85 / 2018-11-28 21:42:05

DC Dielectric Properties of UV-photoinitiated Crosslinking Polyethylene Grafting Maleic Anhydride

UV-photoinitiated crosslinking polyethylene, space charge, molecular chain relaxation, maleic anhydride, deep trap

Compared with traditional cable insulation crossinking technology such as silane crosslinking and peroxide crosslinking, UV-photoinitiated crosslinking has the advantages of fast production, low investment cost, energy saving and environmental protection. Its principle is that the photoinitiator will convert to triplet excited states from ground state excited by UV light and capture the hydrogen atom of polyethylene chain, and then the polyethylene chains could interconnect into network structure from linear structure. With the improving performance of UV light source, UV-photoinitiated crosslinking method can achieve sufficient crossinking of insulation above 16mm thickness. Furthermore, maleic anhydride could be grafted onto polyethylene chain during UV-photoinitiated crosslinking reaction due to the existence of photoinitiator, which will be beneficial to the improvement of insulation DC properties. It means that UV-photoinitiated crosslinking process has significant advantages in the production of high voltage DC cable insulation layer. It is well known to all that insulation material will capture carriers and form space charge under high DC electric field, leading to the distortion of electric field and insulation failure. However, research on the space charge characteristics of crosslinking polyethylene is limited to peroxide crosslinking polyethylene based on LDPE (low density polyethylene) up to mow. Therefore, it’s of great significance to study the space charge characteristics of UV-photoinitiated crosslinking polyethylene (denoted by UV-XLPE). In present paper, UV-XLPE and UV-photoinitiated crosslinking polyethylene grafting maleic anhydride (denoted by UV-XLPE-g-MAH) based on LLDPE (linear low density polyethylene) matrix was prepared to investigate the space charge distribution employing PEA (pulsed electro-acoustic) method in comparison with a common commercial DC XLPE material (denoted by C-XLPE) based on LDPE (low density polyethylene) matrix, and their differences in space charge characteristics were analyzed through thermal stimulation current method (TSC). PEA results reveal that space charge accumulation and electric field distortion are aggravated as temperature increasing, while the space charge accumulation in UV-XLPE and UV-XLPE-g-MAH can be more efficiently suppressed than C-XLPE. The space charge distribution of UV-XLPE-g-0.5wt%MAH was significantly improved compared with UV-XLPE, but the accumulation of space charge at 40 °C and 60 °C increased when the grafted MAH content is higher. TSC test demonstrates that space charge density of specimen increases intensively near it’s α relaxation peak. The higher α relaxation temperature and the resulting more effectively suppression of space charge accumulation for UV-XLPE are attributed to the lower molecular chain relaxation compared with C-XLPE, leading to the less increments of physical defects and traps with increasing temperature. It is considered that the polar groups of MAH can introduce uniform and dense deep traps in material, so that charged lattice and coulomb field could be formed through capturing carriers, leading to the suppression of charge injection and carrier mobility. It is worth mentioning is that the space charge density of UV-XLPE has no significant difference below 80 °C because it’s α relaxation peak appears at 90 °C which is higher than the actual temperature (about 70 °C) of the cable insulation layer during operation. On the contrary, the space charge density of C-XLPE increases sharply above 60 °C due to its α relaxation temperature at 50 °C. The research in this paper has certain reference value for the development of DC power cable insulation materials.