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Liquid dielectrics are used for filling transformers, circuit breakers and as impregnates in high voltage cables and capacitors.
For transformer, the liquid dielectric is used both for providing insulation between the live parts of the transformer and the grounded parts besides carrying out the heat from the transformer to the atmosphere thus providing cooling effect.
For circuit breaker, again besides providing insulation between the live parts and the grounded parts, the liquid dielectric is used to quench the arc developed between the breaker contacts.

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The liquid dielectrics mostly used are petroleum oils.
Other oils used are synthetic hydrocarbons and halogenated hydrocarbons and for very high temperature applications silicone oils and fluorinated hydrocarbons are also used.
The three most important properties of liquid dielectric are
(i) The dielectric strength
(ii) The dielectric constant
(iii) The electrical conductivity.
Other important properties are viscosity, thermal stability, specific gravity, flash point etc.

The most important factors which affect the dielectric strength of oil are the, presence of fine water droplets and the fibrous impurities.
The presence of even 0.01% water in oil brings down the dielectric strength to 20% of the dry oil value and the presence of fibrous impurities brings down the dielectric strength much sharply

Breakdown Mechanism in liquid� dielectrics

Electronic Breakdown

Once an electron is injected into the liquid, it gains energy from the electric field applied between the electrodes.
It is presumed that some electrons will gain more energy due to field than they would lose during collision.
These electrons are accelerated under the electric field and would gain sufficient energy to knock out an electron and thus initiate the process of avalanche.
The threshold condition for the beginning of avalanche is achieved when the energy gained by the electron equals the energy lost during ionization (electron emission) and is given by

Suspended Solid Particle Mechanism

Commercial liquids will always contain solid impurities either as fibres or as dispersed solid particles.
The permittivity of these solids (E₁) will always be different from that of the liquid (E₂).
Let us assume these particles to be sphere of radius r. These particles get polarized in an electric field E and experience a force which is given as

and this force is directed towards a place of higher stress if ε₁ > ε₂ and
towards a place of lower stress if ε₁ < ε₂ when ε₁ is the permittivity of gas bubbles.
The force given above increases as the permittivity of the suspended particles (ε₁) increases.

Thus, the force will tend the particle to move towards the strongest region of the field. In a uniform electric field which usually can be developed by a small sphere gap, the field is the strongest in the uniform field region.
Here dE/dx → 0 so that the force on the particle is zero and the particle remains in equilibrium.
Therefore, the particles will be dragged into the uniform field region.
Since the permittivity of the particles is higher than that of the liquid, the presence of particle in the uniform field region will cause flux concentration at its surface.

Other particles if present will be attracted towards the higher flux concentration.
If the particles present are large, they become aligned due to these forces and form a bridge across the gap.
The field in the liquid between the gap will increase and if it reaches critical value, breakdown will take place.
If the number of particles is not sufficient to bridge the gap,
the particles will give rise to local field enhancement and
if the field exceeds the dielectric strength of liquid, local breakdown will occur near the particles and
thus it will result in the formation of gas bubbles which have much less dielectric strength and hence finally lead to the breakdown of the liquid.

The movement of the particle under the influence of electric field is opposed by the viscous force posed by the liquid and since the particles are moving into the region of high stress, diffusion must also be taken into account.
We know that the viscous force is given by (Stoke’s relation)

F_V= 6πnrν where η is the viscosity of liquid, r the radius of the particle and v the velocity of the particle.

However, if the diffusion process is included, the drift velocity due to diffusion will be given by

where D = KT/6πηr a relation known as Stokes-Einstein relation. Here K is Boltzmann’s constant and T the absolute temperature.
At any instant of time, the particle should have one velocity and, therefore, equation v = v_d

It is clear that the breakdown strength E depends upon the concentration of particles N, radius r of particle, viscosity η of liquid and temperature T of the liquid.
It has been found that liquid with solid impurities has lower dielectric strength as compared to its pure form.
Also, it has been observed that larger the size of the particles impurity the lower the overall dielectric strength of the liquid containing the impurity.