dielectric breakdown in liquids

Liquid Breakdown affecting Factors

Breakdown due to gaseous inclusions

In impure liquid dielectrics there may be gas or vapor bubbles, which are form from dissolved gases. fluctuations in temperature and pressure, or other causes. Dielectric breakdown in liquids are discuss below.

The electric field Eb in a gas bubble that is submerge in a liquid of permittivity 01 is given by

dielectric breakdown in liquids
dielectric breakdown in liquids

where E0 is the field in the liquid in the absence of the bubble.

The electrostatic forces on the bladder cause it to elongate in the direction of the electric field. Expansion continues when a sufficient electric field is apply and the gas in the bladder (which has a lower dielectric strength) breaks down to a critical length. This discharge causes the liquid molecules to decompose and lead to complete degradation.

Breakdown due to liquid globules

If an insulating liquid contains a ball from another liquid in suspension, the decomposition may be due to instability of the ball in the electric field.

Consider a spherical liquid ball of permittivity 02 that is immersed in a liquid dielectric of permittivity 01. When exposed to an electric field between parallel electrodes, the field inside the sphere is given by

dielectric breakdown in liquids
Equation

where E0 is the field in the liquid in the absence of the sphere.

Electrostatic forces cause the ball to elongate and take the form of a prolate spheroid (i.e., an elongated spheroid). When the field is enlarged, the sphere is lengthened to increase the ratio of the longest diameter to the shortest diameter of the spheroid. For the same field E, the relation is a function of e2/e1.

Graph
Graph

Variation of ratio of diameters of spheroid

If 02 >> 01 (usually 02/01> 20) and the field exceeds a critical value, there is no stable shape and the ball continues to expand, eventually joining the electrodes and causing the space to collapse. If 02/01 >> 20, the critical field in which the ball becomes unstable no longer depends on the ratio and is indicated by Ecrit.

equation
equation

where 1 = surface tension of the ball (N / m) 01 = relative permittivity of the insulating liquid R = initial radius of the ball (m).

Example – dielectric breakdown in liquids

Example
Example

Therefore, we can see that a drop of water with a radius of only 1 m (quite unobservable) can significantly reduce the dielectric strength of the liquid dielectric. In fact, a water ball with a radius of only 0.05 m, which cannot be observed, is destroy at a value of around 1 MV / cm, which corresponds to the dielectric strength of the pure liquid. Therefore, even submicroscopic water sources such as condensed degradation products or solid hygroscopic contaminants can have a significant impact on degradation conditions. A ball that is unstable at an applied field value elongates rapidly, and then electrode gap break channels develop at the end of the ball. Channel propagation leads to complete failure.

Breakdown due to solid particles

Solid contaminants cannot be avoided in commercially available liquids and are present as fibers or as dispersed solid particles. If the impurity looks like a spherical particle of permittivity 02 and is present . In a liquid dielectric of permittivity 01, it experiences a force

where E = created field, r = radius of the particle.

Generally 02> 01, so the force would move the particle towards the stronger field areas. The particles continue to move in this way and align in the direction of the field. A stable chain of particles would be create that could cause a break at a critical length.

Due to the tendency to contamination, liquids over 100 kV / cm are seldom use alone in devices with a continuous power supply. However, up to 1 MV / cm can be use in conjunction with solids, which can act as barriers to prevent the formation of solid contaminants and to locate bubbles that may form.

Purification of a liquid for testing – dielectric breakdown in liquids

Removal of dust

Small dust particles can charge and cause local voltages that can cause degradation. They can also be fuse into conductive bridges between electrodes. Careful filtration can remove dust particles larger than 1m. The liquid resistance increases and greater stability is achieve.

Removal of dissolved gasses

Liquid insulation generally contains dissolved gas in small but significant amounts. Some gases, like nitrogen and hydrogen, don’t seem to interfere greatly with electrical properties, but oxygen and carbon dioxide can cause the force to change significantly. Therefore, it is necessary to control the amount of gases present. This is done by distillation and degassing.

Removal of ionic impurities

Ionic impurities in the liquid (especially easily dissociated waste water) lead to abnormal conductivity and heating of the liquid. Water can be remove by drying agents, vacuum drying and by freezing with low temperature distillation.

Ionic impurities in the liquid (especially easily dissociated waste water) lead to abnormal conductivity and heating of the liquid. Water can be remove by drying agents, vacuum drying and by freezing with low temperature distillation.

Reference dielectric breakdown in liquids

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