Partial discharges usually occur in voids in the insulation of cables and windings as the di-electric constant of the voids are significantly lesser than the surrounding insulating media. They can also appear across bubbles in liquid insulating media such as in breakers or transformers. In gaseous media, they can appear as corona around an electrode. Corona can be seen in wet weather in power lines.
Partial Discharge occurs usually in insulation which has been subjected to mechanical and electrostatic stress or which has been weakened by premature aging due to adverse environmental conditions. Once initiated, Partial Discharge results in the formation of electrical trees which can accelerate the failure of the cable causing a short-circuit between two conductors or an earth fault. They can appear as "tracking" or distinct pathways in the cable insulation
Partial discharge can also be caused by improperly terminated/jointed connections in HV cables and windings.
The currents which flow during Partial discharge are extremely small and last for very short periods of time of the order of nanoseconds. This makes them difficult to measure. Partial Discharges generally dissipate energy in the form of heat, light and sound.
There are a wide range of methods(Partial discharge Analtysis) to detect and monitor Partial Discharge. Acoustic sensors detect the ultrasonic frequency noises which occur when a partial discharge is taking place. There are also inductive and capacitive sensors which can detect Partial Discharges