Partial discharge characteristics during electrical tree growth in LDPE

Partial discharge (PD) measurements were
performed during an electrical treeing process in a classical
needle-plane low density polyethylene sample under AC voltage.
This treeing process features an evolution from the propagation
of a non-conducting electrical tree to the generation of a
conducting electrical tree. PD characteristics are discussed with
regard to different treeing stages during the process. It is found
that changes in PD characteristics occur before transitions in the
visual aspect of treeing development. In particular, statistical
parameters obtained from measurements reveal a polarity
dependence in PD patterns in the AC cycle. In general, during
non-conducting tree growth, positive and negative discharges are
not symmetric in magnitude or phase of occurrence. Positive
discharge patterns are more stable in time than negative
discharge patterns. The discharge frequency and magnitude
seems to have an inverse relationship for negative discharges.
Negative PD patterns were found to change during periods of fast
tree growth. This was not the case for positive PD patterns.