Breakdown mechanisms in commercial liquids

Commercial liquid dielectrics (transformer oil, cable oil) are never perfectly pure. They contain solid particles, dissolved gases, moisture and gas bubbles, so their breakdown strength (about 100–200 kV/cm in tests) is much lower than that of pure liquids (about 1 MV/cm). Their breakdown is governed by these impurities, not by the liquid molecules.

Mechanisms of breakdown in commercial liquids

1. Suspended particle mechanism
2. Cavitation and bubble mechanism
3. Thermal mechanism
4. Stressed oil volume mechanism

1. Suspended particle mechanism

• Solid impurities (fibres, dust, wet cellulose) with permittivity $\varepsilon_2$ higher than that of the oil $\varepsilon_1$ become polarized.
• In a non-uniform field a particle of radius $r$ is pulled towards the region of highest stress by the force

$$F = r^3\,\frac{\varepsilon_2 - \varepsilon_1}{2\varepsilon_1 + \varepsilon_2}\,E\,\frac{dE}{dx}$$

• Particles line up end to end and form a bridge between the electrodes. Current through the bridge heats it, moisture vaporizes and breakdown follows.
• Breakdown strength therefore falls with more particles and longer time of stress.

2. Cavitation and bubble mechanism

Bubbles form in the oil from: gas pockets on electrode surfaces, electrostatic repulsion of space charge, dissociation of oil by electron collision, and vaporization by corona at sharp points.

• The bubble has lower permittivity, so the stress in it is higher.
• The bubble elongates along the field (Kao's theory) and when the voltage drop along it reaches the Paschen minimum of the gas, it ionizes.
• Ionization produces more gas, the bubble grows and bridges the gap.

3. Thermal mechanism

Under very short pulses, high current density at cathode micro-points causes local heating and vaporization, forming a vapour channel.

4. Stressed oil volume mechanism

Breakdown strength depends on the volume of oil stressed above about 90% of the maximum. A larger stressed volume contains more weak links (particles), so strength falls as stressed volume increases.

Breakdown phenomenon in brief

 Impurities in oil (particles, gas, water)
          |
 Field polarizes/elongates them
          |
 Particle chain or elongated bubble
          |
 Local high current / ionization
          |
 Heating, more gas, channel grows
          |
 Gap bridged -> breakdown (arc)

In practice the breakdown strength of oil is improved by filtering, degassing and drying, and is checked by the standard breakdown voltage (BDV) test.