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In view of the problem and difficulty of isothermal relaxation current method in evaluating the insulation state of distribution cables, a new sample of domestic unaged 10 kV distribution cable was used as the test object in this paper, and the isothermal relaxation current of the cable section and the high-field conductance current of the insulation slice during the 288 hours of accelerated thermal ageing process at 140℃ were detected. The variation trend and range of current relaxation component, ageing factor A, and threshold electric field Et during thermal ageing were analyzed. The results show that there are three obvious polarization relaxation peaks in the isothermal relaxation current, and the time constants τ1, τ2, and τ3 are in the range of 7-12 s, 31-39 s, and 210-536 s, respectively. During the thermal ageing process, the material traps corresponding to isothermal relaxation current Peak2 and Peak3 change greatly, and the change of Peak3 is the most obvious. The ageing factor A based on the isothermal relaxation current is in the range of 1.72-3.17. The A decreases at first and then increases with the increase of ageing time, and the corresponding conductance threshold electric field Et shows the opposite trend of rising at first and then decreasing, which indicates that there is a clear correlation between the threshold electric field Et and the ageing factor A, they are both the macroscopic appearances of internal molecular structure of the insulating material. The ageing factor A of the domestic cable before ageing is 1.91, which is close to the "aged" state of the German standard DIN VDE 0276, and the A drops to 1.72 after ageing for 48 h, which reaches the "better" state of the German standard. The high A of the domestic cable may be related to the residues of cross-linking by-products of cables, so the degassing process should be improved in the production of domestic cables.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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