Space Charge Behavior of DC Cable's XLPE Material at High Temperatures

In order to study the space charge behavior of imported DC cable's XLPE material at high temperatures, we prepared undegassed and degassed XLPE samples for the tests, including DC conduction, dielectric and space charge during steady and transient processes at high temperatures. The results show that both the conduction current and dielectric loss of undegassed XLPE are larger than those of degassed XLPE. Negative charge accumulation is observed in the bulk of degassed and undegassed XLPE under -30 MV/m, and the distortion ratio of internal electric fields is below 25% at 30℃ and 70℃. A positive space charge packet appears and subsequently moves quickly within the first 2 s polarization under -100 MV/m at 70℃. However, the injection and migration of a negative space charge packet can only be observed when the applied field raises to -150 MV/m. Here, both polarities of space charge packets follow negative differential mobility. The analyses reveal that degassing treatment can effectively reduce the amount of crosslinking byproducts in XLPE, resulting in the decrease of ionization and conductivity in low frequency regions. A small amount of negative charge accumulating in the bulk is produced mainly by the ionization of crosslinking byproducts and impurities, but the positive charge is injected from the anode with the increase of electric field, and the rate of impurity ionization is higher than the positive charge injection rate. During the initial stage of polarization processes under -150 MV/m at 70℃, the fast transit of positive charge leads to the severe distortion of the local field near the cathode (up to 61.5%), resulting in a large amount of negative charge injection and the formation of a negative space charge packet in the bulk. © 2018, High Voltage Engineering Editorial Department of CEPRI. All right reserved.