Impact of DC Polarity on Electrical Treeing in LDPE

A comparison is made of electrical tree initiation and growth in LDPE under pure positive and negative DC stresses. Branch trees formed from needles in DC fields, and they comprised channels of ~2 $\mu$m diameter. Positive DC trees exhibited long initial lengths followed by little growth, while negative DC trees had shorter initial lengths with further tree growth. At −45kV, tree initiation length and the length at which tree growth ceases yield an inverse linear fit. PD events were measured before tree initiation in negative DC fields in some cases, and a linear relation was found between initial tree length and the logarithm of the maximum PD magnitude prior to initiation. An electric field threshold for tree growth is proposed. Based on this and an understanding of space charge injection and accumulation, the difference in tree initiation and growth with polarity is explained.