Partial Discharge in Solid Insulation

• The partial discharge (PD) is originated in voids/cavities, cracks or inclusions within a solid dielectric.
• Since the discharge is for only limited portion/area, it is known as partial discharge (PD).

• In case of epoxy material, gas-filles voids are caused due to air leaking into mould during curing process.
• Air leakage into mould causes cavity.
• The partial discharge can also occur along the surface of the dielectric.
• Consider a dielectric material placed between two electrodes as shown in Figure A.

1. When voltage is applied to the dielectric, the voltage stress appearing across the void is significantly higher than the remaining mass of the dielectric.
2. This is because the dielectric constant of void is considerably less than surrounding dielectric.
3. If the voltage across the void increases above corona inception voltage (CIV) of the gas present in the void, the partial discharge takes place across the void.
4. This can cause progressive deterioration of the dielectric material. The equivalent circuit of this process is represented in Figure B.

1. C_V represents capacitance of void and C_Y represents capacitance of dielectric in series with void.
2. C_b represents capacitance of remaining mass of dielectric.
3. When partial discharge takes place, high frequency current are produced for a very short duration of time and after discharge, this current disappears.
4. Let applied voltage across the dielectric is V volt and the voltage across void is V₁ volt.

d₁ = thickness of dielectric

d₂ = thickness of dielectric

∈₀ = permittivity of void air medium

∈_r = permittivity of dielectric medium

∈₁ = ∈₀ ∈_r