What is Partial Discharge (PD) and what are the different types of Partial Discharges in power cables?

According to IEC 60270 standard:

“Partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation between conductors and which may or may not occur adjacent to a conductor.”

PD occurs whenever there is a stressed region due to some impurity/cavity inside the insulation or when there is a protrusion outside it. The stressed region is formed if there are sharp edges or protrusions around the conductor.

Types of Partial Discharges in Power Cables

The term ‘partial discharge’ includes a wide group of discharge phenomena

• Internal Discharges: occurring in voids or cavities within solid or liquid dielectrics;
• Surface Discharges: appearing at the boundary of different insulation materials;
• Corona Discharges: occurring in gaseous dielectrics in the presence of inhomogeneous fields;
• Treeing: continuous impact of discharges in solid dielectrics forming discharge channels.

Explanation of PD phenomenon

In a typical situation of PD, imagine there is an internal cavity/void or impurity in insulation. When High Voltage is applied on HV conductor, a field is also induced on the cavity. Further, when the field increases the breakdown capacity of this defect, it breaks down and discharges different forms of energy which result in partial discharge.

The detection and measurement of discharges is based on the exchange of energy taking place during the discharge. These exchanges are manifested as:

• electrical pulse currents
• dielectric losses
• M. radiation (light)
• sound (noise)
• increased gas pressure
• chemical reactions