Electrical tree growth characteristics in XLPE cable insulation under DC voltage conditions

The growth characteristics of electrical trees in XLPE cable insulation under DC voltage conditions were studied by a combination of treeing test and microscopic observation system. In the treeing test samples, the tip radius of the needle electrode was 5 μm, and the pin-plane distance was 2 mm. When a 70 kV DC voltage was applied on the samples for totally 10 hours, either continuously or intermittently, no trees appeared. So the trees for growth study were all initially triggered by a 50 kV standard lightning impulse. The DC voltages were applied periodically, so as to make it possible for the required observation and recording. Under the DC voltages, the electrical trees developed gradually from a single slender branch to a sparse bushlike structure, and tree channels were found to be non-conductive. The growth rates were rather slow compared with AC trees under the same condition. Grouping experiments showed that the tree length increased with the increasing number of cycles and amplitude of DC voltage, and it was also related to the voltage duration in each cycle at 60 kV. This tree growth pattern has been well explained by a nonconductive tree model and the equivalent circuit, and the discharges occurred in the tree channels are thought to be the driving force for tree growth. Under the condition of an unexpected grounding of the needle electrode, a new tree would initiate instantly from the needle tip, while an existed tree would grow very rapidly. This is correlated to the material damage caused by the sudden release of the injected charges. The work is useful for the recommendations on test arrangement and operation strategy of HVDC cables.