Electrical Tree Driven Breakdown of XLPE Under Step Stress Breakdown Test at Multiple Frequencies

The electrical treeing of a 33 kV XLPE cable is investigated on a pin-plane electrode system in the frequency range of 50–650 Hz. Cable specimens are created with needle electrodes having a tip radius of 1 µm and a tip distance of 0.75 ± 0.1 mm. Step stress tests are conducted at a step increase of 1 kV peak voltage for step times of 15 seconds and 75 seconds. The breakdown instances of the specimens are recorded to obtain the time to breakdown (TTB), breakdown voltage and the polarity of the breakdown voltage. In addition, the breakdown path is observed using a stereo microscope. It is found that the normalized TTB and the breakdown voltage exhibit a decreasing trend with respect to the frequency for step stress tests. A high percentage of samples are broken down in the negative half cycle of the applied voltage. Microscopic imaging shows that the corresponding electrical trees are branch type trees for all the frequencies. The observations for normalized TTB and voltage variation is attributed to the phenomenon of space charge limiting electrical field variation with frequency.