The effect of frequency on the breakdown of XLPE cable insulation with artificial defects

It is of great importance to assess the reliability of on-site testing for extruded insulation cables. In the present work, the effect of the applied field frequency on the breakdown performance of cross-linked polyethylene (XLPE) insulation was investigated. An inserted needle was embedded into a 10 kV commercial XLPE cable along the radial direction to simulate the real defects. The shape of the artificial defects was precisely controlled by a designed mold with a micrometer. Breakdown tests were conducted in transformer oil at room temperature using a frequency-tuned resonant test system and damped AC voltage system in the frequency range from 20 to 300 Hz. After breakdown measurements, specimens were cut into thin layers to investigate microstructure of their breakdown paths by an optical microscope. We found that the breakdown voltage of specimens with inserted needle was sensitive to the applied field frequency and voltage waveforms. The maximum breakdown voltages under variable frequency AC voltage locate at 25Hz and 240 Hz, while that under damped AC voltage locates around 180 Hz. It was found that DC ramp breakdown voltage is higher than impulses breakdown voltage, while ac breakdown voltage is the lowest. It was interesting to note that, different kinds of tree-like sub-paths appear on the flank of the main breakdown path of the specimen with the inserted needle at different frequencies. Frequency dependence of breakdown performance of XLPE cable with inserted needle defect was discussed.