Molecular Dynamics-based Microscopic Characterization Of Low-temperature Composite Insulation Under Air-gap Discharge Development

In recent years,superconducting power technology has made great progress,and polypropylene laminated paper(PPLP),as a commonly used primary insulation material,has been widely used in the design of high-temperature superconducting DC cable insulation,whose insulation reliability is closely related to the safe operation of the system.When superconducting cables are in operation,the insulation material is susceptible to deterioration due to the combined effects of multiple stresses such as electrical,thermal,and mechanical stresses over a long period of time,leading to partial discharges and further insulation degradation.At present,most of the studies on the degradation characteristics of low-temperature composite insulation are experimental,and the damage characteristics of the material and its internal molecular cleavage mechanism under the discharge process have yet to be further investigated.In this paper,we analyze the insulation degradation characteristics of PPLP under liquid nitrogen impregnation and simulate the molecular cleavage process in the degradation process by using molecular dynamics method,and the main results are as follows:(1)In this paper,according to the actual operating environment of superconducting DC cables,a low-temperature composite insulation partial discharge simulation test platform was built,a typical air gap defect fault model was produced,repetitive air gap discharge tests were carried out,and air gap discharge development characteristics were analyzed in terms of discharge energy and statistical characteristics;(2)For the degradation damage characteristics of PPLP specimens impregnated with liquid nitrogen,the insulation characteristics of test samples of different voltage levels under the action of discharge were analyzed by scanning electron microscopy,three-dimensional optical surface profiler,Fourier infrared spectroscopy and high-resistance tester equipment,and the surface microscopic morphology,surface roughness,structure,generation,and volume resistivity and surface resistivity of insulation materials were completed.The results show that the insulating materials can be used in pre-breakdown tests.The results show that the insulating material becomes significantly porous in the pre-breakdown stage,the structure of each relevant functional group is destroyed,and the volume/surface resistivity drops abruptly,which leads to the deterioration of the insulation properties;(3)In order to investigate the deterioration mechanism of insulating materials at the molecular level,molecular dynamics simulation software was used to carry out a simulation study on the molecular cleavage process of PPLP surface insulating paper cellulose and interlayer polypropylene corresponding to different stages of discharge development,and the effect of different temperature intervals on the interaction between PPLP surface insulating paper and interlayer polypropylene film was analyzed.It is found that the C-O bond requires less energy to break,while the C-C bond is relatively stable and requires more energy to break;the interaction between the PPLP surface insulating paper and the interlayer polypropylene film fluctuates widely with the increase of the simulated temperature,and the intermolecular interaction force decreases.The degradation characteristics of the insulating material and the molecular cleavage process are determined by combining experiments and simulations,which provide a theoretical basis for exploring the degradation mechanism of the insulating material.