JPH02836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a reinforcing insulating layer in a crosslinked polyethylene cable connection.

When forming a connection portion of a crosslinked polyethylene cable, the connection portion including the cable portion may be heated to a high temperature to mold a reinforcing insulating layer. For example, when forming a straight connection, a reinforcing insulating layer is provided by wrapping rubber or plastic tape on the outside of the conductor connection, spanning the insulating layer of the cables to be connected, and then heat molding. In this way, the cable insulation layer and the reinforcing insulation layer are fused together. Also, instead of the above tape,
Extrude molten rubber or plastic into a mold,
In some cases, a reinforcing insulating layer is provided by injection, followed by heat molding. Furthermore, in order to prevent oil or water from entering the joints at both ends of the cable connection section, the cable may be adhesively fixed by mold-fusion-bonding a metal fitting or a polymeric material to the cable.

Conventionally, in this type of molding work, the decomposition gas of the crosslinking agent remaining in the crosslinked polyethylene insulating layer of the cable may create minute voids in the insulator, so 2 to 3 kg from the outside was used. / ^cm2
The generation of voids has been prevented by applying the above pressure. Therefore, it is believed that if sufficient pressure is applied uniformly throughout, no voids will be formed in the crosslinked polyethylene insulator or at the interface with the external electrode. However, if the pressure is uneven or there are areas where no pressure is applied, gas will be generated due to heating and voids will be generated in the insulator, causing a significant decrease in the insulation performance of the cable connection.

The present invention has been made in order to fundamentally solve the above-mentioned problem when molding reinforcing insulating layers in cross-linked polyethylene cable connection parts, and the gist thereof is to After the amount of gas produced by decomposition of the crosslinking agent remaining in the crosslinked polyethylene insulation layer has been reduced to 0.1ml or less per 1g of crosslinked polyethylene under standard conditions,
A method for forming a reinforcing insulating layer in a cross-linked polyethylene cable connection section, which comprises forming the reinforcing insulating layer by heating at a temperature of 190° C. or lower.

Crosslinked polyethylene is usually made by blending 100 parts of polyethylene with 2 to 3 parts of dicumyl peroxide (DCP). This reaction is as follows.

In this way, when polyethylene is mixed with dicumyl peroxide and crosslinked by heating, the decomposition residues in the crosslinked polyethylene are acetophenone cumyl alcohol, methane, α-methylstyrene, water, and the like. Among these, methane has a low boiling point and is gaseous at room temperature.

The amount of methane gas generated is determined by the amount of crosslinking agent added. Since the amount of crosslinking agent (DCP) is approximately 2 g per 100 g of polyethylene, the amount of methane gas generated when DCP decomposes from 100 g of polyethylene is as follows.

DCP1mol (270g) ---→ Decomposition CH ₄ 1mol (16g, 22.4) 2g/270g×22400ml≒170ml/PE100g Therefore, methane gas per 1g of cross-linked polyethylene
Approximately 1.7ml is generated under standard conditions.

The saturated dissolution amount of crosslinked polyethylene material varies depending on the temperature. Figure 1 shows experimental data on the amount of saturated dissolved gas in crosslinked polyethylene.The amount dissolved decreases with temperature, reaching 0.1 ml/g at 190°C. That is, if the gas amount is below this saturated dissolution amount, all the gas produced by decomposition of the crosslinking agent will be dissolved, and no voids will be formed in the crosslinked polyethylene. Therefore, when heat molding is performed at 190°C or lower, the amount of gas in cross-linked polyethylene should be reduced to 0.1
It must be kept below ml/gram. similarly
0.2ml/g below 180℃, below 170℃
0.27ml/g, 0.33ml/g below 160℃
0.48ml/g below 150℃, below 140℃
It can be seen that if the gas amount is set to 0.42 ml/g or less, no voids will be generated when the mold is heated.

On the other hand, molding work is more efficient at higher heating temperatures, so it is desirable to mold at a higher temperature.
If the amount of gas in crosslinked polyethylene is large, there is a risk that voids will be generated.

There are various methods for removing gas from crosslinked polyethylene, but the following two methods are shown below. One method is to leave the cable at room temperature and wait for the gas to escape naturally.

Based on Ficks' diffusion equation regarding the diffusion of polymer materials, the relationship between the initial gas amount Q ₀ in the cable insulator and the gas amount Q in the insulator after being left for several days is approximately expressed by the following equation.

Q/Qo=8/π ² exp (−Dπ ² /l ² t) D: Diffusion coefficient (2.24 mm ² /day at room temperature) l: Thickness of sample (mm) t: Number of days left standing (day) Here If the initial gas amount Qo is 1.7 ml, then the number of days of storage and the amount of gas in the insulator will be as shown in Figure 2, taking a 30 mm thick cable as an example.

Using this formula to find the number of days for which the amount of gas in cross-linked polyethylene is 0.1 ml/g or less, it is 12 days for an insulation thickness of 10 mm, 48 days for an insulation thickness of 20 mm, and 48 days for an insulation thickness of 30 mm.
It will be 100 days. Therefore, when molding is carried out at temperatures below 190°C, it is sufficient to use cables that have been left for this many days.

Next, if you leave it at room temperature, it will take several days, and if you cannot secure enough days, there is a method of forcing it to dry. Figure 3 shows the measurement data for drying at 80℃ and the amount of gas generated. This measurement result shows that drying at 80°C for 3 days is necessary to reduce the gas amount to 0.1ml/g or less.

The temperature of 80°C was determined as the limit temperature to prevent crosslinked polyethylene from becoming soft and deformed during drying.

Example 2 A 75KV cross-linked polyethylene insulated cable (27mm thick) was left at room temperature for 100 days, and molding work was performed to confirm that there were no problems. Additionally, it was dried using a 66KV cross-linked polyethylene cable at 80°C for 3 days and molded, and it was confirmed that there were no voids.

According to this invention, even if pressure is no longer applied to the mold part due to a molding error or the like, voids will not occur and connection work can be carried out with peace of mind by controlling only the highest point of mold temperature. can.

[Brief explanation of drawings]

FIG. 1 shows the temperature characteristics of the saturated dissolved gas amount of crosslinked polyethylene. Figure 2 shows the number of days a cross-linked polyethylene insulated cable with an insulation thickness of 30 mm is left unused and the decrease in gas content. Figure 3 shows the relationship between the number of drying days for a crosslinked polyethylene cable and the amount of gas generated.

Claims (1)

[Claims] 1 After reducing the amount of decomposed gas of the crosslinking agent remaining in the crosslinked polyethylene insulating layer of the crosslinked polyethylene cables that are connected to each other to 0.1 ml or less per 1 g of crosslinked polyethylene under standard conditions, a reinforcing insulating layer is applied to the reinforcing insulating layer. Above the melting point of polyethylene
A method for forming a reinforcing insulating layer at a cross-linked polyethylene cable joint, the method comprising heating at a temperature of 190°C or lower.