JPH036605B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention resides in a polyolefin insulator,
This relates to an interfacial electric field mitigation method using a diffusion layer that can effectively alleviate electric field concentration caused by protrusions and irregularities on the electrode surface, especially at the interface between the semiconducting layer and the insulating layer of a crosslinked polyethylene cable equipped with a semiconducting layer. , which is highly effective in improving insulation deterioration caused by electric field concentration.

Electric power plastic cables that use polyolefin for insulation, such as XLPE cables, do not use oil like other types of cables, especially oil-impregnated insulated paper cables, so-called OF cables, which have been widely used in the past, so they are easier to install and maintain. It is easy to manage and has low power transmission losses. For this reason, in recent years it has become widely used for power distribution, especially in urban areas.Recently, based on this track record, even higher voltage XLPE cables, such as those for ultra-high voltage, are being developed, and some of them have already been developed. It has been put into practical use.

However, with current XLPE power cables, etc., the V-t characteristics, that is, the insulation life characteristics with respect to applied voltage, exhibit a drooping characteristic as shown in curve a shown in Figure 1, and differ over time compared to OF cables shown in curve b. Insulation deterioration is large. Therefore, the design potential gradient for the insulator must be made smaller than that of the OF cable, and the thickness of the insulating layer must be increased accordingly. As a result, it goes without saying that power transmission loss increases compared to OF cables, but at the same time, the outer diameter inevitably increases, which creates major bottlenecks in transportation and installation work. This is disadvantageous for cables.

By the way, the main reason for the above-mentioned drooping of the v-t characteristics in such XLPE cables, etc. is,
Irregularities at the interface between the semiconducting layer and the insulating layer, especially semiconducting irregularities that remain on the surface of the conductive layer 1 during manufacturing and protrude into the insulating layer 2, as shown in Figure 2, which shows a cross section of the cable. Research to date has revealed that this is due to the concentration of the electric field at the sex protrusion 3. Note that 5 in FIG. 2 is a conductor. Therefore, efforts have been made to prevent the formation of semiconductive protrusions by improving manufacturing conditions, etc., but it is technically extremely difficult to eliminate them completely, and it is said that improvements have now reached their limits. It is no exaggeration to say so. Accordingly
In order to achieve even higher voltages in XLPE cables, etc., it is necessary to take new methods that have not been seen before.

The present invention provides a new structure that can make semiconductive protrusions, which are considered to be the most harmful to XLPE cables, harmless without making any changes to the cable manufacturing process, and aims to increase the voltage of XLPE cables. It is made to be moisturized. Next, the details will be explained using the drawings.

The semiconducting protrusion 3 is harmful because it protrudes into the insulating layer 2 and the electric field strength at its tip is much greater than the electric field strength in the surrounding insulating layer 2. That is, if the length of the protrusion 3 is l, the radius of curvature of the tip of the protrusion 3 is r (l≫r), and the electric field strength of the insulator layer 2 near the protrusion 3 is E _p , then
The electric field strength E _nax at the tip of the protrusion is given by E _nax = E _p・l/2r1/[l _o (l/r)−2 (1), for example, l=1000μm, r=
If it is 5 μm, E _nax is 10.E _p , and E _nax is proportional to E _p . Therefore, if the electric field strength E _p of the insulator layer 2 near the semiconducting protrusion can be reduced, the electric field strength at the tip of the protrusion 3 can be reduced accordingly, and the deterioration of the insulator layer 2 due to this can be prevented and the design can be improved. The potential gradient can be made higher than before.

The feature of the present invention is that due to diffusion from the interface with the semiconducting layer 1, the partial enlarged sectional view of the cable shown in FIG. As shown in the relationship diagram with the distance of
A diffusion layer 4 with a continuous dielectric constant gradient in which the protrusion 3 is embedded is formed using a substance having a higher dielectric constant than the constituent material of the insulator layer 2, and the dielectric constant of the insulator layer 2 around the protrusion 3 is By increasing the rate of electric field strength
The point is that E _p is lowered, and for example, the following method is used to implement this.

