At 150 Hz case, shown in Table 1 and Figure 2, it was
obvious that tree initiation and growth have begun after
several minutes. One sample breaks down after 10 min,
while the other did not show any initiation of electrical
tree. This may be attributed to the impact of needle tip
position inside the insulation.
However, the other 2 samples have shown a shape of
electrical tree as classified in [13]. It was of branch type,
which considered damaging to electrical insulation due
to high speed of penetration in the crystalline material.
Other trees at 250 Hz and 350 Hz were classified as bush
type tree and bush-branch tree as shown in Figures 3 and
4. This type of trees has property of propagation with
high speed both vertically and horizontally allowing
creating high number of pits and channels in the
insulation, due to intense partial discharge and photo
ionization at needle tip.