Requirements for silicone rubber materials for Cold shrinkable cable accessories

With the increase of population density, the power consumption in big cities is increasing day by day, and the load density is growing rapidly, so it is increasingly urgent to adopt the transmission mode with higher transmission capacity. The high-voltage power cable transmission system is the main form of electric energy transmission from super-large cities to urban centers in the future, and is also a main way of electric energy transmission of hydropower stations at present. Silicone rubber is also increasingly used in high voltage cold shrinkable cable accessories.

The performance characteristics of silicone rubber:

1. Silicone rubber is produced by the convergence of hydrolyzed dimethyl dichlorosilane and methyl phenyl dichlorosilane, and its molecular main chain is composed of Si-O bonds. Due to the unique molecular structure of silicone rubber, it has the following properties;

2. Electrical insulation. Silicone rubber is a saturated non-polar material with high electrical resistivity and excellent electrical insulation. Its electrical insulation performance changes little when damp, frequency change, temperature rise, in arc discharge is not easy to coke, even if the combustion, the generated silica is still an insulator. At the same time, silicone rubber material has excellent corona resistance and arc resistance.

3. Heat and cold resistance. Silicone rubber molecule main chain Si-O bond has high bond energy, so it has good heat resistance. Under high temperature conditions, its performance retention rate is high. At 150℃, the tensile strength of silicone rubber is much higher than that of universal rubber, and the silicone rubber is resistant to high temperature and pressure change. Because the bond length of silicone rubber Si-O-Si is long, the bond Angle is large, the flexibility is good, and the intermolecular force is weak, the low temperature resistance of silicone rubber is also very good, and it still maintains good elasticity at -60℃ ~ -70℃.

4. Weather resistance. There is no saturation bond in the main chain of silicone rubber, and Si-O-Si bond is very stable to oxygen, ozone and UV, so it has excellent weather resistance without any additives. Long time exposure to UV and wind and rain, its physical and mechanical properties have little change. When corona discharge occurs in ozone, organic rubber ages quickly and has little effect on silicone rubber.

5. Hydrophobicity. Silicon rubber molecular chain of small electronegativity, hydrogen bond energy is small, can produce a special surface properties and many materials non-stick, has excellent hydrophobicity. Silicone rubber also has a unique detest water migration, when the outdoor appearance of silicone rubber products dirty, the internal small molecules can be diffused to the outer surface, and spread to the surface of the dirty layer, so in the case of pollution and humidity, the surface can still maintain a high resistivity.

Requirements for silicone rubber materials for cold shrinkable cable accessories:

1. Power cable accessories are a special product. The internal part of the product should withstand the high electric field generated by the shielded fracture of the cable in the operation process and the high temperature generated by the cable core through the large current (normal operation temperature of 90℃, short-time short-circuit temperature of 250℃).

2. Power cable terminals are usually installed outdoors. After installation, the product is exposed to the atmosphere, to withstand the sun in summer, to withstand snow and ice in winter; The product shall withstand the high voltage of lightning impact that may be produced (110kV cable terminal requires to withstand 550kV impact voltage); Also withstand the system equipment may produce operating overvoltage.

3. Under the action of high electric field, the related electrical performance changes of silicone rubber insulator and stress control body need to be further studied. In recent years, we have seen a good phenomenon, the State Grid Wuhan High Voltage Research Institute, Tsinghua University and other scientific research institutions have strengthened the research on the electrical properties of silicon rubber basic materials under various conditions.

4. Special requirements of cold shrinkable cable accessories on silicone rubber. The amount of 10-35kV power cable accessories is very large in China, and the biggest usage is cold shrinkable cable accessories at present. Cold shrinkable cable accessories have an important requirement for silicone rubber: it can still be reset after two years of expansion and storage, and it can still be shrunk and pressed on the cable insulation. This special requirement is a challenge for our silicone rubber technicians.

5. The main reasons for the failure of power cables and cold shrinkable cable accessories in the middle period of operation (5-25 years) are the aging of the insulation branches of the cable body and the surface discharge of the cable accessories caused by the breathing effect of the incoming tide. In the later period of operation (25 years later), the main insulation branches and electric heating aging of the high voltage cable body are aging, and the cable accessories are mainly aging materials. Whether the performance of silicone rubber materials used in cold shrinkable cable accessories is stable after 30 years (40 years) is worth our attention.