221 / 2018-09-25 14:39:45

The effects of nano-SiO2 on the electrical treeing resistance of XLPE

cross-linked polyethylene,periodic grounded DC tree,AC electrical tree,nano-SiO2

Electrical tree is a typical electrically induced degradation in cross-linked polyethylene (XLPE) insulation under AC or DC high electric field. It is the main reason that limits the voltage level of AC and DC extruded power cable. In order to further improve the working voltage level of ac and dc XLPE insulated cables, many polymer modification methods are tried to improve the electrical tree resistance of XLPE, among which nano-SiO2 is confirmed as an effective method to improve the electrical properties of XLPE. Therefore, the electrical tree resistance of the nano-SiO2 modified XLPE insulation under AC and DC voltage should be systematically tested, which has a good reference value for the practical application of the composite material under higher voltage grade.
In order to systemically investigate the effects of nano-SiO2 on the AC, DC electrical treeing resistance properties of XLPE, nano-SiO2/XLPE composite with the doping rate of 1wt% was prepared using parallel twin screw extruder. An electrical tree real-time observation system was designed. With untreated XLPE and commercial DC cable XLPE insulation as reference, the doping effects of nano-SiO2 on the electrical tree initiation and growth characteristics under AC voltage and periodic grounded DC voltage were tested under different voltages using needle-plate electrode system.
The grounded DC tree test results indicate that: The periodic grounded DC electrical tree initiation and propagation are more difficult and less harmful than the AC electrical tree. The periodic grounded DC electrical tree initiation of commercial DC cable XLPE is more difficult and the tree growth speed is much slower than that of untreated XLPE. Nano-SiO2/XLPE composite has significant inhibition effect on the imitation and growth of the periodic grounded DC electrical tree, its periodic grounded DC electrical tree initiation property is superior to the commercial cable material, and its growth speed of periodic grounded DC electrical tree is between the untreated XLPE and commercial DC cable XLPE. The grounded DC electrical tree inhibition effect of commercial DC cable XLPE is probably that its purity is higher, and this commercial material has been specifically controlled in the synthesis stage, making the physical defects and chemical impurities inside the material significantly less, thus provided with a good ability to restrain the accumulation of space charge. Therefore, the space charge accumulation near the needle tip under DC stress is less than that of pure XLPE, and the damage caused by the charge release at the short circuit moment is relatively weaker. As for nano-SiO2/XLPE composite, a large number of deep traps are densely and uniformly introduced after the addition of nano-SiO2. As a result, the space charge is less injected from the needle tip and immediately trapped in a very limited area under DC stress, making the destruction area much smaller than untreated XLPE during each stressing period.
The AC electrical tree test results indicate that: initiation voltage of AC electrical tree of nano-SiO2/XLPE is 13.6% higher than that of untreated XLPE, the AC tree length of nano-SiO2/XLPE is lower than the untreated XLPE in the early propagation stage. However, the AC tree length of SiO2/XLPE exceeds that of untreated XLPE after a certain amount of stress time. The inhibition effect of nano-SiO2/XLPE composite is attributed to the barrier effect of inorganic nanoparticles, while the lose of the AC tree inhibition effect after the electrical tree adequately stretched is ascribed to the differences in the electrical tree structures in the two tested materials.