Direct Current Properties Of Crosslinked Polyethylene Grafted With Multifunctional Molecule

The key to restricting the development of cross-linked polyethylene(XLPE)high-voltage DC cables towards high voltage levels is the accumulation of space charges and field strength reversal in the insulation layer.Some polar groups carried in polar molecules can introduce local state energy levels into polymer materials,inhibit carrier migration,reduce space charge,and improve electrical properties by grafting them on XLPE,which has become a research hotspot in recent years.However,the influence of different types of polar molecules or polar groups on the quantum chemistry properties of the modified materials is still unclear,and the relationship between the type,size and distribution of the local state energy levels in the materials and the DC electrical properties of the materials is still unclear.There is a lack of feasible methods to optimize the grafted polar molecules with good electrical property modification effect under the guidance of theoretical calculations.In order to obtain graft monomers with excellent comprehensive performance,this article first calculated 9 different types of polar compounds carrying different polar groups through theoretical analysis and simulation.The study found that the distribution of polar groups should be in a central asymmetric structure,ensuring that they have high dipole moments,wide electrostatic potential distribution,and deep trap energy levels of grafting modified materials.Subsequently,based on the principle of asymmetric distribution and containing multiple polar groups,a small molecule compound diacetone acrylamide(DAAM)with high dipole moment and carrying both amine and carbonyl groups was selected.XLPE and XLPE-g-DAAM were prepared through mixing and thermochemical crosslinking process.Firstly,the torque rheological curve,thermal extension and gel content tests were used to investigate the crosslinking performance of the grafting modified materials.The results showed that the graft reaction did not affect the crosslinking degree of the material.The results of infrared spectroscopy showed that DAAM could be grafted onto XLPE macromolecular chains.Tensile tests and dynamic thermo-mechanical tests shows that the mechanical and physical properties of the grafting material are similar to those of XLPE.Finally,the thermogravimetric test of small molecule DAAM shows that the grafted monomer DAAM have a higher gasification temperature.Furthermore,the DC performance of the grafting materials in a wide temperature range was systematically tested.The space charge test results at 30℃and 80℃ showed that the grafting modified materials could form a space charge shield on the surface of the sample,which effectively inhibited the further injection of electrodes into the sample.However,the introduction of excessive polar groups strengthen the effect of shallow hole trap,and the hole injection of the anode is significantly intensified.Conductance current test at wide temperature shows that DAAM graft can significantly reduce the conductance current of XLPE material,reduce the temperature sensitivity of the material,improve the degree of nonlinearity of the electric field,and inhibit the inversion of the electric field in the insulation layer.The results of DC breakdown strength test showed that the electrical strength of the grafted modified material was significantly improved at 30℃,and the shape parameters in a wide temperature range were significantly improved.