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In order to study the ageing characteristics of cross-linked polyethylene (XLPE) cable and explore the microscopic factors affecting the dielectric properties of materials, the XLPE cables with different ageing states were conducted isothermal relaxation current (IRC) and partial discharge (PD) tests. A pure polyethylene molecular chain P1, an aged polyethylene molecular chain P2 containing hydroxyl groups, and a further thermal-oxidative aged polyethylene molecular chain P3 containing carboxyl groups were established, and polyethylene molecules with different ageing states were carried out quantum chemical calculation. The results show that with the increase of ageing time, the time constant of depolarization current and ageing factor A increase, and the trap number and energy level depth in insulating medium increase. Besides, there are more partial discharges with higher magnitude occurring in the aged cable. The carbon-oxygen group generated by thermal oxygen ageing in polyethylene molecular chain is an important reason for the increase of trap number and energy level depth, and the type of functional group will affect the distribution of trap energy level directly. Under the effect of electric field, the stability of the aged polyethylene molecular chain becomes worse, the dipole moment is larger, and the charge transfer is more obvious, which make the charges in the aged cable insulation more likely to be trapped and accumulate, resulting in electric field concentration and increase of partial discharge level.
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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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