**1. Introduction**

High-voltage direct current (HVDC) transmission plays a significant role in the power system [1–5]. In particular, HVDC cable transmission is feasible over long distances and large capacities due to the absence of reactive power and low transmission losses [6,7]. Typical medium and high-voltage power polyethylene (PE)cable cross-sectional constructions include: (1) conductors, (2) conductor shield, (3) insulation, (4) insulation shield, (5) metal shield, and (6) enclosure material [8]. In the construction of high-voltage power cables, the semiconductive layer can suppress the injection of carriers from the metal electrode into the insulating layer and can effectively prevent local electric field distortion between the conductor and the insulating layer.

However, the electrical resistance of the semiconductive layer can suddenly increase to 90 ◦C, which causes the cable to heat up and leads the interface to partially melt. This phenomenon is called the positive temperature coefficient (PTC) effect [9]. The PTC effect of semiconductive composites is usually weakened by increasing the content of carbon black (CB) or by using high-structure carbon black [10,11]. However, the amount of CB added to the semiconductive shielding layer affects its processing and mechanical properties. The conductivity of the ionic conductor increases with increasing temperature. In this work, the influence of the (CB—La0.6Sr0.4CoO3 (LSC)) co-filled on the electrical properties of semiconductive composites was studied, in which LSC was used as a second filler to suppress the PTC effect. The perovskite oxide LaCoO3 has been widely used because of its high ionic and electrical conductivity. The ideal perovskite structure is shown in Figure 1 [12–14]. When La in LaCoO3 is partly replaced by the Sr, the lattice spacing becomes larger and the oxygen vacancies in the crystal increase [15]. Oxygen vacancies and lattice defects of LSC can provide more conductive channels for electrons, which facilitates electron migration when Sr-doped LaCoO3 is added to a semiconductive composite material.

**Figure 1.** Structure of LaCoO3.

On the other hand, space charge accumulation of the insulating layer is another key factor affecting the stable operation of the cable. Insulating layers tend to accumulate space charge, which causes distortion of the electric field [16,17]. Eventually, the insulating layer is easily aging or mangled [18–20]. Until recently, most studies have been limited to the insulation nano-doped polyethylene, which has been attracting more and more attention. It has been reported that polyethylene-doped inorganic nanoparticles such as MgO, ZnO and SiO2 can significantly suppress the accumulation of space charge in the insulation [21–25]. Some researchers add SrFe16O19 to the semiconductive layer to reduce the injection of space charge in the insulating layer by using the Lorentz force of magnetic particles on the charge [26]. However, in this study, we suppressed the injection of space charge by using the LSC modified semiconductive layer. The injection of charges into the insulating layer is reduced, through the Coulomb effect between LSC particles in semiconductive materials and injected charges. This work provides a new idea for the development of semiconductive materials.

### **2. Materials and Methods**
