Suppression of Surface Charge Accumulation of Dry and Oil-impregnated Nomex Paper by Surface Fluorination

Nomex paper has been recognized as an excellent insulation material that was evidenced by its electrical properties and been widely used in high voltage insulation system such as dry-type transformers and oil-immersed transformers. Because of its high surface and volume resistivity, the surface charge accumulation on Nomex paper was difficult to dissipate, which was believed to have great influence on its electrical performance. Surface fluorination has been considered to be an effective approach to modify the surface of paper and to suppress the surface charge. To understand the influence of surface fluorination on surface charge characteristic of dry Nomex paper and oil-impregnated Nomex paper, Nomex T410 paper was fluorinated using a F₂/N₂ mixture with duration of 15, 30 and 45 min. The oil-impregnated samples were prepared by impregnating the untreated and fluorinated Nomex paper in insulation oil. Attenuated total reflection infrared spectroscopy (ATR-IR, Bruker ALPHA) was used to characterize the surface chemical bond changing caused by fluorination. The surface potential decay characteristics were investigated. The ATR-IR spectra of untreated and fluorinated Nomex paper evidence that the C-Fx bonds were induced to Nomex surface by substitution for hydrogen atoms, accompanying with aromatic chain scission and ring opening reaction. The measured surface potential decay characteristics shows that both positive and negative potential decay were accelerated by surface fluorination. For both untreated and fluorinated Nomex paper, the surface potential decay of dry Nomex paper was faster than oil-impregnated Nomex paper. The surface trap energy and trap distribution of the samples were calculated by isothermal surface potential decay (ISPD) method. The result shows that the fluorination may shallow the traps by modify the chemical chain while the oil-impregnated samples had higher trap energy because some charges were trapped by the insulation oil.