A count of partial discharges tells you how many there are and how large, but not what produces them. The phase-resolved partial discharge pattern, or PRPD, adds the missing information: it plots each discharge by its amplitude and by the angle of the voltage waveform at which it occurred, accumulated over many cycles.
The resulting cloud of points has a characteristic shape depending on the mechanism. An internal void gives two symmetrical groups on the rising slopes of each half-cycle; surface discharge gives asymmetric, broader groups; a metallic point gives discharges concentrated in a single half-cycle; and a free particle gives a scattered pattern with no clear relationship to phase.
That is why PRPD is a diagnostic tool and not merely a detection one: it allows a decision on whether what is seen is a defect that progresses quickly — the point — or one that tolerates monitoring, and it separates the real signal from external noise, which is not synchronised with the network.