SUMMARYIt is well known that water tree propagation in XLPE cable is significantly influenced by inorganic impurities in water. Therefore, we investigated both changes in concentration and deviation of isotopic content of inorganic elements in XLPE samples by water tree experiments in a clean environment. The concentration of several kinds of elements (e.g., Li, Na, Mg, Al, K, Ca, Fe, Ni, Pb, and Bi) in water-treed sample showed anomalous increase or decrease dependent on cation (K
SUMMARYBow-tie tree (BTT) generated from contaminant, such as metal, carbon, amber (overcured resin) or void is a deterioration factor of XLPE cable. In particular, BTT in contact with inner or outer semiconductive shield could significantly lower residual AC breakdown voltage of HV power cables. To evaluate the influence of water and ion diffusion on generation and progress of BTT, we investigated the relationship between water content of XLPE and the generation of BTT under various accelerated aging conditions. The number of BTT in XLPE samples with accelerated aging under the open condition, involving evaporation of water in which samples were immersed, was very large compared with the closed condition. Furthermore, when samples were intermittently immersed in water, the number of BTT in samples was large compared with samples immersed continuously. In these experiments the generation of BTT seemed to have nothing to do with changes in water content before and after accelerated aging. Therefore, it was suggested that diffusion of ions rather than water in XLPE played an important role in the generation of BTT.
Bow-tie tree (BTT) generated from contaminant, e.g., metal, carbon, amber (over cured resin) or void in insulator is a deterioration factor of XLPE power cable. However, essential role of water in generation and progress of BTT is not yet sufficiently cleared. In order to investigate the role of water we paid attention to difference in chemical properties of light water (H 2 O) and heavy water (D 2 O), moreover we evaluated influence of isotopic effect due to hydrogen and deuterium on behavior of BTT generation. Fig. 1 shows configuration of accelerated aging test. XLPE samples, in which copper powder of 500ppm was contaminated as BTT cores, were dipped in light water mixed with 0, 1, 10 or 100wt% of heavy water. A cell for voltage application using separable beaker was immersed in tap water maintained at 60℃, and AC electric field (2kV/mm, 2400Hz ) was applied to XLPE sample for 240 hours. From these results we considered that the depression effect due to hydrogen isotope appeared by inhibiting ionization and elution of BTT cores, because salt-solubility and ionic mobility of heavy water are about 15 to 20% smaller than those of light water. Therefore, the essential role of water seemed to be production and transport of ions in XLPE. Bow-tie tree (BTT) generated from contaminant, e.g., metal, carbon, amber(over cured resin) or void in insulator is a significant deterioration factor of XLPE power cable. However, essential role of water in generation and progress of BTT is not yet sufficiently cleared. In order to investigate the role of water we paid attention to difference in chemical properties of light water (H2O) and heavy water (D2O), moreover we evaluated influence of isotopic effect due to hydrogen and deuterium on behavior of BTT generation. In accelerated aging test the number of BTT in XLPE sample, in which copper powder of 500ppm was contaminated as BTT cores, dipped in heavy water (D2O:100wt%) decreased to one third compared with light water(D2O:0wt%).Furthermore, the maximum length of BTT decreased with increase in concentration of heavy water. The experimental results show that heavy water exerted a depression effect on generation and progress of BTT. We considered that the depression effect due to hydrogen isotope appeared by inhibiting ionization and elution of BTT cores, because salt-solubility and ionic mobility of heavy water are about 15 to 20% smaller than those of light water. Therefore, the essential role of water seemed to be production and transport of ions in XLPE.
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