Conduction and Breakdown Commercial Liquids
Q. Explain clearly suspended particle mechanism of liquid breakdown.
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| Figure A |
1. Suspended particle theory
In commercial liquids, the presence of solid impurities cannot be avoided These impurities will be present as fibers or as dispersed solid particles.
- The permittivity of these particles (∈2) will be different from the permittivity of the liquid (∈1). If we consider these impurities to be spherical particles of radius and if the applied field is E then the particles experience a force F, where
{F = 1 (∈2 – ∈1) / 2r3 2 ∈1 + ∈2 grad E2}
- This force is directed towards areas of maximum stress, if ∈2 > ∈1 for eg. in case of presence of solid particles like paper in liquid.
- On the other hand, if only gas bubbles are present in the liquid i.e. ∈2 > ∈1, the force will be in direction of areas of lower stress. If the voltage is continuously applied (dc) or the duration of voltage is long (ac), then this force drives the particles towards areas of maximum sad stress.
- If the number of particles present are large , they become aligned due to these forces and thus form a stable chain bridging the electrode pap causing a breakdown between the electrodes.
- If there is only a single conducting particle between the electrodes, it will give rise to local field enhancement depending on its shape. If this field exceeds the breakdown system of the liquid, local breakdown will occur near the particle and this will result in the formation of gas bubbles which may lead to the breakdown of the liquid.
- The values of breakdown system of liquids containing solid impurities was found to be much less than the values for pure liquids. The impurity particles reduce the breakdown system and it was also observed that the larger the size of the particle the lower were the breakdown strength.
2. Cavitation and Bubble Theory / Cavity Breakdown
- In many liquids, the breakdown system depends strongly on the applied hydrostatic pressure, suggesting that change of phase of the medium is Involved in the breakdown process, which in other words means that a kind of vapour bubble formed is responsible for breakdown.
- The following process are responsible for formation of the vapour bubbles :
(a) Gas pockets at the surfaces of the electrodes.
(b) Electrostatic repulsive forces between space changes which may be sufficient to overcome the surface tension.
(c) Gaseous products due to the dissociation of liquid molecules by electron
(d) Vaporization of the liquid by corona type discharge from sharp points and irregularities on the electrode surface.
Once a bubble is formed it will elongate in the direction of the electric field under the influence of electrostatic forces. The volume of the bubble remains constant during elongation. Breakdown occurs when the voltage drop along the length of the bubble becomes equal to the minimum value on the Paschen’s curve for the gas in the bubble. The breakdown is given as :
E0 = 1 / (∈1 –∈2) [2πσ (2∈1 + ∈2) / r {π / 4 √Vb / 2r E0 – 1}]
where,
σ = Surface tension of the liquid
∈1
∈2= Permittivity of the gas bubble.
r = initial radius of the bubble as a sphere.
Vb = voltage drop in the bubble (corresponding to minimum on the Paschen’s curve).
From this equation it can be seen that the breakdown strength depends on the initial size of the bubble which in turn is influenced by the hydrostatic pressure and temperature of the liquid.
- This theory does not take into account the production of the initial bubble and hence the results given by this theory do not agree well with experimental results.
3. Stressed Oil Volume Theory
- In commercial liquids where minute traces of impurities are present, the breakdown strength is determined by the “largest possible impurity” or “weak link”.
- Electrical breakdown system of oil was proposed to be defined by the weakest region in the oil, namely the region which is stressed to the maximum and by the volume of oil included in that region.
- In non – uniform fields, the stressed oil volume is taken as the volume which is contained between the maximum stress (Emax) contour and 0.9 Emax contour. According to this theory the breakdown system is inversely proportional to the electric stressed oil volume.
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| Figure B |
- The breakdown voltage is highly influenced by the gas content in the oil viscosity of the oil and the presence of other impurities. These being uniformly distributed, inverse in the stressed oil volume consequently results in a reduction in the breakdown voltage. The variation of breakdown voltage stress with stressed oil volume is shown in Figure B.