Preliminary Investigations of the Electrical Tree Growth at Frequencies of 50 Hz and 2.2 kHz by means of Partial Discharge Analysis

During fault situations within high voltage direct current (HVDC) links, transient overvoltages with frequency components of several kHz, can occur and incept electrical trees inside the cable insulation. Since the relationship between applied voltage and frequency and the growth of the electrical trees is not completely understood, a deeper investigation is necessary. In particular, starting from cross-linked polyethylene (XLPE) samples obtained from a commercial HVDC cable, electrical treeing tests have been performed by applying a voltage level with two different frequencies: 50 Hz and 2.2 kHz. During the tests, the partial discharge (PD) activity has been monitored. The obtained results show that the time to breakdown obtained from the samples subjected to the 50 Hz power supply is shorter than those obtained with 2.2 kHz. Analyzing the PD activity and the sample by means of digital microscope, different tree shapes were visible, which influence the degradation time of the performed test. The obtained results of this preliminary study suggest that increasing the frequency does not necessarily lead to a reduction of the time to breakdown but influences the shape and the growth of the electrical trees.