Factors Affecting XLPE Insulation Thermal Shrinkage

The cross-linked polyethylene cable material (XLPE) can make the aggregate structure of the insulating material in a reasonable state through the cross-linking process, and can increase the long-term working temperature of the power cable to 90 ℃, and the instantaneous short-circuit temperature is 170 ℃ ~ 250 ℃. The performance remains unchanged while other performances are improved and enhanced. Therefore, the use of cross-linked polyethylene (XLPE) insulated cables is becoming wider and wider. However, the author found in the experiment that due to the relatively small contact area between the insulating material and the conductor of the small-area cable, especially when the surface of the single-core conductor is smooth and rounded and the adhesion is insufficient, the thermal shrinkage of the insulation is large, and it is difficult to reach the national standard GB/ T12706-2008 “Rated Voltage 1kv (Um=1.2kV) to 35kV) Extruded Insulated Power Cable,such as 1-35kV underground armoured power cable , and Accessories” stipulates the requirement of not more than 4%, and the cable with larger area and higher voltage level is due to insulation and conductor It is easier to pass the test if the contact area is larger and the insulation thickness is larger.
Cables that fail the insulation heat shrinkage test are prolonged during use. Because of the excessive shrinkage caused by the insulation, the conductors may be exposed, which may cause the danger of telephone calls. Therefore, we must try our best to solve the problems in the production and improve the product quality of the cable.
So what factors will affect the insulation thermal shrinkage, and what causes the insulation thermal shrinkage test to fail?

PE is a crystalline polymer that is subjected to shearing and traction stretching under a heated environment (melting temperature), which makes the crystal grains of PE molecules increase in size along the stretching direction (longitudinal) and decrease in lateral size. The ordering is improved, that is, PE molecules are oriented, which increases the number of crystal nuclei, shortens the crystallization time, and increases the crystallinity and strengthens the orientation. However, when the finished XLPE insulated cable is placed at room temperature, the internal stress (shrinkage stress) generated during the extrusion of the XLPE insulation increases, which makes the crystallized XLPE molecules easy to de-orientate (the trend of shrinking). These factors, and Mainly related to the melting temperature and time, cooling rate, and external force (traction and stretching) in these three aspects:

1) Melting temperature and time
At high melt temperature, the crystalline polymer is a melt containing crystal nuclei, and the longer the melting time, the fewer the number of crystal nuclei. Therefore, during the cable insulation extrusion process, the higher the heating and melting temperature of the XLPE insulation material, the longer the residence time (holding time) at the heating temperature, the less the number of crystal nuclei, and the lower the crystallization performance of PE Cable, which is beneficial to reduce The crystallinity of the insulation can make the insulation thermal shrinkage meet the standard requirements;

2) Cooling rate
The temperature decrease rate of the polymer melt from above the melt temperature to below the glass transition temperature is called the cooling rate, and the cooling rate is the key to the crystallization of the polymer. The cooling rate is not only related to the melt temperature and room temperature, but also related to the crystallization rate and thermal properties of the polymer itself. The crystallization rate of PE itself is very large, and PE insulation can also get a higher crystallinity under extremely fast cooling conditions. Therefore, this situation is especially obvious in winter, and special attention should be paid to the control of the cooling rate during the extrusion process of XLPE cable insulation. The specific heat capacity of PE is large and the thermal conductivity is small. If the PE melt has a slow cooling rate and sufficient cooling is obtained, the relaxation process of the PE molecules will be prolonged, the orientation can be easily de-orientated, the degree of orientation can be reduced, and the generation of PE nuclei can be controlled and the grains can be delayed Grow up;
In addition, the conductor temperature also affects the cooling rate of the XLPE insulation. The conductor temperature is too low. When the high-temperature PE melt at the die of the extruder is coated on the surface of the conductor, the LPE insulation will shrink and shrink due to contact with the low-temperature conductor to produce shrinkage stress, and reduce the adhesion between the XLPE insulation and the conductor. Focus on reducing the resistance to heat shrinkage, and ultimately affect the heat shrinkage performance of the insulation layer of the XLPE insulated cable.


