Figure 9. Electric field distribution along the interface between
the grading material and the insulation for a grading material
conductivity of 10−9 S/m during the 10 ms polarity reversal period.
Since the reversal time is about 1/3 of the dielectric time
constant, which is far too short for the system to maintain a
resistively graded field during the transition, the field distribution
at the end of polarity reversal is close to the capacitive field
distribution caused by the change in voltage (−270 kV from 135
kV to −135 kV) during the transition superimposed on the initial
resistive field distribution which results in a very large field
near the ground shield.