| In order to study the breakdown factors and microscopic mechanism of XLPE insulating materials,a variety of molecules in the amorphous region of XLPE were modeled.Based on the first principles,the characteristic parameters of energy band structure under the action of electric field were calculated and analyzed,and the breakdown field strength was calculated by combining with the neural network.The molecular models of short C-H chains,water and main cross-linked byproducts were established.Firstly,Monte Carlo method was used to establish multiple amorphous cell models in different molecular conformations containing only C-H molecular chains.According to the trend of band gap change and numerical interval,the conformation of C-H molecular chains in amorphous regions of XLPE insulation materials was classified and discussed and statistically analyzed.The breakdown field strength of XLPE amorphous cell model in specific molecular conformation was calculated by BP neural network.Then,the XLPE amorphous cell models with different water molecular mass fractions were established by encapsulating the water molecules and C-H molecular chains.The variation rule of the model band gap under different water molecular mass fractions was analyzed.The breakdown field strength of the water doping model was calculated by BP neural network.Amorphous cell models of crosslinked byproducts phenylcarbinol,αmethylstyrene and acetophenone were established,and their effects on the intrinsic breakdown of XLPE dielectric were analyzed by comparing their band gaps and change rates at different field strengths.The results show that for C-H molecular chains with the same chain length and different molecular conformations,the band gap changes differently with the field intensity.50%of the conformations show a monotonically decreasing trend,20%of the conformations show an obvious trend of first increasing and then decreasing,and 6%of the conformations show a trend of first stabilizing and then sharply decreasing.The band gap of different molecular conformation models under increasing field intensity is statistically dispersed,and most of the data are in more than one banded region bounded by the lower envelope.The neural network model predicted that the band gap corresponding to the typical conformation(No.43)changed from positive to negative when the spot strength reached 0.094 a.u.,indicating the breakdown occurred.Compared with the XLPE amorphous cell model containing only C-H molecular chain,the band gap of the model is significantly reduced due to the presence of water impurities,and the band gap is approximately linear with the change of water molecular mass fraction.When the mass fraction increases by 10 percentage points,the band gap decreases by about 0.7 eV.From the perspective of energy band structure,the breakdown field strength of XLPE dielectric can be reduced by the three crosslinked byproducts,among which,acetophenone has the greatest effect,followed by αmethylstyrene. |
