# Partial Discharge Source Classification in Power Transformers: A Systematic Literature Review
## 2 Conceptualization of Partial Discharge in Transformers  
The IEC 60270 standard defines partial discharge as a localized breakdown of the transformer solid or liquid dielectric material, which moderately bridges the conductors and may or may not occur close to a conductor [11]. PD mainly occurs due to nearby voltage stresses within the insulation or even on the surface of solid insulation, which results in a slow deterioration of the insulation material and reduces the life of the insulation in the transformer, which may ultimately cause complete insulation breakdown and premature failure of a transformer. It emerges as current pulses lasting much shorter than 1 us; this is due to electric fields that are larger than the limit of the insulation and thus induce a partial breakdown of the local dielectric material [12]. The characteristics of PD result in the production of light, heat, sound, electromagnetic and chemical reactions [13] which contribute to further deterioration of the insulation material, but also provide means for sensing and monitoring the source and level of PD within a transformer by methods such as acoustic emission, optical, ultra-high frequency and electrical. Three essential definitions related to PD as defined in [14] are apparent charge, PD inception voltage and PD extinction voltage. Apparent charge 'q' of a PD pulse amounts to the charge which, if introduced between the connections of the test object within a short duration of time, would yield the same voltage measurement across the terminals as the PD pulse itself, and is expressed in pico-coulombs (pC). The PD inception voltage (PDIV) is the smallest voltage at which the magnitude of a partial discharge pulse value is equivalent to or greater than a given low value. The PD extinction value (PDEV) is the smallest voltage at which the magnitude of a chosen PD pulse value is equivalent to or lower than a given low value. Furthermore, an equivalent circuit representing the PD activity is formulated using the premise of PD inception in a void. The PD results in a spark occurring inside the void, thus initiation the flow of current in the conductor. This equivalent circuit is represented by a capacitor 'C3', which is parallel to two other capacitors, 'C1' and 'C2', as shown in Figure 1. 'C1' and'C2' form a voltage divider, with 'C1' representing the capacitance of the void while 'C2' is the capacitance in series with the void. 'C3' is the capacitances within the insulation. The nature and form of electric transformers is that they are made up of a variety of insulation materials. Internally the transformers are filled with insulating oil which makes up the liquid insulation material. Solid insulation materials inside a transformer come in the form of Kraft paper which is primarily wrapped around the conductors to provide electrical insulation strength. Pressboard is also extensively utilized as insulation between layers of conductor, as well as to provide both electrical insulation and mechanical support for the transformer internals. Externally transformers have bushings; some house busbars within ducts while others utilize spacing as a form of isolation between phases. This broad variety of dielectric materials mean that transformers are also exposed to many forms of PD. Ref. [15] classifies PD occurring in transformers into five types, namely corona, surface discharge, internal discharge, electrical treeing and barrier discharge. Corona discharge occurs due to sharp live points which ionize a fluid dielectric such as air or oil. This can occur inside or outside the transformer on bare conductors or terminations. Surface discharge occurs in close proximity to the dielectric material, which is a result of a difference in electric field strength between the insulation and the barrier of the insulation material. Surface discharges are further influenced and propagated by pollution or contamination on the insulation material such as dust or water on bushing insulators.