Partial Discharge Test For Cables

To understand PD , we first need to understand the Conductor and Insulator which are used for electrical transmission and distribution of electric power from one point to other point by using Power cables.
Power cables are made of following

Insulation and Dielectric :
• Electrical insulation
• Metals – which is having loose electrons
• Organic molecules – have shared electrons

Electrical insulators that can be electrostatically charged by an applied electric field are referred to as a dielectric , there are 2 main functions of Insulators
• for Energy storing capacity
• and to have Electrical obstruction

Insulator Types :
Common high HV insulating materials
• Gasses – Air (78% Nitrogen!) – SF6
• Liquids – Mineral oil – Silicon oil
• Solids – Ceramic – Glass – Mica – High polymer plastics – Resin

These above insulators decrease their quality as per utilization and application of
• Thermal stress
• Mechanical stress
• Electrical stress

If this insulation fails i.e breakdown happens causing of electrical failures
• In medium and high voltage equipment, partial discharge (PD) is the first indication of insulation breakdown
• Common phenomenon in solid insulation – electrical treeing

Hence PD is observed as
“A localized electrical discharge that only partially bridges the insulation between conductors as per IEC60270
PD gets developed when voltage stress across the void exceeds the dielectric strength of the insulating material
Physical and chemical changes may happen, which produce emissions that we can detect

PD can be seen in the form of
• discharge due to Void or hole due to improper of joints and termination
• discharge on Surface of termination and joints
• Corona discharge in the air near bus bar or near earthling point