Partial Discharges: Physics and Classification

Partial discharges occur in electrical insulation systems when the electric field strength exceeds the breakdown value of the material, and the current through the gas or vapor appears produced by the process of ionization. The ionization of the material is a process of separating the electrons of the molecules of the insulation material, producing ions and electrons. This process can be easily seen in gases and liquids, where the ionization process occurs more quickly. In solid materials, the ionization is more difficult to observe, because the electrons are more strongly bound to the atoms. The breakdown of the material is a stochastic process, i.e. it cannot be accurately predicted. It is possible to estimate the time of occurrence of the breakdown by knowing the electric field strength and the characteristics of the material. This chapter presents the main terms and concepts related to PDs. We focus on the physics of the discharges, specifically when equipment is subjected to high voltage. Special attention will be given to PDs that occur in stator insulation, providing an overview of their sources as well as a description of typical PD patterns.