The sample consisted of a void between a PE surface and a
metallic surface, which was called VMD electrode. In the
sample, the thickness of PE film was about 6 μm, and the
diameter and height of void was about 5 mm and 0.15 mm,
respectively. The waveform generator could produce ac
voltage with variable cycle, which was amplified by the high
voltage amplifier. The output voltage amplitude of 2.7 kV was
used to make sure that PD could take place and there was no
“back discharge” [20].