In fact,
the decay of surface charge always exists during a PD
sequence, which may result from surface conduction, bulk
conduction and some other factors. If the voltage cycle
increases, the interval time of discharges may become larger.
Correspondingly, surface charge decay will become much
more serious, leading to the reduction of locally enhanced
field and thus the change of the effect of residual surface
charge on PD location. Therefore, investigating the influence
of surface charge on PD characters when the voltage cycle
varies is important to clarify the mechanism. Besides, it is
helpful to understand the experimental results when using the
Variable Frequency Phase Resolved Partial Discharge
Analysis to diagnosis insulation condition [8].