•     Void Discharge   

This discharge usually occurs due to the manufacturer’s defects in solid insulation. Commonly found in Cables, Bushings, GIS Junction insulation. Highly destructive to insulation. Voids typically continue to grow until failure. If a void PD is discovered, the insulator should be replaced.   

•   Corona Discharge  

Corona is a discharge to air from the sharp surface of a conductor. Corona is typically not a problem besides the sound and the radio frequency emission. Corona signals have many distinct characteristics, allowing it to be easily differentiated from other PD signals.. Corona disturbances do not interfere with other PD measurements. 

•   Surface Discharge  

Also known as "surface tracking". Discharge along the surface of insulation can be very destructive. Usually caused by contamination or weathering of insulator surface. It is different from corona because it tends to track to grounded metal, while corona discharges to air. Corona conditions can evolve into surface PD as they become more severe. This can happen on any MV and HV equipment when strength of insulation breaks down in high humidity environments. or poor maintenance of equipment can lead to this phenomenon.  Moisture intrusion is also common cause of surface PD.   

•   Floating Discharge  

Occurs when exposed load carrying conductor is exposed to another conductive surface of different potential not connected to said conductor. Types: Metal to Insulation and Metal to Metal. Usually caused by a manufacturing defect or non-grounded piece of metal within the field. Floating Discharge is the most common type of PD. Often caused by human interaction, such as conductors not positioned properly or foreign or loose object inside of insulation.  

•    Particle Discharge  

Occurs in GIS (SF6 gas)  and oil insulated transformers. Caused when conductive particles are left inside system. Allows PD to jump from particle to particle.  Purifying and processing the oil or SF6 is recommended.