Environmental protection is the future trend of power equipment development, and is also a research hotspot in the field of power cable insulation in recent years. Due to the excellent electrical properties and recyclability, polypropylene (PP) based composites are regarded as promising insulating materials for eco‐friendly next‐generation power cables. However, the high modulus and hardness of pure PP make it difficult to be directly employed as cable insulations, which needs to be further optimised. General methods of mechanical performance regulation often result in the deterioration of electrical performance, such as breakdown strength, space charge and so on. Therefore, it is recognised that the major challenge impeding practical applications of PP power cable insulation arises from the synergetic regulation of multi‐performances. The multi‐level structures influencing the multi‐performances of PP are introduced by the authors and the researches on the performance enhancement of PP through nanoscale structure regulation in recent years are reviewed in detail. Seven kinds of modification methods including nano‐doping, chemical grafting, in‐suit copolymerisation, heat treatment, nucleating agent, voltage stabiliser and elastomer blending are paid special attention. Based on the full understanding of the research status, the challenges and issues of future research are put forward for eco‐friendly PP power cable insulation.

The behavior of intrinsic and extrinsic defect relaxations in ZnO ceramics has been systematically investigated. The defect relaxations are characterized by various parameters, and electric modulus is proposed to be more sensitive to the defect relaxations. Four defect relaxations with various energy levels are found at different temperatures. Two defect relaxations appearing below 243 K with energy levels (L1, L2) about 0.24 eV and 0.35 eV are suggested to be intrinsic defects relating to interstitial Zn and vacancy oxygen, which are not affected by additives and impulse current degradation. Another two defect relaxations appearing above 353 K are proposed to be extrinsic defects associated with energy level (L3) at intergranular phase and energy level (L4) at grain-boundary interfaces, respectively. The extrinsic defect relaxations are closely related with additives and electrical performance. Only L4 was reduced from 0.84 eV to 0.76 eV by impulse current degradation while other defect energy levels keep unchanged. It is further proposed that L4 is mainly responsible for the electrical properties and stability of ZnO ceramics, and it mainly contributes to the grain boundary resistance of ZnO ceramics. A schematic energy band diagram is demonstrated for interpreting the intrinsic and extrinsic defect relaxations in ZnO ceramics.

A general aging model of the double Schottky barrier was proposed to unveil the long-term aging behaviors of $\mathrm{Zn}\mathrm{O}$ varistor ceramics, especially for those ones with steadily decreasing power loss. For those samples, the barrier height and electrical properties were even enhanced rather than commonly deteriorated, which were beyond the classic ion migration model. In this paper, inspired by the unique reversible aging of them, interface states are proposed to remain stable in those samples. The major mobile ions, which have been in debate, are further identified to be ${\mathrm{Zn}}_i^{\cdot }$ ions. Based on these assumptions, a quantitative dynamic ion migration-diffusion model is proposed. The calculated power loss steadily decreases with aging time, which well supported our proposal. When the interface states are not combined with those mobile ions, the formation of a “U-shape” ion spatial distribution in depletion layers is found to be responsible for the unique aging phenomena, i.e., a reduction in the depletion layer and interfacial charge, a rise in the depletion layer width, and an increase in the barrier height. However, continuously increasing power loss would be generated if the mobile ions combined with the interface states. Therefore, a general mechanism on the aging of the double Schottky barrier is unveiled that it is a competition process between consumption of the interface states and the dynamics of mobile ions in depletion layers.

The decrease in electrical properties caused by the toughening of polypropylene (PP) is a difficult problem for the modification of PP used for cable insulation. In this research, an isotactic PP, a cross-linked polyethylene (XLPE) and two impact PP copolymers (IPCs) with an ethylene–propylene rubber phase content of 15 and 30% were prepared to assess the possibility of IPCs to be used as cable insulating material. The tensile properties and breakdown strength were evaluated, meanwhile, the rubber phase content dependence of the crystalline structure, morphology and trap distribution were also investigated. Results show that IPCs with a 15% rubber phase content (IPC15) can achieve the simultaneous improvement of elongation at break and breakdown strength compared with isotactic PP, which can be attributed to the special crystalline structure. According to the results of differential scanning calorimetry (DSC) and FTIR, it is proposed that the lamella thickness of IPC15 is maximal and some ethylene segments exist in PP crystals of IPC15 as crystalline structure defects, which is responsible for this enhanced breakdown strength. The morphology results reveal that rubber microspheres are found to coexist with spherulites, which can promote the relative sliding among lamellas under external force and further results in the increase in the elongation at break.

Thermal ageing of cross-linked polyethylene (XLPE) cable insulation is an important issue threatening the safe operation of power cables. In this paper, thermal ageing of XLPE was carried out at 160 °C in air for 240 h. The influence of oxygen diffusion on thermal ageing of XLPE was investigated by Ultraviolet–visible spectrophotometer (UV–Vis), tensile testing, and Fourier transformed infrared spectroscopy (FTIR). It was observed that the degradation degree not only depended on ageing time but also on sample positions. The thermally aged samples were more oxidized in the surface region, presented a darker color, more carbon atoms appeared in the conjugate cluster, had smaller elongation at break and tensile strength, and a larger carbonyl index. As ageing time increased, the non-uniform oxidation of the XLPE samples became more prominent. The degree of non-uniform oxidation caused by oxygen diffusion was quantitatively studied by first order oxidation kinetic. The calculated results demonstrated that carbonyl index measured by FTIR was more sensitive to non-uniform oxidation with a shape parameter in the range of 1–2. The result shown in this paper is helpful for interpreting and predicting the non-uniform ageing behavior of high voltage XLPE cables.