| Along with the large-scale application of medium and high frequency power electronic equipment in power system, frequency characteristic of electrical tree aging has attracted more and more attention. However, the study was conducted at room temperature so far, than which the temperature in real high voltage power cable is higher. For the most common AC power cable currently, XLPE cable could long run at 90℃, which influences the insulation property of silicone rubber in cable accessory insulation. Therefore, it’s meaningful to study the influence of voltage frequency to the electrical tree aging characteristic at 90℃.In order to study the effect of voltage frequency on the electrical tree aging characteristics in silicone rubber at 90℃, needle-plate electrode was used to simulate the defect and linear rising voltage which is 50V/s was applied with the frequency ranging from 50 Hz to 100 kHz, counting the initiation voltage and electrical tree shape at different frequencies and comparing with those at 30℃. The results show that the initiation voltage decreases monotonously with the rising frequency, which is different from the periodical decreasing characteristic at 30℃, and at the same frequency initiation voltage at 90℃ is 17 percent, at least, lower than that at 30℃. After growing for a few minutes, there exist four kinds of tree shape: branch, pine, mixed pine-bush, and bush. The mixed pine-bush tree only occurs at 90 ℃. Regardless of temperature, tree shape tends to be thicker with frequencies, gradually transforming from branch tree to bush tree. Through analyzing, at 30℃ the main tree initiation mechanism change with the rising frequency, which means the frequent Maxwell inducing stress at the point of needle plays a dominant role in the sharp reduction of initiation voltage between 450Hz-2kHz, while at 90℃ the temperature rising generated from the dielectric loss results in the chain relaxing, bringing out the monotonous decreasing of tree initiation voltage between 50Hz-100 kHz, namely, the tree initiation voltage at 100 kHz is 20 percent lower than that at 50 Hz.In order to further study the influence of voltage frequency to the electrical tree growing characteristic of silicone rubber, the paper apply 10 kV constant voltage to silicone rubber specimen with the frequency ranging from 50 Hz to 2kHz, counting the curve that electrical tree growing length changes with time, the shape of electrical tree shape before breakdown and electrical tree image under reflection light, finally comparing with the situation at 30℃.When voltage frequency rises, the growing speed of electrical tree increases, and the electrical tree shape tend to be denser, in which the increasing partial discharge frequency results supposedly. Compared to the electrical tree at room temperature, the one at high temperature has a smaller growing speed, a thicker channel and a denser shape. Through analysis, the elastic modulus of silicone rubber is larger at high temperature, leading that the growth of electrical tree is harder compared to room temperature. However, the local chain relaxing of silicone rubber is more violent at high temperature, leading that the radial partial discharge cracks main chain, in addition with large increasing of slow-growing time at high temperature, leading to a thicker electrical tree channel and a denser electrical tree shape. There is electrical tree carbonizing only at high temperature. Through analysis, the chemical bond of the silicone rubber at high temperature has a higher chemical activity, which could be cracked easier. The high energy electrons crack the C-Si bond, resulting in graphite carbon deposition in some electrical tree channel. |
