115 / 2018-08-24 21:18:35

Thermal-oxidative Aging Effected on Dielectric Properties of Polyolefin Used For Nuclear Cables Insulation

dielectric properties,thermal-oxidative aging,polyolefin insulation,nuclear cables

Nuclear cables were one of the key electrical facilities, which were used not only in severe environment, but also provided with high security. To ensure the nuclear power plants safety, the service life of nuclear cables required more than 60 years. The evaluation of aging state for the nuclear cables insulation, was of great significance for the normal operation of the whole nuclear power system. The service environment of nuclear cable was complex, including temperature, irradiation, mechanical stress and other environmental factors. The chemical reactions of oxidation and decomposition, the physical changes of brittleness hardening and cracking, and other degradations of the nuclear cable insulation would be caused from single or comprehensive environmental factor. The dielectric properties, mechanical properties and service life of nuclear cables were seriously affected by the physical and chemical change caused by the aging of insulation materials. Research had showed that the thermal-oxidative aging was the main factor of nuclear cable aging in the life cycle.
In order to evaluate the thermal aging properties of nuclear cable insulation, the artificial accelerated aging was carried out and the chemical and dielectric properties in different aging stage were compared. The polyolefin used in nuclear cables insulation was selected for research. The plate sample was prepared from hot pressing and the cross linking process was completed by irradiation. The samples was cut into different sizes according to test requirements. Recommended according to IEC60216-2001 standard, the thermal oxidative aging process was carried out at 165℃. The changes of infrared spectra, volume resistivity, breakdown strength, dielectric constant and loss were measured at aging time of 0h, 48h, 96h, 168h, 336h, 504h, 672h and 840h.
High temperature and oxygen accelerated the change of molecular chain structure of the polyolefin, and the thermo-oxidative reaction took place in the molecular chain under long-term aging. Compared with infrared spectra of samples in different aging stage, the intensity of hydroxyl and carbonyl absorption peak produced by thermo-oxidative ageing increased continuously with the heating time extending. The main reason of the sharply increase of oxygen-containing group in the molecular chain, was the C-C bond broke to form free radicals and they could combine with oxygen to generate hydroxyl and carbonyl group. Oxygen polar groups were introduced into ends of the chain. The thermo-oxidative reaction caused chemical and physical changes of samples. The Chemical aging was mainly characterized by cracking and branching of internal molecular chains. Physical aging was characterized by chain arrangement, which reduced free volume and increased the density. The change of molecule chain effected the dielectric response under applied strength. The dielectric property results showed that, the volume resistivity of the sample increased, and the breakdown field strength decreased slightly. Under test conditions, aging resulted in difficulty of orientation polarization, which was shown as a slight decrease of dielectric constant and loss.