Polypropylene (PP), with high breakdown strength, low dissipation and good processibility, is one of the most widely used dielectric material for power equipment, especially in power capacitors and power cables. The improvement of PP-based dielectric material can benefit the properties enhancement of power capacitors and cables, and thus to meet with the rapid development of the power system. Nanocomposite provided a promising orientation to reach the target and recent research approaches of PP nanocomposite for power equipment were reviewed in this paper. In this paper, we linked the nanofillers to the improved properties of PP nanocomposite, and categorised the research works into nanoclay/PP composites, metal oxide/PP nanocomposite, conductive particles/PP nanocomposite, and PP core-shell nanocomposites chronologically, corresponding to the enhanced thermal and mechanical property, breakdown strength property and energy storage property, respectively. Based on the achieved approaches, prospective for future research was proposed, providing a worth-considering direction for the future work.

Polypropylene (PP) contains promising application prospects in thermoplastic cables for high voltage direct current (HVDC) power transmission because of its outstanding thermal and dielectric properties. However, the problem of poor toughness and space charge has restricted the application of pure PP in HVDC cables. In this paper, polyolefin elastomer (POE) and nano-silica were blended thoroughly and added into a PP mixture by a melting method. Scanning electron microscopy (SEM) was employed to observe the dispersion of POE and nanoparticles. Thermal properties were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Mechanical properties were evaluated by tensile tests. The elastomeric properties of composites were improved as the dispersed POE could transfer and homogenize external mechanical forces. DC breakdown results showed that the fail strength of composite with 10 phr POE and 1 phr nano-silica was obviously enhanced. The pulsed electro-acoustic (PEA) results showed that the injection and accumulation of space charge was increased by the introduction of POE, while it was restrained by the collective effect caused by nano-silica filling. X-ray diffraction (XRD) spectrograms showed that secondary ordered structures existed in the composites of PP, POE, and nano-silica, and that the ordered structure around the nanoparticles contributed to the enhancement of breakdown strength. The mechanical and dielectric properties were modified synergistically, which made the modified PP a propitious insulation material for HVDC cables.

A ternary system composed of isotactic polypropylene (PP), maleic anhydride grafted polypropylene (PP-g-MAH), and nano-ZrO 2 was designed. To investigate the influence of each composition, we systematically designed orthogonal tests with 25 samples with various PP-g-MAH (0-70 wt %) and nano-ZrO 2 concentrations (0-5 wt %). Microscopic observation showed that the introduction of PP-g-MAH could distinctively benefit the dispersion of nanoparticles. This can be understood by chemical bonds between PP-g-MAH and the nano-ZrO 2 surface, which was evidenced by infrared spectroscopy. Meanwhile, both the crystalline properties and aggregation structures were improved within this ternary system. Macroscopically, a great enhancement of DC breakdown strength (BDS) as high as 43.3% was achieved when the PP-g-MAH concentration was 50 wt % and the nano-ZrO 2 concentration was 0.5 wt %. Moreover, the effects of PP-g-MAH and nano-ZrO 2 were revealed respectively. The increase of nano-ZrO 2 content could cause the first increase and then decrease of BDS. The influence of PP-g-MAH on breakdown strength was obviously shown by the analysis of variance, and the rising PP-g-MAH concentration could lead to the nano-ZrO 2 content with the highest BDS shifted to higher loading, indicating that the modified dispersion of nanoparticles played the dominant role in the breakdown performance improvement.

Electrical tree occurring in polymer insulation limits the applications and reduces the service lifetime of power equipment. To improve electrical tree resistance in polymer insulation, polyethylene (PE) nanocomposites with nano-montmorillonite (MMT)layered fillers are compounded and investigated in terms of electrical tree characteristics. By an organization process, intercalation and coupling agents are used to modify the inorganic-layered MMT nanofillers in sequence. Polyethylene/nanomontmorillonite (PE/MMT) composites with different MMT contents are prepared by melting intercalation. Fourier transform infrared spectrophotometry is used to determine the chemical characteristics of the MMT in different surface-modification stages. The crystalline morphologies and MMT distributions in the composites are characterized by polarizing microscopy and scanning electron microscopy. The electrical tree propagating characteristics and corresponding conduction properties of the PE and PE/MMT composites are investigated. Results show that the electrical tree resistance of PE/MMT composites significantly improves. The electrical tree length decreases and the fractal dimension increases in PE/MMT composites compared with pure PE. After performing electrical tree initiation, conductive current slightly increases in composites but decreases in PE. This result suggests that the characteristics and conductive mechanism of electrical tree differ between PE and PE/MMT.Index Terms -Polyethylene/nano-montmorillonite composites, electrical tree, conductive current, conductive mechanism of tree channels.

Investigations into surface flashover characteristics and underlying mechanisms in the range from atmospheric to near vacuum pressure are significant for developing high-power equipment based on the application of gases used for flashover prevention. In this work, polyetherimide (PEI) films with controllable outgassing properties were prepared. Then, the gas-related factors, namely, gas pressure, the type of gases, and outgassing were clarified by investigating the DC surface flashover of films in a wide pressure range (10−4–105 Pa) in different gas atmospheres. In all conditions, the variation of flashover voltage was found to be divided into three regions with pressure increasing, while its deeper reason is that the number density of molecules of the gas layer is affected by outgassing and ambient gas pressure, which affects the collision of electrons. In region I, the surface flashover voltage is closely related to outgassing and shows the highest value. In region II, the surface flashover is jointly affected by outgassing and ambient gas, along with a rapid decrease of flashover voltage with increasing pressure. In region III, the surface flashover is dominated by ambient gas, and the surface flashover voltage shows a U-shaped curve with increasing pressure. Transition points of the three regions shift significantly with the change of outgassing properties. In region I, the desorption of adsorbed gas has little effect on the surface flashover voltage, while the escape of dissolved gas dominates. Finally, a model of the diverse effects of gases on surface flashover in a wide pressure range was established.