Fast measurements of partial discharge currents in solid dielectric samples containing voids

The mechanism of partial discharges inside a void is investigated using a uniform-field electrode system. To allow fast measurements of the partial discharge current, one of the electrodes is split into a central measuring disk and surrounding ring which acts as a coupling capacitor. The minimum permissible diameter of the disk depends on the gap width (Ramo-Schockley effect). A 400-MHz Tektronix oscilloscope 7844 or a digitizer (7912AD) is used, allowing measurements with rise times down to about 1 ns. Fast current measurements in polyethene sheets at 50-Hz AC voltages are reported. The rise time (10-90%) is about 1 to 1.5 ns. The measured charge does not differ greatly from theoretical calculations of the apparent charge (67-76 pC range as opposed to a calculated value of about 66 pC). This indicates that the entire void area discharge and not just part of it.<>