The structure of the insulating layer 2 is added to the constituent material of the semiconductive layer 1 by an appropriate method such as mixing it into the constituent material of the semiconductive layer 1 in advance or injecting it during the extrusion molding process of the semiconductive layer 1 during cable manufacturing. It has a higher dielectric constant [5 to 6] than the material polyolefin,
Moreover, by adding benzine alcohol or l-sulfur which has diffusion permeability into the crosslinked polyolefin, or by coating it on the surface of the formed insulating layer 2 during extrusion molding of the insulating layer 2, it is possible to perform the same process as before. Cables are manufactured using a method that Then, depending on the temperature during thermal cross-linking of the insulator layer 2, a substance having a higher dielectric constant than the insulator layer 2 is automatically diffused from the semiconductor layer interface into the insulator layer using its diffusion permeability. The dielectric constant of the insulator near the interface of the semiconducting layer is increased, and the semiconducting protrusion 3 is covered with this high dielectric constant layer 4, thereby reducing the electric field strength near the protrusion 3, and as a result, the protrusion 3 It is designed to reduce the harmfulness of

In this way, when the rate of increase in the relative permittivity is C, the electric field strength E _S near the interface between the semiconducting layer 1 and the insulating layer 2 will be approximately equal to the increase in the relative permittivity as shown in equation (2). Rate C
Since it is reduced in inverse proportion to E _S E _p /C (2), the semiconductive protrusion 3 can be rendered harmless.

FIG. 5 shows the electric field strength at the interface of the semiconducting layer 1 on the conductor 5 side in FIG. An example of the relationship between the electric field strength and the distance from the internal semiconducting layer surface showing the improved results using benzyl alcohol, a high dielectric constant material that has the property of chemically bonding with the constituent materials during thermal crosslinking. It is a diagram. As is clear from comparing the curve a before the improvement and the curve b showing the state after the improvement, the thickness of the diffusion layer 4 and the height of the semiconductive protrusion 3 (the range L of the interfacial defect existing distance) It can be seen that by making the protrusion larger, the electric field near the protrusion 3 can be reduced and rendered harmless. It was found that almost the same results were obtained when Sata l-Shiyouno was added. Generally, the height of the semiconductive protrusion 3 produced by conventional manufacturing technology is 100 μm or less, so the thickness of the diffusion layer 4 is
It can be made harmless by setting it to about 100 μm. In this case, the loss angle (tan δ) may increase due to the diffusion layer 4 made of a high dielectric constant material, but since the thickness of the insulating layer 2 is generally several mm or more, the increase in tan δ can be almost ignored. , the increase in loss caused by this is not a problem.

Therefore, according to the present invention, (1) the design potential gradient is improved, and the diameter is smaller than before, which is advantageous for transportation and installation.
It is now possible to provide XLPE cables, promoting higher voltage. (2) There is no need to make any changes to the conventional cable manufacturing process, which is very advantageous in manufacturing. (3) Since crosslinking is performed at the same time as diffusion, the electric field relaxation effect at the interface can be stabilized and prolonged, resulting in high reliability. (Four)
Freely control the diffusion layer by selecting the diffusion material,
Various excellent effects can be obtained, such as being able to obtain a cable with desired characteristics, and the present invention is capable of simultaneously forming the three layers (inner semiconducting layer, insulating layer, and outer semiconducting layer) described above. In addition to manufacturing cables using an extrusion method, for manufacturing methods in which a semiconductive layer is formed by tape winding, for example, a high dielectric constant substance to be diffused into the tape forming the semiconductive layer is added. It can be applied by methods such as

As is clear from the above description, the present invention has the excellent advantage of effectively alleviating electric field concentration caused by protrusions and irregularities on the electrode surface that exist in cross-linked polyethylene and other plastic insulators. This has a remarkable effect on reducing the outer diameter of XLPE cables that use polyethylene as the insulating layer by improving the design potential gradient.

[Brief explanation of the drawing]

Fig. 1 is an example of the v-t characteristics of a conventional XLPE cable and an OF cable, Fig. 2 is a sectional view of a conventional XLPE cable, Fig. 3 is a partially enlarged sectional view of an XLPE cable according to the present invention, and Fig. 4 5 is a diagram showing the relationship between the relative dielectric constant in the radial direction due to the diffusion of a high dielectric constant material and the distance from the surface of the semiconducting layer, and FIG. FIG. 3 is a relationship diagram of distance from a semiconducting layer interface. DESCRIPTION OF SYMBOLS 1... Semiconductive layer, 2... Insulator layer, 3... Semiconductive protrusion, 4... High dielectric constant material diffusion layer, 5... Conductor.

Claims (1)

[Claims] 1. In a cross-linked plastic power cable formed from a conductor, an inner semiconducting layer, a polyolefin insulating layer, and an outer semiconducting layer, benzine alcohol or l-sulfur is added to the inner and outer semiconducting layers. , which is diffused into a polyolefin insulating layer during thermal crosslinking to provide a diffusion layer with a continuous dielectric constant gradient.