Discharge And Breakdown Due to Internal Discharge And Errosion Breakdown
- In practice it is difficult to produce a continuous homogeneous solid dielectric block.
- Usually voids or cavities are present in a solid dielectric or at the boundaries between dielectric and electrodes.
- They are of various shapes and sizes and volumes.
- These voide contain gas or medium of lower dielectric strength and the dielectric constant of medium into voids is lower than that of the insulation.
- Hence the electric field strength in the void is higher than that across the dielectric and there will be a field concentration in it owing to difference in the permittivities.
- The field stress ES in the cavity (i.e. in the void) is determined by the ratio of permittivity and the shape of cavity.
- Shape is important due to depolarising effect of the void. It is maximum for the disc shape cavity with its axis of symmetry parallel to the field direction.
- Therefore even under normal working voltages the field in the wire may exceed and corona discharge will occur across the void. When its peak stress equals the breakdown strength of the air and the voltage at which this occurs incretion is known as the discharge voltage.
- The partial discharge repeat every cycle, causing gradual erosion and chemical decoration of the cavity surface. The temperature produced at the extremities of a discharge cause local and progressive degradation and melting of the material which ultimately leads to breakdown.
- Let us consider dielectric between two conductors as shown. Divide the insulation into three parts an electric network of C1, C2, and C3 where
C1 =capacitance of void or cavity
C2 = the capacitance of the rest of the dielectric
where V = applied voltage
V1 = Voltage across the void and therefore V1
is given by
V1 = Vd1/d1+( ∈0/
∈1)d2
d1-d2 = Thickness of the void and
dielectric cavity
∈0, ∈1 = permittivities
of void and dielectric cavity d1<<d2
If the cavities are filled with a gas then
V1 = V∈2 (d1/d2)
Where ∈r = relative permittivity of dielectric
D1,d2 = are thickness of void and dielectric
cavity
∈0, ∈1 = permittivity of
void and dielectric d1<<d2
If cavity is filled with gas V1 = V∈r [d1/d2]
∈r = Relative permittivity of dielectric
when a voltage V is applied,
V1 reaches the breakdown strength of the medium in
the cavity
(V1) and breakdown occurs.
V1 is called discharge inception voltage.
- When the applied voltage is A.C., the breakdown occurs both half cycles and the number of distances will depend on the applied voltage.
 |
| Figure A |
- When first breakdown occurs across the cavity the breakdown voltage across it becomes zero. And once V1 becomes zero, the spark gets extinguished and again the voltage rises till breakdown occurs again.
- This process repeats again and again both in the positive and negative half cycles.
- These internal discharges are called as partial discharges and will have some effect as treeing on the insulation.
- When breakdown occurs in the void the electrons and ions are formed. They will have sufficient energy and when they reach the void surfaces, they may break the chemical bonds.
- In such discharge there will be more heat dissipated in the cavities and this will carbonize the surface of the voids and will cause erosion of the material. Channels and pits formed on the cavity surface increase the conduction Chemical degradation may also occur as a result of active discharge products formed during breakdown.
- All these effects will result into gradual erosion of metal and consecutively reduce thickness of insulation leading to breakdown.
- Life of insulation with internal discharge depends upon applied voltage and number of discharge i.e. number of cycle of voltage.
 |
| Figure B |