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I'm studying partial discharge, and it's not yet clear to me what the difference is between an electrical breakdown and a partial discharge.

Is partial discharge just a "small" electrical breakdown?

Wikipedia states that

In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress, which does not bridge the space between two conductors

"which does not bridge the space between two conductors" is really unclear to me and an explanation of it might help me to understand the whole concept of partial discharge better.

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According to the IEC 60270 standard, partial discharges are “localized electrical discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial discharges are in general a consequence of local electrical stress concentrations in the insula- tion or on the surface of the insulation."

Source: Omicron - a link to further information is given here.

It can be said, that partial discharge results in a local breakdown. Partial discharge is a local electrical discharge that only partially bridges the insulation.

However, through the time partial discharge will harm the insulation so strong that it might lead to a total breakdown.

Picture: A partial discharge within solid insulation. When a spark jumps the gap within the gas-filled void, a small current flows in the conductors, attenuated by the voltage divider network Cx, Cy, Cz in parallel with the bulk capacitance Cb

A partial discharge within solid insulation. When a spark jumps the gap within the gas-filled void, a small current flows in the conductors, attenuated by the voltage divider network Cx, Cy, Cz in parallel with the bulk capacitance Cb Source-Wikipedia

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  • \$\begingroup\$ The link below the IEC quote does not lead directly to the source of the quote. It is better to name the source (Omicron?). \$\endgroup\$ – Charles Cowie Jan 25 at 15:08
  • \$\begingroup\$ Yes you are right. Does it make sense to directly link to the IEC standard? I mean you would have to pay for it to read it... \$\endgroup\$ – Johannes Jan 25 at 15:14
  • \$\begingroup\$ It would make sense to attribute IEC only if you had quoted the standard directly. I see you have revised the answer. That is better. It looks like it is difficult or impossible to link the PDF linked from the link you provided. It would be better to say "a link to further information is given here." or some such thing. \$\endgroup\$ – Charles Cowie Jan 25 at 15:25
  • \$\begingroup\$ What does it mean that only partially bridge in the insulation? Even in the case of a partial discharge there is a shortcut? \$\endgroup\$ – Tommaso Bendinelli Jan 25 at 16:15
  • \$\begingroup\$ PD can be caused through an increase of the field strength or local reduction of the dielectric strength. An example for the decrease of the dielectric strength would be the void mentioned in the picture of my post. Imagine this void is an air gap within the insulation (e.g. cast resin of a transformer). So cast resin has a far better dielectric strength than air. The air gap is a shortcut because their will be a concentration of the electric field. If the voltage "across" the gap is higher than the corona inception voltage PD will occur. It will harm the insulation and cause the void to grow. \$\endgroup\$ – Johannes Jan 25 at 17:48
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"which does not bridge the space between two conductors" is really unclear to me and an explanation of it might help me to understand the whole concept of partial discharge better.

The space between two conductors often consists of several layers of insulation. For example, there may be the insulation on one conductor, air between the two conductors, and the insulation of the second conductor.

In the following illustration from the Wikipedia article, three layer have developed because of a gas-filled void in a single layer of insulation. There could also be a situation with two conductors separated by air. If the insulation of one conductor deteriorates at a small weak spot, the other two layers may prevent current from flowing, but they would be exposed to a higher voltage. The caption describes a small current flowing, attenuated by the capacitive voltage divider. That is leakage current that may not be immediately harmful, but is likely to cause additional deterioration leading to a complete failure of the insulation system and a short circuit.

In some situations, there may be additional modes of current attenuation. If the partial discharge is caused by a repetitive, short-time voltage spike that is riding on a lower continuous voltage source, the source impedance of the spike and the undamaged portion of the insulation may attenuate the fault current. Once the insulation has been damaged to a greater extent, the average voltage of the supply could then drive a much higher fault current.

enter image description here

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