A Physical Model for PD in Tree Channel and its Effects on Tree Growth

Abstract

A model for partial discharge (PD) propagation in a single channel of an electrical tree is proposed. PD is induced by the high field near the needle tip, then propagates along the channel by field enhancement due to charge injection by PD, and stops propagating when the field becomes less than a certain value due to field divergence. This model suggests that the field distribution and the tree growth process are closely related to the behavior of PD. By considering the influences of gas pressure and the applied-voltage frequency on the behavior of PD, various phenomena of tree growth, such as the bush formation under higher gas pressure and low frequency, are qualitatively explained. This model also suggests a relation between PD and the fractal nature of treeing breakdown.

Model for partial discharges associated with treeing breakdown: II. tree growth affected by PDs

“…However, this model cannot explain why no obvious reduction of PD magnitude is observed when a bush is formed. On the other hand, the effect of charge injection was also considered to explain the phenomenon [11,[18][19][20][21]. It was assumed that more space charge injected under the higher voltage might moderate the field distribution at the tree tips, reduce the field at the tree tips and lead to bush formation.…”

“…Tree shape is also affected by the frequency of the applied voltage. Figure 1 shows that the bush-like tree is likely to appear when the frequency is reduced from 60 Hz to 15 Hz [20]; but when the frequency is increased from 60 Hz to 400 Hz (or 800 Hz), a transition from a tree-like tree to a bush-like tree is also reported [10]. Although the average number of PDs in a half-cycle does not change much with an increase in frequency [22] (in the other words, the number of PDs in a unit time is proportional to the frequency), the tree length (l) after a certain period does not increase in proportion to the frequency (f ) (e.g.…”