Thermal Mechanism of Breakdown
Q. Explain the various theories that explain breakdown in commercial liquid dielectrics.
This mechanism is based on the experimental observations of extremely large currents just before breakdown . This high circuit pulses are believed to originate from the tips of the microscopic projections on the cathode surface with densities of the order of 1A / cm3
- These high density circuit pulses give rise to localized heating of the oil which may lead to the formation of vapour bubbles. The vapour bubbles are formed when the energy exceeds 107 J/cm2.
- When a bubble is formed, breakdown follows , either because of its elongatia a critical size or when it completely bridges the gap between the electrodes, either case, it will result in the formation of a spark. According to mechanism, the breakdown system depends on the prior and the molecular structure of liquid.
- For e.g. : It n-alkenes the breakdown strength was observed to depend on ti chain length of the molecule.
- This theory is applicable only at very small lengths (≤ 100 µm) and does explain the reduction in breakdown system with increased gap lengths.