Influence of structural change by AC voltage prestressing on electrical-tree inception voltage of LDPE with water-tree degradation

We investigated the influence of AC voltage prestressing on the electrical-tree inception voltage in LDPE. When AC voltage prestress was not imposed, the impulse-tree inception voltage increased with the size of water tree due to the electrical field relaxation. On the other hand, when AC voltage prestress was imposed, the impulse-tree inception voltage did not change with the water-tree size and was lower than that without AC voltage prestressing. The FTIR absorption measurements showed several absorptions between 650 and 1500 cm^-1, which did not exist in the sample without AC voltage prestressing. Besides, the TEM observation revealed that the AC voltage prestressing increased the density and size of micro voids and some of them were aligned in the direction of applied electric field. These results suggest that AC voltage prestressing causes structural change in the water tree and enhances material degradation, leading to lower tree-inception voltage.