solid dielectric insulation

Liquid dielectric Breakdown

Breakdown in liquid dielectric

In highly purified liquid dielectrics, decomposition is control by gas-like phenomena and the electrical resistance is high .(on the order of 1 MV / cm). Unfortunately, liquids are easily contaminate and can contain solids, other suspend liquids, and dissolved gases. The effect of these impurities is relatively low for short-term pulses (10 s). Breakdown in liquid dielectric is advancedly describe below.

However, when the voltage is apply continuously, the solid contaminants align at right angles to the equipotentials and distort the field, so that failure occurs at a relatively low voltage. Particle disposition is a fairly slow process and resistance is unlikely to be affect at voltages lasting less than 1 ms.

Under the influence of the electric field, dissolved gases can escape from the solution and form a bubble. The gas in the bladder has less force than the liquid, so more gas is generated and the bladder grows, ultimately leading to collapse. Due to the tendency to contaminate, liquids are generally not used alone above 100 kV / cm only in continuous power devices, but are used at much higher voltages (up to 1 MV / cm) along with solids that can be use, to act as barriers that prevent the accumulation of solid contaminants and the location of bubbles that can form. The main function of the liquid in such arrangements is to fill the cavities.

Breakdown of Commercial liquids

When a potential difference is applied to a pair of electrodes immersed in an insulating liquid, a small line current is first observed. When the voltage continuously increases, a spark occurs between the electrodes at a critical voltage. The passage of a spark through a liquid involves the following.

(a) Flow of a relatively large amount of electricity, determined by the properties of the circuit,

(b) a bright light path from electrode to electrode,

(c) the development of gas bubbles and the formation of solid decomposition products ( if the liquid is of the required chemical nature)

(d) formation of small depressions in the electrodes,

(e) a pulse pressure through the liquid with an accompanying explosive noise.

Tests in highly purified transformer oil show this

(a) Dielectric strength has a small but clear dependence on the electrode material,

(b) dielectric strength decreases with increasing electrode distance,

(c) dielectric strength is independent of hydrostatic pressure for degassed oil, but increases with pressure when petroleum gases like contains nitrogen or oxygen in solution.

In the case of a commercially available insulating liquid, which may not be subject to a very complex cleaning treatment, the dielectric strength depends more on the type of impurities contained in it than on the type of liquid itself.

These contaminants, which lead to the degradation of commercial liquids below their inherent strength, can be divided into the following 3 categories.

Degradation of commercial liquids – liquid dielectric

(a) Impurities whose dielectric strength is less than that of the liquid itself (for example, gas bubbles). The decomposition of contaminants can trigger complete decomposition of the liquid.

(b) Unstable impurities in the electric field (for example, water balls). Contamination instability can result in a low resistance bridge across the electrodes and a complete rupture.

(c) impurities that lead to local amplification of the electric field in a liquid (eg, conductive particles). The expanded field can cause a local disturbance and therefore trigger a complete disturbance.

These are discussed in order in the following sections.

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 do 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 breakdown in liquid dielectric

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