Conductivity measurements and space charge inference in polymeric-insulated HVDC model cables

The general objective of this work is to obtain a better understanding of polymeric-insulated HVDC cable characteristics, especially related with conduction current and space charge features. It goes with the building of a model of conductivity vs. field and temperature in order to forecast field distribution in cables. Conduction current was measured in miniature of HVDC power cables with construction 1.5mm polymer insulation thickness, 0.7mm inner semiconductor, 0.15mm outer semiconductor and conductor diameter of 1.4mm. Current measurements were performed with applied voltage varying from 2kV to 30kV and temperature in the range from 30°C to 90°C. The time of polarization (voltage applied) and depolarization (short-circuit) were 1h/1h. The results show that there is not a single conduction mechanism for all applied fields and in the all temperature range. The conductivity is clearly non-linear at voltages above about 10kV (field at inner semicon of 10kV/mm), the threshold being dependent on temperature. With the increase in thermal and electrical stresses, transient currents appear characteristic of processes with massive charge injection at the electrode and therefore the treatment of transport based on a homogeneous conductivity is no longer a sufficient hypothesis.