Thermal Breakdown
- No solid is perfect electrical insulator, hence under an electric field small conduction currents always flow in a solid.
- This flow of current causes local area’s and heating created by the collision of electrons with the lattice by I2R losses. Under A. C. voltage by the energy absorbed by the solid in an A. C. field.
- It can be represented by circuit in which insulator is regarded as parallel combination of resistance and pure lossless capacitance.
- Under a constant voltage ‘V’, power W generated in solids V2/R or ∈2/ρ/m2 where ∈ is stress in V/m are ρ = resistivity is Ω m.
- See the following figure of R and C and vector relation.
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| Figure A |
- Under A.C. voltage Vm Cos ot will not be exactly 90° out of 0º phase with current. The difference between the actual phase angle and 90° is loss angle 𝛿.
- P. F. of circuit is cos ϕ but for small value of 𝛿.
Cos 𝛿 =
tan 𝛿 = ∈”/∈’ where
- € ” and ∈‘ are the real and imaginary parts of relative permittivity.
- The mean power dissipated W = (W/2)C Vm2 . tan 8 watts . where W/2π frequency of applied voltage.
- Energy losses are greater on AC than DC stress owing to oscillations of molecular dipoles in solids.
- This energy loss is dissipated as heat which raises temperature of the dielectric.
- The process may be cumulative, which will raise temperature further and breakdown may occur due to thermal runway. The thermal breakdown can be illustrated by thermal balance Figure B.
- Straight-line (H) represents heat that is dissipated from the surface of the solid to ambient atmosphere.
- The heat inside the soid is removed by conduction to the surface and then by convection etc. to the surrounding medium.
- W1 is the heat generated in the cable for arranged of conductor temperature.
- The increase of W1 with H represents stable overating temperature. Φ1 in the solid because heat generated is balanced by dissipation of heat.
- When the applied voltage is increased the heat curve rises to W2 under the stable temperature θ2.
- The system becomes unstable leading to breakdown under the voltage is increased such that the curve for W3 touches the rear dissipation like H at the critical temperature θ3.
- A flat p. f ./ temperature curve is important and is achieved by composite oil paper insulation
- A steep H / temperature curve is required i.e. low thermal resistance is difficult since high electrical resistance automatically excludes low thermal resistance.
The curves discussed are represented as below :
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| Figure B |
This mechanism is dominant in insulting system under a.c. where the neat generated per unit volume in a given dielectric in a unit time.
W = E2 f ∈r tan
𝛿/ 1.8 x 1012 watts/cm2
For d.c. W = ∈2/ρ
- Thermal breakdown occur at high temperature.
- Breakdown strength depends on size of shape of sample on geometry, thermal. Properties of the electrodes and the ambient medium in a.c. breakdown strength is less than d.c. as power factor increases with frequency.