The effect of voltage frequency on PD parameters [7-11],
e.g. PD pulse shape, PD inception voltage and so on, has been
studied by many researchers, which is usually explained in
terms of statistical time lag and surface charge decay. But due
to the limitation of traditional PD measuring technique [12-
17], the discharge site in a void cannot be precisely located,
hence it is difficult to investigate the effect of previous PD
location on the subsequent one and thus to clarify the PD
mechanism.