The positive temperature coefficient (PTC) effect of the semiconductive layers of high-voltage direct current (HVDC) cables is a key factor limiting its usage when the temperature exceeds 70 °C. The conductivity of the ionic conductor increases with the increase in temperature. Based on the characteristics of the ionic conductor, the PTC effect of the composite can be weakened by doping the ionic conductor into the semiconductive materials. Thus, in this paper, the PCT effects of electrical resistivity in perovskite La0.6Sr0.4CoO3 (LSC) particle-dispersed semiconductive composites are discussed based on experimental results from scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a semiconductive resistance test device. Semiconductive composites with different LSC contents of 0.5 wt%, 1 wt%, 3 wt%, and 5 wt% were prepared by hot pressing crosslinking. The results show that the PTC effect is weakened due to the addition of LSC. At the same time, the injection of space charge in the insulating sample is characterized by the pulsed electroacoustic method (PEA) and the thermally stimulated current method (TSC), and the results show that when the content of LSC is 1 wt%, the injection of space charge in the insulating layer can be significantly reduced.

The semi-conductive layer located between the wire core and the insulation layer in high voltage direct current (HVDC) cable plays a vital role in uniform electric field and affecting space charges behaviors. In this work, the research idea of adding ionic conductive particles to semi-conductive materials to improve the conductive network and reduce the energy of the moving charge inside it and to suppress charge injection was proposed. Semi-conductive composites doped with different La0.8Sr0.2MnO3 (LSM) contents were prepared. Resistivity at different temperatures was measured to investigate the positive temperature coefficient (PTC) effect. Pulse electro-acoustic (PEA) method and thermal-stimulation depolarization currents (TSDC) tests of the insulation layers were carried out. From the results, space charge distribution and TSDC currents in the insulation samples were analyzed to evaluate the inhibitory effect on space charge injection. When LSM content is 6 wt. %, the experimental results show that the PTC effect of the specimen and charge injection are both being suppressed significantly. The maximum resistivity of it is decreased by 53.3% and the insulation sample has the smallest charge amount, 1.85 × 10−7 C under 10 kV/mm—decreased by 40%, 3.6 × 10−7 C under 20 kV/mm—decreased by 45%, and 6.42 × 10−7 C under 30 kV/mm—decreased by 26%. When the LSM content reaches 10 wt. %, the suppression effect on the PTC effect and the charge injection are both weakened, owing to the agglomeration of the conductive particles inside the composites which leads to the interface electric field distortion and results in charge injection enhancement.

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Space charge accumulation in the insulating layers of high-voltage directed current (HVDC) cables is the key factor that leads to their degradation and limits the operation safety. Hence, it is necessary to effectively inhibit carrier implantation to these layers. In this study, a new method is proposed to suppress the charge implantation to the cable insulation layer, a certain amount of Li4Ti5O12 nanopowder is doped in the semiconductive screen to enhance the internal conductive network and homogenize the electric field. The influences of Li4Ti5O12 particles in the semiconductive composite screen on the positive temperature coefficient effect and space charge injection process are explored. The obtained results reveal that the charge quantity in the insulation layer corresponding to the minimum peak resistivity is reduced by 49.7%, 58.9%, and 46.9% after the polarization at a Li4Ti5O12 content of 4 wt.% and electric field strengths of 10, 20, and 30 kV/mm, respectively.