Incorporating inorganic nanoparticles (NPs) into polymer matrices provides a promising solution for suppressing space charge effects that can lead to premature failure of electrical insulation used in high voltage direct current engineering. However, realizing homogeneous NP dispersion is a great challenge especially in high-molecular-weight polymers. Here, we address this issue in crosslinked polyethylene by grafting matrix-compatible polymer brushes onto spherical colloidal SiO₂ NPs (10–15 nm diameter) to obtain a uniform NP dispersion, thus achieving enhanced space charge suppression, improved DC breakdown strength, and restricted internal field distortion (≤10.6%) over a wide range of external DC fields from −30 kV/mm to −100 kV/mm at room temperature. The NP dispersion state is the key to ensuring an optimized distribution of deep trapping sites. A well-dispersed system provides sufficient charge trapping sites and shows better performance compared to ones with large aggregates. This surface ligand strategy is attractive for future nano-modification of many engineering insulating polymers.

Adding nucleating agents (NAs) is one of the most efficient ways to obtain improved mechanical, optical, and thermal properties of isotactic polypropylene (iPP). While it is well appreciated that electrical property is critically affected by crystalline modification, the role between them remains unclear. Here, we address this issue by incorporating commercial α-NA and β-NA into iPP, both of which exhibit strong nucleation ability, e.g., reducing the size of crystalline agglomerates from 45.3 μm (Pure-iPP) to 2.5 μm (α-iPP) and 7.6 μm (β-iPP), respectively. Mechanical testing results show that while β-modification decreases the tensile strength a little, it does enhance the elongation at break (200%) and toughness (25.3% higher), relative to its unfilled counterparts. Moreover, a well-dispersed β-iPP system obtains a comprehensive improvement of electrical properties, including dielectric breakdown strength, space charge suppression, and internal field distortion under a high external field (−100 kV/mm) due to newly-generated deep charge trapping sites. This crystalline modification strategy is attractive for future development of many engineering insulating polymers.

Despite similar material composition and insulation application, the alternating current (AC) cross-linked polyethylene (XLPE) and direct current (DC) XLPE materials cannot replace each other due to different voltage forms. Herein, this work presents a systematical investigation into the effects of thermal aging on the material composition and properties of 500 kV-level commercial AC XLPE and DC XLPE materials. A higher content of antioxidants in the AC XLPE than in the DC XLPE was experimentally demonstrated via thermal analysis technologies, such as oxidation-induced time and oxidation-induced temperature. Retarded thermal oxidation and suppression of space charge effects were observed in thermally aged AC XLPE samples. On the other hand, the carbonyl index of DC XLPE dramatically rose when thermal aging was up to 168 h. The newly generated oxygen-containing groups provided deep trapping sites (~0.95 eV) for space charges and caused severe electric field distortion (120%) under −50 kV/mm at room temperature in the aged DC XLPE samples. For the unaged XLPE materials, the positive space charge packets were attributed to the residue crosslinking byproducts, even after being treated in vacuum at 70 °C for 24 h. Thus, it was reasoned that the DC XLPE material had a lower crosslinking degree to guarantee fewer crosslinking byproducts. This work offers a simple but accurate method for evaluating thermal oxidation resistance and space charge properties crucial for developing high-performance HVDC cable insulation materials.

Electrical tree aging problem under high-frequency voltages seriously affects the reliability of packaging insulation for power electronics modules. In this article, boron nitride (BN) sheet/epoxy resin (ER) composites with low doping concentration are prepared and characterized. Varied voltage frequencies (50 Hz–130 kHz) are selected, and electrical tree characteristics of composites at different voltage frequencies are studied. Results show that as the frequency increases from 50 Hz to 130 kHz, electrical tree initiation voltage of pure ER decreases by 39.8%. However, as the frequency increases, tree channels first become denser, and then, the tree branches decrease. Moreover, electrical tree shape presents sturdy single-branch-like tree at 130 kHz. By introducing BN sheet, the tree initiation voltage increases, especially at higher frequencies. The tree scale of BN sheet/ER is smaller than that of pure ER at all frequencies. Combining the dielectric properties and tree results of different materials, the influence of frequency on tree characteristics of ER is discussed, and the role of BN sheet in the bulk is revealed.

To further explore the long-term stability of nano-dielectrics, experiments were carried out to investigate the moisture absorption characteristics and electrical properties of silicone rubber (SiR) doped with different inorganic nanoparticles. Thermogravimetric analysis (TGA) is utilized to research the moisture absorption characteristics including mass fraction and binding forms. The trap depth and electron orbitals are calculated by density functional theory to explain the influence mechanism of water molecules on SiR. It is found that the addition of nanoparticles will increase the moisture content of SiR. Additionally, the nano-TiO₂-doped SiR absorbs more water and binds with water relatively more loosely than nano-Al₂O₃. There is a degradation of space charge inhibition capability and breakdown strength after moisture absorption, which might be explained by shallow traps brought by water molecules and the narrowed forbidden bandwidth of SiR.

The breakdown property of oil-impregnated paper is a key factor for converter transformer design and operation, but it is not well understood. In this paper, breakdown voltages of oil-impregnated paper were measured at different temperatures. The results showed that with the increase of temperature, electrical, electro-thermal and thermal breakdown occurred successively. An electro-thermal breakdown model was proposed based on the heat equilibrium and space charge transport, and negative differential mobility was introduced to the model. It was shown that carrier mobility determined whether it was electrical or thermal breakdown, and the model can effectively explain the temperature-dependent breakdown.