This test is useful to IDENTIFY FAULTS THAT THE STANDARD TESTS (AC Dielectric Strength Test and Surge Test) ARE NOT ABLE TO IDENTIFY.
In the picture 1 it is shown a typical fault: there’s a wire out of the slot, touching the stack. If this small part of wire is scratched, also a standard AC Dielectric Strength Tester is able to identify this fault. If instead, as commonly happens, this small piece of wire is still well insulated, the voltage applied during the AC Dielectric Strength Test, even if quite high, can be not sufficient to brake the remaining insulation material and this fault cannot be identified. With the measurement of Partial Discharges done at the same time of the AC Dielectric Strength Test – with E.D.C. method – this fault is always identified and filtered.
In the picture 2 it is shown the signal of partial discharge (blue peaks) and the voltage (red signal).