Partial discharges are small electrical discharges that can occur in the interior or on surfaces of electrical insulation systems (International Electrotechnical Commission, High-voltage test techniques-partial discharge measurements. IEC, Publication-60270, 2000). PDs can also originate from high-voltage metallic surfaces, producing air ioniozation around electrodes. These discharges can arise from many causes, such as impurities, air humidity, temperature and mechanical stress (Rotating Electrical Machines - Part 27-2, On-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines. IEC/TS 60034-27-2. International Electrotechnical Commission. Technical Report, 2012). PDs have been known since the nineteenth century and are commonly found in high voltage electrical systems of different applications. It is important to detect them in order to avoid potential damages to high voltage systems, such as hydrogenerators.
According to Institute of Electrical and Electronics Engineers (IEEE Guide for the Measurement of Partial Discharges in AC Electric Machinery. IEEE Std 1434-2014 (Revision of IEEE Std 1434-2000), pp. 1–89, 2014), PDs occur along with various physical manifestations, such as electrical pulses, radio-frequency (RF) signals, acoustic emissions, visible light and chemical reactions involving generator’s cooling gases (mainly air and hydrogen). In this Chapter we discuss electromagnetic methods for PD signal measurements. Electromagnetic measurements may be performed using PD sensors that detect the electric current components of PDs that propagate through the stator windings or by antennae or field sensors which detect the associated radiated electromagnetic waves (Rotating Electrical Machines - Part 27-2, On-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines. IEC/TS 60034-27-2. International Electrotechnical Commission. Technical Report, 2012). Methods for performing measurements of displacement currents in capacitors connected to stator windings are standardized to evaluate the windings insulation condition in rotating machines. Although procedures regarding detection of radiated signals using antennae are still not standardized, it is known that the spectra of radiated electromagnetic disturbances produced by a single PD may range from approximately 100 kHz up to several hundreds of MHz (Sena et al., Energies 14(21), 2021). Therefore, with appropriately designed antennas, it is possible to detect PD occurrences from the radiated electromagnetic waves.
