The influence of AC and DC voltages on electrical treeing in low density polyethylene

Electrical tree growth is a well-documented process leading to failure of high voltage polymeric insulation under AC stresses. However, tree growth in HVDC insulation failure is not well understood. This work considers electrical tree degradation in polymeric insulation subjected to combined AC and DC voltages. Tests in LDPE samples of needle-plane geometry yield three types of electrical trees, which grow depending on the magnitude of the AC components, irrespective of the DC voltages. In tests with an AC component of 10kV (peak amplitude), 12kV and 15kV trees are distinguished by both the conductivity of tree channels and the tree shape, and are referred to as either conducting trees, non-conducting branch trees or non-conducting bush trees respectively. With 10kV AC, tree initiation was significantly accelerated by superimposing −20kV DC. With +20kV DC, the incepted trees had more bifurcations, but there was no major change to tree initiation time. Space charge measurements on thin LDPE films provide a basis for understanding the difference between tree initiation with +DC and −DC voltages. The subsequent propagation of conducting trees were not influenced by biasing the 10kV AC. With 12kV AC and 15kV AC, the growths of non-conducting branch and bush trees were both accelerated by positive biasing. A retardation was observed in bush tree growth with −15kV DC. The effects of DC voltage polarity on tree morphology are different for branch and bush trees. Different relationships between PD magnitudes and tree growth were also found between branch and bush trees. The tree length determined PD magnitudes in branch trees and the evolution of PD in bush trees suggest that the additional DC stress has no impact to PD magnitudes. Nevertheless, DC bias effects on the symmetry between positive and negative discharges have been evidenced through PD analysis for bush trees. Moreover, the modifications on the shape of bush trees are believed to be associated with the changes in PD asymmetry. This work has illustrated the importance of AC ripples in the failure mechanisms in HVDC insulation.