Effect of Nano-Al² O³ Doping Modification on AC/DC Superimposed Breakdown Characteristics of Insulating Paper

This study explored the way to improve the AC and DC superposition breakdown strength by introduction of nanoAl ₂ O ₃ additive. Insulating paper hand-sheet with and without nano-Al ₂ O ₃ (2wt. % were made in the laboratory. The AC and DC superposition breakdown strength of different AC content and under different temperature), dielectric properties including dielectric constant and loss and volume resistivity at room temperature (25°C) were tested. Based on the cellulose fiber and the insulating oil structure, an AC and DC superposition circuit model has been built. Results shows that t the AC and DC superposition breakdown strength of the insulating paper linearly decreases with the increase of the AC content, at room temperature, and the breakdown strength of the modified paper is always higher than that of the conventional paper. By introducing nano-Al ₂ O ₃ the relative dielectric constant was reduced from 3.23 to 3.03 at 50Hz, the dielectric loss was changed from 0.021 to 0.013, which was reduced by about 38.10% and the resistivity has increased from 6.87×10 ¹³ (Ω·m) to (10.21×10 ¹³ (Ω·m). As a result, doping of nano-Al ₂ O ₃ improves the electric field distribution of insulating oil and fiber in the oil-immersed paper under AC superimposed DC voltage and improves the electric field strength of fiber, which successfully enhanced the breakdown strength of insulation paper under AC superimposed DC voltage.