A concept of sub-fractal structure-a fractal structure growing in a space w
ith a fractal dimension (base-fractal)-was put forward to describe the shap
e of a single partial discharge in an electrical tree with a base-fractal d
imension. A computer simulation model was proposed to study the relation be
tween the sub-fractal and the base-fractal dimensions. The tree growth rate
was suggested to be related to the difference between these two fractal di
mensions.