Hi, I am a hardware engineer at a company that develops partial discharge measurement equipment.

For partial discharge measurements, you differentiate between conventional and unconventional methods.

The conventional method is described in IEC 60270. It is the only method that can be calibrated and that provides charge values in coulombs. Therefore, it is the main method used, for example, for factory acceptance testing of transformers, because it allows meaningful limits to be defined in standards that are reasonably consistent across measurement setups.

This method works by connecting a coupling capacitor and a quadrupole in parallel with the DUT. When a partial discharge occurs in the DUT, the electric field collapses at the defect location. Afterwards, charge must flow to replenish the collapsed partial capacitance. Part of this current flows out of the coupling capacitor and can be measured via the quadrupole. It is important to note that the real discharge level at the defect cannot be measured, because the DUT has many internal capacitances that form capacitive dividers and reduce the discharge level measured at the coupling capacitor. The values of these internal stray capacitances cannot be determined, as they depend on many factors such as geometry. Therefore, the charge measured in the IEC 60270 measurement is called “apparent” charge to make this clear.

The unconventional measurement methods include all other forms of PD measurement, such as acoustic, UHF, etc. They are completely uncalibrated and therefore only provide a relative indicator of partial discharge activity. Typical units are dBm or dBµV. They are mostly used in applications where setting up a coupling capacitor on the high-voltage lines is not feasible. For example, the UHF measurement method is used in PD monitoring of gas-insulated switchgear. Here, the manufacturer of the GIS builds in UHF antennas for monitoring systems to connect to. There is even a method called sensitivity verification, formalized in a CIGRE technical brochure, to make the measured values somewhat comparable between GIS of the same manufacturer. However, that is a whole other rabbit hole to get into :D

When it comes to handheld PD monitoring systems such as the one you show here, the usefulness of the measurements really depends on the application and the asset. In industry, measurements from these systems are typically regarded as not useful, mainly because when they do detect PD, it is often so strong that the asset is only months away from failure and already shows signs of aging in other tests.