Electrical treeing and partial discharges in DC-XLPE under constant DC voltage and repetitive DC ramp voltage

Inhalt:
With an increasing number of installed extruded HVDC cable systems gaining knowledge about DC breakdown mechanisms and possibilities and limitation of DC voltage testing including partial discharge measurement becomes more and more important. Based on visual observations and electrical measurements on short pieces of cable samples featuring an artificial defect it is shown that electrical treeing and subsequent breakdown can occur in DC-XLPE under constant DC voltage as well. The treeing inception voltages under DC are comparatively high, though trees form at much lower DC voltages when transient stresses are superimposed. Tree structures at defects caused by transient stresses then grow further at much lower DC voltages compared to the DC treeing inception voltage of the defect itself. The DC treeing is accompanied by partial discharges (PD) with a very low repetition rate and a characteristic charge growth, which correlates to the size of the tree. In addition, electrical treeing and partial discharges were investigated under repeatedly increasing and decreasing DC voltage (unipolar repetitive DC ramp voltage), for which the times until breakdown were found to be lower and the PD repetition rate to be higher than under constant DC stress.