Breakdown failure in insulation material is one of the key problems that threaten the safe operation of high-voltage direct current cable. In this work, the effect of boron nitride nanosheets (BNNSs) concentration, space charge and temperature on DC breakdown strength have been explored. Cross-linked polyethylene (XLPE)/BNNS nanocomposites were prepared by the melt blending method, and the basic characteristics of nanoparticles and composite were characterised. The experimental results indicate that DC breakdown strength of nanocomposite can be effectively improved when a small amount of BN nanosheet is doped into the matrix. The breakdown strength of the sample reaches the maximum value of 407.52 kV/mm when BNNS content is 0.5 wt%, which is about 33% higher than that of pure XLPE. Further, the effect of space charge on the breakdown of nanocomposites has been studied by pre-injecting charges. For the samples with different BNNS contents, all the breakdown strength present ascending trend when the polarity of the applied voltage is the same as that of the pre-injected charges. Besides, it can be found that the breakdown strength of the XLPE/BNNSs composite decreases significantly at 50°C, which is due to more charge accumulation at 50°C. It reaches 2.06 × 10 −8 C which increases by about 2.2 times than the room temperature.

Nanocomposite dielectric is considered the most promising insulation material for high‐voltage direct current cables. Herein, boron nitride nanoparticles (BNNPs) and boron nitride nanosheets (BNNSs) are used as nanofillers to study the effect of nanofiller concentration and morphology on the electrical properties of cross‐linked polyethylene (XLPE) composite based on experiments and simulation. The experimental results indicate that nanofiller concentration has a great influence on permittivity, while the influence of the morphology at the same concentration is small. The maximum relative permittivity occurs at 1 wt%. They are 2.35 and 2.32, respectively, for XLPE/BNNSs and XLPE/BNNP, increasing by about 11% and 9% than that of XLPE matrix. The morphology of BN significantly influence the breakdown performance, and the breakdown strength of the nanocomposite increases first and then decreases with the increase in doping concentration. The maximum breakdown strength of the composite occurs at 0.5 wt%. They are 403.8 and 349.2 kV mm−1, respectively, for XLPE/BNNSs and XLPE/BNNP, which is about 33% and 13% higher than that of XLPE matrix. Molecular simulations show that the free volume of XLPE/BNNSs is lower than that of XLPE/BNNP. In addition, the sheet structure can effectively block the carrier transport and prolong the discharge path.

No abstract

Temperature and electric field are important factors affecting space charge accumulation in high voltage dc (HVDC) cable insulation. Charge conduction and accumulation in XLPE have been studied by correlating the polarization and depolarization processes. The experimental results indicate that the steady current of XLPE increases about 2 or 3 orders with the increasing temperature from 25 °C to 90 °C, and the stronger electric field, the less the increased amplitudes. The trap level in XLPE stressed by different electric fields and temperatures have a tiny change from 0.89 eV to 1.15 eV. Besides, there exists an inflection temperature of charge accumulation in XLPE, around 50 °C∼ 60 °C. At the room temperature, the trapped charges are difficult to release from the traps, and these homo-charges near the electrode can depress the further injection of charges. With the increase of temperature, part of trapped charges near the interface will gradually migrate towards the bulk of the material, and more charges are injected. When the temperature exceeds around 50 °C, the molecular movement is accelerated which can dramatically enhance the hopping probability of charges between the adjacent traps, and little accumulated charges are left.

No abstract