 and 2, which contains a crosslinking agent, a crosslinking aid, and/or a blowing agent in required amounts. 4. A composition in which carbon black is blended with the compositions of items 1 to 3.