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 dielectrics 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
(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.