Jv. Champion et Sj. Dodd, Simulation of partial discharges in conducting and non-conducting electrical tree structures, J PHYS D, 34(8), 2001, pp. 1235-1242
Electrical treeing is of interest to the electrical generation, transmissio
n and distribution industries as it is one of the causes of insulation fail
ure in electrical machines, switchgear and transformer bushings. Previous e
xperimental investigations of electrical treeing in epoxy resins have found
evidence that the tree structures formed were either electrically conducti
ng or non-conducting, depending on whether the epoxy resin was in a flexibl
e state (above its glass transition temperature) or in the glassy state (be
low its glass transition temperature). In this paper we extend an existing
model, of partial discharges within an arbitrarily defined non-conducting e
lectrical tree structure, to the case of electrical conducting trees. With
the inclusion of tree channel conductivity, the partial discharge model cou
ld simulate successfully the experimentally observed partial discharge acti
vity occurring in trees grown in both the flexible and glassy epoxy resins.
This modelling highlights a fundamental difference in the mechanism of ele
ctrical tree growth in flexible and glassy epoxy resins. The much lower res
istivities of the tree channels grown in the glassy epoxy resins may be due
to conducting decomposition (carbonized) products condensing on the side w
alls of the existing channels, whereas, in the case of non-conducting tree
channels, subsequent discharges within the main branches lead to side-wall
erosion and a consequent widening of the tubules. The differing electrical
characteristics of the tree tubules also have consequences for the developm
ent of diagnostic tools for the early detection of pre-breakdown phenomena.