3) External force (traction and stretching)
In the cable insulation production process, the molecules are oriented along the force side under the action of external force (traction and stretching), which will promote the formation of nuclei, increase the speed of crystal nucleus generation, increase the number of crystal nuclei, and shorten the knot time and crystal Degree increases. Extrusion tube extrusion is commonly used in current electric winding production enterprises. Compared with extrusion molds, the insulation heat recovery of Si-XLPE insulated electric windings produced by extrusion tube molds that must be stretched during the extrusion process It is much larger. The disadvantage of the poor compactness of the plastic layer of the extrusion tube extrusion is likely to cause the marginal heat shrinkage test to fail, but because the process inspection and final inspection do not do the type test item, the insulation heat shrinkage test is ignored, even if it is extrusion In order to improve the production speed and the smoothness of the extrusion surface, the compression mold is generally several millimeters larger than the insulation outer diameter of the cable. In this way, the insulation outer diameter is ensured during the insulation production process, and insulation is inevitable. It will be stretched, and the molecules are still affected by external forces during the stretching process, resulting in orientation, which makes the insulation thermal shrinkage of the produced cable also larger, even far exceeding the standard requirements.

Cable Insulation Resistance Decreases Due to Moisture

The insulation resistance of directly buried power cables is reduced and the phenomenon of cable line failures often occurs, especially when the cable insulation is damp, it is easy to cause the insulation resistance of directly buried power cables to decrease and substandard

1. The cable raw materials are damp

The raw materials used for cable insulation and sheathing are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from this modification. When manufacturing materials, material manufacturers go through the processes of compounding, mixing, granulation, cooling and drying, as well as during the transportation and storage of the materials, dampness of varying degrees often occurs, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before squeezing the material on the cable conductor. The extrusion unit is equipped with a material drying device to prevent the extruded insulation layer and sheath from happening. No defects such as bubbles and blisters, and no bubbles on the surface. This is the rigid process regulation of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.

2. the cable manufacturing process is damp

In the insulation extrusion process, the insulation layer is scratched, causing holes or degumming of the insulation layer, and the insulation core enters the cooling water tank, which causes the insulation resistance to drop. Or when the protective layer is squeezed, the protective layer is damaged and water enters, so that the insulating layer is damp and the insulation resistance decreases. When manufacturing a multi-core cable, even if the insulation layer is extruded intact, when the insulated core is twisted into a cable, and when the sheath is extruded, damage may occur and the water may enter and become damp, so the finished cable cannot pass the factory withstand voltage test .

3. Moisture during cable construction

In the process of direct buried cable construction, if the cable trench is excavated, the cable burying operation, the cable intermediate joint and the terminal joint are not made standardly, it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable trench is filled with water, water in the cable will definitely occur. After the insulation is damp, the surface resistance of the cable insulation will decrease and the surface leakage current will increase, and the insulation resistance will decrease, which will also cause the electric field distortion between the conductor and the insulation layer. The uneven electric field distribution in the insulation will cause free discharge inside the insulation and even lead to cable breakdown. After-sales service practice has proved that more than 95% of the direct-buried cable insulation resistance drop accidents are caused by improper construction.

 

As we all know, water and damp in the process of cable manufacturing and laying operation are the main factors that endanger the electrical performance and service life of the cable. Both cable manufacturers and users attach great importance to this.
Practical experience has proved that the main reasons for the ingress of water and moisture in the cable are as follows.

1) Material purity
If the cable insulation material is mixed with impurities, especially metal impurities, even the pigments of different colors used by China Wire and Cable manufacturer  will directly affect the electrical performance of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer will absorb moisture when the cable is damp, and form numerous conductive points; second, the m

etallic impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive channels in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn leads to insulation breakdown.

2) The material is damp
If the cable insulation material has been damp and is not dried before being squeezed on the conductor, there will be quality defects such as a large number of pores in the insulation layer, an unsmooth extrusion surface, reduced mechanical strength, and even cracking. Therefore, when extruding the cable insulation layer, the cable manufacturer must dry the material. When extruding low-smoke and halogen-free materials, more attention should be paid to drying. These are the basic technical knowledge of cable manufacturers.