PD 101 Series – #01: What is Partial Discharge?

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Partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress.

The definition of partial discharge in IEC 60270-2000 is “Localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor“, and it may cause irreversible damage to liquid and solid insulation systems.

Partial discharge occurs as a consequence of High electric field strength

Local over-heating (creation of voids and bubbles) Defects in the insulation material

Delamination Abrasion

Mechanical stress (vibration) Water treeing

Partial Discharge analysis allows us to detect critical defects and assess a condition of the insulation systems. In many cases PD phenomena are the preliminary stage of a complete insulation breakdown, and a result transformers, generators, instrument transformers, cable systems, and switchgear have been checked for PD for many years.

In general, type of partial discharge can be broken down into 2 categories, one is internal partial discharge and the other is external partial discharge.

The void discharge and treeing are the most dangerous processes for assets’ insulation systems being almost impossible to accurately localize them. Based on PD measurements (on/off line) the assets can be taken out of service at the most suitable moment of time (before accidents).


The dielectric of a capacitor includes a gas void (upper left side figure) and the equivalent circuit diagram of this dielectric looks like the lower left side figure. The capacitors CS and CF form a capacitive divider. Thus the U1 drop voltage on CF is lower than the applied voltage Ut (right side picture).

If the electric field strength in the insulation becomes higher than the dielectric strength of the gas inside the void, the total breakdown will appear inside the void. In this moment, the switch “S” closes and the voltage “U1”1 across the void capacitance drops. This drop in voltage “U1” will extinguish the discharge.

The process is reloaded when the electric field strength in the insulation becomes again higher than the dielectric strength of the gas inside the void. This process appears at the zero crosses of Ut and depends on the voltage gradient (around the peaks the voltage gradient tends to zero).

Partial discharge is indicated as charge Q[Coulomb] and calculated by integral formula above.


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