IEEE Std 1434-2014 pdf download – IEEE Guide for the Measurement of Partial Discharges in AC Electric Machinery.
5. Overview of PD detection methods PDs are accompanied by several physical manifestations: electrical pulses and resulting radio frequency (RF) pulses, acoustic pulses, light, as well as chemical reactions within the cooling gases that are either air or hydrogen. The following subclauses are a summary of how some of these manifestations can be measured as a means of quantifying the PD activity in a stator winding or individual coil or bar. 5.1 Electrical pulse sensing Because a PD involves a flow of electrons and ions across a small distance in a finite period of time, a small current flows every time the PD occurs. The total current will be governed by the transport of a certain number of picocoulombs of charge. The current flow creates a voltage pulse across the impedance of the insulation system. One of the primary means of detecting PD is to measure the small voltage pulse that accompanies every PD, or the resulting current pulse. These quantities are measured in circuits remote from the PD source. Note that in a typical coil, bar, or winding, there may be many hundreds of PD per second; thus, many hundreds of electrical pulses may be detected each second. The voltage pulse can be detected by means of high-voltage capacitors, which are normally connected to the phase terminal or elsewhere in the winding. The capacitor has a high impedance to the power frequency voltage, but appears as a low impedance to the high-frequency PD voltage pulses. Alternatively, a high-frequency current transformer can be installed on the lead that connects the neutral of the machine to the grounding impedance, on the phase leads or in other suitable locations to detect the pulse currents accompanying the PD. The output of the capacitors or the high-frequency current transformer are voltage or current pulses, respectively, which can be measured with an oscilloscope, spectrum analyzer, or pulse height analyzer. Variations of these methods can be used for on-line and off-line tests.