Breather, as the name suggest is just an arrangement though which a Transformer breath air. How does a Transformer Breath?
As the load on a Transformer increases, the temperature of Transformer oil increases which cause expansion in oil volume and thus the expanding oil volume pushes the air outside from the Conservator of Transformer. On contrary, if the load on Transformer decreases, the oil volume decreases which cause vacuum in the Transformer oil tank upper part. Due to this vacuum the outside atmospheric air rush toward the Transformer conservator tank. In this way a Transformer breath.
Like Human being need fresh air, likewise a Transformer also need fresh air to breath. For Transformer health, the air must be moisture and dust free. But as the atmospheric air always contains moisture and dust, we need to filter it out. Ingress of moisture will reduce the quality of Transformer insulation oil which will result in degradation in Dielectric Strength of Transformer Oil. Thus we need to care for Transformer heath by providing a mask to filter out the atmospheric air. This filter is Silica Gel Breather.
Construction of Silica Gel Breather:
The construction of Silica Gel Breather of Transformer is very simple from design point of view. It is nothing but a pot of Silica Gel through which, air passes during breathing of transformer. The silica gel is a very good absorber of moisture.
Freshly regenerated Silica Gel is very efficient; it may dry down air to a dew point of below −40°C. A well maintained Silica Gel Breather will generally operate with a dew point of −35°C as long as a large enough and regenerated Silica Gel has been used.
As shown in figure above, there is an oil pot just below the Silica Gel pot. This oil pot serves the purpose of removing the dust particles from the atmospheric air and called Oil Seal Cup. The dust particles get trapped in the oil of Oil Seal Cup.
How does Silica Gel Breather Work?
Silica Gel crystal has a great capacity of absorbing moisture due to its hygroscopic nature. Hygroscopic materials are those which has capacity to absorb moisture.
When atmospheric air passes through the crystals of Silica Gel in the breather; the moisture of the air is absorbed by them. Therefore, the air which reaches to the conservator is quite dry; the dust particles in the air get trapped by the oil in the oil seal cup. The oil in the oil sealing cup acts as barrier between silica gel crystal and air when there is no flow of air through silica gel breather.
Earlier used Silica Gel contained Cobalt Chloride which is a known human toxin. The color of this Silica Gel crystal is dark blue when regenerated but, when it absorbs moisture, i.e. exhausted its color becomes pink. This type i.e. Blue Silica Gel is not used these days.
In 1998, the European Union determined that Blue Indicating Silica Gel, which is contains Cobalt Dichloride, a known human toxin, and prohibited its use within European markets. Orange Indicating Silica Gel conforms to the European Union requirements for moisture indicating desiccants. Similar to Blue Silica Gel, Orange Silica gel is used in Transformer breathers.
Orange Indicating Silica Gel, sometimes known as Yellow Indicating Silica Gel, is comprised of mineral Silicon Dioxide that is purified and processed into a beaded form. When the beads are dry, they appear orange in color, and when they have become saturated with moisture i.e. exhausted, the beads will change to a green color as shown in figure above.