Internal partial discharges are electrical discharges localized in cavities that only partially bridge the insulation between two conductors. We say that they partially bridge the insulation because they do not break the insulating system.
How to generate internal partial discharges.
They normally appear in air gaps or impurities due to defects in the insulation system.
They are related to the aging of the insulation. And they are also the cause and effect of its degradation process. They are a cause for concern because they can eventually trigger electrical treeing and lead to breakage itself.
- There is a free electron available in the cavity, which is filled with gas.
- The field inside the cavity exceeds the inception fiel.
- The electron is accelerated by the electric field, gaining kinetic energy.
- This electron will impact other gas molecules.
- If the kinetic energy is enough to ionize the gas, at least one new free electron will be available.
- If, again, the new free electrons gain enough kinetic energy to ionize other molecules in the gas, an electron avalanche occurs.
One of the mechanisms that allow a free electron inside the gas is the ionization by photon or gas molecule collision. It can also be due to the Schottky emission of electrons from the surfaces of a metal or dielectric material.
Types of cavities according to their shapes.
Depending on their origin, the cavities have different shapes:
- Spherical: polymeric or resin insulated systems with internal defects or poor impregnation.
- Flat, prolate: cavities created from treeing ageing.
- Flat, oblate: delaminations due to thermal cycling at the interfaces of the insulation system.
The electric field inside the cavity varies depending on the shape of the cavity.
Representation and identification of internal partial discharges.
We represent the partial discharges using a color map. From the graph we can see how the internal partial discharges have certain characteristics that help us to identify them:
- They are symmetrical (in most cases).
- They show a moderate dispersion.
- Appear before voltage zeros as voltage increases.
They are not affected by environmental conditions.
